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CITY OF RENTON
AGENDA - City Council Regular Meeting
7:00 PM - Monday, June 13, 2022
Council Chambers, 7th Floor, City Hall – 1055 S. Grady Way
Please note that this regular meeting of the Renton City Council is being offered as a hybrid
meeting and can be attended in person at the Council Chambers, 7th floor of City Hall, 1055 S
Grady Way, Renton, 98057 or remotely through Zoom.
Zoom Participants: Speakers providing audience comments through Zoom must click the link
to the registration form (linked below), fill it out, and submit it by 5 p.m. on the day of the
Council meeting. The public may also submit comments in writing to cityclerk@rentonwa.gov
by 5 p.m. on the day of the meeting. Registration is not required for those who wish to speak
during public hearings.
Registration for Audience Comment
Registration will be open at all times, but speakers must register by 5 p.m. on the day of a
Council meeting in order to be called upon. Anyone who registers after 5 p.m. on the day of
the Council meeting will not be called upon to speak and will be required to re-register for the
next Council meeting if they wish to speak at that next meeting.
• Request to Speak Registration Form
• You may also copy/paste the following URL into your browser:
https://forms.office.com/g/bTJUj6NrEE
• You may also call 425-430-6501 or email jsubia@rentonwa.gov or
cityclerk@rentonwa.gov to register. Please provide your full name, city of residence,
email address and/or phone number, and topic in your message.
• Instructions for Virtual Attendance
For those wishing to attend by Zoom, please (1) click this link:
https://us02web.zoom.us/j/84938072917?pwd=TUNCcnppbjNjbjNRMWpZaXk2bjJnZz09 (or
copy the URL and paste into a web browser) or (2) call-in to the Zoom meeting by dialing 253-
215-8782 and entering 849 3807 2917 Passcode 156708, or (3) call 425-430-6501 by 5 p.m. on
the day of the meeting to request an invite with a link to the meeting.
Those providing audience comments will be limited to 5 minutes each speaker unless an
exception is granted by the Council. Attendees will be muted and not audible to the Council
except during times they are designated to speak. Advance instructions for how to address
the Council will be provided to those who sign up in advance to speak and again during the
meeting.
1. CALL TO ORDER AND PLEDGE OF ALLEGIANCE
2. ROLL CALL
3. PROCLAMATION
a) Juneteenth 2022 - June 19, 2022
4. ADMINISTRATIVE REPORT
a) Administrative Report
5. AUDIENCE COMMENTS
• All remarks must be addressed to the Council as a whole, if a response is requested
please provide your name and address, including email address, to the City Clerk to
allow for follow‐up.
• Speakers must sign-up prior to the Council meeting.
• Each speaker is allowed five minutes.
• When recognized, please state your name & city of residence for the record.
NOTICE to all participants: Pursuant to state law, RCW 42.17A.555, campaigning for any
ballot measure or candidate in City Hall and/or during any portion of the council meeting,
including the audience comment portion of the meeting, is PROHIBITED.
6. CONSENT AGENDA
The following items are distributed to Councilmembers in advance for study and review, and
the recommended actions will be accepted in a single motion. Any item may be removed for
further discussion if requested by a Councilmember.
a) Approval of Council Meeting minutes of June 6, 2022.
Council Concur
b) AB - 3144 Mayor Pavone Mayor Pavone reappoints Al Dieckman to the Parks Commission
for a term expiring June 1, 2026.
Council Concur
c) AB - 3141 Community & Economic Development Department recommends approval of an
amendment to CAG-22-028, with KPG Psomas, Inc., in the amount of $303,333.08, which
re-assigns the contract due to a corporate merger, and modifies the Scope of Work to
include preparation of final bid documents and oversight assistance during the bid
process.
Refer to Finance Committee
d) AB - 3143 Finance Department recommends approval of an ordinance amending Renton
Municipal Code (RMC) Chapters 5-25 for Business and Occupation (B&O) which increases
the B&O tax rates to 0.07% for retail sales and 0.121% for all other tax classifications,
effective January 1, 2023.
Refer to Finance Committee
e) AB - 3145 Finance Department reports that Council approved an interfund loan between
Fund 424 Golf Course Capital Improvement and 000 General Fund in 2021 through
Resolution No. 4432 for the purchase of new golf carts. The funds were not transferred
until 2022 when the golf carts were received. Fund 424 has now received the funds and
began repayment in March 2022 per the amortization schedule in Resolution No. 4432.
None; Information Only
7. UNFINISHED BUSINESS
Topics listed below were discussed in Council committees during the past week. Those topics
marked with an asterisk (*) may include legislation. Committee reports on any topics may be
held by the Chair if further review is necessary.
a) Committee of the Whole: Allocation of Lodging Tax Funding for 2022
b) Finance Committee: Vouchers; ESD Reorganization - IT Administrative Support; Philip
Arnold Park Playground Equipment Replacement; Grant Contract: Coulon North Water
Walk Legislative Appropriation
c) Planning & Development Committee: Continued Pooling of Senate House Bill 1406 Sales
Tax Credit Funds with South King Housing and Homeless Partners; Retail Pet Sales;
Docket 17 Group A; Adopting 2022 Renton Surface Water Design Manual (D-208)*
d) Public Safety Committee: Edward Byrne Memorial Justice Assistance Grant (JAG)
Program FY 2021 Local Solicitation
8. LEGISLATION
Ordinance for first reading and advancement to second and final reading:
a) Ordinance No. 6070: Adopting 2022 Renton Surface Water Design Manual (D-208) (See
Item 7.c)
Ordinance for first reading:
b) Ordinance No. 6071: B & O Tax Update (See Item 6.d)
Ordinances for second and final reading:
c) Ordinance No. 6068: Code Interpretations D-205 (First Reading 6/6/2022)
d) Ordinance No. 6069: 2022 Salary Table - Police (First Reading 6/6/2022)
9. NEW BUSINESS
(Includes Council Committee agenda topics; visit rentonwa.gov/cityclerk for more
information.)
10. ADJOURNMENT
COMMITTEE OF THE WHOLE MEETING AGENDA
(Preceding Council Meeting)
6:00 p.m. - MEETING REMOTELY
Hearing assistance devices for use in the Council Chambers are available upon request to the City Clerk
CITY COUNCIL MEETINGS ARE TELEVISED LIVE ON GOVERNMENT ACCESS CHANNEL 21
To view Council Meetings online, please visit rentonwa.gov/councilmeetings
ArmondoPavoneMayorWhereas,weacknowledgethenoticeoffreedomgiventotheslavesoftheStateofTexasonJune19,1865;andWhereas,wegivehonorandrespectonthisdaytothesufferingofourancestorsandweacknowledgetheevilsofslaveryanditsaftermath;and‘T4”hereas,onthiscelebration,wethinkaboutthemomentin1888when300BlackmenfromVirginia,NorthCarolina,andKentuckyweredeterminedtoescapetheterrorismofthepost-CivilWarSouthastheyboardedtrainsfortheWashingtonterritoryandsettledinKingCountyminingcamps;andWhereas,thestoryandoutspokennessofJamesShepperson,aneducatedBlackmanwhosettledinRoslyn,WAin1888,inspiredmanyotherblackstofleethesouthatgreatriskandpursuenewopportunitiesinWashingtonstateasminers;andW’hereas,weacknowledgeAfricanAmericanfreedom,contributions,andachievementswithinthiscommunity,pastandpresent;andWhereas,duringthisJuneteenthevent,weappreciatetheAfricanAmericanexperienceandcelebratetheinclusionofallraces,ethnicities,andnationalities;andWhereas,wecommittoworkingtogethertowardequityforallinexpandingeconomic,educational,andcareeropportunitiesforallthoseinourcommunity;fJ”fow,therefore,I,ArmondoPavone,MayoroftheCityofRenton,doherebyproclaimJune19,2022,tobeJuneteentñ2022intheCityofRenton,andIencourageallcitizenstojoinmeinthiscelebration.Inwitnesswhereof,IhavehereuntosetmyhandandcausedthesealoftheCityofRentontobeaffixedthis13thdayofJune,2022.(*Armodone,MayorCityoRenn,WashingtonARntonCityHaIl,7thFloor1055SouthGradyWayRentonWA98057rentonvd—Pjrir’iProctamationAGENDA ITEM #3. a)
Mayor’s Office
Memorandum
DATE: June 13, 2022
TO: Ryan McIrvin, Council President
Members of Renton City Council
FROM: Armondo Pavone, Mayor
Ed VanValey, Chief Administrative Officer
SUBJECT: Administrative Report
• Juneteenth is the celebration commemorating the emancipation of the last enslaved
African Americans on June 19, 1865, in Galveston, Texas. Several community groups
have activities planned for June 18:
Annual Community BBQ: presented by Doug Baldwin Jr. and United Way of King
County. Saturday, June 18, 11 a.m. to 3 p.m., Renton Memorial Stadium. Tickets are
$10 per person.
"And Still We Rise:" Free community celebration. Saturday, June 18, at noon, BLM
Mural (302 Lake Street).
Juneteenth Celebration and Festival: Free. Presented by Renton School District.
Saturday, June 18, 2 to 6 p.m., Campbell Elementary (6418 S 124th St, Seattle
98178). Activities will include a BBQ, games, a job fair, & more. For more
information and to register, go to coopartnerships.org.
• On Tuesday June 28, the Urban Forestry Division will be doing tree removal for public
and road user safety on Maple Valley Highway on the westbound side, west of the
parking area for Riverview Park. Traffic will be reduced to one lane in either direction on
the eastbound side from 9:30 am until 2:30 pm. There will also be brief closures of all
four lanes of traffic due to the position of the tree being removed.
• Information about preventative street maintenance, traffic impact projects, and road
closures happening this week can be found at http://rentonwa.gov/traffic. All projects
are weather permitting and unless otherwise noted, streets will always remain open.
Preventative street maintenance, traffic impact projects, and road closures will be at the
following locations:
Monday, June 13 through Friday, June 17, 7:00 am to 2:30 pm. Single intermittent
lane closures on the southernmost eastbound lane closure on NE Sunset Boulevard
between Aberdeen Ave NE overpass and Harrington Avenue NE due to utility
construction. Questions may be directed to Brad Stocco, 425-282 -2373 .
AGENDA ITEM #4. a)
Ryan McIrvin, Council President
Members of Renton City Council
Page 2 of 2
June 13, 2022
Monday, June 13 through Friday, June 17 , 7:00 am to 4:00 pm. Asphalt repairs on
S. 3rd Pl between Rainier Ave S and Shattuck Ave S will impact east and westbound
traffic on S. 3rd St . Flaggers will be there directing traffic.
Monday, June 13 through Friday, June 17, 8:30 am to 3:00 pm. Intermittent lane
closures on Logan Avenue N just south of N 8th Street due to roadway construction.
Questions may be directed to Brad Stocco, 425-282-2373.
Monday, June 13 through Friday, June 17, 8:30 am to 3:00 pm. Intermittent lane
closures on N 8th Street between Logan Avenue N and Park Avenue N due to
roadway construction. Questions may be directed to Brad Stocco, 425-282-2373.
Monday, June 13 through Friday, June 17, 8:30 am to 3:00 pm. Intermittent lane
closures in both directions of Duvall Avenue NE between NE 12th Street and NE
Sunset Blvd due to utility construction. Questions may be directed to Tom Main,
206-999 -1833.
Monday, June 13 through Friday, June 17, 8:30 am to 3:00 pm. Intermittent lane
closure at the 4100 block of Lincoln Ave NE due to utility and roadway construction.
Question s may be directed to Kip Braaten, 206-503-1746.
Downtown Utility Improvement Project. Completion of pavement restoration on
S. 3rd Street is dependent on weather and contractor scheduling. Expect
intermittent lane closures, parking restrictions, and detours as the contractor moves
around the downtown core to finish work. Additional information can be found at
rentonwa.gov/duip.
Duvall Avenue NE Project. Long-term double lane closure on the west side of Duvall
Ave NE between NE 9th Street and Sunset Boulevard for road construction. One
lane of traffic will be provided in each direction, tentative through August 29.
On-going Street Closure through October 4, 2023 (City of Renton Resolution No.
4446). FULL STREET CLOSURE on Sunset Lane NE between NE 10th Street and
Harrington Place NE in support of the Solera Development Project (LUA20-000305).
Questions may be directed to Brad Stocco, 425-282-2373.
AGENDA ITEM #4. a)
June 6, 2022 REGULAR COUNCIL MEETING MINUTES
CITY OF RENTON
MINUTES - City Council Regular Meeting
7:00 PM - Monday, June 6, 2022
Council Chambers, 7th Floor, City Hall – 1055 S. Grady Way
CALL TO ORDER AND PLEDGE OF ALLEGIANCE
Mayor Pavone called the meeting of the Renton City Council to order at 7:00 PM and led the
Pledge of Allegiance.
ROLL CALL
Councilmembers Present:
Ryan McIrvin, Council President
James Alberson, Jr., Council Position No. 1
Carmen Rivera, Council Position No. 2
Valerie O'Halloran, Council Position No. 3
Ed Prince, Council Position No. 5
Ruth Pérez, Council Position No. 6
Kim-Khánh Vǎn, Council Position No. 7
(McIrvin, Vǎn, and Pérez attended remotely)
Councilmembers Absent:
ADMINISTRATIVE STAFF PRESENT
Armondo Pavone, Mayor
Ed VanValey, Chief Administrative Officer
Shane Moloney, City Attorney
Jason Seth, City Clerk
Melissa McCain, Deputy City Clerk
Martin Pastucha, Public Works Administrator
Chief Jon Schuldt, Police Department Administrator
Commander Chandler Swain, Police Department
Attending remotely:
Judith Subia, Council Liaison
Kelly Beymer, Parks & Recreation Administrator
Kristi Rowland, Deputy Chief Administrative Officer
Cailín Hunsaker, Parks & Trails Director
AGENDA ITEM #6. a)
June 6, 2022 REGULAR COUNCIL MEETING MINUTES
Kim Gilman, HR Labor Manager
Ron Straka, Public Works Utility Systems Director
Vanessa Dolbee, Planning Director
Angie Mathias, Long Range Planning Manager
PROCLAMATION
a) LGBTQIA+ Pride Month Proclamation - June 2022: A proclamation by Mayor Pavone was read
declaring June 2022 to be LGBTQIA+ Pride Month in the City of Renton, encouraging all
residents to join in this special observance and recognize the numerous contributions of
LGBTQIA+ individuals in the City. Alex Shockey, of Valley Medical Center, accepted the
proclamation with appreciation.
MOVED BY RIVERA, SECONDED BY MCIRVIN, COUNCIL CONCUR IN THE
PROCLAMATION. CARRIED.
PUBLIC HEARING
a) Battery Energy Supply System (BESS) Moratorium: This being the date set, and proper
notices having been posted and published in accordance with local and State laws, Mayor
Pavone opened the public hearing to consider the Battery Energy Supply System (BESS)
Moratorium.
Senior Planner Katie Buchl-Morales introduced herself to Council and noted her contact
information for the public. She reported that emergency Ordinance No. 6061 was adopted on
April 18, 2022 which did the following:
• Established a moratorium on the acceptance of the following items related to BESS:
o Land Use Applications
o Building Permit Applications
o Business License Applications
Continuing, Ms. Buchl-Morales stated that State law requires a public hearing be held on the
issue within 60 days of adopting the moratorium, and this hearing satisfies that requirement.
She also noted that the moratorium will expire on October 18, 2022 unless lifted or extended
by Council. She also reported that the purpose of the moratorium is to allow staff adequate
time to understand and develop a work program to determine appropriate zoning and
development for BESS facilities. Concluding, Ms. Buchl-Morales explained the next steps and
asked if Council had any questions on this topic.
Mayor Pavone invited public comment:
• Nicola Robinson, Renton, spoke in favor of the moratorium and expressed many
safety concerns as to why the proposed BESS facility should be situated in a location
that is not as close to a residential neighborhood.
• Azure Allender, Renton, spoke in favor of the moratorium and expressed
environmental concerns related to battery storage supply systems.
AGENDA ITEM #6. a)
June 6, 2022 REGULAR COUNCIL MEETING MINUTES
• Ernesto Podaca, Renton, spoke in favor of the moratorium and expressed fire safety
and potential landslide concerns related to locating a battery storage supply system in
the proposed location.
• Paige Selden, Renton, expressed concerns about fire and environmental hazards that
could make the BESS facility unsafe at its proposed location. She expressed support
for the moratorium.
• Lori Goeman, Renton, echoed the concerns of the previous speakers, and expressed
support for the moratorium.
• Jack McCollough, unknown, discussed the benefits of battery storage supply systems
and urged Council to learn more about the technology, noting that a clean energy
future would not be possible without these types of facilities.
• Nancy Quinn, Renton, spoke in favor of the moratorium and expressed concerns
related to the safety of BESS facilities.
• Michael Gillette, Renton, urged Council to vote no on the proposed BESS facility
development, and to keep the moratorium in place.
• City Clerk Seth summarized correspondence from Renton resident Nicola Robinson
and Tenaska, the BESS proposal applicant.
There being no further public comment or deliberations, it was
MOVED BY PRINCE, SECONDED BY O'HALLORAN, COUNCIL CLOSE THE PUBLIC
HEARING. CARRIED.
ADMINISTRATIVE REPORT
CAO Ed VanValey reviewed a written administrative report summarizing the City’s recent
progress towards goals and work programs adopted as part of its business plan for 2022 and
beyond. Items noted were:
• National Secure Your Load Day is today, June 6. This day honors people whose lives
are impacted or taken by unsecured vehicle loads and road debris and encourages all
drivers to properly secure their loads every time they drive. Throughout the month of
June, the Renton Police Dept. Traffic Unit, as well as our patrol officers, will be doing
extra emphasis on the roads to make sure all loads are secured properly.
• Renton Farmers Market returns to Piazza Park, corner of South 3rd Street and Logan
Ave South, when it opens its 21st market season on Tuesday, June 7. Vendors will
have fresh flowers, fresh produce, honey, baked goods, ciders, wine and cheese,
crafts, and delicious ready-to-eat food. SNAP and EBT shoppers can use the SNAP
Market Match incentive program, which offers a dollar-for-dollar match of up to $40
per day to be used for fresh produce. In addition, the Kid’s Patch activity area
returns, and there’ll be live music from 4:30 to 6:30 pm. Park for free in the nearby
City Center Parking Garage (655 South 2nd St.). The market is open every Tuesday
from 3:00 to 7:00 pm through September 27. Find more information on Renton
Farmers Market on their website, Instagram or Facebook.
• Join your neighbors and police officers for coffee and conversation at “Coffee with a
Cop” on June 8 at The Rose Gift House & Coffee, 226 Main Ave S, from 10 am to noon.
Representatives from the Renton Police department will be available to answer your
questions about crime prevention, traffic enforcement, investigations, recruitment or
community programs, and services that are available to the Renton community.
AGENDA ITEM #6. a)
June 6, 2022 REGULAR COUNCIL MEETING MINUTES
• Solid Waste Utility’s second event in their series of three mini Recycle events will be
on Saturday, June 11, from 10 am to 2 pm at Renton Technical College north parking
lot, NE 6th Place & Monroe Avenue NE. Bring mattresses, cardboard and vehicle tires
for recycling. The first event on May 7 served approximately 315 participants and
prevented over 10 tons of scrap metal and 790 pounds of Styrofoam™ from going to
landfill disposal. In addition, 87 pounds of food were donated by Recycle Event
participants to The Salvation Army Food Bank.
• Sunset Community Church, located at 1032 Edmonds Avenue NE, is hosting a
free Community Pancake Breakfast on Saturday, June 11 from 8:00 to 10:00
am.
• Preventative street maintenance will continue to impact traffic and result in
occasional street closures.
AUDIENCE COMMENTS
• Claudia Donnelly, unincorporated King County, Renton, spoke about the extensive
history of water runoff issues on her property she claims are indirectly caused by
developments in Renton and unincorporated King County that are near her property.
MOVED BY PRINCE, SECONDED BY RIVERA, COUNCIL ALLOW MS. DONNELLY AN
ADDITIONAL TWO MINUTES TO FINISH HER COMMENTS. CARRIED.
Continuing, Ms. Donnelly displayed several photographs depicting the flooding that
occurs on her property due to surface water runoff issues.
• Kamran Emad, unincorporated King County, spoke about safety issues on SE 128th St
and requested Council assistance with traffic calming measures. Mayor Pavone noted
that he would have someone from Public Works reach out to Mr. Emad.
CONSENT AGENDA
Items listed on the Consent Agenda were adopted with one motion, following the listing.
a) Approval of Council Meeting minutes of May 23, 2022. Council Concur.
b) AB - 3139 Community & Economic Development Department recommended approval of the
second round of 2022 Lodging Tax Fund allocation recommendations; and approval to
execute the related contracts. Refer to Committee of the Whole.
c) AB - 3121 Equity, Housing, and Human Services Department recommended pooling one
hundred percent (100%) of Senate House Bill (SHB) 1490 sales tax credit funds with South
King Housing and Homelessness Partners (SKHPP) under the existing 2021 interlocal
agreement CAG-21-177; and requested authorization to provide an update and
recommendation to Council by June 15, 2024 on continued pooling of funds for 2025 and
beyond. Refer to Planning & Development Committee.
d) AB - 3138 Executive Services Department recommended approval to reallocate approved
budget dollars to support the implementation of an Executives Services Department
reorganization that adds one Administrative Secretary I (grade a09) to the Information
Technology Division. Refer to Finance Committee.
AGENDA ITEM #6. a)
June 6, 2022 REGULAR COUNCIL MEETING MINUTES
e) AB - 3135 Finance Department recommended setting a public hearing date of June 27, 2022
to solicit input on the preliminary 2023/2024 biennial budget. Council Concur; Set Public
Hearing on 6/27/2022.
f) AB - 3137 Human Resources / Risk Management Department recommended approval of the
Renton Police Guild Commissioned Employees' 2021-2023 contract; and adoption of an
ordinance amending the 2022 salary table to reflect collectively bargained changes. Council
Concur.
g) AB - 2995 Parks & Recreation Department - PPNR recommended approval of a grant
agreement with the Washington State Department of Commerce (DOC) to accept $1,339,000
in grant funds, with $26,780 retained by DOC for contract administration, for the Coulon Park
North Water Walk Repairs and Enhancements project. Refer to Finance Committee.
h) AB - 3132 Parks & Recreation Department - PPNR recommended approval to execute a
contract with Northwest Playground Equipment, Inc., in the amount of $376,852.92, for the
replacement of playground equipment at Philip Arnold Park. Refer to Finance Committee.
i) AB - 3136 Police Department recommended approval of the Edward Byrne Memorial Justice
Assistance (JAG) Program FY 2021 Local Solicitation agreement with the City of Seattle, to
accept $32,263 in grant funds to support police programs such as the domestic violence
victim advocate services and training. Refer to Public Safety Committee.
j) AB - 3134 Public Works Utility Systems Division submitted proposed revisions to the Renton
Municipal Code (RMC) regarding implementation of a Stormwater Pollution Source Control
Program. The Planning Commission, following review of the proposed program, will provide
code revision recommendations to Council. Refer to Planning Commission and Planning &
Development Committee.
MOVED BY MCIRVIN, SECONDED BY PRINCE, COUNCIL CONCUR TO APPROVE THE
CONSENT AGENDA, AS PRESENTED. CARRIED.
UNFINISHED BUSINESS
a) Committee of the Whole Chair McIrvin presented a report concurring in the staff
recommendation to approve and move forward with the city facility renaming process for the
Renton Senior Activity Center to honor Don Persson, as established in Policy 600-04.
MOVED BY MCIRVIN, SECONDED BY PRINCE, COUNCIL CONCUR IN THE
COMMITTEE RECOMMENDATION. CARRIED.
b) Utilities Committee Chair Alberson presented a report concurring in the staff recommendation
to approve Amendment No. 6 to CAG-15-224 with Tetra Tech, Inc. in the amount of $337,210 for
Phase 3 of the Cedar River 205 Project Levee Certification.
MOVED BY ALBERSON, SECONDED BY RIVERA, COUNCIL CONCUR IN THE
COMMITTEE RECOMMENDATION. CARRIED.
LEGISLATION
Ordinances for second and final reading:
a) Ordinance No. 6066: An ordinance was read amending Chapter 3-7 of the Renton Municipal
Code to update Public Works Department Divisions pursuant to Reorganizations, authoring
corrections, providing severability, and establishing an effective date.
AGENDA ITEM #6. a)
June 6, 2022 REGULAR COUNCIL MEETING MINUTES
MOVED BY O'HALLORAN, SECONDED BY RIVERA, COUNCIL ADOPT THE
ORDINANCE AS READ. ROLL CALL: ALL AYES. CARRIED.
b) Ordinance No. 6067: An ordinance was read amending 2022 City of Renton Salary Table to
implement an authorized reorganization of the Public Works Department and reinserting an
existing position that was previously inadvertently omitted from the Salary Table, and
establishing an effective date.
MOVED BY O'HALLORAN, SECONDED BY PÉREZ, COUNCIL ADOPT THE ORDINANCE
AS READ. ROLL CALL: ALL AYES. CARRIED.
Ordinances for first reading:
c) Ordinance No. 6068: An ordinance was read amending Chapter 4-1 of the Renton Municipal
Code; Subsections 4-2-115.F.2, 4-4-055.A, 4- 6-060.J.1, and 4-7-060.B of the Renton Municipal
Code; Definitions in Sections 4-11-010, 4-11-040, and 4-11-140 of the Renton Municipal Code;
and Section 9-2-4 of the Renton Municipal Code, codifying administrative code
interpretations from 2019 to 2022; authorizing corrections; providing for severability, and
establishing an effective date.
MOVED BY PRINCE, SECONDED BY ALBERSON, COUNCIL REFER THE ORDINANCE
FOR SECOND AND FINAL READING AT THE NEXT COUNCIL MEETING. CARRIED.
d) Ordinance No. 6069: An ordinance was read amending the 2022 City of Renton Salary Table
to reflect collectively bargained changes and establishing an effective date.
MOVED BY MCIRVIN, SECONDED BY PRINCE, COUNCIL REFER THE ORDINANCE
FOR SECOND AND FINAL READING AT THE NEXT COUNCIL MEETING. CARRIED.
NEW BUSINESS
Please see the attached Council Committee Meeting Calendar.
MOVED BY RIVERA, SECONDED BY MCIRVIN, COUNCIL REFER THE ISSUE OF
ESTABLISHING AN ALL-GENDER BATHROOM AT THE RENTON COMMUNITY
CENTER TO THE ADMINISTRATION FOR REVIEW AND TO REPORT FINDINGS TO
THE COMMUNITY SERVICES COMMITTEE. CARRIED.
ADJOURNMENT
MOVED BY PRINCE, SECONDED BY RIVERA, COUNCIL ADJOURN. CARRIED.
TIME: 8:15 P.M.
Jason A. Seth, MMC, City Clerk
Jason Seth, Recorder
06 Jun 2022
AGENDA ITEM #6. a)
Council Committee Meeting Calendar
June 6, 2022
June 13, 2022
Monday
3:00 PM Public Safety Committee, Chair Perez
Location: Council Conference Room/Videoconference
1. Edward Byrne Memorial Justice Assistance Grant (JAG) Program FY 2021
Local Solicitation
2. Addressing Gun Violence
3. RFA Briefing
4. Emerging Issues in Public Safety
4:15 PM Finance Committee, Chair O’Halloran
Location: Council Conference Room/Videoconference
1. ESD Reorganization - IT Administrative Support
2. Philip Arnold Park Playground Equipment Replacement
3. Grant Contract: Coulon North Water Walk Legislative Appropriation
4. Vouchers *
5. Emerging Issues in Finance
5:00 PM Planning & Development Committee, Chair Prince
Location: Council Conference Room/Videoconference
1. Continued Pooling of Senate House Bill 1406 Sales Tax Credit Funds with
South King Housing and Homeless Partners
2. Retail Pet Sales
3. Docket 16 Group A Update
4. Stormwater Pollution Source Control Program Title IV Code Amendments
Briefing
5. Emerging Issues in CED
6:00 PM Committee of the Whole, Chair McIrvin
Location: Council Conference Room/Videoconference
1. Allocation of Lodging Tax Funding for 2022
2. Renton Municipal Arts Commission Update
7:00 PM Council Meeting
Location: Council Chambers/Videoconference
* revised 06/07/22
AGENDA ITEM #6. a)
AB - 3144
City Council Regular Meeting - 13 Jun 2022
SUBJECT/TITLE: Parks Commission Reappointment-Al Dieckman
RECOMMENDED ACTION: Council Concur
DEPARTMENT: Mayor Pavone
STAFF CONTACT: April Alexander, Executive Assistant
EXT.: x6520
FISCAL IMPACT SUMMARY:
None
SUMMARY OF ACTION:
Mayor Pavone reappoints Al Dieckman to the Parks Commission for a term expiring 6/1/26.
EXHIBITS:
A. Recommendation Memo
STAFF RECOMMENDATION:
Confirm Mayor Pavone's reappointment of Al Dieckman to the Parks Commission for a term expiring 6/1/26.
AGENDA ITEM #6. b)
Parks & Recreation
Department Memorandum
DATE: May 31, 2021
TO: Armondo Pavone, Mayor
CC: Ed VanValey, Chief Administrative Officer
Jason Seth, City Clerk
Roberta Graver, Administrative Assistant to Parks and Recreation
FROM: Kelly Beymer, Parks and Recreation Administrator
SUBJECT: Reappointment of Park Commissioner Al Dieckman
I, along with staff, would like to request your consideration to recommend to the City
Council the reappointment for Park Board Commissioner Al Dieckman.
Al realizes the importance of parks and recreational opportunities in the community and
serves the city by promoting these efforts at every opportunity. His commitment to the
community is an asset to the Commission, staff, and citizens of Renton.
Al continues to be a front‐runner and leader in support of parks and recreations and
volunteers to oversee the success of the Park Rangers Program. He looks forward to
training volunteers to patrol the Cedar River Trail and working closely with staff from
the Police and Parks and Trails Division to ensure the safety of residents, monitor the
surroundings, and provide information on park safety to the public.
We feel very fortunate Al is willing to volunteer his time in this capacity and continues to
be a valuable advisory proponent for the city’s parks, trails, open space, and recreation
programming efforts.
Should you have any questions or concerns, please feel free to call me at x6617.
AGENDA ITEM #6. b)
AB - 3141
City Council Regular Meeting - 13 Jun 2022
SUBJECT/TITLE:Amendment 1 to CAG-22-028 / Downtown Streetscape Phase
II
RECOMMENDED ACTION: Refer to Finance Committee
DEPARTMENT: Community & Economic Development Department
STAFF CONTACT:Amanda Askren , Assistant Director, Economic Development
EXT.:7369
FISCAL IMPACT SUMMARY:
Contract compensation is amended so that the maximum amount payable to Consultant is increased by
$303,333.08 from $39,460.00 to $342,793.08, plus any applicable state and local sales taxes. Payment will be
from the CBDG funding allocation.
SUMMARY OF ACTION:
The Parties wish to amend Agreement CAG-22-028 to assign the Consultant in recognition of corporate
merger; modify the Scope of Work, including to approve subconsultants; and, increase the compensation. The
purpose of these amendments are to ensure timely and efficient completion of final bid documents and
provide assistance to implement the bid process.
EXHIBITS:
A. Issue Paper
B. Proposed Amendment
C. Exhibit A-1
STAFF RECOMMENDATION:
Authorize the Mayor and City Clerk to approve the addendum to CAG-22-028, with KPG Psomas, Inc., in the
amount of $303,333.08, which re-assigns the contract due to a corporate merger, and modify the Scope of
Work to include preparation of final bid documents and oversight assistance during the bid process.
AGENDA ITEM #6. c)
DEPARTMENT OF COMMUNITY
& ECONOMIC DEVELOPMENT
M E M O R A N D U M
DATE:June 1, 2022
TO:Ryan McIrvin, Council President
Members of Renton City Council
VIA:Armondo Pavone, Mayor
FROM:Chip Vincent, Community and Economic
Development Administrator
STAFF CONTACT:Amanda Askren, Acting Economic Development
Director
SUBJECT:Amendment 1 to CAG-22-028 / Downtown
Streetscape Phase II
ISSUE:
Should the City authorize an amendment to
Agreement CAG-22-028 with KPG Psomas, Inc for
the purposes of a corporation name change,
amended Scope of Work and amended
compensation?
RECOMMENDATION:
Authorize the Mayor and City Clerk to approve the
addendum to CAG-22-028.
BACKGROUND SUMMARY:
Agreement CAG-22-028 marks the beginning of
Phase II of the Downtown Streetscape Project.
The Consultant for CAG 22-028, KPG PS, KPG Inc,
was acquired by KPG Psomas Inc., a California
corporation registered to do business in the State
of Washington, as evidenced by the Articles of
Merger included in Exhibit A-1 “Scope of Work”,
and by this Amendment #1 the City has formally
consented to the assignment of the Agreement to
KPG Psomas, Inc.
AGENDA ITEM #6. c)
The Parties wish to amend Agreement CAG-22-028
to assign the Consultant in recognition of
corporate merger; modify the Scope of Work,
including to approve subconsultants; and increase
the compensation. The purpose of these
amendments are to ensure timely and efficient
completion of final bid documents and provide
assistance to implement the bid process.
Council action was taken on October 19, 2020 after
a public hearing. The Council action was to adopt
the 2021 CBDG funding allocation for the specified
use of Streetscape Phase II. The contract with King
County to receive these funds is in final routing for
signature at which time the contract with KPG can
be finalized and payment will be from the CBDG
funding allocation.
AGENDA ITEM #6. c)
AMENDMENT NO. 1 TO AGREEMENT FOR DOWNTOWN CORE
STREETSCAPE PHASE 2
THIS AMENDMENT, dated for reference purposes only as March 23, 2022, is by and between
the City of Renton (the “City”), a Washington municipal corporation, and Psomas DBA KPG
Psomas, Inc. (“Consultant”), a Washington foreign corporation. The City and the Consultant are
referred to collectively in this Amendment as the “Parties.” Once fully executed by the Parties,
this Amendment is effective as of the last date signed by both parties.
Whereas the City engaged the services of the Consultant under Agreement CAG-22-028, dated
January 27, 2022, to provide necessary services for Downtown Core Streetscape Phase 2
(referred to herein as the “Agreement”).
Whereas the Parties wish to amend the Agreement to: assign the Consultant in recognition of
corporate merger; and, modify the Scope of Work, including to approve subconsultants; and,
increase the compensation. The purpose of these amendments are to ensure timely and
efficient completion of final bid documents and provide assistance to implement the bid
process.
NOW THEREFORE, It is mutually agreed upon that CAG-22-028 is amended as follows:
1. Parties: Consultant for CAG 22-028, KPG PS, was acquired by KPG Psomas Inc., a
California corporation registered to do business in the State of Washington, as
evidenced by the Articles of Merger included in Exhibit A-1 “Scope of Work”, and by this
Amendment #1 the City has formally consented to the assignment of the Agreement to
KPG Psomas, Inc.
2. Scope of Work: Section 1, Scope of Work, is amended to modify the Work, and approve
the named subconsultants as specified in Exhibit A-1, which is attached and
incorporated herein.
3. Compensation: Section 4, Compensation, is amended so that the maximum amount of
compensation payable to Consultant is increased by $303,333.08 from $39,460.00 to
$342,793.08, plus any applicable state and local sales taxes. The additional
compensation shall be paid based upon Work actually performed according to the
rate(s) or amounts specified in Exhibit D-1 of the Agreement for Consultant and
subcontractor.
AGENDA ITEM #6. c)
PAGE 2 OF 2
4.All terms of the Agreement not explicitly modified herein shall remain in full force and
effect and such terms shall apply to Work performed according to this Amendment as if
fully set forth herein.
IN WITNESS WHEREOF, the Parties have voluntarily entered into this Amendment as of the
date last signed by the Parties below.
CITY OF RENTON
By:_____________________________
CONSULTANT
By:____________________________
Mayor Armondo Pavone Nate Mazer
Vice President
_____________________________
Date
_____________________________
Date
Attest
_____________________________
Jason A. Seth
City Clerk
Approved as to Legal Form
By: __________________________
M. Patrice Kent
Sr. Assistant City Attorney
Contract Template Updated 06/17/2021
Legal Ref #2022; CAG 22-028 Legal Ref #1841
AGENDA ITEM #6. c)
City of Renton
Downtown Core Streetscape Phase 2 March 23, 2022
KPG Psomas Inc. Project Number: 21089 Page 1 of 6
EXHIBIT A-1
CITY OF RENTON
DOWNTOWN CORE STREETSCAPE PHASE 2
CAG-22-028
KPG PSOMAS, INC, FORMERLY KPG, INC.
SCOPE OF WORK
SUPPLEMENT NO.1
MARCH 23, 2022
A. PROJECT BACKGROUND / DESCRIPTION
The City of Renton (“City”) obtained a U.S. Department of Housing and Urban Development (HUD)
grant for the Downtown Core Streetscape Engineering Services (“Project”). The Project will
complete a Bid Package for construction of Williams Avenue S, from S 2nd Street to S 3rd Street and
installation of wayfinding elements as identified by the City.
Primary components of the Bid Package will include:
·New and/or modified sidewalks
·Street furniture which may include bike racks, bollards, benches, alternative seating,
streetscape/planter fencing, receptacles (trash, ash, recycle)
·Plant types, street trees (tree pit, tree grate), planting areas and irrigation
·Pedestrian & roadway lighting
·Grind and overlay of Williams Avenue S within the project limits
·Installation of wayfinding signs that can be installed given that S 2nd St and S 3rd St will
remain as one-way streets, and the Burnett Linear Park improvements will not be
completed yet, per the Core Vision Plan. This scope includes the installation of the
following wayfinding signs based on the City’s Wayfinding Design Plans, but will be
vetted and approved by the City:
Advanced Directional Signs: 19 locations (3 potentially on existing traffic signal
mast arms, 3 replacements of existing signs, and 13 on new poles and
foundations)
Vehicular Directional Signs: 17 locations (10 on existing light poles, and 7 on
new poles and foundations)
Pedestrian Directional Signs: 9 locations, with new poles and foundations
B. ASSUMPTIONS
The following assumptions were identified to provide direction with design:
The guiding document for design will be the City of Renton Downtown Streetscape Design
Standards, the Renton Downtown Civic Core Vision and Action Plan, and the City’s Wayfinding
Design Plans.
AGENDA ITEM #6. c)
City of Renton
Downtown Core Streetscape Phase 2 March 23, 2022
KPG Psomas Inc. Project Number: 21089 Page 2 of 6
The project is funded through the King County Consortium and City of Renton Community
Development Block Grant Programs with funds obtained from HUD.
HUD will complete the Environmental Review and Consultant will assist with supporting
documents as requested by HUD.
There is no design DBE Goal established for this project.
Plans will be developed with AutoCAD 2018 Civil 3D using KPG drafting standards.
Stormwater analysis or reporting will not be required for this project.
The project does not include any water or sewer improvements. Minor stormwater system
modifications may be included, as needed, only where existing curb, gutter, sidewalks, and
roof drains are disturbed.
This project does not include any traffic signal improvements.
KPG Psomas will coordinate with all other downtown projects underway.
No right-of-way acquisitions or new easements are anticipated. However, temporary
construction permits will be obtained as needed.
C. KPG PSOMAS DELIVERABLES
Deliverables prepared by the Consultant are identified at the end of each Task.
D. CITY OF RENTON PROVIDED ITEMS:
The City of Renton will be responsible for the following:
Submittal reviews, comments, and approvals (1 to 2 sets of comments per submittal)
Public notices, property owner mailings, postage, and hosting a project website
Meeting room arrangements
Public outreach arrangements for Open House room, virtual meeting, or field walk
Right-of-entries for surveying, if required
Where Advanced Directional Signs and Vehicular Directional Signs are proposed to be
installed on existing poles, as-built pole and foundation information will be provided by the
City to verify adequacy to support new signs.
E. SCOPE OF WORK
TASK 1 – MANAGEMENT / COORDINATION / ADMINISTRATION
The estimated project duration is 10 months to complete to bid documents.
1.1 The Consultant will provide continuous project management for the duration of the
project (estimate 10 months). The Consultant will prepare monthly progress
reports identifying work completed in the previous month, work in progress,
upcoming work elements, and reporting of any delays, problems, or additional
information needs. These reports will be submitted with the Consultant invoices.
AGENDA ITEM #6. c)
City of Renton
Downtown Core Streetscape Phase 2 March 23, 2022
KPG Psomas Inc. Project Number: 21089 Page 3 of 6
1.2 Prepare for and attend monthly design coordination meetings with City Staff and
other project stakeholders (estimate 10 meetings). For meetings initiated by the
City requiring Consultant participation, the City shall provide a minimum of 2 days’
notice.
1.3 The Consultant will create and update a project schedule.
1.4 The Consultant will conduct regular project team meetings with internal staff and
subconsultants.
1.5 The Consultant will provide internal quality assurance/quality control (QA/QC)
reviews of all major deliverables prior to submittal to the City.
Task 1 Deliverables:
Monthly progress reports
Project Schedule and Updates
Meeting notes
TASK 2 – FINAL DESIGN
This task covers the effort required to prepare a Bid Package for construction of streetscape
improvements at Wells Avenue S, from S 2nd Street to S 3rd Street, and installation of wayfinding
elements as approved by the City.
The Consultant shall prepare Plans, Specifications and Estimates for review and approval by the
City. Plans shall be formatted to provide sufficient detail for convenient field layout of all
proposed facilities. City Standard Details and WSDOT standard plans will be supplemented with
project specific details as required. All design plans and documents will be signed by a licensed
professional engineer in the State of Washington.
2.1 The Consultant shall prepare 50% plans and cost estimate for review and approval by
the City.
2.2 The Consultant shall prepare 90% plans and cost estimate for review and approval by
the City. The 90% submittal will address all comments received from the 50% review.
2.3 The Consultant shall prepare 100% plans and cost estimate for review and approval
by the City. The 100% submittal will address all comments received from the 90%
review.
The anticipated plan sheets may include the following:
1 Cover Sheet
1 Legend and Abbreviations
1 Sheet Index, Alignment Plan, and Survey Control
1 Typical Roadway Sections
1 Roadway Details
1 Site Preparation and TESC Plan
AGENDA ITEM #6. c)
City of Renton
Downtown Core Streetscape Phase 2 March 23, 2022
KPG Psomas Inc. Project Number: 21089 Page 4 of 6
1 Roadway and Drainage Plan
1 Illumination Plan
1 Illumination Details
1 Urban Design Plan
2 Urban Design Details
1 Landscape Plan
1 Landscape Details
1 Irrigation Plan
2 Irrigation Details
1 Channelization and Signing Plan
15 Wayfinding Plans and Details
33 Total Sheets
2.4 The Consultant shall prepare Bid Document plans and cost estimate for review and
approval by the City. The Bid Document submittal will address all comments received
from the 100% review.
2.5 The Consultant shall prepare specifications for the 90%, 100%, and Bid submittals based
on the most current WSDOT Standard Specifications, HUD, and federal funding
requirements. The City shall provide a contract boilerplate and all applicable City
general special provisions.
2.6 If needed, the Consultant shall prepare three (3) temporary construction permit (TCP)
exhibits to support with the acquisition services effort by Commonstreet.
Task 2 Deliverables:
30% and 60% Review Submittals - Plans and Construction Cost Estimate (PDF)
90% and 100% Review Submittals – Plans, Specifications, and Cost Estimate (PDF)
Bid Document – Plans, Specifications, and Cost Estimate (PDF)
TCP Exhibits (PDF, up to 3 parcels)
TASK 3 – UTILITY COORDINATION
The Consultant will coordinate with private utilities affected by the proposed Downtown Core
Streetscape Phase 2 improvements. Efforts included under this Task are as follows:
3.1 Perform utility coordination and provide design plans to utility purveyors at each
submittal phase. Prepare for and attend utility coordination meetings (2 assumed)
with franchise utilities to discuss utility conflicts and relocations.
3.2 Potholing and Utility Conflict Plan: This plan will be submitted to all purveyors with
utilities in the project limits to assess whether said utilities may conflict with the
proposed improvements.
Task 3 Deliverables:
Potholing and Utility Conflict Plan (PDF)
AGENDA ITEM #6. c)
City of Renton
Downtown Core Streetscape Phase 2 March 23, 2022
KPG Psomas Inc. Project Number: 21089 Page 5 of 6
Task 3 Assumptions:
Survey required for utility relocations is not included
Any franchise utility expansion which is required to be incorporated into the project is not
included
TASK 4 – PUBLIC OUTREACH
The Consultant will support Public Outreach efforts led by the City. Specific outreach efforts have
not been specifically identified for this project, but may include, preparation of presentation
materials and illustrative figures, and attendance at Open House meetings or field walks to
provide information to community members and stakeholders. An estimated budget has been
included as part of Task 4.
TASK 5 – BID ASSISTANCE
5.1 The Consultant shall provide bid assistance including:
· Coordinate with advertisement at Builders Exchange of Washington (BXWA).
The City is responsible for the fees related to advertising the project through
BXWA.com.
· Preparing addenda and respond to bidder questions relayed through the City.
It is assumed KPG will prepare up to two (2) addenda.
· Attending the bid opening, verify bids for accuracy, prepare bid tabulation,
and provide recommendation for award.
· Consolidate all addenda items and prepare a Conformed Set of Plans,
Specification, and Cost Estimate for the City’s use and files.
Task 5 Deliverables:
Up to two (2) Addenda and Answers to Questions During Bidding (PDF)
Bid Tabulation (PDF)
Recommendation for Award Letter (PDF)
Conformed Set: Plans, Specifications, Estimate (PDF)
MANAGEMENT RESERVE
The City may require additional services from the Consultant. The scope of these services will be
determined based on the unanticipated project needs or other considerations at the sole
discretion of the City. This work may include items identified in the current WE authorizations as
well other items, which may include, but are not necessarily limited to the following:
· Supplemental survey and basemapping
· Geotechnical engineering
· Design of Art Features or part thereof (electrical/structural/civil)
· Additional right-of-way services or additional structural engineering services
· Construction Management Services
AGENDA ITEM #6. c)
City of Renton
Downtown Core Streetscape Phase 2 March 23, 2022
KPG Psomas Inc. Project Number: 21089 Page 6 of 6
These services will be authorized by the City under management reserve. At the time these
services are required, the Consultant shall provide a detailed scope of work and an estimate of
costs. The Consultant shall not proceed with the work until the City has authorized the work and
issued a notice to proceed.
AGENDA ITEM #6. c)
ARTICLES OF MERGER
OF
KPG, P.S.
(a Washington professional services corporation)
INTO
PSOMAS
(a California corporation)
Pursuant to Section 25.15.426 of the Revised Code of Washington, the undersigned corporations
execute and submit for filing the following Articles of Merger:
1. On December 31, 2021, PSOMAS acquired all of the capital stock of KPG, P.S.
2. The merger is permitted by the laws of the State of California under whose laws PSOMAS is
incorporated and was duly approved by the board of directors of PSOMAS, pursuant to the laws
of the State of California, its Articles of Incorporation, as amended, and Bylaws, as amended.
Attached hereto is a true and correct copy of the Resolution of the Board of Directors of Psomas
approving the merger.
3. The merger is permitted by the laws of the State of Washington under whose laws KPG, P.S. is
incorporated and was duly approved by the shareholders of KPG, P.S., pursuant to the laws of the
State of Washington, its Articles of Incorporation, as amended, and Bylaws, as amended.
Attached hereto is a true and correct copy of the Resolution of the Board of Directors of KPG,
P.S. approving the merger.
4. KPG, P.S., a Washington professional services corporation, will operate during its winding down
period as KPG, INC., a Washington corporation.
5. PSOMAS, a California corporation, will continue to operate in the State of Washington as
PSOMAS DBA KPG PSOMAS, INC., a California corporation.
6. PSOMAS’ registered agent in the State of Washington is:
Corporation Service Company
300 Deschutes Way SW, Suite 208 MC-CSC1
Tumwater, WA 98501
7. The effective date of the merger shall be January 1, 2022.
AGENDA ITEM #6. c)
ARTICLES OF MERGER
December 31, 2021
Page 2
____________________________
PSOMAS
Chad Wilson
Vice President and Corporate Secretary
____________________________
KPG, P.S.
Sessyle Asato
Chief Executive Officer and Corporate Secretary
AGENDA ITEM #6. c)
UNANIMOUS WRITTEN CONSENT
OF THE
BOARD OF DIRECTORS OF PSOMAS,
a California corporation.
December 14, 2021
THE UNDERSIGNED, being all of the members of the Board of Directors of Psomas, a California
corporation (the “Corporation”), hereby adopt the following resolutions, pursuant to Section 307(b) of the
General Corporation Law of California, effective December 31, 2021:
WHEREAS, certain officers of the Corporation have presented a proposal for the Corporation to acquire
all of the capital stock of KPG, P.S., a Washington professional corporation (“KPG”);
WHEREAS, the officers have presented the Board with the terms and conditions of the Stock Purchase
Agreement and related ancillary agreements proposed to be executed by and between the Corporation,
KPG, Nelson Davis as Seller Representative, and the shareholders of KPG;
WHEREAS, after duly considering the proposed terms and conditions, the Board agrees that it would be
in the Corporation’s best interests to consummate the transactions contemplated by the terms and
conditions of the Stock Purchase Agreement and ancillary agreements referenced therein, provided that
KPG employees will not receive credit for hours of service performed for KPG prior to becoming an
employee of the Corporation, for purposes of vesting under the Corporation’s Employee Stock Ownership
Plan, consistent with prior acquisitions by the Corporation;
NOW, THEREFORE, BE IT RESOLVED, that the Corporation hereby consents to the acquisition of
the capital stock of KPG on the terms and conditions of the Stock Purchase Agreement and the Board
hereby approves, ratifies and confirms (i) the adoption of the terms and conditions of the Stock Purchase
Agreement in substantially the form presented to the directors, (ii) the execution and delivery, by the
proper officers of the Corporation, in the name and on behalf of the Corporation, of the Stock Purchase
Agreement and all ancillary agreements thereto, and (iii) all transactions contemplated by the terms and
conditions of the Stock Purchase Agreement;
RESOLVED FURTHER, that the Chief Executive Officer, the Chief Financial Officer, and the
Secretary of the Corporation (each such person, an “Authorized Person”) be and each hereby is (acting
singly or together with another Authorized Person) authorized, directed, and empowered, in the name and
on behalf of the Corporation, to:
(i) negotiate, execute, and deliver the Stock Purchase Agreement, with such terms and conditions as the
Authorized Person executing the same approves, such approval to be conclusively evidenced by such
Authorized Person’s execution of the Stock Purchase Agreement; and
(ii) execute and deliver any other agreements, documents, amendments, instruments, and writings, and to
perform such other acts as such Authorized Person may deem necessary or advisable to consummate the
transactions contemplated in the Stock Purchase Agreement, or to carry out the purposes of the Stock
Purchase Agreement, and these resolutions, and to perform the obligations of the Corporation under the
agreements, documents, instruments, and any other writings referred to in these resolutions (in each case,
in such form and on such terms and conditions as such Authorized Person taking such action deems
necessary or advisable);
RESOLVED FURTHER, that the Corporation’s performance of any obligations and the execution,
delivery, filing, or other authentication of any documents, prior to the date of this Consent, by any
Authorized Person (acting singly or together with another Authorized Person), in the name and on behalf
AGENDA ITEM #6. c)
Unanimous Written Consent of the Board of Directors of Psomas
December 14, 2021
Page 2
of the Corporation, in furtherance of the Stock Purchase Agreement or any of the foregoing resolutions,
be and hereby are approved, ratified, and confirmed in all respects;
RESOLVED FURTHER, that any and all actions taken by the officers of this Corporation, or any of
them, as deemed by such officers to be necessary or advisable to effectuate the transactions contemplated
by the foregoing resolutions, whether prior or subsequent to this action by this Board of Directors, are
hereby authorized, approved and ratified, and the taking of any and all such actions and the performance
of any and all such things in connection with the foregoing shall conclusively establish such officers'
authority therefor from this Corporation and the approval and ratification thereof by the Board of
Directors;
RESOLVED FINALLY, that the Bylaws of the Corporation be amended to include the following
provision: “The designated engineer and/or land surveyor, respectively, named in a resolution of
the Board as being in responsible charge, or an engineer or land surveyor under the designated
engineer or land surveyor's direct supervision, shall make all engineering or land surveying
decisions pertaining to engineering or land surveying activities in the state of Washington.”
This Unanimous Written Consent shall be filed with the Minutes of the proceedings of the Board of
Directors, and the actions taken hereby shall have the same force and effect as if taken at a meeting duly
called and held.
Ryan E. McLean
David A. Moritz
Matthew D. Clark
Mike Lucki
Ann Johnston
Steve Margaroni
Alejandro Angel
Byron Tobey
Donald Lee Whiteley
AGENDA ITEM #6. c)
AGENDA ITEM #6. c)
AGENDA ITEM #6. c)
EXHIBIT D-1
PRIME CONSULTANT COST COMPUTATIONS
Client: City of Renton
Project: Downtown Core Streetscape Phase 2 - Supplement #1
Date: March 23, 2022
Senior EngineerSr. Project EngineerDesign EngineerTechnicianSurvey ManagerSurvey Crew II (W/Equip)Senior Survey TechnicianUrban Design ManagerProject Landscape ArchitectLandscape TechnicianResident EngineerSenior CAD TechnicianBusiness ManagerSenior Admin203 179 126 100 246 238 128 195 141 100 136 134 174 112 Hours Fee
1.1 Project Mgmt and Admin Services (est 10 months) 10 6 6 22 3,746.00$
1.2 Coordination Meetings with City (est 10)10 4 14 2,594.00$
1.3 Project Schedule 3 3 609.00$
1.4 Internal Design Team and Subconsultant Coordination 8 8 8 8 4 4 4 44 6,608.00$
1.5 QA/QC 16 16 8 16 56 9,848.00$
47 24 8 8 0 0 0 12 8 4 16 0 6 6 139 23,405.00$
2.1 50% Plans and Cost Estimate 16 40 60 40 12 60 60 60 348 46,808.00$
2.2 90% Plans and Cost Estimate 16 60 60 60 12 60 60 24 352 47,564.00$
2.3 100% Plans and Cost Estimate 8 32 32 32 8 16 24 152 20,800.00$
2.4 Bid Document Plans and Cost Estimate 8 8 24 16 4 4 4 6 74 10,228.00$
2.5 Specifications (90%, 100%, and Bid)20 40 16 24 10 110 18,844.00$
2.6 Temp. Construction Permit Exhibits 2 16 18 1,958.00$
68 182 176 164 0 0 0 52 164 148 0 90 0 10 1054 146,202.00$
3.1 Franchise Utility Coordination and Meetings (est 2)2 2 4 658.00$
3.2 Potholing and Utility Conflict Plan 4 4 24 16 48 6,152.00$
6 4 26 16 0 0 0 0 0 0 0 0 0 0 52 6,810.00$
4.1 Public Outreach 4 4 4 16 16 5,952.00$
4 0 4 0 0 0 0 4 16 16 0 0 0 0 0 5,952.00$ Task Total
Task Total
Task 2 - Design
Task Total
Task 3 - Utility Coordination
Task Total
Task 4 - Public Outreach
Task
No.Task Description
Labor Hour Estimate
Total Hours and Labor Fee
Estimate by Task
Task 1 - Management / Coordination / Administration
City of Renton
Downtown Core Streetscape Phase 2 - Supplement #1
Page 1 of 2 KPG Psomas Inc.AGENDA ITEM #6. c)
EXHIBIT D-1
PRIME CONSULTANT COST COMPUTATIONS
Client: City of Renton
Project: Downtown Core Streetscape Phase 2 - Supplement #1
Date: March 23, 2022
Senior EngineerSr. Project EngineerDesign EngineerTechnicianSurvey ManagerSurvey Crew II (W/Equip)Senior Survey TechnicianUrban Design ManagerProject Landscape ArchitectLandscape TechnicianResident EngineerSenior CAD TechnicianBusiness ManagerSenior Admin203 179 126 100 246 238 128 195 141 100 136 134 174 112 Hours Fee
Task
No.Task Description
Labor Hour Estimate
Total Hours and Labor Fee
Estimate by Task
5.1 Bid Assistance 6 2 8 4 6 26 3,820.00$
6 2 8 0 0 0 0 0 4 0 0 0 0 6 26 3,820.00$
127 212 218 188 0 0 0 64 176 152 16 90 6 22 1,271 186,189.00$
25,000.00$
Widener and Associates (Environmental) 19,228.08$
Commonstreet (ROW)4,938.00$
Chudgar Engineering (Structural)47,028.00$
Potholing Services (TBD)20,000.00$
91,194.08$
Mileage at current IRS rate 450.00$
Reproduction Allowance 500.00$
950.00$
303,333.08$ Total Estimated Budget
Total Labor Hours and Fee
Subconsultants
Total Subconsultant Expense
Reimbursable Direct Non-Salary Costs
Total Reimbursable Expense
Task 5 - Bid Assistance
Task Total
Management Reserve
Total Management Reserve
City of Renton
Downtown Core Streetscape Phase 2 - Supplement #1
Page 2 of 2 KPG Psomas Inc.AGENDA ITEM #6. c)
Cowling & Co. LLC dba Widener & Associates Transportation & Environmental Planning
1902 120th Place SE. Suite 202 Everett WA. 98208 Tel (425) 332-3961
KPG
DOWNTOWN CORE STREETSCAPE PHASE 2
ENGINEERING SERVICES
Scope of Work
Environmental Services
Widener and Associates, under a subconsultant agreement with KPG Inc., will assist the City of
Renton in the preparation of environmental documentation and permitting of the project by
providing the following services:
1 Design Assistance and Early Agency Coordination
Early input into the formation of project alternatives will be provided to ensure each alternative
includes provisions to minimize impacts to the surrounding environment. This coordination
within the various design elements of the project will identify and incorporate minimization
measures early in the alternative development phase of the project and will ensure that an
appropriate range of alternatives are developed prior to the agency coordination.
The Consultant, with assistance from Widener and Associates, will assist the City in presenting
the alternatives to both the state and federal permitting agencies to identify the regulatory issues
associated with each alternative. Potential minimization measures for two alternatives will also
be identified during the coordination with agency representatives.
Deliverable(s):
The Consultant's permitting specialist will prepare meeting minutes and memoranda
documenting the coordination activities with state and federal agencies, as required.
Task 2 SEPA Checklist
The Consultant shall complete appropriate SEPA documentation including all needed studies,
modeling, and analysis in accordance with State Environmental Policy Act (RCW 43.21C) and
SEPA Rules (WAC 197-11). The Consultant will coordinate with the city to address comments on
the SEPA Checklist and provide support for the SEPA process.
Deliverable(s):
•A draft of the SEPA Checklist will be provided.
•The final SEPA documentation will be provided incorporating City comments.
Task 3 Section 106 Process (Cultural and Historic Resources)
This work would include preparation of a Section 106 exemption letter in accordance with the
State Historic Preservation Office standards and guidelines. The work will include the following
subtasks:
AGENDA ITEM #6. c)
Pg. 2
1. Pertinent literature on the archaeology, ethnography, and history of the project area will
be reviewed to determine the existence of archaeological sites and to refine the
probability of archaeological resources and traditional cultural places in the project areas.
2. An exemption letter will be written discussing the projects footprint and sent to the City
for comments. Once the City’s comments are addressed it will be sent to HUD for final
comments and approval.
3. If a 106 exemption is not attainable then a supplement will be needed to complete a full
cultural resource report.
Deliverable(s):
Draft Section 106 report exemption letter
Final Section 106 report exemption letter
Task 3 HUD NEPA
Services will be provided to prepare the NEPA memorandum by reviewing technical reports
related to the project, applying project-specific data to the form, and coordinating approval for
the project by both the City and HUD.
Deliverable(s):
HUD NEPA draft memorandum
HUD NEPA final memorandum
AGENDA ITEM #6. c)
Exhibit A
Project Name Downtown Core Streetscape Ph. 2
Client KPG - Olivia Paraschiv
Location City of Renton Date 9/20/2021
Environmental
Manager
Senior
Biologist
Project
Biologist
Hours Hours Hours
1. Design Assistance & Agency Coordination
Draft 0 10 5
Final 0 8 5
2. SEPA Checklist
Draft 2 5 15
Final 2 4 5
3. Section 106 Process (exemption)
Draft APE Letter Exempton 2 5 10
Final APE Letter Exempton 2 2 5
4. HUD NEPA
Draft 3 8 40
Final 5 5 5
HUD Comments Addressed 4 8 15
Total hours 20 55 105
Summary Hours Direct Cost
Environmental Manager 20 170.40$ 3,408.00$
Senior Biologist 55 120.00$ 6,600.00$
Project Biologist 105 86.80$ 9,114.00$
Total Labor 19,122.00$
Expenses
Mileage 106.08$
TOTAL ESTIMATED COST 19,228.08$
WIDENER & ASSOCIATES
AGENDA ITEM #6. c)
Commonstreet Consulting, LLC
www.csrow.com
Date September 13, 2021
To Olivia Paraschiv, P.E., KPG
From Hutch Goodman, Commonstreet Consulting, LLC
Re Scope and Fee Proposal: City of Renton - Downtown Core Streetscape – Phase 2
SCOPE OF SERVICES - SUMMARY
Thank you for the opportunity to provide acquisition services for the City on this streetscape project for the City
of Renton. Commonstreet’s Team will provide temporary construction permit (TCP) services for (3) larger parcels
associated with the Downtown Core Streetscape – Phase 2 Project. This scope and fee proposal assumes that no
valuations, acquisitions or easements are required and that no certification oversight will be needed.
SCOPE OF SERVICES - DETAIL
Project Management
• Attend kick off meeting with Project Team, discuss design impacts/ommunications approaches for each owner
• Lead acquisition status meetings, provide regular status reporting and oversight into State and HUD compliance
Title Review and TCP Package Preparations
• Research owners of record, prepare acquisition files according to City and HUD standards
• Prepare TCP packages, conveyance documents and related documentation
Negotiations/Memorandum of Understanding (MOU) Drafts
• Contact property owners and tenants impacted by City’s access requests, present offer packages
• Negotiation TCP terms, facilitate document execution, prepare MOU’s, as needed
Project Close Out
• Coordinate with City on final project close-out requirements, QC documents
• Provide City with final, closed out, acquisition files, both digital and hard copies
Assumptions
• Preminary title reports, right of way plans and exhibits for TCP’s to be provided by City
SCOPE AND FEE SCHEDULE
Project Management Project Manager @ $169.11/hour @ 5 hours $846.00
Title Review/Offer Package Preparations (3 Parcels) Senior Agent @ $133.49/hour @ 3 hours $400.00
Negotiations/Settlement Justifications (3 Parcels) Senior Agent @ $133.49/hour @ 27 hours $3,604.00
Sub Total $4,850
Direct Reimbursables Travel
(Mileage):
78 Miles $0.560 $43.68
$45.00
$88.68
Postage 3 $7.50 $22.50
Direct Reimbursable Subtotal:
GRAND TOTAL $4,938
AGENDA ITEM #6. c)
CHUDGAR ENGINEERING COMPANY
Civil & Structural Engineers
1510 140th Ave NE Suite 203 Bellevue WA 98005
Phone: (425) 590-9650 Email: samir.chudgar@chudgar-eng.com
City of Renton
Downtown Core Streetscape Engineering Services Phase 2 Prepared:SC
Dated:9/14/21
Submitted to Olivia Paraschiv, KPG
Signs
Structural Engineering
Scope, Labor Hours and Budget Estimate
The following labor hours estimate has been developed based upon anticipated Scope of Work for design
of sign foundations and evaluation of existing pole for new signs, based on documents provided by KPG
on 9/10/21. Various signs types are proposed and sign dimensions vary. 13 signs may be potentially
installed on existing illumination poles or signal mast arms, for which structural evaluation is required. 29
locations will need new foundations, and 3 are replacing existing signs. Foundation designs are already
completed with Phase 1. Base condition may vary from what is shown in Phase 1. New design and
drawings will be prepared that modify certain limited information from Phase 1 drawings.
Evaluate certain number of conditions for signs on existing signal mast arm poles, by verifying adequacy
of pole and foundation to support new signs. Wind loading is the primary loading. Pole analysis will
include comparision of proposed new loading against original design loading or City of Renton Standards.
Existing pole and thier foundation information is assumed to be made available by City of Renton.
Existing pole and / or foundation may not be adequate to support new sign. Strengthening for pole and / or
foundation will not be performed.
This scope and estimate assumes that a combination of maximum 10 signs will be designed and evaluated.
Various signs that have similarity in sizes and placement may be grouped together under one design.
1
Reviw sign drawings. Analyze sign for applicable loadings. Develop connections and anchorage. Design
foundation. Develop drawings with anchorage and foundation details as necessary. Prepare Structural
General Notes. Structural General Notes will include typical material designations; separate
construction specifications will not be prepared. Cost estimates will be prepared for each foundation as
lump sum cost to be added to sign cost. Structural calculations and drawings will be stamped and signed
by registered Structural Engineer licensed in State of Washington.
Renton Streetscape Phase 2 Page 1 of 2 Scope Estimate
AGENDA ITEM #6. c)
CHUDGAR ENGINEERING COMPANY
Civil & Structural Engineers
1510 140th Ave NE Suite 203 Bellevue WA 98005
Phone: (425) 590-9650 Email: samir.chudgar@chudgar-eng.com
City of Renton
Downtown Core Streetscape Engineering Services Phase 2 Prepared:SC
Dated:9/14/21
Submitted to Olivia Paraschiv, KPG
Signs
Structural Engineering
Estimate
Overhead Rate (Safe Harbor Rate)110.00%
Fixed Fee 15.00%
Task Task Description Hours
Principal
Structural
Engineer
Senior
Structural
Engineer
Structural
Design
Engineer Subtotal
Pay Rate $125.00 $56.09 $40.00
Overhead $137.50 $61.70 $44.00
Fixed Fee $39.38 $17.67 $12.60
Hourly Billing Rate $301.88 $135.46 $96.60
1 Structural Engineering 40 104 216 360
Total Hours 40 104 216 360
Total Labor Cost (DSC)$5,000.00 $5,833.36 $8,640.00 $19,473.36
Overhead Cost $5,500.00 $6,416.70 $9,504.00 $21,420.70
Fixed Fee $1,575.00 $1,837.51 $2,721.60 $6,134.11
Total Labor Cost (DSC)$19,473.36
Overhead Rate (Safe Harbor Rate)110.00%
Overhead Cost (OH)$21,420.70
Fixed Fee 15.00%$6,134.11
Sub-total $47,028.16
Reimbursable Expenses $0.00
Total Budget Estimate $47,028
Designation and Hourly Rate
Renton Streetscape Phase 2 Page 2 of 2 Scope Estimate
AGENDA ITEM #6. c)
AB - 3143
City Council Regular Meeting - 13 Jun 2022
SUBJECT/TITLE: Business and Occupation Tax Rate Increase
RECOMMENDED ACTION: Refer to Finance Committee
DEPARTMENT: Finance Department
STAFF CONTACT: Fred Hall, Tax and Licensing Manager
EXT.: 6858
FISCAL IMPACT SUMMARY:
The proposed rate change is estimated to increase general fund revenues by $1.3 million in 2023 and $2.1
million in 2024.
SUMMARY OF ACTION:
Currently, 77% of the city's general fund revenues come from taxes such as sales tax, property tax, and B&O
tax. However there are limitations on how much some of these taxes can be increased year over year, and
these limitations create a long-term structural imbalance between our revenues and the cost of providing
essential city services as the city’s tax revenues continue to increase at a slower rate than expenditures
annually. Currently the long-range budget projection anticipates needing to use available reserves, increasing
exponentially each year, to keep pace with the rising costs.
The city implemented a B&O tax effective January 1, 2016 to help provide a much -needed resource to pay for
ongoing operating costs and capital improvements. Although the current code does allow for an increase of
this tax annually, the city has not made any rate changes since implementation other than raising the
maximum tax any one taxpayer would pay in a year. An increase of the B&O tax rate at this time is
recommended to help ensure the same level of service and to provide a resource for capital repairs and
maintenance. Even with the proposed tax rate increase, the city will continue to have one of the lowest tax
rates in comparison to other cities in our region along with the highest tax filing threshold.
EXHIBITS:
A. Issue Paper
B. Ordinance
STAFF RECOMMENDATION:
Approve and adopt the ordinance amending RMC Chapters 5-25 for business and occupation taxes, and authorize an
increase of the B&O tax rates to 0.07% for retail sales and 0.121% for all other tax classifications, effective
1/1/2023.
AGENDA ITEM #6. d)
Finance Department
Memorandum
DATE:May 31, 2022
TO:Ryan McIrvin, Council President
Members of Renton City Council
VIA:Armondo Pavone, Mayor
FROM:Kari Roller, Finance Administrator
STAFF CONTACT:Fred Hall, Tax and License Manager
SUBJECT:Business and Occupation Tax Rate Increase
ISSUE
Should the city increase the Business and Occupation (B&O) tax rate effective 1/1/2023?
RECOMMENDATION
Staff recommends council authorize an increase of B&O tax rates, effective 1/1/2023 as
follows:
Tax Classification Current Tax Rate New Tax Rate
Wholesaling 0.085%0.121%
Retailing 0.050%0.070%
Service & Other 0.085%0.121%
Manufacturing/Processing for Hire 0.085%0.121%
Printing/Publishing 0.085%0.121%
Extracting/Extracting for Hire 0.085%0.121%
OVERVIEW
The city implemented a B&O tax effective January 1, 2016. Although the current code
does allow for an increase of this tax annually, the city has not made any rate changes
since implementation other than raising the maximum tax any one taxpayer would pay
in a year.
The city’s tax revenues continue to increase at a slower rate than our expenditures
annually. Currently the long-range projection anticipates needing to use available fund
balance in order to keep pace with the raising costs. An increase of the B&O tax rate at
this time is recommended to help ensure the same level of service.
AGENDA ITEM #6. d)
Ryan McIrvin, Council President
Members of Renton City Council
Page 2 of 2
June 8, 2022
If approved, staff recommend the tax rate change effective date of 1/1/2023 in order to
give staff time to provide further outreach to the business community and to implement
the rate changes within the city’s online payment portal.
FISCAL IMPACT
The proposed rate change is estimated to increase general fund revenues by $1.3
million and $2.1 million for 2023 and 2024 respectively.
CONCLUSION
Staff recommend council to authorize an increase of the B&O tax rates to 0.07% for
retail sales and 0.121% for all other tax classifications, effective 1/1/2023.
AGENDA ITEM #6. d)
1
CITY OF RENTON, WASHINGTON
ORDINANCE NO. ________
AN ORDINANCE OF THE CITY OF RENTON, WASHINGTON, AMENDING
SUBSECTION 5‐25‐4.A OF THE RENTON MUNICIPAL CODE BY ADJUSTING THE
BUSINESS AND OCCUPATION TAX RATE, AUTHORIZING CORRECTIONS,
PROVIDING FOR SEVERABILITY, AND ESTABLISHING AN EFFECTIVE DATE.
THE CITY COUNCIL OF THE CITY OF RENTON, WASHINGTON, DO ORDAIN AS FOLLOWS:
SECTION I. All portions of the Renton Municipal Code in this ordinance not shown in
strikethrough and underline edits remain in effect and unchanged.
SECTION II. Subsection 5‐25‐4.A of the Renton Municipal Code is amended as shown
below. All other provisions in 5‐25‐4 shall remain in effect and unchanged.
5‐25‐4 IMPOSITION OF THE TAX – TAX OR FEE LEVIED; BUSINESS LICENSE FEE
EXEMPTION:
A. Except as provided in RMC 5‐25‐4.B (Tax Thresholds) and 5‐25‐4.C
(Amounts in Excess of Cap Annual Tax Cap and Rate Reduction), effective January
1, 2016 at 12:01 a.m. there is levied upon and shall be collected from every person
a tax for the act or privilege of engaging in business activities in the City of Renton,
whether the person’s office or place of business be within or without the City. The
tax shall be in amounts to be determined by application of rates against gross
proceeds of sale, gross income of business, or value of products, including by‐
products, as the case may be, as follows:
1. Upon every person engaging within the City in business as an extractor;
as to such persons, the amount of the tax with respect to such business shall be
equal to the value of the products, including by‐products, extracted within the City
AGENDA ITEM #6. d)
ORDINANCE NO. ________
2
for sale or for commercial or industrial use, multiplied by the rate of .121 of one
percent (.00121) .085 of one percent (.00085). The measure of the tax is the value
of the products, including by‐products, so extracted, regardless of the place of sale
or the fact that deliveries may be made to points outside the City.
2. Upon every person engaging within the City in business as a
manufacturer; as to such persons, the amount of the tax with respect to such
business shall be equal to the value of the products, including by‐products,
manufactured within the City, multiplied by the rate of .121 of one percent
(.00121) .085 of one percent (.00085). The measure of the tax is the value of the
products, including by‐products, so manufactured, regardless of the place of sale
or the fact that deliveries may be made to points outside the City.
3. Upon every person engaging within the City in the business of making
sales at wholesale; as to such persons, the amount of tax with respect to such
business shall be equal to the gross proceeds of such sales of the business without
regard to the place of delivery of articles, commodities or merchandise sold,
multiplied by the rate of .121 of one percent (.00121) .085 of one percent (.00085).
4. Upon every person engaging within the City in the business of making
sales at retail; as to such persons, the amount of tax with respect to such business
shall be equal to the gross proceeds of such sales of the business, without regard
to the place of delivery of articles, commodities or merchandise sold, multiplied
by the rate of .070 of one percent (.00070) .050 of one percent (.00050).
AGENDA ITEM #6. d)
ORDINANCE NO. ________
3
5. Upon every person engaging within the City in the business of (a)
printing, (b) both printing and publishing newspapers, magazines, periodicals,
books, music, and other printed items, (c) publishing newspapers, magazines and
periodicals, (d) extracting for hire, and (e) processing for hire; as to such persons,
the amount of tax on such business shall be equal to the gross income of the
business multiplied by the rate of .121 of one percent (.00121) .085 of one percent
(.00085).
6. Upon every person engaging within the City in the business of sales of
retail services; as to such persons, the amount of tax with respect to such business
shall be equal to the gross proceeds of sales multiplied by the rate of .121 of one
percent (.00121) .085 of one percent (.00085).
7. Upon every other person engaging within the City in any business
activity other than or in addition to those enumerated in the above subsections;
as to such persons, the amount of tax on account of such activities shall be equal
to the gross income of the business multiplied by the rate of .121 of one percent
(.00121) .085 of one percent (.00085). This subsection includes, among others,
and without limitation whether or not title to material used in the performance of
such business passes to another by accession, merger or other than by outright
sale, persons engaged in the business of developing, or producing custom
software or of customizing canned software, producing royalties or commissions,
and persons engaged in the business of rendering any type of service which does
not constitute a sale at retail, a sale at wholesale, or a retail service.
AGENDA ITEM #6. d)
ORDINANCE NO. ________
4
B. Tax Thresholds: This chapter shall not apply to any person engaging in any
one (1) or more business activities which are otherwise taxable pursuant to
RMC 5‐25‐4.A, whose value of products, including by‐products, gross proceeds of
sales, and gross income of the business, less any deductions, as the case may be,
from all activities conducted during any calendar year, is less than or equal to the
threshold amount of five hundred thousand dollars ($500,000).
C. Annual Tax Cap and Rate Reduction: For the calendar years 2022‐2024,
the tax imposed under this chapter for a single taxpayer shall not exceed the
following maximum tax amounts for each respective year according to the
following schedule: 2022 – seven million dollars ($7,000,000); 2023 – nine million
dollars ($9,000,000); 2024 – eleven million dollars ($11,000,000). Starting in 2025
and for each subsequent year thereafter, once a taxpayer has paid twelve million
dollars ($12,000,000) in taxes imposed under this chapter in the given year, the
rates specified in RMC 5‐25‐4.A shall be discounted by seventy‐five percent (75%)
for remaining gross proceeds of sale, gross income of business, or value of
products, including by‐products, as the case may be.
SECTION III. Upon approval of the City Attorney, the City Clerk is authorized to direct
the codifier to make necessary corrections to this ordinance, including the corrections of
scriveners or clerical errors; references to other local, state, or federal laws, codes, rules, or
regulations; or ordinance numbering and section/subsection numbering and references.
SECTION IV. If any section, subsection, sentence, clause, phrase, or word of this
ordinance should be held to be invalid or unconstitutional by a court or competent jurisdiction,
AGENDA ITEM #6. d)
ORDINANCE NO. ________
5
such invalidity or unconstitutionality thereof shall not affect the constitutionality of any other
section, subsection, sentence, clause, phrase, or word of this ordinance.
SECTION V. This ordinance shall be in full force and effect on January 1, 2023. No later
than five (5) days prior to such effective date, a summary of this ordinance consisting of its title
shall be published in the City’s official newspaper.
PASSED BY THE CITY COUNCIL this _______ day of ___________________, 2022.
Jason A. Seth, City Clerk
APPROVED BY THE MAYOR this _______ day of _____________________, 2022.
Armondo Pavone, Mayor
Approved as to form:
Shane Moloney, City Attorney
Date of Publication:
ORD‐FINANCE:2222:5/25/22
AGENDA ITEM #6. d)
AB - 3145
City Council Regular Meeting - 13 Jun 2022
SUBJECT/TITLE:Interfund Loan Status Update
RECOMMENDED ACTION: None; Information Only
DEPARTMENT: Finance Department
STAFF CONTACT:Nate Malone, Budget & Accounting Manager
EXT.:6936
FISCAL IMPACT SUMMARY:
N/A
SUMMARY OF ACTION:
Council approved an interfund loan between fund 424 Golf Course Capital Improvement and 000 General
Fund in 2021 through resolution 4432 for the purchase of new golf carts. The funds were not transferred to
fund 424 until 2022 as the golf course did not need the funds until the golf carts were received. Fund 424
received funds, and began repayment in March 2022 per the amortization schedule in resolution 4432 and will
continue to make monthly payments throughout the 5 year repayment period.
EXHIBITS:
N/A
STAFF RECOMMENDATION:
None - Information Only.
AGENDA ITEM #6. e)
1
CITY OF RENTON, WASHINGTON
ORDINANCE NO. ________
AN ORDINANCE OF THE CITY OF RENTON, WASHINGTON, AMENDING
SUBSECTIONS 4‐6‐030.C AND 4‐6‐030.I AND SECTION 4‐11‐190 OF THE RENTON
MUNICIPAL CODE, ADOPTING BY REFERENCE THE 2022 CITY OF RENTON
SURFACE WATER DESIGN MANUAL, AND ADOPTING BY REFERENCE THE JULY
2021 KING COUNTY STORMWATER POLLUTION PREVENTION MANUAL,
AUTHORIZING CORRECTIONS, PROVIDING FOR SEVERABILITY, AND
ESTABLISHING AN EFFECTIVE DATE.
WHEREAS, the Surface Water Utility is updating the City of Renton Surface Water Design
Manual (RSWDM); and
WHEREAS, the RSWDM is used by private developers and the City when designing new
storm systems, upgrading existing stormwater infrastructure, or performing drainage review on
projects; and
WHEREAS, the City’s current (2019‐2024) Municipal Stormwater Permit, issued by the
Department of Ecology, requires the adoption of a manual equivalent to the 2019 Ecology
Stormwater Management Manual for Western Washington (SWMMWW); and
WHEREAS, with this Ordinance the City primarily seeks to update the City’s Surface Water
Design Manual for consistency with the SWMMWW, and revise RMC 4‐6‐030, Drainage (Surface
Water) Standards, and RMC 4‐11‐190, Definitions S, for consistency with the RSWDM update;
and
WHEREAS, this matter was duly referred to the Planning Commission for investigation
and study, and the matter was considered by the Planning Commission; and
WHEREAS, pursuant to RCW 36.70A.106, on May 13, 2022, the City notified the State of
Washington of its intent to adopt amendments to its development regulations; and
AGENDA ITEM # 8. a)
ORDINANCE NO. ________
2
WHEREAS, the Planning Commission held a public hearing on May 18, 2022, considered
all relevant matters, and heard all parties in support or opposition, and subsequently forwarded
a recommendation to the City Council;
NOW, THEREFORE, THE CITY COUNCIL OF THE CITY OF RENTON, WASHINGTON, DO
ORDAIN AS FOLLOWS:
SECTION I. All portions of the Renton Municipal Code in this ordinance not shown in
strikethrough and underline edits remain in effect and unchanged.
SECTION II. Subsections 4‐6‐030.C and 4‐6‐030.I of the Renton Municipal Code are
amended as shown below. All other provisions in 4‐6‐030 remain in effect and unchanged.
C. ADOPTION OF SURFACE WATER DESIGN MANUAL:
The 2016 King County, Washington, Surface Water Design Manual as it exists
or may be amended by the City of Renton Amendments to the King County Surface
Water Design Manual, dated December 12, 2016, and further amended by the City
on December 13, 2021, is adopted by reference and referred to hereafter as by
the City of Renton for consistency with the current version of the King County
Surface Water Design Manual. The Surface Water Design Manual shall be filed
with the City Clerk and available for viewing on the City’s website.
I. ADOPTION OF STORMWATER POLLUTION PREVENTION MANUAL:
The April 2016 July 2021 King County Stormwater Pollution Prevention
Manual, hereby referred to as the Stormwater Pollution Prevention Manual, is
hereby adopted by reference. One copy of the manual shall be filed with the City
Clerk.
AGENDA ITEM # 8. a)
ORDINANCE NO. ________
3
SECTION III. The definition of “Surface Water Design Manual” in section 4‐11‐190 of the
Renton Municipal Code is amended as shown below. All other definitions in 4‐11‐190 remain in
effect and unchanged.
SURFACE WATER DESIGN MANUAL: A manual, as it exists or may be amended,
adopted by reference by the City of Renton, which provides stormwater permit
implementation and management guidance consistent with the current version of
the King County Surface Water Design Manual. Shall be the 2016 King County,
Washington, Surface Water Design Manual, amended by the City of Renton
Amendments to the King County Surface Water Design Manual, dated December
12, 2016.
SECTION IV. The 2022 City of Renton Surface Water Design Manual, attached hereto as
Attachment A and incorporated by this reference, is hereby adopted.
SECTION V. The July 2021 King County Stormwater Pollution Prevention Manual,
attached hereto as Attachment B and incorporated by this reference, is hereby adopted.
SECTION VI. Upon approval of the City Attorney, the City Clerk is authorized to direct
the codifier to make necessary corrections to this ordinance, including the corrections of
scriveners or clerical errors; references to other local, state, or federal laws, codes, rules, or
regulations; or ordinance numbering and section/subsection numbering and references.
SECTION VII. If any section, subsection, sentence, clause, phrase, or word of this
ordinance should be held to be invalid or unconstitutional by a court or competent jurisdiction,
such invalidity or unconstitutionality thereof shall not affect the constitutionality of any other
section, subsection, sentence, clause, phrase, or word of this ordinance.
AGENDA ITEM # 8. a)
ORDINANCE NO. ________
4
SECTION VIII. This ordinance shall be in full force and effect five (5) days after publication
of a summary of this ordinance in the City’s official newspaper. The summary shall consist of this
ordinance’s title.
PASSED BY THE CITY COUNCIL this _______ day of ___________________, 2022.
Jason A. Seth, City Clerk
APPROVED BY THE MAYOR this _______ day of _____________________, 2022.
Armondo Pavone, Mayor
Approved as to form:
Shane Moloney, City Attorney
Date of Publication:
ORD‐CED:2218:6/7/22
AGENDA ITEM # 8. a)
ATTACHMENT A
2022 CITY OF RENTON SURFACE WATER
DESIGN MANUAL
AGENDA ITEM # 8. a)
2022
CITY OF RENTON
SURFACE WATER
DESIGN MANUAL
CITY OF RENTON
PUBLIC WORKS DEPARTMENT
SURFACE WATER UTILITY
Adopted on June 22, 2022
AGENDA ITEM # 8. a)
Note:
Some pages in this document have been purposely skipped or blank pages inserted so that this
document will copy correctly when duplexed.
AGENDA ITEM # 8. a)
2022 City of Renton Surface Water Design Manual 6/22/2022
INTRODUCTION
TABLE OF CONTENTS
AND OVERVIEW
CITY OF RENTON
SURFACE WATER
DESIGN MANUAL
AGENDA ITEM # 8. a)
TABLE OF CONTENTS AND OVERVIEW
6/22/2022 2022 City of Renton Surface Water Design Manual
(T his page intentionally left blan k. )
AGENDA ITEM # 8. a)
CITY OF RENTON SURFACE WATER DESIGN MANUAL
2022 City of Renton Surface Water Design Manual 6/22/2022
TABLE OF CONTENTS
CHAPTER 1
DRAINAGE REVIEW AND REQUIREMENTS
Section Page 1.4 Adjustment Process 1-99
1.4.1 Adjustment Authority 1-99
1.4.2 Criteria for Granting Adjustments 1-99
1.4.3 Adjustment Application Process 1-100
1.4.4 Adjustment Review Process 1-101
1.4.5 Appeals 1-101
1.1 Drainage Review 1-11
1.1.1 Projects Requiring Drainage Review 1-12
1.1.2 Drainage Review Types and
Requirements 1-12
1.1.3 Drainage Review Required By Other
Agencies 1-22
1.1.4 Drainage Design Beyond Minimum
Compliance 1-22
1.2 Core Requirements 1-23
1.2.1 Core Requirement #1: Discharge at the
Natural Location 1-23
1.2.2 Core Requirement #2: Offsite Analysis 1-24
1.2.3 Core Requirement #3: Flow Control
Facilities 1-35
1.2.4 Core Requirement #4: Conveyance
System 1-50
1.2.5 Core Requirement #5: Construction
Stormwater Pollution Prevention 1-54
1.2.6 Core Requirement #6: Maintenance and
Operations 1-59
1.2.7 Core Requirement #7: Financial
Guarantees and Liability 1-61
1.2.8 Core Requirement #8: Water Quality
Facilities 1-63
1.2.9 Core Requirement #9: On-Site BMPs 1-73
1.3 Special Requirements 1-89
1.3.1 Special Requirement #1: Other Adopted
Area-Specific Requirements 1-89
1.3.2 Special Requirement #2: Flood Hazard
Area Delineation 1-90
1.3.3 Special Requirement #3: Flood
Protection Facilities 1-91
1.3.4 Special Requirement #4: Source
Controls 1-92
1.3.5 Special Requirement #5: Oil Control 1-94
1.3.6 Special Requirement #6: Aquifer
Protection Area 1-97
AGENDA ITEM # 8. a)
TABLE OF CONTENTS AND OVERVIEW
6/22/2022 2022 City of Renton Surface Water Design Manual
TABLE OF CONTENTS
CHAPTER 2
DRAINAGE PLAN SUBMITTAL
CHAPTER 3
HYDROLOGIC ANALYSIS & DESIGN
Section Page Section Page
2.1 Plans for Permits and Drainage
Review 2-3
2.1.1 Plans Required for Pre-Application
Submittal 2-3
2.1.2 Site Plans Required for Drainage
Review 2-3
2.2 Plans Required with Construction
Permit Application 2-5
2.2.1 Subdivision, PUD, and Binding Site
Plans 2-6
2.2.2 Short Subdivisions 2-7
2.2.3 Commercial Site Development 2-7
2.2.4 Single-Family Residential 2-7
2.2.5 Other Permits 2-7
2.3 Drainage Review Plan Specifications 2-9
2.3.1 Engineering Plan Specifications 2-10
2.3.2 Projects in Targeted Drainage Review
(TDR) 2-35
2.4 Plans Required After Drainage
Review 2-37
2.4.1 Plan Changes After Permit Issuance 2-37
2.4.2 Final Corrected Plan Submittal 2-37
2.4.3 Final Plat, Short Plat, and Binding Site
Plan Submittals 2-38
3.1 Hydrologic Design Standards and
Principles 3-3
3.1.1 Hydrologic Impacts and Mitigation 3-3
3.1.2 Flow Control Standards 3-5
3.1.3 Hydrologic Analysis Using Continuous
Models 3-5
3.2 Runoff Computation and Analysis
Methods 3-9
3.2.1 Rational Method 3-11
3.2.2 Continuous Models and the Runoff
Files Method 3-19
3.2.3 The Approved Model 3-30
3.2.4 The HSPF Model 3-30
3.3 Hydrologic Design Procedures and
Considerations 3-33
3.3.1 General Hydrologic Design Process 3-33
3.3.2 Flow Control Design Using the Runoff
Files Method 3-34
3.3.3 Conveyance System Design with the
Runoff Files Method 3-37
3.3.4 Safety Factors in Hydrologic Design 3-38
3.3.5 Design Options for Addressing
Downstream Drainage Problems 3-38
3.3.6 Point of Compliance Analysis 3-38
3.3.7 Onsite Closed Depressions and
Ponding Areas 3-41
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TABLE OF CONTENTS
CHAPTER 4
CONVEYANCE SYSTEM ANALYSIS &
DESIGN
CHAPTER 5
FLOW CONTROL DESIGN
Section Page Section Page
4.1 Route Design and Easement
Requirements 4-3
4.1.1 Route Design 4-3
4.1.2 Easement and Setback Requirements 4-3
4.2 Pipes, Outfalls, and Pumps 4-7
4.2.1 Pipe Systems 4-7
4.2.2 Outfall Systems 4-30
4.2.3 Pump Systems 4-36
4.3 Culverts and Bridges 4-37
4.3.1 Culverts 4-37
4.3.2 Culverts Providing for Fish
Passage/Migration 4-50
4.3.3 Bridges 4-52
4.4 Open Channels, Floodplains, and
Floodways 4-55
4.4.1 Open Channels 4-55
4.4.2 Floodplain/Floodway Analysis 4-71
5.1 Detention Facilities 5-3
5.1.1 Detention Ponds 5-3
5.1.2 Detention Tanks 5-17
5.1.3 Detention Vaults 5-21
5.1.4 Control Structures 5-25
5.1.5 Parking Lot Detention 5-35
5.1.6 Roof Detention 5-35
5.1.7 Simple Detention Pond for Cleared
Areas 5-35
5.1.8 Alternative Detention Systems 5-42
5.2 Infiltration Facilities 5-45
5.2.1 General Requirements for Infiltration
Facilities 5-45
5.2.2 Infiltration Ponds 5-56
5.2.3 Infiltration Tanks 5-59
5.2.4 Infiltration Vaults 5-62
5.2.5 Infiltration Trenches 5-64
5.2.6 Alternative Infiltration Systems 5-65
5.2.7 Small Infiltration Basins 5-66
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TABLE OF CONTENTS
CHAPTER 6
WATER QUALITY DESIGN
Section Page Section Page
6.1 Water Quality Menus 6-3
6.1.1 Basic Water Quality Menu 6-5
6.1.2 Enhanced Basic Water Quality Menu 6-8
6.1.3 Sensitive Lake Protection Menu 6-10
6.1.4 Sphagnum Bog Protection Menu 6-14
6.1.5 High-Use Menu 6-16
6.1.6 Pretreatment Facilities 6-18
6.2 General Requirements for WQ
Facilities 6-19
6.2.1 Water Quality Design Flows and
Treatment Volumes 6-19
6.2.2 Sequence of Facilities 6-22
6.2.3 Setbacks, Slopes, and Embankments 6-24
6.2.4 Facility Liners 6-28
6.2.5 Flow Splitter Designs 6-32
6.2.6 Flow Spreading Options 6-36
6.3 Vegetated Flowpath Facility Designs 6-41
6.3.1 Basic Bioswales 6-41
6.3.2 Wet Bioswales 6-57
6.3.3 Lateral Inflow Bioswales 6-59
6.3.4 Standard Filter Strips 6-60
6.3.5 Narrow Area Filter Strips 6-68
6.4 Wetpool Facility Designs 6-69
6.4.1 Wetponds — Basic and Large 6-69
6.4.2 Wetvaults 6-84
6.4.3 Stormwater Wetlands 6-90
6.4.4 Combined Detention and Wetpool
Facilities 6-96
6.5 Filtration Facility Designs 6-101
6.5.1 General Requirements For Filtration
Facilities 6-101
6.5.2 Sand Filters — Basic and Large 6-102
6.5.3 Sand Filter Vaults 6-118
6.5.4 Linear Sand Filters 6-123
6.6 Oil Control Facility Designs 6-127
6.6.1 Catch Basin Inserts 6-127
6.6.2 Oil/Water Separators 6-127
6.7 Proprietary Facility Designs 6-141
6.7.1 Ecology Requirements 6-141
6.7.2 City of Renton Requirements 6-141
6.8 Bioretention Facility Designs 6-145
6.8.1 Bioretention 6-145
6.9 WSDOT WQ Facility Designs 6-159
6.9.1 Media Filter Drain 6-159
6.9.2 Compost-Amended Filter Strips 6-169
6.9.3 Compost-Amended Biofiltration Swales 6-170
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DEFINITIONS
APPENDICES
APPENDIX A
Maintenance Requirements for Stormwater
Facilities and On-site BMPs
APPENDIX B
Master Drainage Plan Objective, Criteria,
Components and Review Process
APPENDIX C
Simplified Drainage Requirements
APPENDIX D
Construction Stormwater Pollution Prevention
Standards
REFERENCE
1. Surface Water Runoff Policy
2. Adopted Critical Drainage Areas
3. Other Adopted Area Specific Drainage
Requirements
4. Other Drainage Related Regulations and
Guidelines
A Grading Code Soil Amendment Standard
B Clearing & Grading Seasonal Limitations
C Landscape Management Plan Guidelines
D Shared Facility Maintenance Responsibility
Guidance
5. Wetland Hydrology Protection Guidelines
6. Hydrologic/Hydraulic Design Methods
A Infiltration Rate Test Methods
B Pond Geometry Equations
C Introduction to Level Pool Routing
D Supplemental Modeling Guidelines
REFERENCE (continued)
7. Engineering Plan Support
A King County Standard Map Symbols
B Standard Plan Notes and Example Construction
Sequence
C Stormfilter Facility Access and Cartridge
Configuration
8. Forms and Worksheets
A Technical Information Report (TIR) Worksheet
B Offsite Analysis Drainage System Table
C Water Quality Facility Sizing Worksheets
D Flow Control and Water Quality Facility
Summary Sheet and Sketch
E CSWPP Worksheet Forms
F Adjustment Application Form and Process
Guidelines
G Dedication and Indemnification Clause –
Final Recording
H Bond Quantities Worksheet
I Maintenance and Defect Agreement
J Drainage Facility Covenant
K Drainage Release Covenant
L Drainage Easement
M On-Site BMP Covenant and Maintenance
Instructions (recordable format)
N Impervious Surface Limit Covenant
O Clearing Limit Covenant
P River Protection Easement
Q Leachable Metals Covenant
R Agreement to Construct Improvements
9. Interim Changes to Requirements
A Blanket Adjustments
B Administrative Changes
10. King County-Identified Water Quality Problems
11. Materials
A (VACANT)
B (VACANT)
C Bioretention Soil Media Standard
Specifications
D (VACANT)
E Roofing Erodible or Leachable Materials
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12. (VACANT)
13. (VACANT)
14. Supplemental Approved Facilities
A City of Renton Approved Proprietary Facilities
for Use on Private Development Projects
B City of Renton Approved Proprietary Facilities
for Use in Public Projects
AGENDA ITEM # 8. a)
CITY OF RENTON SURFACE WATER DESIGN MANUAL
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INTRODUCTION
OVERVIEW
The intent of this document is to provide requirements and guidance for the design, construction, and
maintenance of on-site best management practices (BMPs), flow control facilities and water quality
treatment facilities that are approved through the development permit process. This document is based on
the 2021 King County Surface Water Design Manual with some modifications. Revisions have been made
to the 2021 King County Surface Water Manual to reflect City of Renton-specific requirements.
PURPOSE OF AND NEED FOR THIS DOCUMENT
The City’s adoption this manual is required to comply with federal stormwater regulations. Specifically,
the City’s Phase II National Pollutant Discharge Elimination System (NPDES) municipal stormwater
permit establishes regulations for jurisdictions that:
1. Own and operate a storm drain system;
2. Discharge to surface waters;
3. Are located in urbanized areas; and
4. Have a population greater than 1,000.
Washington State’s Department of Ecology (Ecology), who oversees stormwater requirements in the state,
has developed the 2019 Stormwater Management Manual for Western Washington, which complies with
the NPDES requirements. In addition, Ecology has approved the 2021 King County Surface Water Design
Manual as equivalent to the 2019 Stormwater Management Manual for Western Washington.
ORGANIZATION
The chapters of this manual are organized as follows:
Chapter 1 – DRAINAGE REVIEW AND REQUIREMENTS
Describes the basic drainage requirements that implement the City of Renton’s adopted
surface water runoff policies and explains how these requirements are applied to
proposed projects through the drainage review process.
Chapter 2 – DRAINAGE PLAN SUBMITTAL
Describes the requirements and specifications for submittal of design plans for drainage
review, including report and plan formats, and scopes.
Chapter 3 – HYDROLOGIC ANALYSIS AND DESIGN
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Presents the acceptable methods of hydrologic analysis used to estimate runoff and
design flow control, conveyance, and water quality facilities.
Chapter 4 – CONVEYANCE SYSTEM ANALYSIS AND DESIGN
Presents the acceptable methods, details, and criteria for analysis and design of
conveyance systems.
Chapter 5 – FLOW CONTROL DESIGN
Presents the acceptable methods, details, and criteria for analysis and design of flow
control facilities.
Chapter 6 – WATER QUALITY DESIGN
Presents the acceptable methods, details, and criteria for analysis and design of water
quality facilities.
DEFINITIONS – A comprehensive list of the words, terms, and abbreviations accompanied by their
meaning as applied in this manual.
APPENDICES:
APPENDIX A – MAINTENANCE REQUIREMENTS FOR STORMWATER FACILITIES AND
ON-SITE BMPs
Contains the thresholds and standards for maintenance of all flow control facilities,
on-site BMPs, conveyance systems, and water quality facilities required in this
manual.
APPENDIX B – MASTER DRAINAGE PLAN OBJECTIVES, CRITERIA AND COMPONENTS,
AND REVIEW PROCESS
Describes in a general outline, the objectives, criteria, components and review
process for Master Drainage Plans prepared for Urban Planned Developments and
very large projects.
APPENDIX C – SIMPLIFIED DRAINAGE REQUIREMENTS
Describes, the simplified drainage requirements for smaller projects that qualify for
Simplified Drainage Review.
APPENDIX D – CONSTRUCTION STORMWATER POLLUTION PREVENTION
STANDARDS
Describes, the required measures to be implemented during construction to prevent
discharges of sediment-laden runoff from the project site. It also describes
effective management practices for spill control and chemical pollutants used
during construction that may be needed to supplement the required erosion and
sedimentation control measures.
REFERENCE – Includes materials that are strictly for reference only and have not been adopted by the public
rule adopting this manual. The applicant is responsible to ensure that the most current materials
are used in preparing a permit application.
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KEY REVISIONS
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3
KEY REVISIONS
This section identifies the key revisions that the City has made to the 2021 King County Surface Water
Design Manual. These revisions were necessary to address specific City of Renton requirements and to
address deficiencies within the 2021 King County Surface Water Design Manual.
1. Aquifer Protection Area – The City of Renton Surface Water Design Manual includes an additional
special requirement (Special Requirement #6) related to the Aquifer Protection Area. Certain facilities
are restricted in Zone 1 of the Aquifer Protection Area or may require a liner in Zone 1 Modified or
Zone 2 of the Aquifer Protection Area.
2. On-site BMPs – Core Requirement #9 was renamed to On-site BMPs instead of Flow Control BMPs
to avoid confusion with Flow Control Facilities (Core Requirement #3).
3. Additional On-site BMP Options for Core Requirement #9 –
a) Rain Gardens
b) Soil Amendment (included in King County Code, but details were not included in the King
County Surface Water Design Manual)
c) Tree Retention Credit
d) Vegetated Roofs (optional)
4. LID Performance Standard – The LID performance standard is optional, but is not required for any
projects located within the City of Renton.
5. Flow Control Standards – The City developed specific terminology for flow control standards in
Core Requirement #3:
a) Flow Control Duration Standard – Matching Forested Site Conditions
b) Flow Control Duration Standard – Matching Existing Site Conditions
c) Peak Rate Flow Control Standard – Matching Existing Site Conditions
d) Flood Problem Flow Control Standard
6. Additional Water Quality Facility Options for Core Requirement #8 –
a) The following facilities are available as options on the Basic WQ Menu:
i. Bioretention
ii. WSDOT WQ Facilities – Media Filter Drain, Compost-amended Vegetated Filter Strips, and
Compost-amended Biofiltration Swales
b) The following facilities are available as options on the Enhanced Basic WQ Menu:
i. Bioretention
ii. Proprietary Facilities
iii. WSDOT WQ Facilities – Media Filter Drain, Compost-amended Vegetated Filter Strips, and
Compost-amended Biofiltration Swales
c) The following facilities are available as options on the Sensitive Lake Protection Menu:
i. Proprietary Facilities
7. Proprietary Facilities – Added specific proprietary facilities to Chapter 6, Appendix A, and
Reference Section 14-A.
8. Alternative Detention and Infiltration Systems – Revised Section 5.2.6 Alternative Infiltration
Systems and added new Section 5.1.8 Alternative Detention Systems for proprietary flow control
facilities that deviate from the standard design requirements for other flow control facilities.
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9. Element #11 of Core Requirement #2 – This Construction Stormwater Pollution Prevention element
was modified to be more inclusive of other infiltration facilities that are not considered to be On-site
BMPs.
10. Bioretention Design and Construction – The City incorporated the following modifications to the
bioretention design site suitability factors and design criteria:
a) Specific information added regarding perpendicular utility crossings
b) Minimum bottom with is 18 inches
c) Minimum ponding depth is 2 inches
d) Maximum side slopes are 2.5 to 1
e) Minimum berm top width is 6 inches
f) Minimum shoulder between road edge and bioretention side slope is 6 inches
g) Water tolerant plant list has been revised
h) Underdrains are allowed for bioretention facilities designed to meet Core Requirement #8
i) Added a section on construction sequencing
11. Permeable Pavement Design and Construction – The City incorporated the following modifications
to the permeable pavement design criteria:
a) Run-on is not allowed from pervious surfaces
b) Underdrains are allowed in specific settings
c) Added design criteria regarding the overflow
d) Added figures depicting permeable pavement cross sections and permeable pavement with check
dams
e) Added a reference to ASTM C1701 and ASTM C1781 for infiltration rate verification
f) Added a section on construction sequencing
12. Core Requirement #6 and #7 – Language specific to the City of Renton has been incorporated into
Core Requirement #6 (Maintenance and Operations) and Core Requirement #7 (Financial Guarantees
and Liabilities).
13. Basic WQ Thresholds – The thresholds for basic WQ treatment in Core Requirement #8 have been
adjusted for consistency with the Amendments to the 2009 King County Surface Water Design
Manual and the 2012 Stormwater Management Manual for Western Washington, as amended in 2014.
14. Target impervious surface – The City of Renton does not require including existing impervious
surfaces added on or after January 8, 2001 in the definition of target impervious surfaces.
15. Continuous modeling timestep – Precipitation with a 15-minute precipitation is available for the
entire City of Renton, so references to the 1-hour timestep were removed.
16. Continuous modeling precipitation series – The City of Renton allows either rain gage data (Sea-
Tac Airport) or the 158-year extended precipitation timeseries (Puget East) to be used for modeling.
17. Allowable pipe materials – The City of Renton added a table of allowable pipe materials and
minimum cover to Chapter 4.
18. Conveyance requirements – The City of Renton added requirements to Chapter 4 for changes in pipe
size, structures, pipe cover, pipe clearances, pipe system connections, and pump systems.
19. Fencing requirements – The City of Renton added specific fencing requirements to Chapter 5 and
Chapter 6 for detention ponds and wet ponds related to a City policy decision.
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20. Seeding – The City of Renton revised seeding rates, timing, and mixes in Chapter 5, Chapter 6, and
Appendix C, and Appendix D.
21. Removing terminology that does not apply – Terminology related to the Urban Growth Area, Urban
Planned Development (UPD), Agricultural Projects, Critical Drainage Areas, Large Rural Lots,
Stormwater Compliance Plans (SWCPs), Experimental Design Adjustments, recreational tracts, and
Landscape Management Plan was removed.
22. Removed BMPs that do not apply – Catch Basin Inserts (Chapter 6), Narrow Area Filter Strips
(Chapter 6), and Farmland Dispersion (Appendix C).
23. References
a) Provided links to the City’s website for covenants, easements, agreements, and worksheets
OTHER APPLICABLE REFERENCES
The City also adopts, by reference, the 2021 King County Stormwater Pollution Prevention Manual with
amendments, for determining source control requirements.
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CHAPTER 1
DRAINAGE REVIEW
AND REQUIREMENTS
CITY OF RENTON
SURFACE WATER
DESIGN MANUAL
Section Page
1.1 Drainage Review 1-11
1.1.1 Projects Requiring Drainage Review 1-12
1.1.2 Drainage Review Types and
Requirements 1-12
1.1.3 Drainage Review Required By Other
Agencies 1-22
1.1.4 Drainage Design Beyond Minimum
Compliance 1-22
1.2 Core Requirements 1-23
1.2.1 Core Requirement #1: Discharge at the
Natural Location 1-23
1.2.2 Core Requirement #2: Offsite Analysis 1-24
1.2.3 Core Requirement #3: Flow Control
Facilities 1-35
1.2.4 Core Requirement #4: Conveyance
System 1-50
1.2.5 Core Requirement #5: Construction
Stormwater Pollution Prevention 1-54
1.2.6 Core Requirement #6: Maintenance and
Operations 1-59
1.2.7 Core Requirement #7: Financial
Guarantees and Liability 1-61
1.2.8 Core Requirement #8: Water Quality
Facilities 1-63
1.2.9 Core Requirement #9: On-Site BMPs 1-73
1.3 Special Requirements 1-89
1.3.1 Special Requirement #1: Other Adopted
Area-Specific Requirements 1-89
1.3.2 Special Requirement #2: Flood Hazard
Area Delineation 1-90
1.3.3 Special Requirement #3: Flood
Protection Facilities 1-91
1.3.4 Special Requirement #4: Source
Controls 1-92
1.3.5 Special Requirement #5: Oil Control 1-94
1.3.6 Special Requirement #6: Aquifer
Protection Area 1-97
1.4 Adjustment Process 1-99
1.4.1 Adjustment Authority 1-99
1.4.2 Criteria for Granting Adjustments 1-99
1.4.3 Adjustment Application Process 1-100
1.4.4 Adjustment Review Process 1-101
1.4.5 Appeals 1-101
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CHAPTER 1
DRAINAGE REVIEW AND REQUIREMENTS
This chapter describes the drainage review procedures and types, the drainage requirements, and the
adjustment procedures necessary to implement surface water runoff policies codified in Chapter 4-6-030
of the City of Renton Municipal Code (RMC). It also provides direction for implementing the more
detailed procedures and design criteria found in subsequent chapters of this manual.
Chapter Organization
The information presented in Chapter 1 is organized into four main sections as follows:
Section 1.1, “Drainage Review”
Section 1.2, “Core Requirements”
Section 1.3, “Special Requirements”
Section 1.4, “Adjustment Process”
Each of these sections begins on an odd page so the user can insert tabs if desired for quicker reference.
Formatting of Chapter Text
The text of Chapter 1 and subsequent chapters has been formatted using the following conventions to aid
the user in finding, understanding, and properly applying the thresholds, requirements, and procedures
contained in this manual:
Italic is used to highlight the following: (a) terms when they are first introduced and defined within
the same paragraph; (b) special notes that supplement or clarify thresholds, requirements, and
procedures; (c) sentences considered important for purposes of understanding thresholds,
requirements, and procedures; and (d) titles of publications.
Bold italic is used to highlight terms considered key to understanding and applying drainage review
thresholds, requirements, and procedures. These are called “key terms” and are defined below. This
convention applies after the key term is defined and does not necessarily apply to tables and figures.
Bold is used to highlight words and phrases that are not key terms but are considered important to
emphasize for purposes of finding and properly applying thresholds, requirements, and procedures.
Key Terms and Definitions (a complete list of definitions follows Chapter 6)
Proper application of the drainage review and requirements in this chapter requires an understanding of the
following key terms and their definitions. Other key terms may be defined in subsequent chapters. All
such key terms are highlighted in bold italic throughout the manual. Other important terms that are not key
terms are defined in the text when they are first introduced. These are highlighted in italic when they are
first introduced but are not highlighted throughout the manual. All terms defined in this chapter are also
found in the “Definitions” section of this manual as are other important terms defined throughout the
Manual.
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Aquifer Protection Area (APA) means the portion of an aquifer within the zone of capture and recharge
area for a well or well field owned or operated by the City of Renton, as depicted in the Wellhead
Protection Area Zones layer of COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>).
Arterial – A high traffic-volume road or street primarily for through traffic. The term generally includes
roads or streets considered collectors. It does not include local access roads which are generally
limited to providing access to abutting property. Arterial streets are depicted in the Arterials layer of
COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>).
Bioretention – An on-site and water quality treatment best management practice consisting of a shallow
landscaped depression designed to temporarily store and promote infiltration of stormwater runoff.
Standards for bioretention design, including soil mix, plants, storage volume and feasibility criteria,
are specified in Appendix C of this manual. Bioretention can be used to meet Core Requirement #3,
#8 and/or 9.
CED means the Community and Economic Development Department.
Certified Erosion and Sediment Control Lead (CESCL) means an individual who has current certification
through an approved erosion and sediment control training program that meets the minimum training
standards established by the Washington State Department of Ecology (Ecology). A CESCL is
knowledgeable in the principles and practices of erosion and sediment control. The CESCL must have
the skills to assess site conditions and construction activities that could impact the quality of
stormwater and, the effectiveness of erosion and sediment control measures used to control the quality
of stormwater discharges. Certification is obtained through an Ecology approved erosion and sediment
control course.
Civil engineer means a person licensed by the state of Washington as a professional engineer in civil
engineering.
Construct or modify means to install a new drainage pipe or ditch or make improvements to an existing
drainage pipe or ditch, for purposes other than maintenance, that either serves to concentrate
previously unconcentrated surface water or stormwater runoff or serves to increase, decrease or
redirect the conveyance of surface water or stormwater runoff.
Conveyance system nuisance problem means a flooding or erosion problem that does not constitute a
severe flooding problem or severe erosion problem and that results from the overflow of a
constructed conveyance system for runoff events less than or equal to a 10-year event. Examples
include inundation of a shoulder or lane of a roadway, overflows collecting in yards or pastures,
shallow flows across driveways, minor flooding of crawl spaces or unheated garages/outbuildings, and
minor erosion.
Development The division of a parcel of land into two (2) or more parcels; the construction,
reconstruction, conversion, structural alteration, relocation or enlargement of any structure; any
mining, excavation, landfill or land disturbance and any use or extension of the use of land.
Development review engineer – The City of Renton employee responsible for the conditioning, review,
inspection, and approval of right-of-way use permits, and road and drainage improvements
constructed as part of development permits administered by CED.
Effective impervious surface – Those impervious surfaces that are connected via sheet flow or discrete
conveyance to a drainage system. Impervious surfaces are considered ineffective if: 1) the runoff is
fully dispersed as described in Appendix C of this manual; 2) residential roof runoff is infiltrated in
accordance with the full infiltration BMP described in Appendix C of this manual; or 3) approved
continuous runoff modeling methods indicate that the entire runoff file is infiltrated.
Erodible or leachable materials, wastes, or chemicals are those materials or substances that, when
exposed to rainfall, measurably alter the physical or chemical characteristics of the rainfall runoff
(Examples include but are not limited to erodible soil, uncovered process wastes, manure, fertilizers,
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oily substances, ashes, kiln dust, garbage dumpster leakage, commercial-scale vehicle and animal
wash waste, galvanized structural, architectural, cabinet, and utility steel, architectural copper, bronze,
brass, and lead, treated lumber, etc.).
Erosion hazard area is the critical area designation, defined and regulated in RMC 4-3-050, that is applied
to areas underlain by soils that are subject to severe erosion when disturbed. Erosion hazard areas are
depicted in the Erosion Hazard - High layer of COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>).
Existing site conditions means those that existed prior to May 1979 as determined from aerial photographs
and, if necessary, knowledge of individuals familiar with the area, unless a drainage plan for land
cover changes has been approved by the City of Renton since May 1979 as part of a City permit or
approval (or County-approved permit if in an area that has been annexed by the City). If so, existing
site conditions are those created by the site improvements and drainage facilities constructed per the
approved drainage plan.
Exposed means subject to direct or blown-in precipitation and/or direct or blown in runoff. Not fully
covered.
Exposed area or exposed material means not covered sufficiently to shield from rainfall and stormwater
runoff. At a minimum, full coverage to not be considered exposed requires a roof with enough
overhang in conjunction with walls of sufficient height to prevent rainfall blow-in; and the walls must
extend into the ground or to a berm or footing to prevent runoff from being blown in or from running
onto the covered area.
Flood hazard area is the critical area designation, defined and regulated in RMC 4-3-050, that is applied
to areas subject to flooding. Flood hazard areas are depicted in the Flood layer of COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>).
Flow control facility means a drainage facility designed in accordance with the drainage requirements in
this manual to mitigate the impacts of increased stormwater runoff generated by site development. A
“flow control facility” is designed either to hold water for a considerable length of time and then
release it by evaporation, plant transpiration or infiltration into the ground or to hold runoff for a short
period of time and then release it to the conveyance system.
Forested site conditions means those that existed on the site prior to any development in the Puget Sound
region, assumed to be forest cover (see “historical site conditions”).
Fully covered means covered sufficiently to shield from rainfall and stormwater runoff. At a minimum,
full coverage requires a roof with enough overhang in conjunction with walls of sufficient height to
prevent rainfall blow-in; and the walls must extend into the ground or to a berm or footing to prevent
runoff from being blown in or from running onto the covered area. Not exposed.
Fully dispersed means the runoff from an impervious surface or nonnative pervious surface has dispersed
per the criteria for fully dispersed surface in Section 1.2.3.2 of this manual.
Groundwater protection areas include the Cedar Valley Sole Source Aquifer Project Review Area
designated by the federal Environmental Protection Agency, Wellfield Capture Zones as mapped by
the Washington State Department of Health, and the Aquifer Protection Areas as mapped by the City.
High-use site means a commercial or industrial site that (1) has an expected average daily traffic (ADT)
count equal to or greater than 100 vehicles per 1,000 square feet of gross building area; (2) is subject
to petroleum storage or transfer in excess of 1,500 gallons per year, not including delivered heating
oil; or (3) is subject to use, storage, or maintenance of a fleet of 25 or more vehicles that are over
10 tons net weight (trucks, buses, trains, heavy equipment, etc.). Also included is any road intersection
with a measured ADT count of 25,000 vehicles or more on the main roadway and 15,000 vehicles or
more on any intersecting roadway, excluding projects proposing primarily pedestrian or bicycle use
improvements. For the purposes of this definition, commercial and industrial site means that portion
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of a site’s developed area associated with an individual commercial or industrial business (e.g., the
area occupied by the business’s buildings and required parking).
Historical site conditions means those that existed on the site prior to any development in the Puget Sound
region. For lands not currently submerged (i.e., outside the ordinary high water mark of a lake,
wetland, or stream), historical site conditions shall be assumed to be forest cover unless reasonable,
historical, site-specific information is provided to demonstrate a different vegetation cover. The
historical site conditions exception in the King County Surface Water Design Manual does not apply
to the City.
Impaired waterbody or impaired receiving water means where the receiving waterbody is either (1) listed
as impaired according to Ecology's Water Quality Assessment categories 2, 4, or 5 for water or
sediment, as documented in the state’s Water Quality Assessment 303(d)/305(b) Integrated Report and
as displayed in WA Ecology’s electronic database and map viewer of these waterbodies, and/or (2)
where subject to any other local, state, or federal cleanup plan or contaminated site designation.
Impervious surface means a non-vegetated surface area that either prevents or retards the entry of water
into the soil mantle as under natural conditions before development; or that causes water to run off the
surface in greater quantities or at an increased rate of flow compared to the flow present under natural
conditions prior to development (see also new impervious surface). Common impervious surfaces
include, but are not limited to, roof, walkways, patios, driveways, parking lots, or storage areas, areas
that are paved, graveled or made of packed or oiled earthen materials or other surfaces that similarly
impede the natural infiltration of surface water or stormwater. For the purposes of flow control and
water quality treatment modeling and applying the impervious surface thresholds and exemptions
contained in this manual, permeable pavement, vegetated roofs, and pervious surfaces with
underdrains designed to collect stormwater runoff are considered impervious surface. An open
uncovered flow control or water quality facility is not considered impervious surface for the purposes
of applying impervious surface thresholds and exemptions but shall be modeled as impervious surface
for the purposes of computing runoff.
Land disturbing activity means any activity that results in a change in the existing soil cover, both
vegetative and non-vegetative, or the existing soil topography. Land disturbing activities include, but
are not limited to demolition, construction, clearing, grading, filling, excavation, and compaction.
Land disturbing activity does not include tilling conducted as part of agricultural practices, landscape
maintenance, or gardening.
Landslide hazard is the critical area designation, defined and regulated in RMC 4-3-050, that is applied to
areas subject to severe risk of landslide due to topography, soil conditions, and geology. Landslide
hazard areas are depicted in the Landslide layer of COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>).
Landslide hazard drainage area means an area where overland flows from a project may pose a
significant threat to health and safety because of its close proximity to a landslide hazard.
Local drainage system means any natural or constructed drainage feature that collects and concentrates
runoff from the site and discharges it downstream.
Low Impact Development (LID) – A stormwater and land use management strategy that strives to mimic
pre-disturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration
by emphasizing conservation, use of onsite natural features, site planning, and distributed stormwater
management practices that are integrated into a project design.
LID Best Management Practices – Distributed stormwater management practices, integrated into a
project design, that emphasize pre-disturbance hydrologic processes of infiltration, filtration, storage,
evaporation and transpiration. LID BMPs are referred to as on-site BMPs in this manual and include,
but are not limited to, bioretention, permeable pavements, limited infiltration systems, roof
downspout controls, dispersion, soil quality and depth, and minimum disturbance foundations.
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LID Principles – Land use management strategies that emphasize conservation, use of onsite natural
features, and site planning to minimize impervious surfaces, native vegetation loss, and stormwater
runoff.
Maintenance means those usual activities taken to prevent a decline, lapse, or cessation in the use of
currently serviceable structures, facilities, BMPs, equipment, or systems if there is no expansion of
any of these, and there are no significant hydrologic impacts. Maintenance includes the repair or
replacement of non-functional facilities and BMPs, and the replacement of existing structures (e.g.,
catch basins, manholes, culverts) with different types of structures, if the repair or replacement is
required to meet current engineering standards or is required by one or more environmental permits
and the functioning characteristics of the original facility or structure are not changed. For the
purposes of applying this definition to the thresholds and requirements of this manual, CED will
determine whether the functioning characteristics of the original facility, structure, or BMP will
remain sufficiently unchanged to consider replacement as maintenance.
Note: The following pavement maintenance practices are exempt from drainage review:
Pothole and square cut patching
Overlaying existing non-permeable asphalt or non-permeable concrete pavement with asphalt
or concrete without expanding the area of coverage
Shoulder grading
Reshaping/regrading drainage systems
Crack sealing
Resurfacing with in-kind material without expanding the road prism, pavement preservation
activities that do not expand the paved prism
Vegetation maintenance.
The following pavement maintenance practices are not categorically exempt from drainage review:
Removing and replacing a paved surface to base course or lower, or repairing the pavement
base (i.e., “replaced impervious surfaces”).
Extending the edge of pavement without increasing the size of the paved area
Resurfacing that meets the definition of new impervious surface in this manual.
Major receiving water means a large receiving water that has been determined by the City of Renton to be
safe for the direct discharge of increased runoff from a proposed project without a flow control
facility, subject to the restrictions on such discharges set forth in Core Requirement #3, Section 1.2.3.
A list of major receiving waters is provided in Section 1.2.3.1. Major receiving waters are also
considered safe for application of Basic WQ treatment in place of otherwise required Enhanced Basic
WQ treatment (see Section 1.2.8.1), except where the receiving water meets the definition of
impaired waterbody or impaired receiving water, specifically with regard to heavy metals.
Multifamily project (or land use) means any project or land use that requires or would require a
commercial building permit or commercial site development permit for development of residential
dwelling units that are not detached single family dwelling units or attached two-unit (duplex)
buildings.
Native vegetated surface means a surface in which the soil conditions, ground cover, and species of
vegetation are like those of the original native condition for the site. More specifically, this means
(1) the soil is either undisturbed or has been treated according to the “native vegetated landscape”
specifications in Appendix C, Section C.2.1.8; (2) the ground is either naturally covered with
vegetation litter or has been top-dressed between plants with 4 inches of mulch consistent with the
native vegetated landscape specifications in Appendix C; and (3) the vegetation is either (a) comprised
predominantly of plant species, other than noxious weeds, that are indigenous to the coastal region of
the Pacific Northwest and that reasonably could have been expected to occur naturally on the site or
(b) comprised of plant species specified for a native vegetated landscape in Appendix C. Examples of
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these plant species include trees such as Douglas fir, western hemlock, western red cedar, alder, big-
leaf maple and vine maple; shrubs such as willow, elderberry, salmonberry and salal; and herbaceous
plants such as sword fern, foam flower, and fireweed.
Natural discharge area means an onsite area tributary to a single natural discharge location.
Natural discharge location means the location where surface and storm water runoff leaves (or would
leave if not infiltrated or retained) the site or project site under existing site conditions.
New impervious surface means the conversion of a pervious surface to an impervious surface; or the
addition of a more compacted surface, such as resurfacing by upgrading from dirt to gravel, asphalt, or
concrete; upgrading from gravel to asphalt, or concrete; or upgrading from a bituminous surface
treatment (“chip seal”) to asphalt or concrete.
New pervious surface means the conversion of a native vegetated surface or other native surface to a
nonnative pervious surface (e.g., conversion of forest or meadow to pasture land, grass land,
cultivated land, lawn, landscaping, bare soil, etc.), or any alteration of existing nonnative pervious
surface that significantly increases surface and storm water runoff (e.g., conversion of pasture land,
grass land, or cultivated land to lawn, landscaping, or bare soil; or alteration of soil characteristics).
New PGIS means new impervious surface that is pollution-generating impervious surface or any
alteration of existing pollution-generating impervious surface that changes the type of pollutants or
results in increased pollution loads and/or concentrations.
New PGPS means new pervious surface that is pollution-generating pervious surface or any alteration of
existing pollution-generating pervious surface that changes the type of pollutants or results in
increased pollution loads and/or concentrations.
Offsite means any area lying upstream of the site that drains onto the site and any area lying downstream
of the site to which the site drains including frontage improvements.
Onsite means the entire site that includes the proposed development.
On-site BMP means a small scale drainage facility or feature that is part of a development site strategy to
use processes such as infiltration, dispersion, storage, evaporation, transpiration, forest retention, and
reduced impervious surface footprint to mimic pre-developed hydrology and minimize stormwater
runoff.
Permeable pavement means pervious concrete, porous asphalt, permeable pavers or other forms of
pervious or porous paving material intended to allow passage of water through the pavement section.
It often includes an aggregate base that provides structural support and acts as a stormwater reservoir.
Pervious Surface – Any surface material that allows stormwater to infiltrate into the ground. Examples
include lawn, landscape, pasture, and native vegetation areas. This designation excludes permeable
pavement, vegetated roofs, and pervious surfaces with underdrains designed to collect stormwater
runoff (see “impervious surface”).
Pollution-generating impervious surface (PGIS) means an impervious surface considered to be a
significant source of pollutants in stormwater runoff. Such surfaces include those that are subject to
vehicular use, industrial activities, or storage of erodible or leachable materials, wastes, or chemicals
and that receive direct rainfall or the run-on or blow-in of rainfall. A covered parking area would be
considered PGIS if runoff from uphill could regularly run through it or if rainfall could regularly blow
in and wet the pavement surface. Metal roofs are also considered PGIS unless they are coated with an
inert, non-leachable material (see Reference Section 11-E); or roofs that are exposed to the venting of
significant amounts of dusts, mists, or fumes from manufacturing, commercial, or other indoor
activities. PGIS includes vegetated roofs exposed to pesticides, fertilizers, or loss of soil. Other
roofing types that may pose risk but are not currently regulated are listed Reference Section 11-E.
Lawns, landscaping, sports fields, golf courses, and other areas that have modified runoff
characteristics resulting from the addition of underdrains that have the pollution generating
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characteristics described under the “pollution-generating pervious surface” definition are also
considered PGIS.
Pollution-generating pervious surface (PGPS) means a non-impervious surface considered to be a
significant source of pollutants in surface and storm water runoff. Such surfaces include those that are
subject to vehicular use, industrial activities, storage of erodible or leachable materials, wastes, or
chemicals, and that receive direct rainfall or the run-on or blow-in of rainfall; or subject to use of
pesticides and fertilizers, or loss of soil. Such surfaces include, but are not limited to, the lawn and
landscaped areas of residential, commercial, and industrial sites or land uses, golf courses, parks,
sports fields (natural and artificial turf), cemeteries, and grassed modular grid pavement.
Project means any proposed action to alter or develop a site. The proposed action of a permit application
or an approval, which requires drainage review.
Project site means that portion of a site and any offsite areas subject to proposed project activities,
alterations, and improvements including those required by this manual. Offsite areas subject to
proposed project activities, include, but are not limited to, frontage improvements required by the
City.
Rain Garden means a shallow, landscaped depression with compost-amended native soils and adapted
plants. The depression is designed to pond and temporarily store stormwater runoff from adjacent
areas, and to allow stormwater to pass through the amended soil profile. Rain gardens can only be
used to meet Core Requirement #9.
Receiving waters means bodies of water, surface water systems, or groundwater receiving water from
upstream man-made or natural systems.
Redevelopment project means a project that proposes to add, replace, or modify impervious surfaces (e.g.,
building, parking lot) for purposes other than a residential subdivision or maintenance on a site that is
already substantially developed in a manner consistent with its current zoning or with a legal non-
conforming use, or has an existing impervious surface coverage of 35% or more. The following
examples illustrate how this definition may apply to residential and commercial sites.
Redevelopment Project that
Adds New Impervious Surface
Redevelopment Project that
Replaces Impervious Surface
Redevelopment Project that
Adds and Replaces Impervious
Surface
Replaced impervious surface means any existing impervious surface on the project site that is proposed
to be removed and re-established as impervious surface, excluding impervious surface removed for
the sole purpose of installing utilities or performing maintenance on underground infrastructure. For
structures, removed means the removal of buildings down to the foundation. For other impervious
surfaces, removed means the removal down to base course or bare soil. For purposes of this
definition, base course is the layer of crushed rock that typically underlies an asphalt or concrete
pavement. It does not include the removal of pavement material through grinding or other surface
modification unless the entire layer of PCC or AC is removed. Replaced impervious surface also
includes impervious surface that is moved from one location to another on the project site where the
Existing
House
Residential Site
New
Bldg.
Existing
Parking
New
Bldg. Existing
Pervious
Area
(65%)
Existing
Impervious
Area (35%)
Commercial Site
Existing Bldg.
Existing
Parking
New
Parking
Existing
Bldg.
Existing
Impervious
Area (35%)
Commercial Site
New
Bldg.
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following two conditions are met: (A) runoff characteristics and volumes remain the same or are
improved in the area where the existing impervious surface is removed , and (B) impervious surface
at the new location is either designated as non-pollution generating or the pollution generating
characteristics remain unchanged compared to that of the original location.
Replaced PGIS means replaced impervious surface that is pollution-generating impervious surface.
Sensitive lake means a designation applied by the City to lakes that are particularly prone to
eutrophication from development-induced increases in phosphorus loading.
Severe building flooding problem means there is flooding of the finished floor area1 of a habitable
building,2 or the electrical/heating system of a habitable building for runoff events less than or equal to
a 100-year event. Examples include flooding of finished floors of homes and commercial or industrial
buildings, or flooding of electrical/heating system components in the crawl space or garage of a home.
Severe erosion problem means there is an open drainage feature with evidence of or potential for
erosion/incision sufficient to pose a sedimentation hazard to downstream conveyance systems or pose
a landslide hazard by undercutting adjacent slopes. Severe erosion problems do not include roadway
shoulder rilling or minor ditch erosion.
Severe flooding problem means a severe building flooding problem or a severe roadway flooding
problem.
Severe roadway flooding problem means there is flooding over all lanes of a roadway,3 or a sole access
driveway4 is severely impacted, for runoff events less than or equal to the 100-year event. A severely
impacted sole access driveway is one in which flooding overtops a culverted section of the driveway,
posing a threat of washout or unsafe access conditions due to indiscernible driveway edges, or
flooding is deeper than 6 inches on the driveway, posing a severe impediment to emergency access.
Single family residential project means any project that (a) constructs or modifies a single family
dwelling unit or attached two-unit (duplex) building, (b) makes improvements (e.g., driveways, roads,
outbuildings, play courts, etc.) or clears native vegetation on a lot that contains or will contain a single
family dwelling unit or attached two-unit (duplex) building, or (c) is a plat, short plat, or boundary line
adjustment that creates or adjusts lots that will contain single family dwelling units or attached two-
unit (duplex) buildings.
Site (a.k.a development site) means a single parcel; or, two or more contiguous parcels that are under
common ownership or documented legal control; or a portion of a single parcel under documented
legal control separate from the remaining parcel, used as a single parcel for a proposed project for
purposes of applying for authority from the City to carry out a proposed project. For projects located
primarily within dedicated rights-of-way, the length of the project site and the right-of-way
boundaries define the site.
Steep slope hazard area is the critical area designation, defined and regulated in RMC 4-3-050, that is
applied to areas where extra protection of sensitive slopes is required. Steep slope hazard areas are
depicted in the Regulated Slopes layer of COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>).
Structure means a catch basin or manhole in reference to a storm drainage system.
1 Finished floor area, for the purposes of defining severe building flooding problem, means any enclosed area of a building
that is designed to be served by the building’s permanent heating or cooling system.
2 Habitable building means any residential, commercial, or industrial building that is equipped with a permanent heating or
cooling system and an electrical system.
3 Roadway, for the purposes of this definition, means the traveled portion of any public or private road or street classified as
such in the City of Renton Standard Details and City of Renton Transportation Department guidelines.
4 Sole access driveway means there is no other unobstructed, flood-free route for emergency access to a habitable building.
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Subject to vehicular use means the surface is regularly used by motor vehicles including but not limited to
motorcycles, cars, trucks, busses, aircraft, tractors, and heavy equipment. The following surfaces are
considered regularly used by motor vehicles: roads, un-vegetated road shoulders, bike lanes within the
traveled lane of a roadway, driveways, parking lots, unrestricted access fire lanes, vehicular equipment
storage yards, and airport taxiways and runways. The following surfaces are not considered regularly
used by motor vehicles: paved bicycle pathways separated from and not subject to drainage from
roads for motor vehicles, fenced or restricted access fire lanes, and maintenance access roads with a
recurring use of no more than one routine vehicle access per week.
Target impervious surface means that portion of a site’s new and/or replaced impervious surface from
which runoff impacts are required to be mitigated by a particular set of drainage requirements (flow
control facility, water quality facility, and/or on-site BMP).
Type of Development Target Impervious Surface
New development New plus replaced impervious surface
Redevelopment with < 5,000 sf impervious or
improvements < 50% of the assessed value of
the existing site improvements
New impervious surface
Redevelopment with ≥ 5,000 sf impervious
and improvements ≥ 50% of the assessed
value of the existing site improvements
New plus replaced impervious surface
Target pervious surface means all areas subject to clearing and grading that have not been covered by an
impervious surface, incorporated into a drainage facility, or engineered as structural fill or slope.
Threshold discharge area means an onsite area draining to a single natural discharge location, or
multiple natural discharge locations that combine within one-quarter-mile downstream (as
determined by the shortest flowpath). The examples below illustrate this definition. This term is used
to clarify how the thresholds, exemptions, and exceptions of this manual are applied to project sites
with multiple discharge locations.
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Example of a Project Site
with a Single Natural
Discharge and a Single
Threshold Discharge Area
Example of a Project Site
with Multiple Natural
Discharges and a Single
Threshold Discharge Area
Example of a Project Site
with Multiple Natural
Discharges and Multiple
Threshold Discharge Areas
Transportation redevelopment project means a stand-alone transportation improvement project that
proposes to add, replace, or modify impervious surface, for purposes other than maintenance, within
a length of dedicated public or private road right-of-way that has an existing impervious surface
coverage of thirty-five percent or more. Road right-of-way improvements required as part of a
subdivision, commercial, industrial, or multifamily project may not be defined as a separate
transportation redevelopment project.
Zone 1 of the Aquifer Protection Area means the land area situated between a well or well field owned by
the City of Renton and the one-year groundwater travel time contour and not otherwise designated as
Zone 1 Modified, as depicted in the Wellhead Protection Area Zones layer of COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>).
Zone 1 Modified of the Aquifer Protection Area means the land area situated between a well or well field
owned by the City of Renton and the one-year groundwater travel time contour and designated as
Zone 1 Modified, as depicted in the Wellhead Protection Area Zones layer of COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>).
Zone 2 of the Aquifer Protection Area means the land area situated between the one-year groundwater
travel time contour and the boundary of the zone of potential capture for a well or well field owned or
operated by the City, as depicted in the Wellhead Protection Area Zones layer of COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>).
Natural
Discharge
Area
THRESHOLD
DISCHARGE
AREA
(Shaded)
Natural
Discharge
Location
Natural
Discharge
Locations
Natural
Discharge
Area 1
Natural
Discharge
Area 2
THRESHOLD
DISCHARGE
AREA
(Shaded)
Natural
Discharge
Area 1
Natural
Discharge
Area 2
THRESHOLD
DISCHARGE
AREA 1
(Shaded)
THRESHOLD
DISCHARGE
AREA 2
¼ Mile Downstream
(shortest flow path)
Natural
Discharge
Locations
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1.1 DRAINAGE REVIEW
Drainage review is the evaluation by City of Renton staff of a proposed project’s compliance with the
drainage requirements of this manual. The City of Renton department responsible for drainage review is
the Community and Economic Development (CED) Department unless otherwise specified in
RMC 4-6-060. Drainage review by CED is an integral part of its permit review process for development
projects. This section describes when and what type of drainage review is required for a proposed project
and how to determine which drainage requirements apply.
The section covers the following topics related to drainage review:
“Projects Requiring Drainage Review,” Section 1.1.1
“Drainage Review Types and Requirements,” Section 1.1.2
“Drainage Review Required By Other Agencies,” Section 1.1.3
“Drainage Design Beyond Minimum Compliance,” Section 1.1.4
Guide to Using Section 1.1
The following steps are recommended for efficient use of Section 1.1:
1. Determine whether your proposed project is subject to the requirements of this manual by seeing if it
meets any of the thresholds for drainage review specified in Section 1.1.1. Making this
determination requires an understanding of the key terms defined at the beginning of this chapter.
2. If drainage review is required per Section 1.1.1, use the flow chart in Figure 1.1.2.A to determine what
type of drainage review will be conducted by the City. The type of drainage review defines the scope
of drainage requirements that will apply to your project as summarized in Table 1.1.2.A.
3. Check the more detailed threshold information in Section 1.1.2 to verify that you have determined the
correct type of drainage review.
4. After verifying the type of drainage review, use the information in Section 1.1.2 to determine which
core requirements (found in Section 1.2) and which special requirements (found in Section 1.3)
must be evaluated for compliance by your project. To determine how to comply with each applicable
core and special requirement, see the more detailed information on these requirements contained in
Sections 1.2 and 1.3 of this chapter.
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1.1.1 PROJECTS REQUIRING DRAINAGE REVIEW
Drainage review is required for any proposed project (except those proposing only maintenance)
that is subject to a City of Renton development permit or approval, including but not limited to
those listed at right, AND that meets any one of the following conditions:
1. The project adds or will result in 2,000 square feet5 or
more of new impervious surface, replaced impervious
surface, or new plus replaced impervious surface, OR
2. The project proposes 7,000 square feet5 or more of land
disturbing activity, OR
3. The project proposes to construct or modify a drainage
pipe/ditch that is 12 inches or more in size/depth, or
receives storm water runoff or surface water from a
drainage pipe/ditch that is 12 inches or more in
size/depth, OR
4. The project contains or is adjacent to a flood, erosion, or
steep slope hazard area as defined in RMC 4-3-050, or
projects located within a Landslide Hazard Drainage
Area or Aquifer Protection Area, OR
5. Condition #5 does not apply to the City,6 OR
6. The project is a redevelopment project proposing
$100,0007 or more of improvements to an existing high-
use site.
If drainage review is required for the proposed project, the
type of drainage review must be determined based on project
and site characteristics as described in Section 1.1.2. The type
of drainage review defines the scope of drainage requirements
that must be evaluated for compliance with this manual.
1.1.2 DRAINAGE REVIEW TYPES AND
REQUIREMENTS
For most projects resulting in 2,000 square feet or more of new plus replaced impervious surface, the full
range of core and special requirements contained in Sections 1.2 and 1.3 must be evaluated for compliance
through the drainage review process. However, for some types of projects, the scope of requirements
applied is narrowed to allow more efficient, customized review. Each of the following five drainage
review types tailors the review process and application of drainage requirements to a project’s size,
location, type of development, and anticipated impacts to the local and regional surface water system:
“Simplified Drainage Review,” Section 1.1.2.1
“Targeted Drainage Review,” Section 1.1.2.2
“Directed Drainage Review,” Section 1.1.2.3
“Full Drainage Review,” Section 1.1.2.4
“Large Project Drainage Review,” Section 1.1.2.5.
5 The thresholds for new impervious surface, replaced impervious surface, and land disturbing activity shall be applied by
project site and in accordance with the definitions of these surfaces and activities.
6 Footnote 6 is not used.
7 This is the “project valuation” as declared on the permit application submitted to CED. The dollar amount of this threshold may
be adjusted on an annual basis using the local consumer price index (CPI).
City of Renton Permits
and
Approvals
Building Permits/Combination
Building Permits
Construction Permits
Demolition Permits
Flood Control Zone Permits
Grading/Filling Permit
Land Use Permit
Mining, Excavation or Grading Permit
or License
Planned Urban Development
Rezones
Right-of-Way Permits
Right-of-Way Use Application
Site Plan Approvals
Shoreline Permits
Short Subdivision Developments
(Short Plat)
Special Permits
Subdivision Developments (Plats)
Temporary Permits when involving
land disturbance
Other City of Renton permits as
required
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Each project requires only one of the above drainage review types, with the single exception that a project
that qualifies for Simplified Drainage Review may also require Targeted Drainage Review. Figure 1.1.2.A
can be used to determine which drainage review type is required. However, this may entail consulting the
more detailed thresholds for each review type specified in the above-referenced sections.
Table 1.1.2.A can be used to quickly identify which requirements are applied in each type of drainage
review. The applicant must evaluate the requirements “checked” for a particular drainage review type to
determine what is necessary for compliance.
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FIGURE 1.1.2.A FLOW CHART FOR DETERMINING TYPE OF DRAINAGE REVIEW REQUIRED
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TABLE 1.1.2.A REQUIREMENTS APPLIED UNDER EACH DRAINAGE REVIEW TYPE
Simplified
Single family residential projects that result in 2,000 sf of new plus replaced impervious
surface or 7,000 sf of land disturbing activity but do not exceed the new plus replaced PGIS,
new PGPS, and new pervious surface thresholds specified in Sec. 1.1.2.1. Note: The project may
also be subject to Targeted Drainage Review.
Directed
Single family residential projects that result in 2,000 sf of new plus replaced impervious
surface or 7,000 sf of land disturbing activity that are not subject to Simplified Drainage Review
or Large Project Drainage Review. The project may also be subject to Targeted Drainage Review.
Targeted
New and redevelopment projects that are not subject to Directed, Full or Large Project Drainage
Review, AND have characteristics of one or more of the following categories of projects:
1. Projects containing or adjacent to a flood, erosion, or steep slope hazard area; or projects
within a Landslide Hazard Drainage Area or Aquifer Protection Area .
2. Projects that construct or modify a drainage pipe/ditch that is 12″ or larger or receive runoff
from a 12″ or larger drainage pipe/ditch.
3. Redevelopment projects with $100,000 in improvements to a high-use site.(1)
Full
All projects that result in 2,000 sf of new plus replaced impervious surface or 7,000 sf of land
disturbing activity but are not subject to Simplified Drainage Review, Directed Drainage Review,
OR Large Project Drainage Review.
Large Project Projects that result in 50 acres of new impervious surface within a subbasin or multiple
subbasins that are hydraulically connected.
DRAINAGE REVIEW TYPE
Simplified Directed
Targeted
Full
Large
Project Categ 1 Categ 2 Categ 3
SIMPLIFIED DRAINAGE
REQUIREMENTS SEE NOTE 4
CORE REQUIREMENT #1
Discharge at Natural Location (4) (2,3) *(2)
CORE REQUIREMENT #2
Offsite Analysis (4) (2,3) *(2) (3) (3) (3)
CORE REQUIREMENT #3
Flow Control Facilities (4) (2,3) *(2) (3) (3)
CORE REQUIREMENT #4
Conveyance System (4) (2,3) *(2)
CORE REQUIREMENT #5
Construction Stormwater
Pollution Prevention
(4) (2,3)
CORE REQUIREMENT #6
Maintenance & Operations (4) (2,3) *(2)
CORE REQUIREMENT #7
Financial Guarantees & Liability (4) (2,3) *(2) (3) (3) (3) (3)
CORE REQUIREMENT #8
Water Quality Facilities (4) (2,3) *(2) (3) (3)
CORE REQUIREMENT #9
On-site BMPs (4)
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TABLE 1.1.2.A REQUIREMENTS APPLIED UNDER EACH DRAINAGE REVIEW TYPE
DRAINAGE REVIEW TYPE
Simplified Directed
Targeted
Full
Large
Project Categ 1 Categ 2 Categ 3
SPECIAL REQUIREMENT #1
Other Adopted Area-Specific
Requirements
(4) (2,3) (3) (3) (3)
SPECIAL REQUIREMENT #2
Flood Hazard Area Delineation (4) (2,3) (3) (3) (3)
SPECIAL REQUIREMENT #3
Flood Protection Facilities (4) (2,3) (3) (3) (3)
SPECIAL REQUIREMENT #4
Source Control (4) (2,3) (3) (3) (3) (3) (3)
SPECIAL REQUIREMENT #5
Oil Control (4) (2,3)
(3) (3) (3)
SPECIAL REQUIREMENT #6
Aquifer Protection Areas (2,3) (3) (3) (3) (3) (3)
(1) Category 3 projects installing oil controls that construct or modify a 12-inch pipe/ditch are also Category 2 projects.
(2) May be applied by CED based on project or site-specific conditions. Documentation of compliance required.
(3) These requirements have exemptions or thresholds that may preclude or limit their application to a specific project.
(4) A proposed project subject to Simplified Drainage Review that complies with the Simplified drainage requirements detailed in Appendix
C is presumed to comply with all the core and special requirements in Sections 1.2 and 1.3 except those requirements that would apply
to the project if it is subject to Targeted Drainage Review as specified in Section 1.1.2.2.
1.1.2.1 SIMPLIFIED DRAINAGE REVIEW
Simplified Drainage Review is for small residential building projects or clearing projects that meet the
threshold requirements below. The core and special requirements applied under Full Drainage Review are
replaced with simplified drainage requirements that can be applied by a non-engineer. These requirements
include simple stormwater dispersion, infiltration, and site design techniques called flow control Best
Management Practices (BMPs), which provide the necessary mitigation of flow and water quality impacts
for small projects. Also included are simple measures for erosion and sediment control (ESC). This
simplified form of drainage review acknowledges that drainage impacts for many small project proposals
can be effectively mitigated without construction of costly flow control and water quality facilities.
The Simplified Drainage Review process minimizes the time and effort required to design, submit, review,
and approve drainage facilities for these proposals. In most cases, the requirements can be met with
submittals prepared by contractors, architects, or homeowners without the involvement of a civil engineer.
Note: some projects subject to Simplified Drainage Review may also require Targeted Drainage Review if
they meet any of the threshold criteria in Section 1.1.2.2.
Threshold
Simplified Drainage Review is required for any single family residential project that will result in
2,000 square feet8 or more of new impervious surface, replaced impervious surface, or new plus replaced
8 The thresholds of 2,000 and 7,000 square feet shall be applied by project site. All other thresholds specified in terms of
square feet of impervious or pervious surface shall be applied by threshold discharge area and in accordance with the
definitions of these surfaces in Section 1.1. Note: the calculation of total impervious surface may exclude any such added
impervious surface that is confirmed by CED staff to be already mitigated by a City approved and inspected flow control facility
or on-site BMP.
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impervious surface, or 7,000 square feet8 or more of land disturbing activity, AND that meets the
following criteria:
The project will result in less than 5,000 square feet of new plus replaced pollution generating impervious
surface, will result in less than ¾ acre of new pollution generating pervious surfaces, AND meets one of
the following three additional criteria:
1. The project meets the Basic Exemption from flow control in Core Requirement #3 : a) the project
results in less than 5,000 square feet of new plus replaced impervious surface, AND b) less than
¾ acres of new pervious surface will be added. Note the Basic Exemption thresholds are applied by
project site.
2. For projects on predominately till soils:
The project results in no more than 7,947 square feet of target impervious surfaces as defined below
AND proposed pervious area is equal to or less than 14,941 – 1.88 x (total target impervious
surfaces).
3. For projects on predominately outwash soils:
The project results in no more than 6,872 square feet of target impervious surfaces as defined below
AND proposed pervious area is equal to or less than 20,343 – 2.96 x (total target impervious
surfaces).
Determination of Target Impervious Surface
If the project is a New Development project,
then target impervious surfaces include new plus proposed replaced impervious surface.
If the project is a Redevelopment project where
o New impervious surface is less than 5,000 square feet or
o Valuation of improvements is less than 50% of the assessed value of the existing site
improvements,
then target impervious surfaces include new impervious surface.
If the project is a Redevelopment project where
o New impervious surface is greater than or equal to 5,000 square feet and
o Valuation of improvements is greater than or equal to 50% of the assessed value of the existing
site improvements,
then target impervious surfaces include new plus proposed replaced impervious surface.
Note: for the purposes applying this threshold to a proposed single family residential subdivision (i.e., plat
or short plat project), the impervious surface coverage assumed on each created lot shall be 4,000 square
feet or the maximum allowed per RMC 4-2-110A, whichever is less. A lower impervious surface coverage
may be assumed for any lot in which the lower impervious surface coverage is set as the maximum
through a declaration of covenant recorded for the lot. Also, the new pervious surface assumed on each
created lot shall be the entire lot area, except the assumed impervious portion and any portion in which
native conditions are preserved by a clearing limit per RMC IV, a covenant or easement recorded for the
lot, or a tract dedicated by the proposed subdivision.
Scope of Requirements
IF Simplified Drainage Review is required, THEN the proposed project must comply with the simplified
project submittal and drainage design requirements detailed in Simplified Drainage Requirements adopted
as Appendix C to this manual. These requirements include simplified BMPs/measures for flow control and
erosion and sediment control.
Presumption of Compliance with Core and Special Requirements
The simplified drainage requirements applied under Simplified Drainage Review are considered sufficient
to meet the overall intent of the core and special requirements in Sections 1.2 and 1.3, except under certain
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conditions when a proposed project has characteristics that trigger Targeted Drainage Review (see the
threshold for Targeted Drainage Review in Section 1.1.2.2) and may require the involvement of a civil
engineer. Therefore, any proposed project that is subject to Simplified Drainage Review as determined
above and complies with the Simplified drainage requirements detailed in Appendix C is presumed to
comply with all the core and special requirements in Sections 1.2 and 1.3 except those requirements that
would apply to the project if it is subject to Targeted Drainage Review as specified in Section 1.1.2.2.
1.1.2.2 TARGETED DRAINAGE REVIEW
Targeted Drainage Review (TDR) is an abbreviated evaluation by CED permit review staff of a proposed
project’s compliance with selected core and special requirements. Projects subject to this type of drainage
review are typically Simplified Drainage Review proposals or other small projects that have site-specific
or project-specific drainage concerns that must be addressed by a civil engineer or CED engineering
review staff. Under Targeted Drainage Review, engineering costs associated with drainage design and
review are kept to a minimum because the review includes only those requirements that would apply to the
particular project.
Threshold
Targeted Drainage Review is required for any proposed project that is subject to drainage review as
determined in Section 1.1.1, but is not subject to Directed, Full or Large Project Drainage Review as
determined in Sections 1.1.2.3, 1.1.2.4 and 1.1.2.5, AND that has the characteristics of one or more of the
following project categories:
TDR Project Category #1: Projects that contain or are adjacent to a flood hazard, erosion hazard
area, or steep slope hazard area as defined in RMC 4-3-050; OR projects located within a Landslide
Hazard Drainage Area or Aquifer Protection Area. Note: at the discretion of CED, this category may
also include any project in Simplified Drainage Review that has a design or site-specific issue that
must be addressed by a civil engineer. A project is considered adjacent to a flood, erosion, or steep
slope hazard area if any portion of the project site is within 50 feet.
TDR Project Category #2: Projects that propose to construct or modify a drainage pipe/ditch that is
12 inches or more in size/depth or receives surface and storm water runoff from a drainage pipe/ditch
that is 12 inches or more in size/depth.
TDR Project Category #3: Redevelopment projects that propose $100,000 or more of improvements
to an existing high-use site.
Scope of Requirements
IF Targeted Drainage Review is required, THEN the applicant must demonstrate that the proposed project
complies with the selected core and special requirements corresponding to the project category or
categories that best match the proposed project. The project categories and applicable requirements for
each are described below and summarized in Table 1.1.2.A.
Note: If the proposed project has the characteristics of more than one project category, the requirements
of each applicable category shall apply.
Compliance with these requirements requires the submittal of engineering plans and calculations stamped
by a civil engineer, unless deemed unnecessary by CED and the City of Renton. The engineer need only
demonstrate compliance with those core and special requirements that have been predetermined to be
applicable based on specific project characteristics as detailed below. The procedures and requirements for
submitting engineering plans and calculations can be found in Section 2.3.
TDR Project Category #1
This category includes projects that are too small to trigger application of most core requirements, but
may be subject to site-specific floodplain or drainage requirements related to certain critical areas, or
other area-specific drainage requirements adopted by the City. Such projects primarily include single
family residential projects in Simplified Drainage Review.
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IF the proposed project meets the characteristics of TDR Project Category #1, THEN the applicant
must demonstrate that the project complies with the following requirements:
“Core Requirement #5: Construction Stormwater Pollution Prevention,” Section 1.2.5
“Special Requirement #1: Other Adopted Area-Specific Requirements,” Section 1.3.1
“Special Requirement #2: Floodplain/Floodway Analysis,” Section 1.3.2
“Special Requirement #3: Flood Protection Facilities,” Section 1.3.3
“Special Requirement #4: Source Control,” Section 1.3.4
“Special Requirement #6: Aquifer Protection Area,” Section 1.3.6.
In addition, CED may require the applicant to demonstrate compliance with any one or more of the
remaining seven core requirements in Section 1.2 based on project or site-specific conditions. For
example, if the proposed project discharges to an erosion or steep slope hazard area as defined in
RMC 4-3-050, CED may require compliance with “Core Requirement #1: Discharge at the Natural
Location” (Section 1.2.1). This may in turn require compliance with “Core Requirement #2: Offsite
Analysis” (Section 1.2.2) if a tightline is required by Core Requirement #1. If a tightline is found to be
infeasible, CED may instead require a flow control facility per “Core Requirement #3: Flow Control”
(Section 1.2.3). If a tightline is feasible, “Core Requirement #4: Conveyance System” (Section 1.2.4)
would be required to ensure proper size and design. Any required flow control facility or tightline
system may also trigger compliance with “Core Requirement #6: Maintenance and Operations”
(Section 1.2.6), “Core Requirement #7: Financial Guarantees and Liability” (Section 1.2.7), and
possibly “Core Requirement #8, Water Quality” (Section 1.2.8) if runoff from pollution-generating
impervious surfaces is collected.
The applicant may also need to address compliance with any applicable critical areas requirements in
RMC 4-3-050 as determined by CED.
TDR Project Category #2
This category is intended to apply selected core and special requirements to those projects that
propose to construct or modify a drainage system of specified size, but are not adding sufficient
impervious surface to trigger Full Drainage Review or Large Project Drainage Review.
IF the proposed project meets the characteristics of TDR Project Category #2, THEN the applicant
must demonstrate that the proposed project complies with the following requirements:
“Core Requirement #1: Discharge at the Natural Location,” Section 1.2.1
“Core Requirement #2: Offsite Analysis,” Section 1.2.2
“Core Requirement #4: Conveyance System,” Section 1.2.4
“Core Requirement #5: Construction Stormwater Pollution Prevention,” Section 1.2.5
“Core Requirement #6: Maintenance and Operations,” Section 1.2.6
“Core Requirement #7: Financial Guarantees and Liability,” Section 1.2.7
“Special Requirement #4: Source Control,” Section 1.3.4.
TDR Project Category #3
This category is intended to improve water quality by applying source control and oil control
requirements to redevelopment projects located on the most intensively used sites developed prior to
current water quality requirements. These are referred to as high-use sites.
IF the proposed project meets the characteristics of TDR Project Category #3, THEN the applicant
must demonstrate that the proposed project complies with the following requirements:
“Core Requirement #5: Construction Stormwater Pollution Prevention,” Section 1.2.5
“Core Requirement #6: Maintenance and Operations,” Section 1.2.6
“Core Requirement #7: Financial Guarantees and Liability,” Section 1.2.7
“Special Requirement #4: Source Control,” Section 1.3.4
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“Special Requirement #5: Oil Control,” Section 1.3.5.
Note: In some cases, CED may determine that application of these requirements does not require
submittal of engineering plans and calculations stamped by a civil engineer.
1.1.2.3 DIRECTED DRAINAGE REVIEW
Directed Drainage Review (DDR) is an evaluation of a proposed single family residential project by CED
permit review staff to determine a specialized list of submittal (plans, technical reports, etc.) and
engineering requirements that ensures compliance with all core and special requirements in this chapter.
Projects subject to this type of drainage review are single family residential projects that do not qualify
for Simplified Drainage Review.
CED staff will review proposals and determine the following: whether the project is exempt from a given
core or special requirement based on exemptions and exceptions listed in this Manual; whether a pre-
engineered solution is available and feasible for meeting a given core or special requirement; whether a
licensed civil engineer is required to comply with a given core or special requirement; and the type of
technical report and plan submittal required to document compliance with the core and special
requirements. Depending upon a project’s site specific conditions, DDR may result in requirements for
engineering or documentation that range from following the requirements of Appendix C to those required
for full drainage review. CED will provide and/or require documentation of the DDR process and decision
making to be included in the project file that demonstrates how compliance with all core and special
requirements in this Manual are achieved.
Under Directed Drainage Review, engineering costs associated with drainage design and review are
minimized because the review is tailored to the particular project.
Threshold
Directed Drainage Review is required for any single family residential project that results in 2,000 square
feet or more of new plus replaced impervious surface or 7,000 square feet or more of land disturbing
activity (refer to Section 1.1.1) but is not subject to Simplified Drainage Review or Large Project Drainage
Review as determined in Sections 1.1.2.1 and Section 1.1.2.5.
Scope of Requirements
IF Directed Review is required, THEN the proposed project must comply with the following requirements:
All nine core requirements in Section 1.2
All six special requirements in Section 1.3
CED may require submission of engineering plans and calculations stamped by a civil engineer to
demonstrate compliance with these requirements. The procedures and requirements for submittal of
engineering plans and calculations are as directed by CED in the DDR process.
1.1.2.4 FULL DRAINAGE REVIEW
Full Drainage Review is the evaluation by City staff (CED unless otherwise specified in RMC 4-6-060) of
a proposed project’s compliance with the full range of core and special requirements in this chapter. This
review addresses the impacts associated with changing land cover on typical sites.
Threshold
Full Drainage Review is required for any proposed project, including a redevelopment project, that is
subject to drainage review as determined in Section 1.1.1, OR that meets one or more of the following
criteria:
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The project will result in 2,000 square feet9 or more of new impervious surface, replaced impervious
surface, or new plus replaced impervious surface but is not subject to Simplified Drainage Review or
Directed Drainage Review as determined in Sections 1.1.2.1 and 1.1.2.3, OR
The project will result in 7,000 square feet9 or more of land disturbing activity but is not subject to
Simplified Drainage Review or Directed Drainage Review as determined in Sections 1.1.2.1 and
1.1.2.3.
Scope of Requirements
IF Full Drainage Review is required, THEN the applicant must demonstrate that the proposed project
complies with the following requirements:
All nine core requirements in Section 1.2
All six special requirements in Section 1.3
Engineering plans and calculations stamped by a civil engineer must be submitted to demonstrate
compliance with these requirements. The procedures and requirements for submittal of engineering plans
and calculations are found in Section 2.3.
1.1.2.5 LARGE PROJECT DRAINAGE REVIEW
Large Project Drainage Review is applied to development proposals that are large and/or involve resources
or problems of special sensitivity or complexity. Because of the large size and complexities involved, there
is usually a greater risk of significant impact or irreparable damage to sensitive resources. Such proposals
often require a more definitive approach to drainage requirements than that prescribed by the core and
special requirements in Sections 1.2 and 1.3; it may be appropriate to collect additional information about
site resources, use more sophisticated models, and prepare special studies not specified in this manual.
Large Project Drainage Review entails preparation of a master drainage plan (MDP) or limited scope
MDP that is reviewed and approved by CED.
Threshold
Large Project Drainage Review is required for any proposed project that is subject to drainage review as
determined in Section 1.1.1, AND that would, at full buildout, result in 50 acres or more of new
impervious surface within a single subbasin or multiple subbasins that are hydraulically connected10
across subbasin boundaries. Hydraulically connected means connected through surface flow or water
features such as wetlands or lakes.
Scope of Requirements
IF Large Project Drainage Review is required, THEN the applicant must do the following:
1. Prepare a MDP, limited scope MDP, or special study in accordance with the process and requirements
described in the MDP guidelines, Master Drainage Planning for Large or Complex Site
Developments, available from King County Department of Natural Resources and Parks (DNRP) or
CED. The MDP or special study shall be completed, or a schedule for completion identified and
agreed to by CED, prior to permit approval. Note: Generally, it is most efficient for the MDP process
to parallel the State Environmental Policy Act (SEPA) process.
2. Demonstrate that the proposed project complies with all the core and special requirements in
Sections 1.2 and 1.3, with some potential modifications as follows:
Core Requirement #2, Offsite Analysis, is typically modified during MDP scoping.
Core Requirement #3, Flow Control, may be modified to require more sophisticated hydrologic
modeling.
Core Requirement #5, ESC, may be modified to require enhanced construction monitoring.
9 The thresholds of 2,000, 5,000, and 7,000 square feet shall be applied by project site.
10 Hydraulically connected means connected through surface flow or water features such as wetlands or lakes.
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Core Requirement #7, Financial Guarantees and Liability, may be modified to implement a
monitoring fund.
Special pre- and post-development monitoring may also be required if deemed necessary by CED
to adequately characterize sensitive site and downstream resources, and to ensure that onsite
drainage controls and mitigation measures are effective in protecting sensitive or critical
resources. Detailed guidelines for monitoring are appended to the MDP guidelines referenced
above.
1.1.3 DRAINAGE REVIEW REQUIRED BY OTHER AGENCIES
Drainage review for a proposed project’s impact on surface and storm waters may be addressed by
processes or requirements apart from the City’s. Agencies such as those listed below may require some
form of drainage review and impose drainage requirements that are separate from and in addition to the
City’s drainage requirements. The applicant is responsible for coordinating with these agencies and
resolving any conflicts in drainage requirements.
Agency Permit/Approval
Seattle/King County Department of Public
Health
Onsite Sewage Disposal and Well permits
Washington State
Department of Transportation Developer/Local Agency Agreement
Department of Fish and Wildlife Hydraulic Project Approval
Department of Ecology Short Term Water Quality Modification
Approval
Dam Safety permit
UIC Well Registration
NPDES Stormwater permit
Department of Natural Resources Forest Practices Class IV permit
United States Army Corps of Engineers Sections 10, 401, and 404 permits
1.1.4 DRAINAGE DESIGN BEYOND MINIMUM COMPLIANCE
This manual presents the City of Renton’s minimum standards for engineering and design of drainage
facilities. While the City believes these standards are appropriate for a wide range of development
proposals, compliance solely with these requirements does not relieve the professional engineer submitting
designs of his or her responsibility to ensure drainage facilities are engineered to provide adequate
protection for natural resources and public and private property.
Compliance with the standards in this manual does not necessarily mitigate all probable and significant
environmental impacts to aquatic biota. Fishery resources and other living components of aquatic systems
are affected by a complex set of factors. While employing a specific flow control standard may prevent
stream channel erosion or instability, other factors affecting fish and other biotic resources (e.g., increases
in stream flow velocities) are not directly addressed by this manual. Likewise, some wetlands, including
bogs, are adapted to a very constant hydrologic regime. Even the most stringent flow control standard
employed by this manual does not prevent increases in runoff volume, which can adversely affect wetland
plant communities by increasing the duration and magnitude of water level fluctuations. Thus, compliance
with this manual should not be construed as mitigating all probable and significant stormwater impacts to
aquatic biota in streams and wetlands; additional mitigation may be required.
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1.2 CORE REQUIREMENTS
This section details the following nine core requirements:
“Core Requirement #1: Discharge at the Natural Location,” Section 1.2.1
“Core Requirement #2: Offsite Analysis,” Section 1.2.2
“Core Requirement #3: Flow Control,” Section 1.2.3
“Core Requirement #4: Conveyance System,” Section 1.2.4
“Core Requirement #5: Construction Stormwater Pollution Prevention,” Section 1.2.5
“Core Requirement #6: Maintenance and Operations,” Section 1.2.6
“Core Requirement #7: Financial Guarantees and Liability,” Section 1.2.7
“Core Requirement #8: Water Quality,” Section 1.2.8
“Core Requirement #9: On-site BMPs,” Section 1.2.9
1.2.1 CORE REQUIREMENT #1:
DISCHARGE AT THE NATURAL LOCATION
All storm water runoff and surface water from a project must be discharged at the natural location so as
not to be diverted onto or away from downstream properties. The manner in which stormwater runoff and
surface water are discharged from the project site must not create a significant adverse impact to downhill
properties or drainage facilities (see “Discharge Requirements” below). Drainage facilities as described
above means a constructed or engineered feature that collects, conveys, stores, treats, or otherwise
manages surface water or stormwater runoff. “Drainage facility” includes, but is not limited to, a
constructed or engineered stream, lake, wetland, or closed depression, or a pipe, channel, ditch, gutter,
flow control facility, on-site BMP, water quality facility, erosion and sediment control facility, and any
other structure and appurtenance that provides for drainage.
Note: Projects that do not discharge all project site runoff at the natural location will require an approved
adjustment of this requirement (see Section 1.4). CED may waive this adjustment, however, for projects in
which only a small portion of the project site does not discharge runoff at the natural location and the
runoff from that portion is unconcentrated and poses no significant adverse impact to downstream
properties.
Intent: To prevent adverse impacts to downstream properties caused by diversion of flow from one
flowpath to another, and to discharge in a manner that does not significantly impact downhill properties or
drainage systems. Diversions can cause greater impacts (from greater runoff volumes) than would
otherwise occur from new development discharging runoff at the natural location. Diversions can also
impact properties that rely on runoff water to replenish wells and ornamental or fish ponds.
DISCHARGE REQUIREMENTS
Proposed projects must comply with the following discharge requirements (1, 2, and 3) as applicable:
1. Where no conveyance system exists at the abutting downstream property line and the natural
(existing) discharge is unconcentrated, any runoff concentrated by the proposed project must be
discharged as follows:
a) IF the 100-year peak discharge11 is less than or equal to 0.2 cfs under existing conditions and will
remain less than or equal to 0.2 cfs under developed conditions, THEN the concentrated runoff
may be discharged onto a rock pad or to any other system that serves to disperse flows.
11 Peak discharges for applying this requirement are determined using the approved runoff model with 15-minute time steps as
detailed in Chapter 3.
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b) IF the 100-year peak discharge is less than or equal to 0.5 cfs under existing conditions and will
remain less than or equal to 0.5 cfs under developed conditions, THEN the concentrated runoff
may be discharged through a dispersal trench or other dispersal system provided the applicant can
demonstrate that there will be no significant adverse impact to downhill properties or drainage
systems.
c) IF the 100-year peak discharge is greater than 0.5 cfs for either existing or developed conditions,
or if a significant adverse impact to downhill properties or drainage systems is likely, THEN a
conveyance system must be provided to convey the concentrated runoff across the downstream
properties to an acceptable discharge point.12 Drainage easements for this conveyance system
must be secured from downstream property owners and recorded prior to engineering plan
approval.
2. IF a proposed project, or any natural discharge area within a project, is located within a Landslide
Hazard Drainage Area and drains over the erodible soils of a landslide hazard with slopes steeper
than 15%, THEN a tightline system must be provided through the landslide hazard to an acceptable
discharge point unless one of the following exceptions applies. The tightline system must comply with
the design requirements in Core Requirement #4 and in Section 4.2.2 unless otherwise approved by
CED. Drainage easements for this system must be secured from downstream property owners and
recorded prior to engineering plan approval.
Exceptions: A tightline is not required for any natural discharge location where CED approves an
alternative system based on a geotechnical evaluation/recommendation from a licensed geotechnical
engineer that considers cumulative impacts on the hazard area under built out conditions AND one of
the following conditions can be met:
a) Less than 2,000 square feet of new impervious surface will be added within the natural
discharge area, OR
b) The developed conditions runoff from the natural discharge area is less than 0.1 cfs for the
100-year runoff event and will be infiltrated for runoff events up to and including the 100-year
event, OR
c) The developed conditions runoff volume13 from the natural discharge area is less than 50% of the
existing conditions runoff volume from other areas draining to the location where runoff from the
natural discharge area enters the landslide hazard onto slopes steeper than 15%, AND the
provisions of Discharge Requirement 1 are met, OR
d) CED determines that a tightline system is not physically feasible or will create a significant
adverse impact based on a soils report by a geotechnical engineer.
3. For projects adjacent to or containing a landslide hazard, steep slope hazard area, or erosion hazard
area as defined in RMC 4-3-050, the applicant must demonstrate that onsite drainage facilities and/or
on-site BMPs will not create a significant adverse impact to downhill properties or drainage systems.
1.2.2 CORE REQUIREMENT #2: OFFSITE ANALYSIS
All proposed projects must submit an offsite analysis report that assesses potential offsite drainage and
water quality impacts associated with development of the project site, and that proposes appropriate
12 Acceptable discharge point means an enclosed drainage system (i.e., pipe system, culvert, or tightline) or open drainage
feature (e.g., ditch, channel, swale, stream, river, pond, lake, or wetland) where concentrated runoff can be discharged without
creating a significant adverse impact.
13 For the purposes of applying this exception, the developed conditions runoff volume is the average annual runoff volume as
computed per Chapter 3. The analysis is performed using the entire period of record. The total volume is divided by the
number of full water years being analyzed to determine the annual average runoff volume. Any areas assumed not to be
cleared when computing the developed conditions runoff volume must be set aside in an open space tract or covenant in order
for the proposed project to qualify for this exception. Preservation of existing forested areas in Landslide Hazard Drainage
Areas is encouraged.
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mitigation of those impacts. The initial permit submittal shall include, at minimum, a Level 1
downstream analysis as described in Section 1.2.2.1 below. If impacts are identified, the proposed
projects shall meet any applicable problem-specific requirements specified in Section 1.2.2.2 for
mitigation of impacts to drainage problems and Section 1.2.2.3 for mitigation of impacts to water quality
problems.
Intent: To identify and evaluate offsite flooding, erosion, and water quality problems that may be created
or aggravated by the proposed project, and to ensure appropriate measures are provided for preventing
creation or aggravation of those problems. In addition, this requirement is intended to ensure appropriate
provisions are made, as needed, to mitigate other identified impacts associated with the quantity and
quality of surface and storm water runoff from the project site (e.g., impacts to the hydrology of a wetland
as may be identified by a “critical area report” per RMC 4-3-050).
The primary component of an offsite analysis report is the downstream analysis, which examines the
drainage system within one-quarter mile downstream of the project site or farther as described in
Section 1.2.2.1 below. It is intended to identify existing or potential/predictable downstream flooding,
erosion, and water quality problems so that appropriate mitigation, as specified in Sections 1.2.2.2 and
1.2.2.3, can be provided to prevent aggravation of these problems. A secondary component of the offsite
analysis report is an evaluation of the upstream drainage system to verify and document that significant
flooding and erosion impacts will not occur as a result of the proposed project. The evaluation must extend
upstream to a point where any backwater effects created by the project cease.
EXEMPTION FROM CORE REQUIREMENT #2
With the exception of:
Projects that trigger Core Requirement #3 (Flow Control Facilities) which must at minimum perform
offsite analysis sufficient to identify and address “Downstream Drainage Problems Requiring Special
Attention (Section 1.2.2.1.1), Problem Type 4 (Potential Impacts to Wetland Hydrology problem),”
and
Projects that trigger Core Requirement # 8 (Water Quality Facilities) which must at minimum perform
offsite analysis sufficient to identify and address “Downstream Water Quality Problems Requiring
Special Attention (Section 1.2.2.1.2),”
a proposed project is exempt from Core Requirement #2 if any one of the following is true:
1. The City of Renton determines there is sufficient information for them to conclude that the project
will not have a significant adverse impact on the downstream and/or upstream drainage system, OR
2. The project adds less than 2,000 square feet of new impervious surface, AND less than ¾ acre of new
pervious surface, AND does not construct or modify a drainage pipe/ditch that is 12 inches or more
in size/depth or that receives runoff from a drainage pipe/ditch that is 12 inches or more in size/depth,
AND does not contain or lie adjacent to a landslide hazard, steep slope hazard area, or erosion
hazard area as defined in RMC 4-3-050, OR
3. The project does not change the rate, volume, duration, or location of discharges to and from the
project site (e.g., where existing impervious surface is replaced with other impervious surface having
similar runoff-generating characteristics, or where pipe/ditch modifications do not change existing
discharge characteristics).
1.2.2.1 DOWNSTREAM ANALYSIS
The level of downstream analysis required depends on specific site and downstream conditions. Each
project submittal must include at least a Level 1 downstream analysis. Upon review of the Level 1
analysis, CED may require a Level 2 or Level 3 analysis. If conditions warrant, additional, more detailed
analysis may be required.
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The Level 1 downstream analysis is a qualitative survey of each downstream system and is the first step
in identifying flooding problems, erosion problems, or potential impacts to wetland hydrology problems as
described below under “Downstream Drainage Problems Requiring Special Attention.” The Level 1
analysis also identifies water quality problems as described below under “Downstream Water Quality
Problems Requiring Special Attention.” Each Level 1 analysis is composed of four tasks at a minimum:
Task 1: Define and map the study area
Task 2: Review all available information on the study area
Task 3: Field inspect the study area
Task 4: Describe the drainage system, and its existing and predicted drainage and water quality
problems.
Upon review of the Level 1 analysis, CED may require a Level 2 or 3 downstream analysis, depending on
the presence of existing or predicted flooding, erosion, or nuisance problems identified in the Level 1
analysis.
Levels 2 and 3 downstream analysis quantify downstream flooding, erosion, or nuisance problems by
providing information on the severity and frequency of an existing problem or the likelihood of creating a
new problem. A Level 2 analysis is a rough quantitative analysis (non-survey field data, uniform flow
analysis). Level 3 is a more precise analysis (e.g., survey field data, backwater analysis) of significant
problems. If conditions warrant, additional, more detailed analysis may be required beyond Level 3. For
Levels 2 and 3 downstream analyses, an additional Task 5, addressing mitigation of existing and potential
flooding, erosion, or nuisance problems, will be required.
Extent of Downstream Analysis
The downstream analysis must consider the existing conveyance system(s) for a minimum flowpath
distance downstream of one-quarter mile and beyond that, as needed, to reach a point where the project
site area constitutes less than 15% of the tributary area. This minimum distance may be increased as
follows:
Task 2 of a Level 1 downstream analysis (described in detail in Section 2.3.1.1) is a review of all
available information on the downstream area and is intended to identify existing drainage and water
quality problems. In all cases, this information review shall extend one mile downstream of the project
site. The existence of flooding or erosion problems further downstream may extend the one-quarter-
mile/15% minimum distance for other tasks to allow evaluation of impacts from the proposed
development upon the identified flooding or erosion problems. The existence of documented water
quality problems beyond the one-quarter-mile/15% distance may in some cases require additional
mitigation of impacts as determined necessary by CED based on the type and severity of problem.
If a project’s impacts to flooding or erosion problems are mitigated by improvements to the
downstream conveyance system, the downstream analysis will extend a minimum of one-quarter mile
beyond the improvement. This is necessary because many such improvements result in a reduction of
stormwater storage or an increase in peak flows from the problem location.
At their discretion, CED may extend the downstream analysis beyond the minimum distance specified
above on the reasonable expectation of drainage or water quality impacts.
A detailed description of the scope of offsite analysis and submittal requirements is provided in
Section 2.3.1.1. Hydrologic analysis methods and requirements for Levels 2 and 3 downstream analyses
are contained in Chapter 3; hydraulic analysis methods are contained in Chapter 4.
1.2.2.1.1 DOWNSTREAM DRAINAGE PROBLEMS REQUIRING SPECIAL ATTENTION
While the area-specific flow control facility requirement in Core Requirement #3 (Section 1.2.3.1) serves
to minimize the creation and aggravation of many types of downstream drainage problems, there are some
types that are more sensitive to creation/aggravation than others depending on the nature or severity of the
problem and which flow control facility standard is being applied. In particular, there are four types of
downstream drainage problems for which the City has determined that the nature and/or severity of the
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problem warrants additional attention through the downstream analysis and possibly additional mitigation
to ensure no creation/aggravation:
1. Conveyance system nuisance problem.
2. Severe erosion problem.
3. Severe flooding problem.
4. Potential Impacts to Wetland Hydrology problem.
These four types of downstream drainage problem are further described below and precisely defined at the
beginning of Chapter 1.
Conveyance System Nuisance Problem (Type 1)
Conveyance system nuisance problems are minor but chronic flooding or erosion problems that result
from the overflow of a constructed conveyance system that is substandard or has become too small as a
result of upstream development. Such problems warrant additional attention because of their chronic
nature and because they result from the failure of a conveyance system to provide a minimum acceptable
level of protection.
If a conveyance system nuisance problem is identified or predicted downstream, the need for additional
mitigation must be evaluated as specified in Section 1.2.2.2 under “Drainage Problem-Specific Mitigation
Requirements“. This may entail additional onsite flow control or other measures as needed to prevent
creation or significant aggravation of the problem.
For any other nuisance problem that may be identified downstream, this manual does not require
mitigation beyond the area-specific flow control facility requirement applied in Core Requirement #3
(Section 1.2.3.1) because preventing aggravation of such problems (e.g., those caused by the elevated
water surfaces of ponds, lakes, wetlands, and closed depressions or those involving downstream erosion)
can require two to three times as much onsite detention volume, which is considered unwarranted for
nuisance problems. However, if under some unusual circumstance, the aggravation of such a nuisance
problem is determined by CED to be a significant adverse impact, additional mitigation may be required.
Severe Erosion Problem (Type 2)
Severe erosion problems can be caused by conveyance system overflows or the concentration of runoff
into erosion-sensitive open drainage features. Severe erosion problems warrant additional attention
because they pose a significant threat either to health and safety or to public or private property.
If a severe erosion problem is identified or predicted downstream, additional mitigation must be
considered as specified in Section 1.2.2.2 under “Drainage Problem-Specific Mitigation Requirements“.
This may entail additional onsite flow control or other measures as needed to prevent creation or
aggravation of the problem.
Severe Flooding Problem (Type 3)
Severe flooding problems (i.e., a severe building flooding problem or severe roadway flooding problem)
can be caused by conveyance system overflows or the elevated water surfaces of ponds, lakes, wetlands, or
closed depressions. Severe flooding problems warrant additional attention because they pose a significant
threat either to health and safety or to public or private property.
If a severe flooding problem is identified or predicted downstream, the need for additional mitigation must
be evaluated as specified in Section 1.2.2.2 under “Drainage Problem-Specific Mitigation Requirements“.
This may entail consideration of additional onsite flow control or other measures as needed to prevent
creation or significant aggravation of the problem.
Potential Impacts to Wetlands Hydrology Problem (Type 4)
Potential impacts to wetlands hydrology can be caused by changes in the rate, duration, and quantity of
stormwater discharged from the project site to a wetland.
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Where wetlands are identified on the site, the applicant shall submit a critical area report at a level
determined by CED to adequately evaluate the proposal and probable impacts.
Where wetlands are identified off the site AND the project is not exempt from Core Requirement #3, the
applicant shall submit a critical area report at a level determined by CED to adequately evaluate the
proposal and probable impacts.
Projects or threshold discharge areas within projects discharging to wetlands, unless exempt from
providing a flow control facility per Core Requirement 3, must demonstrate that the existing wetland
hydroperiod is maintained in accordance with the wetland hydrology protection guidelines in Reference
Section 5.
Based upon the critical area report and, if applicable, the analysis of project compliance with the wetland
hydrology protection guidelines in Reference Section 5, CED will determine if changes in the rate,
duration, and/or quantity of surface and storm water runoff from a proposed project or threshold
discharge area within a proposed project could significantly alter the hydrology of a wetland-- in which
case, CED will require (as described in Section 1.2.2.2 under “Drainage Problem-Specific Mitigation
Requirements”), implementation of additional flow control or other measures to mitigate the adverse
impacts of this alteration in accordance with the wetland hydrology protection guidelines in Reference
Section 5.
1.2.2.1.2 DOWNSTREAM WATER QUALITY PROBLEMS REQUIRING SPECIAL ATTENTION
A water quality problem, for the purposes of impact mitigation in this manual, is a situation in which a
waterbody of the State is documented by the Federal Government, State, or City to be exceeding or at
concern of exceeding the State’s numeric water quality standards, or is subject to a federal, state, or City
cleanup program or action. Water quality problems and associated water quality standards encompass
surface water, groundwater, and sediment quality. The goal of this manual is to prevent creation or
significant aggravation of such problems to the maximum extent practicable. While the area-specific water
quality facility requirement in Section 1.2.8.1, the source controls required in Section 1.3.4, and the oil
controls required in Section 1.3.5 all serve to minimize the creation and aggravation of many types of
downstream water quality problems, there are some types that are either not addressed by these
requirements (e.g., temperature problems) or warrant additional measures/considerations to minimize the
proposed project’s impacts to the maximum extent practicable. In particular, there are currently 7 types of
downstream water quality problems for which the City has determined that additional attention needs to be
given to preventing or minimizing increases in the pollutant or pollutants of concern discharging from the
site. These are as follows:
1. Bacteria Problem
2. Dissolved Oxygen Problem
3. Temperature Problem
4. Metals Problem
5. Phosphorus Problem
6. Turbidity Problem
7. High pH Problem
These problems are defined below and the mitigation of impacts to them is addressed in Section 1.2.2.3.
Bacteria Problem (Type 1)
A bacteria problem is defined as a stream reach, lake, or other waterbody of the state that is either
(1) currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern
for exceedance of the state’s numeric water quality standard for fecal coliform as documented in the
state’s Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology’s
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electronic database and map viewer14 of these waterbodies, or (2) where subject to any other local, state, or
federal cleanup plan or contaminated site designation for fecal coliform.
Dissolved Oxygen (DO) Problem (Type 2)
A dissolved oxygen problem is defined as a stream reach, lake, or other waterbody of the state that is either
(1) currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern
for exceedance of the state’s numeric water quality standard for dissolved oxygen as documented in the
state’s Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology’s
electronic database and map viewer14 of these waterbodies, or (2) where subject to any other local, state, or
federal cleanup plan or contaminated site designation for DO.
Temperature Problem (Type 3)
A temperature problem is defined as a stream reach, lake, or other waterbody of the state that is either
(1) currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern
for exceedance of the state’s numeric water quality standard for temperature as documented in the state’s
Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology’s electronic
database and map viewer14 of these waterbodies, or (2) where subject to any other local, state, or federal
cleanup plan or contaminated site designation for temperature.
Metals Problem (Type 4)
A metals problem is defined as a stream reach, lake, or other waterbody of the state that is either
(1) currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern
for exceedance of the state’s numeric water quality standards for metals (e.g., copper, zinc, lead, mercury,
etc.) as documented in the state’s Water Quality Assessment 303(d)/305(b) Integrated Report and as
displayed in WA Ecology’s electronic database and map viewer16 of these waterbodies, or (2) where
subject to any other local, state, or federal cleanup plan or contaminated site designation for metals.
Phosphorus Problem (Type 5)
A phosphorus problem is defined as a stream reach, lake, or other waterbody of the state that is either
(1) currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern
for exceedance of the state’s numeric action standard for total phosphorus as documented in the state’s
Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology’s electronic
database and map viewer,14 of these waterbodies, or (2) where subject to any other local, state, or federal
cleanup plan or contaminated site designation for total phosphorus.
Turbidity Problem (Type 6)
A turbidity problem is defined as a stream reach, lake, or other waterbody of the state that is either
(1) currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern
for exceedance of the state’s numeric water quality standard for turbidity as documented in the state’s
Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology’s electronic
database and map viewer14 of these waterbodies, or (2) where subject to any other local, state, or federal
cleanup plan or contaminated site designation for turbidity.
High pH Problem (Type 7)
A High pH problem is defined as a stream reach, lake, or other waterbody of the state that is either
(1) currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern
for exceedance of the state’s numeric water quality standard for high pH as documented in the state’s
Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology’s electronic
database and map viewer16 of these waterbodies, or (2) where subject to any other local, state, or federal
cleanup plan or contaminated site designation for pH.
14 The link to the Query Tool is <https://apps.ecology.wa.gov/ApprovedWQA/ApprovedPages/ApprovedSearch.aspx>; select all
appropriate mediums. The Map Tool is at <https://apps.ecology.wa.gov/waterqualityatlas/wqa/map>.
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1.2.2.2 DRAINAGE PROBLEM IMPACT MITIGATION
A proposed project must not significantly aggravate existing downstream drainage problems or create
new problems as a result of developing the site. This manual does not require development proposals to fix
or otherwise reduce the severity of existing downstream drainage problems, although doing so may be an
acceptable mitigation.
Principles of Impact Mitigation for Drainage Problems
Aggravation of an existing downstream drainage problem means increasing the frequency of occurrence
and/or severity of the problem. Increasing peak flows at the location of a problem caused by conveyance
system overflows can increase the frequency of the problem’s occurrence. Increasing durations of flows at
or above the overflow return frequency can increase the severity of the problem by increasing the depth
and duration of flooding. Controlling peaks and durations through onsite detention can prevent
aggravation of such problems by releasing the increased volumes from development at return frequencies
below the conveyance overflow return frequency, which limits their effect to just causing the conveyance
system to flow full for a longer period of time.
When a problem is caused by high water-surface elevations of a volume-sensitive water body, such as a
lake, wetland, or closed depression, aggravation is the same as for problems caused by conveyance
overflows. Increasing the volume of flows to a volume-sensitive water body can increase the frequency of
the problem’s occurrence. Increasing the duration of flows for a range of return frequencies both above
and below the problem return frequency can increase the severity of the problem; mitigating these impacts
requires control of flow durations for a range of return frequencies both above and below the problem
return frequency. The net effect of this duration control is to release the increased volumes from
development only at water surface elevations below that causing the problem, which in turn can cause an
increase in these lower, but more frequently occurring, water surface elevations. This underscores an
unavoidable impact of development upstream of volume-sensitive water bodies: the increased volumes
generated by the development will cause some range of increase in water surface elevations, no matter
what detention standard is applied.
Creating a new drainage problem means increasing peak flows and/or volumes so that after development,
the frequency of conveyance overflows or water surface elevations exceeds the thresholds for the various
problem types discussed in Section 1.2.2.1. For example, application of the Peak Rate Flow Control
Standard requires matching the existing site conditions 2- and 10-year peak flows. The 100-year peak
flow is only partially attenuated, and the flow increase may be enough to cause a severe flooding problem
as described in Section 1.2.2.1.1. The potential for causing a new problem is often identified during the
Level 1 downstream analysis, where the observation of a reduction in downstream pipe sizes, for example,
may be enough to predict creation of a new problem. A Level 2 or 3 analysis will typically be required to
verify the capacity of the system and determine whether 100-year flows can be safely conveyed.
Significance of Impacts to Existing Drainage Problems
The determination of whether additional onsite mitigation or other measures are needed to address an
existing downstream drainage problem depends on the significance of the proposed project’s predicted
impact on that problem. For some identified problems, CED will make the determination as to whether the
project’s impact is significant enough to require additional mitigation. For Type 1, 2, and 3 downstream
drainage problems described in Section 1.2.2.1.1, this threshold of significant impact or aggravation is
defined below. For a Type 4, “Potential Impacts to Wetland Hydrology problem,” CED will make this
determination based on required critical area report findings, whether the project is in compliance with the
wetland hydrology protection guidelines found in Reference Section 5, the project’s relative contribution
to the identified wetland’s hydrology, and the mitigation proposed in meeting other requirements (e.g.,
flow control facilities and on-site BMPs).
For conveyance system nuisance problems, the problem is considered significantly aggravated if there
is any increase in the project’s contribution to the frequency of occurrence and/or severity of the problem
for runoff events less than or equal to the 10-year event. Note: Increases in the project’s contribution to
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this type of problem are considered to be prevented if sufficient onsite flow control and/or offsite
improvements are provided as specified in Table 1.2.3.A.
For severe erosion problems, the problem is considered significantly aggravated if there is any increase in
the project’s existing contribution to the flow duration15 of peak flows ranging from 50% of the 2-year
peak flow up to the full 50-year peak flow at the eroded area. Note: Increases in the project’s contribution
to this type of problem are considered to be prevented if Flow Control Duration Standard or offsite
improvements are provided as specified in Table 1.2.3.A.
For severe building flooding problems, the problem is considered significantly aggravated if there is any
increase in the project’s existing contribution16 to the frequency, depth, or duration of the problem for
runoff events less than or equal to the 100-year event.
For severe roadway flooding problems, the problem is considered significantly aggravated if any of the
following thresholds are exceeded and there is any increase in the project’s existing contribution19 to the
frequency, depth, or duration of the problem for runoff events less than or equal to the 100-year event:
The existing flooding17 over all lanes of a roadway or overtopping the culverted section of a sole
access driveway is predicted to increase in depth more than a quarter-inch or 10% (whichever is
greater) for the 100-year runoff event.
The existing flooding over all lanes of a roadway or severely impacting a sole access driveway is
more than 6 inches deep or faster than 5 feet per second for runoff events less than or equal to the
100-year event. A severely impacted sole access driveway is one in which flooding overtops a
culverted section of the driveway, posing a threat of washout or unsafe access conditions due to
indiscernible driveway edges, or flooding is deeper than 6 inches on the driveway, posing a severe
impediment to emergency access.
The existing flooding over all lanes of a sole access roadway18 is more than 3 inches deep or faster
than 5 feet per second for runoff events less than or equal to the 100-year event, or is at any depth for
runoff events less than or equal to the 10-year event.
DRAINAGE PROBLEM-SPECIFIC MITIGATION REQUIREMENTS
1. IF a proposed project or threshold discharge area within a project drains to one or more of Type 1,
Type 2, or Type 3 downstream drainage problems described in Section 1.2.2.1 as identified through a
downstream analysis, THEN the applicant must do one of the following:
a) Submit a Level 2 or Level 3 downstream analysis per Section 2.3.1 demonstrating that the
proposed project will not create or significantly aggravate the identified downstream drainage
problem(s), OR
b) Show that the natural discharge area or threshold discharge area draining to the identified
problem(s) qualifies for an exemption from Core Requirement #3: Flow Control (Section 1.1.1) or
an exception from the applicable area-specific flow control facility requirement per Section
1.2.3.1, OR
15 Flow duration means the aggregate time that peak flows are at or above a particular flow rate (e.g., the amount of time over the
last 50 years that peak flows were at or above the 2-year flow rate). Note: flow duration is not considered to be increased if it is
within the tolerances specified in Chapter 3.
16 Increases in the project’s contribution are considered to be prevented if sufficient onsite flow control and/or offsite
improvements are provided as specified for severe flooding problems in Table 1.2.3.A. For severe flooding problems
located within the mapped 100-year floodplain of a major receiving water (see Table 1.2.3.B) or the mapped 100-year
floodplain of a major stream for which there is an adopted basin plan, increases in the project’s contribution are considered
negligible (zero) regardless of the flow control standard being applied, unless CED determines there is a potential for increased
flooding separate from that associated with the existing 100-year floodplain.
17 Existing flooding, for the purposes of this definition, means flooding over all lanes of the roadway or driveway has occurred in
the past and can be verified by County records, County personnel, photographs, or other physical evidence.
18 Sole access roadway means there is no other flood-free route for emergency access to one or more dwelling units.
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c) Document that the applicable area-specific flow control facility requirement specified in Core
Requirement #3 is adequate to prevent creation or significant aggravation of the identified
downstream drainage problem(s) as indicated in Table 1.2.3.A with the phrase, “No additional
flow control needed,” OR
d) Provide additional onsite flow control necessary to prevent creation or significant aggravation of
the downstream drainage problem(s) as specified in Table 1.2.3.A and further detailed in
Section 3.3.5, OR
e) Provide offsite improvements necessary to prevent creation or significant aggravation of the
identified downstream drainage problem(s) as detailed in Chapter 3 unless identified as not
necessary in Table 1.2.3.A, OR
f) Provide a combination of additional onsite flow control and offsite improvements sufficient to
prevent creation or significant aggravation of the downstream drainage problem(s) as
demonstrated by a Level 2 or Level 3 downstream analysis.
2. IF it is identified that the manner of discharge from a proposed project may create a significant
adverse impact as described in Core Requirement #1, THEN CED may require the applicant to
implement additional measures or demonstrate that the impact will not occur.
3. IF it is identified through a critical area review as described under “Potential Impacts to Wetlands
Hydrology Problem (Type 4),” that changes in the rate, duration, and/or quantity of surface and
storm water runoff from a proposed project or threshold discharge area within a proposed project
could significantly alter the hydrology of a wetland (Type 4 problem), THEN CED shall require the
applicant to implement additional flow control or other measures to mitigate the adverse impacts of
this alteration in accordance with the wetland hydrology protection guidelines in Reference Section 5.
Intent: To ensure provisions are made (if necessary) to prevent creation or significant aggravation of the
four types of downstream drainage problems requiring special attention by this manual, and to ensure
compliance with the discharge requirements of Core Requirement #1.
In addressing downstream drainage problems per Problem-Specific Mitigation Requirement 1 above,
additional onsite flow control will often be the easiest provision to implement. This involves designing the
required onsite flow control facility to meet an additional set of performance criteria targeted to prevent
significant aggravation of specific downstream drainage problems. To save time and analysis, a set of
predetermined flow control performance criteria corresponding to each of the three types of downstream
drainage problems is provided in Table 1.2.3.A and described in more detail in Chapter 3.
Note that in some cases the area-specific flow control facility requirement applicable to the proposed
project per Section 1.2.3.1 is already sufficient to prevent significant aggravation of many of the defined
downstream drainage problem types. Such situations are noted in Table 1.2.3.A as not needing additional
onsite flow control or offsite improvements. For example, if the project is located within a Flow Control
Duration Standard Area subject to the Flow Control Duration Standard per Section 1.2.3.1.B, and a
conveyance system nuisance problem is identified through offsite analysis per Core Requirement #2, no
additional onsite flow control is needed, and no offsite improvements are necessary.
1.2.2.3 WATER QUALITY PROBLEM IMPACT MITIGATION
As stated in Section 1.2.2.1, the goal of this manual is to prevent creation and/or significant aggravation of
water quality problems to the maximum extent practicable. This is accomplished through a number of
mitigation requirements, including (1) the area-specific water quality facility requirement in
Section 1.2.8.1, (2) any mitigation required by other adopted area-specific requirements per Special
Requirement #1, Section 1.2.9, (3) the source controls required in Special Requirement #4, Section 1.3.4,
(4) the oil control required in Special Requirement #5, Section 1.3.5, and (5) the water quality problem-
specific mitigation requirements presented in this section. Note that this manual does not require
development proposals to fix or otherwise reduce the severity of existing downstream water quality
problems, although doing so may be an acceptable mitigation.
AGENDA ITEM # 8. a)
1.2.2 CORE REQUIREMENT #2: OFFSITE ANALYSIS
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WATER QUALITY PROBLEM-SPECIFIC MITIGATION REQUIREMENTS
IF a proposed project drains to one or more of the 7 types of downstream water quality problems defined
in Section 1.2.2.1 as identified through a downstream analysis, THEN the applicant must comply with the
following problem-specific mitigation requirements that apply. Note that CED may require additional
measures if the opportunity exists to further mitigate the pollutants of concern associated with these types
of problems.
Bacteria Problem (Type 1)
IF the proposed project drains to a bacteria problem located within the quarter mile/15% distance
downstream (or beyond as deemed necessary by CED), THEN the following requirements must be met as
applicable:
1. IF a water quality facility is required per Core Requirement #8, THEN a sand filter or stormwater
wetland shall be used to meet the area-specific water quality facility requirement. Sand filters are the
preferred option. Other treatment options for meeting the area-specific facility requirement may be
used in lieu of a sand filter or stormwater wetland only if combined with an emerging technology
treatment method that provides equivalent removal of fecal coliform as demonstrated through an
experimental design adjustment per Section 1.4.
2. IF the proposed project is a residential subdivision, THEN signage shall be provided in the
subdivision’s public areas (i.e., recreation/open space areas and right-of-way) requesting that pet
waste be picked up in order to protect downstream water quality. The extent and location of this
signage shall be reviewed and approved by CED.
3. IF the proposed project is a multifamily development with a recreation/open area or is a park
improvement, THEN signage shall be provided requesting that pet waste be picked up in order to
protect downstream water quality. The extent and location of this signage shall be reviewed and
approved by CED.
Dissolved Oxygen (DO) Problem (Type 2)
IF the proposed project drains to a DO problem located within the quarter mile/15% distance downstream
(or beyond as deemed necessary by CED), THEN the following requirements must be met as applicable:
1. IF the proposed project includes a wetpond or wetvault, THEN the wetpool depth shall not exceed
6 feet, AND the outflow system shall include a measure designed to promote aeration of the
facility’s discharges for 2-year runoff events and smaller. One way to do this is to create a drop in
flow elevation within a manhole by placing the outlet invert of the incoming pipe a minimum of
12 inches above the 2-year headwater elevation of the outgoing pipe. Alternatively, if the outflow
system discharges to an open channel, the same drop in flow elevation could be achieved by placing
the outlet invert a minimum of 12 inches above the 2-year tailwater elevation created by the channel.
Other equivalent approaches may be used as approved by CED.
2. IF the proposed project includes a wetvault, THEN the required ventilation area specified in
Chapter 6 shall be doubled.
3. IF the DO problem is documented to be caused by excessive phosphorus and a water quality facility
is required per Core Requirement #8, THEN a water quality facility option from the Sensitive Lake
Protection menu shall be a component of the required treatment system.
Temperature Problem (Type 3)
IF the proposed project drains to a temperature problem located within the quarter mile/15% distance
downstream (or beyond as deemed necessary by CED), THEN the following requirements must be met as
applicable:
1. IF a water quality facility is required per Core Requirement #8, THEN use of a wetpond is
prohibited unless it will be at least 50% shaded at midday in the summer or its discharges will flow
AGENDA ITEM # 8. a)
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through 200 feet or more of open channel that is at least 50% shaded at midday in the summer. CED
shall review and approve the extent and location of this shading.
2. IF the proposed project includes open drainage features, THEN vegetation or other means shall be
used where practicable to maximize shading of the drainage features, except bioswales and filter
strips. The extent and location of this shading shall be reviewed and approved by CED.
Metals Problem (Type 4)
IF the proposed project drains to a metals problem located within the quarter mile/15% distance
downstream (or beyond as deemed necessary by CED), THEN the following requirements must be met as
applicable:
1. IF a water quality facility is required per Core Requirement #8, THEN a water quality facility option
from the Enhanced Basic WQ menu shall be a component of the project’s required treatment system.
2. IF the proposed project is a residential subdivision, THEN a covenant shall be recorded for each lot
and common area tract prohibiting use of leachable heavy metals (e.g., galvanized metals) that will
be exposed to the weather (use the covenant in Reference Section 8-Q).
3. IF the proposed project includes road right-of-way improvements, THEN use of leachable heavy
metals (e.g., galvanized metals) that will be exposed to the weather (e.g., guard rails, street lights,
etc.) shall be avoided.
Phosphorus Problem (Type 5)
IF the proposed project drains to a phosphorus problem located within the quarter mile/15% distance
downstream (or beyond as deemed necessary by CED), THEN the following requirements must be met as
applicable:
1. IF a water quality facility is required per Core Requirement #8, THEN the project shall be assumed to
be located within a designated Sensitive Lake WQ Treatment Area for the purposes of applying the
area-specific water quality treatment requirement in Section 1.2.8.1.
2. For the purposes of applying the Erosion and Sediment Control Standards in Appendix D, the project
shall be assumed to be located within a designated Sensitive Lake WQ Treatment Area.
Turbidity Problem (Type 6)
IF the proposed project drains to a turbidity problem located within the quarter mile/15% distance
downstream (or beyond as deemed necessary by CED) AND the downstream flow path from the project
site to the turbidity problem is through a landslide hazard, steep slope hazard area, erosion hazard area
or any actively eroding area, THEN the project shall provide a tightline system through the area in
accordance with the same criteria and exceptions specified in Core Requirement #1, Discharge
Requirement 2 for projects located within a designated Landslide Hazard Drainage Area. Other means
for safely conveying project site discharges through the area of concern for erosion may be proposed
subject to approval by CED.
High pH Problem (Type 7)
IF the proposed project drains to a pH problem located within the quarter mile/15% distance downstream
(or beyond as deemed necessary by CED) AND the proposed project includes a concrete vault structure
for stormwater control purposes, THEN the vault’s submerged surfaces shall be coated or otherwise
treated to prevent alteration of pH.
AGENDA ITEM # 8. a)
1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES
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1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES
All proposed projects, including redevelopment projects, must provide onsite flow control facilities to
mitigate the impacts of storm and surface water runoff generated by new impervious surface, new
pervious surface, and replaced impervious surface targeted for flow mitigation as specified in the
following sections. Flow control facilities must be provided and designed to perform as specified by the
area-specific flow control facility requirement in Section 1.2.3.1 and in accordance with the applicable
flow control facility implementation requirements in Section 1.2.3.2.
Intent: To ensure the minimum level of control needed to protect downstream properties and resources
from increases in peak, duration, and volume of runoff generated by new development. The level of
control varies depending on location and downstream conditions identified under Core Requirement #2.
EXEMPTION FROM CORE REQUIREMENT #3
There is a single exemption from the flow control provisions of Core Requirement #3:
Basic Exemption
A proposed project is exempt if it meets the following criteria:
1. Less than 5,000 square feet of new plus replaced impervious surface will be created, AND
2. Less than ¾ acres of new pervious surface will be added.
1.2.3.1 AREA-SPECIFIC FLOW CONTROL FACILITY REQUIREMENT
Projects subject to Core Requirement #3 must provide flow control facilities as specified by the area-
specific facility requirements and exceptions for the designated flow control area in which the proposed
project or threshold discharge area of the proposed project is located as described in Subsections A, B,
and C below.
Guide to Applying the Area-Specific Flow Control Facility Requirement
The flow control facility requirement varies across the City according to the flow control area within
which the project or a threshold discharge area of the project is located. There are currently four such
flow control areas, three of which are depicted in the Flow Control Application layer in COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>).
These are referred to as follows.
1. Flow Control Duration Standard – Matching Forested site conditions for areas draining to streams
and subject to flow-related water quality problems such as erosion or sedimentation.
2. Flow Control Duration Standard – Matching Existing site conditions in designated highly
urbanized areas draining to streams that are currently stable or showing no impacts caused by high
flows.
3. Peak Rate Flow Control Standard – Matching Existing site conditions 2, 10 and 100-year peak-
rate runoff for areas draining to constructed (man-made) or highly modified drainage systems so as
not to create a downstream flooding problem.
4. Flood Problem Flow Control Standard – The City may apply this standard where projects discharge
to a severe flooding or erosion problems. The standard includes matching existing site conditions for
100-year peaks in addition to fulfilling requirements for the flow control duration standard, either
matching forested or existing site conditions based on the downstream flow control area designation.
AGENDA ITEM # 8. a)
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Guide to Applying the Area-Specific Flow Control Facility Requirement (cont.)
Note that the minimum required performance of the facility as specified by this requirement may need to
be increased to ensure that downstream drainage problems are not created or significantly aggravated as
set forth in Section 1.2.2.2, “Drainage Problem-Specific Mitigation Requirements.” Table 1.2.3.A provides
a quick guide for selecting the flow control performance criteria necessary to meet both the area-specific
flow control facility requirement and the problem-specific mitigation requirement. This is further
explained in Step 4 below.
For efficient application of the flow control facility requirement, the following steps are recommended:
1. Check the Direct Discharge Exemption in Section 1.2.3.1 to determine if and/or which portions of
your project are exempt from the flow control facility requirement. If exempt from the flow control
facility requirement, proceed to Step 6.
2. Refer to the Flow Control Application layer in COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>) to determine the flow
control area in which your project is located.
3. Consult the detailed requirement and exception language for the identified flow control area to
determine if and how the flow control facility requirement applies to your project. This requirement
and exception language is detailed on subsequent pages for each of the flow control areas. If a flow
control facility is not applicable per the area-specific exceptions, proceed to Step 6.
4. If downstream drainage problems were identified through offsite analysis per Core Requirement #2
and are proposed to be addressed through onsite flow control, use Table 1.2.3.A to determine if and
what additional flow control performance is necessary to mitigate impacts (i.e., to prevent creation or
aggravation of the identified problems).
5. Use Section 1.2.3.2 to identify the applicable requirements for implementing the flow control facility
requirement. These requirements cover facility siting, analysis and design, unusual situations, and
other site-specific considerations.
6. Use Core Requirement #9 to identify the on-site BMPs that must be applied to your project site
regardless of whether a flow control facility is required.
AGENDA ITEM # 8. a)
1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES
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TABLE 1.2.3.A
SUMMARY OF FLOW CONTROL PERFORMANCE CRITERIA ACCEPTABLE FOR IMPACT
MITIGATION(1)
IDENTIFIED
PROBLEM
DOWNSTREAM
AREA-SPECIFIC FLOW CONTROL FACILITY REQUIREMENT
Peak Rate Flow
Control Standard
Areas
Flow Control
Duration Standard
Matching Existing
Condition Areas
Flow Control Duration
Standard Matching
Forested Condition
Areas
Flood Problem Flow
Control Standard
Areas
No Problem
Identified
Apply the minimum
area-specific flow
control performance
criteria.
Apply the Peak Rate
Flow Control
Standard, which
matches the 2-, 10-,
and 100-year peaks
Apply the Flow
Control Duration
Standard, which
matches the flow
duration of pre-
developed rates for
existing site
conditions over the
range of flows
extending from 50%
of 2-year up to the
full 50-year flow AND
matches peaks for
the 2- and 10-year
return periods.
Apply the Flow Control
Duration Standard
which matches the flow
duration of pre-
developed rates for
forested (historical)
site conditions over
the range of flows
extending from 50% of
2-year up to the full 50-
year flow AND matches
peaks for the 2- and
10-year return periods
Apply the existing or
forested (historical)
site conditions Flow
Control Duration
Standard (whichever
is appropriate based
on downstream flow
control areas) AND
match existing site
conditions 100-year
peaks
Type 1 Drainage
Problem
Conveyance
System Nuisance
Problem
Additional Flow
Control
Hold 10-year peak to
overflow Tr peak(2)(3)
Additional Flow
Control
The City may require
design adjustments
to meet the Flow
Control Duration
Standard matching
forested (historical)
conditions.
No additional flow
control or other
mitigation is needed
No additional flow
control or other
mitigation is needed
Type 2 Drainage
Problem
Severe Erosion
Problem
Additional Flow
Control
Apply the Flow
Control Duration
Standard matching
forested (historical)
conditions (3)(4)
Additional Flow
Control
Apply the Flow
Control Duration
Standard matching
forested (historical)
conditions.(3)(4)
No additional flow
control is needed, but
other mitigation may be
required(4)
No additional flow
control is needed, but
other mitigation may
be required(4)
Type 3 Drainage
Problem
Severe Flooding
Problem
Additional Flow
Control
Apply the Flow
Control Duration
Standard matching
forested (historical)
conditions. If
flooding is from a
closed depression,
make design
adjustments as
needed to meet the
“special provision for
closed
depressions”(3)(5)
Additional Flow
Control
Apply the Flow
Control Duration
Standard matching
forested (historical)
conditions. If
flooding is from a
closed depression,
make design
adjustments as
needed to meet the
“special provision for
closed
depressions”(3)(5)
Additional Flow
Control
If flooding is from a
closed depression,
make design
adjustments as needed
to meet the “special
provision for closed
depressions”(3)(5)
Additional Flow
Control
If flooding is from a
closed depression,
make design
adjustments as
needed to meet the
“special provision for
closed depressions”
(3)(5)
AGENDA ITEM # 8. a)
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TABLE 1.2.3.A
SUMMARY OF FLOW CONTROL PERFORMANCE CRITERIA ACCEPTABLE FOR IMPACT
MITIGATION(1)
IDENTIFIED
PROBLEM
DOWNSTREAM
AREA-SPECIFIC FLOW CONTROL FACILITY REQUIREMENT
Peak Rate Flow
Control Standard
Areas
Flow Control
Duration Standard
Matching Existing
Condition Areas
Flow Control Duration
Standard Matching
Forested Condition
Areas
Flood Problem Flow
Control Standard
Areas
Type 4 Potential
Impact to Wetland
Hydrology as
Determined through
a Critical Area
Review per RMC
Title IV
Additional Flow
Control
The City may require
design adjustments
per the wetland
hydrology protection
guidelines in
Reference Section 5.
Additional Flow
Control
The City may require
design adjustments
per the wetland
hydrology protection
guidelines in
Reference Section 5.
Additional Flow
Control
The City may require
design adjustments per
the wetland hydrology
protection guidelines in
Reference Section 5.
Additional Flow
Control
The City may require
design adjustments
per the wetland
hydrology protection
guide-lines in
Reference Section 5.
Notes:
(1) More than one set of problem-specific performance criteria may apply if two or more downstream drainage problems are identified
through offsite analysis per Core Requirement #2. If this happens, the performance goals of each applicable problem-specific criterion
must be met. This can require extensive, time-consuming analysis to implement multiple sets of outflow performance criteria if additional
onsite flow control is the only viable option for mitigating impacts to these problems. In these cases, it may be easier and more prudent
to implement the Flow Control Duration Standard matching forested conditions standard in place of the otherwise required area-
specific standard. Use of the Flow Control Duration Standard matching forested conditions standard satisfies the specified
performance criteria for all the area-specific and problem-specific requirements except if adjustments are required per the special
provision for closed depressions described below in Note 5.
(2) Overflow Tr is the return period of conveyance system overflow. To determine Tr requires a minimum Level 2 downstream analysis as
detailed in Section 2.3.1.1. To avoid this analysis, a Tr of 2 years may be assumed.
(3) Offsite improvements may be implemented in lieu of or in combination with additional flow control as allowed in Section 1.2.2.2 and
detailed in Section 3.3.5.
(4) A tightline system may be required regardless of the flow control standard being applied if needed to meet the discharge requirements
of Core Requirement #1 or the outfall requirements of Core Requirement #4, or if deemed necessary by the City of Renton where the
risk of severe damage is high.
(5) Special Provision for Closed Depressions with a Severe Flooding Problem:
IF the proposed project discharges by overland flow or conveyance system to a closed depression experiencing a severe flooding
problem AND the amount of new impervious surface area proposed by the project is greater than or equal to 10% of the 100-year
water surface area of the closed depression, THEN use the “point of compliance analysis technique” described in Section 3.3.6 to verify
that water surface levels are not increasing for the return frequencies at which flooding occurs, up to and including the 100-year
frequency. If necessary, iteratively adjust onsite flow control performance to prevent increases. Note: The point of compliance analysis
relies on certain field measurements taken directly at the closed depression (e.g., soils tests, topography, etc.). If permission to enter
private property for such measurements is denied, the City of Renton may waive this provision and apply the Flow Control Duration
Standard matching forested conditions standard with a mandatory 20% safety factor on the storage volume.
AGENDA ITEM # 8. a)
1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES
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DIRECT DISCHARGE EXEMPTION
Any onsite natural drainage area is exempt from the flow
control facility requirement if the area drains to one of the
major receiving waters listed in Table 1.2.3.B, AND meets
the following criteria for direct discharge20 to that receiving
water:
1. The flowpath from the project site discharge point to the
edge of the 100-year floodplain of the major receiving
water will be no longer than a half mile, except for
discharges to Lake Washington, AND
2. The conveyance system between the project site and the
major receiving water will extend to the ordinary high
water mark, and will be comprised of manmade
conveyance elements (pipes, ditches, etc.) and will be
within public right-of-way or a public or private drainage easement, AND
3. The conveyance system will have adequate capacity21 to convey the 25-year peak flow (per Core
Requirement #4, Conveyance System), for the entire contributing drainage area, assuming build-out
conditions to current zoning for the equivalent area portion (the area that is contained within an arc
formed by the shortest, straight line distance from the conveyance system discharge point to the
furthermost point of the proposed project) and existing conditions for the remaining area, AND
4. The conveyance system will be adequately stabilized to prevent erosion, assuming the same basin
conditions as assumed in Criteria (c) above, AND
5. The direct discharge proposal will not divert flows from or increase flows to an existing wetland or
stream sufficient to cause a significant adverse impact.
A. PEAK RATE FLOW CONTROL STANDARD AREAS
The Peak Rate Flow Control Standard is a peak-rate matching standard intended to prevent increases of
peak flows for specific events rather than match flow-durations over a range of flows. The standard is
appropriate for use in areas where the concern is flooding rather than stream bed erosion. Within the City
of Renton, this standard is allowed for those areas that are highly urbanized prior to 1985 and that drain to
pipes or non-fish bearing constructed conveyance systems leading to the lower Cedar River, Lake
Washington or the portion of the Green River Valley floor located in Renton.
Minimum Required Performance
Facilities in Peak Rate Flow Control Standard Areas must comply with the following flow control
performance standards and assumptions unless modified by offsite analysis per Core Requirement #2 (see
Table 1.2.3.A):
Peak Rate Flow Control Standard: Match the developed peak discharge rates to existing site
conditions peak discharge rates for 2-, 10-, and 100-year return periods.
Intent
The Peak Rate Flow Control Standard is intended to protect flow-carrying capacity and limit
increased erosion within the downstream conveyance system for runoff events less than or equal to the
100-year event. Matching the 2-, 10-, and 100-year peak flows is intended to prevent increases in
return-frequency peak flows less than or equal to the 100-year peak flow down to the 2-year peak
19 Footnote 22 is not used.
20 Direct discharge means undetained discharge from a proposed project to a major receiving water.
21 Note: The City does not charge a special use fee.
TABLE 1.2.3.B
MAJOR RECEIVING WATERS19
Cedar River downstream of Taylor
Creek confluence
Johns Creek downstream of
Interstate-405 (I-405) east right-of-
way
Lake Washington
Note: The major receiving waters listed
above do not include side adjacent or
associated channels, spring- or
groundwater-fed streams, or wetlands.
AGENDA ITEM # 8. a)
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flow. This level of control is also intended to prevent creation of new conveyance system nuisance
problems as described in Section 1.2.2.1.
Effectiveness in Addressing Downstream Drainage Problems
While the Peak Rate Flow Control Standard provides reasonable protection from many
development-induced conveyance problems (up to the 100-year event), it does not prevent increases in
runoff volumes or flow durations that tend to aggravate the three types of downstream drainage
problems described in Section 1.2.2.1. Consequently, if one or more of these problems are identified
through offsite analysis per Core Requirement #2, additional onsite flow control and/or offsite
improvements will likely be required (see “Drainage Problem-Specific Mitigation Requirements” in
Section 1.2.2.2).
Target Surfaces
Facilities in Peak Rate Flow Control Standard Areas must mitigate (either directly or in effect) the
runoff from the following target surfaces within the threshold discharge area for which the facility is
required:
1. New impervious surface that is not fully dispersed per the criteria in Section 1.2.3.2.C as specified in
Appendix C. For individual lots within residential subdivision projects, the extent of new impervious
surface shall be assumed as specified in Chapter 3. Note, any new impervious surface such as a
bridge or boardwalk that spans the ordinary high water of a stream, pond, or lake may be excluded as
a target surface if the runoff from such span is conveyed to the ordinary high water area in
accordance with Criteria (b), (c), (d), and (e) of the “Direct Discharge Exemption“ (p 1-39).
2. New pervious surface that is not fully dispersed as specified in Appendix C. For individual lots
within residential subdivision projects, the extent of new pervious surface shall be assumed to be the
entire lot area, except the assumed impervious portion and any portion in which native conditions are
preserved by covenant, tract, or easement. In addition, the new pervious surface on individual lots
shall be assumed to be 100% grass.
Exceptions
The following exceptions apply only in Peak Rate Flow Control Standard Areas:
1. The facility requirement in Peak Rate Flow Control Standard Areas is waived for any threshold
discharge area in which the target surfaces subject to this requirement will generate no more than a
0.15-cfs increase (when modeled using 15 minute time steps) in the existing site conditions 100 -year
peak flow (modeled using same time step unit (e.g., 15 -minute) used to calculate the developed flow).
Note: for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C
may be modeled in accordance with the on-site BMP sizing credits in Core Requirement #9,
Table 1.2.9.A.
2. The facility requirement in Peak Rate Flow Control Standard Areas may be waived for any
threshold discharge area of a redevelopment project in which all of the following criteria are met:
a) The target surfaces subject to the Peak Rate Flow Control Standard Areas facility requirement
will generate no more than a 0.15-cfs increase (when modeled using 15 -minute time steps) in the
existing site conditions 100 -year peak flow (modeled using same time step unit (e.g., 15 -minute)
used to calculate the developed flow) at any natural discharge location from the project site
(note: for the purposes of this calculation, target surfaces served by on-site BMPs per
Appendix C may be modeled in accordance with the on-site BMP sizing credits in Core
Requirement #9, Table 1.2.9.A, AND
b) The increased runoff from target surfaces will not significantly impact a critical area, severe
flooding problem, or severe erosion problem.
AGENDA ITEM # 8. a)
1.2.3 CORE REQUIREMENT #3: FLOW CONTROL FACILITIES
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B. FLOW CONTROL DURATION STANDARD AREAS
The flow control duration standard requires runoff from urban developments to be detained and released at
a rate that matches the flow duration of predeveloped rates over the range of flows extending from ½ of
the 2-year up to the 50-year flow. Also match developed peak discharge rates to predeveloped peak
discharge rates for the 2- and 10-year return periods. Flow duration specifies the cumulative amount of
time that various flows are equaled or exceeded during a long-term simulation using historical rainfall.
The target flow duration may be the “historical” (i.e., fully forested condition) or in specific situations it
may be the existing site or “pre-project” condition as described below. The Flow Control Application layer
in COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>) shows the areas
where the “forested” and “existing” conditions are allowed.
Forested land cover – Runoff from the developed site will be controlled and released at a rate that
matches the flow duration for a forested (“historical”) land cover. The “historical” land cover is the default
standard required by the technical requirements of the NPDES permit. The standard is applicable to those
areas draining to streams that have erodible channels where runoff from urban areas has the potential to
destabilize the channel.
Existing land cover – Runoff from the developed site will be controlled and released at a rate that
matches the flow duration for the site conditions existing before the development. These are areas that
have been developed for years and drain to stream channels that have become stabilized to a new
hydrologic regime. Ecology has proposed that the existing land cover can be used in basins that have had
at least 40% total impervious surface area for the 20 years preceding Ecology’s adoption of the 2005
Stormwater Management Manual for Western Washington (called the 40/20 rule) and the stream channels
receiving the runoff are considered stable from the standpoint of excessive erosion or sedimentation. In
developing the “40/20 rule” for highly urbanized basins, Ecology conducted a preliminary analysis and
produced maps that identify those areas that may meet the criteria. Portions of Renton were included in the
initial maps prepared by Ecology. These maps have been adjusted to better represent the areas that were
40% impervious in 1985 as well as drainage basin divides within the City.
Flow control facilities designed to the “40/20 rule” will only have to mitigate for the added impervious
surface. As a result, these flow control facilities will be smaller than those required to be designed to
match runoff from a fully forested site.
Minimum Required Performance
Facilities in Flow Control Duration Standard Areas must comply with the following flow control
performance standard and assumptions unless modified by offsite analysis per Core Requirement #2 (see
Table 1.2.3.A):
Flow Control Duration Standard Matching Forested Site Conditions: Developed discharge
durations shall not exceed predeveloped durations for the range of predeveloped discharge rates from
50% of the 2-year peak flow up to the full 50-year peak flow. Developed peak discharge rates shall
not exceed predeveloped peak discharge rates for the 2- and 10-year return periods. Assume forested
(historical) site conditions as the predeveloped condition.
Flow Control Duration Standard Matching Existing Site Conditions: Developed discharge durations
shall not exceed predeveloped durations for the range of predeveloped discharge rates from
50% of the 2-year peak flow up to the full 50-year peak flow. Developed peak discharge rates
shall not exceed predeveloped peak discharge rates for the 2- and 10-year return periods.
Assume existing site conditions as the predeveloped condition.
Intent
The Flow Control Duration Standard flow control standard assuming historical site conditions is
intended to limit the amount of time that erosive flows are at work generating erosion and
sedimentation within natural and constructed drainage systems. Such control is effective in preventing
development-induced increases in natural erosion rates and reducing existing erosion rates where they
may have been increased by past development of the site. This is accomplished by maintaining at
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historical predevelopment levels the aggregate time that developed flows exceed an erosion-causing
threshold (i.e., 50% of the historical 2-year peak flow). Maintaining natural erosion rates within
streams and their tributary areas is important for preventing increases in stream channel erosion and
sediment loading that are detrimental to salmonid habitat and production.
Effectiveness in Addressing Downstream Drainage Problems
While the Flow Control Duration Standard flow control standard assuming historical site
conditions provides a reasonable level of protection for preventing most development-induced
problems, it does not necessarily prevent increases in existing site conditions 100-year peak flows that
can aggravate severe flooding problems as described in Core Requirement #2, nor does it necessarily
prevent aggravation of all severe erosion problems. Consequently, if one or more of these problems
are identified through offsite analysis per Core Requirement #2, additional onsite flow control and/or
offsite improvements will likely be required (see “Drainage Problem-Specific Mitigation
Requirements” in Section 1.2.2.2).
Target Surfaces
Facilities in Flow Control Duration Standard Areas22 must mitigate (either directly or in effect) the
runoff from the following target developed surfaces within the threshold discharge area for which the
facility is required:
1. New impervious surface that is not fully dispersed per the criteria on Section 1.2.3.2.C as specified in
Appendix C. For individual lots within residential subdivision projects, the extent of new impervious
surface shall be assumed as specified in Chapter 3. Note, any new impervious surface such as a
bridge or boardwalk that spans the ordinary high water of a stream, pond, or lake may be excluded as
a target surface if the runoff from such span is conveyed to the ordinary high water area in
accordance with Criteria (b), (c), (d), and (e) of the “Direct Discharge Exemption“ (p 1-39).
2. New pervious surface that is not fully dispersed as specified in Appendix C. For individual lots
within residential subdivision projects, the extent of new pervious surface shall be assumed to be the
entire lot area, except the assumed impervious portion and any portion in which native conditions are
preserved by covenant, tract, or easement. In addition, the new pervious surface on individual lots
shall be assumed to be 100% grass.
3. Replaced impervious surface that is not fully dispersed as specified in Appendix C on a non-
redevelopment project in which the total of new plus replaced impervious surface is 5,000 square feet
or more, OR new pervious surface is ¾ acre or more.
4. Replaced impervious surface that is not fully dispersed on a transportation redevelopment project in
which new impervious surface is 5,000 square feet or more and totals 50% or more of the existing
impervious surface within the project limits.
5. Replaced impervious surface that is not fully dispersed as specified in Appendix C, on a parcel
redevelopment project in which the total of new plus replaced impervious surface is 5,000 square feet
or more and whose valuation of proposed improvements (including interior improvements and
excluding required mitigation improvements) exceeds 50% of the assessed value of: (a) the existing
project site improvements on commercial or industrial projects, or (b) the existing site improvements
on other projects.
Exceptions
The following exceptions apply only in Flow Control Duration Standard Areas:
1. The historical site conditions exception does not apply to the City.
22 Note: Any threshold discharge area that appears to be located within a Flow Control Duration Standard Area according to the
Flow Control Application layer in COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>) but
drains entirely by non-erodible manmade conveyance to a major receiving water (listed on page 1-51) is considered to be
located within a Peak Rate Flow Control Standard Area.
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2. The facility requirement in Flow Control Duration Standard Matching Existing Site Conditions
Areas is waived for any threshold discharge area in which there is no more than a 0.15-cfs
difference (when modeled using 15 minute time steps) in the sum of developed 100-year peak flows
for those target surfaces subject to this requirement and the sum of historical site conditions 100-year
peak flows (modeled using same time step unit (e.g., 15 minute) used to calculate the developed flow)
for the same surface areas.
Note: for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C
may be modeled in accordance with the on-site BMP sizing credits in Core Requirement #9,
Table 1.2.9.A.
3. The facility requirement in Flow Control Duration Standard Matching Forested Site Conditions
Areas is waived for any threshold discharge area in which there is no more than a 0.15-cfs
difference (when modeled using 15 minute time steps) in the sum of developed 100-year peak flows
for those target surfaces subject to this requirement and the sum of forested (historical) site conditions
100-year peak flows (modeled using same time step unit (e.g., 15 minute) used to calculate the
developed flow) for the same surface areas. Note: for the purposes of this calculation, target surfaces
served by on-site BMPs per Appendix C may be modeled in accordance with the on-site BMP sizing
credits in Core Requirement #9, Table 1.2.9.A.
4. The facility requirement in Flow Control Duration Standard Areas may be reduced or waived for
any threshold discharge area where a plan or study approved by the City and Ecology shows that a
lower standard (e.g., Peak Rate Control Standard or targeting existing site conditions instead of
forested conditions) is sufficient or no facility is necessary to protect or allow for restoration of water
body beneficial uses and habitat functions essential to salmonids.
5. The regional facilities plan exception does not apply to the City.
6. The facility requirement in Flow Control Duration Standard Areas as applied to replaced
impervious surface may be reduced by the CED Manager/designee using the adjustment process
detailed in Sections 1.4.3 and 1.4.4 of the adjustment process, if the cost of flow control facilities to
mitigate all target surfaces exceeds that necessary to mitigate only for new impervious surface plus
new pervious surface and also exceeds 1/3 of the valuation of proposed improvements (including
interior improvements) or twice the cost of a facility to mitigate equivalent surfaces on a new
development site, whichever is less. The amount of reduction shall be limited such that the cost of
flow control facilities is at least equal to that necessary to mitigate only for new impervious surface
plus new pervious surface, and beyond this amount, is no greater than 1/3 of the valuation of proposed
improvements (including interior improvements) or twice the cost of a facility to mitigate equivalent
surfaces on a new development site, whichever is less.
C. FLOOD PROBLEM FLOW CONTROL STANDARD AREAS
Flood Problem Flow Control Standard Areas are designated by the City of Renton where the City has
determined that a higher average level of flow control is needed to prevent aggravation of existing
documented flooding problems. At this time, the City has not mapped specific areas, but may apply this
standard when a project discharges to a severe flooding or erosion problem.
Within Flood Problem Flow Control Standard Areas, or where required by the City to protect
aggravation of a downstream problem, required flow control facilities must comply with the following
minimum requirements for facility performance and mitigation of targeted surfaces, except where such
requirements or the facility requirement itself is waived or reduced by the area-specific exceptions at the
end of this subsection.
Minimum Required Performance
Facilities in Flood Problem Flow Control Standard Areas must comply with the following flow control
performance standard and assumptions unless modified by offsite analysis per Core Requirement #2 (see
Table 1.2.3.A):
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Flood Problem Flow Control Standard: Apply the Flow Control Duration Standard, AND match
the developed 100-year peak discharge rate to the predeveloped 100-year peak discharge rate. If the
Flood Problem Flow Control Area is located within a Flow Control Duration Standard Area and
does not drain entirely by non-erodible manmade conveyance to a major receiving water (see
Table 1.2.3.B), then historical site conditions shall be assumed as the predeveloped condition except
for the purposes of matching 100-year peak discharge rates. For all other situations and for the
purposes of matching 100-year peak discharge rates, existing site conditions may be assumed.
Intent
The Flood Problem Flow Control Standard is intended to prevent significant increases in existing
water surface levels for 2-year through 100-year return frequencies. Such increases are expected to
occur as the volume of runoff discharging to the water body is increased by upstream development.
Because inflow rates to these water bodies are typically much higher than the outflow rates, increased
runoff volumes from upstream development are, in effect, stacked on top of existing volumes in the
water body, resulting in higher water surface levels. The duration-matching and 100-year peak-
matching criteria of the Flood Problem Flow Control Standard counteract this stacking effect by
slowing the arrival of additional runoff volumes. Because it can prevent significant aggravation of
existing flooding, the Flood Problem Flow Control Standard is also applicable to other flow
control areas where severe flooding problems have been identified per Core Requirement #2.
Effectiveness in Addressing Downstream Drainage Problems
If the Flood Problem Flow Control Standard is implemented onsite, no additional measures are
required to prevent aggravation of the three types of downstream drainage problems described in Core
Requirement #2. The one exception is for a wetland or lake that is a closed depression with a severe
flooding problem, and the proposed project is adding impervious surface area amounting to more
than 10% of the 100-year water surface area of the closed depression. In this case, additional onsite
flow control or offsite improvements may be necessary as determined by a “point of compliance
analysis” (see “Special Provision for Closed Depressions” in Table 1.2.3.A, and see Section 3.3.6,
“Point of Compliance Analysis”).
Target Surfaces
Facilities in Flood Problem Flow Control Standard Areas must mitigate (either directly or in effect) the
runoff from the following target developed surfaces within the threshold discharge area for which the
facility is required:
1. If the Flood Problem Flow Control Standard Area is located within a Flow Control Duration
Standard Area, then the target surfaces are the same as those required for facilities in Flow Control
Duration Standard Areas (see Section 1.2.3.1.B) unless otherwise allowed by the area-specific
exceptions for Flow Control Duration Standard Areas. Note: Any Flood Problem Flow Control
Standard Area that appears to be located within a Flow Control Duration Standard Area according to
the Flow Control Application layer in COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>), but drains entirely by non-
erodible manmade conveyance to a major receiving water (see Table 1.2.3.B), is considered to be
located within a Peak Rate Flow Control Standard Area.
2. If the Flood Problem Flow Control Standard Area is located within a Peak Rate Flow Control
Standard Area or drains entirely by non-erodible manmade conveyance to a major receiving water,
then the target surfaces are the same as those required for facilities in Peak Rate Flow Control
Standard Areas (see Section 1.2.3.1.A).
Exceptions
The following exceptions apply only in Flood Problem Flow Control Standard Areas:
1. If the Flood Problem Flow Control Standard Area is located within a Flow Control Duration Standard
Matching Existing Site Conditions Area or Peak Rate Flow Control Area, then the facility
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requirement is waived for any threshold discharge area in which there is no more than a 0.15-cfs
difference (when modeled using 15 minute time steps) in the sum of developed 100-year peak flows
for the target surfaces subject to this requirement and the sum of historical site conditions 100-year
peak flows (modeled using same time step unit (e.g., 15 minute) used to calculate the developed flow)
for the same surface areas. Agricultural zoned projects in current agricultural use may use existing site
conditions as the predeveloped condition for purposes of this exception calculation.
Note: for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C
may be modeled in accordance with the on-site BMP sizing credits in Core Requirement #9,
Table 1.2.9.A. Also, any Flood Problem Flow Control Standard Area that appears to be located
within a Flow Control Duration Standard Area according to the Flow Control Application layer in
COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>), but drains
entirely by non-erodible manmade conveyance to a major receiving water (see Table 1.2.3.B), is
considered to be located within a Peak Rate Flow Control Standard Area.
2. If the Flood Problem Flow Control Standard Area is located within a Peak Rate Flow Control
Standard Area, then the facility requirement is waived for any threshold discharge area in which the
target surfaces subject to this requirement will generate no more than a 0.15-cfs increase (when
modeled using 15-minute time steps) in the existing site conditions 100-year peak flow (modeled
using same time step unit (e.g., 15-minute) used to calculate the developed flow.
Note: for the purposes of this calculation, target surfaces served by on-site BMPs per Appendix C
may be modeled in accordance with the on-site BMP sizing credits in Core Requirement #9,
Table 1.2.9.A.
3. Any required application of the Flood Problem Flow Control Standard Areas facility requirement
to replaced impervious surface may be waived if the City has adopted a plan and implementation
schedule approved by the state Department of Ecology for fulfilling this requirement with regional
facilities.
4. Any required application of the Flood Problem Flow Control Standard Areas facility requirement
to replaced impervious surface may be reduced by CED using the procedures detailed in Sections
1.4.3 and 1.4.4 of the adjustment process, if the cost of flow control facilities to mitigate all target
surfaces exceeds that necessary to mitigate only for new impervious surface plus new pervious
surface and also exceeds 1/3 of the valuation of proposed improvements (including interior
improvements) or twice the cost of a facility to mitigate the same surfaces on a new development site,
whichever is less. The amount of reduction allowed by this exception shall be limited such that the
cost of flow control facilities is at least equal to that necessary to mitigate only for new impervious
surface plus new pervious surface, and beyond this amount, is no greater than 1/3 of the valuation of
proposed improvements (including interior improvements) or twice the cost of a facility to mitigate
equivalent surfaces on a new development site, whichever is less.
1.2.3.2 FLOW CONTROL FACILITY IMPLEMENTATION REQUIREMENTS
Flow control facilities shall be designed and implemented in accordance with the following requirements,
allowances, and flexible compliance provisions:
A. ONSITE VS. OFFSITE IMPLEMENTATION
All required flow control facilities must be implemented onsite except where the requirements below can
be met by direct discharge to a regional or shared facility constructed to provide flow control for the
proposed project. Regional facilities are typically constructed as part of a City-approved plan or study
(e.g., basin plan, stormwater compliance plan, or master drainage plan). Shared facilities may be
constructed under a City-developed shared facility drainage plan or under an agreement between two or
more private developers.
1. The regional or shared facility must be of adequate size and design to meet the current flow control
requirements for the proposed project. Note: the current flow control requirements are those specified
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by Core Requirement #3 of this manual unless superseded by other adopted area-specific flow control
requirements per Special Requirement #1 (see Section 1.3.1). In some cases where the current flow
control requirements differ from those used to originally design the regional or shared facility,
additional analysis and possible retrofitting of the facility may be required to ensure adequate size and
design. In other cases where the current flow control requirements are not significantly different or are
less stringent, adequate size and design may already be documented by an adopted City basin plan or
master drainage plan, an approved shared facility drainage plan, or a detailed drainage analysis
approved by the City for a separate permitted development.
2. The regional or shared facility must be fully operational at the time of construction of the proposed
project. In the case of a shared facility, the proposed project must comply with the terms and
conditions of all contracts, agreements, and permits associated with the shared facility. If the offsite
facility is an existing City-owned facility, the City may charge a special use fee equal to or based on
the property value of the detention capacity being used.
3. The conveyance system between the project site and the regional facility must meet the same criteria
specified for direct discharge to a major receiving water except for Criterion (a) (see “Direct
Discharge Exemption” in Section 1.2.3.1). In the case of a shared facility, the criteria are the same,
except the conveyance system need only have adequate capacity and erosion protection for buildout of
the participating portion23 of the contributing drainage area.
B. METHODS OF ANALYSIS AND DESIGN
Flow control facilities must be analyzed and designed using a continuous flow simulation method such as
HSPF (Hydrologic Simulation Program FORTRAN) or the simplified HSPF-based runoff files method.
An overview of the runoff files method is found in Chapter 3. Specifications for use of the approved
modeling software is provided in the software documentation and augmented with limited SWDM-specific
guidance in Reference Section 6-D. Detailed design specifications for flow control facilities are found in
Chapter 5.
C. SIZING CREDITS FOR FULLY DISPERSED SURFACES
A fully dispersed surface (either impervious or nonnative pervious) is one that conforms to the BMP
strategy for “full dispersion” detailed in Appendix C, Section C.2.1. This strategy calls for minimizing the
area of onsite developed surface relative to native vegetated surface, together with the application of
dispersion techniques that utilize the natural retention/detention capacity of the native vegetated surface to
mitigate the runoff effects of the developed surfaces. Developed surfaces conforming to this strategy are
considered to have a negligible impact downstream, and therefore, may be modeled as forest and are not
subject to the area-specific flow control facility requirement (Section 1.2.3.1) or the area-specific water
quality facility requirement (Section 1.2.8.1). In order for developed surfaces to qualify as fully dispersed,
they must meet the basic criteria listed below and further detailed in Appendix C, Section C.2.1.
Criteria for Fully Dispersed Surfaces
1. The total area of impervious surface being fully dispersed must be no more than 15% of the total
area of native vegetated surface being preserved by a clearing limit by a City-approved recorded tract,
easement, or covenant within the same threshold discharge area. The total area of impervious
surface plus nonnative pervious surface24 being fully dispersed must be no more than 35% of a
threshold discharge area.
2. The runoff from a fully dispersed surface must be discharged using one of the following dispersion
devices in accordance with the design specifications and maximum area of fully dispersed surface for
each device set forth in Appendix C, Section C.2.1:
a) Splash blocks
b) Rock pads
c) Gravel filled trenches
23 The participating portion includes those properties that have agreements for use of the shared facility.
24 Nonnative pervious surface means a pervious surface that does not meet the definition of a native vegetated surface.
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d) Sheet flow
Note: The dispersion device must be situated so as to discharge within the same threshold discharge
area of the surface it serves.
3. A native vegetated flowpath segment of at least 100 feet in length (25 feet for sheet flow from a
nonnative pervious surface) must be available along the flowpath that runoff would follow upon
discharge from a dispersion device listed in Minimum Requirement 2 above. The native vegetated
flowpath segment must meet all of the following criteria:
a) The flowpath segment must be over native vegetated surface.
b) The flowpath segment must be onsite or an offsite tract or easement area reserved for such
dispersion.
c) The average slope of the flowpath segment must be no steeper than 15% for any 20-foot reach
of the flowpath segment.
d) The flowpath segment must be located between the dispersion device and any downstream
drainage feature such as a pipe, ditch, stream, river, pond, lake, or wetland.
e) The flowpath segments for adjacent dispersion devices must comply with the minimum spacing
requirements in Appendix C, Section C.2.1. These requirements do not allow overlap of
flowpath segments, except in the case where sheet flow from a nonnative pervious surface
overlaps with the flowpath of any dispersion device listed in Minimum Requirement 2 above. In
this case, the longer of the two overlapping flowpath segments must be extended at least 1 foot for
every 3 feet of distance along the most representative path that runoff would travel from the
upstream end to the discharge end of the nonnative pervious surface.
4. On sites with septic systems, the discharge of runoff from dispersion devices must not be upgradient
of the drainfield. This requirement may be waived by CED if site topography clearly prohibits flows
from intersecting the drainfield.
5. The dispersion of runoff must not create flooding or erosion impacts as determined by CED. If
runoff is proposed to be discharged toward a landslide hazard, erosion hazard area, or steep slope
hazard area (i.e., slopes steeper than 20%), CED may require the applicant to have the proposal
evaluated by a geotechnical engineer, engineering geologist, or CED.
D. SIZING CREDITS FOR USE OF ON-SITE BMPS
Projects that implement on-site BMPs as detailed in Core Requirement #9 and Appendix C, whether
required or optional, may use the on-site BMP sizing credits as described and allowed in Section 1.2.9.4
and Table 1.2.9.A.
E. MITIGATION OF TARGET SURFACES THAT BYPASS FACILITY
On some sites, topography may make it difficult or costly to collect all target surface runoff for discharge
to the onsite flow control facility. Therefore, some project runoff subject to flow control may bypass
required onsite flow control facilities provided that all of the following conditions are met:
1. The point of convergence for runoff discharged from the bypassed target surfaces and from the
project’s flow control facility must be within a quarter-mile downstream25 of the facility’s project
site discharge point, AND
2. The increase in the existing site conditions 100-year peak discharge from the area of bypassed target
surfaces must not exceed 0.4 cfs, AND
25 Note: CED may allow this distance to be extended beyond a half mile to the point where the project site area constitutes less
than 15% of the tributary area.
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3. Runoff from the bypassed target surfaces must not create a significant adverse impact to
downstream drainage systems, salmonid habitat, or properties as determined by CED, AND
4. Water quality requirements applicable to the bypassed target surfaces must be met, AND
5. Compensatory mitigation by a flow control facility must be provided so that the net effect at the
point of convergence downstream is the same with or without the bypass. This mitigation may be
waived if the existing site conditions 100-year peak discharge from the area of bypassed target
surfaces is increased by no more than 0.15 cfs (modeled using 15 minute time steps) and on-site
BMPs as detailed in Appendix C are applied to all impervious surfaces within the area of bypassed
target surfaces. One or combination of the following methods may be used to provide compensatory
mitigation by a flow control facility subject to permission/approvals from other parties as deemed
necessary by CED:
a) Design the project’s flow control facility or retrofit an existing offsite flow control facility as
needed to achieve the desired effect at the point of convergence, OR
b) Design the project’s flow control facility or provide/retrofit an offsite flow control facility to
mitigate an existing developed area (either onsite or offsite) that has runoff characteristics (i.e.,
peak flow and volume) equivalent to those of the bypassed target surfaces but is currently not
mitigated or required to be mitigated to the same flow control performance requirement as the
bypassed target surfaces. Consideration of an offsite area to be mitigated for must take into
account the likelihood of that area redeveloping in the future. Those areas determined by the City
to have a high likelihood of future redevelopment that will provide its own mitigation may not be
used as compensatory mitigation.
F. BYPASS OF RUNOFF FROM NON-TARGET SURFACES
The performance of flow control facilities can be compromised if the contributing area, beyond that which
must be mitigated by the facility, is too large. Therefore, IF the existing 100-year peak flow rate from any
upstream area (not targeted for mitigation) is greater than 50% of the 100-year developed peak flow rate
(undetained) for the area that must be mitigated, THEN the runoff from the upstream area must bypass the
facility. Offsite areas that naturally drain onto the project site must be intercepted at the natural drainage
course within the project site and conveyed in a separate conveyance system and must bypass onsite
stormwater facilities. The bypass of upstream runoff must be designed so that all of the following
conditions are met:
1. Any existing contribution of flows to an onsite wetland must be maintained, AND
2. Upstream flows that are naturally attenuated by natural detention on the project site under
predeveloped conditions must remain attenuated, either by natural means or by providing additional
onsite detention so that peak flows do not increase, AND
3. Upstream flows that are dispersed or unconcentrated on the project site under predeveloped
conditions must be discharged in a safe manner as described in Core Requirement #1 under
“Discharge Requirements“.
4. Bypasses shall be designed in accordance with standards of Core Requirement #4, Conveyance
System
G. MITIGATION TRADES
A project’s flow control facility may be designed to mitigate an existing developed non-target surface area
(either onsite or offsite) in trade for not mitigating part or all of the project’s target surface area, provided
that all of the following conditions are met:
1. The existing developed non-target surface area (i.e., an area of existing impervious surface and/or
nonnative pervious surface) must have runoff discharge characteristics (i.e., peak flow and volume)
equivalent to those of the target surface area for which mitigation is being traded and must not be
currently mitigated to the same flow control performance requirement as the target surface area, AND
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2. Runoff from both the target surface area being traded and the flow control facility must converge
prior to discharge of the runoff from the target surface area being traded onto private property
without an easement or through any area subject to erosion, AND
3. The net effect in terms of flow control at the point of convergence downstream must be the same with
or without the mitigation trade, AND
4. The undetained runoff from the target surface area being traded must not create a significant
adverse impact to downstream drainage systems, salmonid habitat, or properties prior to convergence
with runoff from the flow control facility.
5. Mitigation trade proposals must be reviewed and approved with input from the City of Renton.
6. The existing non-targeted surface area that is mitigated for purposes of the required flow control must
be documented and tracked by CED. Documentation should clarify that future redevelopment of the
existing non-targeted area used for the mitigation trade will incur additional flow control mitigation
requirements if the redevelopment exceeds Core Requirement #3 thresholds. This additional flow
control mitigation must be met in addition to that previously required and provided for the mitigation
trade. Applicants must consider sizing flow control facilities sufficient for both the mitigation trade
area and future development of the existing non-targeted area, if feasible.
H. MANIFOLD DETENTION FACILITIES
A manifold detention facility is a single detention facility designed to take the place of two or more
otherwise required detention facilities. It combines the runoff from two or more onsite drainage areas
having separate natural discharge locations, and redistributes the runoff back to the natural discharge
locations following detention. Because manifold detention facilities divert flows from one natural
discharge location to another and then back, they are not allowed except by an approved adjustment (see
Section 1.4).
I. FACILITY REQUIREMENT IN LANDSLIDE HAZARD DRAINAGE AREAS
Proposed projects subject to Discharge Requirement 2 in Core Requirement #1 must provide a tightline
system unless the 100-year runoff from the project site can be feasibly infiltrated or one of the other
exceptions listed in Section 1.1.2.2. For infiltration to be used as an alternative to the tightline requirement,
it must be feasible per the facility design requirements and limitations specified in Section 5.2. When
evaluating the feasibility of infiltration, multiple facility locations scattered throughout the project site
shall be considered and used where feasible and practical to avoid concentrating infiltrated water in one
location. If multiple facilities are not feasible or practical, then a single infiltration facility meeting the
minimum setback requirements in Section 5.2 may be used where feasible.
Where infiltration is not feasible, it is still possible for a proposed project to qualify for one of the other
exceptions to the tightline requirement specified in Core Requirement #1. If such a project is subject to the
flow control facility requirement in Core Requirement #3, the required facility must be a detention pond
sized to meet, at minimum, the Flow Control Duration Standard Matching Forested site conditions
flow control facility standard with a safety factor of 20% applied to the storage volume. The detention
pond must be sited and designed so as to maximize the opportunity for infiltration in the pond. To
accomplish this, all of the following design requirements must be met:
1. The detention pond must be preceded by either a water quality treatment facility per Core
Requirement #8 or a presettling basin per Section 5.2, AND
2. All detention pond side slopes must be 3H:1V or flatter and must be earthen, AND
3. Detention pond liners that impede infiltration shall not be used, AND
4. The pond bottom shall be at or above the seasonal high groundwater table, AND
5. The detention pond outflow must meet the discharge dispersal requirements specified in Discharge
Requirement 1 of Core Requirement #1.
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1.2.4 CORE REQUIREMENT #4: CONVEYANCE SYSTEM
All engineered conveyance system elements for proposed projects must be analyzed, designed, and
constructed to provide a minimum level of protection against overtopping, flooding, erosion, and structural
failure as specified in the following groups of requirements:
“Conveyance Requirements for New Systems,” Section 1.2.4.1
“Conveyance Requirements for Existing Systems,” Section 1.2.4.2
“Conveyance System Implementation Requirements,” Section 1.2.4.3
Intent: To ensure proper design and construction of engineered conveyance system elements. Conveyance
systems are natural and engineered drainage facilities that provide for the collection and transport of
surface water or stormwater runoff. This core requirement applies to the engineered elements of
conveyance systems (primarily pipes, culverts, and ditches/channels).
1.2.4.1 CONVEYANCE REQUIREMENTS FOR NEW SYSTEMS
All new conveyance system elements,26 both onsite and offsite, shall be analyzed, designed, and
constructed according to the following requirements. Also see Section 4.1 for route design and easement
requirements.
Pipe Systems
1. New pipe systems shall be designed with sufficient capacity to convey and contain (at minimum) the
25-year peak flow, assuming developed conditions for onsite tributary areas and existing conditions
for any offsite tributary areas.
2. Pipe system structures may overtop for runoff events that exceed the 25-year design capacity,
provided the overflow from a 100-year runoff event does not create or aggravate a severe flooding
problem or severe erosion problem as described in Core Requirement #2, Section 1.2.2. Any overflow
occurring onsite for runoff events up to and including the 100-year event must discharge at the natural
location for the project site. In residential subdivisions, this overflow must be contained within an
onsite drainage easement, tract, covenant, or public right-of-way.
3. The upstream end of a pipe system that receives runoff from an open drainage feature (pond, ditch,
etc.) shall be analyzed and sized as a culvert as described below.
Culverts
1. New culverts shall be designed with sufficient capacity to meet the headwater requirements in Section
4.3.1 and convey (at minimum) the 25-year peak flow, assuming developed conditions for onsite
tributary areas and existing conditions for any offsite tributary areas.
2. New culverts must also convey as much of the 100-year peak flow as is necessary to preclude creating
or aggravating a severe flooding problem or severe erosion problem as described in Core
Requirement #2, Section 1.2.2. Any overflow occurring onsite for runoff events up to and including
the 100-year event must discharge at the natural location for the project site. In residential
subdivisions, this overflow must be contained within an onsite drainage easement, tract, covenant, or
public right-of-way.
3. New culverts proposed in streams with salmonids shall be designed to provide for fish passage as
detailed in Section 4.3.2. Note: The City’s critical areas regulations (RMC 4-3-050) or the state
Department of Fish and Wildlife may require a bridge to facilitate fish passage.
26 New conveyance system elements are those that are proposed to be constructed where there are no existing constructed
conveyance elements.
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Ditches/Channels
1. New ditches/channels shall be designed with sufficient capacity to convey and contain, at minimum,
the 25-year peak flow, assuming developed conditions for onsite tributary areas and existing
conditions for any offsite tributary areas.
2. New ditches/channels must also convey as much of the 100-year peak flow as is necessary to preclude
creating or aggravating a severe flooding problem or severe erosion problem as described in Core
Requirement #2, Section 1.2.2. Any overflow occurring onsite for runoff events up to and including
the 100-year event must discharge at the natural location for the project site. In residential
subdivisions, such overflow must be contained within an onsite drainage easement, tract, covenant, or
public right-of-way.
3. In both conditions listed above, ditches must be designed with a 6-inch minimum freeboard.
Tightline Systems Traversing Steep Slopes
New tightline conveyance systems traversing slopes that are steeper than 15% and greater than 20 feet in
height, or are within a steep slope hazard area as defined in RMC 4-3-050, shall be designed with
sufficient capacity to convey and contain (at minimum) the 100-year peak flow, assuming full build-out
conditions27 for all tributary areas, both onsite and offsite. Tightline systems shall be designed as detailed
in Section 4.2.2.
Bridges
New bridges shall be designed to accommodate the 100-year peak flow as specified in Section 4.3.3 and in
accordance with the floodplain development standards in RMC 4-3-050.
1.2.4.2 CONVEYANCE REQUIREMENTS FOR EXISTING SYSTEMS
The following conveyance requirements for existing systems are less rigorous than those for new systems
to allow some salvaging of existing systems that are in useable condition. Existing systems may be utilized
if they are capable of providing a minimum level of protection as-is or with minor modifications.
Existing Onsite Conveyance Systems
No Change in Flow Characteristics: Existing onsite conveyance systems that will not experience a
change in flow characteristics (e.g., peak flows or volume of flows) as a result of the proposed project
need not be analyzed for conveyance capacity.
Change in Flow Characteristics: Existing onsite conveyance systems that will experience a change in
flow characteristics as a result of the proposed project must comply with the following conveyance
requirements:
1. The existing system must be analyzed and shown to have sufficient capacity to convey and contain (at
minimum) the 25-year peak flow assuming developed conditions for onsite tributary areas and
existing conditions for any offsite tributary areas.
2. The applicant must demonstrate that the 100-year peak flow to the existing system will not create or
aggravate a severe flooding problem or severe erosion problem as described in Core Requirement #2,
Section 1.2.2.
3. Minor modifications may be made to the conveyance system to achieve the required capacity stated
above. Examples of minor modifications include raising a catch-basin rim, replacing or relaying a
section of pipe to match the capacity of other pipes in the system, improving a pipe inlet, or enlarging
a short, constricted reach of ditch or channel.
4. Modifications to an existing conveyance system or element that acts to attenuate peak flows, due to
the presence of detention storage upstream, shall be made in a manner that does not significantly
27 Full build-out conditions means the tributary area is developed to its full zoning potential except where there are existing
sensitive areas, open space tracts, and/or native growth protection easements/covenants.
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increase peak flows downstream. For example, if water is detained in a pond upstream of a restrictive
road culvert, then installing an overflow system for the culvert should prevent overtopping of the road
without significantly reducing existing detention storage.
Existing Offsite Conveyance Systems
1. Existing offsite conveyance systems need not be analyzed for conveyance capacity except as required
by Core Requirement #2, or if offsite improvements or direct discharge are proposed per Core
Requirement #3.
2. Improvements made to existing offsite conveyance systems to address the drainage problem-specific
mitigation requirements in Section 1.2.2.2 need only change existing conveyance capacity sufficient to
prevent aggravation of the drainage problem(s) being addressed.
3. Existing offsite conveyance systems proposed to be used for direct discharge to a major receiving
water per Core Requirement #3 shall meet the same conveyance requirements specified in Section
1.2.4.1 for new systems.
1.2.4.3 CONVEYANCE SYSTEM IMPLEMENTATION REQUIREMENTS
Conveyance systems shall be designed and implemented in accordance with the following requirements,
allowances, and flexible compliance provisions:
A. METHODS OF ANALYSIS AND DESIGN
Properly sized conveyance elements provide sufficient hydraulic capacity to convey peak flows of the
return frequencies indicated in Sections 1.2.4.1 and 1.2.4.2. Conveyance capacity shall be demonstrated
using the methods of analysis detailed in Chapter 4. Design flows for sizing conveyance systems shall be
determined using the appropriate runoff computation method specified in Section 3.2.
B. COMPOSITION
Where feasible, conveyance systems shall be constructed of vegetation-lined channels, as opposed to pipe
systems, except in Zone 1 of the Aquifer Protection Area where pipe systems are required. Vegetative
channels shall generally be considered feasible if all of the following conditions are present:
1. The channel gradient generally does not exceed 5 percent, AND
2. Ditches/roadway section must be approved by the City, AND
3. The channel will be accessible for maintenance (see Section 1.2.6), AND
4. The channel will not be subject to erosion.
Exceptions: The following are exceptions to the requirement for vegetative channels:
Conveyance systems proposed under roadways, driveways, or parking areas
Conveyance systems proposed between houses in urban-zoned plats and short plats
Conveyance systems conveying roof runoff only.
Conveyance systems in Zone 1 of the Aquifer Protection Area.
C. INTERFLOW AND INTERCEPTION
Interflow is near-surface groundwater that moves laterally through the soil horizon following the hydraulic
gradient of underlying relatively impermeable soils. When interflow is expressed on the surface, it is
termed a spring or seepage. Any significant springs or seepage areas that impact a roadway or structure
proposed by the project must be intercepted and directed into a conveyance system. Where roadways may
impede the passage of interflow to downstream wetlands or streams, provision for passage of
unconcentrated flows must be made.
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D. PROVISION FOR LOT DRAINAGE WITHIN SUBDIVISIONS
Within subdivision projects,28 provision must be made for the safe conveyance of runoff from the
discharge location of each lot to the subdivision’s main conveyance system or road drainage system. This
may include, but is not limited to, provisional stub-outs from an enclosed roadway drainage system to the
edge of the road right-of-way at each created lot, or lot-line pipes or ditches that collect lot drainage and
convey it to the subdivision’s main conveyance system or road drainage system.
E. OUTFALLS
An outfall is defined as a point where collected and concentrated surface and storm water runoff is
discharged from a pipe system or culvert.
Energy Dissipation: At a minimum, rock erosion protection is required at outfalls from all drainage
systems and elements except where CED determines that erosion protection is being provided by other
means or is not needed. Details on outfall structures are included in Section 4.2.2.
New Point Discharges Over Steep Slopes: Proposed outfalls that will discharge runoff in a location
where the natural (existing) discharge is unconcentrated over a slope steeper than 15% and greater than
20 feet in height, or over a steep slope hazard area (as defined in RMC 4-3-050), must meet the following
criteria:
A tightline conveyance system must be constructed to convey the runoff to the bottom of the slope
unless other measures are approved by CED based on an evaluation/report by a licensed geotechnical
engineer.
The geotechnical analysis must consider cumulative impacts from the project and surrounding areas
under full built-out conditions.
Tightline systems must be designed so that existing baseflow conditions are not significantly changed
and adequate energy dissipation is provided at the bottom of the slope.
Where alternative measures (e.g., dispersal trench) to the tightline system are approved upstream of a
landslide hazard or steep slope hazard area, they may be placed no closer than 50 feet from the top of
the hazard area slope based on an evaluation/report by a licensed geotechnical engineer.
F. OUTFALLS TO THE GREEN RIVER
New stormwater outfalls or modifications to existing stormwater outfalls discharging to the Green River
between River Mile 6 (South Boeing Access Road) and SR 18 are allowed only through the adjustment
process. These outfalls must comply with requirements of the Green River Pump Operations Procedure
Plan, which establishes storage volumes and release rate criteria for developments proposing to construct
or modify outfalls. Copies of the plan are available from DNRP.
G. SPILL CONTROL
Projects proposing to construct or replace onsite conveyance system elements that receive runoff from
non-roof-top pollution-generating impervious surface must provide a spill control device as detailed in
Section 4.2.1.1 prior to discharge from the site or into a natural onsite drainage feature.29 More
specifically, this requirement applies whenever a proposed project does either of the following:
Constructs a new onsite conveyance system that receives runoff from non-roof-top pollution-
generating impervious surface, OR
Removes and replaces an existing onsite conveyance system element that receives runoff from
5,000 square feet or more of non-roof-top pollution-generating impervious surface onsite.
28 For purposes of this requirement, the term subdivision project refers to any project that creates a short plat, plat, or binding
site plan.
29 Natural onsite drainage feature means a natural swale, channel, stream, closed depression, wetland, or lake.
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The intent of this device is to temporarily detain oil or other floatable pollutants before they enter the
downstream drainage system in the event of an accidental spill or illegal dumping. It may consist of a tee
section in a manhole or catch basin, or an equivalent alternative as specified in Section 4.2.1.1.
Note that in addition to this spill control requirement to protect offsite and natural drainage systems,
there are other spill control requirements in this manual for discharges to certain water quality facilities
and all infiltration facilities (see the design criteria for water quality facilities in Chapter 6 and the
general requirements for infiltration facilities in Section 5.2). The application of these requirements must
be such that all stated intents are satisfied.
H. GROUNDWATER PROTECTION
Any reach of new ditch or channel proposed by a project in which the untreated runoff from 5,000 square
feet or more of pollution-generating impervious surface or ¾ acre or more of pollution-generating
pervious surface comes into direct contact with an outwash soil must be lined with either a low
permeability liner or a treatment liner consistent with the specifications for such liners in Section 6.2.4,
except where it can be demonstrated that the soil meets the soil suitability criteria listed in Section 5.2.1.
The intent of this requirement is to reduce the likelihood that pollutants will be discharged to groundwater
when untreated runoff is conveyed in ditches or channels constructed in soils with high infiltration rates.
I. PUMP SYSTEMS
Pump systems may be used to convey water from one location or elevation to another within the project
site provided they meet the design criteria specified for such systems in Section 4.2.3 and will be privately
owned and maintained.
Pump systems discharging flows from the project site that would not have discharged by gravity flow
under existing site conditions will require an approved adjustment to Core Requirement #1 (see
Section 1.4, “Adjustment Process“). These pump systems will be considered only when there is no other
physical gravity alternative and they are necessary to prevent creation or aggravation of a flooding or
erosion problem as specified in Section 1.2.2.
1.2.5 CORE REQUIREMENT #5:
CONSTRUCTION STORMWATER POLLUTION PREVENTION
All proposed projects that will clear, grade, or otherwise disturb the site must provide erosion and
sediment controls to prevent, to the maximum extent practicable, the transport of sediment from the
project site to downstream drainage facilities, water resources, and adjacent properties. All proposed
projects that will conduct construction activities onsite or offsite must provide stormwater pollution
prevention and spill controls to prevent, reduce, or eliminate the discharge of pollutants to onsite or
adjacent stormwater systems or watercourses. To prevent sediment transport and pollutant discharges as
well as other impacts related to land-disturbing and construction activities, Erosion and Sediment
Control (ESC) measures and Stormwater Pollution Prevention and Spill Control (SWPPS) measures
that are appropriate to the project site must be applied through a comprehensive Construction
Stormwater Pollution Prevention (CSWPP) plan as described in Sections 1.2.5.1 and 1.2.5.3 and shall
perform as described in Section 1.2.5.2. In addition, these measures, both temporary and permanent, shall
be implemented consistent with the requirements in Section 1.2.5.3 that apply to the proposed project.
Intent:
To prevent the transport of sediment and other impacts, like increased runoff, related to land
disturbing activities. Erosion of disturbed areas on construction sites can result in excessive sediment
transport to adjacent properties and to surface waters. This sediment can result in major adverse
impacts, such as flooding from obstructed drainage ways, smothering of salmonid spawning beds,
algal blooms in lakes, and exceedances of state water quality standards for turbidity. These impacts
can also result from the increased runoff generated by land disturbing activities on construction sites.
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To prevent, reduce, or eliminate the discharge of pollutants to onsite or adjacent stormwater systems
or watercourses from construction-related activities such as materials delivery and storage, onsite
equipment fueling and maintenance, demolition of existing buildings and disposition of demolition
materials and other waste, and concrete handling, washout and disposal.
1.2.5.1 CSWPP MEASURES
Construction Stormwater Pollution Prevention (CSWPP) measures include Erosion and Sediment
Control (ESC) measures and Stormwater Pollution Prevention and Spill (SWPPS) measures.
ESC Measures
Each of the following categories of ESC measures must be considered for application to the project site as
detailed in the Erosion and Sediment Control (ESC) Standards located in the Construction Stormwater
Pollution Prevention Standards adopted as Appendix D of this manual:
1. Clearing Limits
2. Cover Measures
3. Perimeter Protection
4. Traffic Area Stabilization
5. Sediment Retention
6. Surface Water Collection
7. Dewatering Control
8. Dust Control
9. Flow Control
10. Control Pollutants (also see SWPPS Measures below)
11. Protect Existing and Proposed Stormwater Facilities and On-site BMPs
12. Maintain Protective BMPs
13. Manage the Project
SWPPS Measures
Each of the following categories of SWPPS measures must be considered for application to the project site
as detailed in the Stormwater Pollution Prevention and Spill Control (SWPPS) Standards located in the
CSWPP Standards adopted as Appendix D of this manual:
Follow effective pollutant handling and disposal procedures.
Provide cover and containment for materials, fuel and other pollutants.
Manage the project site to maximize pollutant control and minimize pollutant sources.
Protect from spills and drips of petroleum products and other pollutants.
Avoid overapplication or untimely application of chemicals and fertilizers.
Prevent or treat contamination of stormwater runoff by pH modifying sources.
1.2.5.2 CSWPP PERFORMANCE AND COMPLIANCE PROVISIONS
The changing conditions typical of construction sites call for frequent field adjustments of existing ESC
and SWPPS measures or additional ESC and SWPPS measures in order to meet required performance. In
some cases, strict adherence to specified measures may not be necessary or practicable based on site
conditions or project type. In other cases, immediate action may be needed to avoid severe impacts.
Therefore, careful attention must be paid to ESC and SWPPS performance and compliance in accordance
with the following provisions:
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A. CSWPP SUPERVISOR
For projects in Targeted, Full or Large Project Drainage Review, or projects in Directed Drainage Review
as determined by the CED permit reviewer, the applicant must designate a CSWPP supervisor who
shall be responsible for the performance, maintenance, and review of ESC and SWPPS measures and for
compliance with all permit conditions relating to CSWPP as described in the CSWPP Standards. The
applicant’s selection of a CSWPP supervisor must be approved by the City. This approval may be
rescinded for non-compliance, requiring the applicant to select another CSWPP supervisor and obtain City
approval prior to continuing work on the project site. For projects that disturb one acre or more of land,
the CSWPP supervisor must be a Certified Professional in Erosion and Sediment Control (see
<www.cpesc.net> for more information) or a Certified Erosion and Sediment Control Lead whose
certification is recognized by the Department of Ecology or King County.30 The City may also require a
certified ESC professional for sites smaller than one acre of disturbance if CED determines that onsite
ESC measures are inadequately installed, located, or maintained.
For larger, more sensitive sites, the City may require a certified ESC professional with several years of
experience in construction supervision/inspection and a background in geology, soil science, or agronomy
(See Appendix D, Section D.2.3.1 for more information).
B. MONITORING OF DISCHARGES
The CSWPP supervisor shall have a turbidity meter onsite and shall use it to monitor surface and storm
water discharges from the project site and into onsite wetlands, streams, or lakes whenever runoff occurs
from onsite activities and during storm events. If the project site is subject to a NPDES general permit for
construction issued by the Washington State Department of Ecology (Ecology), then the project must
comply with the monitoring requirements of that permit.
The CSWPP supervisor shall also use the specific SWPPS control BMP procedures for monitoring surface
and stormwater discharge for pollutants and acceptable discharge levels. The CSWPP supervisor shall
keep logs as required by the procedures of all measurements taken onsite and make them available to CED
on request.
C. ESC PERFORMANCE
ESC measures shall be applied/installed and maintained to prevent, to the maximum extent practicable, the
transport of sediment from the project site to downstream drainage systems or surface waters or into onsite
wetlands, streams, or lakes or onto adjacent properties. This performance is intended to be achieved
through proper selection, installation, and operation of the above ESC measures as detailed in the CSWPP
Standards (Appendix D) and approved by the City. However, the CSWPP supervisor or the City may
determine at any time during construction that the approved measures are not sufficient and that additional
action is required based on one of the following criteria:
1. If a turbidity test of surface and storm water discharges leaving the project site is greater than the
benchmark value of 25 NTU (nephelometric turbidity units) set by the Washington State Department
of Ecology, but less than 250 NTU, the CSWPP Supervisor shall do all of the following:
a) Review the ESC plan for compliance and make appropriate revisions within 7 days of the
discharge that exceeded the benchmark of 25 NTU, AND
b) Fully implement and maintain appropriate ESC measures as soon as possible but no later than
10 days after the discharge that exceeded the benchmark, AND
c) Document ESC implementation and maintenance in the site log book.
2. If a turbidity test of surface or storm water entering onsite wetlands, streams, or lakes indicates a
turbidity level greater than 5 NTU above background when the background turbidity is 50 NTU or
30 King County recognition of certification means that the individual has taken a King County-approved third party training
program and has passed the King County-approved test for that training program.
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less, or 10% above background when the background turbidity is greater than 50 NTU, then corrective
actions and/or additional measures beyond those specified in Section 1.2.5.1 shall be implemented as
deemed necessary by the City inspector or onsite CSWPP supervisor.
3. If discharge turbidity is 250 NTU or greater, the CSWPP Supervisor shall do all of the following:
a) Notify the City by telephone, AND
b) Review the ESC plan for compliance and make appropriate revisions within 7 days of the
discharge that exceeded the benchmark of 25 NTU, AND
c) Fully implement and maintain appropriate ESC measures as soon as possible but no later than
10 days after the discharge that exceeded the benchmark, AND
d) Document ESC implementation and maintenance in the site log book. AND
e) Continue to sample discharges until turbidity is 25 NTU or lower, or the turbidity is no more than
10% over background turbidity.
4. If the City determines that the condition of the construction site poses a hazard to adjacent property
or may adversely impact drainage facilities or water resources, THEN additional measures beyond
those specified in Section 1.2.5.1 may be required by the City.
D. SWPPS PERFORMANCE
SWPPS measures shall be applied/installed and maintained so as to prevent, reduce, or eliminate the
discharge of pollutants to onsite or adjacent stormwater systems or watercourses or onto adjacent
properties. This performance is intended to be achieved through proper selection, installation, and
operation of the above SWPPS measures as detailed in the CSWPP Standards (Appendix D) and approved
by the City. However, the CSWPP supervisor designated per Section 1.2.5.2.A or the City may determine
at any time during construction that such approved measures are not sufficient and additional action is
required based on the criteria described in the specific SWPPS BMP standard and/or conditions of an
approved adjustment:
E. FLEXIBLE COMPLIANCE
Some projects may meet the intent of Core Requirement #5 while varying from specific CSWPP
requirements contained here and in the CSWPP Standards. If a project is designed and constructed to meet
the intent of this core requirement, the City may determine that strict adherence to a specific ESC
requirement is unnecessary; an approved adjustment (see Section 1.4) is not required in these
circumstances. Certain types of projects are particularly suited to this greater level of flexibility, for
instance, projects on relatively flat, well drained soils, projects that are constructed in closed depressions,
or projects that only disturb a small percentage of a forested site may meet the intent of this requirement
with very few ESC measures. However, SWPPS requirements may actually be emphasized on well-
drained soils, particularly in groundwater or well protection areas, or in close proximity to water bodies.
More information on intent and general ESC and SWPPS principles is contained in the CSWPP Standards
in Appendix D.
F. ROADS AND UTILITIES
Road and utility projects often pose difficult erosion control challenges because they frequently cross
surface waters and are long and narrow with limited area available to treat and store sediment-laden water.
Because of these factors, road and utility projects are allowed greater flexibility in meeting the intent of
Core Requirement #5 as described in the CSWPP Standards.
G. ALTERNATIVE AND EXPERIMENTAL MEASURES
All measures proposed for erosion and sediment control shall conform to the details and specifications in
the CSWPP Standards unless an alternative is approved by the City, and if the alternative is a new
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technology, it must also be approved through Ecology’s CTAPE program (see “Alternative and
Experimental Measures” in the CSWPP Standards, Appendix D).
1.2.5.3 CSWPP IMPLEMENTATION REQUIREMENTS
Proposed projects must identify, install, and maintain required erosion and sediment control and
stormwater pollution prevention and spill control measures consistent with the following requirements:
A. CSWPP PLAN
As specified in Chapter 2, all proposed projects must submit a CSWPP plan for implementing CSWPP
measures. The CSWPP plan is comprised of the ESC plan and the SWPPS plan. The ESC plan must
show the location and details of all ESC measures as specified in Chapter 2 and the CSWPP Standards and
shall include a CSWPP report, which contains additional directions and supporting information like a
detailed construction sequence as proposed by the design engineer and any calculations or information
necessary to size ESC measures and demonstrate compliance with Core Requirement #5. The CSWPP
plan shall also contain plan notes that outline specific permit conditions as outlined in Appendix D
Section D.4.2 Standard ESC and SWPPS Plan Notes. The City may require large, complex projects to
phase construction and to submit multiple ESC plans for the different stages of construction. New CSWPP
plans are not required for changes that are necessary during construction, unless required by the City
inspector.
B. WET SEASON CONSTRUCTION
During the wet season (October 1 to April 30) any site with exposed soils shall be subject to the “Wet
Season Requirements” contained in the ESC Standards. In addition to the ESC cover measures, these
provisions include covering any newly-seeded areas with mulch and seeding as much disturbed area as
possible during the first week of October to provide grass cover for the wet season. Other ESC measures
such as baker tanks and portable sand filters may be required for use during the wet season. A separate
“Wet Season” ESC plan shall be submitted and approved by the City before continuing work on any site
during the wet season.
C. CONSTRUCTION WITHIN CRITICAL AREAS AND BUFFERS
Any construction that will result in disturbed areas on or within a stream or associated buffer, within a
wetland or associated buffer, or within 50 feet of a lake shall be subject to the “Critical Area Restrictions”
contained in the CSWPP Standards. These provisions include phasing the project whenever possible so
that construction in these areas is limited to the dry season.
D. MAINTENANCE
All ESC and SWPPS measures shall be maintained and reviewed on a regular basis as prescribed in the
CSWPP Standards.
E. FINAL STABILIZATION
Prior to obtaining final construction approval, the site shall be stabilized, structural ESC and SWPPS
measures (such as silt fences, sediment traps and concrete waste collection pits) shall be removed, and
drainage facilities shall be cleaned as specified in the CSWPP Standards. A separate ESC plan describing
final stabilization may be required by the City prior to implementation.
F. CONSIDERATION OF OTHER REQUIRED PERMITS
Consideration should be given to the requirements and conditions that may be applied by other agencies as
part of other permits required for land-disturbing activities. In particular, the following permits may be
required and should be considered when implementing CSWPP measures:
A Class IV Special Forest Practices Permit is required by the Washington State Department of
Natural Resources for projects that will clear more than two acres of forest or 5,000 board feet of
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timber. All such clearing is also subject to the State Environmental Policy Act (RCW 43.21C) and will
require SEPA review.
A NPDES General Permit for Construction (pursuant to the Washington State Department of
Ecology’s Construction Stormwater General Permit) is required for projects that will disturb one or
more acres for purposes of constructing or allowing for construction of a development, or projects
disturbing less than one acre that are part of a larger common plan of sale31 that will ultimately disturb
one or more acres.
1.2.6 CORE REQUIREMENT #6: MAINTENANCE AND OPERATIONS
Maintenance and operation of all drainage facilities is the responsibility of the applicant or property
owner, except those facilities for which the City assumes maintenance and operation as described below
and in RMC 4-6-030.M. Drainage facilities must be maintained and operated in accordance with the
maintenance standards in Appendix A of this manual, or other maintenance standards as approved by the
City.
Intent: To ensure that the maintenance responsibility for drainage facilities is clearly assigned and that
these facilities will be properly maintained and operated in perpetuity.
Drainage facilities serving private improvements are not allowed in public right-of-way. On-site BMPs
serving private improvements are also not allowed in the public right-of-way. Under certain situations,
drainage facilities for single family residential subdivisions with public roads may be allowed in the public
right-of-way through the City adjustment/variance process. Examples of conditions in which facilities may
be considered for placement in the public way are:
Dead end streets or cul-de-sacs where future extensions of the road is unlikely and where drainage
facilities will not conflict with existing utility improvements.
Unimproved right-of-way where future improvements are not anticipated and would not conflict with
existing or future utility improvements.
Drainage Facilities to be Maintained by the City of Renton
The City will assume maintenance and operation of the following drainage facilities 32 for any residential
subdivision with public streets, except where the City grants an adjustment per Section 1.4, allowing the
facilities to be maintained by the homeowners association:
Flow control and water quality treatment facilities within a stormwater tractor right-of-way dedicated
to the City.
On-site BMPs serving more than one lot, and serving public improvements within a stormwater tract.
Bioretention facilities in City right-of-way, mitigating for public improvements.
Where serving public improvements, on-site BMP vegetated flow paths for full dispersion within an
easement that includes provisions for access and maintenance. The City maintenance of these
vegetated flow paths will be limited to their functionality. All other maintenance shall remain the
responsibility of the owner(s).
The conveyance system within a drainage easement, tract or improved public road right-of-way
granted to the City.
31 Common plan of development or sale means a site where multiple separate and distinct construction activities may take
place at different times or on different schedules, but still under a single plan. Examples include: 1) phased projects and
projects with multiple filings or lots, even if the separate phases or filings/lots will be constructed under separate contract or by
separate owners (e.g., a development where lots are sold to separate builders); 2) a development plan that may be phased
over multiple years, but is still under a consistent plan for long-term development; and 3) projects in a contiguous area that
may be unrelated but still under the same contract, such as construction of a building extension and a new parking lot at the
same facility.
32 Note: the City of Renton does not assume maintenance of individual lot drainage systems or drainage stub-outs serving single
family residential lot downspout, footing, or yard drains.
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Note: The City may assume maintenance of facilities serving any mix of developments through an
adjustment.
The City will assume maintenance and operation of these facilities two years after final construction
approval by CED and an inspection by the City to ensure the facilities have been properly maintained and
are operating as designed.
Flow control facilities, water quality treatment facilities, and on-site BMPs to be maintained and
operated by the City, along with the required perimeter landscaping (as required per RMC 4-9-150), must
be located in a stormwater tract. For drainage facilities requiring perimeter landscaping, the stormwater
tract shall be granted and conveyed with all ownership and maintenance obligations (excluding
maintenance of the drainage facilities) to the subdivision’s lot owners. An easement under and upon said
tract shall be dedicated to the City for the purpose of operating, maintaining, and repairing the drainage
facilities contained in the stormwater tract. If perimeter landscaping is not required, then the stormwater
tract shall be dedicated to the City along with the maintenance of the drainage facility contained therein.
Required vegetated flow paths for full dispersion and basic dispersion BMPs require a recorded
declaration of covenant that stipulates restrictions on use AND shall be located in an easement that
includes provisions for access and maintenance. City maintenance of these vegetated flow paths will be
limited to their functionality. All other maintenance shall remain the responsibility of the owner(s).
Access roads serving these facilities must also be located in the tract or right-of-way and must be
connected to an improved public road right-of-way.
Conveyance systems to be maintained and operated by the City must be located in a drainage easement,
tract, or right-of-way granted to the City.
Note: the City does not normally assume maintenance responsibility for conveyance systems that are
outside of improved public road right-of-way.
Drainage Facilities to be Maintained by Private Parties
For residential subdivisions of nine lots or less with private streets, planned unit developments, and
commercial and industrial sites, maintenance and operation of flow control and water quality treatment
facilities including on-site BMPs are the responsibility of the property owner (s) and must be located in a
tract or easement that identifies each property owner as having equal and undivided interest.
Shared facilities shall be maintained jointly by the property owners or users of the facility. Shared facilities
must have a City approved maintenance plan or agreement regarding assignment of maintenance and
operation.
All drainage facilities maintained privately, by the City or by other public agencies must be maintained as
specified in Appendix A, “Maintenance Requirements for Stormwater Facilities and On-Site BMPs,” and
as further prescribed in Chapter 6 for water quality facilities, unless otherwise approved by the City. A
copy of the Operation and Maintenance Manual submitted as part of the permit application for flow
control and water quality treatment facilities (see Section 2.3.1) shall be retained on site and shall be
transferred with the property to the new owner. A log of maintenance activity indicating when cleaning
occurred and where waste was disposed of shall also be kept by the owner and be available for inspection
by the City.
All privately maintained on-site BMPs must be maintained as specified in the site/lot’s declaration of
covenant and grant of easement per Section 1.2.9.
The City shall annually inspect all privately maintained drainage facilities for compliance with these
requirements. The City may reduce the inspection frequency based on maintenance records of double the
length of time of the proposed inspection frequency. If the property owner(s) fails to maintain their
facilities to the acceptable standards, the City shall issue a written notice specifying the required remedial
actions and requiring a schedule for timely completion of the actions. If these actions are not performed in
a timely manner, the City shall enter the property to perform the actions needed and bill the property
owner(s) for the cost of the actions. If a hazard to public safety exists, the City shall perform remedial
actions without written notice.
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If the proposed project is a commercial, industrial, or multifamily development or redevelopment, or a
single family residential building permit, a drainage facility declaration of covenant and grant of
easement must be recorded at the King County Office of Records and Elections. Whenever a flow control
facility, water quality treatment facility, or on-site BMP is proposed to be located on a parcel separate
from the parcel or parcels containing the target surfaces mitigated by the facility or BMP, provisions must
be made to ensure that the owner or owners of the target surfaces have a perpetual right to operate and
maintain the facility. This may be done either by recording an easement granting this right to the owner(s)
of the target surfaces, or by conveying the land on which the facility sits (or an interest therein) to the
owner(s) of target surfaces.
If the proposed project is a residential subdivision development, all privately maintained conveyance
systems or other drainage facilities that convey flows through private property must be located in a
drainage easement dedicated to convey surface and stormwater . Individual owners of the properties
containing these easements must maintain the drainage facilities through their property. The legal
instrument creating drainage easements on private property must contain language that requires a private
property owner to obtain written approval from the City prior to removing vegetation (except by routine
mowing) from any drainage easement containing open, vegetated drainage facilities (such as swales,
channels, ditches, ponds, etc.).
Maintenance of On-Site BMPs
Maintenance and operation of all on-site BMPs are the responsibility of the property owner unless
specified above in Section 1.2.6 (Drainage Facilities to be Maintained by the City of Renton).
On-site BMPs are not allowed in City right-of-way unless constructed to mitigate for public
improvements. Maintenance and operation of on-site BMPs constructed in the right-of-way is the
responsibility of the adjacent property owner in accordance with RMC 4-6-060.
1.2.7 CORE REQUIREMENT #7:
FINANCIAL GUARANTEES AND LIABILITY
In accordance with RMC 4-6-030, CED shall require all persons constructing any surface water facilities
(including flow control/water quality facilities, conveyance systems, erosion control, and road drainage),
to post with the City of Renton a bond, assignment of funds or certified check.
The applicant must also maintain liability insurance as described in this Core Requirement #7.
Intent: To ensure financial guarantees are posted to sufficiently cover the cost of correcting, if necessary,
incomplete or substandard drainage facility construction work, and to warrant for two years the
satisfactory performance and maintenance of those newly-constructed drainage facilities. Core
Requirement #7 is also intended to ensure that a liability policy is provided that protects the proponent and
the City from any damages relating to the construction or maintenance of required drainage facilities by
private parties.
Construction Bond for Required Improvements
Before a permit, pursuant to the provisions of RMC 4-6-030, may be issued, the applicant may be required
to execute to the City a construction bond. In some instances, and at the sole option of the City, a
certificate of occupancy, final inspection, or final approval may be issued prior to completion of required
public or site improvements if an acceptable form of guarantee is provided by the applicant.
Amount of Required Construction Bond: The construction bond shall be for not less than 100 percent of
the amount calculated in the bond quantity worksheet (as provided in Reference Section 8-H) of all
required drainage improvements associated with the proposed project. The bond quantity worksheet shall
be provided by the applicant and is subject to review and acceptance by the City.
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Utilization of Funds Provided by the Construction Bond: If the required improvements associated with
the proposed project are not completed by the termination date of the construction bond, the City shall use
the bond to construct the improvements in accordance with the City’s standards.
Release of Construction Bond: The construction bond will be released when the applicant completes the
following:
Correct any defects noted in the final inspection.
Address, to the satisfaction of the City, all deficiencies noted in the final inspection by the City.
Provide to the City as-built drawings, final recorded plat, recorded easements, bill of sale, cost data
inventory of public storm system improvements to be owned and maintained by the City, and recorded
restricted covenant and grant of easement.
Receive a City Final inspection to ensure the drainage facilities have been properly installed and are
operated as desired.
Submission of maintenance bond to the City.
Maintenance Bond
Prior to acceptance by the City of any newly constructed public improvements to be deeded to the City, or
any onsite or offsite private storm drainage improvements, the applicant shall file with the City a
construction maintenance bond. The maintenance bond is to be held by the City for a period of two years.
Amount of Maintenance Bond: The maintenance bond shall be for 20 percent of the amount calculated
in the bond quantity worksheet.
Utilization of Funds Provided by the Maintenance Bond: In the event that required improvements are
not properly maintained during the required maintenance guarantee period, the City shall notify the
developer/owner. If the developer/owner fails to correct the problem within a period of 15 days, the City
shall use the maintenance guarantee to perform the maintenance work. Should any failures occur in regard
to required improvements associated with a development project within the warranty period, the City shall
require the developer/owner to correct all failures. Should the developer/owner fail to perform within a
period of 15 days, the City shall use the maintenance and warranty bond to correct any failures.
Release of Maintenance Bond: Maintenance bond will be released upon completion of the two-year
maintenance bond period following final inspection and correction of any maintenance defects identified
in the final inspection by the City.
Hold Harmless
The permittee shall protect, defend, indemnify, and save harmless the City, its officers, employees, and
agents from any and all costs, claims, judgments, or awards of damages, arising out of or in any way
resulting from the negligent acts or omissions of the permittee. The permittee agrees that its obligations
under this Section extend to any claim, demand, and/or cause of action brought by, or on behalf of, any of
its employees or agents.
Insurance Required
Before a permit shall be issued for any construction, insurance will be required as follows:
1. The applicant shall secure and maintain in force throughout the duration of the permit: Commercial
General Liability insurance written on an occurrence basis with limits no less than one million dollars
($1,000,000) per occurrence/two million dollars ($2,000,000) aggregate.
2. Copies of such insurance policy or policies shall be furnished unto the City with a special
endorsement in favor of the City with the City named as a primary and noncontributory additional
insured on the insurance policy and an endorsement stating such shall be provided to the City.
3. The policy shall provide that it will not be canceled or reduced without 30 days’ advanced written
notice to the City.
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4. Upon showing of a hardship and at the discretion of the Administrator or his/her designee, the
insurance requirements may be reduced or waived for single-family or two-family residential
applications.
Other Important Information about Core Requirement #7
Other requirements include the following:
Cash Bond Returned: The cash bond will be returned to applicant when work is accepted by the
City, less any sums due to the City under the terms of this Core Requirement #7.
Reimbursement of City’s Costs Incurred to Obtain Funds Provided by Guarantees: If the City
finds it necessary to utilize funds provided for any guarantee, and incurs expenses in obtaining and
administering such funds, a portion of these monies shall also be used to reimburse the City for such
recovery costs. If the guarantee is not adequate to cover all necessary costs, the developer/owner is
required to make up the deficit in cash within 30 days of receipt of written notice from the City.
1.2.8 CORE REQUIREMENT #8: WATER QUALITY FACILITIES
All proposed projects, including redevelopment projects, must provide water quality (WQ) facilities to
treat the runoff from those new and replaced pollution-generating impervious surfaces and new
pollution-generating pervious surfaces targeted for treatment as specified in the following sections. These
facilities shall be selected from a menu of water quality facility options specified by the area-specific
facility requirements in Section 1.2.8.1 and implemented according to the applicable WQ implementation
requirements in Section 1.2.8.2.
Intent: To require an efficient, cost-effective level of water quality treatment tailored to the sensitivities
and resource protection needs of the downstream receiving water to which the project site drains, or, in the
case of infiltration, protection of the receiving groundwater system.
Guide to Applying Core Requirement #8
Core Requirement #8 requires that WQ facilities be provided to remove pollutants from runoff discharging
from a project site in accordance with land use-specific WQ facility requirements found in Section 1.2.8.1.
For efficient application of Core Requirement #8, the following steps are recommended:
1. Check the exemption language in Section 1.2.8 to determine if or which threshold discharge areas of
the project site must provide WQ facilities per Core Requirement #8.
2. Use the Basic WQ treatment areas section (Section 1.2.8.1.A) to determine if basic or enhanced
treatment is required.
3. Consult Section 1.2.8.2 for other design requirements, allowances, and flexible compliance provisions
related to implementing water quality treatment.
4. Consult Sections 1.2.2, Core Requirement #2: Offsite Analysis, 1.2.2.1, Downstream Analysis, and
1.2.2.1.2, Downstream Water Quality Problems Requiring Special Attention.
Other Important Information about Core Requirement #8
Core Requirement #8 is the primary component of an overall water quality protection strategy required by
this manual. Other requirements include the following:
Core Requirement #4: Conveyance System, Spill Control Provisions, Section 1.2.4 — This
provision generally applies whenever a project constructs or replaces onsite conveyance system
elements that receive runoff from pollution-generating impervious surfaces. The provision requires
that runoff from such impervious surfaces be routed through a spill control device prior to discharge
from the project site or into a natural onsite drainage feature.
Core Requirement #4: Conveyance System, Groundwater Protection, Section 1.2.4 —This
provision requires that ditches/channels be lined as needed to reduce the risk of groundwater
contamination when they convey runoff from pollution-generating impervious surfaces that comes
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into direct contact with an outwash soil. Facilities that allow runoff to have direct contact with the soil
and open channel conveyance systems that are note concrete lined are not allowed in Zone 1 of the
Aquifer Protection Area.
Special Requirement #4: Source Control, Section 1.3.4 — This requirement applies water quality
source controls from the King County Stormwater Pollution Prevention Manual to commercial,
industrial, and multifamily projects.
Special Requirement #5: Oil Control, Section 1.3.5 — This requirement applies special oil controls to
those projects proposing to develop or redevelop a high-use site.
EXEMPTIONS FROM CORE REQUIREMENT #8
There are four possible exemptions from the requirement to provide a water quality facility per Core
Requirement #8:
1. Surface Area Exemption
A proposed project or any threshold discharge area within the project site is exempt if it meets all of
the following criteria:
a) Less than 5,000 square feet of new plus replaced PGIS will be created, AND
b) Less than ¾ acre of new PGPS will be added.
2. Surface Exemption for Transportation Redevelopment Projects
A proposed transportation redevelopment project or any threshold discharge area within the project
site is exempt if it meets all of the following criteria:
a) The total new impervious surface within the project limits is less than 50% of the existing
impervious surface, AND
b) Less than 5,000 square feet of new PGIS will be added, AND
c) Less than ¾ acre of new PGPS will be added.
3. Cost Exemption for Parcel Redevelopment Projects
A proposed redevelopment project on a single or multiple parcel site or any threshold discharge area
within the project site is exempt if it meets all of the following criteria:
a) The total valuation of the project’s proposed improvements (including interior improvements
and excluding required mitigation improvements) is less than 50% of the assessed value of: (a) the
existing project site improvements on commercial or industrial projects, or (b) the existing site
improvements on other projects, AND
b) Less than 5,000 square feet of new PGIS will be added, AND
c) Less than ¾ acre of new PGPS will be added.
4. Soil Treatment Exemption
A proposed project or any drainage area within a project is exempt if the runoff from pollution-
generating impervious surfaces is infiltrated in soils that meet the “groundwater protection criteria”
outlined below. These soil properties must be met by the undisturbed native soils onsite (i.e. in situ).
Soil may not be imported in order to meet groundwater protection criteria.
Groundwater Protection Criteria: The first 2 feet or more of the soil beneath an infiltration facility
must have a cation exchange capacity greater than 5 (tested using EPA Laboratory Method 9081) and
an organic content of 1.0% or greater (measured on a dry weight basis using ASTM D 2974), AND
must meet one of the following specifications for general protection of groundwater:
a) The soil must have a measured infiltration rate37 of less than or equal to 9 inches per hour,
except in groundwater protection areas where the measured rate must be less than or equal to
2.4 inches per hour, OR
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b) The soil must be composed of less than 25% gravel by weight with at least 75% of the soil
passing the #4 sieve, and the portion passing the #4 sieve must meet one of the following
gradations:
At least 50% must pass the #40 sieve and at least 2% must pass the #100 sieve, OR
At least 25% must pass the #40 sieve and at least 5% must pass the #200 sieve.33
This exemption is not allowed for areas that are infiltrated (1) within one-quarter-mile of a sensitive
lake, or (2) within one-quarter-mile of fresh water with existing or designated aquatic life use whose
land use would otherwise trigger application of a facility from the enhanced basic treatment menu, or
(3) within one-quarter-mile of a phosphorous or metals problem as described in Section 1.2.2.1.2.
1.2.8.1 LAND USE-SPECIFIC WATER QUALITY FACILITY REQUIREMENT
Projects subject to Core Requirement #8 must provide a water quality facility selected from a menu of
water quality facility options identified in the area-specific facility requirements and exceptions for the
WQ treatment area in which the proposed project or threshold discharge area of the proposed project is
located. These WQ treatment areas are listed below and their requirements and exceptions are detailed in
the following subsections:
A. Basic WQ Treatment Areas
B. Sensitive Lake WQ Treatment Areas
C. Sphagnum Bog WQ Treatment Areas.
Intent: To apply an appropriate level of water quality treatment based on the sensitivities of receiving
waters for the drainage area in which the project lies. These drainage areas are identified as WQ treatment
areas on the WQ Applications Map adopted with this manual. In addition to a minimum basic standard,
which applies broadly to most geographic areas, special menus are provided for land uses that generate the
highest concentrations of metals in stormwater and for sites within the watersheds of sensitive lakes, and
sphagnum bog wetlands.
A. BASIC WQ TREATMENT AREAS
Basic WQ Treatment Areas are designated by the City of Renton where a general, cost-effective level of
treatment is sufficient for most land uses. Most direct discharges only require Basic WQ Treatment. Some
land uses, however, will need an increased level of treatment (Enhanced Basic WQ Treatment) because
they generate high concentrations of metals in stormwater runoff and acute concentrations of metals in
streams are toxic to fish.
Required Treatment Menu
Within Basic WQ Treatment Areas, a water quality facility option from the Basic WQ menu shall be used
to treat runoff from the surfaces listed under “Target Surfaces” below, except where such treatment is
waived or reduced by the area-specific exceptions at the end of this subsection and except where the
Enhanced Basic WQ menu is applicable as follows.
If 50% or more of the runoff that drains to any proposed water quality facility is from one or more of the
following land uses, then the Enhanced Basic WQ menu shall be used in place of the Basic WQ menu
for the design of this facility, except if such treatment is waived or reduced by the area-specific exceptions
at the end of this subsection:
1. Commercial, industrial, or multifamily land use.
2. A road with an expected average daily traffic (ADT) count of 7,500 or more vehicles.
33 Measured infiltration rate shall be as measured by the EPA method or the Double Ring Infiltrometer Method (ASTM D3385). For some soils,
an infiltration rate of less than 9 inches per hour may be assumed based on a soil texture determination rather than a rate measurement. For
more details, see the “Groundwater Protection” requirements in Section 5.2.1.
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Treatment Goal and Options
The treatment goal for facility options in the Basic WQ menu is 80% removal of total suspended solids
(TSS) for flows or volumes up to and including the WQ design flow or volume for a typical rainfall year,
assuming typical pollutant concentrations in urban runoff.34 TSS is the general performance indicator for
basic water quality protection because it is the most obvious pollutant of concern. TSS is not a single
pollutant; it is a general term for a highly variable mixture of solid pollutants with variable particle size
and particle density distributions, and to one degree or another containing a variety of sorbed dissolvable
pollutants. The Basic WQ menu includes facilities such as wetponds, combined detention/wetponds,
bioswales, vegetated filter strips, and sand filters. See Chapter 6 for specific facility choices and design
details. Additional facility designs may appear in Reference Section 14 in the future.
The treatment goal for facility options in the Enhanced Basic WQ menu is to accomplish better removal
of heavy metals and potentially other toxic materials than can be achieved by basic treatment, while still
meeting the basic treatment goal of 80% TSS removal. The specific target performance is > 30% reduction
of dissolved copper and > 60% removal of dissolved zinc. Dissolved copper and zinc are indicators of a
wider range of metals typically found in urban runoff that are potentially toxic to fish and other aquatic
life. The Enhanced Basic WQ menu includes options for use of a basic-sized stormwater wetland, a large
sand filter, or a combination of two facilities in series. See Chapter 6 for specific facility options and
designs. Additional facility designs may appear in Reference S14 in the future.
Intent
The Basic WQ menu is intended to be applied to both stormwater discharges draining to surface waters
and those infiltrating into soils that do not provide adequate groundwater protection (see Exemption 4
from Core Requirement #8). Overall, the 80% TSS removal objective, in conjunction with special
requirements for source control and high-use site controls, should result in good stormwater quality for all
but the most sensitive water bodies. Increased water quality treatment is necessary for developments that
generate the highest concentrations of metals and for developments that drain to sensitive lakes and
sphagnum bog wetlands.
Facility options in the Enhanced Basic WQ menu are intended to remove more metals than expected
from those in the Basic WQ menu. Lower metal concentrations reduce the risk to fish from exposure to
both chronic and acute toxic concentrations of metals such as copper and zinc, and very low concentration
copper deleterious olfactory effects. As the toxicity of metals depends on their concentration, this standard
is most effective for project sites with a larger proportion of pollution-generating impervious surface like
roadways and medium to high density subdivisions. The Enhanced Basic WQ menu is intended to apply to
all such project sites that drain by surface flows to a fish-bearing stream. However, projects that drain
entirely by pipe to the major receiving waters listed Table 1.2.3.B may be excused from the increased
treatment and may revert to the Basic WQ menu because concentration effects are of less concern as the
overall flow volume increases; however, this exception is not applicable for metals impaired segments per
Section 1.2.2.1: Downstream Analysis, and 1.2.2.1.2: Downstream Water Quality Problems Requiring
Special Attention, Metals Problem (Type 4).
Target Surfaces
Facilities in Basic WQ Treatment Areas must treat (either directly or in effect) the runoff from the
following target surfaces within the threshold discharge area for which the facility is required:
1. New PGIS that is not fully dispersed per the Criteria for Fully Dispersed Surfaces (see Section
1.2.3.2.C) in Core Requirement #3. For individual lots within residential subdivision projects, the
extent of new PGIS shall be assumed based on expected driveway size as approved by CED.
2. New PGPS that is not fully dispersed and from which there will be a concentrated surface discharge in
a natural channel or man-made conveyance system from the site. For individual lots within residential
34 The influent concentration range for demonstrated pollutant removal is 100 to 200 mg/L. For influent concentrations lower than
100 mg/l the effluent goal is equal to or less than 20 mg/l. For influent concentrations greater than 200 mg/l, the goal is greater
than 80% TSS removal.
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subdivision projects, the extent of new pervious surface shall be assumed to be the entire lot area,
except the assumed impervious portion as specified in Chapter 3 and any portion in which native
conditions are preserved by covenant, tract, or easement.
3. Replaced PGIS that is not fully dispersed on a non-redevelopment project.
4. Replaced PGIS that is not fully dispersed on a transportation redevelopment project in which new
impervious surface is 5,000 square feet or more and totals 50% or more of the existing impervious
surface within the project limits.
5. Replaced PGIS that is not fully dispersed on a parcel redevelopment project in which the total of new
plus replaced impervious surface is 5,000 square feet or more and whose valuation of proposed
improvements (including interior improvements and excluding required mitigation improvements)
exceeds 50% of the assessed value of: (a) the existing project site improvements on commercial or
industrial projects, or (b) the existing site improvements on other projects.
Exceptions
The following exceptions apply only in Basic WQ Treatment Areas:
1. Exception #1 does not apply to the City
2. The Enhanced Basic WQ menu as specified above for certain land uses may be reduced to the Basic
WQ menu for treatment of any runoff that is infiltrated per the standards of Section 5.2. This
exception is not allowed where infiltrating into soils that do not meet the groundwater protection
standards described in Section 5.2.1, if within one-quarter-mile of a fresh water designated for aquatic
life use or that has an existing aquatic life use.
3. The Enhanced Basic WQ menu as specified above for certain land uses may be reduced to the Basic
WQ menu for treatment of any runoff that is discharged directly, via a non-fish-bearing conveyance
system, all the way to the ordinary high water mark of a stream with a mean annual flow of 1,000 cfs
or more (at the discharge point of the conveyance system), a lake that is 300 acres or larger, or a
waterbody that is listed as a major receiving water per Table 1.2.3.B. This exception does not apply
where the receiving water is impaired for metals per Section 1.2.2.1: Downstream Analysis, and
1.2.2.1.2: Downstream Water Quality Problems Requiring Special Attention, Metals Problem (Type
4).
4. The Enhanced Basic WQ menu as specified above for treating runoff from a commercial land use may
be reduced to the Basic WQ menu if all of the following criteria are met:
a) A facility from the Enhanced Basic WQ menu is not feasible, AND
b) No leachable heavy metals are currently used or proposed to be used in areas of the site,
exposed to the weather, AND
c) A covenant is recorded that prohibits future such use of leachable, heavy metals on the site
(use the covenant in Reference Section 8-Q), AND
d) Less than 50% of the runoff draining to the proposed water quality facility is from any area of
the site comprised of one or both of the following land uses:
Commercial land use with an expected ADT of 100 or more vehicles per 1,000 square
feet of gross building area.
Commercial land use involved with vehicle repair, maintenance, or sales.
5. The facility requirement as applied to replaced PGIS may be waived if the City has adopted a plan
and implementation schedule for fulfilling this requirement using regional facilities.
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B. SENSITIVE LAKE WQ TREATMENT AREAS
There are no Sensitive Lake WQ Treatment Areas in the City at the time this manual was adopted;
however, this section has been retained in case of future changes in lake status.
Required Treatment Menu
Within Sensitive Lake WQ Treatment Areas, a water quality facility option from the Sensitive Lake
Protection menu shall be used to treat runoff from the surfaces listed under “Target Surfaces” below,
except where such treatment is waived or reduced by the area-specific exceptions at the end of this
subsection and except where the Enhanced Basic WQ menu is applicable as follows. If 50% or more of the
runoff that drains to any proposed water quality facility is from one or more of the following land uses,
then a water quality facility option common to both the Sensitive Lake Protection menu and Enhanced
Basic WQ menu shall be used for the design of this facility, except if such treatment is waived or reduced
by the area-specific exceptions at the end of this subsection:
1. Commercial, industrial, or multifamily land use.
2. A road with an expected average daily traffic (ADT) count of 7,500 or more vehicles.
Treatment Goal and Options
The treatment goal for facility options in the Sensitive Lake Protection menu is 50% annual average total
phosphorus (TP) removal assuming typical pollutant concentrations in urban runoff.35 This goal was
chosen as a realistic and cost-effective level of phosphorus removal. The Sensitive Lake Protection menu
includes options for using either Basic WQ facilities of larger size, combinations of two facilities in
series,36 or a single facility in combination with land use planning elements that reduce phosphorus. See
Chapter 6 for specific facility options and design details.
On some developments or portions thereof that have surface uses that generate the highest concentrations
of metals in stormwater runoff, the treatment goal is expanded to include > 30% reduction of dissolved
copper and > 60% removal of dissolved zinc. This expanded goal requires use of a water quality facility
option that is common to both the Sensitive Lake Protection menu and the Enhanced Basic menu.
Intent
A project discharging runoff via surface flow contributes phosphorus loading to a sensitive lake regardless
of distance from the lake. If discharge is via infiltration through coarse soils, it is also possible that
phosphorus would be transported through the ground for some distance without attenuation. This
groundwater transport distance is considered to be typically no more than one-quarter mile. Therefore,
onsite treatment using the Sensitive Lake Protection menu is required prior to infiltration within one-
quarter mile of a sensitive lake. Infiltration through finer soils is expected to provide significant
attenuation of TP, so the general groundwater protection criteria specified in Section 1.2.8 under “Soil
Treatment Exemption” are considered sufficient for infiltration through finer soils.
Where the treatment goal is expanded to include > 30% reduction of dissolved copper and > 60% removal
of dissolved zinc, the facility options common to both the Sensitive Lake Protection menu and the
Enhanced Basic WQ menu should meet this goal as well as the lake protection goal of 50% removal of
annual average total phosphorous. The intent behind the enhanced heavy metals removal goal and why it is
applied is described in Section 1.2.8.1.
35 Phosphorus concentrations of between 0.10 and 0.50 mg/L are considered typical of Seattle area runoff (Table 1, “Water
Quality Thresholds Decision paper,” King County Surface Water Management Division, April 1994).
36 In series means that the entire treatment water volume flows from one facility to the other in turn.
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Target Surfaces
Facilities in Sensitive Lake WQ Treatment Areas must mitigate (either directly or in effect) the runoff
from the following target surfaces within the threshold discharge area for which the facility is required:
1. New PGIS that is not fully dispersed per the Criteria for Fully Dispersed Surfaces (see Section
1.2.3.2.C) in Core Requirement #3. For individual lots within residential subdivision projects, the
extent of new PGIS shall be assumed based on expected driveway size as approved by CED.
2. New PGPS that is not fully dispersed and from which there will be a concentrated surface discharge
in a natural channel or man-made conveyance system from the site. For individual lots within
residential subdivision projects, the extent of new pervious surface shall be assumed to be the entire
lot area, except the assumed impervious portion as specified in Chapter 3 and any portion in which
native conditions are preserved by covenant, tract, or easement. Note: where the runoff from target
PGPS is separated from the runoff from target PGIS, the Basic WQ menu may be used in place of the
Sensitive Lake Protection menu for treatment of runoff from the target PGPS (see the area-specific
exceptions at the end of this subsection).
3. Replaced PGIS that is not fully dispersed, on a non-redevelopment project.
4. Replaced PGIS that is not fully dispersed on a transportation redevelopment project in which new
impervious surface is 5,000 square feet or more and totals 50% or more of the existing impervious
surface within the project limits.
5. Replaced PGIS that is not fully dispersed, on a parcel redevelopment project in which the total of
new plus replaced impervious surface is 5,000 square feet or more and whose valuation of proposed
improvements (including interior improvements and excluding required mitigation improvements)
exceeds 50% of the assessed value of: (a) the existing project site improvements on commercial or
industrial projects, or (b) the existing site improvements on other projects.
Exceptions
The following exceptions apply only in Sensitive Lake WQ Treatment Areas:
1. The Basic WQ menu may be used in place of the Sensitive Lake Protection menu for treatment of
any runoff that is infiltrated according to the standards in Section 5.2. This exception is not allowed
where infiltrating into soils that do not meet the groundwater protection standards described in Section
5.2.1, if within one-quarter-mile of a phosphorous sensitive receiving water or a tributary to that
receiving water.
2. Application of the Enhanced Basic WQ menu as specified above for certain land uses may be
waived for treatment of any runoff that is infiltrated according to the standards in Section 5.2 (A
facility from the Sensitive Lake Protection menu is still required unless that requirement has been
reduced to the Basic WQ Menu by another exception). This exception is not allowed where infiltrating
into soils that do not meet the groundwater protection standards described in Section 5.2.1, if within
one-quarter-mile of a fresh water designated for aquatic life use or that has an existing aquatic life use.
3. Application of the Enhanced Basic WQ menu as specified above for certain land uses may be
waived for treatment of any runoff that is discharged, via a non-fish-bearing conveyance system, all
the way to the ordinary high water mark of a stream with a mean annual flow of 1,000 cfs or more (at
the discharge point of the conveyance system), a lake that is 300 acres or larger, or a waterbody that is
listed as a major receiving water per Table 1.2.3.B (A facility from the Sensitive Lake Protection menu
is still required unless that requirement has been reduced to the Basic WQ Menu by another
exception). This exception is not applicable for WQ impaired segments per Section 1.2.2.1:
Downstream Analysis, and 1.2.2.1.2: Downstream Water Quality Problems Requiring Special
Attention, Metals Problem (Type 4).
4. The Enhanced Basic WQ menu as specified above for treating runoff from a commercial land use
may be waived (A facility from the Sensitive Lake Protection menu is still required unless that
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requirement has been reduced to the Basic WQ Menu by another exception) if the all of the following
criteria are met:
a) No leachable metals (e.g., galvanized metals) are currently used or proposed to be used in areas of
the site, exposed to the weather, AND
b) A covenant is recorded that prohibits future such use of leachable metals on the site, exposed to
the weather (use the covenant in Reference Section 8-Q), AND
c) Less than 50% of the runoff draining to the proposed water quality facility is from any area of the
site comprised of one or both of the following land uses:
Commercial land use with an expected ADT of 100 or more vehicles per 1,000 square feet of
gross building area.
Commercial land use involved with vehicle repair, maintenance, or sales.
5. The Basic WQ menu may be used for treatment of any runoff from target PGPS that is treated
separately from the runoff from target PGIS.
6. Exception #6 does not apply in the City.
7. The facility requirement as applied to replaced PGIS may be waived if the City has adopted a plan
and implementation schedule for fulfilling this requirement using regional facilities.
Note: If a lake management plan has been prepared and adopted by the City, additional treatment and/or
other water quality measures may be required as specified in the plan and pursuant to Special
Requirement #1, Section 1.2.9.
C. SPHAGNUM BOG WQ TREATMENT AREAS
There are no Sphagnum Bog WQ Treatment Areas in the City at the time this manual was adopted;
however, this section has been retained in case of future changes. Sphagnum Bog WQ Treatment Areas
are areas of King County from which runoff drains to or otherwise comes into contact with the vegetation
of a sphagnum bog wetland37 larger than 0.25 acres in size.38 These wetlands support unique vegetation
communities, and they tend to develop in areas where water movement is minimized. Although sphagnum
bog wetlands are typically isolated from significant sources of surface and ground water and receive their
main water supply from rainfall, there are instances where they are components of larger wetlands and
may be subject to inundation by those wetlands during high intensity or long duration runoff events.
Sphagnum bog wetlands are generally uncommon in the Puget Sound area; of all the inventoried wetlands
in King County, only a small percentage have sphagnum bog wetland components.39
Only a portion of all sphagnum bog wetlands have been identified and mapped by King County.
Consequently, many of these wetlands and their contributing drainage areas must be identified during the
wetland identification and delineation for a project site and during offsite analysis as required in Core
Requirement #2. A list of identified sphagnum bog wetlands is included on the WQ Applications Map and
in the 1997 King County Bog Inventory, updated November 2002, available at
http://your.kingcounty.gov/dnrp/library/2002/kcr249-2002.pdf ; however, if a wetland that meets the
definition of a sphagnum bog wetland is found downstream of a project site and runoff from the project
site drains to or otherwise comes into contact with the wetland’s vegetation, the project site is considered
to be within a Sphagnum Bog WQ Treatment Area whether the wetland is listed or not.
37 A sphagnum bog wetland is defined as a wetland dominated by sphagnum moss and which has an associated acid-loving
plant community. See the "Definitions" section for more details on how King County defines a sphagnum bog wetland.
38 The size of a sphagnum bog wetland is defined by the boundaries of the sphagnum bog plant community.
39 Approximately 3% of wetlands in the 1990 sensitive areas inventory are either sphagnum bog wetlands or include portions of a
lake or wetland with sphagnum bog wetland characteristics.
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Note: Any threshold discharge area from which runoff drains to or comes into contact with the vegetation
of a sphagnum bog wetland larger than 0.25 acres in size is considered to be within a Sphagnum Bog WQ
Treatment Area regardless of the WQ treatment area indicated by the WQ Applications Map.
Required Treatment Menu
A treatment option from the Sphagnum Bog Protection menu shall be used to treat runoff from the target
surfaces specified below, except where this mitigation is waived or reduced by the area-specific exceptions
at the end of this subsection.
Treatment Goals and Options
The treatment goals for protection of sphagnum bog wetlands include the control of nutrients, alkalinity,
and pH. Although these goals may change as additional information about these wetlands becomes
available, target pollutant removals for sphagnum bog protection are currently as follows:
Total phosphorus reduction of 50%
Nitrate + nitrite reduction of 40%
pH below 6.5
Alkalinity below 10 mg CaCO3/L.
Facility options to meet these goals are limited; therefore, the City discourages developments from
discharging runoff to sphagnum bog wetlands. Where infiltration of developed area runoff is not feasible
or applicable per Section 5.2, water quality facility options include a treatment train 40 of two or three
facilities in series. One of the facilities in the train must be a sand filter. The order of facilities in the
treatment train is important; see Chapter 6 for specific facility options and design details.
Intent
Sphagnum bog wetlands support unique vegetation communities that are extremely sensitive to changes in
alkalinity and nutrients from surface water inputs. The most effective way to prevent these changes is to
infiltrate or redirect developed area runoff so it does not come into contact with the vegetation of a
sphagnum bog wetland. However, this is not practicable for most development projects due to soil
constraints precluding infiltration (see Section 5.2) and the onerous nature of bypassing runoff around a
wetland. Therefore, where runoff contact with sphagnum bog vegetation cannot be avoided, the bog
protection menu seeks to minimize certain changes in the chemistry of developed area runoff to protect
this unique vegetation. This menu applies not only to runoff that drains directly to a sphagnum bog
wetland but to runoff that otherwise comes into contact with the bog’s vegetation, such as through
inundation of the bog by an adjacent water body during high intensity or long duration runoff events.
While water quality facility options emphasize reduction of mineral elements (alkalinity) and nutrients in
the runoff, little is known about their ability to reduce alkalinity or to actually protect sphagnum-based
plant communities. In addition, the effect of frequent water level changes on the sphagnum plant
community is also unknown but could be damaging. Hence, it is best to avoid discharge to sphagnum bog
wetlands whenever possible. Permeable pavements that are tributary to sphagnum bog wetlands should be
types other than Portland cement (PCC) permeable pavement, if feasible.
Target Surfaces
Facilities in Sphagnum Bog WQ Treatment Areas must mitigate (either directly or in effect) the runoff
from the following target surfaces within the threshold discharge area for which the facility is required:
1. New PGIS that is not fully dispersed per the Criteria for Fully Dispersed Surfaces (p. 1-46) in Core
Requirement #3. For individual lots within residential subdivision projects, the extent of new PGIS
shall be assumed based on expected driveway size as approved by CED.
40 A treatment train is a combination of two or more treatment BMPs connected in series (i.e., the design water volume passes
through each facility in turn).
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2. New PGPS that is not fully dispersed and from which there will be a concentrated surface discharge
in a natural channel or man-made conveyance system from the site. For individual lots within
residential subdivision projects, the extent of new pervious surface shall be assumed to be the entire
lot area, except the assumed impervious portion as specified in Chapter 3 and any portion in which
native conditions are preserved by covenant, tract, or easement.
3. Replaced PGIS that is not fully dispersed, on a non-redevelopment project.
4. Replaced PGIS that is not fully dispersed on a transportation redevelopment project in which new
impervious surface is 5,000 square feet or more and totals 50% or more of the existing impervious
surface within the project limits.
5. Replaced PGIS that is not fully dispersed on a parcel redevelopment project in which the total of
new plus replaced impervious surface is 5,000 square feet or more and whose valuation of proposed
improvements (including interior improvements and excluding required mitigation improvements)
exceeds 50% of the assessed value of: (a) the existing project site improvements on commercial or
industrial projects, or (b) the existing site improvements on other projects.
Exceptions
The following exceptions apply only in Sphagnum Bog WQ Treatment Areas:
1. The Basic WQ menu may be used in place of the Sphagnum Bog Protection menu for treatment of
any runoff that is infiltrated in a facility per Section 5.2. This exception is not allowed where
infiltrating into soils that do not meet the groundwater protection standards described in Section 5.2.1,
if within one-quarter-mile of a phosphorous sensitive receiving water or a tributary to that receiving
water. If the infiltration facility is located in soils not meeting the groundwater protection standards
described in Section 5.2.1, and within the prescribed distance of a sensitive lake, then the Sensitive
Lake Protection menu shall be used.
2. The facility requirement for Sphagnum Bog WQ Treatment Areas may be reduced to that of the
surrounding WQ treatment area (i.e., either the Basic WQ Treatment Area or Sensitive Lake
Treatment Area, whichever contains the Sphagnum Bog WQ Treatment Area) for treatment of any
replaced PGIS runoff.
1.2.8.2 WATER QUALITY IMPLEMENTATION REQUIREMENTS
Water quality facilities shall be designed and implemented in accordance with the following requirements,
allowances, and flexible compliance provisions:
A. METHODS OF ANALYSIS AND DESIGN
Water quality facilities shall be analyzed and designed as detailed in Chapter 6.
B. SITING OF WATER QUALITY FACILITIES
Required water quality facilities shall be located so as to treat the runoff from all target surfaces, except as
allowed below under “Treatment Trades” and “Untreated Discharges.”
Any other onsite or offsite runoff draining to a proposed water quality facility must be treated whether it is
from a target pollution-generating surface or not and regardless of whether the runoff has already been
treated by another facility. The facility must be sized for all flows/volumes entering the facility. This is
because treatment effectiveness is determined in part by the total volume of runoff entering the facility.
C. TREATMENT TRADES
The runoff from target pollution-generating surfaces may be released untreated if an existing non-
targeted pollution-generating surface of equivalent size and pollutant characteristics lying within the same
watershed or stream reach tributary area is treated on the project site. Such substitution is subject to all of
the following restrictions:
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1. The existing non-targeted pollution-generating surface is not currently being treated, is not required to
be treated by any phase of the proposed project, is not subject to NPDES or other permit requirements,
and is not under a compliance order or other regulatory action.
2. The existing non-targeted pollution-generating surface that is treated for purposes of the treatment
trade must be documented and tracked by CED. Documentation should clarify that future
redevelopment of the existing non-targeted, treated area used for the treatment trade will incur
additional water quality treatment requirements if the redevelopment exceeds Core Requirement #8
thresholds. Any additional water quality treatment triggered by redevelopment of the non-targeted,
treated area must be achieved by implementing an additional treatment trade.
3. The proposal is reviewed and approved by CED.
D. UNTREATED DISCHARGES
If site topographic constraints are such that runoff from a target pollution-generating surface must be
pumped to be treated by the required water quality facility, then CED may allow the area’s runoff to be
released untreated provided that all of the following conditions are met:
1. Treatment of the constrained area by filter strip, bioswale, or a linear sand filter is not feasible, and a
treatment trade as described above is not possible, AND
2. The untreated target surface is less than 5,000 square feet of new plus replaced PGIS.
E. USE OF PROPRIETARY FACILITIES
Water quality facilities other than those identified in Chapter 6, Reference Section 14-A, or Reference
Section 14-B may be allowed if it can be demonstrated that they are likely to meet the pollutant removal
goal for the applicable receiving water. Use of such facilities requires an adjustment, which requires
approval by the City according to Section 1.4, “Adjustment Process,” and Section 6.7, “Alternative
Facilities.” Any new treatment technologies must be approved through the state Department of Ecology’s
TAPE41 program before the technology can be considered by the City. Monitoring will be required, the
nature of which will depend on the pre-existing Ecology use-level designation, the number of existing
facilities of this design for which monitoring data already exists, and review of the monitoring results from
those facilities. When sufficient data on performance and maintenance requirements have been collected
and if both are acceptable, the new facility may be added to the appropriate water quality menu for
common use through a blanket adjustment or update of this manual. Criteria may be set, which if not met,
may require replacement of the facility with a standard facility from Chapter 6.
F. OWNER RESPONSIBILITY FOR WATER QUALITY
Regardless of the means by which a property owner chooses to meet the water quality requirements of this
manual – whether a water quality facility, a train of facilities, or a treatment trade – it is the responsibility
of the property owner to ensure that runoff from their site does not create water quality problems or
degrade beneficial uses downstream. It is also the responsibility of the property owner to ensure that the
discharge from their property is not in violation of state and federal laws.
1.2.9 CORE REQUIREMENT #9: ON-SITE BMPS
All proposed projects, including redevelopment projects, must provide on-site BMPs to mitigate the
impacts of storm and surface water runoff generated by new impervious surface, new pervious surface,
existing impervious surfaces, and replaced impervious surface targeted for mitigation as specified in the
following sections. On-site BMPs must be selected and applied according to the basic requirements,
41 Ecology W, 2011. Technical Guidance Manual for Evaluating Emerging Stormwater Treatment Technologies: Technology
Assessment Protocol – Ecology (TAPE), Publication No. 11-10-061, 2011 ed. Washington State Department of Ecology,
Lacey, WA, pp. 1–73. <https://fortress.wa.gov/ecy/publications/summarypages/1110061.html>.
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procedures, and provisions detailed in this section and the design specifications for each BMP in Appendix
C, Section C.2.
On-site BMPs are methods and designs for dispersing, infiltrating, or otherwise reducing or preventing
development-related increases in runoff at or near the sources of those increases. On-site BMPs include,
but are not limited to, preservation and use of native vegetated surfaces to fully disperse runoff; use of
other pervious surfaces to disperse runoff; roof downspout infiltration; permeable pavements;
bioretention; limited infiltration systems; and reduction of development footprint.
Intent: To provide mitigation of hydrologic impacts that are not possible/practical to mitigate with a flow
control facility. Such impacts include increases in runoff volumes and peak discharges and decreases in
groundwater recharge. Increased runoff volume and peak discharges leads to higher and more variable
stream velocities at low flows and more frequent water level fluctuations in streams and wetlands. This
causes wash-out and stranding of aquatic species, algal scour and washout of organic matter, loss of
vegetation diversity and habitat quality, and disruption of cues for spawning, egg hatching, and migration.
Decreased groundwater recharge reduces water supply for human use and summer base flows in streams,
which is critical to water temperature, salmonid use of smaller streams, and the habitat quality of
mainstem side channels and wetlands used for spawning, rearing, and flood refuge. On-site BMPs seek to
reduce runoff volumes and flashiness and increase groundwater recharge by reducing imperviousness and
making use of the pervious portions of development sites to maximize infiltration and retention of
stormwater onsite. Thus, the goal is to apply on-site BMPs to new impervious surfaces, new pervious
surfaces, and replaced impervious surfaces, to the maximum extent feasible without causing flooding or
erosion impacts.
EXEMPTIONS FROM CORE REQUIREMENT #9
There are two exemptions from the on-site BMP provisions of Core Requirement #9:
1. Basic Exemption
A proposed project is exempt if it meets the following criteria:
a) Less than 2,000 square feet of new plus replaced impervious surface will be created, AND
b) Less than 7,000 square feet of land disturbing activity will occur.
2. Infiltration Flow Control Facility Exemption
Any impervious surface served by an infiltration facility designed in accordance with the flow control
facility requirement (Section 1.2.3.1), the facility implementation requirements (Section 1.2.3.2), and
the design criteria for infiltration facilities (Section 5.2) is exempt from the on-site BMP requirement.
1.2.9.1 ON-SITE BMP REQUIREMENTS OVERVIEW
Projects that are subject to Core Requirement #9 must apply on-site BMPs to either supplement the flow
mitigation provided by required flow control facilities or provide flow mitigation where flow control
facilities are not required. All such on-site BMPs are detailed in Appendix C of this manual. On-site
BMPs must be implemented per the requirements and approach detailed in Sections 1.2.9.2 and 1.2.9.3
below for individual lots and subdivisions or road improvement projects, respectively. As described within
Sections 1.2.9.2 and 1.2.9.3, there are two methods of satisfying the on-site BMP requirement:
(1) application of BMPs to the maximum extent feasible using lists specific to the project location, size,
and impervious coverage; or (2) using a continuous runoff model to demonstrate compliance with the Low
Impact Development (LID) Performance Standard, described below. Demonstrating compliance with the
LID Performance Standard using modeling is an optional method for all projects.
A. TARGET SURFACES
Target surfaces for application of Core Requirement #9 (On-site BMPs) include new impervious surfaces,
new pervious surfaces, and replaced impervious surfaces.
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Projects that trigger Core Requirement #9 by disturbing 7,000 square feet or more of land, but where new
plus replaced impervious surface is less than 2,000 square feet, may consider basic dispersion as an equal
choice for treating the target impervious surfaces alongside full infiltration, limited infiltration,
bioretention, and permeable pavement. These projects are not required to meet the minimum BMP
implementation requirements described in “Small Lot BMP Requirements” and “Large Lot BMP
Requirements,” (Requirement #5 on both lists), and are not required to comply with Core Requirement #6 .
Target pervious surfaces must be protected in accordance with the soil amendment BMP as detailed in
Appendix C, Section C.2.13.
Projects or threshold discharge areas of projects qualifying as exempt from the flow control facility
requirement using the Direct Discharge Exemption in accordance with Section 1.2.3.1 do not have to
achieve the LID Performance Standard (described below), nor consider bioretention, permeable
pavement, and full dispersion. However, target pervious surfaces must be protected in accordance with
the soil amendment BMP as detailed in Appendix C, Section C.2.13; and target impervious surfaces must
implement full infiltration as detailed in Appendix C, Section C.2.2, Basic Dispersion per Appendix C,
Section C.2.4; perforated pipe connection as detailed in Appendix C, Section C.2.11 for roofs, if feasible;
and Basic Dispersion per Appendix C, Section C.2.4 for other impervious surfaces, if feasible.
B. LOW IMPACT DEVELOPMENT PERFORMANCE STANDARD
The LID Performance Standard is defined as follows:
For the target surfaces subject to Core Requirement #9, Stormwater discharges shall match developed
discharge durations to pre-developed durations for the range of pre-developed discharge rates from 8% of
the 2-year peak flow to 50% of the 2-year peak flow. Assume historical site conditions as the
predeveloped condition.
Projects that opt to demonstrate compliance with the LID Performance Standard using a continuous runoff
model must protect the soil moisture capacity of new pervious in accordance with the soil amendment
BMP as detailed in Appendix C, Section C.2.13. Additionally, any proposed connection of roof
downspouts to the local drainage system must be via a perforated pipe connection as detailed in Appendix
C, Section C.2.11.
Projects that are required or opt to model compliance with the LID Performance Standard are still subject
to meeting applicable area specific flow control requirements as determined in Core Requirement #3
(Section 1.2.3).
Note that when demonstrating compliance with the LID Performance Standard, on-site BMPs are modeled
explicitly, utilizing design infiltration rates as determined and selected per Section 5.2.1. However, when
modeling flow control facility sizing, water quality facility sizing, and the peak flow exceptions from the
area-specific flow control facility requirement in Sections 1.2.3.1.A, B, and C, these BMPs are not
modeled explicitly, but may use modeling credits as allowed and subject to the limitations described in
Section 1.2.9.4 and Table 1.2.9.A. On-site BMPs used to demonstrate compliance with the LID
Performance Standard must meet the implementation requirements described in Section 1.2.9.4.
C. DEMONSTRATING COMPLIANCE WITH THE LID PERFORMANCE STANDARD
Project applicants may opt to use the LID Performance Standard in lieu of the BMP selection and
application requirements described in Sections 1.2.9.2 and 1.2.9.3 below.
D. IMPLEMENTATION
Four kinds of implementation for the on-site BMP requirement are described in this section as follows:
1. For non-subdivision projects making improvements on an individual site/lot, implementation of
this requirement shall be in accordance with the “Individual Lot BMP Requirements” in Section
1.2.9.2, which specify the selection of BMPs and the extent of their application on the site/lot. This
required implementation of on-site BMPs must occur as part of the proposed project and provisions
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must be made for their future maintenance as specified in Section 1.2.9.2. As allowed in Sections
1.2.3 and 1.2.8, credits for the application of on-site BMPs per Table 1.2.9.A may be used to reduce
the size of a required flow control facility, reduce the size of a water quality facility, qualify for a flow
control facility exception or bypass of target surfaces, or reduce the target surfaces subject to flow
control or water quality facility requirements.
2. Subdivision projects and road improvement projects on sites that are 5 acres or larger AND
located outside the Urban Growth Area (UGA). This requirement does not apply to the City of
Renton.
3. For subdivision projects, implementation of on-site BMPs for associated plat infrastructure
improvements (e.g., roads, sidewalks) shall be done per Section 1.2.9.3 and must occur concurrently
and as part of the proposed project, while BMPs associated with the individual lot improvements may
be delayed until construction on the lots. As allowed in Sections 1.2.3 and 1.2.8, credits for the
application of on-site BMPs per Table 1.2.9.A. may be used to reduce the size of a required flow
control facility, reduce the size of a water quality facility, qualify for a flow control facility exception
or bypass of target surfaces, or reduce the target surfaces subject to flow control or water quality
facility requirements. To use these credits, on-site BMPs must be implemented as part of the proposed
project and provisions must be made for their future maintenance as specified in Section 1.2.9.4. For
subdivision projects proposing to take credit for future implementation of BMPs on individual lots,
provisions must be made to ensure their implementation as specified in Section 1.2.9.4.
4. For road improvement projects, implementation of on-site BMPs must occur as part of the
proposed project. As allowed in Sections 1.2.3 and 1.2.8, credits for the application of on-site BMPs
per Table 1.2.9.A may be used to reduce the size of a required flow control facility, reduce the size of
a water quality facility, qualify for a flow control facility exception or bypass of target surfaces, or
reduce the target surfaces subject to flow control or water quality facility requirements. To use these
credits, on-site BMPs must be implemented as part of the proposed project and provisions must be
made for their future maintenance as specified in Section 1.2.9.4.
The information presented in this section is organized as follows:
Section 1.2.9.2, “Individual Lot BMP Requirements”
“Small Lot BMP Requirements,” Section 1.2.9.2.1
“Large Lot BMP Requirements,” Section 1.2.9.2.2
“Large Rural Lot BMP Requirements,” Section 1.2.9.2.3
“Implementation Requirements for Individual Lot BMPs,” Section 1.2.9.2.4
Section 1.2.9.3, “Subdivision and Road Improvement Projects BMP Requirements”
“Small Subdivision Project BMP Requirements,” Section 1.2.9.3.1
“Small Road Improvement and Urban Road Improvement Projects BMP
Requirements,” Section 1.2.9.3.2
“Large Rural Subdivision and Large Rural Road Improvement Projects BMP
Requirements,” Section 1.2.9.3.3
Section 1.2.9.4, “Requirements for Use of BMP Credits”
“Use of Credits by Subdivision Projects,” Section 1.2.9.4.1
“Use of Credits by Projects within Rights-of-Way,” Section 1.2.9.4.2
1.2.9.2 INDIVIDUAL LOT BMP REQUIREMENTS
For projects on individual sites/lots, on-site BMPs must be selected and applied according to the
individual lot BMP requirements in this section. For purposes of applying on-site BMPs to individual
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sites/lots, three categories of requirements have been established based on the size of site/lot subject to
improvements by the project, and the extent of impervious surface coverage resulting from the project on
the site/lot. These categories of requirements are as follows:
Small Lot BMP Requirements (for sites/lots <22,000 square feet)
Large Lot BMP Requirements (for sites/lots 22,000 square feet)
On-site BMPs must be applied in the order of preference and to the extent specified for the category of
individual lot requirements applicable to the proposed project as described in the following subsections.
Note: for lots created by a previous subdivision, some or all of these requirements may have been
addressed by on-site BMPs installed on the lots or within common areas, tracts, or road right-of-way. In
some cases, the type of BMPs required for a subdivision lot have already been established by a recorded
covenant on the lot. See Section 1.2.9.4 for more information on pre-installed or pre-determined BMPs in
subdivisions.
1.2.9.2.1 SMALL LOT BMP REQUIREMENTS
IF the proposed project is on a site/lot smaller than 22,000 square feet, THEN on-site BMPs must be
applied as specified in the requirements below OR the project must demonstrate compliance with the LID
Performance Standard (described in Section 1.2.9.1.B) using an approved continuous runoff model.
Projects on small lots are typically single family residential improvements (e.g., homes, outbuildings, etc.)
but could be a small commercial development.
1. The feasibility and applicability of full dispersion as detailed in Appendix C, Section C.2.1 must be
evaluated for all target impervious surfaces. If feasible and applicable, full dispersion must be
implemented as part of the proposed project. Typically, small lot full dispersion will be applicable
only in subdivisions where enough forest was preserved by tract, easement, or covenant to meet the
minimum design requirements for full dispersion in Appendix C, Section C.2.1.1
2. Where full dispersion of target impervious roof areas is not feasible or applicable, or will cause
flooding or erosion impacts, the feasibility and applicability of full infiltration as detailed in Appendix
C, Section C.2.2 must be evaluated (note, this will require a soils report for the site/lot). If feasible
and applicable, full infiltration of roof runoff must be implemented as part of the proposed project.
3. All target impervious surfaces not mitigated by Requirements 1 and 2 above, must be mitigated to the
maximum extent feasible using one or more BMPs from the following list. Use of a given BMP is
subject to evaluation of its feasibility and applicability as detailed in Appendix C. Feasible BMPs are
required to be implemented. The BMPs listed below may be located anywhere on the site/lot subject
to the limitations and design specifications for each BMP. These BMPs must be implemented as part
of the proposed project.
Full Infiltration per Appendix C, Section C.2.2, or per Section 5.2, whichever is applicable
Limited Infiltration per Appendix C, Section C.2.3,
Rain Gardens per Appendix C, Section C.2.12, sized as follows:
o Rain gardens have a maximum contributing area of 5,000 square feet.
o Rain gardens must have a minimum horizontal projected surface area below the overflow
that is at least 5% of the area draining to it.
Bioretention per Appendix C, Section C.2.6, sized as follows:
o SeaTac regional scale factor equals 1.0: In till soils, provide bioretention volume based on
0.6 inches of equivalent storage depth; in outwash soils provide bioretention volume based
on 0.1 inches of equivalent storage depth,
o SeaTac regional scale factor greater than 1.0: In till soils, provide bioretention volume based
on 0.8 inches of equivalent storage depth; in outwash soils, provide bioretention volume
based on 0.4 inches of equivalent storage depth,
Permeable Pavement per Appendix C, Section C.2.7
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4. All target impervious surfaces not mitigated by Requirements 1, 2 and 3 above, must be mitigated to
the maximum extent feasible using the Basic Dispersion BMP described below. Use of Basic
Dispersion is subject to evaluation of its feasibility and applicability as detailed in Appendix C.
Feasible BMPs are required to be implemented. Basic Dispersion BMPs may be located anywhere on
the site/lot subject to the limitations and design specifications cited in Appendix C. The BMP must be
implemented as part of the proposed project.
Basic Dispersion per Appendix C, Section C.2.4,
5. BMPs must be implemented, at minimum, for an impervious area equal to at least 10% of the site/lot
for site/lot sizes up to 11,000 square feet and at least 20% of the site/lot for site/lot sizes between
11,000 and 22,000 square feet. For projects located in Zone 1 of the Aquifer Protection Area, these
impervious area amounts must be doubled. Doubling of the minimum impervious area required for
BMP implementation in Zone 1 of the Aquifer Protection Area is not required for projects located
within 200 feet of a steep slope hazard area, landslide hazard, or erosion hazard area. If these
minimum areas are not mitigated using feasible BMPs from Requirements 1, 2, 3, and 4 above, one or
more BMPs from the following list are required to be implemented to achieve compliance. These
BMPs must be implemented as part of the proposed project.
Reduced Impervious Surface Credit per Appendix C, Section C.2.9,
Native Growth Retention Credit per Appendix C, Section C.2.10.
Tree Retention Credit per Appendix C, Section C.2.14
6. The soil moisture holding capacity of new pervious surfaces (target pervious surfaces) must be
protected in accordance with the soil amendment BMP as detailed in Appendix C, Section C.2.13.
7. Any proposed connection of roof downspouts to the local drainage system must be via a perforated
pipe connection as detailed in Appendix C, Section C.2.11.
1.2.9.2.2 LARGE LOT BMP REQUIREMENTS
IF the proposed project is on a site/lot that is 22,000 square feet or larger, THEN on-site BMPs must be
applied as specified in the requirements below OR the project must demonstrate compliance with the LID
Performance Standard (described in Section 1.2.9.1.B) using an approved continuous runoff model.
1. The feasibility and applicability of full dispersion as detailed in Appendix C, Section C.2.1 must be
evaluated for all target impervious surfaces. If feasible and applicable for any such surface, then full
dispersion must be applied to that surface and implemented as part of the proposed project. Typically,
full dispersion will be applicable only on the largest sites/lots where there may be enough forest area
available within a threshold discharge area to meet the 15% ratio of fully dispersed impervious area
to native vegetated surface.
2. Where full dispersion of target impervious roof areas is not feasible or applicable, or will cause
flooding or erosion impacts, the feasibility and applicability of full infiltration of roof runoff must be
evaluated in accordance with Appendix C, Section C.2.2, or Section 5.2, whichever is applicable
based on the type of project.42 If feasible and applicable, full infiltration of roof runoff must be
implemented as part of the proposed project.
3. All target impervious surfaces not mitigated by Requirements 1 and 2 above, must be mitigated to the
maximum extent feasible using one or more BMPs from the following list. Use of a given BMP is
subject to evaluation of its feasibility and applicability as detailed in Appendix C. Feasible BMPs are
required to be implemented. The BMPs listed below may be located anywhere on the site/lot subject
to the limitations and design specifications for each BMP. These BMPs must be implemented as part
of the proposed project.
42 For projects subject to Simplified Drainage Review, and for any single family residential project subject to Full or Large
Project Drainage Review, the design requirements and specifications in Appendix C, Section C.2.2 may be used for evaluation
and design of full infiltration on individual lots. For all other projects, full infiltration must be evaluated and designed in
accordance with the infiltration facility standards in Section 5.2.
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Full Infiltration per Section C.2.2, or per Section 5.2, whichever is applicable
Limited Infiltration per Appendix C, Section C.2.3
Bioretention per Appendix C, Section C.2.6, sized as follows:
o SeaTac regional scale factor equals 1.0: In till soils, provide bioretention volume based on
0.6 inches of equivalent storage depth; in outwash soils provide bioretention volume based
on 0.1 inches of equivalent storage depth
o SeaTac regional scale factor greater than 1.0: In till soils, provide bioretention volume based
on 0.8 inches of equivalent storage depth; in outwash soils, provide bioretention volume
based on 0.4 inches of equivalent storage depth,
Permeable Pavement per Appendix C, Section C.2.7
4. All target impervious surfaces not mitigated by Requirements 1, 2, and 3 above, must be mitigated to
the maximum extent feasible using the Basic Dispersion BMP described below. Use of Basic
Dispersion is subject to evaluation of its feasibility and applicability as detailed in Appendix C.
Feasible BMPs are required to be implemented. Basic Dispersion BMPs may be located anywhere on
the site/lot subject the limitations and design specifications cited in Appendix C. The BMP must be
implemented as part of the proposed project.
Basic Dispersion per Appendix C, Section C.2.4
5. BMPs must be implemented, at minimum, for impervious area amounts defined as follows.
For projects that will result in an impervious surface coverage on the buildable portion of the
site/lot of less than 45%, on-site BMPs must be applied to 50% of target impervious surfaces.
For projects that will result in an impervious surface coverage 45-65% on the buildable portion of
the site/lot, on-site BMPs must be applied to 50% of target impervious surfaces reduced by 1.5%
for each 1% of impervious surface coverage above 45% (e.g., impervious coverage of 55%
results in a requirement of on-site BMPs applied to 35% of target impervious surfaces).
For projects that will result in an impervious surface coverage greater than 65% on the buildable
portion of the site/lot, on-site BMPs must be applied to 20% of the target impervious surfaces or
to an impervious area equal to at least 10% of the site/lot, whichever is less.
The buildable portion of the site/lot is the total area of the site/lot minus any critical areas and minus
200 ft. buffer areas from a steep slope hazard, landslide hazard, or erosion hazard area. If these
minimum areas are not mitigated using feasible BMPs from Requirements 1, 2, 3, and 4 above, one or
more BMPs from the following list are required to be implemented to achieve compliance. These
BMPs must be implemented as part of the proposed project.
Reduced Impervious Surface Credit per Appendix C, Section C.2.9,
Native Growth Retention Credit per Appendix C, Section C.2.10,
Tree Retention Credit per Appendix C, Section C.2.14.
6. The soil moisture holding capacity of new pervious surfaces (target pervious surfaces) must be
protected in accordance with the soil amendment BMP as detailed in Appendix C, Section C.2.13.
7. Any proposed connection of roof downspouts to the local drainage system must be via a perforated
pipe connection as detailed in Appendix C, Section C.2.11.
1.2.9.2.3 LARGE RURAL LOT BMP REQUIREMENTS
This requirement does not apply in the City of Renton.
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1.2.9.2.4 43IMPLEMENTATION REQUIREMENTS FOR INDIVIDUAL LOT BMPS
The on-site BMPs required in Sections 1.2.9.2 above must be implemented in accordance with the
following requirements:
1. Implementation Responsibility. All on-site BMPs required for the site/lot must be implemented
(installed) by the applicant as part of the proposed project unless they have already implemented as
part of a subdivision project that created the lot per Section 1.2.9.4.
2. Maintenance Responsibility. Maintenance of all required on-site BMPs is the responsibility of the
owner of the site/lot served by these BMPs. The responsibility for such maintenance must be clearly
assigned to the current and future owners of the site/lot through a “declaration of covenant and grant
of easement” as described in Requirement 3 below.
3. Declaration of Covenant and Grant of Easement. To ensure future maintenance of on-site BMPs
and allow for City inspection of BMPs, a declaration of covenant and grant of easement must be
recorded for each site/lot that contains on-site BMPs. A draft of the proposed covenant must be
reviewed and approved by CED prior to recording. All required covenants must be recorded prior to
final construction approval for the proposed project. If the individual site/lot contains or will contain
flow control or water quality facilities, then the drainage facility covenant in Reference Section 8-J (or
equivalent) must be used, and is designed to achieve the following:
a) Provide notice to future owners of the presence of on-site BMPs on the lot and the responsibility
of the owner to retain, uphold, and protect the on-site BMPs, features, pathways, limits, and
restrictions.
b) Include as an exhibit, a recordable version44 of the following drainage plan information:
The site plan showing all developed surfaces (impervious and pervious) and the location and
dimensions of on-site BMPs, features, flowpaths (if applicable), limits of native growth
retention areas (if applicable), and limits of tree retention areas (if applicable). This plan(s)
must be to scale and include site topography in accordance with the specifications for such
plans in Appendix C, Section C.4.2. Also indicate any areas where City access is excluded
(see paragraph 3.d below). Note: CED may waive this element if, for example, the only on-
site BMP proposed is a limit on impervious surface (reduced footprint).
The on-site BMP design and maintenance details for each on-site BMP per Appendix C,
Section C.4.3. This includes a diagram (if applicable) of each on-site BMP and written
maintenance and operation instructions and restrictions for each device, feature, flowpath (if
applicable), native growth retention area (if applicable) and impervious surface coverage (if
applicable). See Reference Section 8-M for prepared 8-1/2″ x 11″ maintenance instruction
sheets. See City of Renton’s Surface Water Design Standards web site:
<www.rentonwa.gov/swdm> for downloadable BMP details.
Ensure the exhibits are correctly cross-referenced in the declaration of covenant (the site plan is
typically Exhibit A and the design/maintenance details are typically Exhibit B).
c) Require that each on-site BMP be operated and maintained at the owner’s expense in
accordance with the above exhibit.
d) Grant the City the right to enter the property at reasonable times for purposes of inspecting the
on-site BMPs and to perform any corrective maintenance, repair, restoration, or mitigation work
on the on-site BMPs that has not been performed by the property owner within a reasonable time
43 Footnote 48 is not used.
44 Recordable version means one that meets King County’s “Standard Formatting Requirements for Recording Documents”
pursuant to RCW 36.18.010 and 65.04.045, available online at < https://kingcounty.gov/~/media/depts/records-
licensing/recorders-office/documents/Requirements_WAState_Formatting.ashx?la=en> or from the King County Recorder’s
Office. These requirements include specifications for such things as page size (81/2″ x 14″ or smaller), font size (at least 8-
point), and margin width (1″ on all sides of every page if there is a standard cover sheet).
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set by CED, and to charge the property owner for the cost of any maintenance, repair, restoration,
or mitigation work performed by the City.
e) The right to enter typically applies to the entire property, but occasionally accepts areas on the
property agreed upon by the City to be excluded from access. Such areas are to be shown on the
site plan described above.
f) Prohibit any modification or removal of on-site BMPs without written approval from the City.
The approval must be obtained from CED and a covenant must be recorded to reflect the changes.
Approval will be granted only if equivalent protection in terms of hydrologic performance is
provided by other means.
4. Timing of Implementation. All required on-site BMPs must be installed prior to final inspection
approval of constructed improvements. For BMPs that rely on vegetation, the vegetation must be
planted and starting to grow prior to final construction approval.
5. Acceptance standards. On-site BMPs may be inspected during and/or following construction.
Approval of the constructed BMPs will be based on verification that the materials and placement
appear to meet the specifications and that the BMPs appear to function as designed. Onsite
observations may be used to verify that materials are as specified and material receipts checked.
Performance may be evaluated by a site visit while it is raining or by testing with a bucket of water or
garden hose to check pavement permeability or proper connection to BMP devices/features, etc.
6. Drainage concerns. If CED determines that there is a potential for drainage impacts to a neighboring
property, then additional measures may be required. Some on-site BMPs may not be appropriate in
certain situations, and will not be allowed by CED where they may cause drainage problems.
7. Geotechnical concerns. A geotechnical engineer, engineering geologist, or CED must evaluate and
approve on-site BMPs that are proposed: (A) on slopes steeper than 15%; (B) within a setback from
the top of slope equal to the total vertical height of the slope area that is steeper than 15%; or
(C) within 200 feet of a steep slope hazard area, erosion hazard area, or landslide hazard. In
addition, CED may require review by a geotechnical engineer or engineering geologist of any
proposed BMP that infiltrates, disperses, or directs overflow adjacent to or towards a steep slope
hazard area, erosion hazard area, or landslide hazard. CED may also require some projects to route
flows down or around such slopes using non-perforated pipes. Some on-site BMPs may not be
appropriate for these locations, and will not be allowed by CED where flows may cause erosion
problems.
8. Sewage system concerns. If CED determines that there is a potential conflict between onsite sewage
systems and on-site BMPs, additional measures may be required. Some projects may need to route
flows past onsite sewage systems using non-perforated pipes. Also, some on-site BMPs may not be
appropriate for these sites, and will not be allowed where sewage systems may be impacted.
9. Engineering Concerns. While most of the on-site BMPs in Appendix C can be implemented by a
non-engineer, there are some that have structural components that must be designed or evaluated by a
civil engineer or structural engineer. When a BMP is proposed that has such components as
identified in Section C.2 in Appendix C, CED may require submittal of engineering plans for that
component signed and stamped by a civil engineer or structural engineer.
10. Connection to Subsurface Drains. On-site BMPs should not be connected to subsurface drains
(e.g., footing drains) as these connections may adversely affect the performance of the BMPs, and in
some cases may cause reverse flow into the footing drains during storm events.
11. Simplified Drainage Plan. The type, size, and placement of proposed on-site BMPs are to be shown
on the site plan submitted for the proposed project. This plan must be in accordance with the
specifications for such plans outlined in Section C.4 in Appendix C unless otherwise directed by CED.
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1.2.9.3 SUBDIVISION AND ROAD IMPROVEMENT PROJECTS BMP
REQUIREMENTS
For subdivision and road improvement projects, on-site BMPs must be selected and applied according to
the subdivision and road improvement projects BMP requirements in this section. For purposes of
applying on-site BMPs to these projects, two categories of requirements have been established based on
the size of site/lot subject to improvements by the project. These categories of requirements are as follows:
Small Subdivision Project BMP Requirements
Small Road Improvement and Urban Road Improvement Projects BMP Requirements
On-site BMPs must be applied in the order of preference and to the extent specified for the category
of requirements applicable to the proposed project as described in the following subsections.
1.2.9.3.1 SMALL SUBDIVISION PROJECT BMP REQUIREMENTS
On-site BMPs for plat infrastructure improvements (e.g., road and sidewalk etc.) of these projects shall
meet the requirements described in Section 1.2.9.3.2 below for “Small Road Improvement and Urban
Road Improvement Project BMP Requirements.” Implementation of on-site BMPs required for/on the
individual lots of the subdivision may be deferred until a permit is obtained for construction on each lot
and is therefore optional. However, if the applicant wishes to implement or make provision for
implementation of BMPs for the lot improvements as part of the subdivision project for purposes of
receiving BMP modeling credits, the individual lot BMP requirements described in Section 1.2.9.2 and
implementation requirements for subdivision projects described Section 1.2.9.4.1 must be met.
1.2.9.3.2 SMALL ROAD IMPROVEMENT AND URBAN ROAD IMPROVEMENT PROJECTS
BMP REQUIREMENTS
IF the proposed project is a road improvement project that is on a site/parcel less than 5 acres in size,
THEN on-site BMPs must be applied as specified in the requirements below.
1. The feasibility and applicability of full dispersion as detailed in Appendix C, Section C.2.1 must be
evaluated for all target impervious surfaces. If feasible and applicable, full dispersion must be
implemented as part of the proposed project. Typically, small lot full dispersion will be applicable
only in subdivisions where enough forest was preserved by tract, easement, or covenant to meet the
minimum design requirements for full dispersion in Appendix C, Section C.2.1.1.
2. All target impervious surfaces not mitigated by Requirement 1 above, must be mitigated to the
maximum extent feasible using one or more BMPs from the following list. Use of a given BMP is
subject to evaluation of its feasibility and applicability as detailed in Appendix C. Infeasible BMPs are
not required to be implemented. The BMPs listed below may be located anywhere on the site/lot
subject to the limitations and design specifications for each BMP. These BMPs must be implemented
as part of the proposed project.
Full Infiltration per Section C.2.2, or per Section 5.2, whichever is applicable
Limited Infiltration per Appendix C, Section C.2.3,
Bioretention per Appendix C, Section C.2.6, sized as follows:
o SeaTac regional scale factor equals 1.0 : In till soils, provide bioretention volume based on
0.6 inches of equivalent storage depth; in outwash soils provide bioretention volume based
on 0.1 inches of equivalent storage depth,
o SeaTac regional scale factor greater than 1.0: In till soils, provide bioretention volume based
on 0.8 inches of equivalent storage depth; in outwash soils, provide bioretention volume
based on 0.4 inches of equivalent storage depth,
Permeable Pavement per Appendix C, Section C.2.7,
3. All target impervious surfaces not mitigated by Requirements 1 and 2 above, must be mitigated to the
maximum extent feasible using the Basic Dispersion BMP described below. Use of Basic Dispersion
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is subject to evaluation of its feasibility and applicability as detailed in Appendix C. Infeasible BMPs
are not required to be implemented. Basic Dispersion BMPs may be located anywhere on the site/lot
subject to the limitations and design specifications cited in Appendix C. The BMPs must be
implemented as part of the proposed project.
Basic Dispersion per Appendix C, Section C.2.4,
4. The soil moisture holding capacity of new pervious surfaces (target pervious surfaces) must be
protected in accordance with the soil amendment BMP as detailed in Appendix C, Section C.2.13.
1.2.9.3.3 LARGE RURAL SUBDIVISION AND LARGE RURAL ROAD IMPROVEMENT
PROJECTS BMP REQUIREMENTS
This requirement does not apply in the City of Renton.
1.2.9.4 REQUIREMENTS FOR USE OF BMP CREDITS
Projects that implement on-site BMPs, whether required or optional, may use the on-site BMP credits
described in this section subject to the implementation requirements in Section 1.2.9.2.4 (for Individual
Lots), Section 1.2.9.4.1 (for Subdivision Projects), Section 1.2.9.4.2 (for Right-of-Way Projects), and any
restrictions noted in this section or Table 1.2.9.A. For all project types, modeling credits cannot be used
for on-site BMPs that will be privately maintained, with the exception of the full dispersion and full
infiltration BMPs. An alternative approach is to perform continuous runoff modeling per Chapter 3 instead
of applying the on-site BMP credits in Table 1.2.9.A.
Two kinds of credits are available. First, any impervious surface served by an on-site BMP that meets the
design specifications for that BMP in Appendix C may be modeled as indicated and allowed in Table
1.2.9.A. Such credits may be used in the following situations:
1. To compute post-development runoff time series when sizing required flow control facilities.
2. To compute post-development 100-year peak flows when assessing any of the peak flow exceptions
from the area-specific flow control facility requirement in Sections 1.2.3.1.A, B, and C.
3. To compute post-development runoff time series when sizing required flow rate based water quality
facilities (e.g., bioswales) and to re-characterize post developed land types when sizing volume based
water quality facilities (e.g., wetponds, wetvaults).
Use of credits for water quality facility sizing as described above is limited to BMPs that are treating flows
downstream from the BMP and tributary to a required water quality facility.
Second, any impervious or nonnative pervious surface that is fully dispersed per the full dispersion
criteria in Section 1.2.3.2.C is not considered a target surface of the area-specific flow control facility
requirement (Section 1.2.3.1) or the area-specific water quality facility requirement (Section 1.2.8.1).
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TABLE 1.2.9.A ON-SITE BMP SIZING CREDITS(1)
On-Site BMP Type Sizing Credit
Privately Maintained
On-Site BMP Publicly Maintained On-Site BMP
Full dispersion Model fully dispersed
surface as forest(2)
Model fully dispersed surface as forest(2)
Full infiltration(3) Subtract impervious area
that is fully infiltrated
Subtract impervious area that is fully infiltrated
Limited infiltration None Model tributary impervious surface as 90% impervious, 10% grass
Basic dispersion None Model dispersed impervious surface as 90% impervious, 10% grass
Rain garden None None
Bioretention None Model tributary impervious surface as 90% impervious, 10% grass
Permeable pavement (unlined
with no underdrain)
None Model permeable pavement area as 50% impervious, 50% grass.
Run-on from other impervious surfaces does not receive a credit.
Grassed modular grid pavement None Model modular grid pavement as all grass
Rainwater harvesting None Credit only allowed via, and as specified in, an approved drainage
adjustment that details conditions of use
Restricted footprint None Model footprint as restricted (Appendix Section C.2.9.2)
Wheel strip driveways None Model credited area as 50% impervious, 50% grass
Minimum disturbance foundation None Model foundation area as 50% impervious, 50% grass
Open grid decking over pervious
area
None Model deck area as 50% impervious, 50% grass
Native growth retention credit None Model mitigated impervious area as 50% impervious, 50% grass
Perforated pipe connection None None
Notes:
(1) These credits do not apply when determining eligibility for exemptions from Core Requirement #3, Core Requirement #8, or exceptions from the
flow control or water quality facility requirements unless otherwise noted in the exemption or exception. Modeling credits cannot be used for
on-site BMPs that will be privately maintained, with the exception of full dispersion and full infiltration BMPs. Explicit modeling of BMP
infiltration for facility sizing is not allowed. When applying modeling credits for flow control facility sizing, infiltrative BMPs tributary to the facility
that are included in the modeling scenario (including the permeable pavement element with area reduced to 50% impervious area fraction, or
other BMPs (e.g., bioretention, trenches, drywells) treating upstream runoff) must have the infiltration option turned off during the flow routing
analysis for facility sizing to avoid double-counting the BMP infiltration benefit. Alternatively, permeable pavement with infiltration turned off may
be represented by an impervious area land use element of equivalent area.
(2) Surface shall be modeled using the soil type found at that location on the site.
(3) For any project subject to Simplified Drainage Review, and for any single family residential project subject to Directed, Full or Large Project
Drainage Review, the design requirements and specifications in Appendix C, Section C.2.2 may be used for design of full infiltration on individual
lots. For all other projects, including any project where full infiltration is proposed to serve more than one lot, full infiltration must be designed in
accordance with infiltration facility standards in Section 5.2.
1.2.9.4.1 USE OF CREDITS BY SUBDIVISION PROJECTS
If a proposed project is a subdivision project,45 implementation of on-site BMPs for plat infrastructure
improvements (e.g., road, sidewalk, or other non-lot improvements) is required concurrent with the
subdivision improvements. Implementation of on-site BMPs on the individual lots of the subdivision may
be deferred until a permit is obtained for construction on each lot and is therefore optional as part of the
subdivision project.
In order to receive the modeling credits (noted above) for on-site BMPs required for plat infrastructure
improvements (e.g., road, sidewalk, or other non-lot improvements), and/or for individual lot BMPs where
the applicant elects to implement or make provision for implementation of individual lot BMPs as part of
45 For purposes of applying on-site BMPs, the term subdivision or subdivision project refers to any project that is a short plat,
plat, or binding site plan.
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the subdivision project, the following requirements must be met depending on where the BMPs are located
on the site. These requirements are in addition to any restrictions for use of modeling credits noted in
Section 1.2.9.4 and/or Table 1.2.9.A.
A. SUBDIVISION IMPLEMENTATION OF BMPS WITHIN ROAD RIGHT-OF-WAY
These are on-site BMPs installed within public or private road right-of-way as part of the construction of
street and drainage improvements for the subdivision. To receive credit for these BMPs, the subdivision
project must meet all of the following requirements:
1. The BMPs must serve impervious surface located only within the road right-of-way.
2. The BMPs must be shown on the site improvement plans submitted with the engineering plans for
the proposed project as specified in Section 2.3.1.2.
3. If the road right-of-way will be privately maintained, provision must be made for future maintenance
of the BMPs in accordance with Core Requirement #6, Section 1.2.6. As specified in Core
Requirement #6, the City will assume maintenance of such BMPs in certain cases.
4. If the City will be assuming maintenance of the BMPs, the BMPs must comply with the drainage
facility financial guarantee and liability requirements in Core Requirement #7, Section 1.2.7.
B. SUBDIVISION IMPLEMENTATION OF BMPS WITHIN DEDICATED TRACTS
These are on-site BMPs installed on or associated with the features (e.g., forest) of common area tracts
dedicated by the subdivision. Such BMPs may serve future improvements on lots, common area
improvements, or road right-of-way improvements. To receive credit for these BMPs, the subdivision
project must meet all of the following requirements:
1. The BMPs must be shown on the site improvement plans submitted with the engineering plans for
the proposed project as specified in Section 2.3.1.2.
2. Provision must be made for future maintenance of the BMPs in accordance with Core
Requirement #6, Section 1.2.6. When maintenance by the City is specified by Core Requirement #6,
the City will assume maintenance of BMP devices (e.g., dispersion trenches) that are within a tract
dedicated to the City for drainage purposes. The City will not assume maintenance of BMPs located
in common areas dedicated for purposes other than just drainage (e.g., play areas, parks, etc.). Where
City maintenance is specified by Core Requirement #6, the City will assume maintenance for on-site
BMP vegetated flow paths that are within an easement that allows for inspection and maintenance by
the City. The City maintenance of these vegetated flow paths will be limited to their on-site BMP
functionality. All other maintenance shall remain the responsibility of the owner(s).
3. BMPs to be maintained by the City in accordance with Core Requirement #6 must comply with the
drainage facility financial guarantee and liability requirements in Core Requirement #7,
Section 1.2.7.
4. If the BMPs installed within a dedicated tract satisfy some or all of the BMP requirements for
individual lots per Section 1.2.9.2, then a note must be placed on the recorded documents for the
subdivision indicating those lots for which BMPs have been provided.
C. SUBDIVISION IMPLEMENTATION OF BMPS ON INDIVIDUAL LOTS
These are on-site BMPs installed on a subdivision’s proposed lots as part of the subdivision project. For
example, the subdivision developer may elect to pre-install some or all of the on-site BMPs required by
the individual lot BMP requirements in Section 1.2.9.2. To receive credits for these BMPs, the subdivision
project must meet all of the following requirements:
1. The on-site BMPs must be installed and implemented in accordance with the individual lot BMP
requirements in Section 1.2.9.2. This includes recording a declaration of covenant and grant of
easement for each lot with BMPs as specified in Implementation Requirement 3 of Section 1.2.9.2.4.
AGENDA ITEM # 8. a)
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If not all of the required BMPs are installed on a lot as part of the subdivision project, language must
be included in the covenant notifying the future lot owner of additional required BMPs.
2. BMPs to be installed on individual lots as part of the subdivision project must be shown on the site
improvement plans submitted with the engineering plans for the proposed project as specified in
Section 2.3.1.2.
D. SUBDIVISION FUTURE IMPLEMENTATION OF BMPS ON INDIVIDUAL LOTS
These are on-site BMPs stipulated to be installed on some or all of a subdivision’s proposed lots by a
declaration of covenant recorded for each such lot. To receive credits for these BMPs, the subdivision
project must meet all of the following requirements:
1. Demonstrate through a lot-specific assessment that the on-site BMPs stipulated for each lot are
feasible and applicable according to the individual lot BMP requirements in Section 1.2.9.2 and the
BMP design specifications in Appendix C. This lot-specific assessment must be included in the TIR
submitted with engineering plans for the subdivision. The assessment shall include any soils reports,
calculations, or other information necessary to select and properly apply BMPs.
2. Record a declaration of covenant and grant of easement for each lot stipulating the type or types of
BMP being proposed for credit. This covenant must be as specified in Implementation Requirement 3
of Section 1.2.9.2.4, except as follows:
a) The site plan requirement may be waived depending on the BMPs proposed or may be
conceptual, showing only the information necessary to stipulate the type or types of BMP being
proposed for credit. For example, if the BMP is full dispersion, the approximate location of future
impervious surface and the limits of the “native vegetated flowpath segment” (see Appendix C,
Section C.2.1) must be shown. If the BMP is full infiltration, the approximate location of future
impervious surface, septic drain field (if applicable), and infiltration devices must be shown. For
all other BMPs, the “design and maintenance details” (see Item b below) for each proposed BMP
per Appendix C may be sufficient as determined by CED.
b) The on-site BMP design and maintenance details must include the dimensions of all proposed
devices, features, and flowpaths, expressed as unit amounts per square foot of impervious surface
served or as a percentage of the lot size or impervious surface created.
c) The notice to future lot owners must indicate that they are responsible to install the on-site BMP
or BMPs stipulated for the lot prior to final inspection approval of constructed lot improvements.
Alternative BMPs that provide equivalent performance may be proposed at the time of permit
application for proposed lot improvements. In any case, a revised covenant will need to be
recorded to reflect the final approved BMPs and site improvement plan(s).
3. If single family residential lots are being created, a note must be placed on the recorded documents
for the subdivision indicating the following:
“Single family residences and other improvements constructed on the lots created by this
subdivision must implement the flow control best management practices (BMPs) stipulated in the
drainage plan declaration of covenant and grant of easement recorded for each lot. Compliance
with this stipulation must be addressed in the small project drainage plan submitted for drainage
review when application is made for a single family residential building permit for the lot.”
4. If commercial lots are being created, a note must be placed on the recorded documents for the
subdivision indicating the following:
“Improvements constructed on the lots created by this subdivision must implement the flow
control best management practices (BMPs) stipulated in the drainage plan declaration of covenant
and grant of easement recorded for each lot. Compliance with this stipulation must be addressed
in the engineering plans submitted for drainage review when application is made for a permit to
make improvements to the lot.”
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5. If a binding site plan is being created, a note must be placed on the recorded documents for the
subdivision indicating the following:
“Improvements constructed on the lots created by this binding site plan must implement the flow
control best management practices (BMPs) stipulated in the drainage plan declaration of covenant
and grant of easement recorded for each lot. Compliance with this stipulation must be addressed
in the engineering plans submitted for drainage review when application is made for a permit to
make improvements to the lot.”
1.2.9.4.2 USE OF CREDITS BY PROJECTS WITHIN RIGHTS-OF-WAY
If a proposed project is located primarily within an established public or private right-of-way,
implementation of on-site BMPs is as required per Section 1.2.9.3. To receive credit for these BMPs, the
project must meet all of the following requirements in addition to any restrictions for use of modeling
credits noted in Section 1.2.9.4 and/or Table 1.2.9.A.:
1. The BMPs must serve impervious surface located only within the right-of-way.
2. If the right-of-way will be privately maintained, provision must be made for future maintenance of
the BMPs in accordance with Core Requirement #6, Section 1.2.6.
3. If the City will be assuming maintenance of the BMPs, the BMPs must comply with the drainage
facility financial guarantee and liability requirements in Core Requirement #7, Section 1.2.7.
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1.3 SPECIAL REQUIREMENTS
This section details the following six special drainage requirements that may apply to the proposed project
depending on its location or site-specific characteristics:
“Special Requirement #1: Other Adopted Area-Specific Requirements,” Section 1.3.1
“Special Requirement #2: Flood Hazard Area Delineation,” Section 1.3.2
“Special Requirement #3: Flood Protection Facilities,” Section 1.3.3
“Special Requirement #4: Source Control,” Section 1.3.4
“Special Requirement #5: Oil Control,” Section 1.3.5
“Special Requirement #6: Aquifer Protection Area,” Section 1.3.6
1.3.1 SPECIAL REQUIREMENT #1:
OTHER ADOPTED AREA-SPECIFIC REQUIREMENTS
This manual is one of several adopted regulations in the City of Renton that apply requirements for
controlling drainage on an area-specific basis. Other adopted area-specific regulations include
requirements that have a more direct bearing on the drainage design of a proposed project. These
regulations include the following:
Master Drainage Plans (MDPs): MDPs are comprehensive drainage plans prepared for urban
planned developments (UPDs) or other large, complex projects (described in Section 1.1.2.5). Projects
covered by a MDP must meet any adopted requirements specific to that plan.
Basin Plans (BPs): The City of Renton adopts basin plans to provide for the comprehensive
assessment of resources and to accommodate growth while controlling adverse impacts to the
environment. A basin plan may recommend specific land uses, regional capital projects, and special
drainage requirements for future development within the basin area it covers.
Salmon Conservation Plans (SCPs): Salmon conservation plans are comprehensive, ecosystem-
based plans intended to identify and assess the means to protect and restore salmon habitat through
mechanisms such as habitat improvements, regulations, incentives, BMPs, land acquisition, and public
education activities. These plans are developed in collaboration with other jurisdictions within a water
resource inventory area (WRIA) designated by the state under WAC 173-500-040 and spanning
several basins or subbasins.
Lake Management Plans: The City of Renton may adopt lake management plans to provide for
comprehensive assessment of resources and to accommodate growth while controlling adverse
impacts from nutrient loading to selected lakes. A lake management plan may recommend nutrient
control through special drainage and source control requirements for proposed projects within the area
it covers.
Hazard Mitigation Plan: The City’s Hazard Mitigation Plan prepared in accordance with RCW
86.12.200 includes information on reducing flood risks.
Shared Facility Drainage Plans (SFDPs): SFDPs are approved by the City of Renton to allow two
or more projects to share drainage facilities required by this manual. Projects covered by a SFDP must
meet any specific requirements of that plan.
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Threshold Requirement
IF a proposed project is in an area
included in an adopted master drainage
plan, basin plan, salmon conservation
plan, stormwater compliance plan,
hazard mitigation plan, lake
management plan, or shared facility
drainage plan …
THEN the proposed project shall
comply with the drainage requirements
of the master drainage plan, basin plan,
salmon conservation plan, stormwater
compliance plan, hazard mitigation
plan, lake management plan, or shared
facility drainage plan, respectively.
Application of this Requirement
The drainage requirements of adopted MDPs, BPs, SCPs, Hazard Mitigation Plan, lake management plans,
and SFDPs shall be applied in addition to the drainage requirements of this manual unless otherwise
specified in the adopted regulation. Where conflicts occur between the two, the drainage requirements of
the adopted area-specific regulation shall supersede those in this manual.
Examples of drainage requirements found in other adopted area-specific regulations include the following:
More or less stringent flow control
More extensive water quality controls
Forest retention requirements
Infiltration restrictions
Groundwater recharge provisions
Discharge to a constructed regional flow control or conveyance facility.
Adjustments to vary from the specific drainage requirements mandated by BPs, SCPs, FHMPs, and lake
management plans may be pursued through the adjustment process described in Section 1.4 of this manual.
Copies of all adopted basin plans, SCPs, Hazard Mitigation Plan, and lake management plans are available
from the City of Renton.
Projects covered by SFDPs shall demonstrate that the shared facility will be available by the time the
project is constructed and that all onsite requirements are met. Projects covered by a SFDP are still
required to provide any onsite controls necessary to comply with drainage requirements not addressed by
the shared facility.
1.3.2 SPECIAL REQUIREMENT #2:
FLOOD HAZARD AREA DELINEATION
Flood hazard areas are composed of the 100-year floodplain, zero-rise flood fringe, zero-rise floodway,
and FEMA floodway. If a proposed project contains or is adjacent to a flood hazard area as determined by
CED, this special requirement requires the project to determine those components that are applicable and
delineate them on the project’s site improvement plans and recorded maps.
Floodplains are subject to inundation during extreme events. The 100-year floodplain, and floodway if
applicable, is delineated in order to minimize flooding impacts to new development and to prevent
aggravation of existing flooding problems by new development. Regulations and restrictions concerning
development within a 100-year floodplain are found in the critical areas code, RMC 4-3-050.
AGENDA ITEM # 8. a)
1.3.3 SPECIAL REQUIREMENT #3: FLOOD PROTECTION FACILITIES
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Threshold Requirement
IF a proposed project contains or is
adjacent to a flood hazard area for a river,
stream, lake, wetland, closed depression, ,
or if other City of Renton regulations
require study of flood hazards related to
the proposed project …
THEN the 100-year floodplain, and
applicable floodway, shall be determined
and their boundaries, together with the
boundaries of the severe and moderate
channel migration hazard area (if
applicable), shall be delineated on the site
improvement plans and profiles, and on
any final subdivision maps prepared for
the proposed project.
Application of this Requirement
The applicant is required to use the best available floodplain/floodway data when delineating the 100-year
floodplain and floodway boundaries on site improvement plans and profiles, and on any final subdivision
maps. The floodplain/floodway delineation used by the applicant shall be in accordance with
RMC 4-3-050 and associated public rules. If floodplain/floodway data and delineation does not exist, then
a floodplain/floodway analysis shall be prepared by the applicant as described in Section 4.4.2,
“Floodplain/Floodway Analysis.”
1.3.3 SPECIAL REQUIREMENT #3:
FLOOD PROTECTION FACILITIES
Flood protection facilities, such as levees and revetments require a high level of confidence in their
structural integrity and performance. Proper analysis, design, and construction are necessary to protect
against the potentially catastrophic consequences if such facilities should fail.
Threshold Requirement
IF a proposed project will:
Rely on an existing flood protection
facility (such as a levee or revetment)
for protection against hazards posed
by erosion or inundation, OR
Modify or construct a new flood
protection facility …
THEN the applicant shall demonstrate
that the flood protection facility, as
determined by a licensed professional
engineer, conforms with siting, structural
stability, environmental, and all other
relevant standards cited in the following
regulations and documents:
Washington State Integrated
Streambank Protection Guidelines,
Corps of Engineers Manual for
Design and Construction of Levees
(EM 1110-2-1913),
RMC 4-3-050 and
Special Requirement #1 (specifically
the City Hazard Mitigation Plan)
AND, flood containment levees shall meet
or exceed the professional engineering
standards summarized in FEMA National
Flood Insurance mapping regulations …
(44 CFR, subsection 65.10) or FEMA’s
Analysis and Mapping Procedures for
non-Accredited Levee Systems.
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Application of this Requirement
Conformance with the requirements listed above shall be addressed in the Technical Information Report
submitted with the project’s engineering plans (see Section 2.3.1.1).
Conformance also requires that certain easement requirements (outlined in Section 4.1) be met in order
to allow County access to the facility. If the proposed project contains an existing City of Renton flood
protection facility or proposes to rely on a City of Renton flood protection facility, the applicant shall
provide an easement to the City of Renton consistent with the river protection easement requirements
outlined in Section 4.1.
1.3.4 SPECIAL REQUIREMENT #4: SOURCE CONTROLS
Water quality source controls prevent rainfall and runoff water from coming into contact with pollutants,
thereby reducing the likelihood that pollutants will enter public waterways and violate water quality
standards or City stormwater discharge permit limits. A Stormwater Pollution Prevention Manual was
prepared for citizens, businesses, and industries to identify and implement source controls for activities
that often pollute water bodies. The City of Renton provides education about source control
implementation upon request. The City will implement a progressive enforcement policy to require
mandatory source controls through education and outreach, technical assistance, and enforcement.
Threshold Requirement
IF a proposed project requires a
commercial building or commercial site
development permit …
THEN water quality source controls
applicable to the proposed project shall
be applied as described below in
accordance with the King County
Stormwater Pollution Prevention Manual
and Renton Municipal Code, Title IV.
Application of this Requirement
When applicable per the Stormwater Pollution Prevention Manual, structural source control measures,
such as car wash pads or dumpster area roofing, shall be applied to the entire site containing the proposed
project, not just the project site. If the applicant is a tenant or lessee for only a portion of the site, CED
may limit the entire site application of structural source controls to only that portion of the site occupied or
leased by the applicant. All applicable structural source control measures shall be shown on the site
improvement plans submitted for engineering review and approval. Other, nonstructural source control
measures, such as covering storage piles with plastic or isolating areas where pollutants are used or
stored, are to be implemented after occupancy and need not be addressed during the plan review process.
All commercial, industrial, and multifamily projects (irrespective of size) undergoing drainage review are
required to implement applicable source controls.
Activities That May Result In Structural Improvements
There are a number of activities that may require structures and/or specific drainage configurations in
order to protect stormwater and maintain compliance with county code. Roof structure s, wheel washes,
cement pads, shutoff valves, containment berms and indoor mop sinks are all examples of things that need
to be in place prior to commencing the activity. These may require building permits and other approvals
prior to construction.
Below are some highlighted activities and the numbered BMP activity sheets in the Stormwater Pollution
Prevention Manual that provide more detail:
AGENDA ITEM # 8. a)
1.3.4 SPECIAL REQUIREMENT #4: SOURCE CONTROLS
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Commercial Composting
Structural improvements: paved composting and storage pads, leachate collection system, lined collection
ponds, wheel wash system
A-4 Outdoor Storage of Soil, Sand, and Other Erodible Materials
A-24 Commercial Composting
Food and Beverage Manufacturing and Storage
Structural improvements: roofed enclosures, containment, wastewater collection, storage, and disposal
system
A-7 Food and Beverage Manufacturing and Storage
Fueling of Equipment and Vehicles
Structural improvements: Portland cement pads, roofs, spill control devices, trench drains, oil/water
separators
A-17 Stationary Fueling Operations
A-48 Older Stationary Fueling Operations
Greenhouses and Plant Nurseries
Structural improvements: berms, covering, and erosion control measures
A-4 Outdoor Storage of Soil, Sand, and Other Erodible Materials
Horse Stables
Structural improvements: Wash racks connected to sanitary sewer or separate infiltration area, manure
containment areas
A-35 Keeping Livestock in Stables, Pens, Pastures, or Fields
Mining of Sand or Gravel
Structural improvements: Wheel wash system and track-out control, catch basin inserts
A-41 Wheel Wash and Tire Bath Track Out Control
Painting, Finishing, and Coating of Vehicles and Equipment
Structural improvements: Permitted, enclosed paint booths
A-22 Painting, Finishing, & Coating of Vehicles, Products, & Equipment
Restaurants and Food Trucks
Structural improvements: Indoor sinks format and rack washing and mop and wastewater disposal.
A-8 Storage of Solid and Food Wastes (Including Cooking Grease)
A-12 Cleaning or Washing of Food Services Areas and Equipment
Outdoor Storage of Erodible Materials (e.g., compost, bark, sand, etc.)
Structural improvements: Wheel wash system and track-out control, berms, containment areas, covering,
catch basin inserts
A-41 Wheel Wash and Tire Bath Track-Out Control
Outdoor Storage or Processing of Galvanized Materials
Structural improvements: Roofs or other covering, stormwater collection and treatment system
A-21 Manufacturing and Post-Processing of Metal Products
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Storage of Liquid Materials
Structural improvements: Secondary containment, roofed structures, spill control devices
A-2 Outdoor Storage of Liquid Materials in Stationary Tanks
A-3 Storage of Liquid Materials in Portable Containers
Utility Corridor Maintenance
Structural improvements: Road stabilization
A-45 Maintenance of Public and Private Utility Corridors and Facilities
Washing of Cars, Trucks, and Equipment (not just commercial car washes)
Structural improvements: Dedicated wash pads, sewer connection, holding tanks, catch basin inserts
A-13 Vehicle Washing and Steam Cleaning
Wood Treatment and Preserving
Structural improvements: Paved, contained and covered storage and processing areas
A-4 Outdoor Storage of Soil, Sand, and Other Erodible Materials
A-23 Wood Treatment & Preserving
1.3.5 SPECIAL REQUIREMENT #5: OIL CONTROL
Projects proposing to develop or redevelop a high-use site must provide oil controls in addition to any
other water quality controls required by this manual. Such sites typically generate high concentrations of
oil due to high traffic turnover, onsite vehicle or heavy or stationary equipment use, some business
operations, e.g., automotive recycling, or the frequent transfer of liquid petroleum or coal derivative
products.
The traffic threshold in the definition above focuses on vehicle turnover per square foot of building area
(trip generation) rather than ADT alone because oil leakage is greatest when engines are idling or cooling.
In general, all-day parking areas are not intended to be captured by these thresholds except those for diesel
vehicles, which tend to leak oil more than non-diesel vehicles. The petroleum storage and transfer
stipulation is intended to address regular transfer operations like service stations, not occasional filling of
heating oil tanks.
Threshold Requirement
IF a proposed project:
Develops a site that will have high-
use site characteristics, OR
Is a redevelopment project
proposing $100,000 or more of
improvements to an existing high-use
site, OR
Is a redevelopment project that
results in new plus replaced pollution
generating impervious surfaces of
5,000 square feet or more or new
pollution generating pervious surface
of ¾ acre or more improvements to
an existing high-use site …
THEN the project must treat runoff from
the high-use portion of the site using oil
control treatment options from the High-
Use menu (described below and detailed
in Chapter 6).
AGENDA ITEM # 8. a)
1.3.4 SPECIAL REQUIREMENT #4: SOURCE CONTROLS
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High-Use Menu
High-use oil control options are selected to capture and detain oil and associated pollutants. The goal of
this treatment is no visible sheen on runoff leaving the facility, or less than 10 mg/L total petroleum
hydrocarbons (TPH) in the runoff, depending on the facility option used. Oil control options include
facilities that are small, handle only a limited tributary area, and require frequent maintenance, as well as
facilities that treat larger areas and generally have less frequent maintenance needs. Facility choices
include linear sand filters and oil/water separators. See Chapter 6 for specific facility choices and design
details.
Application of this Requirement
For high-use sites located within a larger commercial center, only the impervious surface associated with
the high-use portion of the site is subject to treatment requirements. If common parking for multiple
businesses is provided, treatment shall be applied to the number of parking stalls required for the high-use
business only. However, if the treatment collection area also receives runoff from other areas, the water
quality facility must be sized to treat all water passing through it.
High-use roadway intersections shall treat the intersection itself, plus lanes where vehicles accumulate
during the signal cycle, including all lanes, from the beginning of the left turn pocket (see Figure 1.3.5.A
below). If no left turn pocket exists, the treatable area shall begin at a distance of 75 feet from the stop
line. If runoff from the intersection drains to more than two collection areas that do not combine within the
intersection, treatment may be limited to any two of the collection areas. Oil control facilities shall be
designed for all flows tributary to the oil control facility including flow from otherwise exempt areas that
are not bypassed around the facility.
Note: For oil control facilities to be located in public road right-of-way and maintained by the City of
Renton, only coalescing plate or baffle oil/water separators shall be used unless otherwise approved
through an adjustment. Catch basin inserts are not allowed for oil control.
Methods of Analysis
The traffic threshold for the High-Use menu shall be estimated using information from Trip Generation,
published by the Institute of Transportation Engineers, from a traffic study prepared by a professional
engineer or transportation specialist with experience in traffic estimation, or from documented data from
the City.
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FIGURE 1.3.5.A TREATABLE AREAS FOR HIGH-USE ROAD INTERSECTIONS
AGENDA ITEM # 8. a)
1.3.6 SPECIAL REQUIREMENT #6: AQUIFER PROTECTION AREA
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1.3.6 SPECIAL REQUIREMENT #6: AQUIFER PROTECTION AREA
Aquifer Protection Area(s) (APA) are identified in the RMC 4-3-050. If a proposed project is located
within the APA, this special requirement requires the project to determine those components that are
applicable and delineate them on the project’s site improvements plans. APA zones are depicted in the
Wellhead Protection Area Zones layer of COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>).
Threshold Requirement
IF a proposed project is in Zone 1 of the
APA…
THEN the following are prohibited:
a. Facilities that allow runoff to have
direct contact with the soil, such as
flow control and water quality
treatment ponds, stormwater
wetlands, filter strips, and infiltration
facilities.
b. On-site BMPs that rely on infiltration,
such as bioretention and permeable
pavement.
c. Open channel conveyance systems
that are not concrete lined, such as
ditches and swales.
IF a proposed project is in Zone 1
Modified or Zone 2 of the APA…
THEN the following may require a liner in
accordance with the design criteria in
Section 6.2.4:
a. Facilities that allow runoff to have
direct contact with the soil, such as
flow control and water quality
treatment ponds, stormwater
wetlands, filter strips, and infiltration
facilities.
b. Open channel conveyance systems
that are not concrete lined, such as
ditches as swales.
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1.4 ADJUSTMENT PROCESS
For proposed projects subject to drainage review by the City of Renton this process is provided for the
occasions when a project proponent desires to vary from one of the core or special requirements, or any
other specific requirement or standard contained in this manual. Proposed adjustments should be approved
prior to final permit approval, but they may be accepted up to the time the City approves final construction
or accepts drainage facilities for maintenance.
Types of Adjustments
To facilitate the adjustment process and timely review of adjustment proposals, the following types of
adjustments are provided:
Standard Adjustments: These are adjustments of the standards and requirements contained in the
following chapters and sections of this manual:
o Chapter 1, “Drainage Review and Requirements”
o Chapter 2, “Drainage Plan Submittal”
o Chapter 3, “Hydrologic Analysis and Design”
o Chapter 4, “Conveyance System Analysis and Design”
o Chapter 5, “Flow Control Design”
o Chapter 6, “Water Quality Design”
o Appendix A, “Maintenance Requirements for Stormwater Facilities and On-Site BMPs”
o Appendix B, “Master Drainage Plans”
o Appendix C, “Simplified Drainage Requirements”
o Appendix D, “Construction Stormwater Pollution Prevention Standards”
Requests for standard adjustments will be accepted only for permits pending approval or approved
permits that have not yet expired.
Blanket Adjustments: This type of adjustment may be established by the City based on approval of
any of the above-mentioned adjustments. Blanket adjustments are usually based on previously
approved adjustments that can be applied routinely or globally to all projects where appropriate.
Blanket adjustments are also used to effect minor changes or corrections to manual design
requirements or to add new designs and methodologies to this manual. There is no application process
for establishing blanket adjustments because they are initiated solely by the City.
1.4.1 ADJUSTMENT AUTHORITY
CED shall have full authority to determine if and what type of adjustment is required for any proposed
project subject to drainage review by CED. The authority to grant adjustments for such projects is
distributed as follows:
CED shall have full authority to approve or deny adjustments, except those involving outfalls or pump
discharges to the Green River between River Mile 6 and SR 18 per Section 1.2.4.2.F and 1.2.4.2.I.
CED decisions on those adjustments are subject to approval by the King County Flood Control
District.
1.4.2 CRITERIA FOR GRANTING ADJUSTMENTS
Adjustments to the requirements in this manual may be granted provided that granting the adjustment will
achieve the following:
1. Produce a compensating or comparable result that is in the public interest, AND
2. Meet the objectives of safety, function, appearance, environmental protection, and maintainability
based on sound engineering judgment.
Also, the granting of any adjustment that would be in conflict with the requirements of any other City
department will require review and concurrence with that department.
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Criteria Exception
If it can be demonstrated that meeting the above criteria for producing a compensating or comparable
result will deny reasonable use of a property, approval of the adjustment will require an adjustment
criteria exception to be approved by the City. An adjustment that requires a criteria exception may be
granted following legal public notice of the adjustment request, the proposed decision on the request, and a
written finding of fact that documents the following:
1. There are special physical circumstances or conditions affecting the property such that strict
application of the criteria for producing a compensating or comparable result would deprive the
applicant of all reasonable use of the parcel of land in question, and every effort has been made to find
creative ways to meet the intent of the requirement for which the adjustment is sought, AND
2. Granting the adjustment for the individual property in question will not create a significant adverse
impact to public health, welfare, water quality, and properties downstream or nearby, AND
3. The adjustment requires the best practicable alternative for achieving the spirit and intent of the
requirement in question.
In addition, the written finding of fact must include the following information:
The current (pre-project) use of the site.
How application of the requirement for which an adjustment is being requested denies reasonable use
of the site compared to the restrictions that existed under the 2009 King County Surface Water Design
Manual and City of Renton Amendments to the 2009 King County Surface Water Design Manual.
The possible remaining uses of the site if the criteria exception were not granted.
The uses of the site that would have been allowed under the 2009 King County Surface Water Design
Manual and City of Renton Amendments to the 2009 King County Surface Water Design Manual .
A comparison of the estimated amount and percentage of value loss as a result of the requirements of
this manual versus the estimated amount and percentage of value loss as a result of requirements that
existed under the 2009 King County Surface Water Design Manual and City of Renton Amendments to
the 2009 King County Surface Water Design Manual.
The feasibility for the owner to alter the project to apply the requirements of this manual.46,47
1.4.3 ADJUSTMENT APPLICATION PROCESS
Standard Adjustments
The application process for standard adjustments is as follows:
Requests for standard adjustments will be accepted only for permits pending approval or approved
permits that have not yet expired.
The completed adjustment request application forms must be submitted to CED along with sufficient
engineering information (described in Chapter 2) to evaluate the request. The application shall note
the specific requirement for which the adjustment is sought.
If the adjustment request involves use of a previously unapproved construction material or
construction practice, the applicant should submit documentation that includes, but is not limited to, a
record of successful use by other agencies and/or evidence of meeting criteria for quality and
performance, such as that for the American Association of State Highway and Transportation Officials
(AASHTO) and the American Society of Testing and Materials (ASTM).
If the adjustment requires a criteria exception, additional engineering or other information may be
required by CED to document that denial of reasonable use would occur, that every effort was made
to achieve compliance, and that the best practicable alternative will not cause significant adverse
impact.
A fee reduction may be requested if it is demonstrated that the adjustment request requires little or no
engineering review.
46 Footnote 51 is not used.
47 Footnote 52 is not used.
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Blanket Adjustments
There is no application process for establishing blanket adjustments because they are initiated and issued
solely by the City.
1.4.4 ADJUSTMENT REVIEW PROCESS
All adjustments are governed by the review procedures and time lines set forth by the City. Consistent
with these procedures, the general steps of the review process for specific types of adjustments are
presented as follows.
Standard Adjustments
CED will review the adjustment request application forms and documentation for completeness and
inform the applicant in writing as to whether additional information is required from the applicant in
order to complete the review. The applicant will also be informed if CED determines that special
technical support is required in cases where the adjustment involves a major policy issue or potentially
impacts a City drainage facility.
The CED Development Review Engineer will review and either approve or deny the adjustment
request following determination that all necessary information has been received from the applicant.
If a criteria exception is required for the adjustment, CED will issue a legal public notice of the
adjustment request that indicates the director’s proposed decision on the request, including the written
finding of fact specified in Section 1.4.2. The public notice will include a 15-working-day public
comment period within which a request for reconsideration may be made to the CED director as
described in Section 1.4.5. Absent a request for reconsideration, the director’s decision becomes final
after the two week public comment period.
Approvals of standard adjustments will expire upon expiration of the permit to which they apply.
Blanket Adjustments
Blanket adjustments may be established at the discretion of CED. Blanket adjustments are established by
memorandum based on:
1. Previously approved adjustments and supporting documentation, AND
2. Monitoring results in conjunction with any TAPE or CTAPE results AND
3. Information presenting the need for the blanket adjustment. Typically, blanket adjustments should
apply globally to design or procedural requirements and be independent of site conditions.
CED must approve creation of a blanket adjustment.
Applicants may use any approved blanket adjustment listed in Reference Section 14, by submitting the
form titled “Surface Water Design Manual Requirements/Standards Blanket Adjustment No. ____” to the
CED plan reviewer currently reviewing the specific project proposal, but no further approval is required.
1.4.5 APPEALS
Any appeals from administrative determinations for variances or adjustments related to the Storm
Drainage regulations and codes shall be filed in writing to the Hearing Examiner by any person aggrieved,
or by any officer, department, board or bureau of the City affected by such determination per
RMC 4-8-110.
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AGENDA ITEM # 8. a)
2022 Surface Water Design Manual 6/22/2022
CHAPTER 2
DRAINAGE PLAN
SUBMITTAL
CITY OF RENTON
SURFACE WATER
DESIGN MANUAL
Section Page
2.1 Plans for Permits and Drainage
Review 2-3
2.1.1 Plans Required for Pre-Application
Submittal 2-3
2.1.2 Site Plans Required for Drainage
Review 2-3
2.2 Plans Required with Construction
Permit Application 2-5
2.2.1 Subdivision, PUD, and Binding Site
Plans 2-6
2.2.2 Short Subdivisions 2-7
2.2.3 Commercial Site Development 2-7
2.2.4 Single-Family Residential 2-7
2.2.5 Other Permits 2-7
2.3 Drainage Review Plan Specifications 2-9
2.3.1 Engineering Plan Specifications 2-10
2.3.2 Projects in Targeted Drainage Review
(TDR) 2-35
2.4 Plans Required After Drainage
Review 2-37
2.4.1 Plan Changes After Permit Issuance 2-37
2.4.2 Final Corrected Plan Submittal 2-37
2.4.3 Final Plat, Short Plat, and Binding Site
Plan Submittals 2-38
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CHAPTER 2
DRAINAGE PLAN SUBMITTAL
This chapter details the drainage related submittal requirements for engineering design plans as part of a
permit application to the City of Renton Community and Economic Development (CED) Department. The
intent of these requirements is to present consistent formats for design plans and the technical support data
required to develop the plans. These conventions are necessary to review engineering designs for
compliance with City of Renton ordinances and regulations, and to ensure the intent of the plan is easily
understood and implemented in the field. Properly drafted design plans and supporting information also
facilitate the construction, operation, and maintenance of the proposed system long after its review and
approval. When plans comply with the formats and specifications contained herein, they facilitate review
and approval with a minimum of time-consuming corrections and resubmittals.
Note that this chapter primarily describes how to submit drainage plans for review—what must be
submitted, in what formats, at what times and to what offices. The basic drainage requirements that these
plans must address are contained in Chapter 1, “Drainage Review and Requirements.” The specific design
methods and criteria to be used are contained in Chapters 3, 4, 5, and 6.
Several key forms used in the plan review process are reproduced in Reference Section 8, “Forms and
Worksheets.” The drainage submittal requirements for different types of developments are contained in
this chapter with the exception of Master Drainage Plans, which if required, the scope of the requirements
will be determined by the Surface Water Utility and will generally follow King County’s Master Drainage
Planning for Large or Complex Site Development and requirements. For information on general
requirements for any permit type, refer to the City of Renton website or customer information counter at
CED.
Chapter Organization
The information presented in this chapter is organized into four main sections as follows:
Section 2.1, “Plans for Permits and Drainage Review”
Section 2.2, “Plans Required with Construction Permit Application”
Section 2.3, “Drainage Review Plan Specifications”
Section 2.4, “Plans Required After Drainage Review”
These sections begin on odd pages so the user can insert tabs if desired for quicker reference.
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2.1 PLANS FOR PERMITS AND DRAINAGE REVIEW
CED is responsible for the review of all engineering aspects of private development proposals. Drainage
review is a primary concern of engineering design. This section describes the types of engineered
drainage plans required for engineering review at various permit review stages. Refer to the City website
for other details or requirements, such as the submittal and expiration periods set for each type of permit
application, review fees, right-of-way use requirements, and other code requirements.
2.1.1 PLANS REQUIRED FOR PRE-APPLICATION SUBMITTAL
Most projects require some degree of drainage plans or analysis to be submitted for drainage review; the
extent of which will depend upon the type of permit, size and type of project, and project vicinity to any
critical areas. All proposed developments within the City of Renton have the option to request a free pre-
application meeting to gain feedback on development requirements and feasibility prior to formal
submittal of any required permit application materials. Subdivisions, PUDs, short plats and binding site
plans require conceptual plans (may be engineered or non-engineered) be submitted with the pre-
application meeting request. Preliminary plans provide general information on the proposal, including
location of critical areas, road alignments and right-of-way, site topography, building locations, land use
information, and lot dimensions. They are used to determine the appropriate drainage conditions and
requirements to be applied to the proposal during the drainage review process.
For more information refer to the permit submittal requirements documents that are applicable to the
development proposal (available on the City’s website and from staff in the pre-application meeting, if
held).
2.1.2 SITE PLANS REQUIRED FOR DRAINAGE REVIEW
For drainage review purposes, engineering plans consist of the following:
1. Site improvement plans (see Section 2.3.1.2), which include all plans, profiles, details, notes, and
specifications necessary to construct road, drainage, utilities, and off-street parking improvements.
2. A construction stormwater pollution prevention (CSWPP) plan, which identifies the measures and
BMPs required to prevent the discharge of sediment-laden water and other pollutants associated with
construction/land disturbing activities. The CSWPP plan includes two component plans: an erosion
and sediment control (ESC) plan (see Section 2.3.1.3), which addresses prevention of sediment-
laden discharges; and a stormwater pollution prevention and spill (SWPPS) plan (see
Section 2.3.1.4), which addresses prevention of other pollutant discharges.
3. A technical information report (TIR) (see Section 2.3.1.1), which contains all the technical
information and analysis necessary to develop the site improvement plan and CSWPP plan.
Projects Under Targeted Drainage Review usually require engineering plans, except that only certain
sections of the technical information report are required to be completed and the site improvement plan
may have a limited scope depending upon the characteristics of the proposed project. The scope of these
plans should be confirmed during the pre-application meeting with CED. For other permits, such as
single-family residential permits, the scope of the targeted engineering analysis is usually determined
during CED engineering review.
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Plans Required for Simplified Drainage Review
Simplified drainage plans are a simplified form of site improvement and CSWPP plans (without a TIR or a
separate SWPPS plan) that may be prepared by a non-engineer from a set of pre-engineered design details.
Simplified drainage plans are only allowed for single family in Simplified Drainage Review but may be
required for individual lots created by a subdivision project to show how required on-site BMPs, ESC and
SWPPS measures will be applied to future lot construction.
For single-family residential permits, the level and scope of drainage plan requirements are determined by
CED during drainage review. Some projects subject to Simplified Drainage Review may also require
Targeted Drainage Review.
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2.2 PLANS REQUIRED WITH CONSTRUCTION PERMIT
APPLICATION
This section describes the submittal requirements for construction permit applications at the City of
Renton. Required plans for each permit or project type described in Section 2.2.1 through 2.2.5 are
summarized in Table 2.2.A. The timing for submittal of engineering plans will vary depending on permit
type. For commercial, subdivisions, short plats, and other types of construction permits, this submittal
usually follows the City’s approval of plans described in Section 2.2. For commercial building permits,
engineering plans must be submitted as part of the construction permit application, unless otherwise
approved by CED. For other permit types the drainage plan requirements are determined during the permit
review process.
Note: If engineering plans are required to be submitted with the construction permit application, they must
be accompanied by the appropriate supporting documents (e.g., required application forms, reports, etc.).
For more details, see the City’s website.
Design Plan Certification
All preliminary plans and engineering plans must be stamped by a civil engineer.
All land boundary surveys and legal descriptions used for preliminary and engineering plans must be
stamped by a land surveyor licensed in the State of Washington. Topographic survey data and mapping
prepared specifically for a proposed project may be performed by the civil engineer stamping the
engineering plans as allowed by the Washington State Board of Registration for Professional Engineers
and Land Surveyors.
TABLE 2.2.A DRAINAGE PLAN SUBMITTALS
Type of Permit
or Project
Plans Required with
Construction Permit
Application
Type of
Drainage Review
Plans Required for
Drainage Review
SUBDIVISIONS,
PUDs, AND
BINDING SITE
PLANS
Plat Map(5)
Engineering Plans(1),(2)
Full or Targeted Drainage
Review(2)
Preliminary Plans(5)
Engineering Plans(1)
Large Project Drainage
Review
Preliminary Plans(5)
Master Drainage
Plan(4) or Special
Study
Engineering Plans(1)
SHORT PLATS Site Plan(5) Simplified Drainage Review Simplified Drainage
Plans(3)
Site Plan(5)
Preliminary Reports
Simplified Drainage Review
AND Targeted Drainage
Review(2)
Simplified Drainage
Plans(3)
Engineering Plans(1)
Full or Targeted Drainage
Review(2)
Engineering Plans(1)
COMMERCIAL
PERMITS
Engineering Plans(1),(2) Full or Targeted Drainage
Review
Engineering Plans(1)
SINGLE-
FAMILY
Site Plan(5) for Single-Family
Residential Building Permits
Simplified Drainage Review Simplified Drainage
Plans(3)
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TABLE 2.2.A DRAINAGE PLAN SUBMITTALS
Type of Permit
or Project
Plans Required with
Construction Permit
Application
Type of
Drainage Review
Plans Required for
Drainage Review
RESIDENTIAL
BUILDING
PERMITS
Simplified Drainage Review
AND Targeted Drainage
Review(2) AND Directed
Drainage Review(6)
Simplified Drainage
Plans(3)
Engineering
Plans(1)(6)
Full or Targeted Drainage
Review(2)
Engineering Plans(1)
OTHER
PROJECTS OR
PERMITS
Project-specific (contact CED
or the City’s website)
Full or Targeted Drainage
Review(2)
Engineering Plans(1)
Notes:
(1) Submittal specifications for engineering plans are detailed in Section 2.3.1.
(2) Submittal specifications for Targeted Drainage Review are found in Section 2.3.2.
(3) Specifications for submittal of Simplified drainage plans are found in Appendix C, Simplified Drainage Requirements.
(4) Specifications for submittal of master drainage plans or special studies are found in the King County publication titled
Master Drainage Planning for Large or Complex Site Developments.
(5) Submittal specifications for these plans are found on the City’s website and/or from CED staff in the pre-application
meeting.
(6) Scope of submittals for Directed Drainage Review is determined by CED staff at the City’s Permit Counter and/or
during the plan review process. Submittal specifications per Notes 1, 2, and 3.
2.2.1 SUBDIVISION, PUD, AND BINDING SITE PLANS
Applications for proposed subdivision, PUD, and binding site plan projects must include engineered
preliminary plans, which are used to help determine engineering plan requirements to recommend to the
Hearing Examiner. Preliminary plans shall include the following:
1. A conceptual drainage plan prepared, stamped, and signed by a civil engineer. This plan must show
the location and type of the following:
a) Existing and proposed flow control facilities
b) Existing and proposed water quality facilities
c) Existing and proposed conveyance systems.
The level of detail of the plan should correspond to the complexity of the project.
2. A Level 1 Downstream Analysis as required in Core Requirement #2 and outlined under “TIR
Section 3, Offsite Analysis.” This offsite analysis shall be submitted in order to assess potential offsite
drainage and water quality impacts associated with development of the project, and to help propose
appropriate mitigation of those impacts. A higher level of offsite analysis may be requested by the
City prior to preliminary approval, or as a condition of engineering plan submittal. The offsite analysis
must be prepared, stamped, and signed by a civil engineer.
3. Survey/topographic information. The submitted site plan and conceptual drainage plan shall include
the following:
a) Field topographic base map to accompany application (aerial topography allowed with CED
permission)
b) Name and address of surveyor and surveyor’s seal and signature
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c) Notation for field or aerial survey
d) Datum and benchmark/location and basis of elevation
e) Location of all critical areas
f) Contour intervals per the following chart:
Zoning Designation Contour Intervals
Densities of developed area
of over 4 DU per acre
2 feet at less than 15% slope
5 feet at 15% slope or more
Densities of developed area
of 4 DU or less per acre
5 feet
2.2.2 SHORT SUBDIVISIONS
Applications for proposed short plats1 require a proposed site plan drawn to scale showing geographic
features such as adjacent streets, existing buildings, and critical areas if any are known to be present; and a
Level 1 Downstream Analysis. Site plans are usually engineered, except for projects exempt from
drainage review or projects subject to Simplified Drainage Review for the entire project. The
specifications for submittal of site plans are outlined on the City’s website.
The Level 1 Downstream Analysis is required for all short plats except those meeting the exemptions
outlined in Section 1.2.2 or those subject to Simplified Drainage Review for the entire project. A higher
level of offsite analysis may be requested by CED prior to preliminary approval, or as a condition of
engineering plan submittal.
2.2.3 COMMERCIAL SITE DEVELOPMENT
Applications for commercial permits require that engineering plans be submitted as part of the building
permit application, unless otherwise approved by CED. Most commercial projects will go through Full
Drainage Review and require complete engineering plans. Projects that qualify for limited scope
engineering design should request Targeted Drainage Review during the pre-application meeting with
CED.
2.2.4 SINGLE-FAMILY RESIDENTIAL
Applications for single-family residential permits1 require a non-engineered site plan to be submitted. The
specifications for site plans are outlined on the City’s website.
2.2.5 OTHER PERMITS
Other permit applications1 will require project-specific information. Submittal requirements can be
obtained by contacting CED or consulting the City’s website.
1 The specific level of required drainage analysis and design is usually determined during the preliminary drainage review of the
plans submitted with the application. The overall plan review process may be expedited if the project is submitted with the
appropriate level of detail.
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2.3 DRAINAGE REVIEW PLAN SPECIFICATIONS
This section presents the specifications and contents required of plans to facilitate drainage review. Most
projects subject to Full Drainage Review will require engineering plans that include a “technical
information report (TIR),” “site improvement plans,” and a “construction stormwater pollution prevention
(CSWPP) plan,” which includes an “erosion and sediment control (ESC) plan” and a “stormwater
pollution prevention and spill (SWPPS) plan.” For more information on the types of projects subject to
Full Drainage Review, see Section 1.1.2.4. Additional information is available at the City’s website and
from the staff in the pre-application meeting, if held.
Small projects with specific drainage concerns that are subject to Targeted Drainage Review also require
engineering plans that include the same elements, except that the TIR may be of limited scope. The site
improvement plans, ESC and SWPPS plans may also be of limited scope, but must meet all applicable
specifications. For more information on the types of projects subject to Targeted Drainage Review, see
Section 1.1.2.2.
Projects subject to Simplified Drainage Review may be required to submit “Simplified drainage plans.”
These are simplified drainage and erosion control plans that may be prepared by a non-engineer from a set
of pre-engineered design details, and which do not require a TIR or a separate SWPPS plan. The
Simplified Drainage Requirements booklet available at King County Department of Permitting and
Environmental Review and appended to this manual (Appendix C) contains the specifications for
Simplified drainage plans and details on the Simplified Drainage Review process.
Note: Projects in Simplified Drainage Review may be required to submit engineering plans if they are also
subject to Targeted Drainage Review as determined in Section 1.1.2.2 and Appendix C. Also, short plats in
Simplified Drainage Review will be required to submit engineering plans if roadway construction is a
condition of preliminary approval.
Single-family residential projects that do not qualify for Simplified Drainage Review may qualify for
Directed Drainage Review, which requires a specialized list of submittals (plans, technical reports, etc.)
and engineering requirements determined by CED review staff that ensures compliance with all core and
special requirements of the SWDM. The scope of the submittal requirement is determined during the
initial review of the application. Specifications for the plans and TIR generally follow those described for
the other review types but may be reduced in scope or complexity in accordance with CED’s
determination.
Design Plan Certification
All preliminary plans and engineering plans must be stamped by a civil engineer.
All land boundary surveys, and legal descriptions used for preliminary and engineering plans must be
stamped by a land surveyor licensed in the State of Washington. Topographic survey data and mapping
prepared specifically for a proposed project may be performed by the civil engineer stamping the
engineering plans as allowed by the Washington State Board of Registration for Professional Engineers
and Land Surveyors.
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2.3.1 ENGINEERING PLAN SPECIFICATIONS
For drainage review purposes, engineering plans must consist of the following:
1. A TIR as detailed in Section 2.3.1.1, AND
2. Site improvement plans as detailed in Section 2.3.1.2, AND
3. A CSWPP plan, which includes an ESC plan as detailed in Section 2.3.1.3 and a SWPPS plan as
detailed in Section 2.3.1.4.
Projects in Targeted Drainage Review require a limited scope TIR with site improvement plans and a
CSWPP plan, as detailed in Section 2.3.2. CED may allow a modified site improvement plan for some
projects in Targeted Drainage Review (see Section 2.3.2) or where major improvements (e.g., detention
facilities, conveyance systems, bridges, road right-of-way improvements, etc.) are not proposed.
2.3.1.1 TECHNICAL INFORMATION REPORT (TIR)
The full TIR is a comprehensive supplemental report containing all technical information and analysis
necessary to develop the site improvement plan. This report shall contain all calculations, conceptual
design analysis, reports, and studies required and used to construct a complete site improvement plan
based on sound engineering practices and careful geotechnical and hydrological design. The TIR must be
stamped and dated by a civil engineer.
The TIR shall contain the following ten sections, preceded by a table of contents:
1. Project Overview
2. Conditions and Requirements Summary
3. Offsite Analysis
4. Flow Control, Low Impact Development (LID) and Water Quality Facility Analysis and Design
5. Conveyance System Analysis and Design
6. Special Reports and Studies
7. Other Permits
8. CSWPP Analysis and Design
9. Bond Quantities, Facility Summaries, and Declaration of Covenant
10. Operations and Maintenance Manual.
Every TIR must contain each of these sections; however, if a section does not apply, the applicant may
simply mark “N/A” and a brief explanation shall be provided. This standardized format allows a quicker,
more efficient review of information required to supplement the site improvement plan.
The table of contents should include a list of the ten section headings and their respective page numbers, a
list of tables with page numbers, and a list of numbered references, attachments, and appendices.
When the TIR package requires revisions, the revisions must be submitted in a complete TIR package.
TIR SECTION 1
PROJECT OVERVIEW
The project overview must provide a general description of the proposal, predeveloped and developed site
conditions, site and project site area, size of the improvements, and the disposition of stormwater runoff
before and after development. The overview shall identify and discuss difficult site parameters, the natural
drainage system, and drainage to and from adjacent property, including bypass flows.
The following figures are required:
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Figure 1. TIR Worksheet
Include a copy of the TIR Worksheet (see Reference Section 8-A).
Figure 2. Site Location
Provide a map that shows the general location of the site. Identify all roads that border the site and all
significant geographic features and critical areas (lakes, streams, steep slopes, etc.).
Figure 3. Drainage Basins, Subbasins, and Site Characteristics
This figure shall display the following:
1. Show acreage of subbasins.
2. Identify all site characteristics.
3. Show all areas used for treatment trades or mitigation trades, if applicable.
4. Show all onsite and offsite bypass areas, if applicable.
5. Show all threshold discharge areas (TDAs).
6. Show existing discharge points to and from the site.
7. Show routes of existing, construction, and future flows at all discharge points and downstream
hydraulic structures.
8. Use a minimum USGS 1:2400 topographic map as a base for the figure.
9. Show (and cite) the length of travel from the farthest upstream end of a proposed storm system in the
development to any proposed flow control facility.
Figure 4. Soils
Show the soils within the following areas:
1. The project site.
2. The area draining to the site.
3. The drainage system downstream of the site for the distance of the downstream analysis (see
Section 1.2.2).
Copies of King County Soil Survey maps may be used; however, if the maps do not accurately represent
the soils for a proposed project (including offsite areas of concern), it is the design engineer’s
responsibility to ensure that the actual soil types are properly mapped. Soil classification symbols that
conform to the SCS Soil Survey for King County shall be used; and the equivalent soil type (till, outwash,
or wetlands) per the approved stormwater model shall be indicated (see Table 3.2.2.B).
Projects will need to evaluate the soils on each lot for applicability of the full infiltration and other low
impact on-site BMPs as specified in Core Requirement #9. This soils report, as well as geotechnical
investigations necessary for proposed infiltration facilities, shall be referenced in the TIR Overview and
submitted under Special Reports and Studies, TIR Section VI. A figure in the required geotechnical report
that meets the above requirements may be referenced to satisfy 1, 2, and 3 above.
Projects located in outwash soils may need to provide a low-permeability liner or a treatment liner for
facilities that allow runoff to have direct contact with the soil and open channel conveyance systems that
are not concrete lined, consistent with the specifications for such liners in Section 6.2.4.
TIR SECTION 2
CONDITIONS AND REQUIREMENTS SUMMARY
The intent of this section is to ensure all preliminary approval conditions and applicable requirements
pertaining to site engineering issues have been addressed in the site improvement plan. All conditions and
requirements for the proposed project shall be included.
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In addition to the core requirements of this manual, adopted basin plans and other plans as listed in Special
Requirement #1 should be reviewed and applicable requirements noted. Additionally, critical area
requirements, conditions of plat approval, and conditions associated with development requirements (e.g.,
conditional use permits, rezones, variances and adjustments, SEPA mitigations, etc.) shall be included.
TIR SECTION 3
OFFSITE ANALYSIS
All projects in engineering review shall complete, at a minimum, an Offsite Analysis, except for projects
meeting the exemptions outlined in Section 1.2.2. The Offsite Analysis is usually completed as part of the
initial permit application and review process, and is to be included in the TIR. Note: If offsite conditions
have been altered since the initial submittal, a new offsite analysis may be required.
The primary component of the offsite analysis is the downstream analysis described in detail below.
Upstream areas are included in this component to the extent they are expected to be affected by backwater
effects from the proposed project. Other components of the offsite analysis could include, but are not
limited to, evaluation of impacts to fish habitat, groundwater levels, groundwater quality, or other
environmental features expected to be significantly impacted by the proposed project due to its size or
proximity to such features.
Levels of Analysis
The offsite analysis report requirements vary depending on the specific site and downstream conditions.
Each project submittal shall include at least a Level 1 downstream analysis. Upon review of the Level 1
analysis, CED may require a Level 2 or Level 3 analysis. If conditions warrant, additional, more detailed
analysis may be required. Note: Potential impacts upstream of the proposal shall also be evaluated.
Level 1 Analysis
The Level 1 analysis is a qualitative survey of each downstream system leaving a site. This analysis is
required for all proposed projects and shall be submitted with the initial permit application. Depending
on the findings of the Level 1 analysis, a Level 2 or 3 analysis may need to be completed or additional
information may be required. If further analysis is required, the applicant may schedule a meeting with
CED staff.
Level 2 or 3 Analysis
If drainage problems are identified in the Level 1 analysis, a Level 2 (rough quantitative) analysis or a
Level 3 (more precise quantitative) analysis may be required to further evaluate proposed mitigation
for the problem. CED staff will determine whether a Level 2 or 3 analysis is required based on the
evidence of existing or potential drainage problems identified in the Level 1 analysis and on the
proposed design of onsite drainage facilities. The Level 3 analysis is required when results need to be
as accurate as possible: for example, if the site is flat; if the system is affected by downstream
controls; if minor changes in the drainage system could flood roads or buildings; or if the proposed
project will contribute more than 15 percent of the total peak flow to the drainage problem location.
The Level 2 or 3 analysis may not be required if CED determines from the Level 1 analysis that
adequate mitigation will be provided.
Additional Analysis
Additional, more detailed hydrologic analysis may be required if CED determines that the
downstream analysis has not been sufficient to accurately determine the impacts of a proposed project
on an existing or potential drainage problem. This more detailed analysis may include a point of
compliance analysis as detailed in Section 3.3.6.
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Scope of Analysis
Regardless of the level of downstream analysis required, the applicant shall define and map the study area
(Task 1), review resources (Task 2), inspect the study area (Task 3), describe the drainage system and
problems (Task 4), and propose mitigation measures (Task 5) as described below.
Task 1. Study Area Definition and Maps
For the purposes of Task 2 below, the study area shall extend downstream one mile (minimum
flowpath distance) from the proposed project discharge location and shall extend upstream as
necessary to encompass the offsite drainage area tributary to the proposed project site. For the
purposes of Tasks 3, 4, and 5, the study area shall extend downstream to a point on the drainage
system where the proposed project site constitutes less than 15 percent of the total tributary drainage
area, but not less than one-quarter mile (minimum flowpath distance). The study area shall also extend
upstream of the project site a distance sufficient to preclude any backwater effects from the proposed
project.
The offsite analysis shall include a site map showing property lines, and the best available
topographical map (e.g., from CED and Renton topographic map) with the study area boundaries,
site boundaries, downstream flowpath for a distance of one mile, and potential/existing problems
(Task 4) shown. Other maps, diagrams, photographs and aerial photos may be helpful in describing
the study area.
Task 2. Resource Review
To assist the design engineer in preparing an offsite analysis, Renton has gathered information
regarding existing and potential flooding, erosion, and water quality problems. For all levels of
analysis, all of the resources described below shall be reviewed for existing/potential problems in the
study area (upstream and one mile downstream of the project site):
Adopted basin plans available at King County DLS-Permitting, King County DNRP, and CED.
For areas where there is no adopted basin plan, Basin Reconnaissance Summary Reports may
be useful.
Floodplain/floodway (FEMA) maps available at CED and the library.
Other offsite analysis reports in the same subbasin, if available (check with CED staff).
Sensitive Areas Folio available at King County DLS-Permitting, King County DNRP, and COR
Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>) must be used to
document the distance downstream from the proposed project to the nearest critical areas.
2U.S. Department of Agriculture, King County Soils Survey available at King County DLS-
Permitting and the library.
Wetlands Inventory maps available at CED.
Washington State Department of Ecology’s latest published Clean Water Act Section 303d list of
polluted waters posted at the following website: <https://ecology.wa.gov/Water-
Shorelines/Water-quality/Water-improvement/Assessment-of-state-waters-303d>.
City of Renton Erosion Maps and Landslide Maps.
Potential/existing problems identified in the above documents shall be documented in the Drainage
System Table (see Reference Section 8-B) as well as described in the text of the Level 1
Downstream Analysis Report. If a document is not available for the site, note in the report that the
information was not available as of a particular date. If necessary, additional resources are available
from King County, the Washington State Department of Fisheries and Wildlife (WDFW), the State
Department of Ecology (Ecology), the United States Army Corps of Engineers (Corps), and the public
works departments of other municipalities in the vicinity of the proposed project site.
2 Footnote 2 is not used.
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Task 3. Field Inspection
The design engineer shall physically inspect the existing on- and offsite drainage systems of the study
area for each discharge location. Specifically, he/she shall investigate any evidence of the following
existing or potential problems and drainage features:
Level 1 Inspection:
1. Investigate any problems reported or observed during the resource review.
2. Locate all existing/potential constrictions or lack of capacity in the existing drainage system.
3. Identify all existing/potential downstream drainage problems as defined in Section 1.2.2.1.
4. Identify existing/potential overtopping, scouring, bank sloughing, or sedimentation.
5. Identify significant destruction of aquatic habitat or organisms (e.g., severe siltation, bank
erosion, or incision in a stream).
6. Collect qualitative data on features such as land use, impervious surfaces, topography, and
soil types.
7. Collect information on pipe sizes, channel characteristics, drainage structures, and relevant
critical areas (e.g., wetlands, streams, steep slopes).
8. Verify tributary basins delineated in Task 1.
9. Contact neighboring property owners or residents in the area about past or existing drainage
problems, and describe these in the report (optional).
10. Note the date and weather conditions at the time of the inspection.
Level 2 or 3 Inspection:
1. Perform a Level 1 Inspection.
2. Document existing site conditions (approved drainage systems or pre-1979 aerial
photographs) as defined in Core Requirement #3.
3. Collect quantitative field data. For Level 2, conduct rough field survey using hand tape, hand
level, and rod; for Level 3, collect field survey profile and cross-section topographic data
prepared by an experienced surveyor.
Task 4. Drainage System Description and Problem Descriptions
Each drainage system component and problem shall be addressed in the offsite analysis report in three
places: on a map (Task 1), in the narrative (Task 4), and in the Offsite Analysis Drainage System Table
(see Reference Section 8-B).
Drainage System Descriptions: The following information about drainage system components such
as pipes, culverts, bridges, outfalls, ponds, tanks, and vaults shall be included in the report:
1. Location (corresponding map label and distance downstream/upstream from site discharge)
2. Physical description (type, size, length, slope, vegetation, and land cover)
3. Problems including copies of any relevant drainage complaints
4. Field observations.
Problem Descriptions: All existing or potential drainage and water quality problems (e.g., ponding
water, high/low flows, siltation, erosion, listed water bodies, etc.) identified in the resource review or
field inspection shall be described in the offsite analysis. These descriptions will help in determining if
such problems require special attention per Core Requirement #2 (see Section 1.2.2.1) because they
are one of three defined drainage problem types or one of seven defined water quality problem types.
Special attention may include more analysis, additional flow control, or other onsite or offsite
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mitigation measures as specified by the problem-specific mitigation requirements set forth in
Sections 1.2.2.2 and 1.2.2.3.
The following information shall be provided for each existing or potential drainage problem:
1. Description of the problem (ponding water, high or low flows, siltation, erosion, slides, etc.).
2. Magnitude of or damage caused by the drainage problem (siltation of ponds, dried-up ornamental
ponds, road inundation, flooded property, flooded building, flooded septic system, significant
destruction of aquatic habitat or organisms).
3. General frequency and duration of drainage problem (dates and times the problem occurred, if
available).
4. Return frequency of storm or flow (cfs) of the water when the problem occurs (optional for
Level 1 and required for Levels 2 and 3). Note: A Level 2 or 3 analysis may be required to
accurately identify the return frequency of a particular downstream problem; see Section 3.3.3.
5. Water surface elevation when the problem occurs (e.g., elevation of building foundation, crest of
roadway, elevation of septic drainfields, or wetland/stream high water mark).
6. Names and concerns of involved parties (optional for all levels of analysis).
7. Current mitigation of the drainage problem.
8. Possible cause of the drainage problem.
9. Whether the proposed project is likely to aggravate (increase the frequency or severity of) the
existing drainage problem or create a new one based on the above information. For example, an
existing erosion problem should not be aggravated if Durational flow control is already required
in the region for the design of onsite flow control facilities. Conversely, a downstream flooding
problem inundating a home every 2 to 5 years will likely be aggravated if only Peak Flow
Control is being applied in the region. See Section 1.2.3.1 for more details on the effectiveness of
flow control standards in addressing downstream problems.
The following information shall be provided for each existing or potential water quality problem:
1. Description of the problem as documented by the State, County, or City in the problem’s listing.
This should include the pollutant or pollutants of concern, the nature or category of the listing,
and any other background information provided in the listing.
2. Flow path distance downstream of the project site and percentage of area draining to the problem
that the project site occupies.
3. Possible or probable cause of the water quality problem.
4. Any current mitigation of the water quality problem.
Task 5. Mitigation of Existing or Potential Problems
For any existing or potential offsite drainage problem determined to be one of the three defined
problem types in Section 1.2.2.1, the design engineer must demonstrate that the proposed project
neither aggravates (if existing) nor creates the problem as specified in the drainage problem-specific
mitigation requirements set forth in Section 1.2.2.2. The engineer must review each relevant drainage
complaint found and include a narrative explaining how each complaint problems is addressed or
mitigated. Actual copies of the relevant complaints must be included in the Analysis. To meet these
requirements, the proposed project may need to provide additional onsite flow control as specified in
Table 1.2.3.A (see also Section 3.3.5), or other onsite or offsite mitigation measures as described in
Section 3.3.5.
For any existing or potential water quality problem determined to be one of the seven defined water
quality problem types in Section 1.2.2.1, the design engineer must document how the applicable water
quality problem-specific mitigation requirement in Section 1.2.2.3 will be met.
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TIR SECTION 4
FLOW CONTROL, LOW IMPACT DEVELOPMENT (LID) AND WATER QUALITY
FACILITY ANALYSIS AND DESIGN
Existing Site Hydrology (Part A)
This section of the TIR shall include a discussion of assumptions and site parameters used in analyzing the
existing site hydrology.
The acreage, soil types, and land covers used to determine existing flow characteristics, along with basin
maps, graphics, and exhibits for each subbasin affected by the development, shall be included.
The following information must be provided on a topographical map:
1. Delineation and acreage of areas contributing runoff to the site
2. Flow control facility and BMP location(s)
3. Outfall(s)
4. Overflow route(s)
5. Area(s) used for treatment trade or mitigation trade, if applicable
6. Onsite and offsite bypass area(s), if applicable
7. Threshold discharge area(s)
The scale of the map and the contour intervals must be sufficient to determine the basin and subbasin
boundaries accurately. The direction of flow, the acreage of areas contributing drainage, and the limits of
development shall all be indicated on the map.
Each subbasin contained within or flowing through the site shall be individually labeled and parameters
for the approved stormwater model referenced to that subbasin.
All natural streams and drainage features, including wetlands and depressions, must be shown. Rivers,
closed depressions, streams, lakes, and wetlands must have the 100-year floodplain (and floodway where
applicable) delineated as required in Special Requirement #2 (see Section 1.3.2) and by the critical areas
requirements in RMC 4-3-050.
Developed Site Hydrology (Part B)
This section shall provide narrative, mathematical, and graphical presentations of parameters selected and
values used for the developed site conditions, including acreage, soil types and land covers, roadway
layouts, and all constructed drainage facilities and any required on-site BMPs.
Developed subbasin areas and flows shall be clearly depicted on a map and cross-referenced to computer
printouts or calculation sheets. Relevant portions of the calculations shall be highlighted and tabulated in a
listing of all developed subbasin flows.
All maps, exhibits, graphics, and references used to determine developed site hydrology must be included,
maintaining the same subbasin labeling as used for the existing site hydrology whenever possible. If the
boundaries of the subbasin have been modified under the developed condition, the labeling should be
modified accordingly (e.g., Subbasin “Am” is a modified version of existing Subbasin “A”).
Performance Standards (Part C)
The design engineer shall include brief discussions of the following:
The applicable area-specific flow control facility standard as depicted in the Flow Control
Application layer in COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>) per Section 1.2.3.1, any
modifications to the standard to address onsite or offsite drainage conditions, and applicable on-site
BMP requirements determined from Section 1.2.3.3 and Core Requirement #9; and
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The applicable area-specific water quality treatment menu determined from the Water Quality
Applications Map per Section 1.2.8.1, and any applicable special requirements for source control or
oil control determined from Sections 1.3.4 and 1.3.5.
Flow Control System (Part D)
This section requires:
An illustrative sketch of the flow control facility (or facilities), required on-site BMPs, and
appurtenances. The facility sketch (or sketches) must show basic measurements necessary to calculate
the storage volumes available from zero to the maximum head, all orifice/restrictor sizes and head
relationships, control structure/restrictor orientation to the facility, and facility orientation on the site.
The on-site BMP sketch (or sketches) must show basic measurements and dimensions, orientation on
the site, flowpath lengths, etc.
The applicant shall include all supporting documentation such as computer printouts, calculations,
equations, references, storage/volume tables, graphs, soils data, geotechnical reports and any other
aides necessary to clearly show results and methodology used to determine the storage facility
volumes and on-site BMP applications.
Facility documentation files, flow duration comparison files, peaks files, return frequency or
duration curves, etc., developed with the approved model shall be included to verify the facility meets
the performance standards indicated in Part C.
The volumetric safety factor used in the design shall be clearly identified, as well as the reasoning
used by the design engineer in selecting the safety factor for this project.
If on-site BMP credits are used as allowed in Core Requirement #9, documentation must be provided,
explaining how the credits will be used and how the criteria for use of credits will be met.
If the flow control system is an infiltration facility, the soils data, groundwater mounding analysis, and
other calculations used to determine the design infiltration rate shall be provided.
On-site BMP infeasibility discussion and supporting documentation shall also be included in Part D.
Water Quality System (Part E)
This section requires an illustrative sketch of the proposed water quality facility (or facilities), source
controls, oil controls, and appurtenances. This sketch (or sketches) of the facility, source controls, and oil
controls must show basic measurements and dimensions, orientation on the site, location of inflow,
bypass, and discharge systems, etc.
The applicant shall also include all supporting documentation such as computer printouts, calculations,
equations, references, and graphs necessary to show the facility was designed and sized in accordance with
the specifications and requirements in Chapter 6. If the water quality credit option is used as allowed in
Section 6.1.3, documentation must be provided, identifying the actions that will be taken to acquire the
requisite credits.
TIR SECTION 5
CONVEYANCE SYSTEM ANALYSIS AND DESIGN
This section shall present a detailed analysis of any existing conveyance systems, and the analysis and
design of the proposed stormwater collection and conveyance system for the development. This section
also includes any analysis required for the design of bridges to convey flows and pass sediments and
debris per Section 4.4.3. Analysis information should be presented in a clear, concise manner that can be
easily followed, checked, and verified. This section shall explain the applicable conveyance system
capacity standards per Section 1.2.4. All pipes, culverts, catch basins, channels, swales, and other
stormwater conveyance appurtenances must be clearly labeled and correspond directly to the engineering
plans.
The minimum information included shall be pipe flow tables, flow profile computation tables,
nomographs, charts, graphs, detail drawings, and other tabular or graphic aides used to design and confirm
performance of the conveyance system.
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Verification of capacity and performance must be provided for each element of the conveyance system.
The analysis must show design velocities and flows for all drainage facilities within the development, as
well as those offsite that are affected by the development. If the final design results are on a computer
printout, a separate summary tabulation of conveyance system performance shall also be provided.
TIR SECTION 6
SPECIAL REPORTS AND STUDIES
Some site characteristics, such as steep slopes or wetlands, pose unique road and drainage design problems
that are particularly sensitive to stormwater runoff. As a result, CED may require the preparation of
special reports and studies that further address the site characteristics, the potential for impacts associated
with the development, and the measures that would be implemented to mitigate impacts. Special reports
shall be prepared by people with expertise in the particular area of analysis. Topics of special reports may
include any of the following:
Floodplain delineation in accordance with Section 1.3.2
Flood protection facility conformance in accordance with Section 1.3.3
Critical areas analysis and delineation
Geotechnical/soils (soils documentation supporting on-site BMP design, infiltration rate
determination and infeasibility conclusions may also be located in TIR Section 6)
Groundwater, including groundwater mounding analyses required for infiltration design
Slope protection/stability
Erosion and deposition
Geology
Hydrology
Fluvial geomorphology
Anadromous fisheries impacts
Water quality
Structural design
Structural fill
Aquifer Protection Areas
TIR SECTION 7
OTHER PERMITS
Construction of road and drainage facilities may require additional permits from other agencies for some
projects. These additional permits may contain more restrictive drainage plan requirements. This section
of the TIR should provide the titles of any other permits, the agencies requiring the other permits, and the
permit requirements that affect the drainage plan. Examples of other permits are listed in Section 1.1.3. If a
UIC well registration is required, a copy must be provided.
TIR SECTION 8
CSWPP PLAN ANALYSIS AND DESIGN
This section of the TIR should include the analysis and design information used to prepare the required
construction stormwater pollution prevention (CSWPP) plan . This information should be presented in
two parts associated with the CSWPP plan’s two component plans, the erosion sediment control (ESC)
plan (Part A) and the stormwater pollution prevention and spill control (SWPPS) plan (Part B). See
Sections 2.3.1.3 and 2.3.1.4 for plan specifications and contents.
ESC Plan Analysis and Design (Part A)
This section must include all hydrologic and hydraulic information used to analyze and design the erosion
and sediment control measures, including final site stabilization measures. The TIR shall explain how
proposed ESC measures comply with the Erosion and Sediment Control Standards in Appendix D and
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show compliance with the implementation requirements of Core Requirement #5, Section 1.2.5.
Part A must include the following:
1. Provide sufficient information to justify the overall ESC plan and the choice of individual ESC
measures. At a minimum, there shall be a discussion of each of the measures specified in Section 1.2.5
and their applicability to the proposed project.
2. Include all hydrologic and hydraulic information used to analyze and size the ESC facilities shown
in the engineering plans. Describe the methodology, and attach any graphics or sketches used to size
the facilities.
3. Identify areas with a particularly high susceptibility to erosion because of slopes or soils, as well as
areas to be protected for existing and proposed on-site BMPs. Discuss any special measures taken to
protect these areas as well as any special measures proposed to protect water resources on or near the
site.
4. Identify any ESC recommendations in any of the special reports prepared for the project. In the
project geotechnical report supporting on-site BMP design, provide recommendations to address
mitigation of on-site BMP areas impacted by erosion and/or sedimentation during construction. If
these special reports’ recommendations are not included in the ESC plan, provide justification.
5. If proposing exceptions or modifications to the standards detailed in the Erosion and Sediment
Control Standards in Appendix D, clearly present the rationale. If proposing techniques or products
different from those detailed in the ESC Standards, provide supporting documentation so the City can
determine if the proposed alternatives provide similar protection.
SWPPS Plan Design (Part B)
The stormwater pollution prevention and spill control plan must identify all activities that could
contribute pollutants to surface and storm water during construction. This section of the TIR must provide
sufficient information to justify the selection of specific stormwater pollution prevention (SWPPS) BMPs
proposed to be applied to the pollution-generating activities that will occur with construction of the
proposed project. BMPs applicable to such activities are found in the Construction Stormwater Pollution
Prevention and Spill Control (CSWPP) Standards (Appendix D) and the King County Stormwater
Pollution Prevention Manual (viewable at <https://www.kingcounty.gov/sppm>) adopted pursuant to
RMC 4-6-030.
At a minimum, there shall be a discussion of each anticipated pollution-generating activity and the
pollution prevention BMPs selected to address it. If there are any calculations required for the selected
BMP, include those in the discussion. If an alternative BMP or major modification to one of the City’s
standard BMPs will be used, a written request must be submitted for review and approval, detailing how
the alternative will work. An “Alternative BMP Request Form” is available in the Stormwater Pollution
Prevention Manual.
Updates or revisions to the SWPPS plan may be requested by CED at any time during project
construction if CED determines that pollutants generated on the construction site have the potential to
contaminate surface, storm, or ground water.
The SWPPS plan shall also discuss the receiving waters, especially if the receiving water body is listed
on the 303d list. Information must be provided that shows the plan meets TMDL requirements. Discuss
the 303(d) listed pollutant generated or used onsite and any special handling requirements or BMPs.
TIR SECTION 9
BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT
Bond Quantities Worksheet
Each plan submittal requires a construction quantity summary to establish appropriate bond amounts.
Using the Site Improvement Bond Quantities Worksheet furnished by CED (see the City’s website), the
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design engineer shall separate existing right-of-way and erosion control quantities from other onsite
improvements. In addition, the design engineer shall total the amounts based on the unit prices listed on
the form.
Drainage facilities for single-family residential building permits, which are normally not bonded, shall be
constructed and approved prior to finalization of the permit.
Flow Control and Water Quality Facility Summary Sheet and Sketch
Following approval of the plans, a Flow Control and Water Quality Facility Summary Sheet and Sketch
(see Reference Section 8-D) shall be submitted along with an 81/2″ x 11″ plan sketch for each facility
proposed for construction. The plan shall show a north arrow, the tract, the facility access road, the extent
of the facility, and the control structure location. The approximate street address shall be noted. At project
completion, the Summary Sheet and Sketch shall be updated in the Final Corrected TIR to reflect the
completed project (see Section 2.4.2).
Declaration of Covenant for Privately Maintained Flow Control and WQ Facilities
Any declaration of covenant and grant of easement required for proposed flow control and water quality
facilities per Section 1.2.6 must be included here for review and approval before recording. The
necessary covenant exhibits, and maintenance instructions associated with the facility type (see Reference
Section 5), shall be included with the declaration of covenant. After approval by CED, the declaration of
covenant and grant of easement must be signed and recorded at the office of King County Records and
Elections before finalization of any permit. A copy of the recorded document shall be included in the Final
Corrected TIR (see Section 2.4.2).
Declaration of Covenant for Privately Maintained On-Site BMPs
Any declarations of covenant and grant of easement required for proposed on-site BMPs per Core
Requirement #9 must be included here for review and approval before recording. The necessary
covenant exhibits, and maintenance instructions associated with the on-site BMP type (see Reference
Section 5), shall be included with the declaration of covenant. After approval by CED, all such
documents must be signed and recorded at the office of King County Records and Elections before
finalization of any permit. A copy of the recorded document shall be included in the Final Corrected TIR
(see Section 2.4.2) or otherwise provided to the CED if no TIR was required.
TIR SECTION 10
OPERATIONS AND MAINTENANCE MANUAL
For each flow control and water quality facility and/or BMP that is to be privately maintained, and for
those that have special non-standard features, the design engineer shall prepare an operations and
maintenance manual. The manual should be simply written and should contain a brief description of the
facility or BMP, what it does, and how it works. In addition, the manual shall include a copy of the
Maintenance Requirements for Flow Control, Conveyance, and WQ Facilities (see Appendix A) and
provide an outline of maintenance tasks and the recommended frequency each task should be performed.
This is especially important for on-site BMP and water quality facilities where proper maintenance is
critical to facility performance. For this reason, most of the flow control facility designs in Chapter 5 and
the water quality facility designs in Chapter 6 include “maintenance considerations” important to the
performance of each facility. BMP maintenance instructions by BMP type, prepared in 8-1/2″ x 11″ size
for inclusion in TIRs and declarations of covenant, are also provided in Reference Section 5.
2.3.1.2 SITE IMPROVEMENT PLAN
Site improvement plans shall portray design concepts in a clear and concise manner. The plans must
present all the information necessary for persons trained in engineering to review the plans, as well as
those persons skilled in construction work to build the project according to the design engineer’s intent.
Supporting documentation for the site improvement plans must also be presented in an orderly and concise
format that can be systematically reviewed and understood by others.
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Survey Datum and Precision (RC)
The horizontal component of all surveys shall have as its coordinate base: The North American Datum of
1983/91. All horizontal control for projects must be referenced to or in conjunction with a minimum of
two of the City of Renton’s Survey Control Network monuments. The source of the coordinate values used
will be shown on the survey drawing per RCW 58.09.070. The horizontal component of all surveys shall
meet or exceed the closure requirements of WAC 332-130-060. The control base lines for all surveys shall
meet or exceed the requirements for a Class A survey revealed in Table 2 of the Minimum Standard Detail
Requirements for ALTA/ACSM Land Title Surveys jointly established and adopted by ALTA and ACSM
in 1992 or comparable classification in future editions of said document. The angular and linear closure
and precision ratio of traverses used for survey control shall be revealed on the face of the survey drawing,
as shall the method of adjustment.
The horizontal component of the control system for surveys using global positioning system methodology
shall exhibit at least 1 part in 50,000 precision in line length dependent error analysis at a 95 percent
confidence level and performed pursuant to Federal Geodetic Control Subcommittee Standards for GPS
control surveys as defined in Geometric Geodetic Accuracy Standards & Specifications for Using GPS
Relative Positioning Techniques dated August 1, 1989 or comparable classification in future editions of
said document. The vertical component of all surveys shall be based on NAVD 1988, the North American
Vertical Datum of 1988, and tied to at least one of the City of Renton Survey Control Network
benchmarks. If there are two such benchmarks within 3000 feet of the project site a tie to both shall be
made. The benchmark(s) used will be shown on the drawing. If a City of Renton benchmark does not exist
within 3,000 feet of a project, one must be set on or near the project in a permanent manner that will
remain intact throughout the duration of the project. Source of elevations (benchmark) will be shown on
the drawing, as well as a description of any bench marks established. See the City of Renton Survey and
Drafting Standards.
The site improvement plans consist of all the plans, profiles, details, notes, and specifications necessary to
construct road, drainage structure, and off-street parking improvements. Site improvement plans include
the following:
A base map (described below), and
Site plan and profiles (described below).
Note: Site improvement plans must also include grading plans if onsite grading extends beyond the
roadway.
Modified Site Improvement Plan
CED may allow a modified site improvement plan for some projects in Targeted Drainage Review (see
Section 2.3.2) or Directed Drainage Review, or where major improvements (e.g., detention facilities,
conveyance systems, bridges, road right-of-way improvements, etc.) are not proposed. The modified site
improvement plan must:
1. Be drawn on a 11″ x 17″ or larger sheet,
2. Accurately locate structure(s) and access, showing observance of the setback requirements given in
this manual, the critical areas code (RMC 4-3-050), or other applicable documents,
3. Provide enough information (datum, topography, details, notes, etc.) to address issues as determined
by CED.
GENERAL PLAN FORMAT
Site improvement plans should use City of Renton Drafting Standards as appropriate, and must include
Standard Plan Notes (see Reference Section 7). Each plan must follow the general format detailed below:
1. Plan sheets and profile sheets, or combined plan and profile sheets, specifications, and detail sheets
as required shall be on 22-inch by 34-inch sheets (22″ x 34″). Right-of-way improvements must be on
22-inch by 34-inch sheets (22″ x 34″). Original sheets shall be archive quality reproducibles,
electronic pdf format.
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2. Drafting details shall generally conform to City of Renton Survey and Drafting Standards (see
<https://rentonwa.gov/city_hall/community_and_economic_development/permits/civil_construction >)
with standard text height of 0.125″ (1/8″). Existing features shall be shown with dashed lines or as
half-toned (screened) in order to clearly distinguish existing features from proposed improvements.
Hatch patterns shall not be used.
3. Each submittal shall contain a project information/cover sheet with the following:
a) Title: Project name and CED file number(s)
b) Table of contents (if more than three pages)
c) Vicinity map
d) Name and phone number of utility field contacts (e.g., water, sanitary sewer, gas, power,
telephone, and TV) and the One-Call number (811 or 1-800-424-5555)
e) The City’s preconstruction/inspection notification requirements
f) Name and phone number of the erosion control/CSWPP supervisor
g) Name and phone number of the surveyor
h) Name and phone number of the owner/agent
i) Name and phone number of the applicant
j) Legal description
k) Plan approval signature block for CED
l) Name and phone number of the engineering firm preparing the plans (company logos acceptable)
m) Renton Regional Fire Authority’s’ approval stamp (if required)
n) Statement that mailbox locations have been designated or approved by the U.S. Postal Service
(where required)
o) List of conditions of preliminary approval and conditions of approved adjustments and variances
on all site improvements
4. An overall site plan shall be included if more than three plan sheets are used. The overall plan shall
be indexed to the detail plan sheets and include the following:
a) The complete property area development
b) Right-of-way information
c) Street names and road classification
d) All project phasing and proposed division boundaries
e) All natural and proposed drainage collection and conveyance systems with catch basin numbers
shown
5. Each sheet of the plan set shall be stamped, signed, and dated by a civil engineer. At least one sheet
showing all boundary survey information and tied to two City of Renton monuments must be provided
and stamped by a land surveyor licensed in the State of Washington.
6. Detail sheets shall provide sufficient information to construct complex elements of the plan. Details
may be provided on plan and profile sheets if space allows.
7. The City of Renton title block shall be provided on each plan sheet. Title block can be obtained at the
City’s website. At a minimum, the title block shall list the following:
a) Development title
b) Name, address, and phone number of the firm or individual preparing the plan
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c) A revision block
d) Page (of pages) numbering
e) Sheet title (e.g., road and drainage, grading, erosion and sediment control, stormwater pollution
prevention and spill control)
8. A blank CED approval block (included on the City’s title block) shall be provided on each plan
sheet.
9. The location and label for each section or other detail shall be provided.
10. Critical areas, critical area buffers, and critical area building setbacks as required by RMC 4-3-050
shall be delineated and labeled.
11. All match lines with matched sheet number shall be provided.
12. All division or phase lines and the proposed limits of construction under the permit application shall
be indicated.
13. Wetlands shall be labeled with the number from the City’s wetland inventory, or shall be labeled as
“uninventoried” if not listed on the wetland inventory.
14. The standard plan notes that apply to the project shall be provided on the plans (see Reference
Section 7-B).
15. Commercial building permit applications shall include the designated zoning for all properties
adjacent to the development site(s).
BASE MAP
A site improvement plan base map provides a common base and reference in the development and design
of any project. A base map helps ensure that the engineering plans, grading plans, and CSWPP plans are
all developed from the same background information. This base map shall include the information listed in
Table 2.3.1.A.
TABLE 2.3.1.A BASE MAP REQUIREMENTS
Feature Requirements
Ground Surface
Topography
Provide topography within the site and extending beyond the property lines.
Contour lines must be shown as described in “Plan View: Site Plan and
Roadway Elements.”
Surface Water
Discharge
Provide ground surface elevations for a reasonable “fan” around points of
discharge extending at least 50 feet downstream of all point discharge
outlets.
Hydrologic
Features
Provide spot elevations in addition to contour lines to aid in delineating the
boundaries and depth of all existing floodplains, wetlands, channels, swales,
streams, storm drainage systems and BMPs, roads (low spots), bogs,
depressions, springs, seeps, swales, ditches, pipes, groundwater, and
seasonal standing water.
Other Natural
Features
Show the location and relative sizes of other natural features such as rock
outcroppings, existing vegetation, and trees 12 inches in diameter and
greater that could be disturbed by the project improvements and
construction activities (within tree canopy), noting species.
Flows Provide arrows that indicate the direction of surface flow on all public and
private property and for all existing conveyance systems.
Floodplains/
Floodways
Show the floodplain/floodways as required by the flood hazard portion of the
critical areas code (RMC 4-3-050) and Section 4.4.2.
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General
Background
Information
Show the location and limits of all existing:
Property boundaries
Structures
Easements (including dimensions)
Total property (including dimensions)
Roads and right-of-way
Sanitary sewers and water utilities
Common open space
Public dedications
Other manmade features affecting existing topography/proposed
improvements.
Development
Limitations
Delineate limitations to the development that may occur as identified on the
TIR worksheet, Part 11 (see Reference Section 8-A).
SITE PLAN AND PROFILES
The design engineer shall provide plans and profiles for all construction, including but not limited to the
following information:
Plan View: Site Plan and Roadway Elements
1. Provide property lines, right-of-way lines, and widths for proposed roads and intersecting roads.
Note: the condition of all public right-of-way and the right to use it as proposed must be verified.
2. Provide all existing and proposed roadway features, such as centerlines, edges of pavement and
shoulders, ditchlines, curbs, and sidewalks. In addition, show points of access to abutting properties
and roadway continuations.
3. Show existing and proposed topography contours at 2-foot intervals (5-foot intervals for slopes
greater than 15 percent, 10-foot intervals for slopes greater than 40 percent). Contours may be
extrapolated from USGS mapping, aerial photos, or other topography map resources. However,
contours shall be field verified for roadway and stream centerlines, steep slopes, floodplains, drainage
tracts easements, and conveyance systems. Contours shall extend 20 feet beyond property lines to
resolve questions of setback, cut and fill slopes, drainage swales, ditches, and access or drainage to
adjacent property.
4. Show the location of all existing utilities and proposed utilities (except those designed by the utility
and not currently available) to the extent that these will be affected by the proposed project. Clearly
identify all existing utility poles.
5. Identify all roads and adjoining subdivisions.
6. Show right-of-way for all proposed roadways, using sufficient dimensioning to clearly show exact
locations on all sections of existing and proposed dedicated public roadway.
7. Clearly differentiate areas of existing pavement and areas of new pavement. If the project is a
redevelopment project, delineate areas of replaced impervious surface.
8. For subdivision projects, generally use drawing scales of 1″=20′; however, 1″=50′ is optional for
development of lots one acre or larger. For commercial, multi-family, or other projects, generally use
scales of 1″=20′; however, 1″=10′, 1″ = 30′, 1″=40′ and 1″=50′ are acceptable. Show details for
clarification, including those for intersections and existing driveways, on a larger scale.
Plan View: Drainage Conveyance
1. Sequentially number all catch basins and curb inlets starting with the structure farthest
downstream.
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2. Represent existing storm drainage facilities and BMPs in dashed lines and label with “Existing.”
3. Clearly label existing storm drainage facilities to be removed with “Existing to be removed.”
4. Show the length, diameter, and material for all pipes, culverts, and stub-outs. Include the slope if not
provided on the profile view. Material may be noted in the plan notes. Where an open channel
conveyance system that is not concrete lined is provided, and a low-permeability liner or treatment
liner is required per Section 6.2.4, indicate the limits of such liner(s).
5. Clearly label catch basins as to size and type (or indicate in the plan notes).
6. Clearly label stub-out locations for footing drains and other lot-specific connections to the storm
drainage system. Locate all stub-outs to allow gravity flow from the lowest corner of the lot to the
connecting catch basin.
7. Show datum, benchmark locations, and elevations on each plan sheet.
8. Clearly label all stub-out locations for any future pipe connections.
9. Clearly show on the plans all drainage easements, tracts, access easements, Native Growth Retention
Areas, Critical Area Tracts, Critical Area Setback Areas, and building setback lines. Show
dimensions, type of restriction, and use.
10. Using arrows, indicate the drainage direction of hydraulic conveyance systems.
11. Clearly label storm drainage facilities, on-site BMPs, pipes, and structures as either privately or
publicly maintained.
Plan View: Other
1. Show the location, identification, and dimensions of all buildings, property lines, streets, alleys, and
easements.
2. Show the locations of structures on abutting properties within 50 feet of the proposed project site.
3. Show the location of all proposed drainage facility fencing, together with a typical section view of
each fencing type.
4. Provide section details of all retaining walls and rockeries, including sections through critical
portions of the rockeries or retaining walls.
5. Show all existing and proposed buildings with projections and overhangs.
6. Show the location of all wells on site and within 100 feet of the site. Note wells to be abandoned.
7. Show the location and dimensions of proposed on-site BMPs, features, pathways, limits, and set-
asides.
8. Show the location and dimensions of structural source control BMPs required by the SWPPS
Standards in Appendix D and the King County Stormwater Pollution Prevention Manual.
Profiles: Roadway and Drainage
1. Provide existing centerline ground profile at minimum of 50-foot stations and at significant ground
breaks and topographic features, with average accuracy to within 0.1 feet on unpaved surface and
0.02 feet on paved surface.
2. For publicly maintained roadways, provide final road and storm drain profile with the same
stationing as the horizontal plan, to show stationing of points of curve, tangent, and intersection of
vertical curves, with elevation of 0.01 feet. Include tie-in with intersecting pipe runs.
3. On a grid of numbered lines, provide a continuous plot of vertical positioning against horizontal.
4. Show finished road grade and vertical curve data (road data measured at centerline or edge of
pavement). Include stopping sight distance.
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5. Show all roadway drainage, including drainage facilities and BMPs that are within the right-of-way
or easement.
6. On the profile, show slope, length, size, and type (in plan notes or on a detail sheet) for all pipes and
detention tanks in public right-of-way.
7. Indicate the inverts of all pipes and culverts and the elevations of catch basin grates or lids. It is also
desirable, but not required, to show invert elevations and grate elevations on plan sheets.
8. For pipes that are proposed to be within 2.0 feet of finished grade, indicate the minimum cover
dimensions.
9. Indicate roadway stationing and offset for all catch basins.
10. Indicate vertical and horizontal scale.
11. Clearly label all profiles with respective street names and plan sheet reference numbers, and indicate
all profile sheet reference numbers on plan sheets, if drawn on separate sheets.
12. Locate match points with existing pavements, and show elevations.
13. Show all property boundaries.
14. Label all match line locations.
15. Provide profiles for all 12-inch and larger pipes and for channels (that are not roadside ditches).
16. Show the location of all existing and proposed (if available or critical for clearance) gas, water, and
sanitary sewer crossings.
17. Show energy dissipater locations.
18. Identify datum used and all benchmarks (may be shown on plan view instead). Datum and
benchmarks must refer to established control when available.
19. Use a vertical scale of 1″=5′. As an exception, vertical scale shall be 1″=10′ if the optional 1″=50′
horizontal scale is used on projects with lots one acre or larger. Clarifying details, including those for
intersections and existing driveways, should use a larger scale.
20. Split sheets, with the profile aligned underneath the plan view, are preferred but not required.
DETAILS
The design engineer shall provide details for all construction, including but not limited to the following.
Flow Control, Water Quality, and Infiltration Facility and BMP Details
1. Provide a scaled drawing and supporting details of each detention pond or vault, on-site BMP, and
water quality facility, including the tract boundaries.
2. Show predeveloped and finished grade contours at 2-foot intervals. Show and label maximum design
water elevation.
3. Dimension all berm widths.
4. Show and label at least two cross sections through a pond or water quality facility, or any BMP large
enough to require design elements of ponds and/or water quality facilities. One cross section must
include the restrictor when included in the design.
5. Specify soils and compaction requirements for pond construction and on-site BMP construction.
Specify low-permeability liners or treatment liners as required for facilities that allow runoff to
have direct contact with the soil and open channel conveyance systems that are not concrete lined per
Section 6.2.4.
6. Show the location and detail of emergency overflows, spillways, and bypasses.
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7. Specify rock protection/energy dissipation requirements and details.
8. Provide inverts of all pipes, grates, inlets, tanks, and vaults, and spot elevations of the pond bottom.
9. Show the location of access roads to control manholes and pond/forebay bottoms.
10. Provide plan and section views of all energy dissipaters, including rock splash pads. Specify the
size of rock and thickness.
11. Show bollard locations on plans. Typically, bollards are located at the entrance to drainage facility
access roads.
12. On the pond or water quality facility detail, show the size, type (or in plan notes), slope, and length of
all pipes.
13. Show to scale the section and plan view of restrictor and control structures. The plan view must
show the location and orientation of all inlet pipes, outlet pipes, and flow restrictors.
14. Draw details at one of the following scales: 1″=1′, 1″=2′, 1″=4′, 1″=5′, 1″=10′, or 1″=20′.
Structural Plan Details
Any submittal that proposes a structure (e.g., bridge crossing, reinforced concrete footings, walls, or
vaults) shall include plan sheets that include complete working drawings showing dimensions, steel
placement, and specifications for construction. Structures may require a design prepared and stamped by
a professional structural engineer licensed in the State of Washington, and an application for a separate
commercial building permit.
2.3.1.3 EROSION AND SEDIMENT CONTROL (ESC) PLAN
This section details the specifications and contents for ESC plans. Note that an ESC plan includes the
plan’s drawings plus an ESC report, which provides all supporting information and any additional
direction necessary for implementing ESC measures and meeting ESC implementation requirements. The
ESC plan’s drawings may be simplified by the use of the symbols and codes provided for each ESC
measure in the Erosion and Sediment Control Standards in Appendix D. In general, the ESC plan’s
drawings shall be submitted as a separate plan sheet(s). However, there may be some relatively simple
projects where providing separate grading and ESC plan drawings is unnecessary.
GENERAL SPECIFICATIONS
The site improvement plan shall be used as the base of the ESC plan. Certain detailed information that is
not relevant (e.g., pipe/catch basin size, stub-out locations, etc.) may be omitted to make the ESC plan
easier to read. At a minimum, the ESC plan shall include all of the information required for the base map
(see Table 2.3.1.A), as well as existing and proposed roads, driveways, parking areas, buildings, drainage
facilities and BMPs, utility corridors not associated with roadways, relevant critical areas3 and critical
area buffers, and proposed final topography. A smaller scale may be used to provide better comprehension
and understanding.
The ESC plan shall generally be designed for proposed topography, not existing topography, since rough
grading is usually the first step in site disturbance. The ESC plan shall address all phases of construction
(e.g., clearing, grading, installation of utilities, surfacing, and final stabilization). If construction is being
phased, separate ESC plans may need to be prepared to address the specific needs for each phase of
construction.
The ESC plan outlines the minimum requirements for anticipated site conditions. During construction,
ESC plans shall be revised as necessary by the CSWPP supervisor or as directed by CED to address
changing site conditions, unexpected storm events, or non-compliance with the ESC performance criteria
in Core Requirement #5.
3 Relevant critical areas, for the purposes of drainage review, include aquatic areas, wetlands, flood hazard areas, erosion
hazard areas, landslide hazards, steep slope hazard areas, and critical aquifer recharge areas.
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The ESC plan shall be consistent with the information provided in Section 8 of the TIR and shall address
the following:
1. Identify areas with a high susceptibility to erosion.
2. Provide all details necessary to clearly illustrate the intent of the ESC design.
3. Include ESC measures for all on- and offsite utility construction included in the project.
4. Specify the construction sequence. The construction sequence shall be specifically written for the
proposed project. An example construction sequence is provided in Appendix D.
5. Include ESC standard plan notes (see Reference Section 7-B).
6. Include an inspection and maintenance program for ESC measures, including designation of a
CSWPP supervisor who is a certified ESC professional and identification of phone numbers for 24-
hour contact.
7. Include the basis and calculations for selection and sizing of ESC measures.
MEASURE-SPECIFIC INFORMATION
ESC plan drawings must include the following information specific to applicable ESC measures and
implementation requirements. As noted above, this information may need to be updated or revised during
the life of the project by the CSWPP supervisor or as directed by CED.
Clearing Limits
1. Delineate clearing limits.
2. Provide details sufficient to install and maintain the clearing limits.
Cover Measures
1. Specify the type and location of temporary cover measures to be used onsite.
2. If more than one type of cover measure is to be used onsite, indicate the areas where the different
measures will be used, including steep cut and fill slopes.
3. If the type of cover measures to be used will vary depending on the time of year, soil type, gradient, or
some other factor, specify the conditions that control the use of the different measures .
4. Specify the nature and location of permanent cover measures. If a landscaping plan is prepared, this
may not be necessary.
5. Specify the approximate amount of cover measures necessary to cover all disturbed areas.
6. If netting, blankets, or plastic sheeting are specified, provide typical detail sufficient for installation
and maintenance.
7. Specify the mulch types, seed mixes, fertilizers, and soil amendments to be used, as well as the
application rate for each item.
8. For surface roughening, describe methods, equipment and areas where surface roughening will be
use.
9. If PAM is used, show location(s) and describe application method.
10. When compost blankets are used, show location, application rates, and the name of the supplier to
document that compost meets quality specifications per Reference Section 11-C.
Perimeter Protection
1. Specify the location and type of perimeter protection to be used.
2. Provide typical details sufficient to install and maintain the perimeter protection.
3. If silt fence is to be used, specify the type of fabric to be used.
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4. If compost berms or socks are used, documentation must be provided to ensure the supplier meets
the criteria and compost meets quality standards per Reference Section 11-C.
Traffic Area Stabilization
1. Locate the construction entrance(s).
2. Provide typical details sufficient to install and maintain the construction entrance.
3. Locate the construction roads and parking areas.
4. Specify the measure(s) that will be used to create stabilized construction roads and parking areas.
Provide sufficient detail to install and maintain.
5. If a wheel wash or tire bath system will be installed, provide location, typical details for installation
and maintenance.
6. Provide a list of dust control products that will be used onsite and the location of potential application
areas.
Sediment Retention
1. Show the locations of all sediment ponds and traps.
2. Dimension pond berm widths and all inside and outside pond slopes.
3. Indicate the trap/pond storage required and the depth, length, and width dimensions.
4. Provide typical section views through pond and outlet structures.
5. If chemical or electrocoagulation treatment of sediment-laden waters will be used, approval
documentation from Ecology must be included (see SWPPS plan requirements for chemical storage).
6. Provide details for disposal of contaminated or chemically treated waters (e.g., where Chitosan or
CO2 have been used) (see SWPPS plan requirements for chemical storage).
7. Include appropriate approval documentation from local sewer districts if contaminated or
chemically treated water will be discharged to the sanitary sewer.
8. Provide typical details of the control structure and dewatering mechanism .
9. Detail stabilization techniques for outlet/inlet protection.
10. Provide details sufficient to install cell dividers.
11. Specify mulch or recommended cover of berms and slopes.
12. Indicate the required depth gage with a prominent mark at 1-foot depth for sediment removal.
13. Indicate catch basins that are to be protected.
14. Indicate existing and proposed on-site BMP areas that are to be protected.
15. Provide details of the catch basin and on-site BMP protection sufficient to install and maintain.
16. Provide sediment retention prior to any discharge to the City sewer or local sewer district system.
Surface Water Control
1. Locate all pipes, ditches, interceptor ditches, dikes, and swales that will be used to convey
stormwater.
2. Provide details sufficient to install and maintain all conveyances.
3. Indicate locations of outlet protection and provide detail of protections.
4. Indicate locations and outlets of any possible dewatering systems. Provide details of alternative
discharge methods from dewatering systems if adequate infiltration rates cannot be achieved. Do not
route dewatering water, clean or untreated, through stormwater sediment ponds.
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5. Indicate the location of any level spreaders and provide details sufficient to install and maintain.
6. Show all temporary pipe inverts.
7. Provide location and specifications for the interception of runoff from disturbed areas and the
conveyance of the runoff to a non-erosive discharge point.
8. Provide locations of rock check dams.
9. Provide details, including front and side sections, of typical rock check dams.
Protection of Existing and Proposed On-site BMP Areas
1. Provide perimeter protection at existing and proposed on-site BMP locations
2. Provide cautionary plan notes emphasizing avoidance of negative impacts to receptor soils and
existing vegetation to remain.
BMP Maintenance
1. Provide adequate plan notes for guidance of BMP maintenance methods and schedules.
2. Include an inspection and maintenance program for ESC measures.
Management of the Project
1. Provide plan notes to clarify and emphasize the management responsibilities for the project.
2. Include an inspection and maintenance program for ESC measures, including designation of a
CSWPP supervisor who is a certified ESC professional and identification of phone numbers for 24-
hour contact.
Wet Season Requirements
1. Provide a list of all applicable wet season requirements.
2. Clearly identify that from October 1st through April 30th, no soils shall be exposed for more than two
consecutive working days. Also note that this two-day requirement may be applied at other times of
the year if storm events warrant more conservative measures.
3. Clearly identify that exposed soils shall be stabilized at the end of the workday prior to a weekend,
holiday, or predicted rain event.
4. Weekly maintenance report is required to be submitted to CED.
Critical Areas Restrictions
1. Delineate and label the following critical areas, and any applicable buffers, that are on or adjacent to
the project site: aquatic areas, wetlands, flood hazard areas, erosion hazard areas, landslide hazards,
steep slope hazard areas, and aquifer protection areas per RMC 4-3-050.
2. If construction creates disturbed areas within any of the above listed critical areas or associated
buffers, specify the type, locations, and details of any measures or other provisions necessary to
comply with the critical area restrictions in Appendix D and protect surface waters and steep
slopes.
2.3.1.4 STORMWATER POLLUTION PREVENTION AND SPILL (SWPPS) PLAN
This section details the specifications and contents for SWPPS plans, which together with ESC plans,
comprise the construction stormwater pollution prevention (CSWPP) plan that must be submitted as part
of the engineering plans required for drainage review . Additional guidance for developing the SWPPS
plan can be found in the SWPPS Standards in Appendix D, Construction Stormwater Pollution Prevention
Standards, in the King County Stormwater Pollution Prevention Manual and in the Stormwater
Management Manual for Western Washington (SWMMWW) published by the Washington State
Department of Ecology (Ecology).
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The SWPPS plan must be kept on site during all phases of construction and shall address the
construction-related pollution-generating activities outlined in Subsection A below. The plan must
include a description of the methods the general contractor will use to ensure sub-contractors are aware of
the SWPPS plan. A form or record must be provided that states all sub-contractors have read and agree to
the SWPPS plan.
A SWPPS plan consists of the following three elements, which are further described in Subsections B, C,
and D below:
1. A site plan showing the location and description of BMPs required to prevent pollution and control
spills from construction activities and from chemicals and other materials used and stored on the
construction site. See Subsection B below for more specifics on the SWPPS site plan.
2. A pollution prevention report listing the potential sources of pollution and identifying the
operational, source control, and treatment BMPs necessary to prevent/mitigate pollution from these
sources. See Subsection C below for more specifics on the SWPPS pollution prevention report.
3. A spill prevention and cleanup report describing the procedures and BMPs for spill prevention and
including provisions for cleanup of spills should they occur. See Subsection D below for more
specifics on the SWPPS spill prevention and cleanup report.
A. ACTIVITY-SPECIFIC INFORMATION REQUIRED
At a minimum, the SWPPS plan shall address, if applicable, the following pollution-generating activities
typically associated with construction and include the information specified below for each activity. If
other pollution-generating activities associated with construction of the proposed project are identified, the
SWPPS plan must address those activities in a similar manner.
Storage and Handling of Liquids
1. Identify liquids that will be handled or stored onsite, including but not limited to petroleum products,
fuel, solvents, detergents, paint, pesticides, concrete admixtures, and form oils.
2. Specify types and sizes of containers of liquids that will be stored/handled onsite. Show locations on
the SWPPS site plan.
3. Describe secondary containment methods adequately sized to provide containment for all liquids
stored onsite. Show the locations of containment areas on the SWPPS site plan.
Storage and Stockpiling of Construction Materials and Wastes
1. Identify construction materials and wastes that may be generated or stockpiled onsite. Show the
locations where these materials and wastes will be generated and stockpiled on the SWPPS site plan.
2. Specify type of cover measures to be used to keep rainwater from contacting construction materials
and wastes that can contribute pollutants to storm, surface, and ground water.
3. If wastes are kept in containers, describe how rainwater will be kept out of the containers.
Fueling
1. Specify method of onsite fueling for construction equipment (i.e., stationary tanks, truck mounted
tanks, wet hosing, etc.). If stationary tanks will be used, show their location on the SWPPS site plan.
2. Describe type and size of tanks.
3. Describe containment methods for fuel spills and make reference to the SWPPS site plan for
location information.
4. If fueling occurs during evening hours, describe lighting and signage plan. Make reference to the
SWPPS site plan for location information.
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Maintenance, Repairs, and Storage of Vehicles and Equipment
1. Identify maintenance and repair areas and show their locations on the SWPPS site plan. Use of drip
pans or plastic beneath vehicles is required. A note to this effect must be shown on the SWPPS site
plan.
2. Describe method for collection, storage, and disposal of vehicle fluids.
3. If an area is designated for vehicle maintenance, signs must be posted that state no vehicle washing
may occur in the area. A note to this effect must be shown on the SWPPS site plan.
Concrete Saw Cutting, Slurry, and Washwater Disposal
1. Ensure that washout of concrete trucks is performed off-site or in designated concrete washout areas
only. Identify truck washout areas to ensure such areas are not within a critical aquifer recharge
area. If they are, the washout area must be lined with an impervious membrane. Show location
information on the SWPPS site plan. Locate washout area at least 50 feet from sensitive areas such as
storm drains, open ditches, or water bodies, including wetlands.
2. Specify size of sumps needed to collect and contain slurry and washwater. Show location information
on the SWPPS site plan.
3. Identify areas for rinsing hand tools including but not limited to screeds, shovels, rakes, floats and
trowels. Show the locations of these areas on the SWPPS site plan.
4. Describe methods for collecting, treating, and disposal of waste water from exposed aggregate
processes, concrete grinding and saw cutting, and new concrete washing and curing water.
Handling of pH Elevated Water
New concrete vaults/structures may cause collected water to have an elevated pH. This water cannot be
discharged to storm or surface water until neutralized.
1. Provide details on treating/neutralizing water when pH is not within neutral parameters. Written
approval from Ecology is required before using chemical treatment other than CO 2 or dry ice to adjust
pH.
2. Provide details on disposal of water with elevated pH or of the treated water.
Application of Chemicals including Pesticides and Fertilizers
1. Provide a list of chemicals that may be used on the project site and the application rates.
2. Describe where and how chemicals will be applied. Show location information on the SWPPS site
plan.
3. Describe where and how chemicals will be stored. Show location information on the SWPPS site
plan.
B. SWPPS SITE PLAN
The site plan element of the SWPPS plan shall include all of the information required for the base map
(see Table 2.3.1.A), as well as existing and proposed roads, driveways, parking areas, buildings, drainage
facilities, utility corridors not associated with roadways, relevant critical areas4 and associated buffers,
and proposed final topography. A smaller scale may be used to provide more comprehensive details on
specific locations of each activity and specific prevention measure. In addition to this information, the
following items, at a minimum, shall be provided as applicable:
1. Identify locations where liquids will be stored and delineate secondary containment areas that will be
provided.
4 Relevant critical areas, for the purposes of drainage review, include aquatic areas, wetlands, flood hazard areas, erosion
hazard areas, landslide hazards, steep slope hazard areas, and aquifer protection areas as described in RMC 4-3-050.
AGENDA ITEM # 8. a)
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2. Identify locations where construction materials and wastes will be generated and stockpiled.
3. Identify location of fueling for vehicles and equipment if stationary tanks will be used.
4. Delineate containment areas for fuel spills.
5. Show location of lighting and signage for fueling during evening hours.
6. Delineate maintenance and repair areas and clearly note that drip pans or plastic shall be used
beneath vehicles. Also, clearly note that signs must be posted that state no vehicle washing may occur
in the area.
7. Delineate truck washout areas and identify the location of slurry/washwater sumps and rinsing
areas for tools.
8. Delineate where chemicals will be applied and identify where they will be stored.
9. Identify where spill response materials will be stored.
C. POLLUTION PREVENTION REPORT
This report provides the specifics on pollution prevention and must include the following information in
addition to the activity-specific information specified in Subsection A above:
1. List the possible sources of pollution per Subsection A above and identify the BMPs to be used for
each source to prevent pollution. Include any supporting information (site conditions, calculations,
etc.) for the selection and sizing of pollution prevention BMPs.
2. Identify the personnel responsible for pollution prevention and clearly list the responsibilities of each
person identified. Contact information for these personnel must be clearly identified in the report and
on the SWPPS site plan.
3. Describe the procedures to be used for monitoring pollution prevention BMPs and for responding to a
BMP that needs attention, including keeping records/reports of all inspections of pollution prevention
BMPs (see Reference Section 8-E for examples of worksheets that may be used).
D. SPILL PREVENTION AND CLEANUP REPORT
This report provides the specifics on spill prevention and cleanup and must include the following
information in addition to any activity-specific information in Subsection A above related to spill
prevention:
1. List the possible sources of a spill and identify the BMPs to be used for each source to prevent a spill.
2. Identify personnel responsible for spill prevention and cleanup and clearly list the responsibilities of
each person identified. Contact information for these personnel must be clearly identified in the
report and on the SWPPS site plan. (On typical projects, the primary contact for SWPPS issues will be
the CSWPP supervisor, who may be managing other spill responders to ensure compliance; complex
projects may warrant specialist personnel for specific site applications.)
3. Describe the procedures to be used for monitoring spill prevention BMPs and for responding to a
spill incident, including keeping records/reports of all inspections and spills (see Reference Section
8-E for examples of worksheets that may be used).
4. Identify where spill response materials will be stored. Make reference to the SWPPS site plan for
location information.
5. Identify disposal methods for contaminated water and soil after a spill.
2.3.1.5 LANDSCAPE MANAGEMENT PLANS (IF APPLICABLE)
The City of Renton does not allow landscape management plans to be used as an alternative to the
requirement to formally treat (with a facility) the runoff from pollution generating pervious surfaces
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subject to Core Requirement #8 (see Section 1.2.8). A landscape management plan is an approved plan for
defining the layout and long-term maintenance of landscaping features to minimize the use of pesticides
and fertilizers, and reduce the discharge of suspended solids and other pollutants.
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2.3.2 PROJECTS IN TARGETED DRAINAGE REVIEW
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2.3.2 PROJECTS IN TARGETED DRAINAGE REVIEW (TDR)
This section outlines the specifications and contents of limited scope engineering plans allowed for
projects in Targeted Drainage Review. Table 2.3.2.A specifies the minimum required elements of the
targeted technical information report based on the type of permit or project, and on the three categories of
project characteristics subject to Targeted Drainage Review per Section 1.1.2.2.
TABLE 2.3.2.A MINIMUM ENGINEERING PLAN ELEMENTS(1)
FOR PROJECTS IN TARGETED DRAINAGE REVIEW
Type of
Permit or
Project
Drainage
Review
Type
Project Category 1(2)
Projects in TDR that
contain or are adjacent
to a flood, erosion, or
steep slope hazard
area; or are within a
Landslide Hazard
Drainage Area or
Aquifer Protection
Area
Project Category 2(2)
Projects in TDR that
propose to construct or
modify a 12″ or larger
pipe/ditch, or receive
runoff from a 12″ or larger
pipe/ditch
Project
Category 3(2)
Redevelopment
projects in TDR
that propose
$100,000 or more of
improvements to an
existing high-use
site
SINGLE-
FAMILY
RESIDENTIAL
(SFR)
BUILDING
PERMITS
SHORT
PLATS
Targeted
Drainage
Review
ONLY
TIR Sections 1, 2,
and 6 (minimum)
Simplified ESC
Plan(3) and SWPPS
Plan
Site Improvement
Plan(5)
TIR Sections 1, 2, 3, 5,
6, 7, and 8 (minimum)
Simplified ESC Plan(3)
and SWPPS Plan
ESC Plan(4) for
conveyance work
Site Improvement
Plan(5)
N/A
Targeted
Drainage
Review
COMBINE
D WITH
Simplified
Drainage
Review
TIR Sections 1, 2,
and 6 (minimum)
Simplified ESC
Plan(3) and SWPPS
Plan
Site Improvement
Plan(5)
TIR Sections 1, 2, 3, 5,
6, 7, and 8 (minimum)
Simplified ESC Plan(3)
and SWPPS Plan
ESC Plan(4) for
conveyance work
Site Improvement
Plan(5)
N/A
OTHER
PROJECTS
OR PERMITS
Targeted
Drainage
Review
ONLY
TIR Sections 1, 2, 6,
and 8 (minimum)
ESC Plan(4) and
SWPPS Plan for any
site disturbance work
Site Improvement
Plan(5)
TIR Sections 1, 2, 3, 5,
6, 7, and 8 (minimum)
ESC Plan(4) and
SWPPS Plan for any
site disturbance work
Site Improvement
Plan(5)
TIR Sections 1, 2,
4, 8, and 10
(minimum)
ESC Plan(4) and
SWPPS Plan for
any site
disturbance work
Site Improvement
Plan(5)
Notes:
(1) The above plan elements are considered the recommended minimum for most development cases in Targeted
Drainage Review. CED may add to these elements if deemed necessary for proper drainage review. Predesign meetings
with CED are recommended to identify all required elements.
(2) For more detailed descriptions of project categories, see Section 1.1.2.2. If the proposed project has the characteristics
of more than one category, the plan elements under each applicable category shall apply.
(3) Simplified ESC plans are an element of the Simplified drainage plan as explained in the Simplified Drainage
Requirements booklet (Appendix C).
(4) ESC plans shall meet the applicable specifications detailed in Section 2.3.1.3
(5) Site improvement plans shall meet the applicable specifications detailed in Section 2.3.1.2. CED may allow modified site
improvement plans as described in Section 2.3.1.2.
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2.4 PLANS REQUIRED AFTER DRAINAGE REVIEW
This section includes the specifications and contents required of those plans submitted at the end of the
permit review process or after a permit has been issued.
2.4.1 PLAN CHANGES AFTER PERMIT ISSUANCE
If changes or revisions to the originally approved engineering plans require additional review, the revised
plans shall be submitted to CED for approval prior to construction. The plan change submittals shall
include all of the following:
1. The appropriate Plan Change form(s)
2. One (1) revised TIR or addendum
3. One (1) set of the engineering plans
4. One (1) Complete Electronic copy of all submittal items
5. Other information needed for review.
2.4.2 FINAL CORRECTED PLAN SUBMITTAL
During the course of construction, changes to the approved engineering plans are often required to address
unforeseen field conditions or design improvements. Once construction is completed, it is the applicant’s
responsibility to submit to CED a final corrected plan (or record drawings). These corrected drawings
must be professionally drafted revisions applied to the original approved plan, excluding the CSWPP plan,
and must include all changes made during the course of construction. The final as-built plans must be
stamped, signed, and dated by a civil engineer or land surveyor. A CAD drawing file (.dwg) of the final
as-built plan set must be submitted. The CAD file must contain all the pages of the plan set for road and
drainage infrastructure, but need not contain other sheets. A single hard copy of the draft final corrected
plans may be required by the City to perform onsite field inspections to verify the final corrected plans. A
final corrected TIR, updated to include all changes made to the originally approved TIR during the
course of construction, must be submitted with the final corrected plan. In addition to any design changes
and supporting calculations and documentation, the final corrected TIR shall include a final updated
Stormwater Facility Summary Sheet (see Reference Section 8-D) and signed/recorded copies of all
required easements and declarations of covenant. The electronic copy of the final corrected TIR shall be
in .pdf format.
Additional information regarding the final corrected plan submittal can be found on the City’s website.
Disposition of Approved Engineering Plans for Subdivisions
CED will retain the .pdf copy of the full record drawing set, utilizing it to make copies for public
inspection, distribution, base reference, and permanent public record as required.
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2.4.3 FINAL PLAT, SHORT PLAT, AND BINDING SITE PLAN
SUBMITTALS
Any subdivision to be finalized, thereby completing the subdivision process and legally forming new lots,
requires a final submittal for approval and recording. Binding site plans and short plats also require a
final submittal for approval and recording. The final plat or map page shall contain the elements
summarized and specified in detail on the City’s website. Submittals shall be accompanied by appropriate
fees as prescribed by Renton Municipal Code. Final submittals will be allowed only after the approval of
preliminary plans (for subdivisions only) and any required engineering plans.
All final map sheets and pages shall be prepared by a land surveyor licensed in the State of Washington
and shall conform to all state and local statutes.
The final submittal for recording only applies to subdivisions (plats), binding site plans, and short plats.
This plan is required by state and local statutes.
In addition to the requirements described on the City’s website and in the City of Renton Municipal Code,
submittals for final recording of subdivisions, short plats, and binding site plans must include the
following information:
1. Indicate dimensions of all easements, tracts, building setbacks, tops of slopes, wetland boundaries,
and floodplains.
2. Include pertinent restrictions as they apply to easements, tracts, and building setback lines.
3. State the maximum amount of added impervious surface and proposed clearing per lot as
determined through engineering review. The maximum amount of impervious surface may be
expressed in terms of percentage of lot coverage or square feet.
4. Include a recorded declaration of covenant and grant of easement for each lot on which on-site
BMPs are installed or stipulated per Core Requirement #9, Section 1.2.9.4.1, and each lot for which
on-site BMPs are installed in a separate dedicated tract per Section 1.2.9.4.1.
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2022 City of Renton Surface Water Design Manual 6/22/2022
CHAPTER 3
HYDROLOGIC
ANALYSIS & DESIGN
CITY OF RENTON
SURFACE WATER
DESIGN MANUAL
Section Page
3.1 Hydrologic Design Standards and
Principles 3-3
3.1.1 Hydrologic Impacts and Mitigation 3-3
3.1.2 Flow Control Standards 3-5
3.1.3 Hydrologic Analysis Using Continuous
Models 3-5
3.2 Runoff Computation and Analysis
Methods 3-9
3.2.1 Rational Method 3-11
3.2.2 Continuous Models and the Runoff
Files Method 3-19
3.2.3 The Approved Model 3-30
3.2.4 The HSPF Model 3-30
3.3 Hydrologic Design Procedures and
Considerations 3-33
3.3.1 General Hydrologic Design Process 3-33
3.3.2 Flow Control Design Using the Runoff
Files Method 3-34
3.3.3 Conveyance System Design with the
Runoff Files Method 3-37
3.3.4 Safety Factors in Hydrologic Design 3-38
3.3.5 Design Options for Addressing
Downstream Drainage Problems 3-38
3.3.6 Point of Compliance Analysis 3-38
3.3.7 Onsite Closed Depressions and
Ponding Areas 3-41
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CHAPTER 3
HYDROLOGIC ANALYSIS & DESIGN
This chapter presents the concepts and rationale for the surface water controls and designs required by this
manual and the acceptable methods for estimating the quantity and characteristics of surface water runoff.
These methods are used to analyze existing and to design proposed drainage systems and facilities.
Hydrologic concepts, tools and methodologies, and an overview of the assumptions and data requirements
of the methods, are described for the following tasks:
Calculating runoff time series and flow statistics
Designing detention and infiltration facilities
Approved hydrologic modeling software are listed in Reference Section 6-D. Tools and methodologies
specific to the software can be obtained from the software documentation and trainings provided by the
software providers. At this writing, the approved models for stormwater runoff and water quality design
include WWHM2012 and WWHM4, available from the Washington State Department of Ecology
(Ecology), MGS Flood, available from MGS Engineering Consultants, Inc., and the Hydrologic
Simulation Program (Fortran) (HSPF). Currently, MGS Flood is not approved for modeling bioretention.
It will be allowed for modeling bioretention only at such time that it is formally approved by Ecology for
that use.
Hydrologic tools and methodologies, and the assumptions and data requirements of the methods, are
presented for the following tasks:
Sizing conveyance facilities
Analyzing conveyance capacities.
Chapter Organization
The information presented in this chapter is organized into three main sections:
Section 3.1, “Hydrologic Design Standards and Principles”
Section 3.2, “Runoff Computation and Analysis Methods”
Section 3.3, “Hydrologic Design Procedures and Considerations”
These sections begin on odd pages so the user can insert tabs if desired for quicker reference.
Other Supporting Information
For specific guidance on the mechanics of using the approved modeling software for hydrologic analysis
and design, refer to the associated approved model website and program documentation. See Reference
Section 6-D for limited modeling guidance and requirements as applicable for specific tasks in this
manual.
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3.1 HYDROLOGIC DESIGN STANDARDS AND PRINCIPLES
This section presents the rationale for and approach to hydrologic analysis and design. Topics covered
include the following:
“Hydrologic Impacts and Mitigation,” Section 3.1.1
“Flow Control Standards,” Section 3.1.2
“Hydrologic Analysis Using Continuous Models,” Section 3.1.3
3.1.1 HYDROLOGIC IMPACTS AND MITIGATION
Hydrologic Effects of Urbanization
The hydrologic effects of development can cause a multitude of problems, including minor nuisance
flooding, degradation of public resources, diminished fish production, and significant flooding
endangering life and property. Increased stormwater flows expand floodplains, bringing flooding to
locations where it did not occur before and worsening flood problems in areas already flood-prone.
Increased stormwater flows also hasten channel erosion, alter channel structure, and degrade fish habitat.
Human alteration of the landscape, including clearing, grading, paving, building construction, and
landscaping, changes the physical and biological features that affect hydrologic processes. Soil
compaction and paving reduce the infiltration and storage capacity of soils. This leads to a runoff process
called Horton overland flow whereby the rainfall rate exceeds the infiltration rate, and the excess
precipitation flows downhill over the soil surface. This type of flow rapidly transmits rainfall to the stream
or conveyance system, causing much higher peak flow rates than would occur in the unaltered landscape.
Horton overland flow is almost nonexistent in densely vegetated areas, such as forest or shrub land, where
the vast majority of rainfall infiltrates into the soil. Some of this infiltrated water is used by plants, and
depending on soil conditions, some of it percolates until it reaches the groundwater table. Sometimes the
percolating soil water will encounter a low-permeability soil or rock layer. In this case, it flows laterally as
interflow over the low-permeability layer until it reaches a stream channel. Generally, forested lands
deliver water to streams by subsurface pathways, which are much slower than the runoff pathways from
cleared and landscaped lands. Therefore, urbanization of forest and pasture land leads to increased
stormwater flow volumes and higher peak flow rates.
Land development increases not only peak flow rates but also changes annual and seasonal runoff
volumes. In forested basins in King County, about 55% of the rain that falls each year eventually appears
as streamflow. This percentage is called the yield of a basin. The remaining 45% of the rain evaporates
and returns to the atmosphere. As trees are cleared and the soil is graded to make way for lawns and
pastures, and as part of the land is covered with asphalt or concrete, the basin yield increases. More of the
rain becomes streamflow, and less evaporates. In lowland King County, the yield of a basin covered with
landscaped lawns would be about 65%, while the yield of an impervious basin would be about 85 to 90%.
For these reasons, development without mitigation increases peak stormwater rates, stormwater volumes,
and annual basin yields. Furthermore, the reduction of groundwater recharge decreases summer base
flows.
In summary, the following are the hydrologic impacts of unmitigated development:
Increased peak flows
Increased durations of high flows
Increased stormwater runoff volumes
Decreased groundwater recharge and base flows
Seasonal flow volume shifts
Altered wetland hydroperiods.
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The resulting economic and ecological consequences of these hydrologic changes include the following:
Increased flooding
Increased stream erosion
Degraded aquatic habitat
Changes to wetland species composition.
Mitigation of Hydrologic Effects of Urbanization
Engineered facilities can mitigate many of the hydrologic changes associated with development.
Detention facilities can maintain the rates and/or durations of high flows at predevelopment levels.
Infiltration facilities can control flow volumes and increase groundwater recharge as well as control flow
rates and durations. Conveyance problems can be avoided through analysis and appropriate sizing and
design of conveyance facilities. Engineered mitigation of the hydrologic impacts of development include
the following:
Managing peak flow rates with detention facilities
Managing high flow durations with detention facilities
Reducing flow volumes and maintaining or enhancing groundwater recharge with infiltration facilities
Avoiding flooding problems with appropriately sized and designed conveyance systems
Bypassing erosion problems with tightlines.
Engineered facilities cannot mitigate all of the hydrologic impacts of development. Detention facilities
do not mitigate seasonal volume shifts, wetland water level fluctuations, groundwater recharge reductions,
or base flow changes. Such impacts can be further reduced through the use of low impact development
(LID) techniques, beginning with careful site planning. For instance, clustering of units to reduce
impervious cover while maintaining site density is an effective way to limit hydrologic change. Preserving
native vegetation and minimizing soil disturbance or compaction in pervious areas also reduces hydrologic
change. Such non-engineered mitigation measures are encouraged by the City and are discussed in Core
Requirement #9 and Appendix C of this manual and are referred to as on-site BMPs.
Other on-site BMPs, such as permeable pavements, bioretention, vegetated roofs, and rainwater
harvesting can be effective in reducing increases in surface water volumes. The incorporation of these
concepts in the design of the project is required, as detailed in Core Requirement #9 and Appendix C.
Many of these approaches will result in a reduction in flow control facility size, so the on-site BMP
requirements in Core Requirement #9 and Appendix C should be carefully considered and applied to
maximize the benefits of this approach.
Detention Facility Concepts
The basic concept of a detention facility is simple: water is collected from developed areas and released at
a slower rate than it enters the collection system. The excess of inflow over outflow is temporarily stored
in a pond or a vault and is typically released over a few hours or a few days. The volume of storage needed
is determined by (1) how much stormwater enters the facility (determined by the size and density of the
contributing area), (2) how rapidly water is allowed to leave the facility, and (3) the level of hydrologic
control the facility is designed to achieve.
To prevent increases in the frequency of flooding due to new development, detention facilities are often
designed to maintain peak flow rates at their predevelopment levels for recurrence intervals of concern
(e.g., 2- and 10-year). Such mitigation can prevent increases in the frequency of downstream flooding.
Facilities that control only peak flow rates, however, usually allow the duration of high flows to increase,
which may cause increased erosion of the downstream system. For example, the magnitude of a 2-year
flow may not increase, but the amount of time that flow rate occurs may double. Therefore, stream
systems, including those with salmonid habitat, which require protection from erosion warrant detention
systems that control the durations of geomorphically significant flows (flows capable of moving
sediment). Such detention systems employ lower release rates and are therefore larger in volume.
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3.1.3 HYDROLOGIC ANALYSIS USING CONTINUOUS MODELS
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3.1.2 FLOW CONTROL STANDARDS
Refer to Chapter 1, Section 1.2.3, for flow control standards.1,2
3.1.3 HYDROLOGIC ANALYSIS USING CONTINUOUS MODELS
The Need for Continuous Hydrologic Modeling
This manual prescribes the use of a continuous hydrologic model for most hydrologic analyses rather
than an event model. Event models such as the Santa Barbara Urban Hydrograph (SBUH), King County
Runoff Series (KCRTS) and the Soil Conservation Service (SCS)3 method were used in previous versions
of the King County Surface Water Design Manual. A continuous model was selected for the current
version of the City of Renton SWDM because hydrologic problems in western Washington are associated
with the high volumes of flow from sequential winter storms rather than high peak flows from short
duration, high intensity rainfall events.
The continuous hydrologic analysis tools prescribed in this manual are generically described as the
“approved model”; a list of the approved models is found in Reference Section 6-D (as updated). At this
writing, the approved continuous hydrologic models4 include the Western Washington Hydrologic
Model (WWHM) and MGS Flood, both of which are variants of the Hydrologic Simulation Program-
FORTRAN (HSPF) model. HSPF is also an approved model, but is more complex than other approved
models and is typically used for basin planning and master drainage plan analyses.
Continuous models are well suited to accounting for the climatological conditions in the lowland Puget
Sound area. Continuous models include algorithms that maintain a continuous water balance for a
catchment to account for soil moisture and hydraulic conditions antecedent to each storm event (Linsley,
Kohler, Paulhus, 1982), whereas event models assume initial conditions and only address single
hypothetical storm events. As a result, continuous hydrologic models are more appropriate for evaluating
runoff during the extended wet winters typical of the Puget Sound area.
The drawbacks of event models are summarized as follows:
Event methods inherently overestimate peak flows from undeveloped land cover conditions. The
overestimation is due, in part, to the assumption that runoff from forest and pasture land covers flows
across the ground surface. In actuality, the runoff from forests and pastures, on till soils, is dominated
by shallow subsurface flows (interflow) which have hydrologic response times much longer than those
used in event methods. This leads to an over estimation of predeveloped peak flows, which results in
detention facility release rates being overestimated and storage requirements being underestimated.
A single event cannot represent the sequential storm characteristics of Puget Sound winters.
Event models assume detention facilities are empty at the start of a design event, whereas actual
detention facilities may be partially full as a result of preceding storms.
Testing of event-designed detention facilities with calibrated, long-term continuous hydrologic
simulations demonstrates that these facilities do not achieve desired performance goals.
Event methods do not allow analysis of flow durations or water level fluctuations.
The benefits of continuous hydrologic modeling are summarized as follows:
1 Footnote 1 does not apply.
2 Footnote 2 does not apply.
3 The Soil Conservation Service (SCS) is now known as the National Resources Conservation Service (NRCS). The method
described in Urban Hydrology for Small Watersheds, Technical Release 55 (TR-55), June 1986, published by the NRCS, is
commonly referred to as the “SCS method.”
4 Note that MGS Flood is not currently approved for modeling bioretention. It will be allowed for modeling bioretention only at
such time that it is formally approved by Ecology for that use.
AGENDA ITEM # 8. a)
SECTION 3.1 HYDROLOGIC DESIGN STANDARDS AND PRINCIPLES
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A continuous model accounts for the long duration and high precipitation volume of winter wet
periods characterized by sequential, low-intensity rainfall events. Continuous simulation uses
continuous long-term records of observed rainfall rather than short periods of data representing
hypothetical storm events. As a result, continuous simulation explicitly accounts for the long duration
rainfall events typically experienced in the Pacific Northwest as well as the effects of rainfall
antecedent to major storm events.
HSPF has been shown to more accurately simulate runoff from basins with a wide range of sizes and
land covers using the regional parameters developed by the United States Geologic Survey (USGS).
Continuous simulation allows direct examination of flow duration data for assessing the impacts of
development on stream erosion and morphology. An event model, whether using a 1-day or a 7-day
storm, cannot provide such information.
A continuous model allows water level analysis for wetlands, lakes, and closed depressions whose
water level regime is often dependent on seasonal runoff rather than on 1-day or 7-day event runoff.
Continuous models produce flow control facilities that more accurately and effectively achieve
desired performance goals.
The importance of continuous modeling in the Puget Sound area is illustrated in Figure 3.1.3.A, which
shows a small basin’s runoff response to a series of winter storms and the outflow from a detention pond
designed to control the peak annual flows from this basin. Note that the largest outflow from the detention
pond corresponds not to the peak inflow on 11/6/86, but rather to the high volume of flow from the
sequential storms beginning on 11/19/86. This demonstrates a key difference between continuous and
event based models.
With an event model, designers are accustomed to working with a single design storm event (e.g.,
10-year), which by definition has the same return period once routed through a reservoir (10-year inflow
will always generate 10-year outflow). With a continuous model, flow recurrence estimates are based on
annual peak flow rates, with each time series being analyzed independently. Events that generate annual
peak inflows to a reservoir may not generate annual peak discharges from the reservoir. In other words, the
runoff event containing the 10-year inflow peak, when routed, may not create the 10-year outflow peak.
This is due to natural variability of storm peaks and volumes (e.g., high intensity/short duration
thunderstorms as compared to moderate intensity/long duration winter storms) contained within a
continuous record.
Requirements of Continuous Hydrologic Modeling
For the entire period of simulation, a continuous hydrologic model requires a continuous record of
precipitation and evaporation at discrete time steps small enough to capture the temporal variability of
hydrologic response, and it provides a continuous record of simulated flows at the same time step. The
quicker a basin responds hydrologically (e.g., due to small size, land cover, or lack of detention), the
smaller the time step should be. Time steps of 15 minutes are sufficient for most basins in the Puget Sound
area.
The continuous hydrologic model must include mathematical representations of hydrologic processes to
determine the fate and movement of rainfall. For example, a good continuous hydrologic model must
include representations of infiltration processes to determine how much water infiltrates the soil and how
much runs off the surface. It must represent shallow and deep soil storage as well as the release of
subsurface water to streams via interflow and groundwater flow, and it must also account for the loss of
soil water to the atmosphere via evapotranspiration between rainfall events. The benefit of all this
computation is a complete hydrologic assessment including information on peak flow rates, flow
durations, storm volumes, seasonal volumes, annual volumes, and water levels of receiving bodies.
AGENDA ITEM # 8. a)
3.1.3 HYDROLOGIC ANALYSIS USING CONTINUOUS MODELS
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FIGURE 3.1.3.A EFFECTS OF SEQUENTIAL STORMS ON DETENTION PERFORMANCE
Small Basin Runoff Response:
surface and interflows from 10-acre till site
0
0.5
1
1.5
2
2.5
11/2/86 11/9/86 11/16/86 11/23/86 11/30/86
DateFlow, CFSForest Condition Flows
Detention Pond Outflows
Pond Inflows from
Residential Development
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CITY OF RENTON SURFACE WATER DESIGN MANUAL
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3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS
This section presents the following four runoff computation methods accepted for hydrologic analysis and
design:
The Rational Method described below and detailed in Section 3.2.1
The TR-55 or SBUH methods described below.
The Runoff Files Method described below and detailed in Section 3.2.2
The Hydrologic Simulation Program-FORTRAN (HSPF) model described below and detailed in
Section 3.2.4.
ACCEPTABLE USES OF RUNOFF COMPUTATION METHODS
Acceptable uses of the four runoff computation methods are summarized below and in Table 3.2:
Rational Method: This method is most appropriate for sizing new conveyance systems that drain
smaller, quickly responding tributary areas (i.e., less than 10 acres) where very short, intense storms
tend to generate the highest peak flows. The Rational Method may also be used for conveyance sizing
in any size basin if the attenuation effects of existing storage features within the basin are ignored.
TR-55/SBUH Methods: The Natural Resources Conservation Service (NRCS, formerly the Soil
Conservation Service (SCS)) TR-55 method or the SBUH method of the 1990 King County Surface
Water Design Manual may be used for conveyance sizing where tributary areas are greater than or
equal to 10 acres and if storage features are ignored. The peak flows from these single-event models
are considered conservative for larger tributary areas if the flows are not routed through existing
storage features. The TR-55 method is also used for water quality volume calculation in this manual.
For more background information, refer to NRCS Publication 210-VI-TR-55, Second Edition (June
1986) or the 1990 SWDM.
The Runoff Files Method: This continuous modeling method using the approved model is the most
versatile for quickly performing many of the computations summarized in Table 3.2. For conveyance
sizing and analysis, the peak flows from the approved model are most accurate when the shortest
possible time step is used. Unlike the Rational Method, the approved model may be used for tributary
areas less than 10 acres where there is a significant storage feature(s). The City requires 15-minute
time steps for sizing of all flow control facilities, water quality facilities and conveyance to
provide consistent management of surface water and protect against cumulative increases in peak
flows on a basin-wide basis (see Sections 3.3.1 and 3.3.2). Methods for analysis and design of
detention storage and water levels5 require the use of the approved model. See the user’s
documentation for background and guidance.
HSPF Model: For projects in Large Project Drainage Review (see Section 1.1.2.5), the City may
require HSPF modeling for formulating a Master Drainage Plan (see Master Drainage Planning for
Large Site Developments – Process and Requirement Guidelines available from King County). The
City also generally encourages use of HSPF for tributary areas larger than 200 acres. The HSPF model
can be used wherever the approved model is allowed for sizing and analysis of conveyance systems,
flow control facilities, and water quality facilities using a 15-minute time step. For such projects
draining to a wetland or potentially impacting groundwater resources or stream base flows, the City
may require the collection of actual rainfall and runoff data to be used in developing and calibrating
the HSPF model.
5 One of the simplest and most commonly used level pool routing methods is described in the Handbook of Applied Hydrology
(Chow, Ven Te, 1964) and elsewhere, and summarized in Reference Section 6-C, It is based on the continuity equation and
can be completed with a spreadsheet. Although not approved for design with this manual, it provides a background for
modeled routing techniques.
AGENDA ITEM # 8. a)
SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS
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TABLE 3.2 ACCEPTABLE USES OF RUNOFF COMPUTATION METHODS
TYPE OF
COMPUTATION APPLIED TO RATIONAL
METHOD TR 55/SBUH THE APPROVED
MODEL HSPF
PEAK FLOW
CONVEYANCE
SIZING INC.
TESC(1)
(DESIGN FLOWS)
(See Chapter 4 for
hydraulic analysis
procedures)
Tributary
Areas 10 ac
(measured to
individual
conveyance
elements)
REQUIRED for
undetained
areas,(2) and
OKAY for
detained areas if
no storage
routing(3) is
performed
OKAY if majority of
tributary area is
detained(4)
OKAY if majority of
tributary area is
detained(4)
Tributary
Areas 10 ac
OKAY if no
storage routing(3)
is performed
OKAY if no
storage
routing(3) is
performed
OKAY (storage
routing is allowed)
OKAY (storage
routing is allowed)
LEVEL-POOL
ROUTING
FLOW CONTROL
(NEW/EXIST.) &
WQ FACILITY
SIZING AND
ANALYSIS
Projects in
Full Drainage
Review
OKAY OKAY
Projects in
Large Project
Drainage
Review
MAY BE
ALLOWED(5)
MAY BE
REQUIRED(5)
DOWNSTREAM
ANALYSIS
Projects in
Full or
Targeted
Drainage
Review
OKAY if no
storage routing(3)
is performed
OKAY for
tributary areas
10 ac. if no
storage
routing(3) is
performed
OKAY OKAY
Projects in
Large Project
Drainage
Review
MAY BE
ALLOWED(5) if
used as
described in the
box above
MAY BE
ALLOWED(5) if
as described in
the box above
MAY BE
ALLOWED(5) if
used as described
in the box above
PEAK FLOWS
FOR APPLYING
EXEMPTIONS &
THRESHOLDS
All Projects OKAY OKAY
Notes:
(1) Water quality design flow rates are determined as described in Section 6.2.1.
(2) Undetained areas are those upstream of detention facilities or other storage features.
(3) Storage routing uses the Level Pool Routing technique (described in Reference Section 6-C) or other similar method to account for the
attenuation of peak flows passing through a detention facility or other storage feature.
(4) The majority of the tributary area is considered detained if the runoff from more than 50% of the tributary area is detained by a detention
facility or other storage facility.
(5) For projects in Large Project Drainage Review, the selection of methodology for detention sizing and/or downstream analysis becomes
a site-specific or basin-specific decision that is usually made by CED during the scoping process for master drainage plans. Guidelines
for selecting the approved model, HSPF, or calibrated HSPF are found in the King County publication Master Drainage Planning for
Large or Complex Site Developments, available from King County.
AGENDA ITEM # 8. a)
3.2.1 RATIONAL METHOD
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3.2.1 RATIONAL METHOD
The Rational Method is a simple, conservative method for analyzing and sizing conveyance elements
serving small drainage subbasins, subject to the following specific limitations:
Only for use in predicting peak flow rates for sizing conveyance elements
Drainage subbasin area A cannot exceed 10 acres for a single peak flow calculation
The time of concentration Tc must be computed using the method described below and cannot exceed
100 minutes. It is also set equal to 6.3 minutes when computed to be less than 6.3 minutes.
Note: Unlike other methods of computing times of concentration, the 6.3 minutes is not an initial
collection time to be added to the total computed time of concentration.
RATIONAL METHOD EQUATION
The following is the traditional Rational Method equation:
QR = CIRA (3-1)
where QR = peak flow (cfs) for a storm of return frequency R
C = estimated runoff coefficient (ratio of rainfall that becomes runoff)
IR = peak rainfall intensity (inches/hour) for a storm of return frequency R
A = drainage subbasin area (acres)
“C” Values
The allowable runoff coefficients to be used in this method are shown in Table 3.2.1.A by type of land
cover. These values were selected following a review of the values previously accepted by King County
for use in the Rational Method and as described in several engineering handbooks. The values for single
family residential areas were computed as composite values (as illustrated in the following equation) based
on the estimated percentage of coverage by roads, roofs, yards, and unimproved areas for each density. For
drainage basins containing several land cover types, the following formula may be used to compute a
composite runoff coefficient, Cc:
Cc = (C1A1 + C2A2 +… + CnAn)/At (3-2)
where At = total area (acres)
A1,2,…n = areas of land cover types (acres)
C1,2,…n = runoff coefficients for each area land cover type
“IR” Peak Rainfall Intensity
The peak rainfall intensity IR for the specified design storm of return frequency R is determined using a
unit peak rainfall intensity factor iR in the following equation:
IR = (PR)(iR) (3-3)
where PR = the total precipitation at the project site for the 24-hour duration storm event for the
given return frequency. Total precipitation is found on the Isopluvial Maps in
Figure 3.2.1.A through Figure 3.2.1.D.
iR = the unit peak rainfall intensity factor
AGENDA ITEM # 8. a)
SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS
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The unit peak rainfall intensity factor iR is determined by the following equation:
iR = (aR)(Tc)( )b R (3-4)
where Tc = time of concentration (minutes), calculated using the method described below and
subject to equation limitations (6.3 Tc 100)
aR, bR = coefficients from Table 3.2.1.B used to adjust the equation for the design storm return
frequency R
This “iR” equation was developed by DNRP from equations originally created by Ron Mayo, P.E. It is
based on the original Renton/Seattle Intensity/Duration/Frequency (I.D.F.) curves. Rather than
requiring a family of curves for various locations, this equation adjusts proportionally the Renton/Seattle
I.D.F. curve data by using the 24-hour duration total precipitation isopluvial maps. This adjustment is
based on the assumption that the localized geo-climatic conditions that control the total volume of
precipitation at a specific location also control the peak intensities proportionally.
Note: Due to the mathematical limits of the equation coefficients, values of T c less than 6.3 minutes or
greater than 100 minutes cannot be used. Therefore, real values of Tc less than 6.3 minutes must be
assumed to be equal to 6.3 minutes, and values greater than 100 minutes must be assumed to be equal to
100 minutes.
“Tc” Time of Concentration
The time of concentration is defined as the time it takes runoff to travel overland (from the onset of
precipitation) from the most hydraulically distant location in the drainage basin to the point of discharge.
Note: When Cc (see Equation 3-2) of a drainage basin exceeds 0.60, it may be important to compute Tc
and peak rate of flow from the impervious area separately. The computed peak rate of flow for the
impervious surface alone may exceed that for the entire drainage basin using the value at Tc for the total
drainage basin. The higher of the two peak flow rates shall then be used to size the conveyance element.
Tc is computed by summation of the travel times Tt of overland flow across separate flowpath
segments defined by the six categories of land cover listed in Table 3.2.1.C, which were derived from a
chart published by the Soil Conservation Service in 1975. The equation for time of concentration is:
Tc = T1 + T2 +…+ Tn (3-5)
where T1,2,…n = travel time for consecutive flowpath segments with different land cover
categories or flowpath slope
Travel time for each segment t is computed using the following equation:
Tt = (3-6)
where Tt = travel time (minutes) Note: Tt through an open water body (such as a pond) shall be
assumed to be zero with this method
L = the distance of flow across a given segment (feet)
V = average velocity (fps) across the land cover = kR s o
where kR = time of concentration velocity factor; see Table 3.2.1.C
so = slope of flowpath (feet/feet)
L
V60
AGENDA ITEM # 8. a)
3.2.1 RATIONAL METHOD
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TABLE 3.2.1.A RUNOFF COEFFICIENTS – “C” VALUES FOR THE RATIONAL METHOD
General Land Covers Single Family Residential Areas*
Land Cover C Land Cover Density C
Dense forest
Light forest
Pasture
Lawns
Playgrounds
Gravel areas
Pavement and roofs
Open water (pond, lakes,
wetlands)
0.10
0.15
0.20
0.25
0.30
0.80
0.90
1.00
0.20 DU/GA (1 unit per 5 ac.)
0.40 DU/GA (1 unit per 2.5 ac.)
0.80 DU/GA (1 unit per 1.25 ac.)
1.00 DU/GA
1.50 DU/GA
2.00 DU/GA
2.50 DU/GA
3.00 DU/GA
3.50 DU/GA
4.00 DU/GA
4.50 DU/GA
5.00 DU/GA
5.50 DU/GA
6.00 DU/GA
0.17
0.20
0.27
0.30
0.33
0.36
0.39
0.42
0.45
0.48
0.51
0.54
0.57
0.60
* Based on average 2,500 square feet per lot of impervious coverage.
For combinations of land covers listed above, an area-weighted “Cc x At” sum should be computed based on the equation
Cc x At = (C1 x A1) + (C2 x A2) + …+(Cn x An), where A8 = (A1 + A2 + …+An), the total drainage basin area.
TABLE 3.2.1.B COEFFICIENTS FOR THE RATIONAL METHOD “IR” EQUATION
Design Storm Return Frequency aR bR
2 years
5 years
10 years
25 years
50 years
100 years
1.58
2.33
2.44
2.66
2.75
2.61
0.58
0.63
0.64
0.65
0.65
0.63
TABLE 3.2.1.C KR VALUES FOR TT USING THE RATIONAL METHOD
Land Cover Category kR
Forest with heavy ground litter and meadow 2.5
Fallow or minimum tillage cultivation 4.7
Short grass pasture and lawns 7.0
Nearly bare ground 10.1
Grassed waterway 15.0
Paved area (sheet flow) and shallow gutter flow 20.0
AGENDA ITEM # 8. a)
SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS
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FIGURE 3.2.1.A 2-YEAR 24-HOUR ISOPLUVIALS
AGENDA ITEM # 8. a)
3.2.1 RATIONAL METHOD
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FIGURE 3.2.1.B 10-YEAR 24-HOUR ISOPLUVIALS
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SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS
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FIGURE 3.2.1.C 25-YEAR 24-HOUR ISOPLUVIALS
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3.2.1 RATIONAL METHOD
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FIGURE 3.2.1.D 100-YEAR 24-HOUR ISOPLUVIALS
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SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS
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RATIONAL METHOD EXAMPLE
Compute the peak flow Q25 to size a new roadway cross culvert for a 9.8-acre drainage basin east of Kent,
P25 = 3.42 inches.
Given: AREAS
A1 = 4.3 acres of single family residential area at 3.8 DU/GA
A2 = 2.3 acres of light forest
A3 = 3.2 acres of pasture
At = 9.8 total acres
DESCRIPTION OF FLOWPATH SEGMENTS FOR Tc
L1 = 300 feet s1 = 0.08 forest land cover kR = 2.5
L2 = 200 feet s2 = 0.03 meadow kR = 2.5
L3 = 1000 feet s3 = 0.015 grassed waterway (ditch) kR = 15.0
Compute: COMPOSITE RUNOFF COEFFICIENT Cc
A1: C1 = From Table 3.2.1.A, C for 4.00 DU/GA = 0.48, C for 3.50 DU/GA = 0.45.
Therefore, C1 for 3.80 DU/GA = 0.47 by visual interpolation.
A2: C2 = 0.15
A3: C3 = 0.20
Cc = [(C1 x A1) + (C2 x A2) + (C3 x A3)]/At
= [(0.47 x 4.3) + (0.15 x 2.3) + (0.20 x 3.2)]/9.8 = 0.31
PEAK RAINFALL INTENSITY IR
First, compute Tc:
T1 =
= 7 minutes
T2 =
= 8 minutes
T3 =
= 9 minutes
Tc = T1 + T2 + T3 = 7 + 8 + 9 = 24 minutes
Second, compute iR for R = 25:
i25 = (aR)(Tc)(-bR) = (2.66)(24)- (0.65) = 0.34
Third, compute IR for R = 25:
I25 = (P25)(i25) = (3.42)(0.34) = 1.16
PEAK RUNOFF RATE
Q25 = C I25 A = Cc I25 A = (0.31)(1.16)(9.8) = 3.5 cfs
L
V
L
k sR
1
1
1
16060
300
60 25 0 08
( ) ( . . )
L
V
L
k sR
2
2
2
26060
200
60 25 003
( ) ( . . )
L
V
L
k sR
3
3
3
36060
1000
60 15 0015
( ) ( . )
AGENDA ITEM # 8. a)
3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD
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3.2.2 CONTINUOUS MODELS AND THE RUNOFF FILES METHOD
The approved continuous model/runoff files implementations of HSPF were developed as tools that have
the accuracy and versatility of HSPF but are much simpler to use and provide a framework for efficient
design of onsite stormwater detention facilities. This section describes the Runoff Files Method. The term
runoff files refers to a database of continuous flows presimulated by HSPF. The KCRTS software package
has formerly been a tool for using this flow database. Current approved continuous models are listed in
Reference Section 6-D (as updated); as of this writing, they include the Western Washington Hydrology
Model (WWHM) and MGSFlood6. Projects are required to use the same model throughout unless
otherwise approved through the adjustment process described in Section 1.4.
The Runoff Files method was developed as a hydrologic modeling tool for western King County to
produce results (design flows, detention pond sizing, etc.) comparable to those obtained with the U.S.
Environmental Protection Agency’s HSPF model but with significantly less effort. This is achieved by
providing the user with a set of time series files of unit area land surface runoff (“runoff files”)
presimulated with HSPF for a range of land cover conditions and soil types within King County. The
design flows are estimated and detention facilities are designed by directly accessing and manipulating the
runoff file data by means of the continuous modeling software. Typical basic capabilities of the continuous
modeling software include:
Estimating time series of flows for a specified land use and location within King County
Analyzing flow frequency and duration
Analyzing water surface frequency and duration
Plotting analysis results
Sizing detention facilities.
DEVELOPMENT OF THE RUNOFF FILES
To compile the runoff files, the land surface hydrologic response (represented by a time series of unit area
land surface runoff) was generated by HSPF with regional parameters for a variety of land use
classifications and for a long-term (over 50-year) rainfall station representing the western lowlands of
King County (Sea-Tac Airport). A 158-year extended precipitation timeseries (Puget East) was also
developed by MGS Consulting. The City allows the use of either the 50-year Sea-Tac Airport gage data or
the 158-year simulated timeseries for sizing. The methods for developing the runoff files are specific to the
individual approved models. Consult the program documentation and the software provider’s website
information for the particular model for background on the development of the runoff files for that model.
Runoff time series were generated with data from these and other stations for the following eight soil/land
cover types:
Impervious
Till forest
Till pasture
Till grass
Outwash forest
Outwash pasture
Outwash grass
Wetland.
HSPF and the approved models simulate surface runoff, interflow, and groundwater flow. Groundwater
flow, induced by surface runoff or occurring naturally, is usually lost from the system through the
6 Note that MGS Flood is not currently approved for modeling bioretention. It will be allowed for modeling bioretention only at
such time that it is formally approved by Ecology for that use.
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SECTION 3.2 RUNOFF COMPUTATION AND ANALYSIS METHODS
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analysis, but may require consideration in the analysis if it expresses to the surface. Consult the user’s
guide for application of the interflow and groundwater components of runoff in the approved continuous
model.
3.2.2.1 GENERATING TIME SERIES
Most hydrologic analyses will require time series of flows for different land use conditions. For example,
to size a flow control detention facility to meet the Peak Rate Flow Control Standard, the 2-, 10-, and
100-year peaks from the facility discharge time series must be compared with 2-, 10-, and 100-year peaks
from the predevelopment time series. To generate a flow time series with the approved continuous model,
depending on the model used, the program applies the following:
1. As determined by selecting the project’s location on a map,
The rainfall region within which the project lies (i.e., Sea-Tac) and multiplier (a regional scale
factor applied to the runoff files) to account for variations in rainfall volumes between the project
site and the rainfall station, or
A calibrated area-specific rainfall map developed from the Sea-Tac rainfall data, or
A long-term (158-year) simulated precipitation timeseries (i.e., Puget East), or
Site specific calibrated rainfall data. See the approved model’s documentation for background on
the development of the runoff files for the model.
2. The time step to be used in the analysis. As of this manual update, 15-minute time steps are
required for all applications including detention sizing and volume analysis .
3. The complete historical runoff record used in the analysis:
4. The amount of land (acreage) of each soil/cover group for the subbasin under study, as calculated per
model methodology and the methods described in this chapter.
5. If applicable, the percentage of impervious area that is effectively connected to the drainage system,
typically accounted for by adjusting actual impervious area for the model inputs.
See the user’s documentation for the approved model for methodology and guidance for generating a new
time series. See Reference Section 6-D for specific guidance to be used with this manual.
SELECTION OF PRECIPITATION RECORD AND REGIONAL SCALE FACTOR
As noted in the previous section, runoff files were developed using rainfall data from Sea-Tac Airport.
The regional scale factor is a geographically variable multiplier applied to the flow time series to account
for the variations in rainfall amounts, and hence runoff. Whereas previous models (e.g., KCRTS) required
determination by mapped values as data input, the scaling effects are determined in the currently approved
continuous models (e.g., WWHM and MGS Flood) by selecting the project location within the model. See
the approved model user’s documentation for background and guidance.
Alternatively, the user can select the 158-year simulated precipitation timeseries (Puget East) for sizing.
This precipitation timeseries can be found by selected “Use WS-DOT data” in WWHM or under
“Extended Timeseries” in MGS Flood. A scaling factor does not need to be applied to the Puget East
precipitation timeseries.
CATEGORIZATION OF SOIL TYPES AND LAND COVER
The Runoff Files method typically supports several land use classifications, including till forest, till
pasture, till grass, outwash forest, outwash pasture, outwash grass, wetland, and impervious. These
classifications incorporate both the effects of soil type and land cover. In the SCS method, four different
hydrologic soil groups are defined (A, B, C, and D) based on soil type as mapped by the SCS. The SCS
also defines hydrologic response for about a dozen different land use or cover types. The SCS method
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therefore allows the user a considerably greater degree of flexibility in defining land cover and soil types
than do continuous models. However, the flexibility and apparent detail available with the SCS method
cannot be supported on the basis of the data used to develop that method. The Runoff Files method
minimizes the number of land use classifications, thereby simplifying both the analysis and review of
development proposals.
Soil Groups for the Continuous Model
The following soil characterization is generally true for continuous models; however, consult the model
documentation for specific applicability.
Till Soils
Till soils are underlain at shallow depths by relatively impermeable glacial till. The principal SCS soil
group within the City classified as a till soil is the Alderwood series (SCS hydrological soil group C).
The hydrologic response of till soils in an undeveloped, forested state is characterized by relatively
slight surface runoff, substantial interflow occurring along the interface between the till soil and the
underlying glacial till, and slight groundwater seepage into the glacial till.
Bedrock soils, primarily Beausite and Ovall soils in King County, are underlain by either sandstone or
andesite bedrock, and a large group of alluvial soils.
Alluvial soils are found in valley bottoms. These are generally fine-grained and often have a high
seasonal water table. There has been relatively little experience in calibrating the HSPF model to
runoff from these soils, so in the absence of better information, these soils have been grouped as till
soils. Most alluvial soils are classified by the SCS in hydrologic soil groups C and D.
Outwash Soils
Outwash soils are formed from highly permeable sands and gravels. The principal SCS soil group
classified as an outwash soil is the Everett series. Where outwash soils are underlain at shallow depths
(less than 5 feet) by glacial till or where outwash soils are saturated, they may need to be treated as till
soils for the purpose of application in the model. Refer to the model documentation for specifics.
Wetland Soils
Wetland soils have a high water content, are poorly drained, and are seasonally saturated. For the
purposes of applying continuous modeling in King County, wetland soils can be assumed to coincide
with wetlands as defined in the critical areas code (RMC 4-3-050 ).
The approximate correspondence between SCS soil types and the appropriate soil group for typical
continuous modeling is given in Table 3.2.2.A (refer to the model documentation for specific soil group
application for the model). If the soils underlying a proposed project have not been mapped, or if existing
soils maps are in error or not of sufficient resolution, then a soils analysis and report shall be prepared and
stamped by a civil engineer with expertise in soils to verify underlying soil conditions.
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TABLE 3.2.2.A EQUIVALENCE BETWEEN SCS SOIL TYPES AND TYPICAL
CONTINUOUS MODELING SOIL TYPES
SCS Soil Type
SCS
Hydrologic
Soil Group
Soil Group for
Continuous
Model Notes
Alderwood (AgB, AgC, AgD) C Till
Arents, Alderwood Material (AmB, AmC) C Till
Arents, Everett Material (An) B Outwash 1
Beausite (BeC, BeD, BeF) C Till 2
Bellingham (Bh) D Till 3
Briscot (Br) D Till 3
Buckley (Bu) D Till 4
Earlmont (Ea) D Till 3
Edgewick (Ed) C Till 3
Everett (EvB, EvC, EvD, EwC) A/B Outwash 1
Indianola (InC, InA, InD) A Outwash 1
Kitsap (KpB, KpC, KpD) C Till
Klaus (KsC) C Outwash 1
Neilton (NeC) A Outwash 1
Newberg (Ng) B Till 3
Nooksack (Nk) C Till 3
Norma (No) D Till 3
Orcas (Or) D Wetland
Oridia (Os) D Till 3
Ovall (OvC, OvD, OvF) C Till 2
Pilchuck (Pc) C Till 3
Puget (Pu) D Till 3
Puyallup (Py) B Till 3
Ragnar (RaC, RaD, RaC, RaE) B Outwash 1
Renton (Re) D Till 3
Salal (Sa) C Till 3
Sammamish (Sh) D Till 3
Seattle (Sk) D Wetland
Shalcar (Sm) D Till 3
Si (Sn) C Till 3
Snohomish (So, Sr) D Till 3
Sultan (Su) C Till 3
Tukwila (Tu) D Till 3
Woodinville (Wo) D Till 3
Notes:
1. Where outwash soils are saturated or underlain at shallow depth (<5 feet) by glacial till, they should be treated as till
soils.
2. These are bedrock soils, but calibration of HSPF by King County shows bedrock soils to have similar hydrologic
response to till soils.
3. These are alluvial soils, some of which are underlain by glacial till or have a seasonally high water table. In the absence
of detailed study, these soils should be treated as till soils.
4. Buckley soils are formed on the low-permeability Osceola mudflow. Hydrologic response is assumed to be similar to that
of till soils.
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Land Cover Types in Continuous Modeling
Continuous models support land cover types including forest, pasture, grass, and impervious. These cover
types shall be applied in accordance with Core Requirement #3 and as specified in Table 3.2.2.B.
Predevelopment land cover types are determined by whether the project is in a Peak Rate Flow Control
Standard Area or Flow Control Duration Standard Area and whether the area in question is a target
surface, as defined in Section 1.2.3.1. Target surfaces within Peak Rate Flow Control Standard Areas and
Flow Control Duration Standard Matching Existing Condition Areas and non-target surfaces are modeled
as existing site conditions; for target surfaces in Flow Control Duration Standard Matching Forested
Condition Areas, the predeveloped condition is assumed to be forested (historical) site conditions.
TABLE 3.2.2.B CONTINUOUS MODEL COVER GROUPS AND AREAS OF APPLICATION
Continuous
Model Application
Cover Group Predevelopment Post-Development
Forest All forest/shrub cover,
irrespective of age.
All permanent (e.g., protected by covenant or critical
area designation) onsite forest/shrub cover,
irrespective of age, planted at densities sufficient to
ensure 80%+ canopy cover within 5 years.
Pasture All grassland, pasture land,
lawns, and cultivated or cleared
areas, except for lawns in
redevelopment areas with
predevelopment densities in
excess of 4 DU/GA.
Unprotected forest in rural residential development
shall be considered half pasture, half grass.
Grass Lawns in redevelopment areas
with predevelopment densities in
excess of 4 DU/GA.
All post-development grassland and landscaping and
all onsite forested land not protected by covenant or
designated as a protection area (wellhead, wetland,
or buffer) in RMC 4-3-050.
For purposes of runoff modeling, underdrained
pervious areas may be modeled explicitly to account
for attenuation and infiltration, or may be modeled as
50% impervious/50% grass where either: (a) there is
no added liner, (b) where the added liner is a
treatment liner, or (c) where the added liner is one
that does not restrict infiltration rates below the in situ
soil infiltration rate. Other lined underdrained systems
must be modeled explicitly or as 100% impervious.
Wetland All delineated wetland areas. All delineated wetland areas.
Impervious(1) All impervious surfaces,
including compacted dirt roads,
parking areas, etc., and open
water bodies (ponds and lakes).
For purposes of runoff modeling,
gravel lots, roads and parking
areas shall be modeled as 50%
impervious/50% pasture.
All impervious surfaces, including compacted gravel
and dirt roads, parking areas, etc., and open water
bodies, including onsite detention and water quality
ponds.(2)
(1) Impervious acreage used in computations should be the effective impervious area (EIA). This is the effective area as
determined through layouts of the proposal and on-site BMP credit reductions from Table 1.2.9.A in Chapter 1. Non-
effective impervious areas are considered the same as the surrounding pervious land cover.
(2) To avoid iterations in the facility sizing process, the “assumed size” of the facility need only be within 80% of the final
facility size when modeling its contribution of runoff from direct rainfall.
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The following factors are considered in specifying the above land cover types to be used in hydrologic
analysis with continuous modeling:
Cover types are applied to anticipate ultimate land use conditions. For example, probable clearing of
woodland after development is nominally complete suggests that the post-development land use be
specified as grassland (either pasture or grass) unless the forest cover is protected by covenant.
In areas of redevelopment, there are often significant changes between the predevelopment and post-
development efficiencies of the drainage system. For example, in conversion of low density residential
areas to higher density land use, impervious areas prior to redevelopment may not be efficiently
connected to a drainage system (e.g., downspouts draining to splash blocks, ditched instead of piped
roadway systems). These problems are addressed by defining an “effective impervious fraction” for
existing impervious areas and by generally requiring predevelopment grasslands to be modeled as
pasture land.
All onsite, predevelopment forest/shrub cover and all offsite forest/shrub cover is defined as “forest,”
irrespective of age. Post-development onsite land use is defined as forested only if forested areas are
in a critical area buffer or are otherwise protected and will have a minimum 80% canopy cover within
5 years. In urban areas, unprotected onsite forest cover should be treated as grass in the post-
development analysis. In rural areas, unprotected forest cover should be assumed 50% grass, 50%
pasture.
The HSPF grass parameters were developed by the USGS study of regional hydrology and have
generally been interpreted as providing the hydrologic response for “urban” grasslands (lawns, etc.),
which have relatively low infiltration rates and are drained effectively. The HSPF “pasture”
parameters were developed to provide a hydrologic response intermediate to the USGS forest and
grass parameters, as might be typified by ungrazed or lightly grazed pasture with good grass cover.
Because it is impossible to adequately control grassland management after development, all post-
development grassland should be modeled as “grass” (with the exception of unprotected forest, and
pasture areas on large lots, in rural development as noted above). All predevelopment grassland
should be modeled as “pasture” except for redevelopment of areas with predevelopment land use
densities of 4 DU/GA or greater (which are modeled as grass).
CALCULATION OF IMPERVIOUS AREA
Total Impervious Coverage
Table 3.2.2.C lists percent impervious coverage for use in continuous runoff modeling analysis of existing
residential areas. The tabulated figures are useful in offsite analysis that includes large developed
residential areas, making a detailed survey of impervious coverage impractical.
Impervious coverage for proposed residential, commercial, and industrial development must be estimated
for each specific proposal. Impervious coverage of streets, sidewalks, hard surface trails, etc., shall be
taken from layouts of the proposal. House/driveway or building coverage shall be as follows:
For residential development, the assumed impervious coverage shall be 4,000 square feet per lot or the
maximum impervious coverage permitted by RMC 4-2-110A, whichever is less.
For commercial, multi-family, and industrial development, impervious coverage shall be estimated
from layouts of the proposal.
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TABLE 3.2.2.C ESTIMATED PERCENT IMPERVIOUS COVERAGE FOR OFFSITE
EXISTING RESIDENTIAL AREAS
Zoning Designation Percent Impervious Surface Area
Resource Conservation (RC) Lots 5 acres or more: 20%
Lots 10,000 sq ft: 55%. For each additional 10,000 sq ft
increase in lot size, the impervious coverage shall be
decreased by 1.75% to a minimum of 20% for a 5-acre lot
Lots 10,000 sq ft or less: 55%
Residential-1 (R-1) 30%
Residential-4 (R-4) 55%
Residential-6 (R-6) 65%
Residential-8 (R-8) 75%
Residential-10 (R-10) Detached units: 75%
Attached units: 65%
Residential-14 (R-14) 85%
Effective Impervious Area
The net hydrologic response of an impervious area depends on whether that area is effectively connected
(usually by pipes or a channel) to a storm drainage system. The impervious area that the user inputs to the
continuous model is the “Effective Impervious Area” (EIA).
Non-effective impervious area (i.e., total impervious area less EIA) is assumed to have the same
hydrologic response as the immediately surrounding pervious area. For example, for existing
residential areas with rooftops draining to splash pads on lawns or landscaping, the non-effective portion
of the roof areas would be treated as pasture for predevelopment conditions (if DU/GA < 4.0) and grass for
post-development conditions. Note: Credits for infiltration/dispersion of downspouts on individual lots in
proposed single family residential subdivisions are applied separately on a site-specific basis. Core
Requirement #9 outlines where the use of on-site BMPs may be used to reduce the effective impervious
area of the project..
The effective impervious area can be determined from detailed site surveys.
3.2.2.2 TIME SERIES STATISTICAL ANALYSIS
When using a continuous runoff model to size flow control, water quality, and conveyance facilities,
design flows and durations must be determined through statistical analysis of time series data generated by
the software. Flow frequency analysis is used for determining design peak flows while flow duration
analysis is used for determining durations of flow exceedance.
FLOW FREQUENCY ANALYSIS
Flow frequency is a commonly used but often misunderstood concept. The frequency of a given flow is the
average return interval for flows equal to or greater than the given flow. The flow frequency is actually the
inverse of the probability that the flow will be equaled or exceeded in any given year (the exceedance
probability). For example, if the exceedance probability is 0.01, or 1 in 100, that flow is referred to as the
100-year flow. Assuming no underlying changes in local climate, one would expect to see about 10 peak
annual flows equal to or greater than the 100-year flow in a 1,000-year period. Similarly, the 2-year flow is
the flow with a probability of 0.5, or 1 in 2, of being equaled or exceeded in any given year. In a 100-year
period, one would expect to observe 50 peak annual flows greater than or equal to the 2-year flow. The
number of peak annual flows actually equal to the 2-year flow may be zero, since peak annual flows come
from a continuous spectrum.
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There are many methods for estimating exceedance probabilities and therefore flow frequencies. The
USGS Bulletin 17B methods are commonly used, as are graphical methods using the Gringorten, Cunane,
or Weibull plotting schemes (Maidment, 1993). Graphical methods for flow frequency estimation involve
assigning exceedance probabilities, and therefore return intervals, to each annual peak in a series of annual
peak observations, and then plotting the peak flows against their assigned return periods. This plot is
known as a flow-frequency curve, and it is a very useful tool for analyzing flood probabilities. Examples of
flow-frequency curves for a small basin under various conditions are shown in Figure 3.2.2.A.
Flow-frequency curves are used in continuous flow simulations to determine the effect of land use
change and assess the effectiveness of detention facilities. Using continuous methodology to design
detention facilities to control peak flows, the analyst must match (i.e., not exceed) the post-development
(detained) and predevelopment flow-frequency curves at the frequencies of interest, as shown in
Figure 3.2.2.A, rather than match specific design events as when using an event model.
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FIGURE 3.2.2.A EXAMPLE FLOW FREQUENCY ANALYSIS
Post-developed
Pre-developed
Undetained Developed
Detained Developed
Detained Post-developed
Pre-developed
The 2- and 10-year annual peak
flows are matched; however, the
100-year peak flow is only partially
attenuated in this example, so the
detention volume would need to be
increased to fully meet the Peak
Rate Flow Control Standard
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Flow frequency information is derived from the time series flow file by plotting the peak annual events in
the runoff file and calculating runoff frequencies using a Log Pearson distribution or other statistical
analysis. Typical return periods calculated in continuous models are the 100-year, 50-year, 25-year,
10-year, 5-year, 3-year, 2-year, and lesser storms for low-flow regime, LID and water quality applications.
FLOW DURATION ANALYSIS
Flow duration analysis is important because it identifies the changes in durations of all high flows rather
than simply the change in frequency of the peak annual flows. Channel scour and bank erosion rates rise
proportionally with increases in flow durations. Flow duration analysis can only be conducted with
continuous flow models or from gage records.
A flow duration curve is a plot of flow rate against the percentage of time that the flow rate is exceeded.
In a continuous flow model, the percent exceedance of a given flow is determined by counting the number
of time steps during which that flow is equaled or exceeded and dividing that number by the total number
of time steps in the simulation period. Flow duration curves are usually plotted with a linear flow scale
versus a log scale of percent exceedance. The log scale for exceedance percentage is used because
geomorphically significant flows (flows capable of moving sediment) and flows that exceed the 2-year
flow typically occur less than one percent of the total time.
DURATIONS AND PEAKS FOR FLOW CONTROL STANDARDS
The Flow Control Duration Standard matching existing site conditions and Flow Control Duration
Standard matching forested site conditions per Section 1.2.3.1 requires matching predevelopment and
post-development flow duration curves for all flows from 50% of the 2-year flow up to the full 50-year
flow.
To simplify design, brief excursions7 of post development durations above the target predevelopment
durations are allowed for matching flows greater than 50% of the predevelopment 2-year peak flow. These
excursions shall not increase the duration of discharge by more than 10% at any flow level and must be
strictly below the target duration curve at the low end of the range of control from 50% of the 2-year peak
flow to the 2-year peak flow. This allows efficient design using only two orifices for most applications,
although two-orifice designs may not allow sizing with automatic pond sizing routines; see the software
documentation for guidance. An example of a flow duration analysis is shown in Figure 3.2.2.B.
The Flood Problem Flow Control Standard matches predevelopment and post-development flow durations
over the same range of predevelopment flows as the Flow Control Duration Standard and requires
matching the 100-year post-development peak flow. This standard provides additional storage volume
over the Flow Control Duration Standard facility, which substantially mitigates the impacts of increased
volumes of surface runoff on downstream, volume-sensitive flooding problems.
The Peak Rate Flow Control Standard does not require flow duration analysis because it addresses peak
flows only (the 2-year, 10-year, and 100-year peaks).
The Low Impact Development (LID) performance standard requires that stormwater discharges shall
match (i.e., not exceed) developed discharge durations to pre-developed durations for the range of pre-
developed discharge rates from 8% of the 2-year peak flow to 50% of the 2-year peak flow. No excursions
above the pre-developed durations are allowed.
7 Brief excursions may not result in more than 50% of the target duration curve being exceeded.
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FIGURE 3.2.2.B EXAMPLE FLOW DURATION ANALYSIS
Post-developed
Pre-developed
Pre-developed FC Duration Existing Target
Predeveloped
Return
Frequencies
50% 2-year
2-year
10-year
50-year
10% allowable horizontal
tolerance along portion of
target curve above 2-year
predevelopment peak flow
Strictly below target curve
at low end of range of
control (50% of 2-year peak
flow to 2-year peak flow).
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When evaluating impacts to closed depressions, ponding areas and wetlands, or when evaluating for
tightlined system requirements in critical areas per Core Requirement #1, frequencies of water levels or
determination of average annual runoff volumes must be determined through statistical analysis of time
series data generated using a continuous runoff model.
ASSESSING WATER LEVEL STATISTICS
Stage frequency analysis consists of estimating and plotting recurrence estimates for water levels within a
storage feature in the same manner as flow frequency analysis is conducted for discharges. Stage
frequency analysis is required for assessing runoff impacts to offsite closed depressions and ponding areas
as required under Core Requirements #2 and #3, and as discussed Section 3.3.6, “Point of Compliance
Analysis,” or as required for analyses of wetland impacts pursuant to Core Requirement #9.
ASSESSING ANNUAL AVERAGE RUNOFF VOLUMES
To compute the annual average runoff volume, the volume of runoff (surface + interflow) of a time series
must be computed using the approved model. The analysis is performed using the entire period of record.
The total volume is divided by the number of full water years being analyzed to determine the annual
average runoff volume.
3.2.3 THE APPROVED MODEL
The continuous hydrologic analysis tools prescribed in this manual are generically described as the
“approved model”; a list of the approved models is found in Reference Section 6-D. At this writing, the
approved continuous hydrologic models8 include the Western Washington Hydrologic Model
(WWHM) and MGS Flood, both of which are variants of the Hydrologic Simulation Program-
FORTRAN (HSPF) model. HSPF is also an approved model, but is more complex than other approved
models and is typically used for basin planning and master drainage plan analyses.
General instruction and guidance for use of the approved model is found in the user’s documentation for
the model. Guidance specific to the City for the continuous runoff models approved for use with this
manual is contained in Reference Section 6-D. A brief overview of HSPF follows below.
3.2.4 THE HSPF MODEL
HSPF is the parent model from which the other approved model methods are built. It is a very versatile
continuous hydrologic/hydraulic model that allows for a complete range of hydrologic analysis. This
model has been extensively used in King, Snohomish, and Thurston counties and found to be an accurate
tool for representing hydrologic conditions in this area. The USGS has developed regional parameters to
describe the common soil/cover combinations found in this area. In many cases, these regional parameters
can be used to represent rainfall/runoff relationships in lieu of site-specific calibration parameters.
Unfortunately, the HSPF model is very difficult to use. Design engineers using HSPF should study this
model in detail and obtain training before using it on a project. For these reasons, the HSPF model is
recommended only for large and complex projects where the capabilities of the approved model are too
limited.
The strengths of HSPF relative to the approved model are as follows:
1. HSPF can be calibrated to local conditions.
2. HSPF can model, link, and route many separate subbasins.
8 Note that MGS Flood is not currently approved for modeling bioretention. It will be allowed for modeling bioretention only at
such time that it is formally approved by Ecology for that use.
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3. HSPF includes the groundwater component of streamflow.
4. HSPF can address groundwater connections and perform low-flow analysis.
5. HSPF can handle more complex hydrologic routing (e.g., evaporation, seasonal infiltration, etc.).
The HSPF model is generally recommended for large sites where these additional features are required
for comprehensive hydrologic and/or hydraulic analysis.
Anyone planning a project that is large enough to require Large Project Drainage Review and submittal of
a Master Drainage Plan (MDP) per Section 1.1.2.5 should meet with CED review staff regarding
appropriate hydrologic analysis prior to initiating such analysis. If a project subject to Large Project
Drainage Review drains to a wetland, a salmonid stream with low-flow sensitivities, or a ground water
protection area, it is likely that the City will require a calibrated HSPF model. If such a project drains to
erosion-sensitive streams or has features with complex hydraulics, the City may recommend or require an
HSPF model using the USGS regional parameters. Smaller or less sensitive subbasins within a MDP area
can be analyzed with the approved model.
Additional data is required to develop an HSPF model. At a minimum, development of an HSPF model
requires collection of onsite rainfall data for a period from seven to twelve months. This data is used to
determine which regional long-term rainfall record is most appropriate for modeling the site and for
determining transposition factors for the long-term records. If calibration is required, the onsite rainfall
data is used. Calibration also requires the installation of flow gages and the collection of flow data against
which simulated flows can be compared. HSPF analysis is based on simulations with long-term rainfall
records (greater than 30 years). Long-term precipitation records in HSPF format can be obtained from
King County for the Sea-Tac rain gage and the Puget East 158-year simulated precipitation timeseries.
Land surface representation with HSPF follows the same procedures and classification as used with the
approved model.
Conceptually, the outputs required from an HSPF analysis are consistent with those required from an
approved model analysis, including frequency and durational analysis. Flow and/or water level frequencies
shall be estimated using the full set of annual peaks from the long-term simulations using the USGS
Bulletin 17B methods as well as the Gringorten or Cunane graphical methods. Durational analyses can be
produced from the HSPF model and the results presented graphically. If a wetland is modeled, water level
analyses may be required. Monthly, seasonal, and annual water balance and flow information, if
appropriate, can be calculated with the HSPF model.
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3.3 HYDROLOGIC DESIGN PROCEDURES AND
CONSIDERATIONS
This section presents the design procedures and considerations for sizing flow control facilities to meet the
required hydrologic performance specified in Core Requirement #3, Section 1.2.3. It includes the
following procedures and special considerations for proper hydrologic design:
“General Hydrologic Design Process,” Section 3.3.1
“Flow Control Design Using the Runoff Files Method,” Section 3.3.2
“Conveyance System Design with the Runoff Files Method,” Section 3.3.3
“Safety Factors in Hydrologic Design,” Section 3.3.4
“Design Options for Addressing Downstream Drainage Problems,” Section 3.3.5
“Point of Compliance Analysis,” Section 3.3.6
“Onsite Close Depressions and Ponding Areas,” Section 3.3.7
3.3.1 GENERAL HYDROLOGIC DESIGN PROCESS
This section presents the general process involved in conducting a hydrologic analysis using the runoff
computation and analysis tools described in Section 3.2 to design flow control facilities for a project. The
process is described as follows:
1. Review the core and special requirements in Chapter 1 to determine all requirements that will apply
to the proposed project.
a) Determine the applicable flow control standard (outflow performance criteria and land cover
assumptions).
b) If downstream drainage problems are identified through offsite analysis per Core Requirement #2,
determine if they will necessitate additional onsite flow control or other measures as described in
Section 3.3.5.
2. Determine and demonstrate in the Technical Information Report (see Section 2.3) the predeveloped
conditions per Core Requirement #3, Flow Control (see Section 1.2.3).
3. Identify and delineate the drainage basin for each natural discharge location from the project site.
a) Identify existing drainage features such as streams, conveyance systems, detention facilities,
ponding areas, depressions, wetlands, etc.
b) Identify existing land uses.
c) Identify soil types using SCS soil survey or onsite evaluation.
d) Convert SCS soil types to soil classifications for the approved model.
4. Select and delineate appropriate subbasins, including subbasins tributary to major drainage features
and important conveyance points, and subbasins for separate computation of onsite flows and offsite
flows.
5. Determine hydrologic parameters for each subbasin under predeveloped conditions.
a) Categorize soil types and land cover.
b) Determine total impervious areas and effective impervious areas within each subbasin.
c) Determine areas for each soil/cover type in each subbasin.
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6. Determine the runoff time series for predeveloped conditions at each natural discharge location.
a) Compute the predeveloped condition runoff time series for each subbasin using 15-minute time
steps.
b) For subbasins that drain to a drainage feature with significant detention storage (e.g.,
existing detention facilities, ponding areas, closed depressions), route the runoff time series
through the feature per the storage routing methods in the approved model. This will yield an
attenuated flow series, which becomes the effective runoff time series for that subbasin.
c) Sum the appropriate subbasin runoff time series to obtain the total runoff time series for each
natural discharge location.
d) Determine the 100-year peak flow for each natural discharge location.
7. Repeat Steps 4 through 6 for the proposed post-development condition.
8. Compare the 100-year peak flows for the appropriate predeveloped and post-development conditions
at each natural discharge location.
a) Check the “Discharge Requirements” criteria in Core Requirement #1 to determine the
acceptable manner of discharge from the project site (using existing conditions).
b) Check the flow control exemptions in Core Requirement #3 to determine if a flow control
facility is required (using existing site or historical site conditions, as specified in Core
Requirement #3).
c) Check the requirement for bypass of runoff from non-target surfaces in Core Requirement #3
to determine if runoff from non-target surfaces must be conveyed around onsite flow control
facilities (using existing conditions).
9. If flow control facilities are required, determine their location and make any necessary adjustments to
the developed condition subbasins.
10. Design and size each flow control facility using the methods described in Section 3.2 and the Runoff
Files Method design procedure in Section 3.3.2.
a) Analyze the appropriate predeveloped condition runoff time series to determine target release
rates for the proposed facility. Note: If the target release rates are zero, an infiltration facility will
be required.
b) Compute the post-development runoff time series for the proposed facility.
c) Use the post-development runoff time series and an iterative process to size the facility to meet
the required level of performance set forth in Core Requirement #3. See the approved model
user’s documentation for procedures in sizing flow control facilities using continuous flow time
series.
11. Design required onsite conveyance systems using the appropriate runoff computation method (either
the Rational method or the Runoff Files method with 15-minute time steps) as specified in
Section 3.2.
3.3.2 FLOW CONTROL DESIGN USING THE RUNOFF FILES METHOD
Flow control facility design using the approved modeling software involves four basic steps:
1. Determining the statistical characteristics (peaks or durations) of predevelopment flows (using
15-minute time steps) which set the targets for the facility release rates,
2. Developing preliminary facility volume and orifice configuration,
3. Routing post-development flow time series through the preliminary facility to check performance, and
4. Iteratively revising the facility and checking performance until the target flow conditions are achieved.
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Instead of using individual design rainfall events as in an event model, the design of the facility is based
on simulation of the facility’s performance using the full historical (over 50-years) time series record of
simulated post-development flows, and also on comparison of the outflow record to characteristics of the
predevelopment flow record. Final design is achieved when the outflow time series meets the target flow
specifications.
Detention facility design with a continuous model is based on aggregate flow statistics, not upon
individual storms. When designing detention facilities with a continuous model, the return period of the
peak flow leaving the facility for a particular event may not have the same return period as the peak flow
entering the facility during the same event. Unlike event models, continuous models have natural
variability in the ratio of storm peak and volume. This lack of correspondence in the return periods of peak
inflows and outflows in continuous models means that facility design using a continuous runoff model is
more complicated than with an event method and in general has to be done on an iterative trial-and-error
basis to obtain an optimal (i.e., least volume) design.
The effect of detention facilities in controlling peak flows is dependent on both the volume and peak of the
inflowing hydrograph. Generally, it is high volume storms rather than high intensity storms that cause
detention facilities to fill and overtop. The hydrographs produced by a continuous runoff model show
considerable variability in the relationships between peak flows and storm volumes. For example, one
event produced by high rainfall intensities in a relatively short duration storm may produce high peak
flows with a relatively small hydrograph volume. By contrast, a second rainfall event may have relatively
low intensities but long duration, producing a runoff hydrograph with large volumes and relatively small
peak. Due to this natural variability, the peak annual outflows from a detention facility may not correspond
in time to the annual peaks of the inflow record.
Similarly, the predevelopment peak annual flows may not occur during the same storm as the peak annual
flows for the post-development flow series. This is because the types of storms that produce high flows
from undeveloped land covers are different from those that produce high flows from impervious surfaces.
Forests generate high streamflows in response to long-duration, high-volume rainfall events that soak the
soil profile, whereas impervious surfaces produce the highest flow rates in response to high precipitation
intensity. This is another reason why detention facility design with a continuous runoff model is based on
aggregate flow statistics, not upon individual storm hydrographs.
The following is a typical procedure for hydrologic design of detention/infiltration facilities using a
continuous runoff model. Specific guidance for conducting hydrologic analysis and design with the
approved model is provided in the approved model user’s documentation.
1. Create time series of flows from the predevelopment area using graphic elements that detail the
predevelopment land cover, the post-development area tributary to the facility, any onsite post-
development bypass area, and any offsite flow-through areas.
2. Add any offsite flow-through time series to the predevelopment flow time series using similar
graphic elements to produce a time series of total predevelopment outflows from the project site.
Similarly, add the same offsite flow-through time series to the time series of post-development flows
tributary to the facility to produce a time series of total post-development inflows to the facility.
3. Generate peak annual flow estimates, flow duration curves and flow frequency curves for pre- and
post-development time series.
4. Enter the Facility element for the scenario and specify initial facility specifications for the type of
facility proposed. Use of two orifices is usually sufficient for most designs. If designing an infiltration
facility, the bottom orifice may be elevated or zero orifices may be specified.
5. Route the complete facility inflow time series through the facility. The outflow time series is
automatically saved. Use the analysis tools to evaluate facility performance. When sizing the facility
to account for credits from on-site BMPs per Core Requirement #9 and Appendix C, note that it is
necessary to turn infiltration off for on-line on-site BMPs draining to the facility, to avoid
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counting the flow reduction effect twice. For facilities designed using this manual, explicit modeling
of infiltrative BMPs for downstream flow control facility sizing is not allowed.
6. Adjust orifice configuration and facility size, iterate until desired performance is achieved. Use
of the automatic facility sizing routine in the approved model is helpful.
7. Verify the facility performance by routing the complete time series of inflows and checking the
post-development peak flows and/or durations at the project site boundary against the target flows
and/or durations (see the criteria for “Evaluating Flow Control Performance” provided below). When
explicitly modeling BMPs for compliance with the LID Performance standard, two separate routings
are necessary to evaluate the flow control credit based facility performance and the explicitly
modeled BMPs for the LID Performance standard.
Evaluating Flow Control Performance
Evaluating the performance of facility designs intended to provide flow frequency control is
comparatively straightforward: the post-development facility annual peak flows should be strictly less than
or equal to predevelopment annual peak flows at each of the specified return periods.
Note: Peak flow matching is required per Core Requirement #3. The automatic sizing routines in the
approved continuous runoff models are based on duration matching and do not evaluate for peak flow
compliance. The user must complete this evaluation as an additional step to verify compliance.
Evaluating the design performance of detention facilities providing flow duration control, however,
generally requires several iterations. In fact, considerable time could be spent attempting to match
predevelopment and post-development duration curves. Some flexibility in assessing the adequacy of fit is
clearly needed to expedite both design and review. Therefore, flow duration designs will be accepted as
meeting performance standards when the following conditions are met:
1. The post-development flow duration curve lies strictly on or below the predevelopment curve at the
lower limit of the range of flow control (between 50% of the 2-year and the 2-year).9
2. At any flow value within the upper range of flow control (from the 2-year to the 50-year), the post-
development duration of the flow is no more than 1.1 times the predevelopment flow duration.
3. The target duration curve may not be exceeded along more than 50% of the range of control.
4. Where a facility or BMP is used to meet the LID Performance Standard, the post-development flow
duration curve lies strictly on or below the predevelopment curve for the range of pre-developed
discharge rates for the LID Performance standard (from 8% of the 2-year peak flow to 50% of the
2-year peak flow).10
9 For small projects, the lower limit of the range of control for flow control duration standard matching existing site conditions
is considered met with a minimum diameter (0.25 inches) lower orifice in a low head facility (maximum effective storage depth
of 3 feet) where full duration control cannot be achieved at the lower limit. Predeveloped flow durations, within allowed
tolerances, must be met for all flows above the best achievable lower limit. The LID Performance standard must also be met;
performance results could be influenced by the minimum diameter.
10 See Core Requirement #9 and Appendix C for application of pre-sized on-site BMPs for mitigating the LID Performance
standard in lieu of explicit modeling
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3.3.3 CONVEYANCE SYSTEM DESIGN WITH THE RUNOFF FILES
METHOD
This section provides guidance for use of the Runoff Files method in determining peak flows for the
design and analysis of conveyance elements, overflow structures, and other peak flow sensitive drainage
features.
Rainfall events that create the highest rates of runoff from developed areas are typically shorter in duration
and are characterized by brief periods of high intensity rainfall. To simulate the runoff from higher
intensity, shorter duration rainfall events, a 15-minute time series is used.
The following is the typical procedure for hydrologic design and analysis of conveyance facilities using
the Runoff Files method:
1. Select and delineate appropriate subbasins.
a) Select separate subbasins for major drainage features and important conveyance points.
b) Identify existing land covers offsite and post-development land covers onsite.
c) Identify soil types by using the SCS soil survey or by directly evaluating the site.
d) Convert SCS soil types to the approved model soil classifications.
2. Determine hydrologic parameters for each subbasin.
a) Within the approved model, locate the project to determine appropriate rainfall region and/or
regional scale factor.
b) Categorize soil types and land cover per Table 3.2.2.A and Table 3.2.2.B.
c) Determine total impervious areas and effective impervious areas within each subbasin.
d) Determine areas for each soil/cover type in each subbasin.
3. Determine peak flows for the conveyance element being analyzed.
a) Following the approved model guidance, assemble the post-development scenario including an
element for each subbasin and using 15-minute time steps.
b) Set the point of compliance at the confluence of the post-developed subbasins being routed to the
conveyance element. Run the scenario for the developed subbasins and conduct a flow frequency
analysis on the results of the scenario run. From this analysis the 10-year, 25-year, and 100-year
peak flows can be determined. These design flows can then be used to size or assess the capacity
of pipe systems, culverts, channels, spillways, and overflow structures.
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3.3.4 SAFETY FACTORS IN HYDROLOGIC DESIGN
It is often appropriate to apply safety factors to detention volumes or conveyance design flows. This
manual does not require safety factors for detention or conveyance design, but it does recommend the use
of safety factors when the designer believes the results of the approved model are not sufficiently
conservative given local conditions. The approved model methodology does not include inherent safety
factors as it is meant to account for “average” conditions. On a particular site, the approved model may
overestimate or underestimate flow rates and detention volumes.
Within any soil/cover group, there is a range of hydrologic response dependent on local soil and geologic
conditions for which the approved model methodology does not account. The USGS regional parameters
for HSPF that were used to create the runoff files produce “average” runoff time series that overestimate
peak flows in some basins and underestimate them in others. Similarly, the detention volumes designed
with the approved model for a given conversion type are in the middle of the range of volumes that would
be created if exact local hydrologic conditions were known for every project of that type. Therefore, some
of the detention facilities designed with the approved model are oversized and some are undersized,
depending on variable site conditions.
Because of the uncertainty in local hydrologic response, the City recommends, but does not require, that a
volume safety factor of 10% be applied to all detention facilities. If downstream resources are especially
sensitive, or if the designer believes that the approved model significantly overestimates predevelopment
flows or underestimates post-development flows, a volume safety factor of up to 20% may be appropriate.
If a volume safety factor is applied to a detention facility, the volume should be increased by the given
percentage at each one-foot stage increment. Safety factors for conveyance systems should be evaluated
with respect to the potential damages and costs of failures due to backwatering, overtopping, etc.
Applications of safety factors fall strictly within a professional engineer’s judgment and accountability for
design. Section 4 of the Technical Information Report should state what safety factor was applied to the
design of the flow control facility.
3.3.5 DESIGN OPTIONS FOR ADDRESSING DOWNSTREAM
DRAINAGE PROBLEMS
See Chapter 1, Table 1.2.3.A for options for addressing downstream drainage problems.
3.3.6 POINT OF COMPLIANCE ANALYSIS
The point of compliance is the location where flow control performance standards are evaluated. In most
cases, the point of compliance is the outlet of a proposed detention facility where, for example, 2- and
10-year discharges must match predevelopment 2- and 10-year peak flow rates.
The point of compliance for hydrologic control moves downstream of the detention facility outlet or the
property boundary under the following circumstances:
1. The proposed project discharges to an offsite closed depression with a severe flooding problem per
Section 1.2.2, and the project adds impervious surface greater than or equal to 10% of the 100-year
water surface area of the closed depression (see Table 1.2.3.A). In these cases, the closed depression
becomes the point of compliance, and the engineer must ensure that project site runoff does not
aggravate the flooding problem (or create a new flooding problem).
2. The proposed project includes an onsite runoff bypass, a small developed area that bypasses the flow
control facility (see Section 1.2.3.2). In such cases, runoff from the remainder of the project site is
overdetained so that the sum of the detained and undetained flows meets the required flow control
performance standard. The point of compliance for such projects is where the onsite bypass flows join
the detained flows.
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3. The proposed project bypasses offsite flows around an onsite closed depression, ponding area, or
wetland (see Section 3.3.7). As with onsite bypasses, the point of compliance in this case is where
detained flows converge with the bypassed flows.
The approved model allows multiple points of compliance for evaluating runoff performance within a
scenario. The automatic facility sizing routine in the approved model requires a point of compliance to size
an individual facility; a separate point of compliance is required for downstream evaluation. See the
approved model user’s documentation for modeling application of points of compliance to meet the
requirements of this manual.
Note: When controlling flow durations at a downstream point of compliance to demonstrate no adverse
impact, the 10% tolerance specified for the Flow Control Duration Standard may not be used.
Predevelopment condition flow durations should be matched to the extent feasible for all flows above the
level of concern. The resultant facility should also be checked to verify that the minimum onsite
performance standard has also been met.
OFFSITE CLOSED DEPRESSIONS
If a project drains to an offsite closed depression with existing or potential flooding problems, then the
water surface levels of the closed depression must not be allowed to increase for return frequencies at
which flooding occurs, up to and including the 100-year frequency. This section describes the point of
compliance analysis necessary to size detention facilities discharging to such a closed depression. If the
closed depression is classified as a wetland, other requirements apply per Section 1.2.2, Core
Requirement #2.
The closed depression is first modeled (using the site’s predevelopment condition) to determine the return
frequency at which flooding currently occurs and the water levels associated with return frequencies in
excess of this frequency. These flooding levels and their probabilities dictate the detention performance for
the proposed development. The proposed detention facility is then iteratively sized such that discharge
from the site’s post-development condition does not increase water surface levels for the frequencies at
which flooding occurs—that is, after development, water level frequency curves must match for all
frequencies equal to or greater than the frequency at which flooding occurs (up to the 100-year water
level).
The infiltration rate must be determined in order to accurately model the closed depression. In the case of
a closed depression with an existing flooding problem, the infiltration rate is most realistically depicted by
calibrating the model to known flooding events. This should be done using the full historical runoff files
and setting the closed depression outflow (infiltration) such that recorded or anecdotal levels of flooding
occur during the same storm events in the historical record.
Where a flooding problem might be created by discharge of post-development flows to a closed
depression, and in the absence of information on dates and water surface levels in the closed depression
during past runoff events, infiltration rates must be determined through testing as follows:
For a closed depression without standing water, two or more test pits should be dug in the bottom of
the closed depression to a depth of 10 feet or to the water table, whichever is reached first. The test
pits shall be dug under the supervision of a geotechnical engineer, and a test pit log shall be kept.
Evidence of high water table shall be noted.
If the test pit reveals deep homogeneous permeable material with no evidence of a high water table,
then infiltration tests shall be performed in the bottom of the closed depression at locations of similar
elevation and on opposite sides of the bottom area (as feasible). Surface infiltration rates shall be
determined using the methods for assessing measured infiltration rates included in Section 5.2. The
measured rates should be used directly, without applying correction factors.
If the closed depression has standing water or is a defined as a wetland according to RMC 4-3-050, or
if test pits show evidence of a high water table or underlying impermeable material, then procedures
for determining infiltration rates will be established on a case-by-case basis in coordination with CED.
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In the event that a closed depression with a documented severe flooding problem is located on private
property and all reasonable attempts to gain access to the closed depression have been denied, the
Flood Problem Flow Control Standard shall be applied with a 20% factor of safety on the storage
volume.
ONSITE RUNOFF BYPASS
It is sometimes impractical to collect and detain runoff from an entire project area, so provisions are made
to allow undetained discharge from onsite bypass areas (see Section 1.2.3.2) while overdetaining the
remainder of the runoff to compensate for unmitigated flows. A schematic of an onsite runoff bypass is
shown in Figure 3.3.6.A.
For projects employing onsite runoff bypass, flow control performance standards are evaluated at the point
of compliance, the point where detained and undetained flows from the project site are combined.
Point of Compliance Analysis for Onsite Bypass Areas
1. In the approved model, create a predeveloped condition element for the entire project area including
the predevelopment detained area and the predevelopment bypass area. Route the scenario and apply
the analysis tools to determine flow targets (either flow frequencies or durations, depending on the
applicable design standard) from the predeveloped condition runoff time series.
2. Create and route separate developed condition elements for the detained area and the bypass area,
producing a separate time series for each area.
3. Ensure that the flow characteristics of the developed runoff element for the bypass area do not exceed
the targets determined in Step 1 or the 0.4 cfs threshold in Core Requirement #3. If the bypass area
flows exceed the targets or threshold, then the bypass is not feasible.
4. Estimate allowable release rates from the detention facility for each return period of interest with the
following equation:
Allowable release = (Total Project Area Flow)predeveloped cond. – (Bypass Area Flow)developed cond.
Note: WWHM 2012 and later supports the direct sizing of onsite detention facilities based on the results at
a downstream point-of-compliance. See the WWHM user’s documentation for further details.
1. Develop a preliminary design of the flow control facility based on the estimated release rate(s).
2. Route post-development flows from the detained area through the detention facility to create a
detention facility outflow time series. Provide a downstream point of compliance and route the bypass
area and the facility outflow to the downstream POC.
3. The approved model determines the total project post-development outflow by adding the detention
facility outflow runoff time series to the post-development runoff time series from the bypass area at
the downstream point of compliance. Check characteristics of the total project post-development
outflow against the targets determined in Step 1.
4. If compliance is not achieved (e.g., 2- and 10-year post-development flows exceed 2- and 10-year
predevelopment flows), revise the facility design (or revise the project design to reduce the bypass
area) and repeat Steps 6 through 8.
For WWHM 2012 and later, Steps 6 through 8 have been automated for facility sizing by using the point of
compliance option in the facility element of the model. See the WWHM user’s documentation for guidance.
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3.3.7 ONSITE CLOSED DEPRESSIONS AND PONDING AREAS
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FIGURE 3.3.6.A SCHEMATIC OF AN ONSITE RUNOFF BYPASS
3.3.7 ONSITE CLOSED DEPRESSIONS AND PONDING AREAS
Onsite closed depressions, ponding areas, and wetlands require special consideration when determining
detention performance targets; if altered, they can shift the point of compliance downstream. However, the
critical areas code (RMC 4-3-050) regulates wetlands (note that most closed depressions and ponding
areas are wetlands by definition) and generally does not permit alteration through either filling or gross
hydrologic changes such as bypassing offsite flows. Note: Post-development discharges to offsite closed
depressions, ponding areas, or wetlands (with the exception of those in Flood Problem Flow Control
Areas per the Flow Control Applications Map or those discussed in Section 3.3.6) are normally not
required to meet special performance standards unless there is a severe flooding problem as defined in
Section 1.2.2.
GENERAL REQUIREMENTS
The following general requirements apply to onsite closed depressions, ponding areas, and wetlands
(referred to below as “features”):
1. Flow attenuation provided by onsite wetlands and ponding areas, and storage provided by onsite
closed depressions must be accounted for when computing both existing onsite and offsite flows.
Existing onsite flows must be routed through onsite wetlands and ponding areas to provide
accurate target release rates for the developed site. Note: Closed depressions will have no outflow
for some portions of the site for some events, although overflow may occur during extreme events.
Existing offsite flows will increase at the project boundary if the feature is filled or if the offsite
flows are bypassed around the feature. To compensate, post-development onsite flows must be
stream
NG PE bufferdetention
tract
open
space
BYPASS
AREA
DETAINED
AREA
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overdetained, and the point of compliance will shift downstream to where the detained flows
converge with the bypassed offsite flows.
2. If the onsite feature is used for detention, the 100-year floodplain must be delineated considering
developed onsite and existing offsite flows to the feature. Note: Additional storage volume may be
necessary within the feature, and the point of compliance is the discharge point from the feature.
3. If the detention facility for the proposed project discharges to an onsite wetland, ponding area,
or closed depression that is not altered11 by the proposed project, AND Flow Control Duration
or Flood Problem Flow Control is provided, the point of compliance is the discharge point of the
detention facility, not the outlet of the onsite feature. If Peak Rate Flow Control is being provided,
the point of compliance is the outlet of the onsite feature.
FLOODPLAIN DELINEATION FOR LAKES, WETLANDS, CLOSED DEPRESSIONS, AND
PONDING AREAS
A minor floodplain analysis is required for onsite or adjacent lakes, wetlands, and closed depressions that
do not have an approved floodplain or flood hazard study (see Section 4.4.2; note the exceptions). Minor
floodplain studies establish an assumed base flood elevation below which development is not allowed.
The following are guidelines for minor floodplain analysis of volume sensitive water bodies:
1. Create time series representing tributary flows to the feature from the entire tributary area. Where the
feature is contained entirely onsite and where no offsite flows exist, use the tributary area for the
proposed developed condition.
2. Where the feature is only partially onsite, or where there are offsite flows to the feature, assume the
entire tributary area is fully built out under current zoning , accounting for required open space
and protected critical areas in the basin as well as impervious surfaces and grass.
3. For potential future development, assume detention standards per Section 1.2.3.1. For simplicity
the proposed detention may be simulated with a single assumed detention pond just upstream of
the feature. This pond should be sized to the appropriate detention standard and predevelopment
condition assumption as noted in Section 1.2.3.1 and will require generating a predevelopment time
series for the basin. Large water bodies may provide significant floodwater storage and may also be
included in the analysis. Most existing detention in the basin, with exception of that providing
duration control, will have little effect on the analysis and should be discounted.
4. Sum all subbasin time series to create a single composite time series for the drainage feature.
5. Develop routing curves for the feature. As appropriate, consider infiltration as an outflow for closed
depressions.
6. Route the time series through the storage feature, generate water surface frequency curves, and note
the 100-year water surface elevation.
11 Not altered means existing on- and offsite flows to the feature will remain unchanged and the feature will not be excavated or
filled.
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CHAPTER 4
CONVEYANCE SYSTEM
ANALYSIS & DESIGN
CITY OF RENTON
SURFACE WATER
DESIGN MANUAL
Section Page
4.1 Route Design and Easement
Requirements 4-3
4.1.1 Route Design 4-3
4.1.2 Easement and Setback Requirements 4-3
4.2 Pipes, Outfalls, and Pumps 4-7
4.2.1 Pipe Systems 4-7
4.2.2 Outfall Systems 4-30
4.2.3 Pump Systems 4-36
4.3 Culverts and Bridges 4-37
4.3.1 Culverts 4-37
4.3.2 Culverts Providing for Fish
Passage/Migration 4-50
4.3.3 Bridges 4-52
4.4 Open Channels, Floodplains, and
Floodways 4-55
4.4.1 Open Channels 4-55
4.4.2 Floodplain/Floodway Analysis 4-71
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CHAPTER 4
CONVEYANCE SYSTEM ANALYSIS & DESIGN
This chapter presents the City of Renton’s approved methods for the hydraulic analysis and design of
conveyance systems. A conveyance system includes all portions of the surface water system, either natural
or man-made, that transports surface and storm water runoff.
This chapter contains the detailed design criteria, methods of analysis, and schematic representations for
all components of the conveyance system. In some cases, reference is made to other adopted or accepted
design standards and criteria such as the City of Renton Standard Details and the requirements of the City
of Renton Transportation department and Surface Water Utility as applicable.
The figures included in this chapter are provided as schematic representations and should not be used for
design. Refer to the City of Renton Standard Details for specific design information. The figures provided
in this chapter illustrate one example of how the conveyance system design criteria may be applied.
Although the figures are meant to illustrate many of the most important design criteria, they may not show
all criteria that apply. In general, the figures are not used to specify requirements unless they are indicated
elsewhere in this manual. If this manual refers to a standard detail not included in the City of Renton
Standard Details, the applicant shall use the figure provided in this manual.
Chapter Organization
The information presented in this chapter is organized into four main sections:
Section 4.1, “Route Design and Easement Requirements“
Section 4.2, “Pipes, Outfalls, and Pumps”
Section 4.3, “Culverts and Bridges”
Section 4.4, “Open Channels, Floodplains, and Floodways”
These sections begin on odd pages so the user can insert tabs if desired for quicker reference.
Required vs. Recommended Design Criteria
Both required and recommended design criteria are presented in this chapter. Criteria stated using “shall”
or “must” are mandatory, to be followed unless there is a good reason to deviate as allowed by the
adjustment process (see Section 1.4). These criteria are required design criteria and generally affect
facility performance or critical maintenance factors.
Sometimes options are stated as part of the required design criteria using the language “should” or “may.”
These criteria are recommended design criteria, but are closely related to the required criteria, so they
are placed in the same section.
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SECTION 4.1 ROUTE DESIGN AND EASEMENT REQUIREMENTS
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AGENDA ITEM # 8. a)
CITY OF RENTON SURFACE WATER DESIGN MANUAL
2022 City of Renton Surface Water Design Manual 6/22/2022
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4.1 ROUTE DESIGN AND EASEMENT REQUIREMENTS
This section presents the general requirements for aligning conveyance systems and providing easements
and setbacks to allow for proper maintenance and inspection of all conveyance system elements.
4.1.1 ROUTE DESIGN
The most efficient route selected for new conveyance systems will result from careful consideration of the
topography of the area to be traversed, the legal property boundaries, and access for inspection and
maintenance. Additionally, topography and native soil characteristics beneficial to Low Impact
Development (LID) applications may influence the route. The general requirements for route design are as
follows:
1. Proposed new conveyance systems should be aligned to emulate the natural conveyance system to
the extent feasible. Inflow to the system and discharge from the system should occur at the natural
drainage points as determined by topography and existing drainage patterns.
2. New conveyance system alignments in residential subdivisions should be located adjacent and
parallel to property lines so that required drainage easements can be situated along property lines.
Drainage easements should be located entirely on one property and not split between adjacent
properties.
Exception: Streams and natural drainage channels shall not be relocated to meet this requirement.
3. Aesthetic considerations, traffic routes and on-site BMP strategies may dictate the placement and
alignment of open channels. Appropriate vehicular and pedestrian traffic crossings must be provided
in the design.
4. For any reach or partial reach of new conveyance (ditch, channel or closed pipe system) proposed by a
project, a geotechnical analysis and report is required if the conveyance is located within 200 feet of a
steep slope hazard area or landslide hazard area, OR if the conveyance is located within a setback
distance from top of slope equal to the total vertical height of the slope area that is steeper than 15%.
The geotechnical analysis must consider cumulative impacts from the project and surrounding areas
under full built-out conditions. A low-permeability liner per Section 6.2.4 for the trench or channel
may be required if warranted by soil stability conditions.
4.1.2 EASEMENT AND SETBACK REQUIREMENTS
Proposed projects must comply with the following easement and setback requirements unless otherwise
approved by the City:
1. Any onsite conveyance system element (including on-site BMPs used as conveyance) constructed as
part of a subdivision project shall be located in a dedicated drainage easement, tract, or right-of-way
that preserves the system's route and conveyance capacity as follows:
Onsite conveyance systems within the right-of-way will be inspected and maintained by the City.
Onsite conveyance systems within the drainage easements or tracts will be maintained by the
property owners through the Homeowners Association created by the subdivision (with each
property owner having equal responsibility for maintenance). These conveyance elements include
those entering and exiting the tract from/to the public right-of-way. For conveyance pipes entering
the tract from right-of-way, responsibility begins at the last structure prior to entering the tract.
For conveyance pipes exiting the tract to right-of-way, responsibility ends at the next downstream
structure. The easement shall grant the City rights for inspection.
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SECTION 4.1 ROUTE DESIGN AND EASEMENT REQUIREMENTS
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Exception: Roof downspout, minor yard, and footing drains do not require easements, tracts, or right-
of-way. If easements are provided for these minor drains (or for other utilities such as power, gas or
telephone), they need not comply with the requirements of this section.
Note: except for those facilities that have been formally accepted for maintenance by the City,
maintenance and repair of drainage facilities and BMPs on private property is the responsibility of
the property owner. Except for the inflow pipe and discharge pipe of a City-accepted flow control or
water quality facility, the City does not normally accept maintenance of conveyance systems
constructed through private property.
2. Any onsite conveyance system element (including on-site BMPs used as conveyance) constructed
under a commercial building or commercial development permit shall be covered by the drainage
facility declaration of covenant and grant of easement in Reference Section 8-J (or equivalent) that
provides the City right of access for inspection, maintenance, and repair.
Note: except for those facilities that have been formally accepted for maintenance by the City,
maintenance and repair of drainage facilities on private property is the responsibility of the property
owner.
3. Retained or replaced 12-inch or greater pipe diameter (or equivalent) conveyance system elements that
convey offsite flows on a project site on private property shall be covered by the drainage facility
declaration of covenant and grant of easement in Reference Section 8-J (or equivalent) that provides
the City right of access for inspection, maintenance, and repair. For projects with conveyance system
elements as described above that cannot meet or be relocated to meet the easement and building
setback line (BSBL) requirements in Table 4.1 due to the presence of existing structures, applicants
are required only to record a notice on title that identifies the subject conveyance elements and states
that maintenance and repair of those elements is the responsibility of the property owner. For
conveyance system elements as described above that are on a site but not within the project site,
applicants are required only to record a notice on title that identifies the subject conveyance elements
and states that maintenance and repair of those elements is the responsibility of the property owner.
Note: except for those facilities that have been formally accepted for maintenance by the City,
maintenance and repair of drainage facilities on private property is the responsibility of the property
owner.
4. Any offsite conveyance system element (including on-site BMPs used as conveyance) constructed
through private property as part of a proposed project that conveys runoff from public roads within the
project site shall be located in a drainage easement dedicated to the City. If an offsite conveyance
system through private property is proposed by a project to convey runoff diverted from the natural
discharge location, the City may require a drainage release covenant per Reference Section 8-K as a
condition of approval of the adjustment required in Section 1.2.1.
5. A river protection easement shall be required for all properties adjoining or including major rivers1
that may be dedicated to the City or County as applicable. The County shall review and approve river
protection easements dedicated to the County.
6. Table 4.1 lists the required widths and BSBLs for drainage easements. For all pipes or any channels
or constructed swales greater than 30 feet wide, facilities must be placed in the center of the easement.
For channels or constructed swales less than or equal to 30 feet wide, the easement extends to only
one side of the facility.
Note: The requirement for drainage easements with accompanying widths and BSBLs per Table 4.1
also applies to existing and replaced conveyance elements as described in #3 above.
1 Major rivers are defined in the King County Flood Hazard Management Plan.
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7. Any portion of a conveyance system drainage easement (shown in Table 4.1) shall not be located
within an adjacent property or right-of-way. Building setback lines may cross into adjacent
property.
8. The distance between the easement line and building or other structure footings shall be no less than
the BSBL distance shown in Table 4.1.
Exception: The BSBL distance indicated in Table 4.1 may be measured from the edge of a pipe in the
easement plus 2 feet if all of the following conditions are met:
a) As-builts showing the location of the pipe are submitted
b) A geotechnical/structure analysis demonstrates stability of the proposed structure
c) Access for maintenance/replacement remains unobstructed.
TABLE 4.1 EASEMENT WIDTHS AND BUILDING SETBACK LINES
For Pipes:(1)
Inside Diameter (ID) Easement Width
BSBL
(From Easement)
ID 36″ depth to invert < 8′: 10 feet(2)
depth to invert > 8′: 15 feet
5 feet
36″ < ID 60″ depth to invert < 8′: 10 feet(2)
depth to invert > 8′: 15 feet
7.5 feet
ID > 60″ ID plus 10 feet 10 feet
For Channels and Swales:
Top Width of Channel (W) Easement Width
BSBL
(From Easement)
W 10 feet W plus 10 feet on one side
W if no access required(3)
5 feet
10 feet < W 30 feet W plus 15 feet on one side 5 feet
W > 30 feet W plus 15 feet on both sides 5 feet
For Major Rivers Easement Width
BSBL
(From Easement)
See the King County Flood
Hazard Management Plan for
a list of the major rivers
Varies per site conditions
Minimum 30 feet
from stable top of bank(4)
5 feet
Notes:
(1) Pipes installed deeper than 10 feet require one of the following actions:
Increase the BSBL such that the distance from the BSBL to the centerline of the pipe is at least 1.5 times the depth to
pipe invert, or
Place a restriction on adjacent lots that the footings be placed at a specific elevation, deep enough that the closest
horizontal distance from the footing to the pipe centerline is 1.5 times the difference in elevation of the footing and
pipe invert, or
Place a restriction on adjacent lots that the footings be designed by a geotechnical engineer or licensed engineering
geologist, such that excavation of the pipe may be performed without necessitating shoring of adjacent structures.
(2) Fifteen-foot easement width is required for maintenance access to all manholes, inlets, and culverts.
(3) Access is not required for small channels if the channel gradient is greater than 5% (assumes steep channels will be
self-cleaning).
(4) Stable top of bank shall be as determined by King County.
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AGENDA ITEM # 8. a)
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4.2 PIPES, OUTFALLS, AND PUMPS
This section presents the methods, criteria, and schematic representations for analysis and design of pipe
systems, outfalls, and pump-dependent conveyance systems. The information presented is organized as
follows:
Section 4.2.1, “Pipe Systems”
“Design Criteria,” Section 4.2.1.1
“Methods of Analysis,” Section 4.2.1.2
Section 4.2.2, “Outfall Systems”
“Design Criteria,” Section 4.2.2.1
Section 4.2.3, “Pump Systems”
“Design Criteria,” Section 4.2.3.1
“Methods of Analysis,” Section 4.2.3.2.
4.2.1 PIPE SYSTEMS
Pipe systems are networks of storm drain pipes, catch basins, manholes, inlets, and outfalls designed and
constructed to convey surface water. The hydraulic analysis of flow in storm drain pipes typically is
limited to gravity flow; however, in analyzing existing systems it may be necessary to address pressurized
conditions. A properly designed pipe system will maximize hydraulic efficiency by utilizing proper
material, slope, and pipe size.
4.2.1.1 DESIGN CRITERIA
General
In addition to the design criteria described below, pipe systems shall be design to meet the hydraulic
criteria as described in Section 1.2.4.1. These criteria supersede the methodology descriptions contained in
Chapter 4.
All pipe material, joints, protective treatment, construction workmanship, and inspection requirements
shall be in accordance with the City of Renton Standard Details and the requirements of the City of Renton
Transportation Department of Surface Water Utility as applicable.
Note: The pipe materials and specifications included in this section are for conveyance systems installed
according to engineering plans required for City permits/approvals. Other pipe materials and
specifications may be used by private property owners for drainage systems they construct and maintain
when such systems are not required by or granted to City.
Acceptable Pipe Sizes and Length Between Structures
Acceptable pipe sizes and maximum lengths between structures shall be per Table 4.2.1.A1.
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TABLE 4.2.1.A1 PIPE SIZES AND LENGTHS BETWEEN STRUCTURES
Upstream Structure to
Downstream Structure
Pipe Diameter
(in) Where Allowed
Maximum
Length (ft)
Inlet to CB/MH 8 Private 40
12 Private 60
CB/MH to CB/MH 8 Private/Public(1) 100/66(1)
12 or greater Private/Public 300(2)
Notes:
(1) Minimum diameter for public pipes shall be 12 inches. However, 8-inch diameter public pipes may be
permitted on cross street laterals less than 66 feet to avoid utility conflicts or to meet shallow grade.
(2) Maximum spacing on surface drainage course between catch basins or manholes shall be 150 on
grades less than 1% and 200 feet on grades from 1% to 3%. Otherwise, maximum spacing shall be
300 feet on grades over 3%, or as required by grate flow capacities. Maximum spacing may need to
be reduced depending on street width and inlet capacity analysis in Section 4.2.1.2.
Allowable Pipe Materials and Minimum Cover Requirements
1. The designer shall have the option of constructing storm sewers, drains and culverts of the pipe types
listed below within the cover limits specified. In addition, concrete pipe shall be rubber gasketed and
metal pipe shall be gasketed and securely banded. Leak testing shall be conducted if required by the
City Engineer.
2. The pipe materials included in Table 4.2.1.A2 are allowed for use in meeting the requirements of this
manual. Refer to the current edition of WSDOT/APWA Standard Specifications 7-02, 7-03 and 7-04
for detailed specifications for acceptable pipe materials.
TABLE 4.2.1.A2 ALLOWABLE PIPE MATERIALS AND MINIMUM COVER
Pipe Type
Minimum
Cover (ft) Public Private
Allowed in Zone 1
of the APA
Corrugated Steel Pipe 2.0 Yes Yes Yes
Spiral Rib Steel Pipe 2.0 Yes Yes Yes
Plain Concrete Pipe (PCP) 2.0 No Yes No
Reinforced Concrete Pipe (RCP) 1.0 Yes Yes No
Ductile Iron 1.0 Yes Yes Yes
Line Corrugated Polyethylene Pipe (LCPE) 2.0 Yes Yes Yes
Corrugated Polyethylene Pipe (CPE) – Triple Wall 2.0 Yes Yes Yes
Polypropylene Pipe (PP) – Dual Wall 2.0 Yes Yes Yes
Polyvinyl Chloride Pipe (PVC) 3.0 Yes Yes Yes
Solid Wall High Density Polyethylene Pipe (HDPE) 2.0 Yes Yes Yes
Allowable Pipe Joints
1. Concrete pipe shall be rubber gasketed.
2. Corrugated steel pipe shall be rubber gasketed and securely banded.
3. Spiral rib steel pipe shall be “hat-banded” with neoprene gaskets.
4. Ductile iron pipe joints shall be flanged, bell and spigot, or restrained mechanical joints.
5. PP and CPE pipe joints (lined and single wall, fully corrugated) shall conform to the current
WSDOT/APWA Standard Specifications.
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6. PVC pipe, CPE pipe and PP pipe shall be installed following procedures outlined in ASTM
D2321.Solid wall HDPE pipe shall be jointed by butt fusion methods or flanged according to the City
of Renton Standard Details.
Pipe Alignment
1. Pipes must be laid true to line and grade with no curves, bends, or deflections in any direction.
2. Exception: Vertical deflections in solid wall HDPE and ductile iron pipe with flanged restrained
mechanical joint bends (not greater than 30) on steep slopes, provided the pipe drains.
3. A break in grade or alignment, or changes in pipe material shall occur only at catch basins or
manholes.
Allowable Pipe Slopes and Velocities
Table 4.2.1.A3 presents allowable pipe slopes and velocities by pipe material.
TABLE 4.2.1.A3 ALLOWABLE PIPE SLOPES AND VELOCITIES
Pipe
Material
Minimum
Slope
Allowed(3)
Pipe Slope above which
Pipe Anchors Required and
Minimum Anchor Spacing
Maximum
Slope
Allowed
Minimum
Velocity at
Full Flow(3)
Maximum
Velocity at
Full Flow
Steel or
PVC(1)
0.5% 20%
(1 anchor per 100 LF of pipe)
30%(4) 3 fps 30 fps
Concrete,
CPE, or
PP(1)
0.5% 10%
(1 anchor per 50 LF of pipe)
20%(4) 3 fps 30 fps
Ductile
Iron(2)
0.5% 20%
(1 anchor per pipe section)
None 3 fps None
Solid wall
HDPE(2)
0.5% 20%
(1 anchor per 100 LF of pipe,
cross-slope installations only)
None 3 fps None
Notes:
(1) These materials are not allowed in landslide hazard areas.
(2) Butt-fused or flanged pipe joints are required; above ground installation is recommended on slopes greater than 40%.
(3) Minimum slope and full flow velocity is required unless it cannot be achieved due to outlet control, site topography, burial
depth or other situations or conditions.
(4) A maximum slope of 200% is allowed for these pipe materials with no joints (one section), with structures at each end,
and with proper grouting.
Changes in Pipe Size
1. Increase or decreases in pipe size are allowed only at structures. Exceptions may be allowed as
follows:
Connections to pipe systems may be made without placing a catch basin or manhole on the
mainline by meeting all of the following conditions:
a) The mainline pipe is 48 inches or greater and at least two times the size of the connecting pipe.
b) Make connections in accordance with the manufacture’s recommendations. Standard shop
fabricated tees, wyes and saddles shall be used, except for concrete connections constructed in
accordance with the City of Renton Standard Details.
c) There shall be a catch basin or manhole on the connecting pipe within 2 to 10 feet of the external
wall of the main line.
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d) Offset angle of connecting pipe to mainline, horizontally and vertically shall be less than
45 degrees.
e) Two-point survey control shall be used to set catch basin locations.
2. When connecting pipes at structures, match any of the following (in descending order of
preference): crowns, 80% diameters,2 or inverts of pipes. Side lateral connections3, 12 inches and
smaller, are exempt from this requirement.
3. Drop manholes may be used for energy dissipation when pipe velocities exceed 10 feet per second.
External drop manholes are preferred where maintenance access to the upstream pipe is preserved by
use of a tee section. Internal drop structures may be approved only if adequate scour protection is
provided for the manhole walls. Drop structures must be individually engineered to account for design
variations, such as flow rates, velocities, scour potential and tipping forces.
4. Downsizing pipes larger than 12 inches may be allowed provided pipe capacity is adequate for design
flows.
Note: The above criteria do not apply to detention tanks.
Structures
Table 4.2.1.B lists typical drainage structures with corresponding maximum allowable pipe sizes.
1. Catch basin (or manhole) diameter shall be determined by pipe orientation at the junction structure. A
plan view of the junction structure, drawn to scale, will be required when more than four pipes enter
the structure on the same plane, or if angles of approach and clearance between pipes is of concern.
The plan view (and sections if necessary) must ensure a minimum distance (of solid concrete wall)
between pipe openings of 8 inches for 48-inch and 54-inch catch basins, and 12 inches for 72-inch and
96-inch catch basins.
2. Evaluation of the structural integrity for H-20 loading, or as required by the City of Renton Standard
Details, may be required for multiple junction catch basins and other structures.
3. All solid wall HDPE pipe systems (including buried solid wall HDPE pipe) must be secured at the
upstream end. The downstream end shall be placed in a 4-foot section of the next larger pipe size. This
sliding sleeve connection allows for the high thermal expansion/contraction coefficient of this pipe
material.
4. The maximum slope of the ground surface for a radius of 5 feet around a catch basin grate or solid
lid should be 5:1 (H:V) to facilitate maintenance access. Where not physically feasible, a maximum
slope of 3:1 (H:V) shall be provided around at least 50% of the catch basin circumference.
5. Catch basins (see City of Renton Standard Details) rather than inlets shall be used to collect storm
water from road surfaces, unless approved by the City Engineer.
6. Type 2 (see City of Renton Standard Details) catch basins shall be used where the depth to the invert
of the pipe exceeds 5 feet.
7. Manholes (see City of Renton Standard Details) may be used in lieu of catch basins if they do not
collect surface water. Manholes must be used if inverts are greater than 18 feet.
8. Roof and yard drains, or other concentrated flow from adjacent property shall not discharge over the
surface of roadways, sidewalks, walkways, or shoulders.
2 Match point is at 80% of the pipe diameter, measured from the invert of the respective pipes.
3 Side laterals include any 8-inch or smaller pipe connected to the main conveyance system at a catch basin, or manhole, as
allowed under this manual and/or the City of Renton Standard Details. In addition, 12-inch and smaller pipes that serve a single
inlet point (e.g., roadway simple inlets, footing drains, and lot stubouts including manifold systems serving multiple residential
lots) are also included. Excluded from this definition are inlet pipes that contribute 30% or more of the total flow into a catch
basin, or that collect or convey flows from a continuous source.
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9. Catch basins or manholes are required when joining differing types of pipes.
10. The location of at least two points of all catch basins shall be surveyed to ensure that the catch basin,
frame and grate will properly align with finished curb, horizontally and vertically.
11. Metal castings for drainage structures shall not be dipped, painted, welded, plugged or repaired.
12. Porosity in metal castings for drainage structures shall be considered a workmanship defect subject to
rejection by the inspector.
13. Grates and covers shall be seated properly to prevent rocking, including the replacement of existing
covers with solid metal covers.
14. Unless otherwise specified, vaned grates (see City of Renton Standard Details), shall be used with
standard frame in the traveled way, gutter, or shoulder. Vaned grates shall not be located within
crosswalks.
15. At sag vertical curves, on the end of downgrade cul-de-sacs, or before intersections with a grade four
percent or greater, an analysis shall be done to assure that typical catch basin grates will collect the
surface runoff. To collect excessive volumes of runoff or protect against plugged grates and overflow
situations, the City Road Engineer will require the use of through inlet frames on vertical curbs, (see
City of Renton Standard Details). Where the through-curb inlets cannot be used, place a catch basin at
the low point and two extra inlets located not greater than 0.1 foot above the low point grate within a
spacing of 25 feet.
16. New catch basins and manholes that do not collect runoff shall use solid locking covers (per City of
Renton Standard Details). Existing catch basins, which no longer collect runoff, shall have their frame
and grates replaced with solid covers.
17. All storm drain covers and grates need to be locking regardless of their location.
18. Slot drains may be used when approved by the City Engineer. At a minimum, slot drains shall have
catch basins at either end unless used as a driveway culvert. The maximum distance between catch
basins along a slot drain shall be 50 feet.
TABLE 4.2.1.B ALLOWABLE STRUCTURES AND PIPE SIZES
Catch Basin Type(1)
Maximum Pipe Diameter
Steel, Solid Wall HDPE, PVC, and
Ductile Iron(2)
Concrete,
CPE, PP
Inlet(4) 12″ 12″
Type 1(3) 18″ 12″
Type 1L(3) 24″ 18″
Type 2 – 48-inch dia. 30″ 24″
Type 2 – 54-inch dia. 36″ 30″
Type 2 – 72-inch dia. 54″ 42″
Type 2 – 96-inch dia. 72″ 60″
Notes:
(1) Catch basins (including manhole steps, ladder, and handholds) shall conform to the City of Renton Standard Details.
(2) Generally these pipe materials will be one size larger than concrete, CPE or PP due to smaller wall thickness. However,
for angled connections or those with several pipes on the same plane, this will not apply.
(3) A maximum of 5 vertical feet is allowed between finished grade and invert elevation.
(4) Inlets are normally allowed only for use in privately maintained drainage systems and must discharge to a catch basin
immediately downstream.
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Pipe Design between Structures
1. Minimum velocity at full flow should be 3.0 feet per second (fps). If site constraints result in
velocities less than 3 fps at full flow, impacts from sedimentation in the pipe system shall be
addressed with larger pipes, closer spacing of structures, sediment basins, or other similar measures.
2. Minimum slope for pipes shall be 0.5%, unless otherwise approved by CED in locations with site
constraints provided that the minimum velocity requirement of 3 fps is met.
3. Maximum lengths between structures shall meet the requirements in Table 4.2.1.A1 above. Solid
wall HDPE tightlines down steep slopes are self-cleaning and do not require structures for
maintenance.
Pipe Cover
1. Pipe cover, measured from the finished grade elevation to the top of the outside surface of the pipe,
shall be 2 feet minimum unless otherwise specified or allowed below or as allowed above in Table
4.2.1.A2. Under drainage easements, driveways, parking stalls, or other areas subject to light vehicular
loading, pipe cover may be reduced to 1 foot minimum if the design considers expected vehicular
loading and the cover is consistent with pipe manufacturer’s recommendations. Pipe cover in areas not
subject to vehicular loads, such as landscape planters and yards, may be reduced to 1 foot minimum.
2. All flexible storm sewer pipe and culvert material shall be covered by a minimum of 2 feet of cover
unless the applicant submits detailed plans accompanied by manufacturer’s recommendations
specifying allowable cover less than 2 feet in depth. All non-flexible storm sewer pipe and culvert
material shall be covered by a minimum of 1 foot of cover. Pipe cover over concrete pipe shall
comply with Table 4.2.1.C. For other pipe types, the manufacturer’s specifications or other
documentation shall be provided for proposed cover in excess of 30 feet. Caution: Additional
precautions to protect against crushing during construction may be needed under roadways if the
road bed is included to meet minimum cover requirements. Damaged pipe shall be replaced.
3. For proposed pipe arches, the manufacturer’s specifications or other documentation shall be provided
for proposed cover in excess of 8 feet.
4. Pipe cover over PVC SDR 35 shall be 3 feet minimum and 30 feet maximum.
TABLE 4.2.1.C MAXIMUM COVER (FEET) FOR CONCRETE PIPE
Pipe Diameter
(inches) Plain Class II Class III Class IV Class V
12 18 10 14 21 26
18 18 11 14 22 28
24 16 11 15 22 28
30 11 15 23 29
36 11 15 23 29
48 12 15 23 29
60 12 16 24 30
72 12 16 24 30
84 12 16 24 30
96 12 16 24 30
108 12 16 24 30
Note: See Figure 4.2.1.A for a schematic representation. Only Class IV and V are allowed in public right-of-way.
Pipe Clearances
A minimum 7-foot horizontal separation and 1-foot vertical separation (measured wall to wall) is required
between storm pipe and other utilities with the exception of water lines where a minimum 10-foot
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horizontal separation and 1.5-foot vertical separation (measured wall to wall) is required, unless otherwise
approved by CED in locations with site constraints.
Pipe Bedding, Backfill and Compaction
Pipe bedding and backfill shall be in accordance with the City of Renton Standard Details. Pipe
compaction shall follow the current WSDOT Standard Specifications. Where pipes pass through flood
containment structures, these standards shall be supplemented and modified as necessary in accordance
with standards set forth in Corps of Engineers Manual for Design and Construction of Levees
(EM 1110-2-1913).
Pipe System Connections
Connections to a pipe system shall be made only at catch basins or manholes. No wyes or tees are allowed
except on roof/footing/yard drain systems on pipes 8 inches in diameter or less, with clean-outs upstream
of each wye or tee.
Additional exceptions may be made provided the following conditions are met:
1. The mainline pipe is 48 inches or greater and at least two times the size of the connecting pipe.
2. Make connections in accordance with the manufacturer’s recommendations. Standard shop fabricated
tees, wyes and saddles shall be used, except for concrete connections constructed in accordance with
the City of Renton Standard Details.
3. There shall be a catch basin or manhole on the connecting pipe within 2 to 10 feet of the external wall
of the main line.
4. Offset angle of connecting pipe to mainline, horizontally and vertically shall be less than 45 degrees.
Storm drainage pipe systems shall not penetrate building foundations, except for sump pump discharge
lines used to drain crawl spaces, provided the sump pump system includes a backflow prevention or a
check valve.
An area drain within a crawl space is allowed to connect into the storm drainage system provided the
following conditions are met:
1. The connection is to be tight lined a minimum of ten feet outside of the foundation, and in no case
connected upstream of the footing drain system for the house.
2. The area drain must be routed through a sump pump system prior to connection to the public storm
drainage system, to help prevent backup during surcharge conditions and of potential sewer gas into
the crawl space area.
Pipe Anchors
Table 4.2.1.A3 presents the requirements, by pipe material, for anchoring pipe systems. Figure 4.2.1.B and
Figure 4.2.1.C show schematic representations of pipe anchors.
Spill Control
Where spill control is required as specified in Section 1.2.4.3.G, allowable options are as follows:
a) A tee section (see Figure 5.1.4.A) in or subsequent to the last catch basin or manhole that collects
runoff from non-roof-top pollution-generating impervious surface prior to discharge from the
site or into an onsite natural drainage feature.4 The tee section typically provided in a wetvault or
detention facility may be used to meet the intent of this requirement. Unless otherwise specified,
the riser top of the tee section shall be at or above the headwater elevation for the 10-year design
4 Natural onsite drainage feature means a natural swale, channel, stream, closed depression, wetland, or lake.
AGENDA ITEM # 8. a)
SECTION 4.2 PIPES, OUTFALLS, AND PUMPS
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flow and a minimum of 6 inches below the ceiling of the catch basin or manhole. The bottom end
of the tee section shall be as illustrated in Figure 5.1.4.A.
b) A wall section or other device as approved by the City that provides spill control equivalent to
that of the tee section specified in a) above.
c) A baffle or coalescing plate oil/water separator at or subsequent to the last catch basin or
manhole that collects runoff from non-roof-top pollution-generating impervious surface prior to
discharge from the site or into an onsite natural drainage feature.
d) An active spill control plan. To use this option, the spill control plan and summary of an existing
or proposed training schedule must be submitted as part of the drainage review submittal. At a
minimum, such plans must include the following:
Instructions for isolating the site to prevent spills from moving downstream (shutoff valves,
blocking catch basins, etc.)
Onsite location of spill clean-up materials
Phone numbers to call for emergency response
Phone numbers of company officials to notify
Special safety precautions, if applicable.
Debris Barriers
Debris barriers (trash racks) are required on all pipes 18 to 36 inches in diameter entering a closed pipe
system. Debris barriers for pipes smaller than 18 inches and larger than 36 inches in diameter may be
required depending on conditions and safety concerns. Debris barriers shall have a bar spacing of 6 inches.
See Figure 4.2.1.D for a schematic representation of debris barriers on pipe ends outside of roadways. See
Figure 4.2.1.E and Section 4.3 for a schematic representation of debris barriers on pipe ends (culverts)
projecting from driveway or roadway side slopes.
Outfalls
Outfalls shall be designed as detailed in Section 4.2.2.
Other Details
In addition to the schematic representations provided in Figure 4.2.1.A through Figure 4.2.1.E , standard
construction details are available in the City of Renton Standard Details and APWA/WSDOT Standard
Plans for Road, Bridge and Municipal Construction . Commonly used details include field tapping of
concrete pipe, catch basins and catch basin details, manholes and manhole details, curb inlets, frames,
grates, and covers.
AGENDA ITEM # 8. a)
4.2.1 PIPE SYSTEMS
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FIGURE 4.2.1.A SCHEMATIC REPRESENTATION OF PIPE BEDDING AND BACKFILL DESIGNS
6"6"6"6"6"6"85% RISE6"FOUNDATION LEVEL
FOUNDATION LEVEL
FOUNDATION LEVEL
GRAVEL BACKFILL FOR
PIPE ZONE BEDDING
(SEE NOTE 2)
PIPE ZONE BACKFILL
(SEE NOTE 1)
NOTES:
1. SEE CURRENT WSDOT
STANDARD SPECIFICATIONS
SECTION 7-08.3(3) FOR PIPE
ZONE BACKFILL.
2. SEE CURRENT WSDOT
STANDARD SPECIFICATIONSSECTION 9-03.12(3) FOR GRAVEL
BACKFILL FOR PIPE ZONE
BEDDING.
3. SEE CURRENT WSDOT
STANDARD SPECIFICATIONSSECTION 2-09.4 FOR
MEASUREMENT OF TRENCH
WIDTH.
4. SEE KCSWDM 4.2.1.1 FOR
CLEARANCE BETWEEN PIPES
AND OTHER UTILITIES.METAL PIPE
THERMOPLASTIC PIPE
CONCRETE AND DUCTILE IRON PIPE
PIPE ARCHES
TRENCH WIDTH
(SEE NOTE 3)PIPE ZONETRENCH WIDTH
(SEE NOTE 3)
TRENCH WIDTH
(SEE NOTE 3)PIPE ZONEGRAVEL BACKFILL FOR
PIPE ZONE BEDDING
(SEE NOTE 2)
PIPE ZONE BACKFILL
(SEE NOTE 1)
GRAVEL BACKFILL
FOR PIPE ZONE
BEDDING
(SEE NOTE 2)
GRAVEL BACKFILL
FOR PIPE ZONE
BEDDING
(SEE NOTE 2)
PIPE ZONE
BACKFILL
(SEE NOTE 1)PIPE ZONEPIPE ZONETRENCH WIDTH
(SEE NOTE 3)
NOTE:
ALL DETAILS
NOT TO SCALE6"15%
RISE
15%
O.D.
85% O.D.O.D.50%O.D.50%O.D.FOUNDATION
LEVEL
AGENDA ITEM # 8. a)
SECTION 4.2 PIPES, OUTFALLS, AND PUMPS
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FIGURE 4.2.1.B SCHEMATIC REPRESENTATION OF A PIPE ANCHOR DETAIL
6" MIN.
NOTE: FOR SOLID WALL HDPE, PIPE MUST BE
FREE TO SLIDE INSIDE A 4' LONG SECTION
OF PIPE ONE SIZE DIAMETER LARGER.
SECTION B-B
NTS
SECTION A-A
NTS
CONCRETE BLOCK ANCHOR
NTS
STRAP-FOOTING ANCHOR
NTS
1" MIN. DIAMETER
STEEL ROD (STRAP)
CLAMPED SECURELY
TO PIPE.
CONCRETE FOOTING
KEYED INTO
UNDISTURBED SOIL
AS SHOWN
PIPE BEDDING
CONCRETE
FOOTING
KEYED INTO
UNDISTURBED
SOIL AS SHOWN
CONCRETE
BLOCK
2 x PIPE DIA.
MIN.1' MIN.1' MIN.
6" MIN.
6" MIN.
2' MIN.
6"
MIN.
3' MIN.
6"
MIN.
6" MIN.
(TYPICAL)
AGENDA ITEM # 8. a)
4.2.1 PIPE SYSTEMS
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FIGURE 4.2.1.C SCHEMATIC REPRESENTATION OF CORRUGATED METAL PIPE COUPLING
AND/OR GENERAL PIPE ANCHOR ASSEMBLY
NOTES:
1. THE SMOOTH COUPLING BAND SHALL BE USED IN
COMBINATION WITH CONCRETE PIPE.
2. CONCRETE PIPE WITHOUT BELL AND SPIGOT SHALL
NOT BE INSTALLED ON GRADES IN EXCESS OF 20%.
3. THE FIRST ANCHOR SHALL BE INSTALLED ON THE
FIRST SECTION OF THE LOWER END OF THE PIPE AND
REMAINING ANCHORS EVENLY SPACED THROUGHOUT
THE INSTALLATION.
4. IF THE PIPE BEING INSTALLED HAS A MANHOLE OR
CATCH BASIN ON THE LOWER END OF THE PIPE, THE
FIRST PIPE ANCHOR MAY BE ELIMINATED.
5. WHEN CMP IS USED, THE ANCHORS MAY BE ATTACHED
TO THE COUPLING BANDS USED TO JOIN THE PIPE AS
LONG AS THE SPECIFIED SPACING IS NOT EXCEEDED.
6. ALL PIPE ANCHORS SHALL BE SECURELY INSTALLED
BEFORE BACKFILLING AROUND THE PIPE.
COUPLING BAND
1 1
2" X 6' PIPE
STAKES EACH
SIDE OF CULVERT
FLATTEN TO POINT
MATERIAL TO BE
ASTM A36 1 4"
PLATE GALVANIZED
AFTER FABRICATION
PER ASTM A123
SLOTS TO BE
119 32 X 3 4"78"SMOOTH COUPLING BAND
FOR SMOOTH PIPE
NTS
PLATE DETAIL
NTS
ANCHOR ASSEMBLY
CORRUGATED METAL PIPE
NTS
3"
MATERIAL TO BE ASTM A36
GALVANIZED AFTER
FABRICATION PER ASTM A153
PLATE (SEE DETAIL)
WELD
COLLAR (2" PIPE)
WELD
PIPE
STAKES6 58"ALL HOLES
3 4" DIAM.4 18"1
"
R
2"3/4"12"
4 12"4 12"
17 8"17 8"
1"
12"
7" COUPLING BAND
12" OR 24" COUPLING BAND
AGENDA ITEM # 8. a)
SECTION 4.2 PIPES, OUTFALLS, AND PUMPS
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FIGURE 4.2.1.D SCHEMATIC REPRESENTATION OF A DEBRIS BARRIER (OFF-ROAD RIGHT-OF-
WAY)
3/4" DIAMETER
SMOOTH BARS
CMP OR
LINED CPE
PIPE
PIPE COUPLING
SPOT WELD BARS TO AT
LEAST 2 CORREGATIONS
OF METAL PIPE (TYPICAL)
BOLT TO LINED CPE PIPE
NOTES:
1. THIS DEBRIS BARRIER IS FOR USE OUTSIDE ROADWAYS ON PIPES
18" DIA. TO 36" DIA.. SEE FIGURE 4.2.1.E FOR DEBRIS BARRIERS ON
PIPES PROJECTING FROM DRIVEWAY OR ROADWAY SIDE SLOPES.
2. ALL STEEL PARTS MUST BE GALVANIZED AND ASPHALT COATED
(TREATMENT 1 OR BETTER).
3. LINED CPE PIPE REQUIRES BOLTS TO SECURE DEBRIS BARRIER TO PIPE.
PLAN
NTS
ISOMETRIC
NTS
END VIEW
NTS
SIDE VIEW
NTS
45°3"L/2L
6"
12" MIN.
6" MAX. (TYP.)
AGENDA ITEM # 8. a)
4.2.1 PIPE SYSTEMS
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FIGURE 4.2.1.E SCHEMATIC REPRESENTATION OF A DEBRIS BARRIER
(IN ROAD RIGHT-OF-WAY)
MAY BE REMOVED
3/4" DIAMETER
BAR FRAME
PIPE
COUPLING
2" X 5" ANCHOR STRIPS WELDED TO
3/4" DIA. BAR-FRAME 4 PLACES SPACED
UNIFORMLY. FASTEN W/ 1/2" GALV. OR
NON-CORROSIVE BOLTS & NUTS.
3
1
3/4" DIA. SMOOTH BARS
WITH ENDS WELDED TO
BAR-FRAME
NOTES:
1. CMP OR LINED CPE PIPE END-SECTION SHOWN; FOR CONCRETE PIPE BEVELED
END SECTION, SEE KCRDCS DRAWING NO. 7-001.
2. ALL STEEL PARTS MUST BE GALVANIZED AND ASPHALT COATED (TREATMENT 1 OR BETTER).
BEVELED PIPE END SECTION
6" O.C. MAX.
BAR SPACING
3"-5" FOR 18" DIA.
5"-8" FOR 24" DIA.
7"-9" FOR 30" DIA. & GREATER
1' MIN.
AGENDA ITEM # 8. a)
SECTION 4.2 PIPES, OUTFALLS, AND PUMPS
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4.2.1.2 METHODS OF ANALYSIS
This section presents the methods of analysis for designing new or evaluating existing pipe systems for
compliance with the conveyance capacity requirements set forth in Section 1.2.4, “Core Requirement #4:
Conveyance System.”
DESIGN FLOWS
Design flows for sizing or assessing the capacity of pipe systems shall be determined using the hydrologic
analysis methods described in Chapter 3.
INLET GRATE CAPACITY
The methods described in Chapter 5, Sections 4 and 5, of the Washington State Department of
Transportation (WSDOT) Hydraulics Manual may be used in determining the capacity of inlet grates
when capacity is of concern, with the following exceptions:
1. Use design flows as required in Section 1.2.4 of this manual.
2. Assume grate areas on slopes are 80% free of debris; “vaned” grates, 95% free.
3. Assume grate areas in sags or low spots are 50% free of debris; “vaned” grates, 75% free.
CONVEYANCE CAPACITY
Two methods of hydraulic analysis using Manning's equation are used sequentially for the design and
analysis of pipe systems. First, the Uniform Flow Analysis method is used for the preliminary design of
new pipe systems. Second, the Backwater Analysis method is used to analyze both proposed and existing
pipe systems to verify adequate capacity. See Core Requirement #4, Section 1.2.4, for sizing requirements
of pipe systems.
Note: Use of the Uniform Flow Analysis method to determine preliminary pipe sizes is only suggested as a
first step in the design process and is not required. Results of the Backwater Analysis method determine
final pipe sizes in all cases.
Uniform Flow Analysis Method
In addition to the design criteria described below, new pipe systems shall be design to meet the hydraulic
criteria as described in Section 1.2.4.1.
This method is used for preliminary sizing of new pipe systems to convey the design flow (i.e., the
10-year or 25-year peak flow rate as specified in Core Requirement #4, Section 1.2.4).
Assumptions:
Flow is uniform in each pipe (i.e., depth and velocity remain constant throughout the pipe for a given
flow).
Friction head loss in the pipe barrel alone controls capacity. Other head losses (e.g., entrance, exit,
junction, etc.) and any backwater effects or inlet control conditions are not specifically addressed.
Each pipe within the system is sized and sloped such that its barrel capacity at normal full flow
(computed by Manning's equation) is equal to or greater than the design flow. The nomograph in
Figure 4.2.1.F may be used for an approximate solution of Manning's equation. For more precise results,
or for partial pipe full conditions, solve Manning's equation directly:
AGENDA ITEM # 8. a)
4.2.1 PIPE SYSTEMS
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V = R2/3 S1/2 (4-1)
or use the continuity equation, Q = AV, such that:
Q = A R2/3 S1/2 (4-2)
where Q = discharge (cfs)
V = velocity (fps)
A = area (sf)
n = Manning's roughness coefficient; see Table 4.2.1.D below
R = hydraulic radius = area/wetted perimeter (ft)
S = slope of the energy grade line (ft/ft)
For pipes flowing partially full, the actual velocity may be estimated from the hydraulic properties shown
in Figure 4.2.1.G by calculating Qfull and Vfull and using the ratio Qdesign/Qfull to find V and d (depth of
flow).
Table 4.2.1.D provides the recommended Manning's “n” values for preliminary design using the
Uniform Flow Analysis method for pipe systems. Note: The “n” values for this method are 15% higher in
order to account for entrance, exit, junction, and bend head losses.
TABLE 4.2.1.D MANNING’S “n” VALUES FOR PIPES
Type of Pipe Material Analysis Method
Uniform Flow
(preliminary
design)
Backwater Flow
(capacity
verification)
A. Concrete pipe, lined CPE pipe and lined PP pipe
B. Annular Corrugated Steel Pipe or Pipe Arch:
1. 2-2/3″ x 1/2″ corrugation (riveted):
a. plain or fully coated
b. paved invert (40% of circumference paved):
1) flow at full depth
2) flow at 80% full depth
3) flow at 60% full depth
c. treatment 5
2. 3″ x 1″ corrugation
3. 6″ x 2″ corrugation (field bolted)
C. Helical 2-2/3″ x 1/2″ corrugation and unlined CPE pipe
D. Spiral rib metal pipe and PVC pipe
E. Ductile iron pipe cement lined
F. Solid wall HDPE pipe (butt fused only)
0.014
0.028
0.021
0.018
0.015
0.015
0.031
0.035
0.028
0.013
0.014
0.009
0.012
0.024
0.018
0.016
0.013
0.013
0.027
0.030
0.024
0.011
0.012
0.009
n
49.1
n
49.1
AGENDA ITEM # 8. a)
SECTION 4.2 PIPES, OUTFALLS, AND PUMPS
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Backwater Analysis Method
This method is used to analyze the capacity of both new and existing pipe systems to convey the required
design flow (i.e., either the 10-year or 25-year peak flow, whichever is specified in Core Requirement #4,
Section 1.2.4). In either case, pipe system structures must be demonstrated to contain the headwater
surface (hydraulic grade line) for the specified peak flow rate. Structures may overtop for the 100-year
peak flow as allowed by Core Requirement #4. When this occurs, the additional flow over the ground
surface is analyzed using the methods for open channels described in Section 4.4.1.2 and added to the flow
capacity of the pipe system.
This method is used to compute a simple backwater profile (hydraulic grade line) through a proposed or
existing pipe system for the purposes of verifying adequate capacity. It incorporates a re-arranged form of
Manning's equation expressed in terms of friction slope (slope of the energy grade line in ft/ft). The
friction slope is used to determine the head loss in each pipe segment due to barrel friction, which can then
be combined with other head losses to obtain water surface elevations at all structures along the pipe
system.
The backwater analysis begins at the downstream end of the pipe system and is computed back through
each pipe segment and structure upstream. The friction, entrance, and exit head losses computed for each
pipe segment are added to that segment's tailwater elevation (the water surface elevation at the pipe's
outlet) to obtain its outlet control headwater elevation. This elevation is then compared with the inlet
control headwater elevation, computed assuming the pipe's inlet alone is controlling capacity using the
methods for inlet control presented in Section 4.3.1.2. The condition that creates the highest headwater
elevation determines the pipe's capacity. The approach velocity head is then subtracted from the
controlling headwater elevation, and the junction and bend head losses are added to compute the total
headwater elevation, which is then used as the tailwater elevation for the upstream pipe segment.
The Backwater Calculation Sheet in Figure 4.2.1.H may be used to compile the head losses
and headwater elevations for each pipe segment. The numbered columns on this sheet are described in
Figure 4.2.1.I. An example calculation is performed in Figure 4.2.1.J.
Note: This method should not be used to compute stage/discharge curves for level pool routing purposes .
Instead, a more sophisticated backwater analysis using the computer software provided with this manual
is recommended as described below.
Computer Applications
The King County Backwater (KCBW) computer program includes a subroutine BWPIPE, which may
be used to quickly compute a family of backwater profiles for a given range of flows through a proposed
or existing pipe system. A schematic description of the nomenclature used in this program is provided in
Figure 4.3.1.G. Program documentation providing instructions on the use of this and the other KCBW
subroutines is available from King County Department of Natural Resources and Parks (DNRP).
AGENDA ITEM # 8. a)
4.2.1 PIPE SYSTEMS
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FIGURE 4.2.1.F NOMOGRAPH FOR SIZING CIRCULAR DRAINS FLOWING FULL
AGENDA ITEM # 8. a)
SECTION 4.2 PIPES, OUTFALLS, AND PUMPS
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FIGURE 4.2.1.G CIRCULAR CHANNEL RATIOS
0
0.2
0.4
0.6
0.8
1
1.2
1.4
00.10.20.30.40.50.60.70.80.91
RATIO OF FLOW DEPTH TO DIAMETER (d/D)PROPORTIONAL AREA, DISCHARGE, VELOCITY, HYDRAULIC RADIUSPROPORTIONAL AREA
PROPORTIONAL
DISCHARGE
PROPORTIONAL HYDRAULIC RADIUS
AGENDA ITEM # 8. a)
4.2.1 PIPE SYSTEMS
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FIGURE 4.2.1.H BACKWATER CALCULATION SHEET
AGENDA ITEM # 8. a)
SECTION 4.2 PIPES, OUTFALLS, AND PUMPS
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FIGURE 4.2.1.I BACKWATER CALCULATION SHEET NOTES
Column (1) - Design flow to be conveyed by pipe segment.
Column (2) - Length of pipe segment.
Column (3) - Pipe Size; indicate pipe diameter or span x rise.
Column (4) - Manning's “n” value.
Column (5) - Outlet Elevation of pipe segment.
Column (6) - Inlet Elevation of pipe segment.
Column (7) - Barrel Area; this is the full cross-sectional area of the pipe.
Column (8) - Barrel Velocity; this is the full velocity in the pipe as determined by:
V = Q/A or Col.(8) = Col.(1) / Col.(7)
Column (9) - Barrel Velocity Head = V2/2g or (Col.(8))2/2g
where g = 32.2 ft/sec2 (acceleration due to gravity)
Column (10) - Tailwater (TW) Elevation; this is the water surface elevation at the outlet of the pipe segment. If the pipe's outlet is not submerged by
the TW and the TW depth is less than (D+dc)/2, set TW equal to (D+dc)/2 to keep the analysis simple and still obtain reasonable results
(D = pipe barrel height and dc = critical depth, both in feet. See Figure 4.3.1.F for determination of dc).
Column (11) - Friction Loss = Sf x L [or Sf x Col.(2)]
where Sf is the friction slope or head loss per linear foot of pipe as determined by Manning's equation expressed in the form:
Sf = (nV)2/2.22 R1.33
Column (12) - Hydraulic Grade Line (HGL) Elevation just inside the entrance of the pipe barrel; this is determined by adding the friction loss to the
TW elevation:
Col.(12) = Col.(11) + Col.(10)
If this elevation falls below the pipe's inlet crown, it no longer represents the true HGL when computed in this manner. The true
HGL will fall somewhere between the pipe's crown and either normal flow depth or critical flow depth, whichever is greater. To
keep the analysis simple and still obtain reasonable results (i.e., erring on the conservative side), set the HGL elevation equal to
the crown elevation.
Column (13) - Entrance Head Loss = Ke x V2/2g [or Ke x Col.(9)]
where Ke = Entrance Loss Coefficient (from Table 4.3.1.B). This is the head lost due to flow contractions at the pipe entrance.
Column (14) - Exit Head Loss = 1.0 x V2/2g or 1.0 x Col.(9)
This is the velocity head lost or transferred downstream.
Column (15) - Outlet Control Elevation = Col.(12) + Col.(13) + Col.(14)
This is the maximum headwater elevation assuming the pipe's barrel and inlet/outlet characteristics are controlling capacity. It does
not include structure losses or approach velocity considerations.
Column (16) - Inlet Control Elevation (see Section 4.3.1.2, for computation of inlet control on culverts); this is the maximum headwater elevation
assuming the pipe's inlet is controlling capacity. It does not include structure losses or approach velocity considerations.
Column (17) - Approach Velocity Head; this is the amount of head/energy being supplied by the discharge from an upstream pipe or channel section,
which serves to reduce the headwater elevation. If the discharge is from a pipe, the approach velocity head is equal to the barrel
velocity head computed for the upstream pipe. If the upstream pipe outlet is significantly higher in elevation (as in a drop manhole) or
lower in elevation such that its discharge energy would be dissipated, an approach velocity head of zero should be assumed.
Column (18) - Bend Head Loss = Kb x V2/2g [or Kb x Col.(17)]
where Kb = Bend Loss Coefficient (from Figure 4.2.1.K). This is the loss of head/energy required to change direction of flow in an
access structure.
Column (19) - Junction Head Loss. This is the loss in head/energy that results from the turbulence created when two or more streams are merged into
one within the access structure. Figure 4.2.1.L may be used to determine this loss, or it may be computed using the following
equations derived from Figure 4.2.1.L:
Junction Head Loss = Kj x V2/2g [or Kj x Col.(17)]
where Kj is the Junction Loss Coefficient determined by:
Kj = (Q3/Q1)/(1.18 + 0.63(Q3/Q1))
Column (20) - Headwater (HW) Elevation; this is determined by combining the energy heads in Columns 17, 18, and 19 with the highest control
elevation in either Column 15 or 16, as follows:
Col.(20) = Col.(15 or 16) - Col.(17) + Col.(18) + Col.(19)
AGENDA ITEM # 8. a)
4.2.1 PIPE SYSTEMS
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FIGURE 4.2.1.J BACKWATER PIPE CALCULATION EXAMPLE
AGENDA ITEM # 8. a)
SECTION 4.2 PIPES, OUTFALLS, AND PUMPS
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FIGURE 4.2.1.K BEND HEAD LOSSES IN STRUCTURES
AGENDA ITEM # 8. a)
4.2.1 PIPE SYSTEMS
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FIGURE 4.2.1.L JUNCTION HEAD LOSS IN STRUCTURES
AGENDA ITEM # 8. a)
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4.2.2 OUTFALL SYSTEMS
Properly designed outfalls are critical to ensuring no adverse impacts occur as the result of concentrated
discharges from pipe systems and culverts, both onsite and downstream. Outfall systems include rock
splash pads, flow dispersal trenches, gabion or other energy dissipaters, and tightline systems. A tightline
system is typically a continuous length of pipe used to convey flows down a steep or sensitive slope with
appropriate energy dissipation at the discharge end. In general, it is recommended that conveyance
systems be designed to reduce velocity above outfalls to the extent feasible.
4.2.2.1 DESIGN CRITERIA
General
At a minimum, all outfalls shall be provided with a rock splash pad (see Figure 4.2.2.A ) except as
specified below and in Table 4.2.2.A:
1. The flow dispersal trench shown in Figure 4.2.2.B shall only be used as an outfall as described in
Core Requirement #1, Section 1.2.1.
2. For outfalls with a velocity at design flow greater than 10 fps, a gabion dissipater or engineered
energy dissipater shall be required. Note the gabion outfall detail shown in Figure 4.2.2.D is
illustrative only; a design engineered to specific site conditions is required. Gabions shall conform to
WDSOT/APWA specifications.
3. Engineered energy dissipaters, including stilling basins, drop pools, hydraulic jump basins, baffled
aprons, and bucket aprons, are required for outfalls with velocity at design flow greater than 20 fps.
These should be designed using published or commonly known techniques found in such references as
Hydraulic Design of Energy Dissipaters for Culverts and Channels , published by the Federal
Highway Administration of the United States Department of Transportation; Open Channel Flow, by
V.T. Chow; Hydraulic Design of Stilling Basins and Energy Dissipaters, EM 25, Bureau of
Reclamation (1978); and other publications, such as those prepared by the Soil Conservation Service
(now Natural Resource Conservation Service). Alternate mechanisms, such as bubble-up structures
(which will eventually drain) and structures fitted with reinforced concrete posts, may require an
approved adjustment and must be designed using sound hydraulic principles and considering
constructability and ease of maintenance.
4. Tightline systems shall be used when required by the discharge requirements of Core Requirement #1
or the outfall requirements of Core Requirement #4. Tightline systems may also be used to prevent
aggravation or creation of a downstream erosion problem.
5. Flood closure devices shall be provided on new outfalls passing through existing levees or other
features that contain floodwaters. Such structures shall be designed to the Corps of Engineers Manual
for Design and Construction of Levees (EM 1110-2-1913).
6. Backup (secondary gate) closure devices shall be required for new outfalls through flood
containment levees unless this requirement is specifically waived by the City.
7. New outfalls through levees along the Green River between River Mile 6 and State Route 18 shall
comply with the terms of the adopted Lower Green River Pump Operation Procedures Plan.
Tightline Systems
1. Outfall tightlines may be installed in trenches with standard bedding on slopes up to 40%. In order to
minimize disturbance to slopes greater than 40%, it is recommended that tightlines be placed at
grade with proper pipe anchorage and support.
AGENDA ITEM # 8. a)
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2. Solid wall HDPE tightlines must be designed to address the material limitations, particularly thermal
expansion and contraction and pressure design, as specified by the manufacturer. The coefficient of
thermal expansion and contraction for solid wall HDPE is on the order of 0.001 inch per foot per
Fahrenheit degree. Sliding sleeve connections shall be used to address this thermal expansion and
contraction. These sleeve connections consist of a section of the appropriate length of the next larger
size diameter of pipe into which the outfall pipe is fitted. These sleeve connections must be located as
close to the discharge end of the outfall system as is practical.
3. Solid wall HDPE tightlines shall be designed and sized using the applicable design criteria and
methods of analysis specified for pipe systems in Section 4.2.1.
4. Due to the ability of solid wall HDPE tightlines to transmit flows of very high energy, special
consideration for energy dissipation must be made. A schematic representation of a “gabion mattress
energy dissipater” has been provided as Figure 4.2.2.D . Flows of very high energy will require a
specifically engineered energy dissipater structure, as described above in General Criterion #3.
Caution, the in-stream sample gabion mattress energy dissipater may not be acceptable within the
ordinary high water mark of fish-bearing waters or where gabions will be subject to abrasion from
upstream channel sediments. A four-sided gabion basket located outside the ordinary high water mark
should be considered for these applications.
TABLE 4.2.2.A ROCK PROTECTION AT OUTFALLS
Discharge Velocity
at Design Flow (fps) REQUIRED PROTECTION
Greater
than
Less than
or equal to
Minimum Dimensions(1)
Type Thickness Width Length Height
0 5 Rock
lining(2)
1 foot Diameter
+ 6 feet
8 feet
or
4 x diameter,
whichever is
greater
Crown
+ 1 foot
5 10 Riprap(3) 2 feet Diameter
+ 6 feet
or
3 x diameter,
whichever is
greater
12 feet
or
4 x diameter,
whichever is
greater
Crown
+ 1 foot
10 20 Gabion
outfall
As required As required As required Crown
+ 1 foot
20 N/A Engineered
energy
dissipater
required
(1) These sizes assume that erosion is dominated by outfall energy. In many cases sizing will be governed by conditions in
the receiving waters.
(2) Rock lining shall be quarry spalls with gradation as follows:
Passing 8-inch square sieve: 100%
Passing 3-inch square sieve: 40 to 60% maximum
Passing 3/4-inch square sieve: 0 to 10% maximum
(3) Riprap shall be reasonably well graded with gradation as follows:
Maximum stone size: 24 inches (nominal diameter)
Median stone size: 16 inches
Minimum stone size: 4 inches
Note: Riprap sizing governed by side slopes on outlet channel is assumed to be approximately 3:1.
AGENDA ITEM # 8. a)
SECTION 4.2 PIPES, OUTFALLS, AND PUMPS
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FIGURE 4.2.2.A SCHEMATIC REPRESENTATION OF PIPE/CULVERT DISCHARGE PROTECTION
PLACE ROCK 1' ABOVE CROWN
BOTH SIDES OF CHANNEL FOR "A" < 8'
ONE SIDE OF CHANNEL FOR "A" > 8'
DISCHARGE PIPE
FILTER FABRIC
LINER UNDER
ROCK
SECTION A-A
NTS
PLAN
NTSTOE OF BANKTOE OF BANKTOP OF BANKTOP OF BANKFLOWCL
REQUIRED DIMENSIONS
8' FOR ROCK LINING
12' FOR RIP RAP
SEE TABLE 4.2.2.A.
"A"+"B"
CHANNEL
"B"
(4' MIN.)
2'
MIN.
30°
MIN.
1' MIN.
1' OR 2'
ROCK THICKNESS
SEE TABLE 4.4.1.A
A
A
"A"
AGENDA ITEM # 8. a)
4.2.2 OUTFALL SYSTEMS
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FIGURE 4.2.2.B SCHEMATIC REPRESENTATION OF A FLOW DISPERSAL TRENCH
1. THIS TRENCH SHALL BE CONSTRUCTED TOPREVENT POINT DISCHARGE AND/OR EROSION.
2. TRENCHES MAY BE PLACED NO CLOSER THAN
50 FEET TO ONE ANOTHER (100 FEET ALONG
FLOWLINE).3. TRENCH AND GRADE BOARD MUST BE LEVEL.
ALIGN TO FOLLOW CONTOURS OF SITE.
4. VERTICALLY SLOTTED BOLT HOLES, 2" SLOT
LENGTH, ALLOWED FOR GRADE BOARD LEVELADJUSTMENT. PROVIDE BACKING WASHERS AND
FASTEN SECURELY.
5. SUPPORT POST SPACING AS REQUIRED BY SOIL
CONDITIONS TO ENSURE GRADE BOARDREMAINS LEVEL.
6. 15% MAX UNLESS OTHERWISE EVALUATED AND
APPROVED, SEE SECTION C.2.1.1
NOTES:
MIN 6" PERFORATED PIPE LAID FLAT
CLEAN (<5% FINES) 34" - 1 12" WASHED ROCK
FILTER FABRIC TRENCH LINER
GALVANIZED
BOLTS, SEE
NOTE 4
4" X 4"
SUPPORT
POST
2 0% MAX ,
S E E NO T E 6
15% M A X
A
NOTCHED
GRADE BOARD
2" X 2" NOTCHES
18" O.C.*
END CAP OR PLUG
CLEAN OUT WYE FROM PIPE
MIN 6" PERFORATED PIPE
LAID FLAT/LEVEL
TYPE I CB W/SOLID
COVER (LOCKING)
INFLUENT PIPE (MAX DESIGN
FLOW <0.5 CFS PER TRENCH)
CLEAN OUT WYE FROM PIPE
FLOW TO SECOND
DISPERSAL TRENCH
IF NECESSARY
FLOW TO OTHER BRANCHING
CB'S AS NECESSARY
TYPE I CB
W/SOLID
COVER
18" O.C.*
2"
A
50' MAX.
PLAN
NTS
SECTION A-A
NTS
1'- 0
MIN.
1'- 0
MIN.
PIPE O.D.
2" GRADE BOARD
NOTCHES*1'- 0
MIN.
PIPE O.D.
1'-0
MIN.12" MIN.36" MAX.6"
MIN.
2"x12"
GRADE
BOARD
*FOR WATER QUALITY FACILITIES,
SEE SECTION 6.2.6.1, OPTION A FOR
NOTCH DIMENSIONS AND SPACING
AGENDA ITEM # 8. a)
SECTION 4.2 PIPES, OUTFALLS, AND PUMPS
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FIGURE 4.2.2.C SCHEMATIC REPRESENTATION OF AN ALTERNATIVE
FLOW DISPERSAL TRENCH
1. THIS TRENCH SHALL BE
CONSTRUCTED TO PREVENT POINT
DISCHARGE AND /OR EROSION.
2. TRENCHES MAY BE PLACED NO
CLOSER THAN 50 FEET TO ONE
ANOTHER (100 FEET ALONG
FLOWLINE).
3. TRENCH AND GRADE BOARD MUST
BE LEVEL. ALIGN TO FOLLOW
CONTOURS OF SITE.
4. VERTICALLY SLOTTED BOLT HOLES,
2" SLOT LENGTH, ALLOWED FOR
GRADE BOARD LEVEL ADJUSTMENT.
PROVIDE BACKING WASHERS AND
FASTEN SECURELY.
5. PROVIDE SUPPORT POST SPACING
AS REQUIRED BY SOIL CONDITIONS
TO ENSURE GRADE BOARD
REMAINS LEVEL.
6. 15% MAX UNLESS OTHERWISE
EVALUATED AND APPROVED, SEE
SECTION C.2.1.1
NOTES:
15% M A X
20% M A X
SEE N O TE 6
GALVANIZED
BOLTS, SEE
NOTE 4
2" X 12"
GRADE
BOARD
2" X 2"
NOTCHES
18" O.C.CLEAN (< 5% FINES)
3 4" - 112" WASHED
ROCK
FILTER FABRIC
SECTION A-A
NTS
18" O.C.*
4" X 4" SUPPORT
POST
2"
1'-6"
MIN.12" MIN.36" MAX.2" GRADE BOARD NOTCHES*
*FOR WATER QUALITY FACILITIES,
SEE SECTION 6.2.6.1, OPTION A FOR
NOTCH DIMENSIONS AND SPACING
NOTCH DETAIL
NTS
AGENDA ITEM # 8. a)
4.2.2 OUTFALL SYSTEMS
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FIGURE 4.2.2.D SCHEMATIC REPRESENTATION OF A GABION MATTRESS ENERGY DISSIPATOR
A
SLEEVE OF NEXT LARGER
SIZE DIAMETER PIPE FOR
THERMAL EXPANSION
AND CONTRACTION
EXISTING
GROUND LINE
PLACE FILTER FABRIC BETWEEN
GABIONS AT SOIL BEDDING
GABION MATTRESSPIPE ANCHOR
RIP RAP
NOTE:
IF PIPE DISCHARGES
PERPENDICULAR TO STREAM,
OR GABIONS ARE LOCATED
WITHIN THE ORDINARY HIGH
WATER MARK (OHWM) OR
WILL BE SUBJECT TO
ABRASION FROM UPSTREAM
SEDIMENTS, A FOUR-SIDED
GABION BASKET LOCATED
OUTSIDE THE OHWM SHOULD
BE CONSIDERED.
GABIONS (TYPICAL)
(SIZE AS REQUIRED)
SOLID WALL
HDPE PIPE
GABION MATTRESS
SECTION C-C
NTS
SECTION B-BSECTION A-A
2 x D
A
PLAN VIEW
NTS
B
BA
45° PREFERRED
C
1'
B
3 x D
D
B
C
A
AGENDA ITEM # 8. a)
SECTION 4.2 PIPES, OUTFALLS, AND PUMPS
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4.2.3 PUMP SYSTEMS
As allowed in Core Requirement #4, Section 1.2.4.3, pump systems may be used for conveyance of flows
internal to a site if located on private property and privately maintained. Pump systems discharging to the
Green River between River Mile 6 and State Route 18 (within the Green River Flood Control Zone
District) shall comply with the standards of the adopted Green River Pump Operation Procedures Plan.
4.2.3.1 DESIGN CRITERIA
Proposed pump systems must meet the following minimum requirements:
1. The pump system must be privately owned and maintained.
2. The pump system shall be used to convey water from one location or elevation to another within the
site.
3. The pump system must have a dual pump (alternating) equipped with an external alarm system.
4. The pump system shall not be used to circumvent any other City drainage requirements, and
construction and operation of the pump system shall not violate any other City requirements.
5. The gravity-flow components of the drainage system to and from the pump system must be designed
so that pump failure does not result in flooding of a building or emergency access, or overflow to a
location other than the natural discharge point for the site.
6. The pump system shall have either installed emergency backup power or the ability for portable
backup power generator in the event of a loss of primary power. If portable backup emergency power
is provided, the appliance must include a description of how the backup power will be brought to the
site during an emergency within an emergency response plan (discussed below).
7. The applicant must provide an emergency response plan that details how backup power will be
activated during an emergency and include method for delivering to the site and energizing portable
backup power. The emergency response plan must also describe response for pump failures including
repair and replacement of damaged pumps/motors and generators.
4.2.3.2 METHODS OF ANALYSIS
Pump systems must be sized in accordance with the conveyance capacity requirements for pipe systems
set forth in Section 1.2.4, “Core Requirement #4: Conveyance System.”
AGENDA ITEM # 8. a)
CITY OF RENTON SURFACE WATER DESIGN MANUAL
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4.3 CULVERTS AND BRIDGES
This section presents the methods, criteria, and details for hydraulic analysis and design of culverts and
bridges. The information presented is organized as follows:
Section 4.3.1, “Culverts”
“Design Criteria,” Section 4.3.1.1
“Methods of Analysis,” Section 4.3.1.2
Section 4.3.2, “Culverts Providing for Fish Passage/Migration”
“Design Criteria,” Section 4.3.2.1
“Methods of Analysis,” Section 4.3.2.2
Section 4.3.3, “Bridges”
“Design Criteria,” Section 4.3.3.1
“Methods of Analysis,” Section 4.3.3.2.
4.3.1 CULVERTS
Culverts are relatively short segments of pipe of circular, elliptical, rectangular, or arch cross section. They
are usually placed under road embankments or driveways to convey surface water flow safely under the
embankment. They may be used to convey flow from constructed or natural channels including streams.
The Critical Areas Code (RMC 4-3-050) contains definitions of streams (termed “aquatic areas”) and
requirements for crossing of streams. In addition to those requirements and the design criteria described
below, other agencies such as the Washington State Department of Fish and Wildlife (WDFW) may have
additional requirements affecting the design of proposed culverts.
4.3.1.1 DESIGN CRITERIA
General
In addition to the design criteria described below, culverts shall be designed to meet the hydraulic criteria
as described in Section 1.2.4.1.
1. All circular pipe culverts shall conform to any applicable design criteria specified for pipe systems in
Section 4.2.1.
2. All other types of culverts shall conform to manufacturer's specifications. See the City of Renton
Standard Details for types of culverts allowed in City right-of-way.
Headwater
1. For culverts 18-inch diameter or less, the maximum allowable headwater elevation (measured from
the inlet invert) shall not exceed 2 times the pipe diameter or arch-culvert-rise at design flow (i.e., the
10-year or 25-year peak flow rate as specified in Core Requirement #4, Section 1.2.4).
2. For culverts larger than 18-inch diameter, the maximum allowable design flow headwater elevation
(measured from the inlet invert) shall not exceed 1.5 times the pipe diameter or arch-culvert-rise at
design flow.
3. The maximum headwater elevation at design flow shall be below any road or parking lot subgrade.
AGENDA ITEM # 8. a)
SECTION 4.3 CULVERTS AND BRIDGES
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Inlets and Outlets
1. All inlets and outlets in or near roadway embankments must be flush with and conforming to the slope
of the embankment.
2. For culverts 18-inch diameter and larger, the embankment around the culvert inlet shall be protected
from erosion by rock lining or riprap as specified in Table 4.2.2.A, except the length shall extend at
least 5 feet upstream of the culvert, and the height shall be at or above the design headwater elevation.
3. Inlet structures, such as concrete headwalls, may provide a more economical design by allowing the
use of smaller entrance coefficients and, hence, smaller diameter culverts. When properly designed,
they will also protect the embankment from erosion and eliminate the need for rock lining.
4. In order to maintain the stability of roadway embankments, concrete headwalls, wingwalls, or tapered
inlets and outlets may be required if right-of-way or easement constraints prohibit the culvert from
extending to the toe of the embankment slopes. All inlet structures or headwalls installed in or near
roadway embankments must be flush with and conforming to the slope of the embankment.
5. Debris barriers (trash racks) are required on the inlets of all culverts that are over 60 feet in length
and are 18 to 36 inches in diameter. Debris barriers for pipes smaller than 18 inches and larger than 36
inches in diameter may be required depending on conditions and safety concerns. Debris barriers shall
have a bar spacing of 6 inches. This requirement also applies to the inlets of pipe systems. See Figure
4.2.1.D and Figure 4.2.1.E for schematic representations of debris barriers.
6. For culverts 18-inch diameter and larger, the receiving channel of the outlet shall be protected from
erosion by rock lining specified in Table 4.2.2.A, except the height shall be one foot above maximum
tailwater elevation or one foot above the crown, whichever is higher (See Figure 4.2.2.A ).
4.3.1.2 METHODS OF ANALYSIS
This section presents the methods of analysis for designing new or evaluating existing culverts for
compliance with the conveyance capacity requirements set forth in Section 1.2.4, “Core Requirement #4:
Conveyance System.”
DESIGN FLOWS
Design flows for sizing or assessing the capacity of culverts shall be determined using the hydrologic
analysis methods described in Chapter 3.
CONVEYANCE CAPACITY
The theoretical analysis of culvert capacity can be extremely complex because of the wide range of
possible flow conditions that can occur due to various combinations of inlet and outlet submergence and
flow regime within the culvert barrel. An exact analysis usually involves detailed backwater calculations,
energy and momentum balance, and application of the results of hydraulic model studies.
However, simple procedures have been developed where the various flow conditions are classified and
analyzed on the basis of a control section. A control section is a location where there is a unique
relationship between the flow rate and the upstream water surface elevation. Many different flow
conditions exist over time, but at any given time the flow is either governed by the culvert's inlet geometry
(inlet control) or by a combination of inlet geometry, barrel characteristics, and tailwater elevation (outlet
control). Figure 4.3.1.A illustrates typical conditions of inlet and outlet control. The procedures presented
in this section provide for the analysis of both inlet and outlet control conditions to determine which
governs.
AGENDA ITEM # 8. a)
4.3.1 CULVERTS
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Inlet Control Analysis
Nomographs such as those provided in Figure 4.3.1.B and Figure 4.3.1.C may be used to determine the
inlet control headwater depth at design flow for various types of culverts and inlet configurations. These
nomographs were originally developed by the Bureau of Public Roads—now the Federal Highway
Administration (FHWA)—based on their studies of culvert hydraulics. These and other nomographs can
be found in the FHWA publication Hydraulic Design of Highway Culverts, HDS No. #5 (Report No.
FHWA-IP-85-15) (September 1985), or the WSDOT Hydraulic Manual.
Also available in the FHWA publication, are the design equations used to develop the inlet control
nomographs. These equations are presented below.
For unsubmerged inlet conditions (defined by Q/AD0.5 ≤ 3.5);
Form 1*: HW/D = Hc/D + K(Q/AD0.5)M - 0.5S** (4-3)
Form 2*: HW/D = K(Q/AD0.5)M (4-4)
For submerged inlet conditions (defined by Q/AD0.5 ≥ 4.0);
HW/D = c(Q/AD0.5)2 + Y - 0.5S** (4-5)
where HW = headwater depth above inlet invert (ft)
D = interior height of culvert barrel (ft)
Hc = specific head (ft) at critical depth (dc + Vc2/2g)
Q = flow (cfs)
A = full cross-sectional area of culvert barrel (sf)
S = culvert barrel slope (ft/ft)
K,M,c,Y = constants from Table 4.3.1.A.
The specified head Hc is determined by the following equation:
Hc = dc + Vc
2/2g (4-6)
where dc = critical depth (ft); see Figure 4.3.1.F
Vc = flow velocity at critical depth (fps)
g = acceleration due to gravity (32.2 ft/sec2).
* The appropriate equation form for various inlet types is specified in Table 4.3.1.A below.
** For mitered inlets, use +0.7S instead of -0.5S.
Note: Between the unsubmerged and submerged conditions, there is a transition zone
(3.5 < Q/AD0.5 < 4.0) for which there is only limited hydraulic study information. The transition zone is
defined empirically by drawing a curve between and tangent to the curves defined by the unsubmerged
and submerged equations. In most cases, the transition zone is short and the curve is easily constructed.
AGENDA ITEM # 8. a)
SECTION 4.3 CULVERTS AND BRIDGES
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TABLE 4.3.1.A CONSTANTS FOR INLET CONTROL EQUATIONS*
Unsubmerged Submerged
Shape and Material Inlet Edge Description
Equation
Form K M c Y
Circular Concrete Square edge with headwall 1 0.0098 2.0 0.0398 0.67
Groove end with headwall 0.0078 2.0 0.0292 0.74
Groove end projecting 0.0045 2.0 0.0317 0.69
Circular Corrugated
Steel Pipe
Headwall 1 0.0078 2.0 0.0379 0.69
Mitered to slope 0.0210 1.33 0.0463 0.75
Projecting 0.0340 1.50 0.0553 0.54
Rectangular Box 30 to 75 wingwall flares 1 0.026 1.0 0.0385 0.81
90 and 15 wingwall flares 0.061 0.75 0.0400 0.80
0 wingwall flares 0.061 0.75 0.0423 0.82
CM Boxes 90 headwall 1 0.0083 2.0 0.0379 0.69
Thick wall projecting 0.0145 1.75 0.0419 0.64
Thin wall projecting 0.0340 1.5 0.0496 0.57
Arch Corrugated
Steel Pipe
90 headwall 1 0.0083 2.0 0.0496 0.57
Mitered to slope 0.0300 1.0 0.0463 0.75
Projecting 0.0340 1.5 0.0496 0.53
Bottomless Arch 90 headwall 1 0.0083 2.0 0.0379 0.69
Corrugated Steel
Pipe
Mitered to slope 0.0300 2.0 0.0463 0.75
Thin wall projecting 0.0340 1.5 0.0496 0.57
Circular with Smooth tapered inlet throat 2 0.534 0.333 0.0196 0.89
Tapered Inlet Rough tapered inlet throat 0.519 0.64 0.0289 0.90
* Source: FHWA HDS No. 5
Outlet Control Analysis
Nomographs such as those provided in Figure 4.3.1.D and Figure 4.3.1.E may be used to determine the
outlet control headwater depth at design flow for various types of culverts and inlets. Outlet control
nomographs other than those provided can be found in FHWA HDS No.5 or the WSDOT Hydraulic
Manual.
The outlet control headwater depth may also be determined using the simple Backwater Analysis method
presented in Section 4.2.1.2 for analyzing pipe system capacity. This procedure is summarized as follows
for culverts:
HW = H + TW - LS (4-7)
where H = Hf + He + Hex
Hf = friction loss (ft) = (V2n2L)/(2.22R1.33)
Note: If (Hf+TW-LS) < D, adjust Hf such that (Hf+TW-LS) = D. This will keep the
analysis simple and still yield reasonable results (erring on the conservative side).
He = entrance head loss (ft) = Ke(V2/2g)
Hex = exit head loss (ft) = V2/2g
AGENDA ITEM # 8. a)
4.3.1 CULVERTS
2022 City of Renton Surface Water Design Manual 6/22/2022
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TW = tailwater depth above invert of culvert outlet (ft)
Note: If TW < (D+dc)/2, set TW = (D+dc)/2. This will keep the analysis simple and still
yield reasonable results.
L = length of culvert (ft)
S = slope of culvert barrel (ft/ft)
D = interior height of culvert barrel (ft)
V = barrel velocity (fps)
n = Manning's roughness coefficient from Table 4.2.1.D
R = hydraulic radius (ft)
Ke = entrance loss coefficient (from Table 4.3.1.B)
g = acceleration due to gravity (32.2 ft/sec2)
dc = critical depth (ft); see Figure 4.3.1.F
Note: The above procedure should not be used to develop stage/discharge curves for level pool routing
purposes because its results are not precise for flow conditions where the hydraulic grade line falls
significantly below the culvert crown (i.e., less than full flow conditions).
AGENDA ITEM # 8. a)
SECTION 4.3 CULVERTS AND BRIDGES
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TABLE 4.3.1.B ENTRANCE LOSS COEFFICIENTS
Type of Structure and Design Entrance Coefficient, Ke
Pipe, Concrete, PVC, Spiral Rib, DI, and Lined CPE
Projecting from fill, socket (bell) end 0.2
Projecting from fill, square cut end 0.5
Headwall, or headwall and wingwalls
Socket end of pipe (groove-end) 0.2
Square-edge 0.5
Rounded (radius = 1/12D) 0.2
Mitered to conform to fill slope 0.7
End section conforming to fill slope* 0.5
Beveled edges, 33.7 or 45 bevels 0.2
Side- or slope-tapered inlet 0.2
Pipe, or Pipe-Arch, Corrugated Metal and Other Non-Concrete or D.I.
Projecting from fill (no headwall) 0.9
Headwall, or headwall and wingwalls (square-edge) 0.5
Mitered to conform to fill slope (paved or unpaved slope) 0.7
End section conforming to fill slope* 0.5
Beveled edges, 33.7 or 45 bevels 0.2
Side- or slope-tapered inlet 0.2
Box, Reinforced Concrete
Headwall parallel to embankment (no wingwalls)
Square-edged on 3 edges 0.5
Rounded on 3 edges to radius of 1/12 barrel dimension or beveled edges on
3 sides
0.2
Wingwalls at 30 to 75 to barrel
Square-edged at crown 0.4
Crown edge rounded to radius of 1/12 barrel dimension or beveled top edge 0.2
Wingwall at 10 to 25 to barrel
Square-edged at crown 0.5
Wingwalls parallel (extension of sides)
Square-edged at crown 0.7
Side- or slope-tapered inlet 0.2
* Note: “End section conforming to fill slope” are the sections commonly available from manufacturers. From limited hydraulic tests they
are equivalent in operation to a headwall in both inlet and outlet control. Some end sections incorporating a closed taper in their
design have a superior hydraulic performance.
Computer Applications
The “King County Backwater” (KCBW) computer program available with this manual contains two
subroutines (BWPIPE and BWCULV) that may be used to analyze culvert capacity and develop
stage/discharge curves for level pool routing purposes. A schematic description of the nomenclature used
in these subroutines is provided in Figure 4.3.1.G. The KCBW program documentation available from
King County Department of Natural Resources and Parks (DNRP) includes more detailed descriptions of
program features.
AGENDA ITEM # 8. a)
4.3.1 CULVERTS
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FIGURE 4.3.1.A INLET/OUTLET CONTROL CONDITIONS
AGENDA ITEM # 8. a)
SECTION 4.3 CULVERTS AND BRIDGES
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FIGURE 4.3.1.B
HEADWATER DEPTH FOR SMOOTH INTERIOR PIPE CULVERTS WITH INLET CONTROL
AGENDA ITEM # 8. a)
4.3.1 CULVERTS
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FIGURE 4.3.1.C HEADWATER DEPTH FOR CORRUGATED PIPE CULVERTS WITH INLET
CONTROL
AGENDA ITEM # 8. a)
SECTION 4.3 CULVERTS AND BRIDGES
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FIGURE 4.3.1.D HEAD FOR CULVERTS (PIPE W/“n”= 0.012)
FLOWING FULL WITH OUTLET CONTROL
AGENDA ITEM # 8. a)
4.3.1 CULVERTS
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FIGURE 4.3.1.E HEAD FOR CULVERTS (PIPE W/“n”= 0.024)
FLOWING FULL WITH OUTLET CONTROL
AGENDA ITEM # 8. a)
SECTION 4.3 CULVERTS AND BRIDGES
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FIGURE 4.3.1.F CRITICAL DEPTH OF FLOW FOR CIRCULAR CULVERTS
AGENDA ITEM # 8. a)
4.3.1 CULVERTS
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FIGURE 4.3.1.G COMPUTER SUBROUTINES BWPIPE AND BWCULV: VARIABLE DEFINITIONS
FLOW DATA COEFFICIENTS / INLET DATA
DC - Critical Depth (ft) KE - Entrance Coefficient under Outlet
Control
DN - Normal Depth (ft) KB - Bend Loss Coefficient
TW - Tailwater Depth (ft) KJ - Junction Loss Coefficient
DO - Outlet Depth (ft) K - Inlet Control Equation parameter
(See Table 4.3.1.A)
DE - Entrance Depth (ft) M - Inlet Control Equation parameter
(See Table 4.3.1.A)
HWO - Headwater (ft) assuming Outlet Control C - Inlet Control Equation parameter
(See Table 4.3.1.A)
HWI - Headwater (ft) assuming Inlet Control Y - Inlet Control Equation parameter
(See Table 4.3.1.A)
DXN - Distance (expressed as a fraction of the pipe
length) from the outlet to where the flow profile
intersects with normal depth. DXN will equal one
under full-flow conditions and will equal zero when
a hydraulic jump occurs at the outlet or when
normal depth equals zero (normal depth will equal
zero when the pipe grade is flat or reversed).
Q-Ratio - Ratio of tributary flow to main upstream
flow (Q3/Q1)
VBH - Barrel Velocity Head (ft) based on the average
velocity determined by V=Q/Afull
VUH - Upstream Velocity Head (ft) based on an inputted
velocity.
EHU - Upstream Energy Head (ft) available after bend
losses and junction losses have been subtracted
from VUH.
VCH - Critical Depth Velocity Head (ft)
VNH - Normal Depth Velocity Head (ft)
VEH - Entrance Depth Velocity Head (ft)
VOH - Outlet Depth Velocity Head (ft)
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4.3.2 CULVERTS PROVIDING FOR FISH PASSAGE/MIGRATION
In fish-bearing waters, water-crossing structures must usually provide for fish passage as required for
Washington State Department of Fish and Wildlife (WDFW) Hydraulic Project Approval or as a condition
of permitting under the critical areas code (RMC 4-3-050). Culverts designed for fish passage must also
meet the requirements of Section 1.2.4, “Core Requirement #4: Conveyance System.”
Fish passage can generally be ensured by providing structures that do not confine the streambed—that is,
a structure wide enough so that the stream can maintain its natural channel within the culvert. Bridges,
bottomless arch culverts, arch culverts, and rectangular box culverts (“utility vaults”) can often be used to
accommodate stream channels.
Where it is unfeasible to construct these types of structures, round pipe culverts may be used if high flow
velocities are minimized and low flow depths are maximized. The Hydraulic Code Rules (Title 220 WAC)
detail requirements for WDFW Hydraulic Project Approval. See the WDFW manual “Design of Road
Culverts for Fish Passage” for detailed design methodologies.
Materials
Galvanized metals leach zinc into the environment, especially in standing water situations. High zinc
concentrations, sometimes in the range that can be toxic to aquatic life, have been observed in the region.
Therefore, use of galvanized materials in stormwater facilities is not allowed, and their use in conveyance
systems is discouraged. Where other metals, such as stainless steel, or plastics are available, they should
be used.
4.3.2.1 DESIGN CRITERIA
Table 4.3.2.A lists allowable velocities, flow depths, and hydraulic drops for culverts in fish-bearing
streams. Velocities are for the high flow design discharge; water depths are for the low flow design
discharge. The hydraulic drop (a vertical drop in the water surface profile at any point within culvert
influence) is for all flows between the high and low flow design discharges.
TABLE 4.3.2.A FISH PASSAGE DESIGN CRITERIA
Adult Trout
Adult Pink, Chum
Salmon
Adult Chinook, Coho,
Sockeye, Steelhead
1. Max Velocity (fps)
Culvert Length:
10–60 ft 4.0 5.0 6.0
60–100 ft 4.0 4.0 5.0
100–200 ft 3.0 3.0 4.0
2. Min Flow Depth (ft) 0.8 0.8 1.0
3. Max Hydraulic Drop (ft) 0.8 0.8 1.0
Source: WDFW manual “Design of Road Culverts for Fish Passage” (2003), Chapter 5, p. 21, Table 5-1.
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4.3.2.2 METHODS OF ANALYSIS
High Flow Design Discharge
For gaged streams, the high flow design discharge shall be estimated by the 10% exceedance flow for
October through April inclusive, proportioned by tributary area to the culvert using the technique
described in Section 4.4.2.4 under “Flood Flows from Stream Gage Data.”
For ungaged streams, the high flow design discharge shall be estimated by one of the following:
The 10% exceedance flow for October through April inclusive for the nearest hydrologically similar
gaged stream, proportioned by tributary area
The 5% exceedance flow determined through duration analysis with the approved model
The 10% exceedance flow for October through April inclusive determined with the HSPF model or
the approved model using the full historical record.
Low Flow Design Discharge
For gaged streams, the low flow design discharge shall be estimated by the 95% exceedance flow for
October through April inclusive, proportioned by tributary area.
For ungaged streams, the low flow design discharge shall be estimated by one of the following:
The 95% exceedance flow for October through April inclusive for the nearest hydrologically similar
gaged stream, proportioned by tributary area
The 95% exceedance flow for October through April inclusive, determined by the HSPF model or the
approved model using the full historical record
The following equation, using input data from the approved model (Note: Equation 4-9 is not used):
For the Sea-Tac rainfall region:
Ql = fr (0.46Atf + 0.56Atp + 0.46Atg + 0.72Aof + 0.96Aop + 1.10Aog) / 1000 (4-7)
where Ql = low flow design discharge (cfs)
fr = regional rainfall scale factor from the WWHM2012 Site Information map screen
Atf = area of till forest (acres)
Atp = area of till pasture (acres)
Atg = area of till grass (acres)
Aof = area of outwash forest (acres)
Aop = area of outwash pasture (acres)
Aog = area of outwash grass (acres)
Note: Minimum depths may also be met by providing an “installed no-flow depth,” per Title 220 WAC,
where the static water surface level meets minimum flow depth criteria.
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4.3.3 BRIDGES
Bridges over waterways are considered conveyance structures and are generally constructed to allow the
continuation of a thoroughfare (such as a road). They generally consist of foundation abutments and/or
piers that support a deck spanning the waterway. In addition to the design criteria for conveyance
described below, bridge designs must meet the City of Renton Transportation requirements, AASHTO
Standard Specifications for Highway Bridges or AASHTO LRFD Bridge Design Specifications and the
most current WSDOT/APWA Standard Specifications as well as the requirements of other agencies such
as the Washington State Department of Fish and Wildlife (WDFW).
4.3.3.1 DESIGN CRITERIA
Bridges shall be designed to convey flows and pass sediments and debris for runoff events up to and
including the 100-year event in a manner that does not increase the potential for flooding or erosion to
properties and structures near or adjacent to the bridge, or cause bridge failure. Inadequate conveyance
capacity may cause flooding to increase by restricting flow through the hydraulic openings, by placing
approach fill or abutments in floodplains, by causing changes in channel gradient and alignment or by
trapping debris. A common mode of bridge failure involving debris is the resultant scour and undermining
of piers or abutments where debris accumulates.
Openings between the structural elements of the bridge and the bottom of the channel or floodplain ground
surface must be large enough to allow for passage of water, sediment, and debris. The horizontal openings
are defined by the bridge span, the horizontal distances between piers or abutments.
Bridge clearance is the vertical distance between the 100-year water surface and the low chord of the
bridge. For stream crossing locations where the 100-year peak flow exceeds 100 cfs, the height of a bridge
clearance above rivers and streams shall be a minimum three feet above the 100-year water surface
elevation unless otherwise required by the City based on evaluation of the design criteria in this section.
For stream crossing locations where the 100-year peak flow is 100 cfs or less, there is no specific
clearance requirement.
Hydraulic Capacity
Bridge and approach roads must pass the 100-year flow without creating hydraulic restrictions that cause
or increase flooding. Design of bridge and approach roads shall demonstrate compliance with the
compensatory storage provisions of RMC 4-3-050. Of necessity, bridge and approach roads are sometimes
constructed within 100-year floodplains. In some cases, approach roads will be inundated and the bridge
will not be accessible during extreme events. In other cases, both the bridge and approach roads will be
inundated by the 100-year flood. In these cases, the bridge shall be designed to withstand the expected
condition while inundated. The design shall employ means to facilitate flow over the bridge and to
minimize the potential for erosion of the roadway fill in the approach roads.
Bed Aggradation
Where bed aggradation is probable, the analysis of hydraulic capacity shall assume the bed raised by an
amount expected during a suitable design life (40 years minimum) of the bridge. Aggradation estimates
shall be based on a sediment transport analysis that, where possible, is calibrated to direct cross-section
comparisons over time. This analysis shall extend upstream and downstream a sufficient distance to
adequately characterize bed aggradation that may affect the hydraulic capacity at the bridge location.
Bed aggradation is frequently associated with channel migration. The location and design of bridges and
approach roads shall consider channel migration hazards.
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Debris Passage
Since debris can pass through an opening either partly or totally submerged, the total vertical clearance
from the bottom of the structure to the streambed needs to be considered. Required clearance for debris
shall include an assessment of the maximum material size available, the ability of the stream to transport
it, and the proximity of debris sources. The following factors also must be considered: history of debris
problems in the river reaches upstream and downstream of the proposed bridge location, history of debris
accumulations on an existing bridge structure or nearby structures upstream and downstream from the
proposed bridge location, mapped channel migration hazard and channel migration history of the reach of
stream, and skew of the bridge alignment such that piers in floodplain may be in the path of the debris. For
a detailed qualitative analysis of debris accumulation on bridges, see the U.S. Department of
Transportation, Federal Highway Administration Publication FHWA-RD-97-028, Potential Drift
Accumulation at Bridges, by Timothy H. Diehl (1997).
Safety Margin
When designing bridges to convey flows and pass sediments and debris, a safety margin shall be
considered by the design engineer to account for uncertainties in flow rates, debris hazards, water surface
elevations, aggradation, and channel migration over time. The safety margin should be increased when the
surrounding community is especially susceptible to flood damages that could be exacerbated by a debris
jam at the bridge. Section 5 of the Technical Information Report submitted with the project’s engineering
plans shall include a discussion of the need for a safety margin and the rationale for its selection.
Bridges and Levees
Where bridge structures and approach roads intersect flood containment levees, the bridge structure and
approach roads shall be designed and constructed to preserve existing levels of flood containment
provided by the existing levee.
Where the existing levee currently provides containment of the 100-year flood, the bridge structure and
approach roads shall be designed and constructed to meet FEMA levee and structural performance
standards, including sufficient freeboard on the levee in the bridge vicinity, as provided for in 44 CFR
(also see Section 1.3.3, Special Requirement #3, Flood Protection Facilities).
Bridge Piers and Abutments
Bridge pier and abutment locations are governed by provisions of the City’s critical areas code,
RMC 4-3-050.
4.3.3.2 METHODS OF ANALYSIS
The following methods are acceptable for hydraulic analysis of bridges and approach roads:
1. The Direct Step backwater method described in Section 4.4.1.2 shall be used to analyze the
hydraulic impacts of bridge piers, abutments, and approach roads to the water surface profile.
2. The Army Corps of Engineers Hydraulic Engineering Center publishes technical papers on methods
used to address the hydraulic effects of bridge piers, abutments, and approach roads. The book Open
Channel Hydraulics by V.T. Chow also contains techniques for analyzing hydraulic effects.
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4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS
This section presents the methods, criteria, and details for hydraulic analysis and design of open channels,
and the determination and analysis of floodplains and floodways. The information presented is organized
as follows:
Section 4.4.1, “Open Channels”
“Design Criteria,” Section 4.4.1.1
“Methods of Analysis,” Section 4.4.1.2
Section 4.4.2, “Floodplain/Floodway Analysis”
“No Floodplain Study Required,” Section 4.4.2.1
“Approximate Floodplain Study,” Section 4.4.2.2
“Minor Floodplain Study,” Section 4.4.2.3
“Major Floodplain/Floodway Study,” Section 4.4.2.4.
4.4.1 OPEN CHANNELS
Open channels may be classified as either natural or constructed. Natural channels are generally referred to
as rivers, streams, creeks, or swales, while constructed channels are most often called ditches, or simply
channels. The Critical Areas, Shorelines, and Clearing and Grading Codes as well as Chapter 1 of this
manual should be reviewed for requirements related to streams.
Natural Channels
Natural channels are defined as those that have occurred naturally due to the flow of surface waters, or
those that, although originally constructed by human activity, have taken on the appearance of a natural
channel including a stable route and biological community. They may vary hydraulically along each
channel reach and should be left in their natural condition, wherever feasible or required, in order to
maintain natural hydrologic functions and wildlife habitat benefits from established vegetation.
Constructed Channels
Constructed channels are those constructed or maintained by human activity and include bank stabilization
of natural channels. Constructed channels shall be either vegetation-lined, rock-lined, or lined with
appropriately bioengineered vegetation5.
Vegetation-lined channels are the most desirable of the constructed channels when properly designed
and constructed. The vegetation stabilizes the slopes of the channel, controls erosion of the channel
surface, and removes pollutants. The channel storage, low velocities, water quality benefits, and
greenbelt multiple-use benefits create significant advantages over other constructed channels. The
presence of vegetation in channels creates turbulence that results in loss of energy and increased flow
retardation; therefore, the design engineer must consider sediment deposition and scour, as well as
flow capacity, when designing the channel.
Rock-lined channels are necessary where a vegetative lining will not provide adequate protection from
erosive velocities. They may be constructed with riprap, gabions, or slope mattress linings. The rock
lining increases the turbulence, resulting in a loss of energy and increased flow retardation. Rock
lining also permits a higher design velocity and therefore a steeper design slope than in grass-lined
5 Bioengineered vegetation lining as referenced here applies to channel stabilization methods. See Appendix C, Simplified
Drainage Requirements for bioswale design criteria. Note, for bioswales and other infiltrative BMPs that may be placed in-line
with conveyance, any infiltration option in the modeling shall be turned off when evaluating conveyance capacity.
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channels. Rock linings are also used for erosion control at culvert and storm drain outlets, sharp
channel bends, channel confluences, and locally steepened channel sections.
Bioengineered vegetation lining is a desirable alternative to the conventional methods of rock
armoring. Soil bioengineering is a highly specialized science that uses living plants and plant parts to
stabilize eroded or damaged land. Properly bioengineered systems are capable of providing a measure
of immediate soil protection and mechanical reinforcement. As the plants grow they produce a
vegetative protective cover and a root reinforcing matrix in the soil mantle. This root reinforcement
serves several purposes:
a) The developed anchor roots provide both shear and tensile strength to the soil, thereby providing
protection from the frictional shear and tensile velocity components to the soil mantle during the
time when flows are receding and pore pressure is high in the saturated bank.
b) The root mat provides a living filter in the soil mantle that allows for the natural release of water
after the high flows have receded.
c) The combined root system exhibits active friction transfer along the length of the living roots.
This consolidates soil particles in the bank and serves to protect the soil structure from collapsing
and the stabilization measures from failing.
The vegetative cover of bioengineered systems provides immediate protection during high flows by
laying flat against the bank and covering the soil like a blanket. It also reduces pore pressure in
saturated banks through transpiration by acting as a natural “pump” to “pull” the water out of the
banks after flows have receded.
The King County publication Guidelines for Bank Stabilization Projects primarily focuses on projects on
larger rivers and streams, but the concepts it contains may be used in conjunction with other natural
resource information for stabilization projects on smaller systems. The WDFW Integrated Streambank
Protection Guidelines is another useful reference.
4.4.1.1 DESIGN CRITERIA
General
1. Open channels shall be designed to provide required conveyance capacity and bank stability while
allowing for aesthetics, habitat preservation, and enhancement. Open channels shall be consistent with
the WDFW Integrated Streambank Protection Guidelines.
2. An access easement for maintenance is required along all constructed channels located on private
property. Required easement widths and building setback lines vary with channel top width as shown
in Table 4.1.
3. Channel cross-section geometry shall be trapezoidal, triangular, parabolic, or segmental as shown in
Figure 4.4.1.C through Figure 4.4.1.E. Side slopes shall be no steeper than 3:1 for vegetation-lined
channels and 2:1 for rock-lined channels. Note: Roadside ditches shall comply with the City of Renton
Standard Details.
4. To reduce the likelihood that pollutants will be discharged to groundwater when untreated runoff is
conveyed in ditches or channels constructed in soils with high infiltration rates, a low permeability
liner or a treatment liner shall be provided for any reach of new ditch or channel proposed by a
project in which the untreated runoff from 5,000 square feet or more of pollution-generating
impervious surface comes into direct contact with an outwash soil, except where it can be
demonstrated that the soil meets the soil suitability criteria listed in Section 5.2.1. The low
permeability liner or treatment liner shall be consistent with the specifications for such liners in
Section 6.2.4.
5. Vegetation-lined channels shall have bottom slope gradients of 6% or less and a maximum
velocity at design flow of 5 fps (see Table 4.4.1.A).
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6. Rock-lined channels or bank stabilization of natural channels shall be used when design flow
velocities exceed 5 feet per second. Rock stabilization shall be in accordance with Table 4.4.1.A
or stabilized with bioengineering methods as described above in “Constructed Channels.”
TABLE 4.4.1.A CHANNEL PROTECTION
Velocity at Design
Flow (fps) Required Protection
Greater
than
Less than
or Equal to Type of Protection Thickness
Minimum Height
Above Design
Water Surface
0 5 Grass lining
or
Bioengineered lining
N/A N/A
5 8 Rock lining(1)
or
Bioengineered lining
1 foot 1 foot
8 12 Riprap(2) 2 feet 2 feet
12 20 Slope mattress gabion,
etc.
Varies 2 feet
(1) Rock Lining shall be reasonably well graded as follows:
Maximum stone size: 12 inches
Median stone size: 8 inches
Minimum stone size: 2 inches
(2) Riprap shall be reasonably well graded as follows:
Maximum stone size: 24 inches
Median stone size: 16 inches
Minimum stone size: 4 inches
Note: Riprap sizing is governed by side slopes on channel, assumed to be approximately 3:1.
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Riprap Design6
When riprap is set, stones are placed on the channel sides and bottom to protect the underlying material
from being eroded. Proper riprap design requires the determination of the median size of stone, the
thickness of the riprap layer, the gradation of stone sizes, and the selection of angular stones that will
interlock when placed. Research by the U.S. Army Corps of Engineers has provided criteria for selecting
the median stone weight, W50 (Figure 4.4.1.A). If the riprap is to be used in a highly turbulent zone (such
as at a culvert outfall, downstream of a stilling basin, at sharp changes in channel geometry, etc.), the
median stone W50 should be increased from 200% to 600% depending on the severity of the locally high
turbulence. The thickness of the riprap layer should generally be twice the median stone diameter (D50)
or at least that of the maximum stone. The riprap should have a reasonably well graded assortment of stone
sizes within the following gradation:
1.25 ≤ Dmax/D50 ≤ 1.50
D15/D50 = 0.50
Dmin/D50 = 0.25
Detailed design methodology may be found in the Corps publication EM 1110-02-1601, Engineering and
Design – Hydraulic Design of Flood Control Channels. For a more detailed analysis and design procedure
for riprap requiring water surface profiles and estimates of tractive force, refer to the paper by Maynord et
al. in Journal of Hydraulic Engineering (A.S.C.E.), July 1989.
Riprap Filter Design
Riprap should be underlain by a sand and gravel filter (or filter fabric) to keep the fine materials in the
underlying channel bed from being washed through the voids in the riprap. Likewise, the filter material
must be selected so that it is not washed through the voids in the riprap. Adequate filters can usually be
provided by a reasonably well graded sand and gravel material where:
D15 < 5d85
The variable d85 refers to the sieve opening through which 85% of the material being protected will pass,
and D15 has the same interpretation for the filter material. A filter material with a D50 of 0.5 mm will
protect any finer material including clay. Where very large riprap is used, it is sometimes necessary to use
two filter layers between the material being protected and the riprap.
Example:
What embedded riprap design should be used to protect a streambank at a level culvert outfall where the
outfall velocities in the vicinity of the downstream toe are expected to be about 8 fps?
From Figure 4.4.1.A, W50 = 6.5 lbs, but since the downstream area below the outfall will be subjected to
severe turbulence, increase W50 by 400% so that:
W50 = 26 lbs, D50 = 8.0 inches
The gradation of the riprap is shown in Figure 4.4.1.B, and the minimum thickness would be 1 foot (from
Table 4.4.1.A); however, 16 inches to 24 inches of riprap thickness would provide some additional
insurance that the riprap will function properly in this highly turbulent area.
Figure 4.4.1.B shows that the gradation curve for ASTM C33, size number 57 coarse aggregate (used in
concrete mixes), would meet the filter criteria. Applying the filter criteria to the coarse aggregate
demonstrates that any underlying material whose gradation was coarser than that of a concrete sand would
be protected.
6 From a paper prepared by M. Schaefer, Dam Safety Section, Washington State Department of Ecology.
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FIGURE 4.4.1.A
MEAN CHANNEL VELOCITY VS. MEDIUM STONE WEIGHT (W50) AND EQUIVALENT STONE
DIAMETER
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FIGURE 4.4.1.B RIPRAP/FILTER EXAMPLE GRADATION CURVE
0.1
1
10
0 10 20 30 40 50 60 70 80 90 100
% Finer by WeightGrain Size (inches)20
RIP-RAP
Coarse Aggregate
size number 57
ASTM 14 C-33
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4.4.1.2 METHODS OF ANALYSIS
This section presents the methods of analysis for designing new or evaluating existing open channels
for compliance with the conveyance capacity requirements set forth in Section 1.2.4, “Core
Requirement #4: Conveyance System.”
DESIGN FLOWS
Design flows for sizing and assessing the capacity of open channels shall be determined using the
hydrologic analysis methods described in Chapter 3.
CONVEYANCE CAPACITY
There are three acceptable methods of analysis for sizing and analyzing the capacity of open channels:
1. Manning's equation for preliminary sizing
2. Direct Step backwater method
3. Standard Step backwater method.
Manning's Equation for Preliminary Sizing
Manning's equation is used for preliminary sizing of open channel reaches of uniform cross section and
slope (i.e., prismatic channels) and uniform roughness. This method assumes the flow depth (or normal
depth) and flow velocity remain constant throughout the channel reach for a given flow.
The charts in Figure 4.4.1.C and Figure 4.4.1.D may be used to obtain graphic solutions of Manning's
equation for common ditch sections. For conditions outside the range of these charts or for more precise
results, Manning’s equation can be solved directly from its classic forms shown in Equations (4-1) and (4-
2).
Table 4.4.1.B provides a reference for selecting the appropriate “n” values for open channels. A number of
engineering reference books, such as Open-Channel Hydraulics by V.T. Chow, may also be used as guides
to select “n” values. Figure 4.4.1.E contains the geometric elements of common channel sections useful in
determining area A, wetted perimeter WP, and hydraulic radius (R= A/WP).
If flow restrictions occur that raise the water level above normal depth within a given channel reach, a
backwater condition (or subcritical flow) is said to exist. This condition can result from flow restrictions
created by a downstream culvert, bridge, dam, pond, lake, etc., and even a downstream channel reach
having a higher flow depth. If backwater conditions are found to exist for the design flow, a backwater
profile must be computed to verify that the channel's capacity is still adequate as designed. The Direct Step
or Standard Step backwater methods presented in this section may be used for this purpose.
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TABLE 4.4.1.B VALUES OF ROUGHNESS COEFFICIENT “n” FOR OPEN CHANNELS
Type of Channel
and Description
Manning's
“n”*
(normal)
Type of Channel
and Description
Manning's
“n”*
(normal)
A. Constructed Channels
a. Earth, straight and uniform
1. Clean, recently completed
2. Gravel, uniform section,
clean
3. With short grass, few
weeds
b. Earth, winding and sluggish
1. No vegetation
2. Grass, some weeds
3. Dense weeds or aquatic
plants in deep channels
4. Earth bottom and rubble
sides
5. Stony bottom and weedy
banks
6. Cobble bottom and clean
sides
c. Rock lined
1. Smooth and uniform
2. Jagged and irregular
d. Channels not maintained,
weeds and brush uncut
1. Dense weeds, high as flow
depth
2. Clean bottom, brush on
sides
3. Same as #2, highest stage
of flow
4. Dense brush, high stage
B. Natural Streams
B-1 Minor streams (top width at
flood stage < 100 ft.)
a. Streams on plain
1. Clean, straight, full stage
no rifts or deep pools
2. Same as #1, but more
stones and weeds
3. Clean, winding, some pools
and shoals
4. Same as #3, but some
weeds
5. Same as #4, but more
stones
0.018
0.025
0.027
0.025
0.030
0.035
0.030
0.035
0.040
0.035
0.040
0.080
0.050
0.070
0.100
0.030
0.035
0.040
0.040
0.050
6. Sluggish reaches, weedy
deep pools
7. Very weedy reaches,
deep pools, or floodways
with heavy stand of
timber and underbrush
b. Mountain streams, no
vegetation in channel, banks
usually steep, trees and
brush along banks
submerged at high stages
1. Bottom: gravel, cobbles,
and few boulders
2. Bottom: cobbles with
large boulders
B-2 Floodplains
a. Pasture, no brush
1. Short grass
2. High grass
b. Cultivated areas
1. No crop
2. Mature row crops
3. Mature field crops
c. Brush
1. Scattered brush, heavy
weeds
2. Light brush and trees
3. Medium to dense brush
4. Heavy, dense brush
d. Trees
1. Dense willows, straight
2. Cleared land with tree
stumps, no sprouts
3. Same as #2, but with
heavy growth of sprouts
4. Heavy stand of timber, a
few down trees, little
undergrowth, flood stage
below branches
5. Same as #4, but with
flood stage reaching
branches
0.070
0.100
0.040
0.050
0.030
0.035
0.030
0.035
0.040
0.050
0.060
0.070
0.100
0.150
0.040
0.060
0.100
0.120
* Note: These “n” values are “normal” values for use in analysis of channels. For conservative design of channel capacity,
the maximum values listed in other references should be considered. For channel bank stability, the minimum values
should be considered.
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Direct Step Backwater Method
The Direct Step backwater method may be used to compute backwater profiles on prismatic channel
reaches (i.e., reaches having uniform cross section and slope) where a backwater condition or restriction to
normal flow is known to exist. The method may be applied to a series of prismatic channel reaches in
secession beginning at the downstream end of the channel and computing the profile upstream.
Calculating the coordinates of the water surface profile using this method is an iterative process achieved
by choosing a range of flow depths, beginning at the downstream end, and proceeding incrementally up to
the point of interest or to the point of normal flow depth. This is best accomplished by the use of a table
(see Figure 4.4.1.G) or computer programs (as discussed in “Computer Applications” in Section 4.4.1.2).
To illustrate analysis of a single reach, consider the following diagram:
Equating the total head at cross sections 1 and 2, the following equation may be written:
Sox + y1 + = y2 + + Sf x (4-8)
where, x = distance between cross sections (ft)
y1, y2 = depth of flow (ft) at cross sections 1 and 2
V1, V2 = velocity (fps) at cross sections 1 and 2
= energy coefficient at cross sections 1 and 2
So = bottom slope (ft/ft)
Sf = friction slope = (n2V2)/(2.21R1.33)
g = acceleration due to gravity, (32.2 ft/sec2)
If the specific energy E at any one cross-section is defined as follows:
E = y + (4-9)
and assuming = 1 = 2 where is the energy coefficient that corrects for the non-uniform distribution
of velocity over the channel cross section, Equations 4-10 and 4-11 can be combined and rearranged to
solve for x as follows:
g
V
2
2
1
1g
V
2
2
2
2
g
V
2
2
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Dx = (E2 - E1)/(So - Sf) = E/( So - Sf) (4-10)
Typical values of the energy coefficient are as follows:
Channels, regular section 1.15
Natural streams 1.3
Shallow vegetated flood fringes (includes channel) 1.75
For a given flow, channel slope, Manning's “n,” and energy coefficient , together with a beginning water
surface elevation y2, the values of x may be calculated for arbitrarily chosen values of y1. The coordinates
defining the water surface profile are obtained from the cumulative sum of x and corresponding values
of y.
The normal flow depth, yn, should first be calculated from Manning's equation to establish the upper limit
of the backwater effect.
Standard Step Backwater Method
The Standard Step Backwater Method is a variation of the Direct Step Backwater Method and may be used
to compute backwater profiles on both prismatic and non-prismatic channels. In this method, stations are
established along the channel where cross section data is known or has been determined through field
survey. The computation is carried out in steps from station to station rather than throughout a given
channel reach as is done in the Direct Step method. As a result, the analysis involves significantly more
trial-and-error calculation in order to determine the flow depth at each station.
Computer Applications
Because of the iterative calculations involved, use of a computer to perform the analysis is recommended.
The King County Backwater (KCBW) computer program included in the software package available
with this manual includes a subroutine, BWCHAN, based on the Standard Step backwater method, which
may be used for all channel capacity analysis. It can also be combined with the BWPIPE and BWCULV
subroutines to analyze an entire drainage conveyance system. A schematic description of the nomenclature
used in the BWCHAN subroutine is provided in Figure 4.4.1.H. See the KCBW program documentation
for further information.
There are a number of commercial software programs for use on personal computers that use variations of
the Standard Step backwater method for determining water surface profiles. The most common and widely
accepted program is called HEC-RAS, published and supported by the United States Army Corps of
Engineers Hydraulic Engineering Center. It is one of the models accepted by FEMA for use in performing
flood hazard studies for preparing flood insurance maps.
AGENDA ITEM # 8. a)
4.4.1 OPEN CHANNELS
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FIGURE 4.4.1.C DITCHES — COMMON SECTIONS
PROPERTIES OF DITCHES
DIMENSIONS HYDRAULICS
NO. Side Slopes B H W A WP R R(2/3)
D-1 – – 6.5″ 5′-0″ 1.84 5.16 0.356 0.502
D-1C – – 6″ 25′-0″ 6.25 25.50 0.245 0.392
D-2A 1.5:1 2′-0″ 1′-0″ 5′-0″ 3.50 5.61 0.624 0.731
B 2:1 2′-0″ 1′-0″ 6′-0″ 4.00 6.47 0.618 0.726
C 3:1 2′-0″ 1′-0″ 8′-0″ 5.00 8.32 0.601 0.712
D-3A 1.5:1 3′-0″ 1′-6″ 7′-6″ 7.88 8.41 0.937 0.957
B 2:1 3′-0″ 1′-6″ 9′-0″ 9.00 9.71 0.927 0.951
C 3:1 3′-0″ 1′-6″ 12′-0″ 11.25 12.49 0.901 0.933
D-4A 1.5:1 3′-0″ 2′-0″ 9′-0″ 12.00 10.21 1.175 1.114
B 2:1 3′-0″ 2′-0″ 11′-0″ 14.00 11.94 1.172 1.112
C 3:1 3′-0″ 2′-0″ 15′-0″ 18.00 15.65 1.150 1.098
D-5A 1.5:1 4′-0″ 3′-0″ 13′-0″ 25.50 13.82 1.846 1.505
B 2:1 4′-0″ 3′-0″ 16′-0″ 30.00 16.42 1.827 1.495
C 3:1 4′-0″ 3′-0″ 22′-0″ 39.00 21.97 1.775 1.466
D-6A 2:1 – 1′-0″ 4′-0″ 2.00 4.47 0.447 0.585
B 3:1 – 1′-0″ 6′-0″ 3.00 6.32 0.474 0.608
D-7A 2:1 – 2′-0″ 8′-0″ 8.00 8.94 0.894 0.928
B 3:1 – 2′-0″ 12′-0″ 12.00 12.65 0.949 0.965
D-8A 2:1 – 3′-0″ 12′-0″ 18.00 13.42 1.342 1.216
B 3:1 – 3′-0″ 18′-0″ 27.00 18.97 1.423 1.265
D-9 7:1 – 1′-0″ 14′-0″ 7.00 14.14 0.495 0.626
D-10 7:1 – 2′-0″ 28′-0″ 28.00 28.28 0.990 0.993
D-11 7:1 – 3′-0″ 42′-0″ 63.00 42.43 1.485 1.302
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SECTION 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS
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FIGURE 4.4.1.D DRAINAGE DITCHES — COMMON SECTIONS
AGENDA ITEM # 8. a)
4.4.1 OPEN CHANNELS
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FIGURE 4.4.1.E GEOMETRIC ELEMENTS OF COMMON SECTIONS
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FIGURE 4.4.1.F OPEN CHANNEL FLOW PROFILE COMPUTATION
Q = ____________ n = ____________ So = ____________ = ____________ Yn = ____________
y A R R4/3 V V2/2g E E Sf _
Sf
_
So - Sf
x x
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
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FIGURE 4.4.1.G DIRECT STEP BACKWATER METHOD – EXAMPLE
y A R R4/3 V V2/2g E E Sf _
Sf
_
So - Sf
x x
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
6.0 72.0 2.68 3.72 0.42 0.0031 6.0031 - 0.00002 - - - -
5.5 60.5 2.46 3.31 0.50 0.0040 5.5040 0.4990 0.00003 0.000025 0.00698 71.50 71.5
5.0 50.0 2.24 2.92 0.60 0.0064 5.0064 0.4976 0.00005 0.000040 0.00696 71.49 142.99
4.5 40.5 2.01 2.54 0.74 0.0098 4.5098 0.4966 0.00009 0.000070 0.00693 71.64 214.63
4.0 32.0 1.79 2.17 0.94 0.0157 4.0157 0.4941 0.00016 0.000127 0.00687 71.89 286.52
3.5 24.5 1.57 1.82 1.22 0.0268 3.5268 0.4889 0.00033 0.000246 0.00675 72.38 358.90
3.0 18.0 1.34 1.48 1.67 0.0496 3.0496 0.4772 0.00076 0.000547 0.00645 73.95 432.85
2.5 12.5 1.12 1.16 2.40 0.1029 2.6029 0.4467 0.00201 0.001387 0.00561 79.58 512.43
2.0 8.0 0.89 0.86 3.75 0.2511 2.2511 0.3518 0.00663 0.004320 0.00268 131.27 643.70
The step computations are carried out as shown in the above table. The values in each column of the table
are explained as follows:
Col. 1. Depth of flow (ft) assigned from 6 to 2 feet
Col. 2. Water area (ft2) corresponding to depth y in Col. 1
Col. 3 Hydraulic radius (ft) corresponding to y in Col. 1
Col. 4. Four-thirds power of the hydraulic radius
Col. 5. Mean velocity (fps) obtained by dividing Q (30 cfs) by the water area in Col. 2
Col. 6. Velocity head (ft)
Col. 7. Specific energy (ft) obtained by adding the velocity head in Col. 6 to depth of flow in Col. 1
Col. 8. Change of specific energy (ft) equal to the difference between the E value in Col. 7 and that
of the previous step.
Col. 9. Friction slope Sf, computed from V as given in Col. 5 and R4/3 in Col. 4
Col. 10. Average friction slope between the steps, equal to the arithmetic mean of the friction slope
just computed in Col. 9 and that of the previous step
Col. 11. Difference between the bottom slope, So, and the average friction slope, Sf
Col. 12. Length of the reach (ft) between the consecutive steps;
Computed by x = E/(So - Sf) or by dividing the value in Col. 8 by the value in Col. 11
Col. 13. Distance from the beginning point to the section under consideration. This is equal to the
cumulative sum of the values in Col. 12 computed for previous steps.
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SECTION 4.4 OPEN CHANNELS, FLOODPLAINS, AND FLOODWAYS
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gVECY2*1 2
1
FIGURE 4.4.1.H BWCHAN COMPUTER SUBROUTINE – VARIABLE DEFINITIONS
BWCHAN – VARIABLE DEFINITIONS
YC-IN Critical Depth (ft) at current section based on incoming flow rate.
YC-OT Critical Depth (ft) at current section based on outgoing flow rate.
YN-IN Normal Depth (ft) at current section based on incoming flow rate/channel grade.
YN-OT Normal Depth (ft) at current section based on outgoing flow rate/channel grade.
Y1 Final Water Depth (ft) at current cross section
N-Y1 Composite n-factor of current section for final depth, Y1.
A-Y1 Cross-sectional Area of current section for final depth, Y1.
WP-Y1 Wetted Perimeter (ft) of current section for final depth, Y1.
V-Y1 Average Velocity (fps) of current section for final depth, Y1.
E1 Total Energy Head (ft) at current section
E2 Total Energy Head (ft) at pervious or downstream section.
SF1 Friction Slope of current section.
SF2 Friction Slope of previous or downstream section.
DXY Distance (expressed as a fraction of the current reach length) from the previous
or downstream section to where the flow profile would intersect the final water
depth, Y1, assuming Y1 were to remain constant
EC Energy Coefficient ““
Q-TW The flow rate used to determine Tailwater Height from an inputted HW/TW Data
File.
TW-HT Tailwater Height.
Q-Y1 Flow rate (cfs) in channel at current section, for depth, Y1
VU-Y1 Upstream Velocity (fps) at current section for depth, Y1 (“Adjust” option).
V1-HD Channel Velocity Head (ft) at current section.
VU-HD Upstream Velocity Head (ft) at current section.
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4.4.2 FLOODPLAIN/FLOODWAY ANALYSIS
This section describes the floodplain/floodway studies required by Special Requirement #2, Flood Hazard
Area Delineation, in Section 1.3.2. Floodplain/floodway studies, as required by this manual, establish base
flood elevations and delineate floodplains and/or floodways when the City determines that a proposed
project contains or is adjacent to a flood hazard area for a river, stream, lake, wetland, closed depression,
marine shoreline, or other water feature. Furthermore, when development is proposed within the
floodplain, the floodplain/floodway study is used to show compliance with the critical areas code
(RMC 4-3-050) flood hazard area regulations.
There are four conditions affecting the requirements for floodplain/floodway studies. Each condition is
considered a threshold for determining the type of studies required and the documentation needed to meet
the study requirements. Each study threshold and related study requirements are shown in the table below,
and described further in this section.
Note that any projects or related flood studies that are expected to result in a change to Base Flood
Elevations published in FEMA Flood Insurance Studies and Rate Maps, must also comply with 44 CFR
Part 65.
TABLE 4.4.2.A FLOODPLAIN/FLOODWAY STUDY THRESHOLDS AND
REQUIREMENTS
Threshold Study Requirements
The project site is on land that is
outside of an already delineated
floodplain and above the floodplain's
base flood elevation based on best
available floodplain data.
No floodplain study
required
Show delineation of floodplain on
the site improvement plan and
indicate base flood elevation
Record a notice on title
See Section 4.4.2.1 for more details
The project site is on land that is at
least 10 feet above the ordinary high
water mark or 2 feet above the
downstream overflow elevation of a
water feature for which a floodplain
has not been determined.
Approximate Floodplain
Study per Section 4.4.2.2
Submit an engineering plan with
approximate base flood elevation
Record a notice on title
See Section 4.4.2.2 for more details
The project site does not meet the
above thresholds and is either on land
that is outside of an already
delineated Zone A floodplain (i.e.,
without base flood elevations
determined), or is adjacent to a water
feature for which a floodplain has not
been determined.
Minor Floodplain Study per
Section 4.4.2.3
Backwater model
Submit an engineering plan with
determined base flood elevation1
Record a notice on title
See Section 4.4.2.3 for more details
The project site is on land that is
partially or fully within an already
delineated floodplain of a river or
stream, or is determined by a Minor
Floodplain Study to be partially or fully
within the floodplain of a river or
stream.
Major Floodplain/Floodway
Study per Section 4.4.2.4
Show mapped floodplain/floodway
on the site improvement plan and
indicate base flood elevation
Record a notice on title
See further requirements in
Section 4.4.2.4
For any project site or study that is intended to result in a change to FEMA Flood Insurance Study or Rate
Maps, including changing published based flood elevations, the applicant must comply with documentation
and approval requirements of FEMA regulations 44 CFR Part 65.
1 For marine shorelines, refer to the FEMA Guidelines and Specifications for Flood Hazard Mapping Partners.
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4.4.2.1 NO FLOODPLAIN STUDY REQUIRED
IF the proposed project site is on land that is outside of an already delineated floodplain and is above the
already determined base flood elevation for that floodplain, based on best available floodplain data, THEN
no floodplain study is required.
In this situation, if the already determined floodplain covers any portion of the site, the boundary of that
floodplain and its base flood elevation must be shown on the project's site improvement plan. In addition,
a notice on title must be recorded for the site, alerting future property owners of the presence of a flood
hazard area on the site and its base flood elevation. The notice on title requirement may be waived if the
floodplain is not on any portion of the site.
4.4.2.2 APPROXIMATE FLOODPLAIN STUDY
If the proposed project site is on land that is at least 10 feet above the ordinary high water mark or 2 feet
above the downstream overflow elevation of a water feature for which the floodplain has not been
delineated, then an Approximate Floodplain Study may be used to determine an approximate floodplain
and base flood elevation.
The intent of the Approximate Floodplain Study is to reduce required analysis in those situations where
the project site is adjacent to a flood hazard area, but by virtue of significant topographical relief, is
clearly in no danger of flooding. The minimum 10 feet of separation from ordinary high water reduces the
level of required analysis for those projects adjacent to streams confined to deep channels or ravines, or
near lakes or wetlands. The minimum 2 feet clearance above the downstream overflow elevation is
intended to avoid flood hazard areas created by a downstream impoundment of water behind a road fill or
in a lake, wetland, or closed depression.
Use of the Approximate Floodplain Study requires submittal of an engineering plan7 showing the
proposed project site is at least 10 feet above the ordinary high water elevation of the water feature in
question, or at least 2 feet above the downstream overflow elevation of the water feature, whichever is
less, subject to the following conditions:
1. The design engineer preparing the engineering plan shall determine an approximate base flood
elevation and include a narrative describing his/her level of confidence in the approximate base flood
elevation. The narrative must include, but is not limited to, an assessment of potential backwater
effects (such as might result from nearby river flooding, for example); observations and/or anecdotal
information on water surface elevations during previous flood events; and an assessment of potential
for significantly higher future flows at basin build out. Note: Many of these issues will have been
addressed in a Level 1 downstream analysis, if required. Acceptance of the approximate base flood
elevation shall be at the sole discretion of the City. If the approximate base flood elevation is not
acceptable, a Minor Floodplain Study or Major Floodplain/Floodway Study may be required.
2. That portion of the site that is at or below the assumed base flood elevation must be delineated and
designated as a floodplain on the engineering plan, and a notice on title must be recorded for the site,
notifying future property owners of the approximate floodplain and base flood elevation.
4.4.2.3 MINOR FLOODPLAIN STUDY
IF the proposed project site does not meet the conditions for “no floodplain study required” per
Section 4.4.2.1 or for use of the Approximate Floodplain Study per Section 4.4.2.2, AND the project site
is either on land that is outside of an already delineated Zone A floodplain (i.e., without base flood
7 Engineering plan means a site improvement plan, including supporting documentation, stamped by a licensed civil engineer.
In some instances, CED review staff may determine that the proposed project is sufficiently above the clearances specified in
this exception and may not require an engineering plan. Typically, this is done for projects in Simplified Drainage Review that
clearly exceed minimum clearances and otherwise would not require engineering design.
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elevations determined) or is adjacent to a water feature for which a floodplain has not been determined,
THEN a Minor Floodplain Study may be used to determine the floodplain. However, if the Minor
Floodplain Study determines that all or a portion of the project site is at or below the base flood elevation
of a river or stream and thus within the floodplain, then the applicant must either redesign the project site
to be out of the floodplain or complete a Major Floodplain/Floodway Study per Section 4.4.2.4.
Use of the Minor Floodplain Study requires submittal of an engineering plan and supporting calculations.
That portion of the site that is at or below the determined base flood elevation must be delineated and
designated as a floodplain on the engineering plan, and a notice on title must be recorded for the site,
notifying future property owners of the floodplain and base flood elevation.
Methods of Analysis
For streams without a floodplain or flood hazard study, or for drainage ditches or culvert headwaters,
the base flood elevation and extent of the floodplain shall be determined using the Direct Step backwater
method, Standard Step backwater method, or the King County Backwater computer program, as described
in Section 4.4.1.2.
For lakes, wetlands, and closed depressions without an approved floodplain or flood hazard study, the
base flood elevation and the extent of the floodplain shall be determined using the “point of compliance
technique” described in Section 3.3.6.
4.4.2.4 MAJOR FLOODPLAIN/FLOODWAY STUDY
The floodplain analysis shall be based on the 100-year storm event using existing land use hydrology
except as noted in the paragraph titled “Flood Flows from Adopted Basin Plan Information.”
IF the proposed project site is on land that is partially or fully within an already delineated floodplain of a
river or stream, or determined by a Minor Floodplain Study to be partially or fully within the floodplain
of a river or stream, THEN a Major Floodplain/Floodway Study is required to determine the floodplain,
floodway, and base flood elevation in accordance with the methods and procedures presented in this
section. This information will be used by the City to evaluate the project's compliance with regulations for
development or improvements within the floodplain.
Major Floodplain/Floodway Studies must conform to FEMA regulations described in Part 65 of 44 Code
of Federal Regulations (CFR). In addition, the following information must be provided and procedures
performed.
INFORMATION REQUIRED
The applicant shall submit the following information for review of a floodplain/floodway analysis in
addition to that required for the drainage plan of a proposed project. This analysis shall extend upstream
and downstream a sufficient distance to adequately include all backwater conditions that may affect
flooding at the site and all reaches that may be affected by alterations to the site.
Floodplain/Floodway Map
A Major Floodplain/Floodway Study requires submittal of five copies of a separate floodplain/floodway
map stamped by a licensed civil engineer and a professional land surveyor registered in the State of
Washington (for the base survey). The map must accurately locate any proposed development with respect
to the floodplain and floodway, the channel of the stream, and existing development in the floodplain; it
must also supply all pertinent information such as the nature of any proposed project, legal description of
the property on which the project would be located, fill quantity, limits and elevation, the building floor
elevations, flood-proofing measures, and any use of compensatory storage.
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The map must show elevation contours at a minimum of 2-foot vertical intervals and shall comply with
survey and map guidelines published in the FEMA publication Guidelines and Specifications for Flood
Hazard Mapping Partners. The map must show the following:
Existing elevations and ground contours;
Locations, elevations and dimensions of existing structures, and fills;
Size, location, elevation, and spatial arrangement of all proposed structures, fills and excavations,
including proposed compensatory storage areas, with final grades on the site;
Location and elevations of roadways, water supply lines, and sanitary sewer facilities, both existing
and proposed.
Study Report
A Major Floodplain/Floodway Study also requires submittal of two copies of a study report, stamped by a
licensed civil engineer, which must include calculations or any computer analysis input and output
information as well as the following additional information:
1. Valley cross sections showing the channel of the river or stream, the floodplain adjoining each side of
the channel, the computed FEMA floodway, the cross-sectional area to be occupied by any proposed
development, and all historic high water information.
2. Profiles showing the bottom of the channel, the top of both left and right banks, and existing and
proposed base flood water surfaces.
3. Plans and specifications for flood-proofing any structures and fills, construction areas, materials
storage areas, water supply, and sanitary facilities within the floodplain.
4. Complete printout of input and output (including any error messages) for HEC-RAS. Liberal use of
comments will assist in understanding model logic and prevent review delays.
5. One ready-to-run digital copy of the HEC-RAS input file used in the study. Data shall be submitted
in an electronic format.
6. The applicant shall prepare a written summary describing the model development calibration,
hydraulic analysis, and floodway delineation. The summary shall also include an explanation of
modeling assumptions and any key uncertainties.
DETERMINING FLOOD FLOWS
The three techniques used to determine the flows used in the analysis depend on whether gage data is
available or whether a basin plan has been adopted. The first technique is for basins in adopted basin plan
areas. The second technique is used if a gage station exists on the stream. The third technique is used on
ungaged catchments or those with an insufficient length of record. In all cases, the design engineer shall be
responsible for assuring that the hydrologic methods used are technically reasonable and conservative,
conform to the Guidelines and Specifications for Flood Hazard Mapping Partners, and are acceptable by
FEMA.
Flood Flows from Adopted Basin Plan Information
For those areas where the City or King County has adopted a basin plan since 1986, flood flows may be
determined using information from the adopted basin plan. The hydrologic model used in the basin plan
shall be updated to include the latest changes in zoning, or any additional information regarding the basin
that has been acquired since the adoption of the basin plan.
Flood Flows from Stream Gage Data
Flood flows from stream gage data may be determined using HEC-FFA, which uses the Log-Pearson Type
III distribution method as described in Guidelines for Determining Flood Flow Frequency, Bulletin 17B of
the Hydrology Committee, prepared by the Interagency Advisory Committee on Water Data (1982). Refer
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to the FEMA Guidelines and Specifications for Flood Hazard Mapping Partners to verify the most current
requirements. Use of HEC-FFA is subject to the following requirements:
1. This technique may be used only if data from a gage station in the basin is available for a period of at
least ten years that is representative of the current basin conditions.
2. If the difference in the drainage area on the stream at the study location and the drainage area to a
gage station on the stream at a different location in the same basin is less than or equal to 50 percent,
the flow at the study location shall be determined by transferring the calculated flow at the gage to the
study location using a drainage area ratio raised to the 0.86 power, as in the following equation:
QSS = QG (ASS/AG)0.86 (4-11)
where QSS = estimated flow for the given return frequency on the stream at the study location
QG = flow for the given return frequency on the stream at the gage location
ASS = drainage area tributary to the stream at the study location
AG = drainage area tributary to the stream at the gage location
3. If the difference in the drainage area at the study location and the drainage area at a gage station in the
basin is more than 50 percent and a basin plan has not been prepared, a continuous model shall be
used as described below to determine flood flows at the study location.
4. In all cases where dams or reservoirs, floodplain development, or land use upstream may have altered
the storage capacity or runoff characteristics of the basin so as to affect the validity of this technique, a
continuous model shall be used to determine flood flows at the study location.
Flood Flows from a Calibrated Continuous Model
Flood flows may be determined by utilizing a continuous flow simulation model such as HSPF. Where
flood elevations or stream gage data are available, the model shall be calibrated; otherwise, regional
parameters8 may be used.
DETERMINING FLOOD ELEVATIONS, PROFILES, AND FLOODWAYS
Reconnaissance
The applicant's design engineer is responsible for the collection of all existing data with regard to flooding
in the study area. This shall include a literature search of all published reports in the study area and
adjacent communities, and an information search to obtain all unpublished information on flooding in the
immediate and adjacent areas from federal, state, and local units of government. This search shall include
specific information on past flooding in the area, drainage structures such as bridges and culverts that
affect flooding in the area, available topographic maps, available flood insurance rate maps, photographs
of past flood events, and general flooding problems within the study area. A field reconnaissance shall be
made by the applicant's design engineer to determine hydraulic conditions of the study area, including type
and number of structures, locations of cross sections, and other parameters, including the roughness
values necessary for the hydraulic analysis.
Base Data
Cross sections used in the hydraulic analysis shall be representative of current channel and floodplain
conditions obtained by surveying. When cross-sections data is obtained from other studies, the data shall
be confirmed to represent current channel and floodplain conditions, or new channel cross-section data
shall be obtained by field survey. Topographic information obtained from aerial photographs may be used
in combination with surveyed cross sections in the hydraulic analysis. The elevation datum of all
8 Dinacola, 1990. U.S.G.S., Characterization and Simulation of Rainfall-Runoff Relations for Headwater Basins in Western King
and Snohomish Counties, Washington.
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information used in the hydraulic analysis shall be specified. All information shall be referenced directly
to NAVD 1988 (and include local correlation to NGVD 1929) unless otherwise approved by the City. See
Table 4.4.2.B for correlations of other datum to NAVD 1988.
Methodology
Flood profiles and floodway studies shall be calculated using the U.S. Army Corps of Engineers’ HEC-
RAS computer model (or subsequent revisions).
Floodway Determination
1. Flood hazard areas are identified by the Federal Insurance Administration in a scientific and
engineering report entitled the Flood Insurance Study for the City of Renton, dated September 29,
1989, and any subsequent revision, with accompanying flood insurance maps which are hereby
adopted by reference and declared to be a part of this Section. The flood insurance study is on file at
the Public Works Department.
Previous Floodplain Studies
If differences exist between a study previously approved by the City and the applicant’s design engineer’s
calculated hydraulic floodways or flood profiles, the design engineer shall provide justification and obtain
City approval for these differences.
Zero-Rise Calculation
For a zero-rise analysis, the flow profile for the existing and proposed site conditions shall be computed
and reported to the nearest 0.01 foot. A zero-rise analysis requires only comparisons of the computed
water surface elevations and energy grade lines for the existing and proposed conditions. Such
comparisons are independent of natural dynamics and are not limited by the accuracy of the model’s
absolute water surface predictions.
Adequacy of Hydraulic Model
At a minimum, the City considers the following factors when determining the adequacy of the hydraulic
model and flow profiles for use in floodway analysis:
1. Cross section spacing
2. Differences in energy grade
Note: Significant differences in the energy grade from cross section to cross section are an indication that
cross sections should be more closely spaced or that other inaccuracies exist in the hydraulic model.
1. Methods for analyzing the hydraulics of structures such as bridges and culverts
2. Lack of flow continuity
3. Use of a gradually-varied flow model
Note: In certain circumstances (such as weir flow over a levee or dike, flow through the spillway of a dam,
or special applications of bridge flow), rapidly-varied flow techniques shall be used in combination with a
gradually-varied flow model.
1. Manning's “n” values
2. Calibration of the hydraulic model with past flood events
3. Special applications. In some cases, HEC-RAS alone may not be sufficient for preparing the
floodplain/floodway analysis. This may occur where sediment transport, two-dimensional flow, or
other unique hydraulic circumstances affect the accuracy of the HEC-RAS hydraulic model. In these
cases, the applicant shall obtain City approval of other methods proposed for estimating the water
surface profiles.
AGENDA ITEM # 8. a)
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TABLE 4.4.2.B DATUM CORRELATIONS
(for general reference use only, values are approximate)
Correlation
From To
(Snoq.
Valley)
NAVD
1988* KCAS
U.S.
Engineers
City of
Seattle
NGVD,
USGS &
USC &
GS 1947
Seattle Area
Tide Tables
&
Navigation
Charts 1954
& Later
NAVD 1988* (Upper
Snoqualmie Valley)
– -3.58 3.44 -9.54 -3.49 2.98
KCAS 3.58 – 7.02 -5.96 0.09 6.56
U.S. Engineers -3.22 -7.02 – -12.98 -6.93 -0.46
City of Seattle 9.54 5.96 12.98 – 6.05 12.52
NGVD, USGS &
USC& GS 1947 (adjusted
to the 1929 datum)
3.49 -0.09 6.93 -6.05 – 6.47
Seattle Area Tide Tables
& Navigation Charts 1954
& Later (based on epoch
1924-1942)
-2.98 -6.56 0.46 -12.52 -6.47 –
Design Tidal Tailwater
Elevation
12.08 8.50 15.52 2.54 8.59 15.06
Mean Higher High Water
(MHHW)
8.34 4.76 11.78 -1.20 4.85 11.32
Mean High Water (MHW) 7.49 3.91 10.93 -2.05 4.00 10.47
Mean Low Water (MLW) -0.16 -3.74 3.28 -9.70 -3.65 2.82
Mean Lower Low Water
(MLLW)
-2.98 -6.56 0.46 -12.52 -6.47 0.00
* Varies, contact the City of Renton for datum correlation for this and other areas.
KCAS datum = Sea Level Datum 1929 (a.k.a. NGVD 1929)
KCAS = King County Aerial Survey
NAVD = North American Vertical Datum
NGVD = National Geodetic Vertical Datum
USGS = United States Geologic Survey
USC & GS = US Coast and Geodetic Survey
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AGENDA ITEM # 8. a)
2022 City of Renton Surface Water Design Manual 6/22/2022
CHAPTER 5
FLOW CONTROL
DESIGN
CITY OF RENTON
SURFACE WATER
DESIGN MANUAL
Section Page
5.1 Detention Facilities 5-3
5.1.1 Detention Ponds 5-3
5.1.2 Detention Tanks 5-17
5.1.3 Detention Vaults 5-21
5.1.4 Control Structures 5-25
5.1.5 Parking Lot Detention 5-35
5.1.6 Roof Detention 5-35
5.1.7 Simple Detention Pond for Cleared
Areas 5-35
5.1.8 Alternative Detention Systems 5-42
5.2 Infiltration Facilities 5-45
5.2.1 General Requirements for Infiltration
Facilities 5-45
5.2.2 Infiltration Ponds 5-56
5.2.3 Infiltration Tanks 5-59
5.2.4 Infiltration Vaults 5-62
5.2.5 Infiltration Trenches 5-64
5.2.6 Alternative Infiltration Systems 5-65
5.2.7 Small Infiltration Basins 5-66
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AGENDA ITEM # 8. a)
CITY OF RENTON SURFACE WATER DESIGN MANUAL
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CHAPTER 5
FLOW CONTROL DESIGN
This chapter presents the City approved methods, criteria, and details for hydraulic analysis and design of
flow control facilities pursuant to Core Requirement #3, “Flow Control” (see Section 1.2.3). Flow control
facilities, as described in this manual, are detention or infiltration facilities engineered to meet a specified
discharge performance. Four terms are commonly used to describe flow control facilities in the City:
detention facilities, retention facilities, infiltration facilities, and R/D (Retention/Detention) facilities. A
detention facility, by definition, temporarily stores surface water runoff and discharges it at a reduced rate.
A retention facility stores water longer and effectively has no surface outflow (outflow occurs by
evaporation or soaking into the ground). Infiltration facilities are retention facilities that rely entirely on
the soaking of collected surface water into the ground. The term R/D facility has been used in previous
versions of this manual to generally refer to all flow control facilities.
On-site BMPs, also known as low impact development (LID) BMPs, are methods and designs for
dispersing, infiltrating, or otherwise reducing or preventing development-related increases in runoff at or
near the sources of those increases. On-site BMPs include, but are not limited to, preservation and use of
native vegetated surfaces to fully disperse runoff; use of other pervious surfaces to disperse runoff; roof
downspout infiltration; permeable pavement; bioretention; and reduction of development footprint. On-
site BMPs are required pursuant to Core Requirement #9, “On-Site BMPs” (see Section 1.2.9). Design
criteria for on-site BMPs are included in Appendix C of this manual.
The figures included in this chapter are provided as schematic representations and should not be used for
design. Refer to the City of Renton Standard Details for specific design information. The figures provided
in this chapter illustrate one example of how the flow control facility design criteria may be applied.
Although the figures are meant to illustrate many of the most important design criteria, they may not show
all criteria that apply. In general, the figures are not used to specify requirements unless they are indicated
elsewhere in the manual. If this manual refers to a standard detail not included in the City of Renton
Standard Details, the applicant shall use the figure provided in this manual.
Chapter Organization
The information in this chapter is organized into the following four main sections:
Section 5.1, “Detention Facilities”
Section 5.2, “Infiltration Facilities”
These sections begin on odd pages so the user can insert tabs if desired for quicker reference.
Required vs. Recommended Design Criteria
Both required and recommended design criteria are presented in this chapter. Criteria stated using “shall”
or “must” are mandatory, to be followed unless there is a good reason to deviate as allowed by the
adjustment process (see Section 1.4). These criteria are required design criteria and generally affect
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facility performance or critical maintenance factors. Sometimes options are stated as part of the required
design criteria using the language “should” or “may.” These criteria are recommended design criteria,
but are closely related to the required criteria, so they are placed in the same section.
Use of Materials
Galvanized metals leach zinc into the environment, especially in standing water situations. High zinc
concentrations, sometimes in the range that can be toxic to aquatic life, have been observed in the region.
Therefore, use of galvanized materials in flow control facilities and on-site BMPs should be avoided.
Where other metals, such as aluminum or stainless steel, or plastics are available, they shall be used.
Allowable materials are specified in the Design Criteria for the facility.
AGENDA ITEM # 8. a)
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5.1 DETENTION FACILITIES
This section presents the methods, criteria, and details for design and analysis of detention facilities. These
facilities provide for the temporary storage of increased surface water runoff resulting from development
pursuant to the performance standards set forth in Core Requirement #3, “Flow Control” (see
Section 1.2.3).
There are three primary types of detention facilities described in this section: detention ponds, tanks, and
vaults. The information presented in this section is organized as follows:
Section 5.1.1, “Detention Ponds”
“Design Criteria,” Section 5.1.1.1
“Methods of Analysis,” Section 5.1.1.2
Section 5.1.2, “Detention Tanks”
“Design Criteria,” Section 5.1.2.1
“Methods of Analysis,” Section 5.1.2.2
Section 5.1.3, “Detention Vaults”
“Design Criteria,” Section 5.1.3.1
“Methods of Analysis,” Section 5.1.3.2
Section 5.1.4, “Control Structures”
“Design Criteria,” Section 5.1.4.1
“Methods of Analysis,” Section 5.1.4.2
Section 5.1.5, “Parking Lot Detention”
Section 5.1.6, “Roof Detention”
Section 5.1.7, “Simple Detention Pond for Cleared Areas”
“Design Criteria,” Section 5.1.7.1
“Methods of Analysis,” Section 5.1.7.2
Section 5.1.8, “Alternative Detention Systems”
“Design Criteria,” Section 5.1.8.1
“Methods of Analysis,” Section 5.1.8.2.
5.1.1 DETENTION PONDS
Open ponds are the most desirable detention facilities for controlling runoff from developed areas. The
design criteria in Section 5.1.1.1 are for detention ponds. However, many of the criteria also apply to
infiltration ponds (Section 5.2.2), and water quality wetponds and combined detention/wetponds
(Section 6.4).
Dam Safety Compliance
Detention ponds and other open impoundment facilities must comply with requirements for dam safety
(WAC 173-175). Under current regulations (as of February 2012), if the impoundment has a storage
capacity (including both water and sediment storage volumes) greater than 10 acre-feet above natural
ground level and a dam height of more than 6 feet, then dam safety design and review are required by the
Washington State Department of Ecology (Ecology). If the storage capacity is less than 10 acre-feet above
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natural ground level, then the facility is exempt from Ecology review. If the dam height is less than 6 feet
but capacity is greater than 10 acre-feet, then Ecology reviews on a case-by-case-basis to determine the
hazard potential downstream in the event of a failure.
5.1.1.1 DESIGN CRITERIA
Schematic representations of detention ponds are shown in Figure 5.1.1.A through Figure 5.1.1.D.
Control structure details are described in Section 5.1.4.
General
1. Ponds must be designed as flow-through systems (however, parking lot storage may be utilized
through a back-up system; see Section 5.1.5). Developed flows must enter through a conveyance
system separate from the control structure and outflow conveyance system. Maximizing distance
between the inlet and outlet is encouraged to promote sedimentation.
2. Pond bottoms shall be level and be located a minimum of 0.5 feet below the inlet and outlet to
provide sediment storage.
3. Outflow control structures shall be designed as specified in Section 5.1.4.
4. Detention ponds preceding required water quality treatment facilities must meet the liner requirements
described in Section 6.2.4 (Facility Liners) to ensure groundwater protection.
5. A geotechnical analysis and report is required if located within 200 feet of a steep slope hazard area
or landslide hazard OR if the facility is located within a setback distance from top of slope equal to
the total vertical height of the slope area that is steeper than 15%. The geotechnical analysis must
consider cumulative impacts from the project and surrounding areas under full built-out conditions.
6. The perimeter of all new flow control and/or water quality treatment facilities shall be landscaped in
accordance with RMC 4-4-070 and Section 5.1.
Side Slopes
1. Side slopes (interior and exterior) shall be no steeper than 3H:1V.
2. Pond walls may be vertical retaining walls, provided: (a) they are constructed of reinforced concrete
per Section 5.1.3; (b) a fence is provided along the top of the wall; (c) at least 25% of the pond
perimeter will be a vegetated soil slope not steeper than 3H:1V; and (d) the design is stamped by a
licensed structural civil engineer.
Embankments
1. Pond berm embankments higher than 6 feet shall require design by a geotechnical engineer.
2. For berm embankments 6 feet or less, the minimum top width shall be 6 feet, or as recommended by
a geotechnical engineer.
3. Pond berm embankments must be constructed on native consolidated soil (or adequately compacted
and stable fill soils analyzed by a geotechnical engineer) free of loose surface soil materials, roots, and
other organic debris.
4. Pond berm embankments greater than 4 feet in height must be constructed by excavating a key equal
to 50% of the berm embankment cross-sectional height and width. This requirement may be waived if
specifically recommended by a geotechnical engineer.
5. The berm embankment shall be constructed of soil placed in 6-inch lifts compacted to at least 95%
of maximum dry density, within 2 percentage points of the optimum moisture content, modified
proctor method ASTM D1557. Density tests shall be performed for each lift to confirm compliance
with this specification. The soil used for construction shall have the following soil characteristics: a
minimum of 20% silt and clay, a maximum of 60% sand, a maximum of 60% silt and clay, with
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nominal gravel and cobble content. Note: In general, excavated glacial till is well suited for berm
embankment material.
6. Anti-seepage collars must be placed on outflow pipes in berm embankments impounding water
greater than 8 feet in depth at the design water surface.
Overflow
1. In all ponds, tanks, and vaults, a primary overflow (usually a riser pipe within the control structure;
see Section 5.1.4.2) must be provided to bypass the 100-year, 15-minute developed peak flow over or
around the restrictor system. This assumes the facility will be full due to plugged orifices or high
inflows; the primary overflow is intended to protect against breaching of a pond embankment (or
overflows of the upstream conveyance system, in the case of a detention tank or vault). The design
must provide controlled discharge directly into the downstream conveyance system or another
acceptable discharge point.
2. A secondary inlet to the control structure must be provided in ponds as additional protection against
overtopping should the inlet pipe to the control structure become plugged. A grated opening
(“jailhouse window”) in the control structure manhole functions as a weir (see schematic
representation in Figure 5.1.1.B) when used as a secondary inlet. Note: The maximum circumferential
length of this opening shall not exceed one-half the control structure circumference. A “birdcage”
overflow structure as shown in the schematic representation in Figure 5.1.1.C may also be used as a
secondary inlet.
Emergency Overflow Spillway
1. In addition to the above overflow requirements, ponds must have an emergency overflow spillway
sized to pass the 100-year, 15-minute developed peak flow in the event of total control structure
failure (e.g., blockage of the control structure outlet pipe) or extreme inflows. Emergency overflow
spillways are intended to control the location of pond overtopping and direct overflows back into the
downstream conveyance system or other acceptable discharge point.
2. Emergency overflow spillways must be provided for ponds with constructed berms over 2 feet in
height, or for ponds located on grades in excess of 5%. As an option for ponds with berms less than
2 feet in height and located at grades less than 5%, emergency overflow may be provided by an
emergency overflow structure, such as a Type II manhole fitted with a birdcage as shown in
the schematic representation in Figure 5.1.1.C. The emergency overflow structure must be designed
to pass the 100-year developed peak flow, with a minimum 6 inches of freeboard, directly to the
downstream conveyance system or another acceptable discharge point. Where an emergency overflow
spillway would discharge to a slope steeper than 15%, consideration should be given to providing an
emergency overflow structure in addition to the spillway.
3. The emergency overflow spillway shall be armored in conformance with Table 4.2.2.A. The spillway
shall be armored full width, beginning at a point midway across the berm embankment and extending
downstream to where emergency overflows re-enter the conveyance system (see
Figure 5.1.1.B).
4. Design of emergency overflow spillways requires the analysis of a broad-crested trapezoidal weir as
described in Section 5.1.1.2. Either one of the weir sections shown in the schematic representations in
Figure 5.1.1.B may be used.
Access Requirements
1. Maintenance access road(s) shall be provided to the control structure and other drainage structures
associated with the pond (e.g., inlet, emergency overflow or bypass structures). Manhole and catch
basin lids must be in or at the edge of the access road and at least three feet from a property line. Rims
shall be set at the access road grade.
2. An access ramp is required for removal of sediment with a trackhoe and truck. The ramp must extend
to the pond bottom if the pond bottom is greater than 1,500 square feet (measured without the ramp)
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and it may end at an elevation 4 feet above the pond bottom, if the pond bottom is less than
1,500 square feet (measured without the ramp), provided the pond side slopes are 3H:1V or flatter.
3. Intent: On large, deep ponds, truck access to the pond bottom via an access ramp is necessary so
loading can be done in the pond bottom. On small deep ponds, the truck can remain on the ramp for
loading. On small shallow ponds, a ramp to the bottom may not be required if the trackhoe can load a
truck parked at the pond edge or on the internal berm of a wetpond or combined pond (trackhoes can
negotiate interior pond side slopes).
4. The internal berm of a wetpond or combined detention and wetpond may be used for access if it is no
more than 4 feet above the first wetpool cell, if the first wetpool cell is less than 1500 square feet
(bottom area measured without the ramp), and if it is designed to support a loaded truck, considering
the berm is normally submerged and saturated.
5. Access ramps shall meet the requirements for design and construction of access roads specified
below.
6. All control structures shall have round, solid locking lids with 5/8-inch diameter Allen head cap
screws (see the City of Renton Standard Details).
7. Access shall be limited by a double-posted gate if a fence is required, or by bollards. Bollards shall be
designed in accordance with the City of Renton Standard Details.
Design of Access Roads
Access roads shall meet the following design criteria:
1. Maximum grade shall be 15% for asphalt paving and 12% for gravel or modular grid paving.
2. Outside turning radius shall be 40 feet, minimum.
3. Fence gates shall be located only on straight sections of road.
4. Access roads shall be 15 feet in width on curves and 12 feet on straight sections.
5. A paved apron shall be provided where access roads connect to paved public roadways. The apron
shall be consistent with driveway details in the City of Renton Standard Details.
Construction of Access Roads
Access roads shall be constructed with an asphalt, concrete or gravel surface, or modular grid pavement.
Access roads must conform to the City of Renton Standard Details for residential or rural minor access
streets. Modular grid pavement shall meet manufacturer’s specifications. Where access roads pass over
emergency overflow spillways, a HMA wearing course is required (see Figure 5.1.1.B).
Fencing
1. All ponds and other similar facilities, as determined by the City, shall be fenced. On stormwater
facilities to be maintained by the City, a fence shall be placed at the top of the berm with the
maintenance access road in the inside of the fence; or 5 feet minimum from the top of berm if there
is no maintenance access road allowing proper maintenance access of the facility.
2. Fences shall be 6 feet in height. For example designs, see WSDOT Standard Plan L-2, Type 1 or
Type 3 chain link fence.
3. Access road gates shall be 16 feet in width consisting of two swinging sections 8 feet in width.
Additional vehicular access gates may be required as needed to facilitate maintenance access.
4. Pedestrian access gates (if needed) shall be 4 feet in width.
5. Fence material shall be black or green bonded vinyl chain link. The following apply:
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a) Vinyl coating shall be compatible with the surrounding environment (e.g., green in open, grassy
areas and black in wooded areas). All posts, cross bars, and gates shall be coated the same color as
the vinyl clad fence fabric.
b) Fence posts and rails shall conform to WSDOT Standard Plan L-2 for Types 1, 3, or 4 chain link
fence.
6. Metal baluster fences are allowed where the fence will be owned and maintained by a private
property owner or homeowners association. Fence maintenance requirements shall be a condition of
subdivision approval, and a statement detailing maintenance responsibility of the fence will be placed
in the final plat. Uniform Building Code standards shall apply.
7. Wood fences are allowed in subdivisions where the fence will be owned and maintained by a private
property owner or homeowners association. Fence maintenance requirements shall be a condition of
subdivision approval, and a statement detailing maintenance responsibility of the fence will be placed
in the final plat.
8. Wood fences shall have pressure treated1 posts (ground contact rated) either set in 24-inch deep
concrete footings or attached to footings by steel brackets. Rails and fence boards shall be cedar.
Signage
Detention ponds, infiltration ponds, wetponds, and combined ponds to be maintained by the City shall
have a sign placed for maximum visibility from adjacent streets, sidewalks, and paths. The sign shall meet
the design and installation requirements illustrated in the City of Renton Standard Details. The fence gate
must be posted with a 12 inch by 18 inch “No Trespassing” sign, unless otherwise approved by the City.
Right-of-Way
1. Open detention ponds shall not be located in dedicated public road right-of-way.
2. Detention ponds to be maintained by the City, along with the perimeter landscaping shall be in a
stormwater tract granted and conveyed with all maintenance obligations (excluding maintenance of
the drainage facilities contained therein) to the property owners. Each property lot owner within the
subdivision shall have an equal and undivided interest in the maintenance of the stormwater tract and
landscaping features. Any tract not abutting public right-of-way will require a 15-foot-wide extension
of the tract to an acceptable access location. An underlying easement under and upon said tract shall
be dedicated to the City for the purpose of operating, maintaining, improving and repairing the
drainage facilities contain therein.
3. Detention ponds to be maintained by a private property owner or homeowners association shall create
stormwater facilities within a private tract or easement or construct the detention pond onsite.
Setbacks
1. A setback of 5 feet from the toe of the exterior slope, retaining walls and rockeries to the tract or
property line is required for City-maintained ponds and recommended for privately maintained ponds.
2. The tract or property line on a detention pond cut slope shall be setback 5 feet from the emergency
overflow water surface.
3. The detention pond water surface at the pond outlet invert elevation shall be setback 100 feet from
proposed or existing septic system drainfields. This setback may be reduced with written approval
of the Public Health – Seattle & King County.
4. The detention pond design water surface shall be a minimum of 200 feet from any steep slope hazard
area or landslide hazard. Upon analysis and approval of a licensed geotechnical engineer or
engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider
cumulative impacts from the project and surrounding areas under full built- out conditions.
1 Fence posts represent a rare exception to the rule of no treated lumber. Ground contact requires pressure treatment.
AGENDA ITEM # 8. a)
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5. The detention pond design water surface shall be set back a minimum distance from top of slope equal
to the total vertical height of a slope area that is steeper than 15%. Upon analysis and approval of a
licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The
geotechnical analysis must consider cumulative impacts from the project and surrounding areas under
full built- out conditions.
Seeps and Springs
Intermittent seeps along cut slopes are typically fed by a shallow groundwater source (interflow) flowing
along a relatively impermeable soil stratum. These flows are storm driven and should discontinue after a
few weeks of dry weather. The approved continuous runoff model accounts for this shallow groundwater
component and no special provisions are needed when directing these flows through the flow control
facility. However, more continuous seeps and springs, which extend through longer dry periods, are likely
from a deeper groundwater source. When continuous flows are intercepted and directed through flow
control facilities, adjustments to the approved facility design may be required to account for the additional
base flow (unless already considered in design). If uncertain at the time of construction, the situation may
be monitored while the facility is under maintenance and defect financial guarantee. Adjustments to the
facility may be required prior to the release of the financial guarantee.
Planting Requirements
Exposed earth on the pond bottom and interior side slopes shall be planted or seeded with an appropriate
seed mixture. All remaining areas of the tract must either be planted with grass, or be landscaped in
accordance with the standards below and mulched with a 4-inch cover of hog fuel or shredded wood
mulch.2
Landscaping
Landscaping is not optional; it is required on all stormwater/landscaping tracts. Landscaping is required in
those areas of the tract that will not impact the functionality or maintenance of the drainage facilities. For
stormwater ponds to be maintained by the City, landscaping inside the fence shall be planted with grass,
low-growing shrubs, or groundcovers that are no- to low-maintenance and do not impede other facility
maintenance activities (as required in Section 5.1). Landscaping maintained by the City and comprised of
species other than grass is subject to City approval. Landscaping inside the fence is allowed for storm
water facilities to be privately maintained provided that the landscaping complies with the requirements of
RMC 4‐4‐070F8, Storm Drainage Facilities.
The following requirements shall apply:
1. No trees or shrubs may be planted within 10 feet of inlet or outlet pipes or manmade drainage
structures such as catch basins, spillways or flow spreaders. Species with roots that seek water, such
as willow or poplar, should be avoided within 30 feet of pipes or manmade structures.
2. Planting is restricted on berms that impound water either permanently or temporarily during
storms. If the pond is City-maintained, then landscaping with trees and large shrubs that may
compromise berm integrity are prohibited in the inside slope of the pond and trees are prohibited on
any drainage-related berms.
a) Trees or tall shrubs may not be planted on portions of water-impounding berms taller than four
feet high. Only grasses and low-growing shrubs or groundcovers may be planted on berms taller
than four feet.
Intent: Grasses and low-growing groundcovers allow unobstructed visibility of berm slopes for
detecting potential dam safety problems such as animal burrows, slumping, or fractures in the
berm.
2 Shredded wood mulch is made from shredded tree trimmings, usually from trees cleared onsite. It must be free of garbage and
weeds and may not contain excessive resin, tannin, or other material detrimental to plant growth.
AGENDA ITEM # 8. a)
5.1.1 DETENTION PONDS
2022 City of Renton Surface Water Design Manual 6/22/2022
5-9
b) Trees planted on portions of water-impounding berms less than 4 feet high must be small, not
higher than 20 feet mature height, and have a fibrous root system. Table 5.1.1.A gives some
examples of trees with these characteristics.
Intent: These trees reduce the likelihood of blow-down trees, or the possibility of channeling or
piping of water through the root system, which may contribute to dam failure on berms that retain
water.
3. All landscape material, including grass, must be planted in good topsoil. Native underlying soils may
be made suitable for planting if amended with 2 inches of mature and stable compost tilled into the top
six inches of soil. Compost used should meet specifications in Reference Section 11-C.
4. Soil in which trees or shrubs are planted may require additional enrichment or additional compost
top-dressing. Consult a landscape professional or arborist for site-specific recommendations.
5. For a naturalistic effect as well as ease of maintenance, trees or shrubs must be planted in clumps to
form “landscape islands” rather than evenly spaced.
6. The landscaped islands must be planted above the 100-year water surface and must be a minimum of
six feet apart, and if set back from fences or other barriers, the setback distance must also be a
minimum of six feet. Where tree foliage extends low to the ground, the six feet of setback should be
counted from the outer drip line of the trees (estimated at maturity).
Intent: This landscape design must allow a 6-foot wide mower to pass around and between clumps.
7. Evergreen trees and trees that produce relatively little leaf-fall such as Oregon ash, Cascara, or
Western crabapple are preferred. Large-leaf deciduous trees may not be planted where branches could
extend over interior pond slopes.
8. All trees shall be set back so branches do not extend over the 100-year water surface of the pond to
prevent leaf-drop into the water.
9. Drought tolerant species are recommended.
10. Landscape areas within the tracts of City-maintained ponds in residential subdivision developments
shall be designated “to be maintained by the homeowner’s association.”
11. For ponds to be maintained by the City, landscaping with trees or large shrubs is not allowed inside
the fence.
TABLE 5.1.1.A SMALL TREES AND SHRUBS WITH FIBROUS ROOTS
Small Trees/High Shrubs Low Shrubs
*Red twig dogwood (Cornus stolonifera) *Snowberry (Symphoricarpus albus)
*Serviceberry (Amelanchier alnifolia) *Salmonberry (Rubus spectabilis)
Strawberry tree (Arbutus unedo) Rosa rugosa (avoid spreading varieties)
Highbush cranberry (Vaccinium opulus) Rock rose (Cistus spp.)
Blueberry (Vaccinium spp.) Ceanothus spp. (choose hardier varieties)
*Filbert (Corylus cornuta, others) New Zealand flax (Phormium penax)
Fruit trees on dwarf rootstock
Rhododendron (native and ornamental
varieties)
Ornamental grasses (e.g., Miscanthis,
Pennisetum)
*Native species.
Guidelines for Naturalistic Planting
Two generic kinds of naturalistic planting are outlined below, but other options are also possible. A
booklet discussing stormwater ponds and landscaping possibilities is available at the King County Water
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
6/22/2022 2022 City of Renton Surface Water Design Manual
5-10
and Land Resources Division that can be consulted for additional ideas. Native vegetation is preferred in
naturalistic plantings.
Note: These landscaping criteria must be followed unless a landscape professional judges that long-term
quality of the open space would be improved by deviating from the criteria, AND that if the facility is
maintained by the City, maintenance would not be made more difficult by the deviations.
Open Woodland
In addition to the general landscaping criteria above, the following requirements must be met:
1. Landscaped islands (when mature) should cover a minimum of 30% or more of the tract, exclusive of
the pond area.
2. Tree clumps should be underplanted with shade-tolerant shrubs and groundcover plants. The goal is to
provide a dense understory that need not be weeded or mowed.
3. Landscaped islands should be placed at several elevations rather than “ring” the pond, and the size of
clumps should vary from small to large to create variety.
4. Not all islands need have trees. Shrub or groundcover clumps are acceptable, but lack of shade should
be considered in selecting vegetation.
Note: Landscaped islands are best combined with the use of hog fuel or shredded wood mulch for
erosion control (only for slopes above the flow control water surface). It is often difficult to sustain a
low-maintenance understory if the area was previously hydroseeded.
Northwest Savannah or Meadow
In addition to the general landscape criteria above, the following requirements must be met:
1. Landscape islands (when mature) should cover 10% or more of the tract, exclusive of the pond area.
2. Planting groundcovers and understory shrubs is encouraged to eliminate the need for mowing under
the trees when they are young.
3. Landscape islands should be placed at several elevations rather than “ring” the pond.
4. The remaining tract area should be planted with an appropriate grass seed mix, which may include
northwest meadow or wildflower species. Native or dwarf grass mixes are preferred. Table 5.1.1.B
below gives one acceptable dwarf grass mix. Grass or meadow seed should be applied at a rate of 80
to 100 seeds per square foot. Actual pounds of seed mix per acre will depend on specific species
composition. Note: Amended soil or good topsoil is required for all plantings.
Creation of areas of emergent vegetation in shallow areas of the pond is recommended. Native
wetland plants, such as sedges (Carex sp.), bulrush (Scirpus sp.), water plantain (Alisma sp.), and
burreed (Sparganium sp.) are recommended. If the pond does not hold standing water, a clump of wet-
tolerant, non-invasive shrubs, such as salmonberry or snowberry, is recommended below the
detention design water surface. Note: This landscape style is best combined with the use of grass for
site stabilization and erosion control.
Table 5.1.1.B lists a mix for stormwater tracts and other intermittently wet areas that should be applied at a
rate of 31 pounds of pure live seed per acre.
AGENDA ITEM # 8. a)
5.1.1 DETENTION PONDS
2022 City of Renton Surface Water Design Manual 6/22/2022
5-11
TABLE 5.1.1.B STORMWATER TRACT SEED MIX.
Common Name Species
Percent Species
Composition
American sloughgrass Beckmannia syzigachne 15%
Tufted hairgrass Deschampsia cespitosa 20%
Blue wildrye Elymus glaucus 18%
Native red fescue Festuca rubra var. rubra 20%
Meadow barley Hordeum brachyantherum 12%
Northwestern mannagrass Glyceria occidentalis 15%
Table 5.1.1.C lists a recommended mix for landscaping seed and should be applied at 19 pounds of pure
live seed per acre.
TABLE 5.1.1.C LANDSCAPING SEED MIX.
Common Name Species
Percent Species
Composition
Sideoats grama Bouteloua curtipendula 20%
California oatgrass Danthonia californica 20%
Native red fescue Festuca rubra var. rubra 30%
Prairie Junegrass Koeleria macrantha 30%
Table 5.1.1.D lists a turf seed mix that should be applied at a rate of 10 pounds of pure live seed per acre.
This mix is for use in dry situations where there is no need for watering. This mix requires very little
maintenance.
TABLE 5.1.1.D LOW-GROWING TURF SEED MIX.
Common Name Species
Percent Species
Composition
Hard fescue Festuca brevipila 25%
Sheep fescue Festuca ovina 30%
Native red fescue Festuca rubra var. rubra 25%
Prairie Junegrass Koeleria macrantha 20%
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
6/22/2022 2022 City of Renton Surface Water Design Manual
5-12
FIGURE 5.1.1.A TYPICAL SCHEMATIC REPRESENTATION OF A DETENTION POND
FLOW12'/15' ACCESS MAINTENANCE ROADFLOWFLOWFLOWPOND INLET
PIPE
15% MAX.
SLOPE
ACCESS RAMP
INTO POND
SEE SECTION
5.1.1.1 FOR
SPECIFICATIONS
LEVEL
BOTTOM
SEE FIGURE 5.1.1.B
FOR SECTION CUT
DIAGRAMS
NOTE:
THIS DETAIL IS A SCHEMATIC REPRESENTATION ONLY. ACTUAL CONFIGURATION WILL
VARY DEPENDING ON SPECIFIC SITE CONSTRAINTS AND APPLICABLE DESIGN CRITERIA.
A
A
BB
C
C
ALTERNATE EMERGENCY
OUTFLOW STRUCTURE
FOR PONDS NOT
REQUIRED TO PROVIDE A
SPILLWAY (FIGURE 5.1.1.C)
CONTROL
STRUCTURE
POND DESIGN
WATER SURFACE
6" SEDIMENT
STORAGE
COMPACTED
EMBANKMENT
5' MIN. SETBACK
BETWEEN TOE OF
SLOPE AND TRACT
BOUNDARY
EMERGENCY
OVERFLOW
SPILLWAY
ROCK LINING
PER TABLE
4.2.2.A
TRACT LINES
AS REQUIRED
AGENDA ITEM # 8. a)
5.1.1 DETENTION PONDS
2022 City of Renton Surface Water Design Manual 6/22/2022
5-13
FIGURE 5.1.1.B TYPICAL SCHEMATIC REPRESENTATION OF DETENTION POND SECTIONS
OVERFLOW
WS
DESIGN
WS
ROCK LININGPER TABLE 4.4.1.A
COMPACTEDEMBANKMENT
EMERGENCY OVERFLOW WS
(AS REQUIRED)
DESIGN WS
OVERFLOW
WS
EMERGENCY OVERFLOW
WS (SEE FIGURE 5.3.1.E)
EMERGENCY OVERFLOW SPILLWAY
NTS
SECTION B-B
SECTION B-B HAS 2 OPTIONS
SECTION a-a
NTS
SECTION C-C
NTS
SECTION A-A
NTS
FRAME/GRATE FOR SECONDARY INLET.
PROVIDE VERTICAL BARS IN FRAME @4" O.C. (OTHER FLOW SYSTEMS
ACCEPTABLE IF APPROVED BY DPER).BARS SHALL BE STAINLESS STEEL OR
ALUMINIZED STEEL.SEE ALSO THE SEPARATE OVERFLOW
STRUCTURE SHOWN IN FIGURE 5.3.1.C
CIRCUMFERENCE LENGTH OF
OPENING SIZED FOR 100 YR FLOW
OVERFLOW WS
6" SEDIMENT
STORAGE
OVERFLOW WS
POND DESIGN
WS
EMERGENCY OVERFLOW WS
CONTROL STRUCTURE
6' MIN.
12'/15' MIN. FORACCESS ROAD EXISTINGGROUND
PROFILE
BERM
EMBANKMENTDEBRIS BARRIER
SEE FIGURE 4.2.1.D
TOP WIDTH
OF BERM
L
a
a
2 MIN.
1
KEY,
IF REQUIRED
POND DESIGN
WS
1
10
1
10
2" ASPHALT
(FOR SPILLWAY ON
ACCESS ROAD)
113 36" MIN. FREEBOARD
1' ROCK LINING
EVALUATEDOWNSTREAM
CONVEYANCE PERCORE REQS #2 AND #4
POND OUTLET SIZED
TO CONVEY THE100-YR DEVELOPED
PEAK FLOW
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
6/22/2022 2022 City of Renton Surface Water Design Manual
5-14
FIGURE 5.1.1.C SCHEMATIC REPRESENTATION OF AN OVERFLOW STRUCTURE
UPPER STEEL
BAND 34" X 4"
WIDE
LOWER STEEL BAND 34" X
4" WIDE FORMED TO FIT
IN GROOVE OF C.B. RISER
34" DIA. SMOOTH ROUND BARS
WELDED EQUALLY SPACED.
BARS SHALL BE WELDED TO
UPPER & LOWER BANDS
(24 BARS EVENLY SPACED SEE
NOTE 1)
STANDARD STEPS OR LADDER
SEE NOTE 2
HOOK CLAMP
ANCHORED
TO C.B. RISER
TYPE 2 CB
C.B. RISER
SMOOTH
VERTICAL BARS
1. DIMENSIONS ARE FOR ILLUSTRATION ON 54" DIAMETER CB. FOR DIFFERENT DIAMETER
CB'S ADJUST TO MAINTAIN 45°ANGLE ON "VERTICAL" BARS AND 7" O.C. MAXIMUM SPACING
OF BARS AROUND LOWER STEEL BAND.
2. METAL PARTS MUST BE CORROSION RESISTANT; BARS MUST BE STAINLESS STEEL OR
ALUMINIZED STEEL.
3. THIS DEBRIS BARRIER IS ALSO RECOMMENDED FOR USE ON THE INLET TO ROADWAY
CROSS-CULVERTS WITH HIGH POTENTIAL FOR DEBRIS COLLECTION (EXCEPT ON TYPE 2
STREAMS).
4. THIS DEBRIS BARRIER IS FOR USE OUTSIDE OF ROAD RIGHT-OF-WAY ONLY. FOR DEBRIS
CAGES WITHIN ROAD RIGHT-OF-WAY, SEE KCRDCS DRAWING NO. 7-028.
PROVIDE MAINTENANCE ACCESS
BY WELDING 4 CROSSBARS TO 4
VERTICAL BARS AS SHOWN.
HINGE UPPER ENDS WITH
FLANGES/ BOLTS AND PROVIDE
LOCKING MECHANISM (PADLOCK)
ON LOWER END.
LOCATE STEPS DIRECTLY BELOW.
34" DIAMETER SMOOTH
BARS EQUALLY SPACED
(4" O.C. MAX.)
4 HOOK CLAMPS EVENLY
PLACED SEE DETAIL BELOW
PLAN VIEW
NTS
SECTION A-A
NTS
DETAIL HOOK CLAMP
NTS
NOTES:
A A
45 °
15° (TYP.)SEE N
O
TE 1
24"
SEE NOTE 1
AGENDA ITEM # 8. a)
5.1.1 DETENTION PONDS
2022 City of Renton Surface Water Design Manual 6/22/2022
5-15
FIGURE 5.1.1.D PERMANENT SURFACE WATER CONTROL POND SIGN
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
6/22/2022 2022 City of Renton Surface Water Design Manual
5-16
5.1.1.2 METHODS OF ANALYSIS
Detention Volume and Outflow
The volume and outflow design for detention ponds shall be in accordance with the performance
requirements in Chapter 1 and the hydrologic analysis and design methods in Chapter 3. Restrictor orifice
structure design shall comply with Section 5.1.4. Note: The design water surface elevation is the highest
elevation that occurs in order to meet the required outflow performance for the pond.
Detention Ponds in Infiltrative Soils
Detention ponds may occasionally be sited on till soils that otherwise meet the basic criteria of “sufficient
permeable soil” for a properly functioning infiltration system (see Section 5.2.1). These detention ponds
have a surface discharge and may also utilize infiltration as a second pond outflow. Detention ponds sized
with infiltration as a second outflow must meet all the requirements of Section 5.2 for infiltration ponds,
including a soils report, performance testing, groundwater protection, presettling, and construction
techniques. Detention ponds are not allowed in Zone 1 of the Aquifer Protection Area.
Emergency Overflow Spillway Capacity
The emergency overflow spillway weir section shall be designed to pass the 100-year runoff event for
developed conditions assuming a broad-crested weir. The broad-crested weir equation for the spillway
section in Figure 5.1.1.E, for example, would be:
Q100 = C (2g)1/2 [2/3 LH3/2 + 8/15 (Tan ) H5/2] (5-1)
where Q100 = peak flow for the 100-year runoff event (cfs)
C = discharge coefficient (0.6)
g = gravity (32.2 ft/sec2)
L = length of weir (ft)
H = height of water over weir (ft)
= angle of side slopes
Assuming C = 0.6 and Tan = 3 (for 3H:1V slopes), the equation becomes:
Q100 = 3.21 (LH3/2 + 2.4 H5/2) (5-2)
To find width L for the weir section, the equation is rearranged to use the computed Q100 and trial values of
H (0.2 feet minimum):
L = [Q100 / (3.21 H3/2)] - 2.4 H or 6 feet minimum (5-3)
FIGURE 5.1.1.E SCHEMATIC REPRESENTATION OF A WEIR SECTION FOR EMERGENCY
OVERFLOW SPILLWAY
EMERGENCY OVERFLOW WS
OVERFLOW WS
ROCK LINING
PER TABLE 4.4.1.A
0 1 1
33
L
.7' MIN.
.5' MIN.
.2' MIN. H
AGENDA ITEM # 8. a)
5.1.2 DETENTION TANKS
2022 City of Renton Surface Water Design Manual 6/22/2022
5-17
5.1.2 DETENTION TANKS
Detention tanks are underground storage facilities typically constructed with large diameter corrugated
steel pipe. Schematic representations of detention tanks are shown in Figure 5.1.2.A and Figure 5.1.2.B.
Schematic representations of control structures are shown in Section 5.1.4.
5.1.2.1 DESIGN CRITERIA
General
1. Tanks shall be designed as flow-through systems with manholes in line (see Figure 5.1.2.A) to
promote sediment removal and facilitate maintenance.
Exception: Tanks may be designed as back-up systems if preceded by water quality facilities since
little sediment should reach the inlet/control structure and low head losses can be expected because of
the proximity of the inlet/control structure to the tank.
2. The detention tank bottom shall be located a minimum of 0.5 feet below the inlet and outlet to provide
dead storage for sediment.
3. The minimum pipe diameter allowed for a detention tank is 36 inches.
4. Tanks larger than 36 inches may be connected to each adjoining structure with a short section (2-foot
maximum length) of 36-inch minimum diameter pipe.
5. Outflow control structures shall be as detailed in Section 5.1.4.
Note: Control and access manholes shall have additional ladder rungs to allow ready access to all
tank access pipes when the catch basin sump is filled with water (see Figure 5.1.4.A, plan view).
Materials
Pipe material, joints, and protective treatment for tanks shall be in accordance with Sections 7.04 and 9.05
of the WSDOT/APWA Standard Specification as modified by the City of Renton Standard Details and
AASHTO designations. Such materials include the following:
Lined corrugated polyethylene pipe (LCPE)
Aluminized Type 2 corrugated steel pipe and pipe arch (meets AASHTO designations M274 and
M36)
Reinforced concrete pipe
Narrow concrete vaults (see Section 5.1.3).
Corrugated steel pipe and pipe arch, Aluminized or Galvanized3 with treatments 1, 2 or 5
Spiral rib steel pipe, Aluminized or Galvanized3 with treatments 1, 2 or 5
Structural plate pipe and pipe arch, Aluminized or Galvanized3 with treatments 1, 2 or 5
Structural Stability
Tanks shall meet structural requirements for overburden support, buoyancy, and traffic loading if
appropriate. H-20 live loads must be accommodated for tanks lying under parking areas, roadways, and
access roads. Metal tank end plates must be designed for structural stability at maximum hydrostatic
loading conditions. Flat end plates generally require thicker gage material than the pipe and/or require
reinforcing ribs.
Tanks shall be placed on stable, well consolidated native material with a suitable bedding. Backfill shall be
placed and compacted in accordance with the pipe specifications in Chapter 4. Tanks made of LCPE
3 Galvanized metals leach zinc into the environment, especially in standing water situations. High zinc concentrations,
sometimes in the range that can be toxic to aquatic life, have been observed in the region. Therefore, use of galvanized
materials should be avoided. Where other metals, such as aluminum or stainless steel, or plastics are available, they shall be
used. If these materials are not available, asphalt coated galvanized materials may then be used.
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
6/22/2022 2022 City of Renton Surface Water Design Manual
5-18
require inspection for deformation prior to installation as well as continuous inspection of backfilling to
one foot above the top of the tank. Tanks shall not be allowed in fill slopes, unless analyzed in a
geotechnical report for stability and constructability.
Buoyancy
In moderately pervious soils where seasonal groundwater may induce flotation, buoyancy tendencies must
be balanced either by ballasting with backfill or concrete backfill, providing concrete anchors, increasing
the total weight, or providing subsurface drains to permanently lower the groundwater table. Calculations
must be submitted that demonstrate stability.
Access Requirements
1. The maximum depth from finished grade to tank invert shall be 20 feet.
2. Access risers required within 50 feet from any location in the detention tank and within 5 feet of each
terminal end. Any location within the detention tank shall have a direct line of sight from an access
point, unobstructed by any restrictions such as a wall of baffle.
3. All tank access openings shall have round, solid locking lids with 5/8-inch diameter Allen head cap
screws (see City of Renton Standard Details).
4. Thirty-six-inch minimum diameter CMP riser-type manholes (Figure 5.1.2.B) of the same gage as
the tank material may be used for access along the length of the tank and at the upstream terminus of
the tank if a backup system. The top slab is separated (1-inch minimum gap) from the top of the riser
to allow for deflections from vehicle loadings without damaging the riser tank.
5. All tank access openings must be readily accessible by maintenance vehicles.
Access Roads
Access roads are required to all detention tank control structures and risers. The access roads shall be
designed and constructed as specified for detention ponds in Section 5.1.1.
Right-of-Way
Detention tanks to be maintained by the City shall be located in a stormwater tract granted and converted
with all maintenance obligations (excluding maintenance of drainage facilities contained therein) to the
homeowners association. If perimeter landscaping is required within the stormwater tract, then said tract
shall be owned by the lot owners within the subdivision. Each lot owner shall have equal and undivided
interest on the plat. Any tract not abutting public right-of-way will require a 15-foot wide extension of the
tract to an acceptable access location. An underlying easement under and upon said tract shall be dedicated
to the City for the purpose of operating, maintaining, improving and repairing the drainage facilities
contain therein. The stormwater tract must be owned by the homeowners association. Each lot owner
within the subdivision shall have an equal and undivided interest in the maintenance of the stormwater
tract. Detention tanks to be maintained by a private property owner or homeowners association shall create
stormwater facilities within a private tract or easement or construct the detention tank onsite.
Setbacks
Setbacks (easement/tract width) and building setback lines (BSBLs) for tanks shall be the same as for
pipes (see Section 4.1).
5.1.2.2 METHODS OF ANALYSIS
Detention Volume and Outflow
The volume and outflow design for detention tanks shall be in accordance with the performance
requirements in Chapter 1 and the hydrologic analysis and design methods in Chapter 3. Restrictor and
orifice design shall be according to Section 5.1.4.
AGENDA ITEM # 8. a)
5.1.2 DETENTION TANKS
2022 City of Renton Surface Water Design Manual 6/22/2022
5-19
FIGURE 5.1.2.A SCHEMATIC REPRESENTATION OF A TYPICAL DETENTION TANK
2" MIN. DIAMETER AIR
VENT PIPE WELDED TO
TANK (REQUIRED IF NO
ACCESS RISER ON TANK)
"FLOW-THROUGH" SYSTEM SHOWN SOLID.
DESIGNS FOR "FLOW BACKUP" SYSTEM AND
PARALLEL TANKS SHOWN DASHED
"FLOW-THROUGH" SYSTEM SHOWN SOLID.
CONTROL
STRUCTURE
(FROP-T SHOWN)
MIN. 54" DIA.
TYPE 2 CB
SEE SECTION 5.1.4
SECTION A-A
NTS
PLAN VIEW
NTS
36" MIN.
DIAMETER (TYP.)
0.5' SEDIMENT STORAGE
ACCESS
RISERS
ACCESS RISERS
(MAX SPACING SHOWN BELOW)
ACCESS RISERS
SEE FIGURE 5.1.2.B
CONTROL
STRUCTURE
INLET PIPE
(BACKUP
SYSTEMS,
WHERE
ALLOWED)
TYPE 2 CB
REQUIRED
FOR FLOW
THROUGH
SYSTEM ONLY
INLET PIPE
(FLOW THROUGH)
MIN. DIAMETER
SAME AS
INLET PIPE
OPTIONAL PARALLEL TANK
NOTES:
ALL METAL PARTS CORROSION RESISTANT. STEEL PARTS
STAINLESS STEEL OR ALUMINIZED STEEL, EXCEPT TANK MAY BE
GALVANIZED AND ASPHALT COATED (TREATMENT 1 OR BETTER).
100' MAX.50' MAX.
36"
36"
FLOW
A A
2'
MAX.
2'
MAX.2'
MIN.
2'
MIN.
2 MIN.
DETENTION TANK
SIZE AS REQUIRED
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
6/22/2022 2022 City of Renton Surface Water Design Manual
5-20
FIGURE 5.1.2.B SCHEMATIC REPRESENTATION OF A DETENTION TANK ACCESS DETAIL
1. USE ADJUSTING BLOCKS AS REQUIRED TO BRING FRAME TO GUIDE.
2. ALL MATERIALS TO BE ALUMINUM OR GALVANIZED AND ASPHALT COATED
(TREAMENT 1 OR BETTER), OR STAINLESS STEEL OR ALUMINIZED STEEL.
3. MUST BE LOCATED FOR ACCESS BY MAINTENANCE VEHICLES.
4. MAY SUBSTITUTE WSDOT SPECIAL TYPE IV MANHOLE (RCP ONLY).
PLAN
NTS
SECTION
NTS
36"
24" MAX.
NOTES:
WELD OR BOLT
STANDARD M.H. STEPS
M.H. STEPS 12" O.C.
COMPACTED PIPE BEDDING
STANDARD LOCKING
M.H. FRAME & LID SEE
KCRDCS DWG. NO. 7-022
DETENTION TANK
RISER, 36" DIAM. MIN.,
SAME MATERIAL & GAGE AS TANK
WELDED OR FUSED TO TANK
MAINTAIN 1" GAP BETWEEN
BOTTOM OF SLAB & TOP OF
RISER - PROVIDE PLIABLE
GASKET TO EXCLUDE DIRT
FRAME WITH LOCKING LID
(MARKED "DRAIN")
MOUNTED OVER 24" DIAM.
ECCENTRIC OPENING.
ALIGN VERTICALLY WITH
ACCESS LADDER TO
PROVIDE 2-FOOT ACCESS
CLEARANCE
STANDARD TYPE 2-60" DIAM.
CB CONCRETE TOP SLAB
36" CMP RISER
AGENDA ITEM # 8. a)
5.1.3 DETENTION VAULTS
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5-21
5.1.3 DETENTION VAULTS
Detention vaults are box-shaped underground storage facilities typically constructed with reinforced
concrete. A schematic representation of a detention vault is shown in Figure 5.1.3.A. Schematic
representations of a control structures are shown in Section 5.1.4.
5.1.3.1 DESIGN CRITERIA
General
1. Detention vaults shall be designed as flow-through systems with bottoms level (longitudinally) or
sloped toward the inlet to facilitate sediment removal. Distance between the inlet and outlet shall be
maximized (as feasible).
2. The detention vault bottom shall slope at least 5% from each side towards the center, forming a broad
“v” to facilitate sediment removal.
Note: More than one “v” may be used to minimize vault depth.
Exception: The vault bottom may be flat if removable panels are provided over the entire vault.
Removable panels shall be at grade, have stainless steel lifting eyes, and weigh no more than 5 tons
per panel.
3. The invert elevation of the outlet shall be elevated above the bottom of the vault to provide an
average 6 inches of sediment storage over the entire bottom. The outlet must also be elevated a
minimum of 2 feet above the orifice to retain oil within the vault.
4. The outflow system and restrictor device shall be designed according to the applicable requirements
specified for control structures in Section 5.1.4.
Materials
Minimum 3,000 psi structural reinforced concrete must be used for all detention vaults. All construction
joints must be provided with water stops.
Structural Stability
All vaults shall meet structural requirements for overburden support, buoyancy, and H-20 traffic loading.
Cast-in-place wall sections shall be designed as retaining walls. Structural designs for vaults must be
stamped by a licensed structural engineer unless otherwise approved by the City. Vaults shall be placed on
stable, well-consolidated native material with suitable bedding. Vaults shall not be allowed in fill slopes,
unless analyzed in a geotechnical report for stability and constructability.
Access Requirements
1. Access consisting of a frame, grate and locking cover shall be provided over the inlet pipe and outlet
structure and located in a manner to allow visual inspection. Access openings over control structures
shall meet a minimum 2 ft. offset to any portion of the FROP-T as shown in Figure 5.1.4.A. Access
openings shall be positioned a maximum of 50 feet from any location within the vault; additional
access points may be required on large vaults. If more than one “v” is provided in the vault floor,
access to each “v” must be provided.
2. For vaults with greater than 1250 square feet of floor area, a 5′ by 10′ removable, locking panel
shall be provided. Alternatively, a separate access vault may be provided as shown in Figure 5.1.3.A.
3. For vaults under roadways, the removable panel must be located outside the travel lanes.
Alternatively, multiple standard locking manhole covers (see City of Renton Standard Details) may be
provided. Spacing of manhole covers shall be 12 feet, measured on center, to facilitate removal of
sediment. Ladders and hand-holds need only be provided at the outlet pipe and inlet pipe, and as
needed to meet OSHA confined space requirements. Vaults providing manhole access at 12-foot
spacing need not provide corner ventilation pipes as specified in Item 9 below.
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
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4. All access openings, except those covered by removable panels, shall have round, solid locking
covers (see City of Renton Standard Details), or 3-foot square, locking diamond plate covers. For
raised openings where the depth from the iron cover to the top of the vault exceeds 24 inches, an
access structure equivalent to a Type 2 catch basin or Type 1 manhole shall be used (see City of
Renton Standard Details). The opening in the vault lid need not exceed 24 inches in diameter.
5. Vaults with widths 10 feet or less must have removable lids.
6. The maximum depth from finished grade to the vault invert shall be 20 feet.
7. Internal structural walls of large vaults shall be provided with openings sufficient for maintenance
access between cells. The openings shall be sized and situated to allow access to the maintenance “v”
in the vault floor.
8. The minimum internal height shall be 7 feet from the highest point of the vault floor (not sump), and
the minimum width shall be 4 feet.
Exceptions:
Concrete vaults may be a minimum 3 feet in height and width if used as tanks with access
manholes at each end, and if the width is no larger than the height.
The minimum internal height requirement may be waived for any areas covered by removable
panels.
9. Ventilation pipes (minimum 12-inch diameter or equivalent) shall be provided in all four corners of
vaults to allow for artificial ventilation prior to entry of maintenance personnel into the vault. These
openings shall be capped or otherwise covered, but designed so that maintenance personnel can
remove (and replace) for ventilation purposes as described.
Access Roads
Access roads are required to the access panel (if applicable), the control structure, and at least one access
point per cell, and they shall be designed and constructed as specified for detention ponds in
Section 5.1.1.
Right-of-Way
Detention vaults to be maintained by the City shall be in a stormwater tract granted and converted with all
maintenance obligations (excluding maintenance of drainage facilities contained therein) to the
homeowners association. Each lot owner shall have equal and undivided interest on the plat granted and
converted with all maintenance obligations (excluding maintenance of drainage facilities contained
therein) to the homeowners association. Any tract not abutting public right-of-way will require a 15-foot-
wide extension of the tract to an acceptable access location. An underlying easement under and upon said
tract shall be dedicated to the City for the purpose of operating, maintaining, improving and repairing the
drainage facilities contain therein. The stormwater tract must be owned by the homeowners association.
Each lot owner within the subdivision shall have an equal and undivided interest in the maintenance of the
stormwater tract. Detention vaults to be maintained by a private property owner or homeowners
association shall create stormwater facilities within a private tract or easement or construct the detention
vault onsite.
Setbacks
Setbacks to tract/easement lines for vaults shall be 5 feet; adjacent building setback lines shall be 10 feet.
For privately owned and maintained vaults, building foundations may serve as one or more of the vault
walls.
AGENDA ITEM # 8. a)
5.1.3 DETENTION VAULTS
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5-23
5.1.3.2 METHODS OF ANALYSIS
Detention Volume and Outflow
The volume and outflow design for detention vaults shall be in accordance with the performance
requirements in Chapter 1 and the hydrologic analysis and routing/design methods in Chapter 3.
Restrictor and orifice design shall be according to Section 5.1.4.
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
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5-24
FIGURE 5.1.3.A SCHEMATIC REPRESENTATION OF A TYPICAL DETENTION VAULT
FLOOR GRATE WITH
2' X 2' HINGED ACCESS
DOOR. (1" X 14" METAL
BARS), STAINLESS
STEEL OR ALUMINIZED
STEEL
CAPACITY OF OUTLET
PIPE NOT LESS THAN
DEVELOPED 100 - YR
DESIGN FLOW
FLOW
RESTRICTOR
DESIGN W.S.
HANDHOLDS, STEPS OR
LADDER SEE KCRDCS
DWG 7-011
12"
12"
WALL FLANGE
(TYP.)
FRAMES, GRATES AND ROUND
SOLID COVERS MARKED "DRAIN"
WITH LOCKING BOLTS.
SEE KCRDCS DWGS. 7-022, 7-023
FOR SPECIFICATION
5' X 10' OPENING FOR VAULTS
1250 SF OR GREATER FLOOR AREA
5%
5%
NOTE:
ALL VAULT AREAS MUST BE
WITHIN 50' OF AN ACCESS POINT
OUTLET PIPE
1. ALL METAL PARTS MUST BE CORROSION RESISTANT. STEEL PARTS MUST BE STAINLESS
STEEL OR ALUMINIZED STEEL.
2. PROVIDE WATER STOP AT ALL CAST-IN-PLACE CONSTRUCTION JOINTS.
PRECAST VAULTS SHALL HAVE APPROVED RUBBER GASKET SYSTEM.
3. VAULTS <10' WIDE MUST USE REMOVABLE LIDS.
4. PREFABRICATED VAULT SECTIONS MAY REQUIRE STRUCTURAL MODIFICATIONS TO
SUPPORT 5' X10' OPENING OVER MAIN VAULT. ALTERNATIVELY, ACCESS CAN BE
PROVIDED VIA A SIDE VESTIBULE AS SHOWN.
5. IF SUMP IS SET BACK FROM WALL, TEE WILL REQUIRE ADDITIONAL 3-POINT BRACING
SECURED TO VAULT WALL.
FLOW
FLOW
PROVIDE ACCESS ADJACENT TO
TEE. ASSURE TEE IS VIEWABLE
FROM SURFACE.
10'
4' min.
PLAN VIEW
NTS
SECTION A-A
NTS
FLOW
A
A
FLOW
NOTES:
6" SEDIMENT
STORAGE
6" MIN.
5'
V-SHAPED
BOTTOM
OPTIONAL 5' X 10'
ACCESS VAULT MAY
BE USED IN LIEU OF
TOP ACCESS
2' MIN.
2' MIN.
AGENDA ITEM # 8. a)
5.1.4 CONTROL STRUCTURES
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5.1.4 CONTROL STRUCTURES
Control structures are catch basins or manholes with a restrictor device for controlling outflow from a
facility to meet the desired performance. The restrictor device is typically a tee section with an orifice
plate welded to the bottom (called a “FROP-T”). To meet performance requirements, one or more elbow
sections with orifice plates may need to be mounted on the side of the tee section. The restrictor device
may also be a weir section sized to meet performance requirements.
Schematic representations of control structures are shown in Figure 5.1.4.A through Figure 5.1.4.C.
5.1.4.1 DESIGN CRITERIA
Multiple Orifice Restrictor
In most cases, control structures need only two orifices: one at the bottom and one near the top of the
riser, although additional orifices may best utilize detention storage volume. Several orifices may be
located at the same elevation if necessary to meet performance requirements.
1. Minimum orifice diameter is 0.25 inches. Note: In some instances, a 0.25-inch bottom orifice may
be too large to meet target release rates, even with minimal head. In these cases, the live storage depth
need not be reduced to less than 3 feet to meet performance.
2. Orifices shall be constructed on a tee section as shown in Figure 5.1.4.A or on a baffle as shown in
Figure 5.1.4.B.
3. In some cases, performance requirements may require the top orifice/elbow to be located too high on
the riser to be physically constructed (e.g., a 13-inch diameter orifice positioned 0.5 feet from the top
of the riser). In these cases, a notch weir in the riser pipe may be used to meet performance
requirements (see Figure 5.1.4.E).
4. Consideration shall be given to the backwater effect of water surface elevations in the downstream
conveyance system. High tailwater elevations may affect performance of the restrictor system and
reduce live storage volumes.
Riser and Weir Restrictor
1. Properly designed weirs may be used as flow restrictors (see Figure 5.1.4.C and Figure 5.1.4.E
through Figure 5.1.4.F). However, they must be designed to provide for primary overflow of the
developed 100-year peak flow discharging from the detention facility.
2. The combined orifice and riser (or weir) overflow may be used to meet performance requirements;
however, the design must still provide for primary overflow of the developed 100-year peak flow
assuming all orifices are plugged. Figure 5.1.4.H may be used to calculate the head in feet above a
riser of given diameter and flow.
Access Requirements
1. An access road to the control structure is required for inspection and maintenance, and shall be
designed and constructed as specified for detention ponds in Section 5.1.1.
2. Manhole and catch basin lids for control structures shall be locking, and rim elevations shall match
proposed finish grade.
3. The restrictor tee shall be located immediately adjacent to the 2-foot clear zone at a maintenance
access ladder. Intent: To provide tee visibility from the surface at the access opening, especially
where a solid vault lid or solid manhole lid design may block view; to provide maintenance access
along the full height of the tee.
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
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FIGURE 5.1.4.A SCHEMATIC REPRESENTATION OF A FLOW RESTRICTOR (TEE)
ISOMETRIC
NTS
SECTION A-A
NTS
PLAN VIEW
NTS
ELBOW RESTRICTOR
DETAIL
NTS
1. USE A MIMIMUM OF A 54" DIAMETER TYPE 2 CATCH BASIN.
2. OUTLET CAPACITY: 100-YEAR DEVELOPED PEAK FLOW.
3. METAL PARTS: CORROSION RESISTANT.
STAINLESS STEEL OR ALUMINIZED STEEL.
4. FRAME AND LADDER OR STEPS OFFSET SO:
A. CLEANOUT GATE IS VISIBLE FROM TOP.
B. CLIMB-DOWN SPACE IS CLEAR OF RISER AND CLEANOUT GATE.
C. FRAME IS CLEAR OF CURB.
5. IF METAL OUTLET PIPE CONNECTS TO CEMENT CONCRETE PIPE: OUTLET PIPE TO HAVE SMOOTH
O.D. EQUAL TO CONCRETE PIPE I.D. LESS 1/4".
6. PROVIDE AT LEAST ONE 3" X .090 GAGE SUPPORT BRACKET ANCHORED TO CONCRETE WALL.
(MAXIMUM 3'-0" VERTICAL SPACING)
7. LOCATE ELBOW RESTRICTOR(S) AS NECESSARY TO PROVIDE MINIMUM CLEARANCE AS SHOWN.
8. LOCATE ADDITIONAL LADDER RUNGS IN STRUCTURES USED AS ACCESS TO TANKS AND VAULT
TO ALLOW ACCESS WHEN CATCH BASIN IS FILLED WITH WATER.
9. TEE SHALL BE CONSTRUCTED OF ALUMINUM CMP OR ALUMINIZED STEEL CMP MEETING
WSDOT/APWA STANDARDS.
NOTES:
ADDITIONAL LADDER
RUN (IN SETS) TO
ALLOW ACCESS TO
TANKS OR VAULTS
WHEN CATCH IS
FILLED WITH WATER.
2' MIN. CLEARANCE TO
ANY PORTION
OF FROP-T INCLUDING
ELBOWS
RESTRICTOR PLATE WITH
ORIFICE DIAMETER AS
SPECIFIED (NOT NEEDED IF
FOR SPILL CONTROL ONLY)
INVERT AND
ELEVATION
PER PLANS
PIPE SUPPORTS
SEE NOTE 6
ELBOW
RESTRICTOR
SEE DETAIL
ELEVATION
PER PLANS
PLATE WELDED
TO ELBOW WITH
ORIFICE AS SPECIFIED
REMOVABLE
WATERTIGHT
COUPLING OR
FLANGE
ANGLE AS
NECESSARY
SEE NOTE 7
ROD FOR
CLEANOUT/DRAIN
(ROD BENT AS
REQUIRED FOR
VERTICAL
ALIGNMENT WITH
COVER) SEE
KCRDCS DWG. 7-026
HANDHOLDS, STEPS
OR LADDER SEE
KCRDCS DWG. 7-006
VERTICAL BAR
GRATE FOR
SECONDARY INLET
INLET
PIPE
FRAME & SOLID COVER
MARKED "DRAIN" WITH LOCKING
BOLTS SEE NOTE 3 & KCRDCS
DWGS 7-022, 7-023
12"
12"
12"
A
6" MAX.
DESIGN
WATER
SURFACE
OUTLET PIPE
SEE NOTES 1 & 5
1' MIN. UNDER
PAVEMENT
6" MIN.
2' MIN.
1.5 x D
MIN.
D
ELBOW
RESTRICTOR
SEE DETAIL 2 ' MI N .2' MIN.2' MIN.2" MIN.
6" MIN.
16" MAX.
ACCESS
ADJACENT TO TEE
AGENDA ITEM # 8. a)
5.1.4 CONTROL STRUCTURES
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5-27
FIGURE 5.1.4.B SCHEMATIC REPRESENTATION OF A FLOW RESTRICTOR (BAFFLE)
1. OUTLET CAPACITY: 100 YEAR DEVELOPED PEAK FLOW.
2. METAL PARTS: CORROSION RESISTANT STEEL PARTS
STAINLESS STEEL OR ALUMINIZED STEEL.
3. CATCH BASIN: TYPE 2 MINIMUM 72" DIAMETER TO
BE CONSTRUCTED IN ACCORDANCE WITH
KCRDCS DWG. 7-005 AND AASHTO M-199 UNLESS
OTHERWISE SPECIFIED.
4. ORIFICES: SIZED AND LOCATED AS REQUIRED WITH
LOWEST ORIFICE A MINIMUM OF 2' FROM BASE.
ELBOW RESTRICTOR DETAIL
NTS
ISOMETRIC
NTS
PLAN VIEW
NTS
SECTION B-B
NTS
SECTION A-A
NTS
NOTES:
ORIFICE PLATE 10
GAGE MINIMUM
GALVANIZED STEEL
WITH ORIFICE
DIAMETER 1" MINIMUM
LESS THAN DIAMETER
OF CONCRETE HOLE
FRAME AND ROUND SOLID
COVER MARKED "DRAIN"
WITH LOCKING BOLTS.
SEE KCRDCS DWGS. 7-022, 7-023.
FRAME ELEVATION
PER PLANS
DESIGN W.S.MAX W.S.
OVERFLOW
CONDITIONS
ELBOW
RESTRICTORS
SEE DETAIL BELOW
HANDHOLDS,
STEPS OR
LADDER
SEE KCRDCS
DWG. 7-011
ATTACH SHEAR GATE
CONTROL ROD TO
SUPPORT BRACKET
ON INSIDE OF
ACCESS OPENING
SHEAR GATE WITH
CONTROL ROD FOR
DRAIN. SEE KCRDCS
DWG. 7-026
REMOVABLE
WATER-TIGHT
COUPLING GROUTED
PLATE WELDED
TO ELBOW
WITH ORIFICE
AS SPECIFIED
A A
B
B
FLOW
6" MIN.
6" MIN.
6"
MAX.
2' MIN.
2' MIN.
1' MIN.
1' MIN. UNDER
PAVEMENT
16" MAX.
6"
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
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FIGURE 5.1.4.C SCHEMATIC REPRESENTATION OF A FLOW RESTRICTOR (WEIR)
LOCATE ADDITIONAL LADDER
RUNGS (IN SETS) TO ALLOW
ACCESS TO TANKS OR VAULTS
WHEN SUMP IS FILLED
WITH WATER
SHEAR GATE
LOCATE HORIZONTAL FOR
CLEARANCE WITH
LADDER.
ATTACH ROD TO SUPPORT
BRACKET ON INSIDE OF
ACCESS OPENING
HANDHOLDS,
STEPS OR LADDER (2
PLACES)
SEE KCRDCS DWG. 7-006
FRAME
ELEVATION
PER
PLANS
OUTLET
PIPE
SHEAR GATE WITH
CONTROL ROD
FOR DRAIN.
SEE KCRDCS
DWG. 7-026
I.E. WEIR,
INLET
PIPE AND
DRAIN
= CROWN
OUTLET PIPE
DESIGN W.S.
WEIR
SHAPE
AS NEEDED
FOR
PERFORMANCE
FRAME AND ROUND SOLID
COVER MARKED "DRAIN" WITH
LOCKING BOLTS.
SEE KCRDCS DWGS. 7-022,
7-023
1. OUTLET CAPACITY: 100-YEAR DEVELOPED PEAK FLOW.
2. METAL PARTS: CORROSION RESISTANT STEEL PARTS, STAINLESS STEEL OR ALUMINIZED STEEL.
3. CATCH BASIN: TYPE 2 MIN. 72" DIAMETER TO BE CONSTRUCTED IN ACCORDANCE WITH
KCRDCS DWG 7-005 AND AASHTO M-199 UNLESS OTHERWISE SPECIFIED.
4. BAFFLE WALL: TO BE DESIGNED WITH CONCRETE REINFORCING AS REQUIRED.
5. SPILL CONTROL REQUIREMENTS: SEE SECTION 4.2.1 PIPE SYSTEMS - DESIGN CRITERIA, SPILL
CONTROL
NOTES:
SECTION B-B
NTS
SECTION A-A
NTS
ISOMETRIC
NTS
PLAN VIEW
NTS
FLOW
B
B
A A
6" MIN.
1' MIN. UNDER
PAVEMENT
W
2' MIN.
2' MIN.
AGENDA ITEM # 8. a)
5.1.4 CONTROL STRUCTURES
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5-29
5.1.4.2 METHODS OF ANALYSIS
This section presents the methods and equations for design of control structure restrictor devices.
Included are details for the design of orifices, rectangular sharp-crested weirs, v-notch weirs, sutro weirs,
and overflow risers.
Orifices
Flow through orifice plates in the standard tee section or turn-down elbow may be approximated by the
general equation:
Q = CA (5-4)
where Q = flow (cfs)
C = coefficient of discharge (0.62 for plate orifice)
A = area of orifice (sf)
h = hydraulic head (ft)
g = gravity (32.2 ft/sec2)
Figure 5.1.4.D illustrates a simplified application of the orifice equation, assuming a water surface at the
top of the riser and that the 2-year water surface represents the head in the outlet pipe.
FIGURE 5.1.4.D SIMPLE ORIFICE
The diameter of the orifice is calculated from the flow. The orifice equation is often useful when expressed
as the orifice diameter in inches:
d = (5-5)
where d = orifice diameter (inches)
Q = flow (cfs)
h = hydraulic head (ft)
gh2
h
Q88.36
h = DISTANCE FROM HYDRAULIC GRADE LINE
AT THE 2-YEAR FLOW OF THE OUTFLOW PIPE
TO THE OVERFLOW ELEVATION.
= C
Q = CA 2gh
2g h h(A + A )
t2gh
b b
b + CA
t
b
t
Q
h b
ht
ORIFICE (t)
ORIFICE (b)
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
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Rectangular, Sharp-Crested Weir
The rectangular, sharp-crested weir design shown in Figure 5.1.4.E may be analyzed using standard weir
equations for the fully contracted condition.
FIGURE 5.1.4.E RECTANGULAR, SHARP-CRESTED WEIR
Q = C (L - 0.2H)H3/2 (5-6)
where Q = flow (cfs)
C = 3.27 + 0.40 H/P (ft)
H,P are as shown above
L = length (ft) of the portion of the riser circumference as necessary not to exceed 50% of
the circumference
D = inside riser diameter (ft)
Note that this equation accounts for side contractions by subtracting 0.1H from L for each side of the
notch weir.
V-Notch, Sharp-Crested Weir
V-notch weirs, as shown in Figure 5.1.4.F, may be analyzed using standard equations for the fully
contracted condition.
SECTION
NTS
PLAN VIEW
NTS
P
H
LD
RISER
AGENDA ITEM # 8. a)
5.1.4 CONTROL STRUCTURES
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5-31
FIGURE 5.1.4.F V-NOTCH, SHARP-CRESTED WEIR
Ratio of H/Y
Where values of C may be taken from the following chart:
Q = C Tan (0/2)H , in cfsd
52
SECTION A-A
NTS
d
dC
A
A
Y
0.2 0.4 0.6 0.8
2.9
2.8
2.7
2.6
2.5
2.4
60°
20°
45°
90°
H
0
0
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
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Proportional or Sutro Weir
Sutro weirs are designed so that the discharge is proportional to the total head. This design may be useful
in some cases to meet performance requirements.
The sutro weir consists of a rectangular section joined to a curved portion that provides proportionality for
all heads above the line A-B (see Figure 5.1.4.G). The weir may be symmetrical or non-symmetrical.
FIGURE 5.1.4.G SUTRO WEIR
For this type of weir, the curved portion is defined by the following equation (calculated in radians):
= 1 - Tan-1 (5-7)
where a, b, x and Z are as shown in Figure 5.1.4.G. The head-discharge relationship is:
Q = Cd b Q = Cd b (5-8)
Values of Cd for both symmetrical and non-symmetrical sutro weirs are summarized in Table 5.1.4.A.
Note: When b > 1.50 or a > 0.30, use Cd = 0.6.
b
x
2
a
Z
ag2
31
ah
SEE
EQUATION
BELOW
TOTAL HEADDISCHARGE
SYMMETRICAL NON-SYMMETRICAL
SEE
EQUATION
BELOW
A B
x
h1
b
a
Z
x
b
a
AGENDA ITEM # 8. a)
5.1.4 CONTROL STRUCTURES
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5-33
TABLE 5.1.4.A VALUES OF CD FOR SUTRO WEIRS
Cd Values, Symmetrical
b (ft)
a (ft) 0.50 0.75 1.0 1.25 1.50
0.02 0.608 0.613 0.617 0.6185 0.619
0.05 0.606 0.611 0.615 0.617 0.6175
0.10 0.603 0.608 0.612 0.6135 0.614
0.15 0.601 0.6055 0.610 0.6115 0.612
0.20 0.599 0.604 0.608 0.6095 0.610
0.25 0.598 0.6025 0.6065 0.608 0.6085
0.30 0.597 0.602 0.606 0.6075 0.608
Cd Values, Non-Symmetrical
b (ft)
a (ft) 0.50 0.75 1.0 1.25 1.50
0.02 0.614 0.619 0.623 0.6245 0.625
0.05 0.612 0.617 0.621 0.623 0.6235
0.10 0.609 0.614 0.618 0.6195 0.620
0.15 0.607 0.6115 0.616 0.6175 0.618
0.20 0.605 0.610 0.614 0.6155 0.616
0.25 0.604 0.6085 0.6125 0.614 0.6145
0.30 0.603 0.608 0.612 0.6135 0.614
Riser Overflow
The nomograph in Figure 5.1.4.H may be used to determine the head (in feet) above a riser of given
diameter and for a given flow (usually the 100-year peak flow for developed conditions).
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
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FIGURE 5.1.4.H RISER INFLOW CURVES
1
10
100
0.1 1 10HEAD IN FEET (measured from crest of riser)
Qweir=9.739 DH3/2
Qorifice=3.782 D2H1/2
Q in cfs, D and H in feet
Slope change occurs at weir-orifice transition Q (cubic feet per second)18
21
24
27
30
42
487254
10
12
15
33
36
RISER DIAMETER (inches)AGENDA ITEM # 8. a)
5.1.7 SIMPLE DETENTION POND FOR CLEARED AREAS
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5.1.5 PARKING LOT DETENTION
Private parking lots may be used to provide additional detention volume for runoff events greater than the
2-year runoff event provided all of the following conditions are met:
1. The depth of water detained does not exceed 1 foot at any location in the parking lot for runoff events
up to and including the 100-year event.
2. The gradient of the parking lot area subject to ponding is 1 percent or greater.
3. The emergency overflow path is identified and noted on the engineering plan, and the path complies
with Core Requirements #1 and #2 (see Sections 1.2.1 and 1.2.2).
4. Fire lanes used for emergency equipment are free of ponding water for all runoff events up to and
including the 100-year event.
Note: Flows may be backed up into parking lots by the control structure (i.e., the parking lot need not
function as a flow-through detention pond).
5.1.6 ROOF DETENTION
Detention ponding on roofs of structures may be used to meet flow control requirements provided all of
the following conditions are met:
1. The roof support structure is analyzed by a structural engineer to address the weight of ponded water.
2. The roof area subject to ponding is sufficiently waterproofed to achieve a minimum service life of
30 years.
3. The minimum pitch of the roof area subject to ponding is 1/4-inch per foot.
4. An overflow system is included in the design to safely convey the 100-year peak flow from the roof.
5. A mechanism is included in the design to allow the ponding area to be drained for maintenance
purposes or in the event the restrictor device is plugged.
5.1.7 SIMPLE DETENTION POND FOR CLEARED AREAS
This simplified alternative to the standard detention pond (Section 5.1.1) may be used to satisfy the flow
control facility requirement only for a conversion of forest to pasture or grass, provided that all of the
following conditions are met:
1. The total area draining to any one pond must be no larger than 3 acres and must consist primarily of
vegetated land (e.g., forest, meadow, pasture, grass, garden, crops, etc.) free of impervious surface. If
more than 3 acres of cleared area (i.e., area converted from forest to pasture/grass) is proposed to be
served, multiple simple detention ponds must be used.
2. The area served by the pond must not be located within a Flood Problem Flow Control Area as
determined in Section 1.2.3.1.
3. The pond must not drain to a severe erosion problem or a severe flooding problem as defined in
Section 1.2.2, Core Requirement #2.
4. The pond is not located with Zone 1 of the Aquifer Protection Area.
5. The pond must be constructed in accordance with the design criteria and methods of analysis
specified in this section.
5.1.7.1 DESIGN CRITERIA
Schematic representations of a simple detention pond are shown in Figure 5.1.7.A and Figure 5.1.7.B.
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
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General
1. A geotechnical analysis and report is required if located within 200 feet of a steep slope hazard area
or landslide hazard OR if the facility is located within a setback distance from top of slope equal to
the total vertical height of the slope area that is steeper than 15%. The geotechnical analysis must
consider cumulative impacts from the project and surrounding areas under full built- out conditions.
2. The detention pond design water surface shall be a minimum of 200 feet from any steep slope hazard
area or landslide hazard. Upon analysis and approval of a licensed geotechnical engineer or
engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider
cumulative impacts from the project and surrounding areas under full built- out conditions.
3. The detention pond design water surface shall be set back a minimum distance from top of slope equal
to the total vertical height of a slope area that is steeper than 15%. Upon analysis and approval of a
licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet. The
geotechnical analysis must consider cumulative impacts from the project and surrounding areas under
full built- out conditions.
4. The dispersal trench at the outlet from the storage pond may not be placed closer than 50 feet from
the top of slopes, 20% or greater.
5. The pond, berm, and dispersal trench must be fenced to prevent livestock disturbance.
6. Runoff discharge toward landslide hazard or steep slope hazard areas must be evaluated by a
geotechnical engineer or a qualified geologist. The discharge point may not be placed on or above
slopes greater than 20% or above erosion hazard areas without evaluation by a geotechnical engineer
or qualified geologist and City approval.
Berming and Excavation
1. To the extent feasible, the pond shall be excavated into the ground with minimal berming on the
downslope (outlet) end of the pond. An excavated pond is easier to construct and maintain and is less
likely to cause problems during severe storm events.
2. Where berms are used, the top of berm shall be a minimum of 3 feet wide. The soil shall be well
compacted and planted with an erosion-control seed mix as soon as possible.
3. Whether created by excavation or berming, all pond side-slopes shall be gently sloped, no steeper
than 3 feet horizontal per 1 foot of vertical drop.
4. Prior to constructing the berm, the underlying ground shall be scrapped clean of organic material.
5. At a minimum, a hand-level shall be used to ensure the berm and outlet structure are constructed at
the correct relative elevations.
6. The bottom 6 inches of the pond shall retain standing water in the pond between storms to create a
permanent pool. The volume of the permanent pool is not counted towards the required detention
volume, which is above the permanent pool.
7. The water depth of required detention volume above the permanent pool should average about
18 inches and must be no deeper than 24 inches.
Simple Outlet Control Structure
1. Materials Required:
a) PVC pipe, 4 inch diameter or greater as needed.
b) PVC pipe cap.
c) Small plastic or concrete catch basin with grate, minimum 12-inch width.
AGENDA ITEM # 8. a)
5.1.7 SIMPLE DETENTION POND FOR CLEARED AREAS
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2. Construction Method:
a) Drill or cut a hole just below the rim of the catch basin, sized to connect the PVC pipe.
b) Install the catch basin into the bottom of the pond. The catch basin should be located within a few
feet of the berm at the downslope end of the pond. The top of catch basin must be a minimum of
6 inches above the bottom of the pond to create the permanent pool. Align the hole in the
downslope direction of discharge.
c) Dig a trench for the pipe from the catch basin to the location of the flow spreader.
d) Connect the PVC pipe to the catch basin. PVC pipe should extend about 4 inches into the basin.
e) Drill the appropriate size hole into the PVC cap. Clean hole to remove burrs, without increasing
the size of the opening.
f) Connect the drilled cap to the end of the PVC pipe extending into the catch basin.
g) Extend the PVC pipe to the location of the flow spreader. The pipe shall be laid with a slight slope
towards the flow spreader. A slope of ¼ inch per foot of pipe is recommended and should not
exceed 2 inches per foot.
h) Backfill the trench over the PVC pipe and compact well. Avoid placing large and/or sharp rocks
in the trench to minimize potential for damaging the pipe during compaction.
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
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FIGURE 5.1.7.A SCHEMATIC REPRESENTATION OF A SIMPLE DETENTION POND – PLAN VIEW
SLOPEPLAN VIEW
NTS
A
A
B B
C C
3' MIN BERM
TOP WIDTH
3H:1V MAX. SIDE
SLOPE (TYP.)
SPILLWAY
SMALL
CATCH BASIN
FLOW
SPREADER
AGENDA ITEM # 8. a)
5.1.7 SIMPLE DETENTION POND FOR CLEARED AREAS
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FIGURE 5.1.7.B SCHEMATIC REPRESENTATION OF A SIMPLE DETENTION POND – SECTION
VIEWS
4"
6"
ALL SLOPES 3:1 OR FLATTER
ALL SLOPES 3:1
OR FLATTER
10" HIGH BERM
SECTION A-A
NTS
SECTION B-B
NTS
SECTION C-C
NTS
3
1
3'6"
18"
6"6"
4"
6'
3
1
6"
10" BERM
COMPACTED
EARTHEN
MATERIAL
8' BOARD 2" X 10"
BOTTOM EMBEDDED 6"
INTO THE GROUND
FLOW SPREADER6'1'1'
PVC OUTLET PIPE
COMPACTED
EARTHEN
MATERIAL
SPILLWAY 6"
LOWER
THAN BERM
4"4"4"4"
CATCH BASIN
GRATE
GRASS TYPICAL OF
ALL DISTURBED AREAS
SMALL
CATCH BASINNOTCHED 2" x 10" BY 8'
LONG SPREADER BOARD
4" PVC PIPE CAP WITH
DRILLED
ORIFICE
COMPACTED
EARTHEN BERM
10" HIGH BERM
NATURAL GROUND LINE
END OF BOARD
EMBEDDED 1'
INTO BERM
AGENDA ITEM # 8. a)
SECTION 5.1 DETENTION FACILITIES
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5.1.7.2 METHODS OF ANALYSIS
The detention volume and orifice sizing for the simple detention pond shall be determined as described in
this section. This determination is based on where the pond is located within the City and how much
cleared area (i.e., area of forest converted to pasture or grass) is served by the pond.
Detention Volume
The map in Figure 5.1.7.C provides the minimum pond volume required based on 10,000 square feet of
cleared area. To determine the total pond volume required, locate the project site on the map and multiply
the number from the map by the amount of cleared area that will be served by the pond (if the cleared area
is measured in units of square feet, remember to divide the actual area by 10,000 before multiplying with
map value). If the project site is located between the lines shown on the map, select the larger of the two
pond unit volumes associated with the lines. Do not interpolate the volume if located midway between two
lines.
To determine if the constructed pond has adequate storage, the pond area must be determined by field
measurements. If all side slopes are at 3H:1V or flatter, the pond’s bottom area may be used to determine
the pond volume, Vt, above the permanent pool using the following equation. The resulting volume, Vt,
must be equal to or greater than the required volume determined from Figure 5.1.7.C.
Vt = 1.5 Ab + 3.4 P (5-9)
where Vt = total pond volume available (cu ft)
Ab = bottom area of pond (sq ft)
P = bottom perimeter of pond (ft)
A more accurate volume determination can be made with field measurements and area calculations taken
at two elevations. The first elevation at which the pond area is measured is at the top of the permanent
pool. The second area measurement is taken at the overflow spillway elevation.
Vt = d (5-10)
where Vt = total pond volume available (cu ft)
Aw = area of pond (sq ft) measured at the lowest elevation of the overflow spillway (Ab)
Ab = area of pond (sq ft) measured at the top of the permanent pool
d = depth of reservoir (ft) = 1.5 feet
Orifice Sizing
Table 5.1.7.A provides the orifice diameter to be drilled into the PVC cap. If the orifice diameter matches
the PVC pipe diameter, no cap is required. Otherwise, the PVC pipe diameter must be greater than the
required orifice diameter. Select the orifice diameter based on the cleared area tributary to the pond,
interpolating between the values when designing for intermediate tributary acreage.
2
)(bwAA
AGENDA ITEM # 8. a)
CITY OF RENTON SURFACE WATER DESIGN MANUAL
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FIGURE 5.1.7.C SIMPLE DETENTION POND – MINIMUM VOLUME
AGENDA ITEM # 8. a)
SECTION 5.2 INFILTRATION FACILITIES
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TABLE 5.1.7.A SIMPLE DETENTION POND – UNIT VOLUME AND ORIFICE SIZE
Rainfall Region Seatac West Seatac Landsburg West Landsburg East King
County
Unit Volume per
Acre Cleared* 6300 cft 5400 cft 6300 cft 6500 cft 4700 cft**
Acres Cleared Orifice Diameter (decimal inches and equivalent fractional inches, 1/16″ increments)
10,000 sq ft
(0.23 ac)
0.4375″
(7/16″)
0.375″
(3/8″)
0.4375″
(7/16″)
0.5625″
(9/16″)
0.8125″
(13/16″)
0.25 ac 0.4375″
(7/16″)
0.375″
(3/8″)
0.4375″
(7/16″)
0.5625″
(9/16″)
0.8125″
(13/16″)
1 ac 0.875″
(7/8″)
0.75″
(3/4″)
0.875″
(7/8″)
1.1875″
1-3/16″)
1.6875″
(1-11/16″)
2 ac 1.25″
(1-1/4″)
1.0625″
(1-1/16″)
1.25″
(1-1/4″)
1.6875″
(1-11/16″)
2.4375″
(2-7/16″)
3 ac 1.5625″
(1-9/16″)
1.3125″
(1-5/16″)
1.5625″
(1-9/16″)
2.0625″
(2-1/16″)
3.0″
(3″)
* Unit Volume per acre is based on modeling cleared areas as pasture, assuming soil amendment requirements are met, and 1.5 feet of
storage depth in pond with 3:1 side slopes
** Volume variability in regions of increasing rainfall reflects limited single-orifice riser efficiency at shallow storage depths, particularly in
western regions where runoff peaks and volumes are smaller.
HOW TO USE THIS TABLE:
Locate the project on Figure 5.1.7.C.
Design unit volume per acre cleared is selected from the larger of the two values (i.e., not interpolated) associated
with the Rainfall Region isopluvials bracketing the project location.
Determine design volume by multiplying unit volume by cleared acres tributary to facility.
Select and interpolate the orifice diameter based on acreage cleared for the selected region unit volume.
NOTE: Projects proposing to clear an acre or less may qualify for a flow rate increase exception and waiver of the flow
control facility requirement per SWDM 1.2.3, Core Requirement #3. An engineering analysis specific to the project site or
other approval from CED review staff is required to qualify for the exception.
5.1.8 ALTERNATIVE DETENTION SYSTEMS
Manufactures have developed other systems that have properties in common with vaults and tanks, but
that do not conform to the standards for those facility types. These systems may be approved by CED
using suitable design standards adapted from the established standards for similar systems.
5.1.8.1 DESIGN CRITERIA
General
1. Alternative detention systems shall be designed as flow-through systems to promote sediment
removal and facilitate maintenance.
2. Outflow control structures shall be as detailed in Section 5.1.4.
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Access Requirements
1. The maximum depth from finished grade to invert shall be 20 feet.
2. Access openings required within 50 feet from any location in the facility and within 5 feet of each
terminal end. Any location within the facility shall have a direct line of sight from an access point,
unobstructed by any restrictions such as a wall of baffle. The facility must be able to be flushed
without restriction from an access point.
3. All access openings, except those covered by removable panels, shall have round, solid locking
covers (see City of Renton Standard Details), or 3-foot square, locking diamond plate covers. For
raised openings where the depth from the iron cover to the top of the alternative detention system
exceeds 24 inches, an access structure equivalent to a Type 2 catch basin or Type 1 manhole shall be
used (see City of Renton Standard Details). The opening in the lid need not exceed 24 inches in
diameter.
4. All access openings must be readily accessible by maintenance vehicles.
Structural Stability, Access Roads, Right-of-Way, and Setbacks
Alternative detention systems shall comply with the structural stability, access road, right-of-way, and
setback criteria consistent with either detention tanks (Section 5.1.2) or detention vaults (Section 5.1.3),
whichever is most similar to the alternative detention system.
5.1.8.2 METHODS OF ANALYSIS
The volume and outflow design for alternative detention systems shall be in accordance with the
performance requirements in Chapter 1 and the hydrologic analysis and routing/design methods in Chapter
3. Restrictor and orifice design shall be according to Section 5.1.4.
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AGENDA ITEM # 8. a)
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5.2 INFILTRATION FACILITIES
This section presents the methods, criteria, and details for design and analysis of infiltration facilities.
These facilities are used where soils are suitable for soaking the increased runoff from development into
the ground. Such facilities usually have a detention volume component to allow for temporary storage of
runoff while it is being infiltrated. This detention volume is typically dependent on the infiltration capacity
of the soils and the required facility performance.
There are five types of infiltration facilities allowed for use in complying with Core Requirement #3,
“Flow Control”: infiltration ponds, infiltration tanks, infiltration vaults, infiltration trenches, and small
infiltration basins. In general, ponds are preferred because of the ease of maintenance and the water quality
treatment that surface soil and vegetation provide. Tanks and trenches are useful where site constraints
prevent use of a pond, and small infiltration basins are simple to design but have limited uses.
Infiltration facilities are not allowed in Zone 1 of the Aquifer Protection Area.
The information presented in this section is organized as follows:
Section 5.2.1, “General Requirements for Infiltration Facilities”
Section 5.2.2, “Infiltration Ponds”
“Design Criteria,” Section 5.2.2.1
“Methods of Analysis,” Section 5.2.2.2
Section 5.2.3, “Infiltration Tanks”
“Design Criteria,” Section 5.2.3.1
“Methods of Analysis,” Section 5.2.3.2
Section 5.2.4, “Infiltration Vaults”
“Design Criteria,” Section 5.2.4.1
“Methods of Analysis,” Section 5.2.4.2
Section 5.2.5, “Infiltration Trenches”
“Design Criteria,” Section 5.2.5.1
“Methods of Analysis,” Section 5.2.5.2
Section 5.2.6, “Alternative Infiltration Systems”
“Design Criteria,” Section 5.2.6.1
“Methods of Analysis,” Section 5.2.6.2
Section 5.2.7, “Small Infiltration Basins”
“Design Criteria,” Section 5.2.7.1.
5.2.1 GENERAL REQUIREMENTS FOR INFILTRATION FACILITIES
This section presents the design requirements generally applicable to all infiltration facilities. Included are
the general requirements for determining acceptable soil conditions, determining infiltration rates, and
providing overflow protection, spill control, presettling, groundwater protection, protection from upstream
erosion, and construction.
For site selection and design decisions, a geotechnical and hydrogeologic evaluation and report should be
prepared by a licensed engineer with geotechnical and hydrogeologic experience, or a licensed geologist,
AGENDA ITEM # 8. a)
SECTION 5.2 INFILTRATION FACILITIES
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hydrogeologist, or engineering geologist. The design engineer may utilize a team of certified or registered
professionals in soil science, hydrogeology, geology, and other related fields.
SOILS
The applicant must demonstrate through infiltration testing, soil logs, and the written opinion of a
geotechnical professional that sufficient permeable soil exists at the proposed facility location to allow
construction of a properly functioning infiltration facility.
At a minimum, test pits or borings shall extend 5 feet below the bottom of the infiltration facility, and at
least one test hole should reach the water table. If the water table is very deep, the test hole need not
extend more than one-fourth the maximum width of the pond below the bottom of a pond, or more than
5 feet below the bottom of a tank. Measurements shall be made during the period when the water level is
expected to be at a maximum (usually in late winter or early spring). Projects performing a groundwater
mounding analysis may be required to provide more extensive subsurface exploration as described in the
“Groundwater Mounding Analysis” section below.
For projects that perform a groundwater mounding analysis that demonstrates the design is adequate and
that overtopping does not occur, the basic requirement is a minimum of 3 feet of permeable soil below the
bottom of the facility (bottom of pond or excavation for tank) and at least 3 feet between the bottom of the
facility and the maximum wet-season water table. For projects that do not perform a groundwater
mounding analysis as allowed and described in the “Design Infiltration Rate” section below, the basic
requirement is a minimum of 5 feet of permeable soil below the bottom of the facility (bottom of pond or
excavation for tank) and at least 5 feet between the bottom of the facility and the maximum wet-season
water table.
Any requirements associated with impacts to an erosion hazard area, steep slope hazard area, or
landslide hazard should also be addressed in the soil study.
The geotechnical professional shall provide a report stating whether the location is suitable for the
proposed infiltration facility, and shall recommend a design infiltration rate (see “Design Infiltration
Rate” below).
MEASURED INFILTRATION RATES
Infiltration rate tests are used to help estimate the maximum sub-surface vertical infiltration rate of the soil
below a proposed infiltration facility (e.g., pond or tank); an infiltrative BMP serving either more than one
lot, 10,000 square feet or more of impervious surface, 3/4 acre or more of pervious surface or 5,000
square feet or more of pollution generating impervious surface; any BMP explicitly modeled to
accomplish LID Performance Standard criteria (see Section 1.2.9); or a closed depression. The tests are
intended to simulate the physical process that will occur when the facility is in operation; therefore, a
saturation period is required to approximate the soil moisture conditions that may exist prior to the onset of
a major winter runoff event.
Testing Procedure
1. Excavations shall be made to the bottom elevation of the proposed infiltration facility. The measured
infiltration rate of the underlying soil shall be determined using one of the following: a small or large
scale Pilot Infiltration Test (PIT) as described in the 2014 Stormwater Management Manual for
Western Washington and Reference Section 6-A of this manual. The PIT tests have been shown to
more closely match actual full-scale facility performance than other test methods. A single ring
percolation test using a ring at least 3 feet in diameter (see Reference Section 6-A), may be used to
determine BMP infiltration rates used to demonstrate compliance with the LID Performance Standard.
2. The test hole or apparatus shall be filled with water and maintained at depths above the test elevation
for the saturation periods specified for the appropriate test.
3. Following the saturation period, the rate shall be determined in accordance with the specified test
procedures, with a head of 6 inches of water.
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5.2.1 GENERAL REQUIREMENTS FOR INFILTRATION FACILITIES
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4. The design engineer shall perform sufficient tests at multiple locations in a proposed facility footprint
to determine a representative infiltration rate. At least one test per 5,000 square feet (or fraction
thereof) of proposed facility footprint shall be performed, with a minimum of one test for each
proposed infiltration facility location; and at least 2 tests per acre shall be performed for a closed
depression. Proposed bioretention swales require a minimum of 1 test per 200 linear feet of swale;
with a minimum of one test performed per site. Proposed bioretention facilities require a minimum of
1 test per 5,000 square feet of facility footprint; with a minimum of one test performed per site.
5. At a minimum, a soils log shall be obtained for each required infiltration test location. Additional tests
shall be obtained as necessary to capture significant soil variations in the facility footprint. Soils shall
be logged for a minimum of 5 feet below the bottom of each proposed infiltration facility. The logs
shall describe the SCS series of the soil, indicate the textural class of the soil horizons throughout the
depth of the log, note any evidence of high groundwater level (such as mottling), and estimate the
maximum groundwater elevation, if within the limits of the log.
DESIGN INFILTRATION RATE- INFILTRATION FACILITIES AND CLOSED
DEPRESSIONS
In the past, many infiltration facilities have been built that have not performed as the designer intended.
This has resulted in flooding and substantial public expenditures to correct problems. Monitoring of actual
facility performance has shown that the full-scale infiltration rate is far lower than the rate determined by
small-scale testing. Actual measured facility rates of 10% of the small-scale test rate have been seen. It is
clear that great conservatism in the selection of design rates is needed, particularly where conditions are
less than ideal.
The design infiltration rate determination shall include a groundwater mounding evaluation using an
analytical groundwater model to investigate the effects of the local hydrologic conditions on facility
performance. Groundwater modeling will not be required for facilities serving less than 1 acre of tributary
area and where there is at least 5 feet of separation between the bottom of the proposed facility and the
maximum seasonal groundwater table or low permeability stratum, unless requested by CED review staff,
or as part of an analysis in the event of facility failure at performance testing. A ground water mounding
analysis is advisable for facilities with drainage areas smaller than 1 acre if the depth to a low permeability
layer (e.g., less than 0.1 inches per hour) is less than 10 feet. If the ground water in the area is known to be
greater than 50 feet below the proposed facility, detailed investigation of the ground water regime for flow
control design is not necessary.
The preliminary design infiltration rate is determined by applying correction factors to the measured
infiltration rate. The correction factors account for uncertainties in testing, depth to the water table or
impervious strata, infiltration receptor geometry, and long-term reductions in permeability due to
biological activity and accumulation of fines. Equation 5-11 has been developed to account for these
factors. This equation estimates the maximum design infiltration rate (Idesign); additional reduction in rate
beyond that produced by the equation may be appropriate. Note that the design infiltration rate Idesign must
not exceed 20 inches/hour.
Idesign = Imeasured x Ftesting x Fgeometry x Fplugging (5-11)
Correction factor Ftesting accounts for uncertainties in the testing methods. For the small and large scale
Pilot Infiltration Test (PIT), Ftesting = 0.50. For the Single Ring Percolation Test (See Reference
Section 6-A) (used only for determining BMP infiltration rates for demonstrating compliance with the LID
Performance Standard), Ftesting = 0.30.
When expanding an existing infiltration facility, the historical full-scale infiltration performance of the
existing facility may be considered in lieu of the testing procedures above. However, determination of
Ftesting for the expanded facility shall include consideration of the existing facility and site characteristics,
existing infiltration performance relative to the original design, facility maintenance and site maintenance
history, and any other factors influencing the performance of the existing facility. A value for Ftesting
between 0.5 and 1.0, as determined by CED review staff, reflecting the existing facility history shall be
applied to the historical full-scale measured infiltration rate.
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Fgeometry accounts for the influence of facility geometry and depth to the water table or impervious strata on
the actual infiltration rate. A shallow water table or impervious layer will reduce the effective infiltration
rate of a large pond, but this will not be reflected in a small scale test. Clearly, a large pond built over a
thin pervious stratum with a shallow water table will not function as well as the same pond built over a
thick pervious stratum with a deep water table. Fgeometry must be between 0.25 and 1.0 as determined by the
following equation:
Fgeometry = 4 D/W + 0.05 (5-12)
where D = depth from the bottom of the proposed facility to the maximum wet-season water table
or nearest impervious layer, whichever is less
W = width of the facility
Note: When conducting a mounding analysis, apply Fgeometry in the mounding analysis only if facility
geometry is not captured in the groundwater model inputs.
Fplugging accounts for reductions in infiltration rates over the long term due to plugging of soils. This factor
is:
0.7 for loams and sandy loams
0.8 for fine sands and loamy sands
0.9 for medium sands
1.0 for coarse sands or cobbles, or any soil type in an infiltration facility preceded by a water quality
facility.
DESIGN INFILTRATION RATE – BIORETENTION AND PERMEABLE PAVEMENT
For bioretention facilities used to meet the LID Performance Standard, a corrected design infiltration rate
shall be used for the standard bioretention soil mix (BSM) cited in Reference Section 11-C. The corrected
rate assumes a correction factor of either 2 or 4 is applied to the standard BSM uncorrected rate of
12 inches per hour. A corrected design rate of 3 inches per hour is used where the drainage area to the
bioretention device exceeds any of the following:
10,000 sq. ft. of impervious surface
5,000 sq. ft. of pollution-generating impervious surface
3/4 acre of pervious surface
A corrected BSM design rate of 6 inches per hour is used if the contributing drainage area does not exceed
any of the above-listed areas, OR for bioretention where the contributing area exceeds any of the
thresholds above AND the design includes a presettling facility for solids removal.
The design rate of the in situ soils underlying the bioretention soil mix shall be the measured infiltration
rate multiplied by a correction factor ranging from 0.33 to 1 as recommended by a geotechnical
professional. The selected correction factor should be based on the number of tests in relation to the size of
the bioretention facility and site variability.
For permeable pavement used to meet the LID Performance Standard, the design rate of the in situ soils
underlying the permeable pavement shall be the measured infiltration rate multiplied by a correction
factor ranging from 0.33 to 1 (no correction) as recommended by a geotechnical professional. The selected
correction factor should be based on the number of tests in relation to the size of the permeable pavement
and site variability. A further correction factor of 0.9 to 1 (no correction) is determined based on the
quality of the aggregate base material. A correction factor of 1 for the quality of pavement aggregate base
material is allowable if the aggregate base is clean washed material with 1% or less fines passing the
200 sieve.
AGENDA ITEM # 8. a)
5.2.1 GENERAL REQUIREMENTS FOR INFILTRATION FACILITIES
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GROUNDWATER MOUNDING ANALYSIS
Groundwater mounding analysis is generally required for infiltration facilities that serve 1 acre or more of
tributary area and have less than 15 feet of separation to a restrictive layer or groundwater table, as
described in the “Design Infiltration Rate” section above.
Groundwater modeling (mounding analysis) of the proposed infiltration facility shall be done using the
design infiltration rate (i.e., reduction factors applied to the measured rate) modified to exclude the
correction factor for geometry (Fgeometry) and the estimated maximum groundwater elevation determined
for the proposed facility location. It is assumed the groundwater mounding model inputs will capture the
facility geometry for the analysis, however if this is not true for the chosen model, the correction factor for
geometry shall be included in the infiltration rate. Note the use of the design infiltration rate (rather than
the measured rate) results in a conservative analysis of the pond design, but may not be representative of
the lateral extent of the actual groundwater mounding effect. The design professional is advised to
evaluate the true extent of the mound and its effects on adjacent structures, properties, etc.
MODRET or an equivalent model must be used unless CED approves an alternative analytic technique.
More complex analyses (e.g., MODFLOW) may warrant preliminary discussion with CED to ensure the
modeling strategies are acceptable.
Developed condition hydrographs of the project site shall be exported from the approved model for the
groundwater mounding analysis. Hydrographs for the mounding analysis input shall include, at a
minimum, the complete water year (October 1 through September 30) records containing a) the 100-year
peak rate event and b) the cumulative highest 30-day volume event identified through analysis of the
developed condition runoff (the two events are usually in different water years). The peak rate water year
is readily determined from the flow frequency analysis in the approved model. The cumulative highest
30-day volume analysis can be completed in a spreadsheet using the developed condition hydrograph for
the full historical record exported from the approved model. Due to model limitations on the size of the
input files, a 1-hour timestep shall be used to generate the hydrographs to be exported, unless otherwise
required by CED.
The exported hydrograph file will require minor modification in preparation for import into the
groundwater model; see the specific model’s documentation for guidance (MODRET file preparation for
hydrograph input is described in the appendix for the software user’s guide). See Reference Section 6-D
for modeling guidelines specific for use with this manual.
Note that an iterative process may be required beginning with an estimated design rate, facility sizing with
the approved runoff model, then groundwater model testing.
The mounding analysis report shall be included in the Special Reports section of the technical information
report (TIR, see Section 2.3.1.1). All mounding analysis submittals shall have at least the following
information in one package:
Test pit and boring logs, including actual elevations used on the design plans (not just relative
elevations) documenting subsurface explorations to a depth below the base of the infiltration facility
of at least 5 times the maximum design depth of ponded water proposed for the infiltration facility, but
not less than 10 feet below the base of the facility. At sites with shallow ground water (less than
15 feet from the estimated base of facility), if a ground water mounding analysis is necessary,
determine the thickness of the saturated zone. Note that documentation of the thickness and location
of the saturated zone can generally be beneficial to mounding analysis results.
Logs must include at a minimum, depth of pit or hole, soil descriptions, depth to ground water table
and/or bedrock/impermeable layers, presence of stratification. (Note: Logs must substantiate whether
stratification does or does not exist. The licensed professional may consider additional methods of
analysis to substantiate the presence of stratification that will significantly impact the design of the
infiltration facility).
Continuous sampling (representative samples from each soil type and/or unit within the infiltration
receptor) to a depth below the base of the infiltration facility of 2.5 times the maximum design ponded
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water depth, but not less than 10 feet. For large infiltration facilities serving drainage areas of 10 acres
or more, perform soil grain size analyses on layers up to 50 feet deep (or no more than 10 feet below
the water table).
Map showing location of test pits, borings and infiltration facility
Wet season (October 1 to April 30) maximum water table elevation. Monitoring through at least one
wet season is required, unless substantially equivalent site historical data regarding ground water
levels is available.
If mottling or iron oxide staining is present, and that elevation does not reflect the wet season
maximum water table elevation, include a detailed justification.
Description and documentation supporting all modeling input parameters
LS stamped letter documenting constructed volume, elevations, infiltration area (constructed facilities
only)
PE stamped letter documenting TIR volume, elevations and infiltration area (design reviews only)
PE stamped letter (may be the same letter as the previous bullet) documenting rainfall data and
infiltration rate determination used in the analysis. Rainfall data shall be at a minimum, the complete
water year (October 1 through September 30) records containing a) the 100-year peak rate event and
b) the cumulative highest volume event identified through analysis of the developed condition runoff,
both using 1-hour timesteps minimum. Infiltration rate description shall include the initial measured
rate and details of the reduction factors applied per Section 5.2.1, Design Infiltration Rate.
Actual inflow data (electronic files prepared for model input) used in the mounding analysis modeling
runs.
Separate model runs for the peak rate and highest 30-day cumulative volume periods (two runs unless
the events occur in the same water year).
Justifications for safety factors applied to the infiltration rate applied in the modeling.
Geotechnical professional summary and conclusions
Small scale infiltration test data (inches/hour) with calibration factor for test type, then converted to
Vertical Hydraulic Conductivity (feet/day)
Geotechnical professional documentation of why a particular Horizontal Hydraulic Conductivity to
Vertical Hydraulic Conductivity (HHC:VHC) ratio is applicable.
Without detailed justification, the City will accept for MODRET input an HHC:VHC ratio of 1.5:1 for
homogeneous soils and 3:1 for layered soils. Note, however, the vertical conductivity input KVU is for the
unsaturated condition (typical of small-scale or PIT test results), while the horizontal conductivity input
KHS is for the saturated condition.
Alternatively, if small-scale or PIT is the only test information available, the saturated horizontal hydraulic
conductivity could be estimated by applying two adjustment factors as follows:4
KVS (vertical, saturated) = 1.5 KVU (vertical, unsaturated) (5-13)
KHS (horizontal, saturated) = 1.5 KVS (vertical, saturated) (5-14)
PERFORMANCE TESTING
Performance testing and verification for a facility shall be conducted before final construction approval by
the City, or prior to construction of other project improvements or recording of a subdivision as required
by RMC 4-4-060.
For projects where a mounding analysis is not required at the design phase (i.e., facilities serving less than
1 acre of tributary area and where there is at least 5 feet of separation between the bottom of the proposed
facility and the maximum seasonal groundwater table or low permeability stratum), the completed facility
4 Source: State of Florida Dept. of Transportation, Stormwater Management Facility Drainage Handbook, Jan 2004, p. 70
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must be tested and monitored to demonstrate that the facility performs as designed. If the facility
performance is not satisfactory, the facility will need to be modified or expanded as needed in order to
make it function as designed.
Where a groundwater mounding analysis was used in the design, performance testing and verification in
the bottom of the facility to demonstrate that the soils in the constructed facility are representative of the
design assumptions is required. The evaluation shall include measured infiltration rate testing and
evaluation of in-situ soil characteristics and groundwater table location as described in this section. The
measured infiltration rate test procedure should follow the same methodology as during the design phase
to be comparable. If the facility performance evaluation is not satisfactory, the facility will need to be
modified or expanded as needed in order to make it function as designed.
100-YEAR OVERFLOW CONVEYANCE
An overflow route shall be identified for stormwater flows that overtop the facility when infiltration
capacity is exceeded or the facility becomes plugged and fails. The overflow route must be able to safely
convey the 100-year developed peak flow to the downstream conveyance system or other acceptable
discharge point in accordance with conveyance requirements in Section 1.2.4.
Where the entire project site is located within a closed depression (such as some gravel pits), the
requirement to identify and analyze a 100-year overflow pathway may be waived by CED if (1) an
additional correction factor of 0.5 is used in calculating the design infiltration rate, (2) the facility is
sized to fully infiltrate the 100-year runoff event, and (3) the facility is not bermed on any side. Intent: to
address situations where the infiltration facility may be a highly permeable onsite closed depression, such
as a gravel pit, where all stormwater is currently, and will remain, fully infiltrated.
SPILL CONTROL DEVICE
All infiltration facilities must have a spill control device upstream of the facility to capture oil or other
floatable contaminants before they enter the infiltration facility. The spill control device shall be a tee
section per Figure 5.1.4.A or an equivalent device approved by CED. If a tee section is used, the top of the
riser shall be set above the 100-year overflow elevation to prevent oils from entering the infiltration
facility.
PRESETTLING
Presettling must be provided before stormwater enters the infiltration facility. This requirement may be
met by either of the following:
A water quality facility from the Basic WQ menu (this alternative is recommended; see Section 6.1.1
for facility options).
A presettling pond or vault with a treatment volume equal to 0.25 times the basic water quality design
volume (see Section 6.4.1.1 for information on computing this volume).
If water in the WQ facility or presettling facility will be in direct contact with the soil, the facility must be
lined according to the liner requirements in Section 6.2.4. If the presettling facility is a vault, design of the
vault shall be the same as required for presettling cells in sand filter vaults (see Section 6.5.3.2).
The settling pond or vault shall be designed to pool water 4 to 6 feet deep with an overflow capacity
sufficient to pass the developed 100-year peak flow. Settling facilities must have a length-to-width ratio of
at least 3:1. The inlet(s) and outlet should be situated to maximize the length of travel through the settling
pond or vault. Berms or baffles may be used to lengthen the travel distance if site constraints limit the
inlet/outlet placement. Inlets should be designed to minimize velocity and turbulence.
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PROTECTION FROM UPSTREAM EROSION
Erosion must be controlled during construction of areas upstream of infiltration facilities since sediment-
laden runoff can permanently impair the functioning of the system. Erosion control measures must be
designed, installed and maintained with great care. Various strategies may be employed to protect
infiltration facilities during construction, as described below.
Projects may be phased to limit clearing and minimize the time that soils are exposed. An alternative to
this approach is to serve the undeveloped area with a large sediment trap on an undeveloped tract with the
trap left in place until all clearing and construction is complete and all permanent landscaping is in place.
See Erosion and Sediment Control Standards (Appendix D) for design details. At the completion of all
construction, the sediment trap must be cleaned out (taking care that no sediment enters the drainage
system) and filled in, and the flow routed to the permanent drainage system.
Another alternative for subdivisions is to stage excavation of the pond as follows:
1. Bottom elevation of the pond prior to paving of plat roadways: 3 feet above the final pond bottom
elevation. At this stage of rough grading, the facility may be used to meet sediment retention
requirements.
2. Bottom elevation of the pond during and after paving and prior to construction of 80% of the houses:
18 inches above the final pond bottom elevation with upstream sediment retention, as needed. At this
stage, the pond will serve as an interim flow control facility pending final stabilization of the site.
Note that RMC 4-4-060 requires that flow control facilities be operational prior to the construction of
any improvements.
FACILITY CONSTRUCTION GUIDELINES
Excavation of infiltration facilities should be done with a backhoe working at “arm’s length” to minimize
disturbance and compaction of the completed infiltration surface. If the bottom of the facility will be
less than three feet below final grade, the facility area should be cordoned off so that construction traffic
does not traverse the area. The exposed soil should be inspected by a soils engineer after excavation to
confirm that soil conditions are suitable.
Two simple staff gages for measuring sediment depth should be installed at opposite ends of the bottom
of ponds. The gages may consist of 1-inch pipe driven at least one foot into the soil in the bottom of the
pond, with 12 inches of the pipe protruding above grade.
OFFSITE GROUNDWATER LEVEL IMPACTS
Potential impacts to groundwater levels off the project site should be considered. In general, replacing
vegetation with impervious cover will increase the total annual volume of runoff generated on a site.
Infiltrating this runoff will tend to increase ground water recharge, which may affect groundwater levels
offsite. The impacts of infiltration could include increased water to landslide hazards, increased
groundwater resources available, increased water levels in closed depressions, and higher groundwater
levels. Higher groundwater levels offsite could result in increased flooding of basements, or impaired
functioning of infiltration systems resulting in surface water flooding. Evidence of offsite groundwater
flooding problems should be examined during the offsite analysis required under Core Requirement #2
(see Section 1.2.2).
In general, groundwater level impacts will be very difficult to reduce, and there are no specific
requirements that apply in many cases. The design engineer is encouraged to consider whether there are
any feasible approaches to reduce groundwater flooding impacts, such as moving facilities or changing
facility geometry, retaining forest cover, minimizing impervious coverage, or fixing downstream
problems.
AGENDA ITEM # 8. a)
5.2.1 GENERAL REQUIREMENTS FOR INFILTRATION FACILITIES
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GROUNDWATER PROTECTION
The protection of groundwater quality is recognized as an issue of greater concern that in the past, and
groundwater protection standards are changing rapidly, see Section 1.3.6 Core Requirement #6: Aquifer
Protection Area. Increased safeguards are often required. The applicant should refer to the Wellhead
Protection Area Zones layer of COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>) to determine if the project lies
within a groundwater protection area.
In combination with the general requirements for infiltration facilities, compliance with Core Requirement
#8 (Water Quality Facilities) and Special Requirement #5 (Oil Control), where applicable, of this manual
is required to address protection of groundwater quality where infiltrating runoff from pollution generating
surfaces. Water quality facility requirements, exemptions, and exceptions in Core Requirement #8 are
influenced by whether a project is infiltrating within a groundwater protection area, whether the project
is infiltrating into soils with properties required for groundwater protection, which water quality treatment
menu is applicable, and the infiltration site’s measured distance to either a sensitive lake; a fresh water that
has an existing or designated aquatic life use; or a surface water body impaired for phosphorus or metals.
Soil Properties Required for Groundwater Protection
Soil properties required for groundwater protection both outside of and within groundwater protection
areas are listed below. Groundwater protection areas include the Cedar Valley Sole Source Aquifer
Review Area, Wellhead Protection Areas, and the Aquifer Protection Area.
Note: The soil properties given are primarily for groundwater protection and do not necessarily satisfy
other protection needs. For example, projects infiltrating runoff within a quarter-mile of a Sensitive Lake
may still be required to provide water quality treatment to meet the resource protection needs of the
Sensitive Lake. See Core Requirement #8 (Section 1.2.8) for additional WQ requirements.
Soil Properties Required for Groundwater Protection Outside of Groundwater
Protection Areas
For infiltration facilities located outside of groundwater protection areas, acceptable groundwater
protection is provided by the soil if the first two feet or more of the soil beneath the infiltration facility has
a cation exchange capacity5 greater than 5 and an organic content6 of 1.0% or greater, AND meets one of
the following criteria:
1. The soil has a measured infiltration rate less than or equal to 9 inches per hour7 or is logged as one of
the classes from the USDA Textural Triangle (Figure 5.2.1.A,), excluding sand and loamy sand
(Note: soil texture classes other than sand and loamy sand may be assumed to have an infiltration rate
of less than or equal to 9 inches per hour without doing field testing to measure rates.8), OR
2. The soil is composed of less than 25% gravel by weight with at least 75% of the soil passing the
#4 sieve. The portion passing the #4 sieve must meet one of the following gradations:
At least 50% must pass the #40 sieve and at least 2% must pass the #100 sieve, or
At least 25% must pass the #40 sieve and at least 5% must pass the #200 sieve.
5 Cation exchange capacity shall be tested using EPA Laboratory Method 9081. Note that per EPA method 9081 guidance,
distinctly acidic soils require “the method of cation-exchange capacity by summation (Chapman, 1965, p. 900; see
Paragraph 10.1).”
6 Organic content shall be measured on a dry weight basis using method ASTM D2974 for the fraction passing the #40 sieve.
7 See discussion of the measured infiltration rate in Section 5.2.1.
8 Criteria (a) is based on the relationship between infiltration rates and soil texture. However, there are many other factors, such
as high water table, presence of impervious strata or boulders close to the surface, etc., which also affect infiltration rate. When
any such condition is suspected because soils are coarser than expected from the measured infiltration rate, a sieve analysis
should be done to establish soil characteristics. The judgment of a geotechnical professional shall determine whether a sieve
analysis is warranted. The sieve analysis must meet Criteria (b) above to be considered protective.
AGENDA ITEM # 8. a)
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Note: These soil properties must be met by the undisturbed native soils onsite (i.e. in situ). Soil may not be
imported in order to meet groundwater protection criteria.
Soil Properties Required within Groundwater Protection Areas
For projects located within groundwater protection areas (excluding Zone 1 of the Aquifer Protection
Area), acceptable groundwater protection is provided by the soil if the first two feet or more of the soil
beneath the infiltration facility has a cation exchange capacity greater than 5 and an organic content of
1% or greater, AND meets one of the following criteria:
1. The soil has a measured infiltration rate less than or equal to 2.4 inches per hour or is logged as
one of the classes from the USDA Textural Triangle (Figure 5.2.1.A), excluding sand, loamy sand,
and sandy loam (Note: soil triangle texture classes other than sand, loamy sand, and sandy loam may
be assumed to have an infiltration rate of less than or equal to 2.4 inches per hour without doing field
testing to measure rates), OR
2. The soil has a measured infiltration rate less than or equal to 9 inches per hour, and it must be
composed of less than 25% gravel by weight with at least 75% of the soil passing the #4 sieve. The
portion passing the #4 sieve must meet one of the following gradations:
At least 50% must pass the #40 sieve and at least 2% must pass the #100 sieve, or
At least 25% must pass the #40 sieve and at least 5% must pass the #200 sieve.
Note: The above soil properties must be met by the undisturbed native soils onsite (i.e. in situ). Soil may
not be imported in order to meet groundwater protection criteria.
FIGURE 5.2.1.A USDA TEXTURAL TRIANGLE
AGENDA ITEM # 8. a)
5.2.1 GENERAL REQUIREMENTS FOR INFILTRATION FACILITIES
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Infiltration near Water Supply Wells
The design engineer should consider the following when designing infiltration facilities near water supply
wells:
1. In no case should infiltration facilities be placed closer than 100 feet from drinking water wells and
200 feet from springs used for drinking water supplies. Where water supply wells exist nearby, it is
the responsibility of the applicant’s engineer to locate such wells, meet any applicable protection
standards, and assess possible impacts of the proposed infiltration facility on groundwater quality. If
negative impacts on an individual or community water supply are possible, additional runoff treatment
must be included in the facility design, or relocation of the facility should be considered.
2. All infiltration facilities located within the one-year capture zone of any well should be preceded by a
water quality treatment facility.
Infiltration near Steep Slope Hazard Areas and Landslide Hazards
The following restrictions apply to the design of infiltration systems located near a slope steeper than 15%.
1. Where infiltration facilities are proposed within 200 feet of a steep slope hazard area or a landslide
hazard, OR closer to the top of slope than the distance equal to the total vertical height of a slope area
that is steeper than 15%, a detailed geotechnical evaluation is required. The geotechnical analysis
must consider cumulative impacts from the project and surrounding areas under full built- out
conditions.
2. Individual lot infiltration and dispersion systems rather than a centralized infiltration facility should be
used to the extent feasible, except for lots immediately adjacent to a landslide hazard. The runoff
from such lots should be discharged into a tightline system, if available, or other measures should be
implemented as recommended by a geotechnical engineer, engineering geologist, or CED.
UNDERGROUND INJECTION CONTROL WELL REGISTRATION
The Underground Injection Control Program (UIC) administered by WA Ecology protects groundwater
quality by regulating discharges to UIC wells. WA Ecology adopted revisions to Chapter 173-218 WAC,
the UIC program rules, on January 3, 2006. The newly adopted revisions went into effect on February 3,
2006. These rules require the registration of new injection wells that manage stormwater. Information
regarding these new regulations may be found at Ecology’s UIC Program website,
<https://ecology.wa.gov/Regulations-Permits/Guidance-technicalassistance/Underground-injection-
control-program>.
UIC wells are manmade structures used to discharge fluids into the subsurface. Examples are drywells,
infiltration trenches with perforated pipe, and any structure deeper than the widest surface dimension (see
Chapter I-4 UIC Program in the 2019 Stormwater Management Manual for Western Washington
(SWMMWW). For single family projects, drywells that are located immediately adjacent to buildings and
infiltrate roof runoff directly from the gutters and downspouts do not need Ecology registration. Open
ponds are not considered injection wells.
UIC Program rule requirements apply to all UIC wells. If an existing UIC well receives stormwater and
was in use before 2/3/2006, the well owner must complete a well assessment with Ecology to determine if
the UIC well is a high threat to groundwater. See Chapter 173-218-090 (2) WAC UIC Program,
<http://app.leg.wa.gov/WAC/default.aspx?cite=173-218-090> or visit
<https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Underground-
injectioncontrol-program> for more information. An assessment example is available at
https://apps.ecology.wa.gov/publications/documents/1210012.pdf.
If UIC registration is required by Ecology for the proposed design, a copy of the registration, or the
Ecology-issued System ID provided at registration, shall be provided by the applicant prior to plan
approval or permit issuance by the City (see Section 2.3.1.1 Technical Information Report (TIR), TIR
Section 7 Other Permits and Section 5.4.1).
Note that existing UIC wells that are unable to obtain Ecology rule authorization and UIC Site ID number
without modification may require design review and permit approval per City requirements for such
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modifications. Permitting for the modified facility shall follow the UIC registration requirements guidance
for new facilities.
5.2.2 INFILTRATION PONDS
Infiltration ponds may be constructed by excavating or constructing berms. A schematic representation of
a typical infiltration pond is shown in Figure 5.2.2.A. Infiltration ponds are not allowed in Zone 1 of the
Aquifer Protection Area.
5.2.2.1 DESIGN CRITERIA
General
The following criteria for ponds are in addition to the general requirements for infiltration facilities
specified in Section 5.2.1:
1. The proposed pond bottom must be at least 3 feet above the seasonal high groundwater level and
have at least 3 feet of permeable soil beneath the bottom.
2. Infiltration ponds are not allowed on slopes greater than 25% (4H:1V). A geotechnical analysis and
report is required if located within 200 feet of a steep slope hazard area or landslide hazard OR if the
facility is located within a setback distance from top of slope equal to the total vertical height of a
slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts from
the project and surrounding areas under full built- out conditions.
3. The infiltration surface must be in native soil (excavated at least one foot in depth).
4. Maintenance access shall be provided to both the presettling pond or vault (if provided) and the
infiltration pond.
5. An overflow structure such as that shown in Figure 5.1.1.C shall be provided. In addition, infiltration
ponds shall have an emergency spillway as required for detention ponds in Section 5.1.1.1.
6. The criteria for general design, side slopes, embankments, planting, maintenance access, access
roads, fencing, signage, and right-of-way shall be the same as for detention ponds (see
Section 5.1.1), except as required for the infiltration design.
Setbacks
1. The toe of the exterior slope of an infiltration pond berm embankment shall be set back 5 feet from
the tract, easement, or property line.
2. The tract, easement, or property line on an infiltration pond cut slope shall be set back 5 feet from
the emergency overflow water surface.
3. The infiltration pond design water surface shall be set back 100 feet from proposed or existing septic
system drainfields. This setback may be reduced to 30 feet with approval from the Public Health –
Seattle & King County.
4. The infiltration pond design water surface shall be a minimum of 200 feet from any steep slope
hazard area or landslide hazard. Upon analysis and approval of a licensed geotechnical engineer or
engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider
cumulative impacts from the project and surrounding areas under full built- out conditions.
5. The infiltration pond design water surface shall be set back a minimum distance from top of slope
equal to the total vertical height of a slope area that is steeper than 15%. Upon analysis and approval
of a licensed geotechnical engineer or engineering geologist, this setback may be reduced to 50 feet.
The geotechnical analysis must consider cumulative impacts from the project and surrounding areas
under full built- out conditions.
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6. Building setback lines for adjacent internal lots shall be 20 feet. These may be reduced to the
minimum allowed by zoning if the facility soils report addresses the potential impacts of the facility
phreatic surface on structures so located.
7. The infiltration pond design water surface shall be set back 20 feet from external tract, easement or
property lines. This may be reduced to 5 feet if the facility soils report addresses the potential
impacts of the facility phreatic surface on existing or future structures located on adjacent external
lots.
5.2.2.2 METHODS OF ANALYSIS
The size of the pond shall be determined using the hydrologic analysis and routing methods described for
detention ponds in Chapter 3. The storage volume in the pond is used to detain runoff prior to infiltration.
The stage/discharge curve shall be developed from the design infiltration rate determined according to
Section 5.2.1. At a given stage the discharge may be computed using the area of pervious surface
through which infiltration will occur (which will vary with stage) multiplied by the recommended design
infiltration rate (in appropriate units). Berms (which should be constructed of impervious soil such as till),
maintenance access roads, and lined swales should not be included in the design pervious surface area.
AGENDA ITEM # 8. a)
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FIGURE 5.2.2.A SCHEMATIC REPRESENTATION OF A TYPICAL INFILTRATION POND
NOTE:
DETAIL IS A SCHEMATIC REPRESENTATION ONLY. ACTUAL CONFIGURATION WILL VARY
DEPENDING ON SPECIFIC SITE CONSTRAINTS AND APPLICABLE DESIGN CRITERIA.
SECTION A-A
NTS
PLAN VIEW
NTS
OVERFLOW/EMERGENCY OVERFLOW
PROVIDED PER SECTION 5.1.1.1
INFLOW PIPE
ACCESS ROAD
SEE SECTION 5.1.1.1
FOR SPECIFICATIONS
TRACT/EASEMENT
LINES AS REQUIRED
CONNECTING
SPILLWAY
INFILTRATION POND
OUTFLOW/
OVERFLOW
STRUCTURE
SEE FIGURE
5.1.1.B
FOR DETAILS
SEE FIGURE 5.1.1.B
SETTLING POND
IF REQUIRED
GEOTECHNICAL
DESIGN REQUIRED
FOR BERM HEIGHT>6'
6' MIN.
5' MINIMUM A
A
EXISTING
GROUND
EMERGENCY OVERFLOW SPILLWAY
KEY REQUIRED FOR
BERM HEIGHT>4'
SLOPES 3H:1V (TYP.)
AGENDA ITEM # 8. a)
5.2.3 INFILTRATION TANKS
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5.2.3 INFILTRATION TANKS
Infiltration tanks consist of underground pipe that has been perforated to allow detained stormwater to be
infiltrated. A schematic representation of a typical infiltration tank is shown in Figure 5.2.3.A . Infiltration
tanks are not allowed in Zone 1 of the Aquifer Protection Area.
5.2.3.1 DESIGN CRITERIA
General
The following criteria for tanks are in addition to the general requirements for infiltration facilities
specified in Section 5.2.1:
1. The proposed tank trench bottom shall be at least 3 feet above the seasonal high groundwater level
and have at least 3 feet of permeable soil beneath the trench bottom.
2. Infiltration tanks are not allowed on slopes greater than 25% (4H:1V). A geotechnical analysis and
report is required if located within 200 feet of a steep slope hazard area or landslide hazard OR if the
facility is located within a setback distance from top of slope equal to the total vertical height of a
slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts from
the project and surrounding areas under full built-out conditions. The infiltration surface elevation
(bottom of trench) must be in native soil (excavated at least one foot in depth).
3. Spacing between parallel tanks shall be calculated using the distance from the lowest trench bottom
to the maximum wet season ground water surface (D) and the design width of the trench for a single
tank (W). The tank spacing S = W2/D, where S is the centerline spacing between trenches (or tanks) in
feet. S shall not be less than W, and S need not exceed 2W.
4. Tanks shall be bedded and backfilled with washed drain rock that extends at least 1 foot below the
bottom of the tank, at least 2 feet but not more than 5 feet beyond the sides, and up to the top of the
tank.
5. Drain rock (3 to 11/2 inches) shall be completely covered with filter fabric prior to backfilling.
6. The perforations (holes) in the tank must be one inch in diameter and located in the bottom half of
the tank starting at an elevation of 6 inches above the invert of the tank. The number and spacing of
the perforations should be sufficient to allow complete utilization of the available infiltration capacity
of the soils with a safety factor of 2.0 without jeopardizing the structural integrity of the tank.
7. Infiltration tanks shall have an overflow structure equipped with a solid bottom riser (with clean-out
gate) and outflow system for safely discharging overflows to the downstream conveyance system or
another acceptable discharge point.
8. The criteria for general design, materials, structural stability, buoyancy, maintenance access, access
roads, and right-of-way shall be the same as for detention tanks (see Section 5.1.2,), except for
features needed to facilitate infiltration.
Setbacks
1. Tanks shall be set back 100 feet from proposed or existing septic system drainfields. This setback
may be reduced to 30 feet with approval from the Public Health – Seattle & King County.
2. All tanks shall be a minimum of 200 feet from any steep slope hazard area or landslide hazard. Upon
analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may
be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project
and surrounding areas under full built- out conditions.
3. All tanks shall be set back a minimum distance from top of slope equal to the total vertical height of a
slope area that is steeper than 15%. Upon analysis and approval of a licensed geotechnical engineer or
AGENDA ITEM # 8. a)
SECTION 5.2 INFILTRATION FACILITIES
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engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider
cumulative impacts from the project and surrounding areas under full built- out conditions.
4. Building setback lines for adjacent internal lots shall be 20 feet. These may be reduced to the
minimum allowed by zoning if the facility soils report addresses the potential impacts of the facility
phreatic surface on structures so located.
5. Infiltration tanks shall be set back 20 feet from external tract, easement, or property lines. This may
be reduced to 5 feet if the facility soils report addresses the potential impacts of the facility phreatic
surface on existing or future structures located on adjacent external lots.
5.2.3.2 METHODS OF ANALYSIS
The size of the tank shall be determined using the hydrologic analysis and routing methods described in
Chapter 3, and the stage/discharge curve developed from the recommended design infiltration rate as
described in Section 5.2.1. The storage volume in the tank is used to detain runoff prior to infiltration with
the perforations providing the outflow mechanism. At any given stage, the discharge may be computed
using the area of pervious surface through which infiltration will occur multiplied by the recommended
design infiltration rate (in appropriate units). The area of pervious surface used for determining the
potential infiltration from the tank shall be computed by taking the lesser of the trench width, or two times
the width of the tank, and then multiplying by the length of the tank (assuming infiltration through the
bottom of the trench only).
AGENDA ITEM # 8. a)
5.2.3 INFILTRATION TANKS
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FIGURE 5.2.3.A SCHEMATIC REPRESENTATION OF A TYPICAL INFILTRATION TANK
NOTES:
1. ALL METAL PARTS CORROSION RESISTANT. STEEL PARTS
STAINLESS STEEL OR ALUMINIZED STEEL, EXCEPT TANK MAY
BE GALVANIZED AND ASPHALT COATED (TREATMENT 1 OR
BETTER).
2. FILTER FABRIC TO BE PLACED OVER WASHED ROCK BACKFILL
PRIOR TO BACKFILLING OVER FACILITY.
OVERFLOW STRUCTURE
MIN. 54" DIA. TYPE 2 C.B.
SEE SECTION 5.2.3.1
OUTLET PIPE
OUTLET/OVERFLOW
STRUCTURE WASHED ROCK BEDDING
AND BACKFILL TO TOP OF TANK,
MIN. 2' BEYOND EDGES
SETTLING VAULT
OR TYPE 2 C.B.
IF REQUIRED
OPTIONAL PARALLEL TANK
ACCESS RISERS
(MAX SPACING
100-FT)
ACCESS RISERS
SEE FIGURE
5.1.2.B
TYPE 2 C.B. OR
SETTLING VAULT
IF REQUIRED
WASHED ROCK BEDDING
AND BACKFILL TO TOP
OF TANK
FILTER FABRIC
TOP ONLY
RISER FOR
INFILTRATION ONLY;
FOR COMBINED
DETENTION /
INFILTRATION
SEE FIGURE 5.1.4.A
36" MIN. DIA. (TYP.)
DETENTION TANK
SIZE AS REQUIRED
1" HOLES AS
REQUIRED
6" MIN. DEAD STORAGE
PLAN VIEW
NTS
SECTION A-A
NTS
INLET PIPE
(FLOW THROUGH)
100' MAX.
50'
MAX.
4' MIN.
A A
2.0'
MAX.
2'
MIN.
2" MIN. DIAMETER AIR VENT
PIPE WELDED TO TANK
(REQUIRED IF NO ACCESS
RISER ON TANK)
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5.2.4 INFILTRATION VAULTS
Infiltration vaults consist of a bottomless concrete vault structure placed underground in native infiltrative
soils9. Infiltration is achieved through the native soils at the bottom of the structure.
Infiltration vaults are similar to detention vaults. A schematic representation of a detention vault is shown
in Figure 5.1.3.A. Schematic representations of overflow riser are shown in Section 5.1.4. Infiltration
vaults are not allowed in Zone 1 of the Aquifer Protection Area.
5.2.4.1 DESIGN CRITERIA
General
The following criteria for vaults are in addition to the general requirements for infiltration facilities
specified in Section 5.2.1:
1. The proposed vault bottom shall be at least 3 feet above the seasonal high groundwater level and
have at least 3 feet of permeable soil beneath the bottom.
2. Infiltration vaults are not allowed on slopes greater than 25% (4H:1V). A geotechnical analysis and
report is required if located within 200 feet of a steep slope hazard area or landslide hazard OR if the
facility is located within a setback distance from top of slope equal to the total vertical height of the
slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts from
the project and surrounding areas under full built- out conditions.
3. The vault bottom must be in native soil (excavated at least one foot in depth).
4. A suitable means to dissipate energy at the inlet is required to prevent scour and may be accomplished
by using the detail for the sand filter vault (see Figure 6.5.3.A).
5. Infiltration vaults shall have a solid bottom riser (with clean-out gate) and outflow system for safely
discharging overflows to the downstream conveyance system or another acceptable discharge point.
Structural Stability
All vaults shall meet structural requirements for overburden support, buoyancy, and H-20 vehicle loading.
Cast-in-place wall sections shall be designed as retaining walls. Structural designs for vaults must be
stamped by a licensed structural engineer unless otherwise approved by CED. Bottomless vaults shall be
provided with footings placed on stable, well-consolidated native material and sized considering
overburden support, traffic loading (assume maintenance traffic, if placed outside ROW), and lateral soil
pressures when the vault is dry. Infiltration vaults shall not be allowed in fill slopes unless analyzed in a
geotechnical report for stability. The infiltration surface at the bottom of the vault must be in native soil.
Access Requirements
Same as specified for detention vaults in Section 5.1.3.1.
Access Roads
Same as specified for detention vaults in Section 5.1.3.1.
Right-of-Way
Infiltration vaults to be maintained by the City shall be in a stormwater tract granted and converted with all
maintenance obligations (excluding maintenance of drainage facilities contained therein) to the
homeowners association. Any tract not abutting public right-of-way will require a 15-foot wide extension
of the tract to accommodate an acceptable access location. An underlying easement under and upon said
tract shall be dedicated to the City for the purpose of operating, maintaining, improving and repairing the
drainage facilities contain therein. The stormwater tract must be owned by the homeowners association.
Each lot owner within the subdivision shall have an equal and undivided interest in the maintenance of the
9 See Section 5.2.1 and Reference Section 6 for UIC definition and UIC well registration requirements for infiltration vaults
AGENDA ITEM # 8. a)
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stormwater tract. Infiltration vaults to be maintained by a private property owner or homeowners
association shall create stormwater facilities within a private tract or easement or construct the infiltration
vault onsite.
Setbacks
1. Infiltration vaults shall be set back 100 feet from proposed or existing septic system drainfields. This
setback may be reduced to 30 feet with approval from the Public Health – Seattle & King County.
2. Infiltration vaults shall be a minimum of 200 feet from any steep slope hazard area or landslide
hazard. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this
setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from
the project and surrounding areas under full built- out conditions.
3. Infiltration vaults shall be set back a minimum distance from top of slope equal to the total vertical
height of a slope area that is steeper than 15%. Upon analysis and approval of a licensed geotechnical
engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis
must consider cumulative impacts from the project and surrounding areas under full built- out
conditions.
4. Building setback lines for adjacent internal lots shall be 20 feet. These may be reduced to the
minimum allowed by zoning if the facility soils report addresses the potential impacts of the facility
phreatic surface on structures so located.
5. Infiltration vaults shall be set back 20 feet from external tract, easement, or property lines. This
may be reduced to 5 feet if the facility soils report addresses the potential impacts of the facility
phreatic surface on existing or future structures located on adjacent external lots.
5.2.4.2 METHODS OF ANALYSIS
The size of the vault shall be determined using the hydrologic analysis and routing methods described in
Chapter 3 and the stage/discharge curve developed from the recommended design infiltration rate as
described in Section 5.2.1. The storage volume in the vault is used to detain runoff prior to infiltration. At
any given stage, the discharge may be computed using the area of pervious surface through which
infiltration will occur (the exposed soil comprising the vault bottom) multiplied by the recommended
design infiltration rate (in appropriate units).
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5.2.5 INFILTRATION TRENCHES
Infiltration trenches can be a useful alternative for developments with constraints that make siting a pond
difficult. Infiltration trenches may be placed beneath parking areas, along the site periphery, or in other
suitable linear areas. Infiltration trenches are not allowed in Zone 1 of the Aquifer Protection Area.
5.2.5.1 DESIGN CRITERIA
General
The following criteria for trenches are in addition to the general requirements for infiltration facilities
specified in Section 5.2.1:
1. The proposed trench bottom must be at least 3 feet above the seasonal high groundwater level and
3 feet below finished grade.
2. There must be at least 3 feet of permeable soil beneath the trench bottom.
3. The infiltration surface elevation (bottom of trench) must be in native soil (excavated at least one foot
in depth).
4. Infiltration trenches are not allowed on slopes greater than 25% (4H:1V). A geotechnical analysis
and report is required if located within 200 feet of a steep slope hazard area or landslide hazard OR if
the facility is located within a setback distance from top of slope equal to the total vertical height of
the slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts
from the project and surrounding areas under full built- out conditions. Trenches shall be a minimum
of 2 feet wide and no more than 5 feet wide10.
5. Trenches shall be backfilled with 11/2 – 3/4-inch washed rock, completely surrounded by filter fabric
and overlain by a minimum 1 foot of compact backfill.
6. Level 6-inch minimum diameter rigid perforated distribution pipes shall extend the length of the
trench. Distribution pipe inverts shall be a minimum of 2 feet below finished grade. Provisions (such
as clean-out wyes) shall be made for cleaning the distribution pipe. The pipe capacity shall be
calculated to verify that the distribution pipe has capacity to handle the maximum design flow.
7. Alternative trench-type systems such as pre-fabricated bottomless chambers that provide an
equivalent system may be used at the discretion of CED.
8. Two feet minimum cover shall be provided in areas subject to vehicle loads.
9. Trenches shall be spaced no closer than 10 feet, measured on center.
Setbacks
1. Trench systems shall be set back 100 feet from proposed or existing septic system drainfields. This
setback may be reduced to 30 feet with approval from the Public Health – Seattle & King County.
2. Trench systems shall be a minimum of 200 feet from any steep slope hazard area or landslide hazard.
Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback
may be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the
project and surrounding areas under full built- out conditions.
3. Trench systems shall be setback a minimum distance from top of slope equal to the total vertical
height of a slope area that is steeper than 15%. Upon analysis and approval of a licensed geotechnical
engineer or engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis
must consider cumulative impacts from the project and surrounding areas under full built- out
conditions.
10 See Section 5.2.1 and Reference Section 6 for UIC definition and UIC well registration requirements for infiltration trenches.
AGENDA ITEM # 8. a)
5.2.6 ALTERNATIVE INFILTRATION SYSTEMS
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4. A minimum 5-foot setback is required between any part of the trench system and any property line.
5. Structures shall be set back 20 feet from individual trenches. This may be reduced if the facility soils
report addresses potential impacts of trench phreatic surface on structures so located.
5.2.5.2 METHODS OF ANALYSIS
The sections and lengths of trenches shall be determined using the hydrologic analysis and routing
methods for flow control design described in Chapter 3. The stage/discharge curve shall be developed
from the design infiltration rate recommended by the soils engineer, as described in Section 5.2.1.
Storage volume of the trench system shall be determined considering void space of the washed rock
backfill and maximum design water surface level at the crown of the distribution pipe. At any given stage,
the discharge may be computed using the area of pervious surface through which infiltration will occur
(trench bottom area only) multiplied by the recommended design infiltration rate (in appropriate units).
5.2.6 ALTERNATIVE INFILTRATION SYSTEMS
Manufactures have developed other systems that have properties in common with vaults, tanks, and
trenches, but that do not conform to the standards for those facility types. These systems may be approved
by CED using suitable design standards adapted from the established standards for similar systems 11.
Alternative infiltration systems are not allowed in Zone 1 of the Aquifer Protection Area.
5.2.6.1 DESIGN CRITERIA
General
The following criteria for alternative infiltration systems are in addition to the general requirements for
infiltration facilities specified in Section 5.2.1:
1. The proposed infiltration surface must be at least 3 feet above the seasonal high groundwater level.
2. There must be at least 3 feet of permeable soil beneath the infiltration surface.
3. The infiltration surface elevation must be in native soil (excavated at least one foot in depth).
4. Infiltration systems are not allowed on slopes greater than 25% (4H:1V). A geotechnical analysis
and report is required if located within 200 feet of a steep slope hazard area or landslide hazard OR if
the facility is located within a setback distance from top of slope equal to the total vertical height of
the slope area that is steeper than 15%. The geotechnical analysis must consider cumulative impacts
from the project and surrounding areas under full built- out conditions.
5. Systems shall be backfilled with 11/2 – 3/4-inch washed rock or similar material, completely
surrounded by filter fabric and overlain by a minimum 1 foot of compact backfill.
6. Two feet minimum cover shall be provided in areas subject to vehicle loads.
7. Chambers shall be spaced no more than 10 feet apart as measured from the adjacent edges. Inflow
pipes or a manifold system shall be connected to each infiltration chamber. Inspection and
maintenance access to each chamber shall be provided as deemed necessary by the City.
11 See Section 5.2.1 and Reference Section 6 for UIC definition and UIC well registration requirements for alternative infiltration
systems.
AGENDA ITEM # 8. a)
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Access Requirements
1. The maximum depth from finished grade to invert shall be 20 feet.
2. Access openings required within 50 feet from any location in the facility and within 5 feet of each
terminal end. Any location within the facility shall have a direct line of sight from an access point,
unobstructed by any restrictions such as a wall of baffle. The facility must be able to be flushed
without restriction from an access point.
3. All access openings must be readily accessible by maintenance vehicles.
Structural Stability, Access Roads, Right-of-Way, and Setbacks
Alternative infiltration systems shall comply with the structural stability, access road, right-of-way, and
setback criteria consistent with either infiltration tanks (Section 5.2.3), infiltration vaults (Section 5.2.4),
infiltration trenches (Section 5.2.5) whichever is most similar to the alternative infiltration system.
5.2.6.2 METHODS OF ANALYSIS
The sizing and layout of the system shall be determined using the hydrologic analysis and routing methods
for flow control design described in Chapter 3, using the approved continuous runoff model. The
stage/discharge curve shall be developed from the design infiltration rate recommended by the soils
engineer, as described in Section 5.2.1. Storage volume of the system shall be determined considering
void space of the washed rock backfill and the volume contained in system elements. At any given stage,
the discharge may be computed using the area of pervious surface through which infiltration will occur
multiplied by the recommended design infiltration rate (in appropriate units).
5.2.7 SMALL INFILTRATION BASINS
Small infiltration basins consist of a bottomless, precast concrete catch basin or equivalent structure
placed in an excavation filled with washed drain rock. Stormwater infiltrates through the drain rock into
the surrounding soil. This facility is intended for use with contributing surface areas of less than 5,000
square feet. Presettlement is most easily provided by a catch basin or manhole with a turned-down elbow;
see Figure 5.2.7.Afor a schematic representation. If water quality treatment is required by Core
Requirement #8 or Special Requirement #5, runoff from pollution-generating impervious surfaces must
be treated before it enters the infiltration portion of the system. Small infiltration basins are not allowed in
Zone 1 of the Aquifer Protection Area.
5.2.7.1 DESIGN CRITERIA
The design criteria for small infiltration basins are the same as for infiltration tanks (see Sections 5.2.1 and
5.2.3), except that only one infiltration rate test and soil log is required for each small infiltration basin.
Access into the basins shall be provided for inspection and maintenance. Designs may incorporate Type II
catch basins, but equivalent designs using other materials may be accepted 12.
12 See Section 5.2.1 and Reference Section 6 for UIC definition and UIC well registration requirements. Careful consideration of
the catch basin or structure to be used may avoid the requirement to register.
AGENDA ITEM # 8. a)
5.2.7 SMALL INFILTRATION BASINS
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FIGURE 5.2.7.A SCHEMATIC REPRESENTATION OF A SMALL INFILTRATION BASIN
INLET
SLOPE
= 2%
SECTION
NTS
FILTER FABRIC
48" PRECAST
CATCH BASIN
W/O BOTTOM
FILL EXCAVATION BELOW
AND SURROUNDING BASIN
WITH 1-1/2" TO 3" WASHED
DRAIN ROCK
24" DIA. CATCH BASIN LID
8" PVC ELBOW SHORT BEND
8" PVC PIPE
20"X24" STANDARD
CATCH BASIN LID
WITH OVERFLOW GRATE
OIL COLLECTION
SEDIMENT
COLLECTION
CATCH BASIN
WITH BOTTOM
6"
3'
2'
2'
6"
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AGENDA ITEM # 8. a)
2022 City of Renton Surface Water Design Manual 6/22/2022
CHAPTER 6
WATER QUALITY
DESIGN
CITY OF RENTON
SURFACE WATER
DESIGN MANUAL
Section Page
6.1 Water Quality Menus 6-3
6.1.1 Basic Water Quality Menu 6-5
6.1.2 Enhanced Basic Water Quality Menu 6-8
6.1.3 Sensitive Lake Protection Menu 6-10
6.1.4 Sphagnum Bog Protection Menu 6-14
6.1.5 High-Use Menu 6-16
6.1.6 Pretreatment Facilities 6-18
6.2 General Requirements for WQ
Facilities 6-19
6.2.1 Water Quality Design Flows and
Treatment Volumes 6-19
6.2.2 Sequence of Facilities 6-22
6.2.3 Setbacks, Slopes, and Embankments 6-24
6.2.4 Facility Liners 6-28
6.2.5 Flow Splitter Designs 6-32
6.2.6 Flow Spreading Options 6-36
6.3 Vegetated Flowpath Facility Designs 6-41
6.3.1 Basic Bioswales 6-41
6.3.2 Wet Bioswales 6-57
6.3.3 Lateral Inflow Bioswales 6-59
6.3.4 Standard Filter Strips 6-60
6.3.5 Narrow Area Filter Strips 6-68
6.4 Wetpool Facility Designs 6-69
6.4.1 Wetponds — Basic and Large 6-69
6.4.2 Wetvaults 6-84
6.4.3 Stormwater Wetlands 6-90
6.4.4 Combined Detention and Wetpool
Facilities 6-96
6.5 Filtration Facility Designs 6-101
6.5.1 General Requirements For Filtration
Facilities 6-101
6.5.2 Sand Filters — Basic and Large 6-102
6.5.3 Sand Filter Vaults 6-118
6.5.4 Linear Sand Filters 6-123
6.6 Oil Control Facility Designs 6-127
6.6.1 Catch Basin Inserts 6-127
6.6.2 Oil/Water Separators 6-127
6.7 Proprietary Facility Designs 6-141
6.7.1 Ecology Requirements 6-141
6.7.2 City of Renton Requirements 6-141
6.8 Bioretention Facility Designs 6-145
6.8.1 Bioretention 6-145
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6.9 WSDOT WQ Facility Designs 6-159
6.9.1 Media Filter Drain 6-159
6.9.2 Compost-Amended Filter Strips 6-169
6.9.3 Compost-Amended Biofiltration Swales 6-170
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CHAPTER 6
WATER QUALITY DESIGN
This chapter presents the City of Renton approved methods, criteria, and details for analysis and design of
water quality facilities pursuant to Core Requirement #8, discussed in Section 1.2.8, and Special
Requirement #5, discussed in Section 1.3.5.
Chapter Organization
The information in this chapter is organized into the following nine main sections.
Section 6.1, “Water Quality Menus,” details the area-specific water quality menus referred to in
Core Requirement #8 of Chapter 1, and the High-Use Menu referred to in Special Requirement #5,
also in Chapter 1.
Section 6.2, “General Requirements for WQ Facilities,” presents general design requirements and
details pertinent to all water quality facilities.
Section 6.3, “Vegetated Flowpath Facility Designs,” presents the details for analysis and design of
bioswales and filter strips.
Section 6.4, “Wetpool Facility Designs,” presents the details for analysis and design of wetponds,
wetvaults, stormwater wetlands, and combinations of these facilities with detention facilities.
Section 6.5, “Filtration Facility Designs,” presents the details for analysis and design of sand filters.
Section 6.6, “Oil Control Facility Designs,” presents the details for analysis and design of
coalescing-plate oil/water separators.
Section 6.7, “Proprietary Facility Designs,” discusses general considerations for proprietary
manufactured facilities, including summary notes regarding City requirements for approval for use of
these systems. This section points to Reference Section 14-A and Reference Section 14-B, which
includes design and maintenance considerations for proprietary facilities which have been approved
by the City.
Section 6.8, “Bioretention Facility Designs,” presents the details for analysis and design of
bioretention facilities.
Section 6.9, “WSDOT WQ Facility Designs,” presents the details for analysis and design of media
filter drains, compost-amended vegetated filter strips, and compost-amended biofiltration swales.
Required vs. Recommended Design Criteria
Both required and recommended design criteria are presented in this chapter. Criteria stated using “shall”
or “must” are mandatory, to be used unless there is a good reason to deviate as allowed under the
adjustment process in Section 1.4. These criteria are required design criteria and generally affect facility
performance or critical maintenance factors.
Sometimes options are stated as part of the required design criteria using the language “should” or “may.”
These criteria are recommended design criteria, but are closely related to the required criteria, so they
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are placed in the same section. In some cases, recommended design features are presented under a
separate heading in the “Design Criteria” sections.
Design Criteria Applicable To All Facilities
All facilities must be designed and constructed to allow inspection and maintenance.
Use of Chapter 6 Figures
The figures included in this chapter are provided as schematic representations and should not be used for
design. Refer to the City of Renton Standard Details for specific design information. The figures provided
in this chapter illustrate one example of how the WQ facility design criteria may be applied. Although the
figures are meant to illustrate many of the most important design criteria, they may not show all criteria
that apply. In general, the figures are not used to specify requirements unless they are indicated elsewhere
in the manual. If this manual refers to a standard detail not included in the City of Renton Standard
Details, the applicant shall use the figures provided in the manual.1
Water Quality Facility Sizing Worksheets
Sizing worksheets for the major water quality facilities can be found in Reference Section 8-C of the 2021
King County Surface Water Design Manual at: <https://kingcounty.gov/services/environment/water-and-
land/stormwater/documents/surface-water-design-manual.aspx>. These worksheets are based on the step
by step sizing methods given for the water quality facilities in this Chapter. Most design criteria that are
not required for facility sizing are omitted from the worksheets. It is the designer’s responsibility to make
sure that all the required design criteria for each water quality facility are provided on submitted plans.
Facility sizing credits for water quality facilities may be used as allowed and specified in Chapter 1,
Section 1.2.9.3 “Requirements for Use of BMP Credits.”
Please note that the worksheets are dated in the footer of each page. It is the designer’s responsibility to
ensure that any Manual updates affecting the sizing procedure or design criteria after that date are
incorporated into the worksheet. Updates, errata, and clarifications are posted at the City of Renton’s
Surface Water Design Standards website: <www.rentonwa.gov/swdm>.
If there are instances in which the worksheet differs from the design criteria in the text of this Chapter, the
criteria as given in this Chapter, and as modified by subsequent updates, shall be considered the governing
criteria.
1 Footnote 1 is not used.
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6.1 WATER QUALITY MENUS
This section identifies facility choices and, in some cases, non-structural options that comprise the water
quality (WQ) menus referred to in Chapter 1. The menus covered in this section and summarized in
Figure 6.1.A are as follows:
“Basic Water Quality Menu,” Section 6.1.1
“Enhanced Basic Water Quality Menu,” Section 6.1.2
“Sensitive Lake Protection Menu,” Section 6.1.3
“Sphagnum Bog Protection Menu,” Section 6.1.4
“High-Use Menu,” Section 6.1.5
Guide to Applying Water Quality Menus
1. Check the exemption language on Section 1.2.8 to determine if or which threshold discharge areas of
the project site must provide WQ treatment per Core Requirement #8.
2. Use the Basic WQ treatment areas Section 1.2.8.1.A to determine if basic or enhanced treatment is
required.
3. Consult Section 1.2.8.1 for other design requirements, allowances, and flexible compliance provisions
related to implementing water quality treatment.
4. Read the implementation requirements in Chapter 1 (Section 1.2.8.2) that address pollution
generating pervious surface. For some WQ menus, and in some situations, the facility requirements
for these surfaces are eased.
5. Determine if your project fits the definition of a high-use site (see Special Requirement #5 in
Chapter 1). If it does, or if you elect to provide enhanced oil pollution control, choose one of the
options presented in the High-Use menu, Section 6.1.5. Detailed designs for oil control facilities are
given in Section 6.6.
6. General water quality facility requirements (see Section 6.2) apply to all menus and may affect the
placement of facilities on your site.
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FIGURE 6.1.A WATER QUALITY TREATMENT FACILITY SELECTION FLOW CHART
AGENDA ITEM # 8. a)
6.1.1 BASIC WATER QUALITY MENU
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6.1.1 BASIC WATER QUALITY MENU
Where applied: Basic WQ Treatment Areas are designated by the City of Renton where a general, cost-
effective level of treatment is sufficient for most land uses. Some land uses, however, will need an
increased level of treatment because they generate high concentrations of metals in stormwater runoff and
acute concentrations of metals in streams are toxic to fish. The treatment facility requirements for Basic
WQ Treatment Areas provide for this increase in treatment. For precise details on the application of this
and other water quality menus, refer to Section 1.2.8, “Core Requirement #8: Water Quality.”
Treatment goal: The Basic Water Quality menu facility choices are designed to remove 80 percent of
total suspended solids2 (TSS) for flows or volumes up to and including the WQ design flow or volume
(defined in Section 6.2.1). Flows and volumes in excess of the WQ design flow or volume may be routed
around the WQ facility or may be passed through untreated.
Basis:
“The use of TSS as an ‘indicator’ pollutant for sediment is well established.”3
“The control of TSS leads to indirect control of other pollutants of concern that can adhere to
suspended solids in stormwater runoff.”3
“80% TSS removal level is reasonably attainable using properly designed, constructed and maintained
structural stormwater BMPs (for typical ranges of TSS concentration found in stormwater runoff).” 3
For higher removal rates, there are diminishing returns, and relatively less treatment is gained for
incremental increases in facility size.
WA Ecology’s TAPE4 guidance finds 80% removal to be achievable by and a suitable criterion for
proprietary “emerging” technologies.
BASIC WQ OPTION 1 BIOSWALE
A bioswale is a long, gently sloped, vegetated ditch designed to settle out pollutants from stormwater.
Grass is the most common vegetation used. Design details are given in Section 6.3.1. The wet bioswale
(see Section 6.3.2) is a variation of the basic bioswale for use where soils drain poorly, the longitudinal
slope is slight (1.5 percent or less), water tables are high, or continuous base flow is likely to result in
saturated soil conditions. Under such conditions, healthy grass growth is not possible and wetland plants
are used instead. The lateral inflow bioswale (see Section 6.3.3) may be used in situations such as
roadways and parking lots where water enters the swale along the side rather than at one discrete inflow
point at the head of the swale summarizes when the bioswale and its variations are to be applied.
2 The influent concentration range for demonstrated pollutant removal is 100 to 200 mg/L. For influent concentrations lower than
100mg/l the effluent goal is equal to or less than 20 mg/l. For influent concentrations greater than 200 mg/l, the goal is greater
than 80% TSS removal.
3 Source: Knox County Tennessee Stormwater Management Manual, Volume 2, Technical Guidance. Date unknown. Accessed
2014/02/14.
4 Ecology, WA. 2011. Technical Guidance Manual for Evaluating Emerging Stormwater Treatment Technologies: Technology
Assessment Protocol – Ecology (TAPE). In Publication No. 11-10-061, 1-73. Lacey, WA: Washington State Department of
Ecology. <https://fortress.wa.gov/ecy/publications/summarypages/1110061.html>;
<https://fortress.wa.gov/ecy/publications/publications/1110061.pdf>.
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TABLE 6.1.1.A SELECTION OF BIOSWALE TYPE APPROPRIATE FOR SITE
Site Circumstances Bioswale Type
Flow enters at head of swale
Longitudinal slope 1.5% or less OR
Located downstream of a Flow Control Duration
Standard or Flood Problem Flow Control detention
facility
Wet bioswale (Section 6.3.1)
Flow enters at head of swale
Longitudinal slope between 1 and 2%
Soil saturation or base flows likely in wet season
EITHER wet bioswale (Section 6.3.2),
OR basic bioswale (Section 6.3.1),
depending on site; may require
underdrain or low-flow drain.
Flow enters at head of swale
Longitudinal slope between 2% and 5%
Base flows may or may not be likely in wet season
Not downstream of Flow Control Duration Standard
or Flood Problem Flow Control detention facility
Basic bioswale (Section 6.3.1); may
require low-flow drain, depending on
site
Along a roadway or parking lot with:
Sheet inflow into the bioswale, OR
Numerous discrete inflows with no single inflow
contributing more than about 10% of total swale
flow
Lateral inflow bioswale (Section 6.3.3)
BASIC WQ OPTION 2 FILTER STRIP
A filter strip is a gently sloped grassed area which treats stormwater runoff from adjacent paved areas
before it concentrates into discrete channels; see Section 6.3.4 for design details. TSS removal is achieved
by particle settling.
BASIC WQ OPTION 3 WETPOND
Wetponds are stormwater ponds that maintain a pool of water for most of the year. Stormwater entering
the pond is treated during the relatively long residence time within the pond. Wetpond volume described in
Section 6.2.1 for the Basic treatment menu is determined directly by the approved continuous runoff
model. Alternatively, the manual sizing method provided for use in this manual calculates the wetpond
volume based on a method developed by the Natural Resources Conservation Service (NRCS, formerly
the Soil Conservation Service [SCS]). See Section 6.4.1 for design details.
BASIC WQ OPTION 4 WETVAULT
An underground vault may be used to comply with the Basic Water Quality menu. The treatment volume
is the same as for the basic wetpond; see Section 6.4.2 for design details.
BASIC WQ OPTION 5 STORMWATER WETLAND
A stormwater wetland uses biological processes of plant uptake and bacterial degradation as well as
physical and chemical processes, e.g., stilling, and gravity settling to remove pollutants. The footprint of
the stormwater wetland is sized based on the wetpond sizing, but the depth of water in the second cell is
reduced to encourage plant growth; see Section 6.4.3 for design details.
BASIC WQ OPTION 6 COMBINED DETENTION AND WETPOOL FACILITIES
This option allows the wetpond, wetvault, or stormwater wetland to be placed under the detention facility
live storage. Where site conditions permit its use, this option occupies less space than separate siting of
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detention and water quality facilities. The basic wetpond portion of the combined facility is sized using the
same method as the wetpond in Option 3; see Section 6.4.4 for design details.
BASIC WQ OPTION 7 SAND FILTER
A sand filter is a land depression, pond, or vault, with a bed of sand near the bottom. Stormwater is treated
as it percolates downward through the sand layer. Removal efficiency for sand filters is much less
sensitive to particle density distribution as compared to that of particle settling facilities (e.g., ponds,
vaults, bioswales), which include an assumption that the particle density is close to that of silica sand.
Sand filters may be built as open ponds, underground vaults or linear perimeter trenches; see Section 6.5.2
for basic and large sand filters, Section 6.5.3 for sand filter vaults, and Section 6.5.4 for linear sand filters.
A sand layer may also be installed above an infiltration pond or vault to treat stormwater before it
infiltrates. Note: Presettling is required prior to sand filtration as described in Section 6.5.1.
BASIC WQ OPTION 8 PROPRIETARY FACILITIES
Most proprietary facilities for basic treatment are cartridge filters, although there are some media filter
designs that do not involve cartridges. A cartridge filter system is a flow-through stormwater filtration
system comprised of a manhole or vault that houses one or more media-filled or porous membrane
cartridges through which stormwater is filtered.
Note: a presettling cell or facility is required for both cartridge filters and for non-cartridge media filters.
Approved proprietary facilities are listed in Table 6.1.1.B as well as in Reference Section 14-A and 14-B
of this manual. The City reserves the right to modify the list of proprietary facilities approved for public
maintenance at any time. Section 1.4 of Chapter 1 and Reference Section 8 provide relevant information
on the process necessary to obtain approvals of other proprietary facilities.
TABLE 6.1.1.B PROPRIETARY FACILITIES ON THE BASIC WQ MENU
Proprietary Facility Name Publicly Maintained Privately Maintained
BayFilter Stormwater Treatment System
w/ Enhanced 545 Media Catridge
X X
BioPod X X
Boxless BioPod X X
EcoStorm Plus X
Filterra X X
Filterra Bioscape X
Jellyfish Filter X
Kraken Filter X
Modular Wetlands Linear X X
PerkFilter w/ ZPC Media X X
StormFilter w/ PhosphoSorb Media X
StormFilter w/ ZPG Media X X
StormGarden Biofilter X
StormTree X
Up-Flo Filter w/ Filter Ribbons X
WetlandMod X X
Other Facilities with a General Use Level
Designation (GULD) for Basic Treatment
X
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BASIC WQ OPTION 9 BIORETENTION
A bioretention facility is a shallow landscaped depression designed to temporarily store and promote
infiltration of stormwater runoff; see Section 6.8. Where bioretention is intended to fully meet treatment
requirements for its drainage area, it must be designed, using an approved continuous runoff model, to
pass at least 91% of the influent runoff file through the imported soil mix.
BASIC WQ OPTION 10 WSDOT WQ FACILITIES
WSDOT has developed several water quality facilities that may be used to meet basic water quality. These
facilities include the media filter drain or MFD (formerly known as the Ecology Embankment), compost-
amended vegetated filter strips (CAVFS), and compost-amended biofiltration swales (CABS); see
Section 6.9.
The MFD is a linear flow-through treatment facility that includes four basic components: a gravel no-
vegetation zone, a grass strip, the MFD mix bed, and a conveyance system for flows leaving the MFD mix
(typically a gravel-filled underdrain trench or a layer of crushed surfacing base course). MFDs are
typically used in areas with limited right-of-way such as highway side slopes, medians, ditches and other
linear depressions.
CAVFS and CABS are variations of the basic vegetated filter strip and bioswale, respectively, which
incorporate compost to provide Enhanced Basic WQ treatment. The addition of compost into native soils
also improves plant health and sustainability, increases surface roughness, and improves infiltration
capacity.
6.1.2 ENHANCED BASIC WATER QUALITY MENU
Where applied: The Enhanced Basic Water Quality menu5 is applied where an enhanced level of
treatment is required for those development sites with land uses that generate the highest concentrations of
metals in stormwater runoff and drain by surface flows to a fish-bearing stream. Metals including but not
limited to copper and zinc are toxic to fish and other aquatic biota.6 For precise details on the application
of this and other water quality menus, refer to Section 1.2.8, “Core Requirement #8: Water Quality
Facilities.”
Note: The Enhanced Basic menu is a stand-alone menu. It integrates the Basic menu level of protection
(TSS removal) and the additional measures needed to achieve a higher level of metals removal. When this
menu is required in Basic WQ Treatment Areas per Section 1.2.8.1.A of Core Requirement #8, it is
intended to replace the Basic WQ menu on development sites or portions of development sites that
generate the highest concentrations of metals in stormwater runoff. When this menu is required in
Sensitive Lake WQ Treatment Areas per Section 1.2.8.1.B, it is intended to be combined with the Sensitive
Lake Protection Menu such that a facility design option common to both menus must be used.
Treatment goal: The Enhanced Basic WQ menu is designed to achieve > 30% dissolved copper removal
and > 60% dissolved zinc removal; in addition to Basic treatment (80% TSS removal) for flows up to and
including the WQ design flow or volume (defined in Section 6.2.1). The goal assumes that dissolved
copper concentrations for untreated runoff are between 5 and 20 micrograms per liter (u/L), and that
dissolved zinc concentrations for untreated runoff are between 20 and 300 micrograms per liter (ug/L).
Basis: The treatment goal is expressed in terms of dissolved copper and zinc removal. Copper and zinc are
reliable indicators of a wider range of heavy metals and are typically found in stormwater runoff from
industrial, commercial, and high density residential land uses at levels that are toxic to fish and other
aquatic biota. Many metals are readily adsorbed onto particulates in the runoff, usually the finer fraction of
the particulates. Facility combinations that remove more of the particulate load than the Basic menu,
including the finer fraction, are specified by the Enhanced Basic menu. Facilities providing organic
5 The Enhanced Basic WQ menu targets different pollutants than the lake or bog protection menus. It does not necessarily
provide a higher level of treatment except for the target pollutant, metal contaminants.
6 Other metals, e.g., lead, are toxic to humans and may build up in sediments.
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binding sites that enhance metal adsorption are also specified. The treatment goals have been found by
WA Ecology to be achievable.
ENHANCED BASIC OPTION 1 LARGE SAND FILTER
This option includes use of a large sand filter, large sand filter vault, or large linear sand filter. Sizing
specifications for these facilities can be found in Sections 6.5.2, 6.5.3, and 6.5.4, respectively. Note:
Presettling is required prior to sand filtration as described in Section 6.5.1.
ENHANCED BASIC OPTION 2 STORMWATER WETLAND
Provision of a stormwater wetland (see Section 6.4.3) or combined detention and stormwater wetland
(see Section 6.4.4) satisfies the Basic (TSS) and Enhanced Basic (dissolved copper and zinc) removal
goals without additional facilities.
ENHANCED BASIC OPTION 3 TWO-FACILITY TREATMENT TRAIN
This option uses one of the basic water quality treatment options listed in Table 5.1.2.A followed by a
basic sand filter (see Section 6.5.2), sand filter vault (see Section 6.5.3), or a linear sand filter (see
Section 6.5.4).
TABLE 6.1.2.A
PAIRED FACILITIES FOR ENHANCED BASIC TREATMENT TRAIN, OPTION 3
First Basic WQ Facility: Second WQ Facility:
Bioswale
(Sections 6.3.1, 6.3.2, and 6.3.3)
Basic sand filter or sand filter vault
(Section 6.5.2 or 6.5.3) or proprietary facility7
Filter strip
(Sections 6.3.4 and 6.3.5)
Linear sand filter (Section 6.5.4) with no presettling
cell needed
Linear sand filter (Section 6.5.4) Filter strip (Sections 6.3.4 and 6.3.5)
Basic wetpond (Section 6.4.1) Basic sand filter or sand filter vault
(Section 6.5.2 or 6.5.3) or proprietary facility7
Wetvault (Section 6.4.2) Basic sand filter or sand filter vault
(Section 6.5.2 or 6.5.3) or proprietary facility7
Basic combined detention and wetpool
facility (Section 6.4.4)
Basic sand filter or sand filter vault
(Section 6.5.2 or 6.5.3) or proprietary facility7
Basic sand filter or sand filter vault
(Sections 6.5.2 or 6.5.3). A presettling
cell is required if the sand filter is not
preceded by a detention facility.
Proprietary facility7
Proprietary facility approved by the City
for Basic WQ7 (Section 6.7)
Basic sand filter or sand filter vault
(Section 6.5.2 or 6.5.3)
ENHANCED BASIC OPTION 4 BIORETENTION
Provision of a bioretention facility (see Section 6.4.3) satisfies the Basic (TSS) and Enhanced Basic
(dissolved copper and zinc) removal goals without additional facilities. Stormwater runoff that infiltrates
through the imported soil mix will have received Enhanced Basic treatment.
ENHANCED BASIC OPTION 5 PROPRIETARY FACILITY
Section 6.7, “Proprietary Facility Designs,” discusses general considerations for proprietary manufactured
facilities. Current approvals for publicly and privately maintained systems are included in Table 6.1.2.B
7 See Reference Section 14-A for City-approved proprietary facilities.
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and Reference Section 14-A and 14-B. The City reserves the right to modify the list of proprietary
facilities approved for public maintenance at any time.
TABLE 6.1.2.B PROPRIETARY FACILITIES ON THE ENHANCED BASIC WQ MENU
Proprietary Facility Name Publicly Maintained Privately Maintained
BioPod X X
Boxless BioPod X X
Filterra X X
Filterra Bioscape X
Modular Wetlands Linear X X
StormTree X
WetlandMod X X
Other Facilities with a General Use
Level Designation (GULD) for
Enhanced Treatment
X
ENHANCED BASIC OPTION 6 WSDOT WQ FACILITIES
Provision of a MFD, CAVFS, or CABS (see Section 6.4.3) satisfies the Basic (TSS) and Enhanced Basic
(dissolved copper and zinc) removal goals without additional facilities.
6.1.3 SENSITIVE LAKE PROTECTION MENU
This section is not currently applicable to the City of Renton.
Where applied: The Sensitive Lake Protection menu is applied to the watersheds of lakes that have been
determined to be particularly sensitive to phosphorus and that are being managed to reduce water quality
impacts. This menu applies to stormwater conveyed to the lake by surface flow as well as to stormwater
infiltrated within one-quarter mile of the lake in soils with high infiltration rates (i.e., measured rate
exceeding 9 inches per hour). If stormwater is infiltrated further than one-quarter mile from the lake, then
the Basic WQ menu is applied unless the project is exempt from Core Requirement #8 per Section 1.2.8.
For precise details on the application of this and other area-specific water quality menus, refer to Section
1.2.8, “Core Requirement #8: Water Quality.”
Note: The Sensitive Lake Protection menu is a stand-alone menu. It integrates the Basic WQ menu level
of protection (TSS removal) and the additional protection needed to achieve lake protection goals in the
options described below. When this menu is required as specified in Core Requirement #8 (see
Section 1.2.8), it is intended to replace the Basic WQ menu in the watersheds of sensitive lakes.
Treatment goal: The Lake Protection menu is designed to achieve a goal of 50 percent total phosphorus
(TP) removal for the WQ design flow or volume (defined in Section 6.2.1), assuming typical forms and
concentrations of phosphorus in untreated stormwater runoff.8
Basis: The Lake Protection menu will result in removal of more of the TSS load, including more of the
finer fraction of TSS, than the Basic menu. The additional increment of solids removal will also provide
enough phosphorus removal to meet the TP goal stated above.
8 Typical TP concentrations in untreated Seattle-area runoff are considered to be between 0.10 and 0.50 mg/L. For projects that
are expected to generate higher levels of TP, such as animal husbandry operations, a higher treatment goal may be
appropriate.
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LAKE PROTECTION OPTION 1 LARGE WETPOND
The 50 percent TP removal goal can be satisfied by use of a large wetpond or large combined detention
and wetpond sized so that the wetpond volume is 1.5 times the Basic water quality volume as determined
either by the approved continuous runoff model or as calculated using the manual method described in
Section 6.4.1. See Section 6.4.1.1 for the large wetpond design, and Section 6.4.4.1 for the large combined
pond design.
Note: A large wetvault option is not included in this menu since the biological processes thought to
remove phosphorus do not take place in underground vaults.
LAKE PROTECTION OPTION 2 LARGE SAND FILTER
This option includes use of a large sand filter, large sand filter vault, or large linear sand filter. Sizing
specifications for these facilities can be found in Sections 6.5.2, 6.5.3, and 6.5.4, respectively.
Note: Presettling is required prior to sand filtration as described in Section 6.5.1.
LAKE PROTECTION OPTION 3 TWO-FACILITY TREATMENT TRAIN
This option involves use of one of the basic water quality treatment options, listed in Table 6.1.3.A,
followed by either a basic sand filter (Section 6.5.2) or basic sand filter vault (Section 6.5.3). For dispersed
flows, a linear sand filter may be used as the second facility.
TABLE 6.1.3.A
PAIRED FACILITIES FOR LAKE PROTECTION TREATMENT TRAIN, OPTION 3
First Basic WQ Facility Second WQ Facility
Bioswale (Sections 6.3.1, 6.3.2, and 6.3.3) Basic sand filter or sand filter vault
(Section 6.5.2 or 6.5.3)
Filter strip (Sections 6.3.4 and 6.3.5) Linear sand filter (no presettling cell
needed) (Section 6.5.4)
Linear sand filter (Section 6.5.4) Filter strip (Sections 6.3.4 and 6.3.5)
Basic wetpond (Section 6.4.1) Basic sand filter or sand filter vault
(Section 6.5.2 or 6.5.3)
Wetvault (Section 6.4.2) Basic sand filter or sand filter vault
(Section 6.5.2 or 6.5.3)
Stormwater wetland (Section 6.4.3) Basic sand filter or sand filter vault
(Section 6.5.2 or 6.5.3)
Basic combined detention and wetpool facility
(Section 6.4.4)
Basic sand filter or sand filter vault
(Section 6.5.2 or 6.5.3)
Proprietary facility approved by the City for Basic
WQ9
9 (Section 6.7)
Basic sand filter or sand filter vault
(Section 6.5.2 or 6.5.3)
LAKE PROTECTION OPTION 4 BASIC MENU PLUS PHOSPHORUS CREDIT
This option provides credit to developments that integrate land use and site design measures to prevent or
reduce the levels of phosphorus leaving the site. Credit is also given for the voluntary use of extra levels of
onsite detention, since less in-stream erosion is likely to take place with more highly controlled
stormwater releases. This reduction in in-stream erosion and bank failure translates directly into control of
the phosphorus load delivered to downstream lakes.
The measures for which credit is given are detailed below, along with the point values assigned to each of
the actions. Providing any combination of these measures equaling 10 points or more earns this credit. The
9 See Reference Section 14-A for City-approved proprietary facilities.
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credit excuses the applicant from the requirement to provide a second water quality facility. Thus, even
though the development is located in the watershed of a sensitive lake, the water quality requirements can
be fully met with the provision of a single water quality facility from the Basic Water Quality menu.
Credit-Earning Actions
Several land use actions and source controls are particularly effective in reducing phosphorus. These
actions are not required by this manual or other regulations; they are an alternative to end-of-the-pipe
treatment of stormwater. Credit options for phosphorus-reducing actions are described below.
1. Leaving at least 65 percent of the site undisturbed, including undevelopable land. Full credit, or
10 points, is awarded for leaving 65 percent of a site in undisturbed native vegetation or allowing
native vegetation to re-establish. Critical areas and their buffers may be counted. All areas for
phosphorus credit must be in tracts dedicated to the City or protected by covenant. A descending scale
of points applies where lower percentages of the site are left undisturbed. Possible credit = 1 to 10
points.
2. Providing extra flow control. Credit for providing extra flow control applies only in cases where site
runoff travels via stream or open drainage system to the sensitive lake. Voluntary use of the Flow
Control Duration Standard when the Peak Rate Flow Control Standard would be required = 5
points. Voluntary use of the Flood Problem Flow Control Standard when the Peak Rate Flow
Control Standard would be required = 8 points. Voluntary use of the Flood Problem Flow Control
Standard when the Flow Control Duration Standard would be required = 3 points. Possible credit =
3 to 8 points.
3. Directing runoff from target pollution-generating surfaces to grassy areas with level spreading.
Directing runoff from target pollution-generating areas to grassy areas that are not routinely fertilized
or to areas of native vegetation results in pollutant removals similar to those obtained in swales while
also providing an increased opportunity for infiltration. To use this option, flows must remain
unconcentrated and be spread uniformly over the intended area. (Flow spreader details are given in
Section 6.2.6.)
In general, the vegetated area receiving dispersed flows should be at least 25 percent as large as the
area contributing flow. The receiving area should be increased by one percent for each percent
increase in slope over four percent. The area should be configured so that the length of the flow path is
no longer than the width over which flows are dispersed.
Example:
Assume a parking lot is 100′ 600′, or 60,000 sf. Flows will be dispersed through an adjacent
area of native vegetation with a slope of 8 percent.
The area of vegetation must be at least 17,400 sf
(i.e., 25% + 4% (for the 8% slope) 60,000 sf). Assuming runoff is dispersed continuously along
the wider edge of the parking lot, the flow path would need to be at least 29 feet
(17,400′ 600′). If the water were dispersed along the shorter edge, flow path would be 174 feet
(17,400′ 100′). However, this flow path would be longer than the width over which flows were
dispersed (100′), and would not be a satisfactory option. The parking lot could be graded,
however, so that flows would be dispersed at both of the 100 foot ends, making each flow path
87 feet, which would be acceptable.
Credit is proportional to the total volume of runoff diverted; one point is earned for every 25 percent
of total volume so directed. Possible credit = 1 to 4 points.
4. Providing covered vehicle washing areas connected to the sanitary sewer system. This credit
applies to commercial, industrial, and multifamily sites excluding commercial car washes or other
operations where this action is already required by other regulations . Frequent car-washing can
contribute significant amounts of phosphorus to stormwater. Note that sewer districts may have
pretreatment requirements before allowing connection to the sanitary sewer. Possible
credit = 3 points.
Table 6.1.3.B details the credit options and associated point totals.
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Credit may be applied to the whole site or to a natural discharge area within the site. It may be
advantageous for a developer to concentrate only on a natural discharge area if the point total for that
particular area could equal 10. For example, assume a particular natural discharge area is one half the
total site area. If 65 percent of the land area in the natural discharge area will remain undisturbed, that
natural discharge area is eligible for 10 points (see Table 6.1.3.B). The stormwater from that natural
discharge area could be treated with a single water quality facility from the Basic WQ menu; the second
facility could be waived. The rest of the site would still have the two-facility requirement.
Alternatively, if the entire site were considered, the undisturbed area decreases to 35 percent, eligible for
only 3 points. In this case, the developer would need to implement other controls worth 7 points in order to
waive the second water quality facility for the entire site.
If the credit option is used, it shall be applied for during initial drainage review by CED. The application
shall include a written request for credit based on either the site plan or the grading plan for the project,
and the threshold discharge areas shall be delineated on the plans. The request shall outline where the
credit would be applied and how the point totals are to be achieved. CED would then evaluate the request
and may waive the second water quality treatment requirement for the site or threshold discharge area
based on point totals outlined in Table 6.1.3.B (below). Credit is not given unless requested.
TABLE 6.1.3.B WATER QUALITY CREDIT FOR PHOSPHORUS CONTROL
Credit Option Points
Leaving site undisturbed, in native vegetation. At least 65 % = 10
60% = 9
50% = 7
40% = 5
30% = 3
20% = 1
Directing road runoff to pervious, non-pollution-generating
vegetated area.
100% of volume = 4
75% of volume = 3
50% of volume = 2
25% of volume = 1
Covered car wash area connected to sanitary sewer (multifamily,
commercial, or industrial sites, except for commercial car-wash
businesses).
3
Extra detention with next most restrictive release rate (if
discharge to stream).
Peak Rate Flow Control Flow
Control Duration Standard = 5
Peak Rate Flow Control Flood
Problem Flow Control = 8
Flow Control Duration Standard
Flood Problem Flow Control = 3
LAKE PROTECTION OPTION 5 PROPRIETARY FACILITY
Section 6.7, “Proprietary Facility Designs,” discusses general considerations for proprietary manufactured
facilities. Current approvals for publicly and privately maintained systems are included in Table 6.1.3.C
and Reference Section 14-A and 14-B. The City reserves the right to modify the list of proprietary
facilities approved for public maintenance at any time.
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TABLE 6.1.3.C PROPRIETARY FACILITIES ON THE
SENSITIVE LAKE PROTECTION MENU
Proprietary Facility Name Publicly Maintained Privately Maintained
BayFilter Stormwater Treatment System w/
Enhanced 545 Media Cartrdige
X X
BioPod X X
Boxless BioPod X X
Filterra X X
Filterra Bioscape X
Jellyfish Filter X
Kraken Filter X
Modular Wetlands Linear X X
PerkFilter w/ ZPC Media X X
StormFilter w/ PhosphoSorb Media X
StormGarden Biofilter X
StormTree X
Up-Flo Filter w/ Filter Ribbons X
Other Facilities with a General Use Level
Designation (GULD) for Phosphorus Treatment
X
LAKE PROTECTION OPTION 6 WSDOT WQ FACILITIES
WSDOT has developed the media filter drain that may be used to meet lake protection.
6.1.4 SPHAGNUM BOG PROTECTION MENU
This section is not currently applicable to the City of Renton.
Where applied: The Sphagnum Bog Protection menu10 covers sphagnum bog wetlands11 greater than
0.25 acres in size.12 It applies to stormwater conveyed by surface flow to the sphagnum bog vegetation
community. If stormwater is infiltrated by the project per Section 5.2, then the Basic WQ menu is applied
unless the project is exempt from Core Requirement #8, “Water Quality.” For precise details on the
application of this and other area-specific water quality menus, refer to Section 1.2.8.
Note: The Sphagnum Bog Protection menu is a stand-alone menu. It integrates the Basic WQ menu level
of protection and the additional measures needed to achieve bog protection goals in the options described
below. When this menu is required as specified in Core Requirement #8 (see Section 1.2.8), it is intended
to replace the Basic WQ menu in areas draining to sphagnum bogs.
Treatment goal: If surface water must be discharged to a bog, the treatment goal is to reduce total
phosphorus by 50 percent, reduce nitrate + nitrite by 40 percent, maintain alkalinity below 10 mg/L,
calcium concentrations should be less than 2 mg/L, and maintain pH below 6.0.13
10 The Bog Protection menu targets a different set of pollutants than the Sensitive Lake or Enhanced Basic menus. Since the
targeted pollutants are more difficult to remove, use of larger and/or additional water quality facilities is required.
11 A sphagnum bog wetland is defined as a wetland having a predominance of sphagnum moss creating a substrate upon which
a distinctive community of acid-loving plants is established (see Section 1.2.8.C and "Definitions" for more detail).
12 The size of a sphagnum bog wetland is defined by the boundaries of the sphagnum bog plant community.
13 Calcium, alkalinity, and pH values are from : Kulzer, L., S. Luchessa, S. Cooke, R. Errington, F. Weinmann, and D. Vitt. 2001.
Characteristics of the low-elevation sphagnum-dominated peatlands of western Washington: A community profile. King County,
WA: King County Water and Land Resources Division.
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Basis: In their undeveloped condition, bogs are isolated from surface water, being supplied almost solely
by rainwater. The best strategy for protection of bog water quality is to infiltrate the water quality design
volume while routing high flows around the bog. Although it is not known whether alkalinity or nitrogen
can be reduced sufficiently by the options outlined below, there are no other technologically-feasible
alternatives at this time. An adjustment (see Section 1.4) could be pursued as additional technologies
become available.
SPHAGNUM BOG PROTECTION OPTION 1
LARGE WETPOND FOLLOWED BY LARGE SAND FILTER
This option uses a large wetpond (see Section 6.4.1) or a large combined detention and wetpond (see
Section 6.4.2), sized so that wetpond volume is 1.5 times the Basic water quality volume as determined
either by the approved continuous runoff model or as calculated using the manual method described in
Section 6.4.1. A large sand filtration facility (see Section 6.5.2 or 6.5.3) must follow the pond. In order to
ensure that algae and sources of alkalinity from the pond are not washed from the pond into the bog, the
sand filter must be the last facility. The sand used for filtration must be silica-based sand rather
than an aragonite14 sand.
SPHAGNUM BOG PROTECTION OPTION 2
STORMWATER WETLAND IN SERIES WITH A LARGE SAND FILTER
This option uses a stormwater wetland (see Section 6.4.3) or combined detention and stormwater
wetland (see Section 6.4.4) to remove solids and enhance the concentration of organic acids, and a large
sand filter (see Section 6.5.2) to remove the finer sediment for alkalinity and nutrient reduction. The sand
used for filtration must be silica-based sand rather than an aragonite sand. The order of facilities is
interchangeable since there are both advantages and disadvantages to having the sand filter last in the train.
Note: Presettling is required prior to sand filtration as described in Section 6.5.1 .
SPHAGNUM BOG PROTECTION OPTION 3 LARGE SAND FILTER IN SERIES WITH
A PROPRIETARY FACILITY
This option uses a large sand filter or large sand filter vault followed by a proprietary facility. Sizing
specifications for the large sand filters can be found in Sections 6.5.2 and 6.5.3. Proprietary facilities are
detailed in Reference Section 14-A and 14-B. The sand used for filtration must be silica-based sand
rather than an aragonite sand.
Note: Presettling is required prior to sand filtration as described in Section 6.5.1.
SPHAGNUM BOG PROTECTION OPTION 4
THREE-FACILITY TREATMENT TRAIN
This option uses one of the basic water quality treatment options followed by two other facilities.
Table 6.1.4.A lists the possible choices of facilities for this option.
14 Aragonite is the second most common type of sand, and is composed of calcium carbonate from biota including but not limited
to coral and shellfish. (Sand. (2014, April 12). In Wikipedia, The Free Encyclopedia. Retrieved 20:38, April 15, 2014, from
<http://en.wikipedia.org/w/index.php?title=Sand&oldid=603938376>)
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TABLE 6.1.4.A FACILITY COMBINATIONS FOR BOG PROTECTION
TREATMENT TRAIN, OPTION 4
First Facility Second Facility Third Facility
Bioswale (Sections 6.3.1, 6.3.2, and 6.3.3)
Basic sand filter
(Sections 6.5.2,
6.5.3, or 6.5.4)
Proprietary facility15
Filter strip (Sections 6.3.4 and 6.3.5)
Basic wetpond (Section 6.4.1)
Basic combined detention and wetpool facility (Section 6.4.4)
Wetvault (Section 6.4.2)
Stormwater wetland (Section 6.4.3)
Proprietary facility 16F
16 (Section 6.7)
* Other treatment options may be pursued through an adjustment per Section 1.4.
6.1.5 HIGH-USE MENU
Where applied: The High-Use menu is applied to all new development and redevelopment projects that
have high-use site characteristics, as defined in Chapter 1 (see “Special Requirement # 5, Oil Control”).
Oil control devices are to be placed upstream of other facilities, as close to the source of oil generation as
practical. Gasoline service stations will likely exceed the high-use site threshold.
Note: Where this menu is applicable, it is in addition to the area-specific WQ menus.
Treatment goal: Oil control options given in the High-Use menu are designed to meet the goals of no
visible sheen or less than 10 mg/L total petroleum hydrocarbons (TPH) leaving the site.
OIL CONTROL OPTION 1 CATCH BASIN INSERT
This oil control option is not allowed in the City of Renton.
OIL CONTROL OPTION 2 BAFFLE OIL/WATER SEPARATOR
Baffle oil/water separators (see Section 6.6.2) may be used to treat stormwater runoff from high-use
developments and facilities that produce relatively high concentrations of oil and grease. Baffle separators
historically have been effective in removing oil having droplet sizes of 150 microns or larger. If sized
properly, they can achieve effluent concentrations as low as 10 to 15 mg/L.
OIL CONTROL OPTION 3 COALESCING PLATE OIL/WATER SEPARATOR
Coalescing plate separators (see Section 6.6.2) may be used to treat stormwater runoff from high-use
developments and facilities that can produce relatively high concentrations of oil and grease. Current
technology and design of coalescing plate separators achieve effluent concentrations as low as 10 mg/L
with removal of oil droplet sizes as small as 20 to 60 microns.
OIL CONTROL OPTION 4 LINEAR SAND FILTER
The linear sand filter (see Section 6.5.4) is used in the Core Requirement #8 water quality menus (i.e., the
Basic, Enhanced Basic, Sensitive Lake, and Sphagnum Bog menus), as well as for oil control in the High-
Use menu (Special Requirement #5). However, if used to satisfy Core Requirement #8, the same facility
shall not also be used to satisfy the oil control requirement (Special Requirement #5) unless enhanced
maintenance is ensured. This is to prevent clogging of the filter by oil so that it will function for suspended
solids, metals, and phosphorus removal as well. Quarterly cleaning is required at a minimum unless more
frequent cleaning is specified otherwise by the designer.
15 See Reference Section 14-A for City-approved proprietary facilities.
16 See Reference Section 14-A for approved proprietary facilities.
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OIL CONTROL OPTION 5 WETVAULT WITH BAFFLE
A wetvault may be modified to fulfill requirements for oil control provided the following are true:
1. The criteria given at the end of Section 6.4.2.2 for modification of wetvaults for use as a baffle
oil/water separators shall be met, and
2. Assurance is provided that the maintenance frequency and oil removal frequency for baffle oil/water
separators will be followed (see Section 6.6.2).
OIL CONTROL OPTION 6 PROPRIETARY FACILITIES
Section 6.7, “Proprietary Facility Designs,” discusses general considerations for proprietary manufactured
facilities. Current approvals for publicly and privately maintained systems are included in Table 6.1.5.A
and Reference Section 14-A and 14-B. The City reserves the right to modify the list of proprietary
facilities approved for public maintenance at any time.
TABLE 6.1.5.A PROPRIETARY FACILITIES ON THE HIGH-USE MENU
Proprietary Facility Name Publicly Maintained Privately Maintained
Filterra X X
Filterra Bioscape X
Other Facilities with a General Use Level
Designation (GULD) for Oil Treatment
X
OIL CONTROL OPTION 7 COMPLIANCE WITH OTHER AGENCY REQUIREMENTS
If the site has a National Pollutant Discharge Elimination System (NPDES) industrial stormwater permit
that specifically addresses oil control for the target pollution-generating impervious surface of the site,
compliance with NPDES permit conditions may be adequate to comply with the oil control requirements
of Special Requirement #5. Copies of the site’s NPDES permit requirement and the best management
practices specifically addressing oil control shall be submitted to determine adequacy.
If the area under the covered fueling island drains to the sanitary sewer, then only the remaining high-use
area actually draining to the storm drainage system (normally ingress and egress routes) need comply with
the High-Use menu.
Note: Ecology requires that fueling islands be paved with Portland cement concrete (or equivalent, not
including asphaltic concrete) and must drain to a dead-end sump or spill control separator in compliance
with the UFC or IFC, and recommends draining from the sump to a sanitary sewer. An alternative to
discharge to a sanitary sewer is to collect stormwater from the fuel island containment pad and hold for
proper off-site disposal.
Drains to treatment facilities must have a normally closed shutoff valve. The spill control sump must be
sized in compliance with Section 7901.8 of the Uniform Fire Code (UFC). Alternatively the fueling island
must be designed as a spill containment pad with a sill or berm raised to a minimum of four inches
(Section 7901.8 of the UFC) to prevent the runoff of spilled liquids and to prevent run-on of stormwater
from the surrounding area. (See Ecology’s Stormwater Management Manual for Western Washington,
Volume IV, Section 2.2, S409 BMPs for Fueling At Dedicated Stations. These BMPs are also required by
the City of Renton for new construction.
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6.1.6 PRETREATMENT FACILITIES
PRETREATMENT FACILITIES OPTION 1 PROPRIETARY FACILITY DESIGN
Current approvals for publicly and privately maintained systems are included in Table 6.1.6.A and
Reference Section 14-A and 14-B. The City reserves the right to modify the list of proprietary facilities
approved for public maintenance at any time.
TABLE 6.1.6.A PROPRIETARY FACILITIES ON THE
PRETREATMENT FACILITIES MENU
Proprietary Facility Name Publicly Maintained Privately Maintained
Aqua-Swirl CONCENTRATOR
Stormwater Treatment System
X
BaySeparator Stormwater Treatment
System
X
CDS X
Downstream Defender X
Stormceptor X
Vortechs X
Other Facilities with a General Use
Level Designation (GULD) for
Pretreatment
X
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6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES
This section presents general requirements and other information applicable to the design of water quality
(WQ) facilities. Topics covered include the following:
“Water Quality Design Flows,” Section 6.2.1
“Sequence of Facilities,” Section 6.2.2
“Setbacks, Slopes, and Embankments,” Section 6.2.3
“Facility Liners,” Section 6.2.4
“Flow Splitter Designs,” Section 6.2.5
“Flow Spreading Options,” Section 6.2.6
When detail in the WQ designs is lacking, refer to Chapter 5 for guidance. In cases where requirements are
extremely costly, a less expensive alternative that is functionally equivalent in terms of performance,
environmental effects, health and safety, and maintenance may be sought through the adjustment process
(see Section 1.4).
Proprietary Facility Designs
Current proprietary facility approvals for publicly and privately maintained systems are included in
Reference Section 14-A and 14-B. Other proprietary facilities that have received a general use level
designation (GULD) through the state Department of Ecology’s Technology Assessment Protocol –
Ecology (TAPE) program will be considered for approval by the City through an adjustment process for
water quality treatment. A list of Ecology GULD approved proprietary facilities can be found on the
Department of Ecology website at
<http://www.ecy.wa.gov/programs/wq/stormwater/newtech/index.html>.17F
17
Use of Materials
Galvanized metals leach zinc into the environment, especially in standing water situations. High zinc
concentrations, sometimes in the range that can be toxic to aquatic life, have been observed in the region.18
Therefore, use of galvanized materials in stormwater facilities and conveyance systems is discouraged.
Where other metals, such as aluminum or stainless steel, or plastics are available, they shall be used.
Groundwater Protection
Water quality facilities that allow runoff to have direct contact with the soil, such as wetponds,
biofiltration swales, bioretention facilities, infiltration facilities and stormwater wetlands, are prohibited in
Zone 1 of the Aquifer Protection Area.
6.2.1 WATER QUALITY DESIGN FLOWS AND TREATMENT
VOLUMES
Water Quality Design Flow
The water quality design flow is defined as follows:
Downstream of detention: The full 2-year release rate from the detention facility, determined using
the approved continuous runoff model.
Preceding detention, or when detention facilities are not required : The flow rate from the drainage
basin at or below which 91% of the total runoff volume will be treated. Design criteria for
treatment facilities are assigned to achieve the applicable performance goal at the water quality design
flow rate (e.g., 80 percent TSS removal). At a minimum, 91% of the total runoff volume, as estimated
17 Footnote 18 is not used.
18 Finlayson, 1990. Unpublished data from reconnaissance of Metro Park and Ride lot stormwater characteristics.
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by an approved continuous runoff model with 15-minute time steps calibrated to site conditions, must
pass through the treatment facility(ies) at or below the approved hydraulic loading rate for the
facility(ies).
Design flow rates for water quality facilities designed using this manual are calculated using a
continuous simulation model. Most of the performance research on biofiltration BMPs has been
conducted on facilities that used event-based designs. The volume of treatment runoff can be predicted
from a 24-hour storm with a 6-month return frequency (a.k.a., 6-month, 24-hour storm). However, the
91st percentile flow event (as calculated by the continuous model) tends to be less than the estimated
6-month, 24-hour event flow rate in most cases. To maintain sizing comparable to the performance
research, Ecology has developed a correlation between the 91st percentile flow event calculated using
the approved models and the single-event predicted flow event. This correlation is used in sizing water
quality flow-based facilities in Section 6.3 and is presented in Table 6.2.1.A below. Intermediate
values of the ratio k for WWHM are calculated by linear interpolation.
(Note: This table does not apply to flow-based non-water quality BMPs; see relevant flow rate
requirements for flow-based non-water quality BMPs in Appendix C.)
TABLE 6.2.1.A
ADJUSTMENT FACTOR k FOR CALCULATING MODIFIED WATER QUALITY FLOW
RATE FROM MODELED ON-LINE/OFF-LINE RATES
SBUH Peak/WWHM On-Line 15-Min WQ Flow
Ratio vs 6-Month Precipitation for 0% to 100%
Impervious Areas
SBUH Peak/WWHM Off-Line 15-Min WQ Flow
Ratio vs 6-Month Precipitation for 0% to 100%
Impervious Areas
6-Month, 24-Hr
Precipitation
(72% of the 2-yr), Inches
Ratio, k 6-Month, 24-Hr
Precipitation
(72% of the 2-yr), Inches
Ratio, k
0.80 1.01 0.80 1.95
1.00 1.30 1.00 2.44
1.50 2.02 1.50 3.68
2.00 2.74 2.00 4.92
2.50 3.45 2.50 6.16
2.90 4.03 2.90 7.15
Intermediate values of k for WWHM are calculated by linear interpolation.
SBUH Peak/MGSFlood On-Line and Off-Line 15-Min WQ Flow Ratio vs 6-month Precipitation for
0% to 100% Impervious Areas
For on-line facilities: k = 1.4366 (P72%, 2-yr.) – 0.1369 (Eq. 6-1)
For off-line facilities: k = 2.4777 (P72%, 2-yr.) – 0.0352 (Eq. 6-2)
where: P72%, 2-yr = 72% of the 2-year, 24-hour precipitation depth (in.)
Note: If the 6-month, 24-hour precipitation depth (in.) is known for the project site, that value may be
used instead of P72%, 2-yr.
The ratio between the 91st percentile flow event and the estimated 6-month, 24-hour flow rate varies
with location and percent of impervious area in the modeled drainage basin. The correlations in the
table account for these variations. When designing bioswales and other flow rate based facilities,
multiply the on-line or off-line water quality design flow rate determined with the approved model by
the coefficient k (off-line or on-line) determined from the associated table (see Methods of Analysis
for guidance on selection of on-line or off-line flow rate and application of the associated correlation).
Unless amended to reflect local precipitation statistics, the 6-month, 24-hour precipitation amount
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may be assumed to be 72 percent of the 2-year, 24-hour amount determined either with the approved
model or by interpolating between isopluvials for the 2-year, 24-hour precipitation. Isopluvials for
2-year, 24-hour amounts for Western Washington are reprinted in Section 3.2.1, Figure 3.2.1.A.
Flow Volume to be Treated
When water quality treatment is required pursuant to the core and special requirements of this manual, the
water quality design storm volume, when using an approved continuous runoff model, shall be equal to the
simulated daily volume that represents the upper limit of the range of daily volumes that accounts for 91%
of the entire runoff volume over a multi-decade period of record.
Alternatively, the water quality design volume of runoff can be predicted from a 24-hour storm with a
6-month return frequency (a.k.a., 6-month, 24-hour storm). Wetpool facilities are sized based upon use of
the NRCS (formerly known as SCS) curve number equations for the 6-month, 24-hour storm.19 Treatment
facilities sized by this simple runoff volume-based approach are the same size whether they precede
detention, follow detention, or are integral with the detention facility (i.e., a combined detention and
wetpool facility).
The approved model calculates the water quality design volume directly. Alternatively, the NRCS method
described in Section 6.4.1.1 may be used. Unless amended to reflect local precipitation statistics, the
6-month, 24-hour precipitation amount may be assumed to be 72 percent of the 2-year, 24-hour amount.
Interpolating between isopluvials for the 2-year, 24-hour precipitation and multiplying by 72% yields the
appropriate storm size. Isopluvials detailed for 2-year, 24-hour amounts for western King County
(including the City of Renton) are reprinted in Section 3.2.1, Figure 3.2.1.A. For locations east of the
figure limits, precipitation amounts are more variable; use the 2-year, 24-hour isopluvial map located on
the National Oceanic and Atmospheric Administration (NOAA) website at
<http://www.nws.noaa.gov/oh/hdsc/PF_documents/Atlas2_Volume9.pdf>.
Note that facilities which are sized based on volume and which include routing of flows through a
detention facility, such as the detailed sand filter method, are significantly smaller when located
downstream of detention, even though the same volume of water is treated in either situation. This is
because the detention facility routing sequence stores peaks within the pond and releases them at a slow
rate, reducing the size of the sand filter pond subsequently needed (the volume needed to store the peaks
need not be provided again in the sand filter pond).
Treatable Flows
As stated in Chapter 1, only runoff from target pollution-generating surfaces must be treated using the
water quality facility options indicated in the applicable water quality menu. These surfaces include both
pollution-generating impervious surface and pollution-generating pervious surface. “Target” means that
portion from which runoff must be treated using a water quality facility as specified in Chapter 1.
Pollution-generating impervious surfaces are those impervious surfaces which are subject to vehicular
use, industrial activities, or storage of erodible or leachable materials, wastes, or chemicals; and which
receive direct rainfall or the run-on or blow-in of rainfall. Target pollution-generating impervious
surfaces typically include right-of-way improvements (roads), parking areas and driveways that are not
fully dispersed as specified in Section 1.2.3.2. Metal roofs are also considered to be pollution-generating
impervious surface unless they are coated with an inert, non-leachable material (see Reference
Section 11-E); or roofs that are subject to venting significant amounts of dusts, mists, or fumes from
manufacturing, commercial, or other indoor activities. Pollution-generating pervious surfaces are those
non-impervious surfaces subject to use of pesticides and fertilizers, loss of soil, or the use or storage of
erodible or leachable materials, wastes, or chemicals. Target pollution-generating pervious surfaces
typically include lawns and landscaped areas that are not fully dispersed and from which there will be
some concentrated surface discharge in a natural channel or man-made conveyance system from the site.
19 For more information, see Urban Hydrology for Small Watersheds, Technical Release 55 (TR-55), June 1986, published by the
NRCS. See Table 6.4.1.1.xx for CN values to be used with this manual.
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The following points summarize which site flows must be treated and under what circumstances:
All runoff from target pollution-generating impervious surfaces is to be treated through the water
quality facility or facilities required in Chapter 1 and specified in the Chapter 6 menus.
Runoff from lawns and landscaped areas often overflows toward street drainage systems where it is
conveyed to treatment facilities along with the road runoff. However, sometimes runoff from
commercial areas and residential backyards drains into open space or vegetated buffer areas. In these
cases, buffers may be used to provide the requisite water quality treatment provided:
1. Runoff sheet flows into the buffer or a dispersal trench is provided to disperse flows broadly into
the buffer, and
2. The flow path through the pollution-generating area is limited to 200 feet, and
3. The buffer contains only native vegetation and is not itself subject to application of any fertilizers
or pesticides.
Drainage from impervious surfaces that are not pollution-generating (such as patios, walkways, and
some roofs) or are not target pollution-generating surfaces may bypass the water quality facility.
However, this allowance to bypass does not excuse target impervious surfaces from, meeting the flow
control requirements per Core Requirement #3. Note that metal roofs are considered pollution-
generating unless they are treated to prevent leaching (see Reference Section 11-E), as are roofs that
are subject to venting significant amounts of dusts, mists, or fumes from manufacturing, commercial,
or other indoor activities.
Drainage from areas in native vegetation should not be mixed with untreated runoff from streets and
driveways, if possible. It is best to infiltrate or disperse this relatively clean runoff to maximize
recharge to shallow groundwater, wetlands, and streams.
Where runoff from non-pollution-generating impervious areas (non-PGIS), areas in native vegetation,
or any other area not targeted for water quality treatment reaches a water quality facility, flows from
those areas must be included in the sizing calculations for the facility. Once runoff from non-
pollution-generating areas and non-target pollution-generating areas is combined with runoff from
target pollution-generating areas, it cannot be separated before treatment.
6.2.2 SEQUENCE OF FACILITIES
As specified in the water quality menus, where more than one water quality facility is used, the order is
often prescribed. This is because the specific pollutant removal role of the second or third facility in a
treatment train often assumes that significant solids settling has already occurred. For example,
phosphorus removal using a two-facility treatment train relies on the second facility (sand filter) to remove
a finer fraction of solids than those removed by the first facility.
There is a larger question, however, of whether water quality facilities should be placed upstream or
downstream of detention facilities. In general, all water quality facilities may be installed upstream of
detention facilities, although presettling basins are needed for sand filters and infiltration basins. Not all
water quality facilities, however, can be located downstream of detention facilities. Those facilities that
treat sheet flows, such as filter strips and narrow-area filter strips, will seldom be practical downstream of
detention facilities. Other facilities present special problems that must be considered before placement
downstream is advisable.
Two facilities that fall into this latter category are the basic bioswale (see Section 6.3.1) and the sand filter
or sand filter vault (see Sections 6.5.2 or 6.5.3). For these facilities, the prolonged low flows resulting from
Flow Control Duration Standard or Flood Problem Flow Control Standard may interfere with facility
operation. In the case of basic bioswales, prolonged flows, generally in excess of about two weeks, will
cause the grass to die. This can be dealt with by using the wet bioswale design.
In the case of sand filters, prolonged flows may result in the sand being saturated for long periods.
Saturated sand can become hypoxic or anoxic (lose most or all oxygen) when dissolved oxygen in the pore
water becomes depleted. Under these conditions, some previously trapped phosphorus can become soluble
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and be released,20 resulting in phosphorus releases in excess of influent concentrations. To prevent long
periods of sand saturation, adjustments may be necessary after the sand filter is in operation to bypass
some areas of the filter, allowing them to drain completely. If saturated conditions are present after facility
operation, adjustments to the design shall be required. It may also be possible to employ a different
alternative that uses facilities less sensitive to prolonged flows. Table 6.2.2.A summarizes placement
considerations of water quality facilities in relation to detention.
Oil control facilities must be located upstream of water quality facilities and as close to the source of oil-
generating activity as possible. They should also be located upstream of detention facilities, if possible.
TABLE 6.2.2.A WATER QUALITY FACILITY PLACEMENT
IN RELATION TO DETENTION
Water Quality Facility Preceding Detention Following Detention
Basic bioswale
(Section 6.3.1)
OK OK if downstream of detention
sized to meet Peak Rate Flow
Control Standard. However,
prolonged flows may cause soil
saturation and injure grass.
If downstream of a pond sized to
meet Flow Control Duration
Standard or Flood Problem Flow
Control Standard, the wet
bioswale may be needed
(see Section 6.3.2)
Wet bioswale (Section 6.3.2) OK OK
Lateral inflow bioswale
(Section 6.3.3)
OK No—must be installed before
flows concentrate.
Filter strip or roadway filter strip
(Sections 6.3.4 and 6.3.5)
OK No—must be installed before
flows concentrate.
Basic or large wetpond
(Section 6.4.1)
OK OK—less water level fluctuation
in ponds downstream of detention
may improve aesthetic qualities.
Basic or large combined detention
and wetpond (Section 6.4.4)
Not applicable Not applicable
Wetvault (Section 6.4.2) OK OK
Basic or large sand filter or sand
filter vault (Section 6.5.2 or 6.5.3)
OK, but presettling and
control of floatables
needed
OK—sand filters downstream of a
pond sized to meet Flow Control
Duration Standard or Flood
Problem Flow Control Standard
may require field adjustments if
prolonged flows cause sand
saturation and resultant hypoxic,
anoxic or anaerobic conditions,
interfering with the phosphorus
removal mechanism and likely
resulting in episodic phosphorus
releases in excess of influent
concentrations.
20 Bicudo, D. D. C., et al. (2007). "Undesirable side-effects of water hyacinth control in a shallow tropical reservoir." Freshwater
Biology 52(6): 1120-1133.
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TABLE 6.2.2.A WATER QUALITY FACILITY PLACEMENT
IN RELATION TO DETENTION
Water Quality Facility Preceding Detention Following Detention
Stormwater wetland/pond
(Section 6.4.3)
OK OK—less water level fluctuation
and better plant diversity are
possible if the stormwater
wetland is located downstream of
the detention facility.
Proprietary facility (Section 6.7) OK OK
Bioretention (Section 6.8) OK No
6.2.3 SETBACKS, SLOPES, AND EMBANKMENTS
This section presents the general requirements for water quality facility setbacks, side slopes, fencing, and
embankments.
When locating water quality facilities near wetlands and streams, there is a potential that the wetland or
stream water level may be lowered by draining to the facility. Care in the design and siting of the facility
or conveyance elements associated with the facility is needed to ensure this impact is avoided. Sufficient
setback of the facility from the water body is one method to prevent impact.
When locating water quality facilities near steep slopes, there is a potential for slope erosion or
destabilization as a result of seepage, infiltration or overflow.
SETBACKS FROM TRACT LINE
Water quality facilities that are maintained by the City must be in tracts dedicated to the City. Different
water quality facilities and different types of side slopes (bermed vs. cut) have somewhat different
requirements for setback from the tract line or setbacks for structures on adjacent tracts; these various
requirements are given in Table 6.2.3.A.
Most setbacks from tract lines are for maintenance equipment maneuverability. Setback requirements do
not apply to water quality facilities that are privately maintained, but adequate room for maintenance
equipment shall be considered during site design. Restrictions on the placement of structures on adjacent
internal lots, as specified for infiltration facilities in Sections 5.2.2, 5.2.3, and 5.2.4, do however apply to
privately maintained facilities.
FACILITY SITING
New residential subdivisions with drainage facilities that collect public runoff must place water quality
treatment ponds, vaults, and other similar drainage facilities, along with the required perimeter
landscaping in a separate stormwater tract per RMC 4-6-030. The stormwater tract, including the
landscaped area, must be owned by the homeowners association.
Other types of new development shall create stormwater facilities either within an easement or within a
tract not dedicated to the City per RMC 4-6-030.
SIDE SLOPES, FENCING, AND EMBANKMENTS
Side slopes for water quality facilities should not exceed a slope of 3H:1V. Moderately undulating slopes
are acceptable and can provide a more natural setting for the facility. In general, gentle side slopes
improve the aesthetic attributes of the facility and enhance safety. Fencing may be required for public
safety and/or protecting the integrity and function of the facility.
AGENDA ITEM # 8. a)
6.2.3 SETBACKS, SLOPES, AND EMBANKMENTS
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Intent:
The requirements for slopes, fencing, and embankments are intended to accomplish the following
objectives:
To prevent persons from inadvertently slipping into the pond, either by providing gentle interior side
slopes (3H:1V or gentler) or by fencing or other barrier
To allow easy egress from the pond (gentle side slopes, safety benches, etc.) when access is not
restricted by a fence or other barrier
To ensure interior and exterior slopes or embankments are stable and will not create a hazardous or
damaging situation.
Water quality facilities must meet the following requirements for side slopes, fencing, and embankments:
1. All wetponds, stormwater wetlands, and similar facilities shall be fenced per Section 5.1.1. A 6-foot
tall chain link fence shall be provided around the facility with access gate(s) to allow maintenance per
City of Renton Standard Details.
2. All open (uncovered) sand filters require fencing. The intent is to prevent sand filters from being used
recreationally and to exclude domestic animals.
3. Where required, fencing shall be placed at the top of the berm with the maintenance access road on
the inside of the fence or 5 feet minimum from top of berm if there is no maintenance access road
allowing proper maintenance access of the facility. The specific fencing requirements in Chapter 5
(see Section 5.1.1) also apply to WQ facilities. Non-residential commercial or industrial facilities that
are privately owned and maintained must still comply with the fencing requirements in
RMC 4-6-030.
4. Side slopes (interior and exterior) shall be no steeper than 3H:1V.
5. Pond walls may be vertical retaining walls, provided: (a) they are constructed of reinforced concrete
per Section 5.3.3; (b) a fence is provided along the top of the wall; (c) at least 25% of the pond
perimeter will be a vegetated soil slope not steeper than 3H:1V; and (d) the design is prepared and
stamped by a licensed structural civil engineer.
6. Water quality facilities with embankments that impound water must comply with Washington State
dam safety regulations (WAC 173-175). The cited language below is as of February 2012 and is
excerpted verbatim from the Washington Administrative Code except for substitution of Department
of Ecology for department. When reading, substitute facility for dam, and overflow water surface for
crest:
(1) These regulations are applicable to dams which can impound a volume of ten acre-feet or more of
water as measured at the dam crest elevation. The ten acre-feet threshold applies to dams which can
impound water on either an intermittent or permanent basis. Only water that can be stored above
natural ground level or which could be released by a failure of the dam is considered in assessing the
storage volume.
The ten acre-feet threshold applies to any dam which can impound water of any quality, or which
contains any substance in combination with sufficient water to exist in a liquid or slurry state at the
time of initial containment.
(2) For a dam whose dam height is six feet or less and which meets the conditions of subsection (1) of
this section, the Washington Department of Ecology (Ecology) may elect to exempt the dam from
these regulations.
The decision by Ecology to exempt a dam will be made on a case-by-case basis for those dams whose
failure is not judged to pose a risk to life and minimal property damage would be expected.
If the storage capacity is less than 10 acre-feet above natural ground level, then the facility is exempt
from Ecology review.
AGENDA ITEM # 8. a)
SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES
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TABLE 6.2.3.A SETBACK REQUIREMENTS *
WATER QUALITY
FACILITY
SETBACK FROM TRACT LINE
At Grade or
Underground
If Facility Slope is Cut
into Grade
If Slope is an
Embankment
Bioswale N/A See conveyance system
requirements
(Section 4.1)
5 feet from toe of
exterior slope
Filter strip 5 feet from toe 5 feet from toe N/A
Wetpond N/A 5 feet from emergency
overflow water surface
(WS)
5 feet from toe of
exterior slope
Combined detention and
wetpond
N/A 5 feet from emergency
overflow WS
5 feet from toe of
exterior slope
Stormwater wetland N/A 5 feet from emergency
overflow WS
5 feet from toe of
exterior slope
Wetvault or sand filter
vault
5 feet from
property line
N/A N/A
Sand filter ponding area N/A 5 feet from emergency
overflow WS
5 feet from toe of
exterior slope
Linear sand filter 5 feet from
property line
N/A N/A
Proprietary facility21 5 feet from
property line
N/A N/A
Bioretention N/A 5 feet from emergency
overflow WS
5 feet from toe of
exterior slope
* Greater setback distances are required whenever expressly stated or referenced in this manual or when
required by other City codes or other agencies. Steep slopes, land slide areas, open water features, springs,
wells, and septic tank drainfields are features that often have additional setback requirements.
Geotechnical Setbacks: Except for tanks, vaults, and pipes:
1. Facilities are not allowed on slopes greater than 25% (4:1). A geotechnical analysis and report is required if located
within 200 feet of a steep slope hazard area or landslide hazard OR if the facility is located within a setback
distance from top of slope equal to the total vertical height of a slope area that is steeper than 15%. The
geotechnical analysis must consider cumulative impacts from the project and surrounding areas under full built-out
conditions.
2. The facility design water surface shall be a minimum of 200 feet from any steep slope hazard area or landslide
hazard. Upon analysis and approval of a licensed geotechnical engineer or engineering geologist, this setback may
be reduced to 50 feet. The geotechnical analysis must consider cumulative impacts from the project and
surrounding areas under full built-out conditions.
3. The facility design water surface shall be set back a minimum distance from top of slope equal to the total vertical
height of a slope area that is steeper than 15%. Upon analysis and approval of a licensed geotechnical engineer or
engineering geologist, this setback may be reduced to 50 feet. The geotechnical analysis must consider cumulative
impacts from the project and surrounding areas under full built-out conditions.
Public Health Minimum Setbacks for All Facilities:
Some typical setback distances imposed by the Public Health – Seattle & King County include the following:
Open water features: 100 feet.
Wells: 100 feet.
Springs used for potable water: 200 feet.
Septic tanks: 50 feet, and drainfields: 100 feet; tanks or vaults must not be located so that they could impede
septic drainfield flows.
WA Ecology Stormwater Facility Setback Requirements for Public Health:
21 See Reference Section 14-A for approved proprietary facilities.
AGENDA ITEM # 8. a)
6.2.3 SETBACKS, SLOPES, AND EMBANKMENTS
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TABLE 6.2.3.A SETBACK REQUIREMENTS *
Stormwater infiltration systems shall be set back at least 100 feet from open water features and 200 feet from
springs used for drinking water supply. Infiltration facilities up-gradient of drinking water supplies must comply
with State Health Department requirements (Washington Wellhead Protection Program, Department of Health,
12/93).
Stormwater infiltration systems, and unlined wetponds and detention ponds shall be located at least 100 feet
from drinking water wells and septic tanks and drainfields.
Where one agency’s setback requirements are more or less restrictive than another’s, the more restrictive setback is
required.
AGENDA ITEM # 8. a)
SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES
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6.2.4 FACILITY LINERS
Open channel conveyance systems and facilities that allow runoff to have direct contact with the soil may
require liners for any of three reasons: groundwater quality protection, steep slope or building protection,
and/or stormwater treatment facility performance.
Liners are intended to:
1. Reduce the likelihood that pollutants in stormwater will reach ground water by transmission through
soil from earthen facilities and conveyances.
2. Prevent infiltration where underflow could cause problems with steep slopes or nearby structures.
3. Ensure permanent wet pools for proper functioning of wetponds, treatment wetlands, and pre-settling
ponds.
4. Ensure wet conditions in the second cell of stormwater treatment wetland sufficient to maintain
wetland plant vegetation.
Types of Liners
Low Permeability Liners
Low permeability liners reduce infiltration to a very slow rate, generally less than 0.02 inches per hour
(1.22 cm/day). Low permeability liners may be fashioned from compacted till, clay, geomembrane, or
concrete as detailed in Section 6.2.4.1.
Treatment Liners
Treatment liners are soil layers meeting specific quality criteria. Depending on design requirements,
treatment liners may include in-place native soils, amended soils, or imported soils. Treatment liners are
assumed to treat infiltrating stormwater before it reaches more freely draining soils. Treatment liners have
slow rates of infiltration; the initial measured rate should be less than 2.4 inches per hour (1.7 x 10 -3 cm/s),
but rates are not as slow as with low permeability liners. See Section 6.2.4.2 for details.
Where Liners Are Required for Groundwater Protection
Outside of Groundwater Protections Areas
A liner is required for facilities and conveyance systems that allow untreated runoff from PGIS to have
direct contact with the soil if the soil has an initial infiltration rate22 greater than 9 inches per hour (0.15
inches per minute) and the soil suitability criteria for groundwater protection given in Chapter 5, Section
5.2.1 is not met.
Inside Groundwater Protections Areas
Facilities that allow runoff to have direct contact with the soil, on-site BMPs that rely on infiltration, and
open channel conveyance systems that are not concrete lined are not allowed in Zone 1 of the Aquifer
Protection Area. See Section 1.3.6 for details. Other areas, such as Zone 1 Modified or Zone 2 of the
Aquifer Protection Area, may be required to incorporate liners for groundwater protection. A liner is
required for facilities and conveyance sytems that allow untreated runoff from PGIS to have direct contact
with the soil when the soil infiltration rate exceeds an initial infiltration rate of 2.4 inches per hour (0.04
inches per minute) and the soil suitability criteria for groundwater protection given in Chapter 5, Section
5.2.1, is not met.
Where Liners are Required to Ensure Permanent Pools and Wet Conditions
1. Both cells of a two-cell wetpond and the single cell of a one cell wetpond must retain a permanent
pool of water throughout the wet season. A wetpond is considered non-compliant if the pond level
22 Infiltration rates can either be measured in the field using methods given in Chapter 5 or inferred from the USDA soil textural triangle
included in “Groundwater Protection,” Section 5.2.1.
AGENDA ITEM # 8. a)
6.2.4 FACILITY LINERS
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drops more than 12 inches in any 7-day measurement period. A low permeability liner will be required
to achieve this standard in infiltrative soils.
2. Presettling ponds must retain a permanent pool of water throughout the wet season. A presettling pond
is considered non-compliant if the pond level drops more than 12 inches in any 7-day measurement
period. A low permeability liner will be required to achieve this standard in infiltrative soils.
3. Both cells of a stormwater wetland shall be lined in infiltrative soils as follows:
a) The first cell of a treatment wetland must retain a permanent pool of water throughout the wet
season. It is considered non-compliant if the pond level drops more than 12 inches in any 7-day
measurement period. A low permeability liner will be required to achieve this standard in
infiltrative soils.
b) The second cell must retain water for at least 10 months of the year. A low permeability liner will
be required to achieve this standard in infiltrative soils. A treatment liner is an alternative where
groundwater levels and/or existing soil infiltration rates are sufficient to achieve the standard.
General Design Criteria
1. Table 6.2.4.A identifies the type of liner for use with various water quality treatment facilities. If a
facility requires a liner, a treatment liner shall be provided, except where a low permeability liner is
noted in Table 6.2.4.A.
2. Liners shall be evenly placed over the bottom and/or sides of the treatment area of the facility as
indicated in Table 6.2.4.A. Areas above the treatment volume that are required to pass flows greater
than the water quality treatment flow (or volume) need not be lined, except in groundwater protection
areas which must be lined to the 2 year water surface in a combined facility or overflow water surface
in a non-combined facility. Note: If the liner cannot be anchored at the required elevation, the lining
must be extended to the top of the interior side slope and anchored.
3. For low permeability liners, the following criteria apply:
a) Where the seasonal high groundwater elevation is likely to contact a low permeability liner, liner
buoyancy may be a concern. A low permeability liner shall not be used in this situation unless
evaluated and recommended by a geotechnical engineer.
b) Where grass must be planted over a low permeability liner per the facility design, a minimum of
6 inches of good topsoil or compost-amended23 native soil (2 inches compost23 tilled into 6 inches
of native soil) must be placed over the liner in the area to be planted. Twelve inches is preferred.
c) If an identification sign is required for the facility (see detention pond requirements in
Section 5.1.1), the face of the sign shall bear a note indicating the facility is lined to protect water
quality. In addition, the back of the sign shall include information indicating which facilities are
lined, the extent of lining, the liner material used, the liner thickness (if clay or till), and the type
and distance of the marker above the liner (if a geomembrane). This information need only be
readable by someone standing at arms-length from the sign.
4. If a treatment liner will be below the seasonal high water level, the pollutant removal performance of
the liner must be evaluated by a geotechnical or groundwater specialist and found to be as protective
as if the liner were above the level of the groundwater.
See Sections 6.2.4.1 and 6.2.4.2 for more specific design criteria on the various options for low
permeability liners and treatment liners.
23 Compost must meet the compost quality requirements in Reference Section 11-C. Compost for application of this requirement
in stormwater treatment wetlands must be Specification 1 Compost detailed in Reference Section 11-C.
AGENDA ITEM # 8. a)
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TABLE 6.2.4.A LINING TYPES FOR STORMWATER FACILITIES
Facility Type Area to be Lined Type of Liner
Bioswale Bottom and sides Treatment liner
Wet bioswale Bottom and sides Low permeability liner (If the swale will
intercept the seasonal high groundwater
table, a treatment liner is recommended.)
Lateral inflow bioswale Bottom and sides Treatment liner
Presettling pond or basin Bottom and sides Low permeability liner (If the cell will
intercept the seasonal high groundwater
table, a treatment liner is recommended.)
Wetpond First cell: bottom and sides to WQ design
water surface, (except in groundwater
protection areas which must be lined to
the overflow water surface
Low permeability liner (If the cell will
intercept the seasonal high groundwater
table, a treatment liner is recommended.)
Second cell: bottom and sides to WQ
design water surface, (except in
groundwater protection areas which
must be lined to the overflow water
surface
Low permeability liner (If the cell will
intercept the seasonal high groundwater
table, a treatment liner is recommended.)
Single cell: bottom and sides to WQ
design water surface, (except in
groundwater protection areas which
must be lined to the overflow water
surface
Low permeability liner
Combined detention/WQ facility First cell: bottom and sides to the 2-year
live storage elevation
Low permeability liner (If the cell will
intercept the seasonal high groundwater
table, a treatment liner is recommended.)
Second cell: bottom and sides to the
2-year live storage elevation
Low permeability liner (If the cell will
intercept the seasonal high groundwater
table, a treatment liner is recommended.)
Single cell: bottom and sides to the
2-year live storage elevation
Low permeability liner
Wet vault Not applicable No liner needed
Stormwater wetland Bottom and sides, both cells Low permeability liner (If the facility will
intercept the seasonal high groundwater
table, a treatment liner is recommended.)
Sand filter Pond sides only Treatment liner
Detention pond Bottom and sides to the 2-year live
storage elevation
Treatment Liner
Sand filter vault Not applicable No liner needed
Linear sand filter Not applicable if in vault
Bottom and sides of presettling cell if not
in vault
No liner needed
Low permeability or treatment liner
Proprietary filter (in vault) Not applicable No liner needed
Bioretention Bottom and sides (when required per
Section 6.8)
Low permeability liner
AGENDA ITEM # 8. a)
6.2.4 FACILITY LINERS
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6.2.4.1 DESIGN CRITERIA FOR LOW PERMEABILITY LINER OPTIONS
This section presents the design criteria for each of the following four low permeability liner options:
Compacted till liners
Clay liners
Geomembrane liners
Concrete liners
COMPACTED TILL LINERS
1. Liner thickness shall be 18 inches after compaction.
2. Soil shall be compacted to 95% minimum dry density, modified proctor method (ASTM D-1557).
3. Soil should be placed in 6 inch lifts.
4. Soils may be used that meet the following gradation:
Sieve Size Percent Passing
6 inch 100
4 inch 90
#4 70–100
#200 30–100
CLAY LINERS
1. Minimum dry (un-swollen) thickness of 12 inches
2. Compacted to 95% minimum dry density, standard proctor method ASTM D-698
3. Clay Particles Passing, ASTM D-422, not less than 30 percent
4. Plasticity Index of Clay, ASTM D4318, not less than 15 percent
5. The slope of clay liners must be restricted to 3H:IV for all areas requiring soil cover; otherwise, the
soil layer must be stabilized by another method so that soil slippage into the facility does not occur.
Any alternative soil stabilization method must take maintenance access into consideration.
6. Where clay liners form the sides of ponds, the interior side slope should not be steeper than 3H:1V,
irrespective of fencing. This restriction is to ensure that anyone falling into the pond may climb out
safely.
GEOMEMBRANE LINERS
1. Geomembrane liners shall be UV resistant and have a minimum thickness of 30 mils. A thickness of
40 mils shall be used in areas of maintenance access or where heavy machinery must be operated
over the membrane. Protect the geomembrane liner from puncture, tearing, and abrasion by installing
geotextile fabric on the top and bottom of the geomembrane.
2. Geomembranes shall be bedded according to the manufacturer’s recommendations.
3. Liners shall be installed so that they can be covered with 12 inches of top dressing forming the bottom
and sides of the water quality facility. Top dressing shall consist of 6 inches of crushed rock covered
with 6 inches of native soil. The rock layer is to mark the location of the liner for future maintenance
operations. As an alternative to crushed rock, 12 inches of native soil may be used if orange plastic
“safety fencing” or another highly-visible, continuous marker is embedded 6 inches above the
membrane.
4. If possible, liners should be of a contrasting color so that maintenance workers are aware of any areas
where a liner may have become exposed when maintaining the facility.
AGENDA ITEM # 8. a)
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5. Where top dressing is required, liners shall not be used on slopes steeper than 5H:1V, to prevent the
top dressing material from slipping. Textured liners may be used on slopes up to 3H:1V upon
recommendation by a geotechnical engineer or engineering geologist that the top dressing will be
stable for all site conditions, including maintenance.
CONCRETE LINERS
1. Portland cement concrete liners are allowed irrespective of facility size, and shotcrete may be used on
slopes; however, specifications must be developed by an engineer who certifies the liner against
cracking or losing water retention ability under expected conditions of operation, including facility
maintenance operations. Cautionary design note: weight of maintenance equipment can be up to
80,000 pounds when fully loaded.
2. Asphaltic concrete may not be used for a liner because of asphalt’s permeability to many organic
pollutants, and potential for asphalt to leach pollutants into stored or conveyed water.
3. If grass is to be grown in soil over a concrete liner, slopes must be no steeper than 5H:1V to prevent
the top dressing material from slipping.
6.2.4.2 DESIGN CRITERIA FOR TREATMENT LINER OPTIONS
This section presents the design criteria for the organic soil layer used as a treatment liner.
ORGANIC SOIL LAYER
1. A two-foot thick layer of soil with a minimum organic matter (OM) content of 1.0% AND a minimum
cation exchange capacity (CEC) of 8 milliequivalents per 100 grams (meq/100g) can be used as a
treatment layer beneath a water quality or detention facility. If the soil is amended or imported, the top
8 inches must have a minimum cation exchange capacity of 10 meq/100g and the remainder of the
depth no less than 8 meq/100g. An 18-inch layer with the same CEC and OM profile will suffice for
ditch conveyances, based on unsaturated flow as a result of alternating wet-dry periods.
2. To demonstrate that in-place soils meet the above criteria, one sample per 1,000 square feet of facility
area, or 500 linear feet of ditch, and no fewer than three samples shall be tested. Each sample shall be
a composite of equally spaced subsamples taken throughout the full extent of the treatment layer depth
(usually two to six feet below the expected facility invert for facilities), except stratified composite
sampling is required where the top 8 inches are required to meet a higher CEC level (composite of top
eight inches and separate composite of the remainder below).
3. Organic content shall be measured on a dry weight basis using ASTM D2974.
4. Laboratory results shall be provided for cation exchange capacity (CEC).
5. Certification by a soils testing laboratory that imported soil meets the organic content and CEC criteria
above shall be provided to the local approval authority.
6. Soil amendment may only be compost meeting the requirements of Reference Section 11-C. Compost
for application of this requirement in stormwater treatment wetlands must be Specification 1 Compost
detailed in Reference Section 11-C.
7. If a treatment liner will be below the seasonal high water level, the pollutant removal performance of
the liner must be evaluated by a geotechnical or ground water specialist and found to be as protective
as if the liner were above the level of the ground water.
6.2.5 FLOW SPLITTER DESIGNS
Most water quality facilities may be designed as flow-through, or on-line, systems with flows above the
water quality design flow or volume simply passing through the facility untreated. However, it is
sometimes desirable to restrict flows to water quality treatment facilities and bypass the remaining higher
flows around them (off-line facilities). This can be accomplished by splitting flows in excess of the water
AGENDA ITEM # 8. a)
6.2.5 FLOW SPLITTER DESIGNS
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quality design flow upstream of the facility and diverting higher flows to a bypass pipe or channel. The
bypass typically enters a detention facility or the downstream receiving drainage system, depending on
flow control requirements. In most cases, it is a designer’s choice whether WQ facilities are designed as
on-line or off-line; an exception is oil/water separators, which must be designed off-line.
A crucial factor in designing flow splitters is to ensure that low flows are delivered to the treatment facility
up to the WQ design flow rate. Above this rate, additional flows are diverted to the bypass system with
minimal increase in head at the flow splitter structure to avoid surcharging the water quality facility under
high flow conditions.
Flow splitters are typically catch basins or vaults with concrete baffles. In place of baffles, the splitter
mechanism may be a half tee section with a solid top and an orifice in the bottom of the tee section. A full
tee option may also be used (see “Design Criteria” below). Two possible schematic representations for
flow splitters are shown in Figure 6.2.5.A and Figure 6.2.5.B. Other designs that achieve the result of
splitting low flows, up to the WQ design flow, into the WQ treatment facility and divert higher flows
around the facility may be considered (an adjustment per Section 1.4 may be required upon evaluation by
CED staff).
6.2.5.1 METHODS OF ANALYSIS
Flow splitters are modeled with the approved model using the design flow rates as described in
Section 6.2.1. The stage/discharge relationship of the outflow pipes should be determined using the
backwater analysis techniques in Chapter 4. The orifice shall be sized per Section 5.1.4.2. Weirs should be
analyzed as sharp-crested weirs.
6.2.5.2 DESIGN CRITERIA
General
1. A flow splitter shall be designed to deliver the required water quality design flow rate specified in
Section 6.2.1 to the WQ treatment facility.
2. The top of the weir shall be located at the water surface for the design flow. Remaining flows enter
the bypass line. Flows shall be modeled using 15-minute time steps.
3. The maximum head shall be minimized for flow in excess of the water quality design flow.
Specifically, flow to the WQ facility at the 100-year water surface shall not increase the design WQ
flow by more than 10%.
4. Either design shown in Figure 6.2.5.A or Figure 6.2.5.B shall be used.
5. Special applications, such as roads, may require the use of a modified flow splitter. The baffle wall
may be fitted with a notch and adjustable weir plate to proportion runoff volumes other than high
flows.
6. For ponding facilities, backwater effects must be included in designing the height of the standpipe in
the catch basin.
7. Ladder or step and handhold access (per City of Renton Standard Details) shall be provided. If the
weir wall is higher than 36 inches, two ladders, one to either side of the wall, are required.
Material Requirements
1. The splitter baffle shall be installed in a Type 2 catch basin or vault.
2. The baffle wall shall be made of reinforced concrete or another suitable material resistant to
corrosion, and have a minimum 4-inch thickness. The minimum clearance between the top of the
baffle wall and the bottom of the catch basin cover shall be 4 feet; otherwise, dual access points shall
be provided.
3. All metal parts shall be corrosion resistant. Examples of preferred materials include aluminum,
stainless steel, and plastic. Zinc and galvanized materials, bronze and brass, and cadmium-plated
hardware shall not be used unless there is no substitute, because of aquatic toxicity. Painting or other
AGENDA ITEM # 8. a)
SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES
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coating of metal parts shall not be allowed because of poor longevity and lack of standardization or
assurance of non-toxic coatings.
FIGURE 6.2.5.A SCHEMATIC REPRESENTATION OF FLOW SPLITTER, OPTION A
SECTION A-A
NTS
PLAN VIEW
NTS
INFLOW
NOTE:
THE WATER QUALITY DISCHARGE PIPE MAY REQUIRE AN ORIFICE PLATE TO BE INSTALLED ON
THE OUTLET TO CONTROL THE HEIGHT OF THE DESIGN WATER SURFACE (WEIR HEIGHT). THE
DESIGN WATER SURFACE SHOULD BE SET TO PROVIDE A MINIMUM HEADWATER/DIAMETER
RATIO OF 2.0 ON THE OUTLET PIPE.
TO WQ FACILITY
TO BYPASS CONVEYANCE
SYSTEM OR DETENTION POND
A
4" MIN. THICKNESS
REINFORCED CONCRETE
BAFFLE WALL OR OTHER
SUITABLE MATERIAL
BYPASS PIPE
4' MIN.
OR PROVIDE SEPARATE
ACCESS TO EITHER SIDE
OF BAFFLE WALL
WQ DESIGN WATER
SURFACE ELEVATION
HANDHOLD AND STEPS OR
LADDER ACCESS (PROVIDE
LADDERS TO BOTH SIDES
OF WALL IF WEIR >36" HIGH)
ROUND SOLID LID
(KCRDCS DWG 7-022 AND 7-023)
INFLOW
REINFORCED BAFFLE WALL
GROUTED TO MANHOLE
STRUCTURE (BOTH ENDS)
TYPE 2 C.B.4' MIN.2' MIN.
A
TO WQ FACILITY
AGENDA ITEM # 8. a)
6.2.5 FLOW SPLITTER DESIGNS
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FIGURE 6.2.5.B SCHEMATIC REPRESENTATION OF FLOW SPLITTER, OPTION B
INFLOW
SECTION A-A
NTS
PLAN VIEW
NTS
TO WATER QUALITY
FACILITY
TO WATER QUALITY
FACILITY
TO BYPASS CONVEYANCE
SYSTEM OR DETENTION POND
INFLOW
*NOTE:
DIAMETER OF STANDPIPE SHOULD BE LARGE ENOUGH TO MINIMIZE
HEAD ABOVE WQ DESIGN WS AND TO KEEP WQ DESIGN FLOWS FROM
INCREASING MORE THAN 10% DURING 100-YEAR FLOWS.
A 2d*6"6"7' MIN.2' MIN.d
SOLID BOTTOM
(PROVIDE
MAINTENANCE
ACCESS)
LADDER (TYP.)
TOP OF PIPE AT WQ DESIGN
WATER SURFACE ELEVATION
ROUND SOLID LID (SEE
KCRDCS DWG 7-022 AND 7-023)
BAFFLE TO CONTROL FLOATABLES, OR
PROVIDE SPILL CONTROL UPSTREAM
"TEE" SECTION WITH CLEANOUT
(OR REMOVABLE BEND-DOWN
ELBOW)
ORIFICE SIZED TO PASS
WQ DESIGN FLOW
TOP OF RISER AT
DESIGN WS ELEVATION
TYPE II CB
NO BASE CHANNEL REQ'D
DIA. OF STANDPIPE*
(2 X DIA. OF OUTLET PIPE
RECOMMENDED AS
STARTING POINT, SEE
NOTE)
BAFFLE TO CONTROL
FLOATABLES
AGENDA ITEM # 8. a)
SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES
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6.2.6 FLOW SPREADING OPTIONS
Flow spreaders function to uniformly spread flows across the inflow portion of water quality facilities
(e.g., sand filter, bioswale, or filter strip).
There are five flow spreader options presented in this section:
Anchored section: Anchored plate or board (Option A)
Concrete sump box (Option B)
Notched curb spreader (Option C)
Through-curb ports (Option D)
Interrupted curbing (Option E)
Options A through C may be used for spreading flows that are concentrated. Any one of these options may
be used when spreading is required by the facility design criteria. Options A through C may also be used
for unconcentrated (sheet) flows, and in some cases must be used, such as to correct for moderate grade
changes along a filter strip.
Options D and E are only for flows that are already unconcentrated when they enter a filter strip or lateral
inflow bioswale. Other flow spreader options are possible with approval from CED.
6.2.6.1 DESIGN CRITERIA FOR FLOW SPREADER OPTIONS
General Design Criteria
1. Flow must not escape around ends or through any breaks in a flow spreader.
2. Where flow enters the flow spreader through a pipe, it is recommended that the pipe be submerged to
the extent practical to dissipate energy as much as possible.
3. For higher velocity inflows (greater than 5 cfs for the 100-yr storm), a Type 1 catch basin should be
positioned in the spreader, and the inflow pipe should enter the catch basin with flows exiting through
the top grate. The top of the grate should be lower than the level spreader plate, or if a notched
spreader is used, lower than the bottom of the v-notches.
4. Table 4.2.2.F in Chapter 4 provides general guidance for rock protection at outfalls.
OPTION A ANCHORED PLATE OR BOARD (FIGURE 6.2.6.A)
1. An adjustable-level anchored plate or board flow spreader shall be preceded by a sump having a
minimum depth of 8 inches and minimum width of 24 inches. If not otherwise stabilized, the sump
area shall be lined to reduce erosion and to provide energy dissipation.
2. The top surface of the flow spreader plate or board shall be level, projecting a minimum of 2 inches
above the ground surface of the water quality facility, or v-notched with notches 6 to 10 inches on
center and 1 to 6 inches deep (use shallower notches with closer spacing). Alternative designs are
allowed. The anchored plate or board level shall be adjustable using slotted bolt holes in the anchored
plate or board.
3. A flow spreader plate or board shall extend horizontally beyond the bottom width of the facility to
prevent water from eroding the side slope. The horizontal extent should be such that the bank is
protected for all flows up to the 100-year flow or the maximum flow that will enter the WQ facility.
4. Flow spreader plates or boards shall be securely fixed in place by bolts through slotted holes for
adjustability in establishing and maintaining level.
5. Flow spreader plates or boards may be made of either wood, metal, fiberglass reinforced plastic, or
other durable material. If wood, untreated 4 by 10-inch cedar heartwood lumber or cedar landscape
timbers are acceptable.
AGENDA ITEM # 8. a)
6.2.6 FLOW SPREADING OPTIONS
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6. Anchor posts shall be 4-inch square concrete or tubular stainless steel. Other material resistant to
decay may be used if approved by CED.
OPTION B CONCRETE SUMP BOX (FIGURE 6.2.6.B)
1. The wall of the downstream side of a rectangular concrete sump box shall extend a minimum of
2 inches above the treatment bed. This serves as a weir to spread the flows uniformly across the bed.
The wall shall have an adjustable anchored plate or board as described in Option A above. The
adjustable anchored plate or board shall be securely fixed to the concrete wall and meet the same
material specifications as described in Option A above.
2. The downstream wall of a sump box shall have “wing walls” at both ends. Side walls and returns
shall be slightly higher than the weir so that erosion of the side slope is minimized.
3. Concrete for a sump box may be either cast-in-place or precast, but the bottom of the sump shall be
reinforced with wire mesh for cast-in-place sumps.
4. Sump boxes shall be placed over bases that consists of 4 inches of crushed rock, 5/8-inch minus to help
ensure the sump remains level.
OPTION C NOTCHED CURB SPREADER (FIGURE 6.2.6.C)
Notched curb spreader sections shall be made of extruded concrete laid side by side and level. Typically
five “teeth” per four-foot section provide good spacing. The space between adjacent “teeth” forms a v-
notch.
OPTION D THROUGH-CURB PORTS (FIGURE 6.2.6.D)
Sheet flows from paved areas entering filter strips or lateral inflow bioswales may use curb ports or
interrupted curbs (Option E) to allow flows to enter the strip or swale. Curb ports use fabricated openings
that allow concrete curbing to be poured or extruded while still providing an opening through the curb to
admit water to the WQ facility.
Openings in the curb shall be at regular intervals but at least every 6 feet (minimum). The width of each
curb port opening shall be a minimum of 11 inches. Approximately 15 percent or more of the curb section
length should be in open ports, and no port should discharge more than about 10 percent of the flow.
OPTION E INTERRUPTED CURB (NO FIGURE)
Interrupted curbs are sections of curb placed to have gaps spaced at regular intervals along the total width
(or length, depending on facility) of the treatment area. At a minimum, gaps shall be every 6 feet to allow
distribution of flows into the treatment facility before they become too concentrated. The opening shall be
a minimum of 11 inches. As a general rule, no opening should discharge more than 10 percent of the
overall flow entering the facility.
AGENDA ITEM # 8. a)
SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES
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FIGURE 6.2.6.A SCHEMATIC REPRESENTATION OF FLOW SPREADER OPTION A: ANCHORED
PLATE
ALTERNATIVE DESIGN
CATCH BASIN RECOMMENDED FOR
HIGHER FLOW SITUATIONS (GENERALLY
FOR INFLOW VELOCITIES OF 5 FPS OR
GREATER FOR 100-YEAR STORM).
*SAND FILTER MAY USE OTHER
SPREADING OPTIONS
EXAMPLE OF ANCHORED PLATE
USED WITH A SAND FILTER*
(MAY ALSO BE USED WITH OTHER
WATER QUALITY FACILITIES).
PLAN VIEW
NTS
SECTION A-A
NTS
(sand bed)
A SEE TABLE 4.2.2.A FOR
GUIDANCE ON ROCK
PROTECTION AT OUTFALLS
EXTEND INTO SLOPE TO
PROTECT FROM THE
100-YEAR FLOW OR THE
HIGHEST FLOW ENTERING
WATER QUALITY FACILITY
EDGE OF SAND
RIPRAP AS
SPECIFIED IN WQ
FACILITY DESIGNS
POND SIDE SLOPES
V-NOTCHED OR LEVEL
PLATE SPREADER PER
SECTION 6.2.6.1
ANCHOR POSTS
SPACED 6' O.C. OR AT
EACH END IF WIDTH <6'
INLET PIPE
SAND LAYER
GRAVEL LAYER
EXISTING
GRADE
LEVEL SPREADER PLATE
BOLTED TO ANCHOR POST
ANCHOR POST EMBEDDED 2'
(MIN.) INTO EXISTING
GROUND. SEE TEXT FOR
ALLOWED POST MATERIALS.
ROCK RIP RAP
2" MIN.
INLET
PIPE
2" MIN.
8" MIN.
24"
MIN.
AGENDA ITEM # 8. a)
6.2.6 FLOW SPREADING OPTIONS
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FIGURE 6.2.6.B SCHEMATIC REPRESENTATION OF FLOW SPREADER OPTION B: CONCRETE
SUMP BOX
NOTE:
EXTEND SIDES INTO SLOPE. HEIGHT OF
SIDE WALL AND WING WALLS MUST BE SUFFICIENT
TO HANDLE THE 100-YEAR FLOW OR THE HIGHEST
FLOW ENTERING THE FACILITY.
EXAMPLE OF A CONCRETE SUMP FLOW
SPREADER USED WITH A BIOFILTRATION SWALE
(MAY BE USED WITH OTHER WQ FACILITIES)
PLAN VIEW
NTS
SECTION A-A
NTS
SECTION B-B
NTS
WING WALL
A
B
B
SIDE WITH WING WALLS
SEE NOTE
SWALE BOTTOM
CONCRETE SUMP
OUTFALL RIPRAP PAD
INLET PIPE
SEE TABLE 4.2.2.A
FOR GUIDANCE ON
ROCK PROTECTION
AT OUTFALLS
INLET PIPE WING WALL OUTLINE
CONCRETE SUMP
(4" WALL THICKNESS)
8"
MIN.
24"
MIN.
2" MIN. CLEARANCE.
2" MIN.
AGENDA ITEM # 8. a)
SECTION 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES
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FIGURE 6.2.6.C SCHEMATIC REPRESENTATION OF FLOW SPREADER OPTION C: NOTCHED
CURB SPREADER
FIGURE 6.2.6.D SCHEMATIC REPRESENTATION OF FLOW SPREADER OPTION D: THROUGH-
CURB PORT
NOTE:
SEE TABLE 4.2.2.A
FOR GUIDANCE ON ROCK
PROTECTION AT OUTFALLS
FRONT VIEW A-A
NTS
SECTION B-B
NTS
2 - #5 REBAR OR
REINFORCE AS NECESSARY
sand
30INFLOW
60
6"PLAN VIEW
NTS
B
B
48"/SECTION (TYP.)12"12"8"CURB PORT
NTS
REINFORCED
CONCRETE
CURB
OPENING 11" MIN.
GRASS FILTER STRIP
MA X .
6 ' O .C .
AGENDA ITEM # 8. a)
CITY OF RENTON SURFACE WATER DESIGN MANUAL
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6.3 VEGETATED FLOWPATH FACILITY DESIGNS
This section presents the methods, details of analysis, and design criteria for bioswales and filter strips.
Included in this section are the following specific facility designs:
“Basic Bioswales,” Section 6.3.1
“Wet Bioswales,” Section 6.3.2
“Lateral Inflow Bioswales,” Section 6.3.3
“Standard Filter Strips,” Section 6.3.4
GENERAL CONSIDERATIONS
Vegetated flowpath facilities are subject to a number of concerns that do not affect other facilities. Failure
mechanisms can include adverse change in plant community, vegetation loss, erosion or channelization,
detrimental change in slope or cross-section from siltation, and ponding. The relationship between the
surface soil, subsurface soil, groundwater interactions, vegetation type, weather, and shading all contribute
to the success or failure of a vegetated flowpath facility. Successful establishment of vegetation requires
seeding or planting at a time of year that ensures optimal moisture and temperature/sunlight for growth.
Typical maintenance requires mowing, mechanical weed control, and silt removal (e.g., in the bed of a
bioswale, or to remove a ‘micro-berm’ forming at the entry edge of a filter strip), which may require re-
planting. While there is initial control over vegetation type, the plant community can change on its own
over time, and soil profile and content can change over time (compaction from mowing, siltation, holes
from voles, etc.). Solutions are site-specific, may require seasonal observation, covering the full range of
climatic conditions, and even then, something that works in a normal rainfall year may not to work in an
excessively dry or wet year.
The information presented for each facility is organized into the following two categories:
1. Methods of Analysis: Contains a step-by-step procedure for designing and sizing each facility.
2. Design Criteria: Contains the details, specifications, and material requirements for each facility, plus
construction and maintenance considerations as applicable.
6.3.1 BASIC BIOSWALES
A bioswale is an open, gently sloped, vegetated channel designed for treatment of stormwater (see the
schematic representations in Figure 6.3.1.A through Figure 6.3.1.E). The primary pollutant removal
mechanism is sedimentation enhanced by plant stems and to a lesser extent by potential trapping and
adhesion of pollutants to the plants and thatch. Bioswales generally do not remove dissolved pollutants
effectively, although some infiltration to underlying soils may occur depending on the nature of those soils
and any required facility liners.
Applications and Limitations
Data suggest that the performance of bioswales is highly variable from storm to storm. Ecology and the
City of Renton recommend considering other treatment methods that perform more consistently, such as
sand filters or wet ponds, before using a bioswale.
A bioswale is designed so that water will flow evenly across the entire width of a densely-vegetated area.
A swale may be designed for both treatment and conveyance of onsite stormwater flow. This combined
use can reduce development costs by eliminating the need for separate conveyance systems.
Bioswales are best applied on a relatively small scale (generally less than 5 acres of impervious surface).
They fit well along roadways, driveways, and parking lots. Swales are more costly to apply in situations
where the swale channel would be deep; in deep swales, self-shading can inhibit the necessary grass
AGENDA ITEM # 8. a)
SECTION 6.3 VEGETATED FLOW PATH FACILITY DESIGNS
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growth, resulting in poor pollutant removal performance. Some specific considerations for bioswale
applications are as follows:
A bioswale shall not be located in a shaded area. For healthy vegetation growth, a swale should
receive a minimum of 6 hours sunlight daily throughout the year, throughout the length of the swale.
To maintain healthy vegetation growth, a basic bioswale must dry between storms. It shall not
receive continuous base flows (such as seepage from a hill slope throughout the winter) or be located
in a high groundwater area, because saturated soil conditions will kill grass. If these conditions are
likely to occur, design options given under “Design Criteria” shall be used, or the wet bioswale design
may be used (see Section 6.3.2, for details).
Stormwater runoff carrying high concentrations of oil and grease kills vegetation and impairs the
treatment capability of a swale. Where a high use site is tributary to a proposed bioswale, an oil
control facility option listed in Section 6.6 shall be installed to treat the subject runoff prior to entering
the bioswale.
Modifying an existing drainage ditch to create an engineered bioswale may be difficult due to
physical constraints and because ditches often serve as conveyance for higher flows from larger offsite
areas.
Utilities may be located in swale side slopes above the WQ design depth. However, the repair or
placement of utilities in swale side slopes requires aggressive implementation of erosion control
practices to prevent soil and sediment from reaching the treatment area of the swale.
Note: Consult the water quality menus in Section 6.1 for information on how this facility may be used to
meet Core Requirement #8. Also see for guidance on which type of bioswale (basic, wet or lateral inflow)
to use for a given set of site characteristics.
6.3.1.1 METHODS OF ANALYSIS
Bioswale sizing is based on several variables, including the peak water quality design flow, longitudinal
slope, vegetation height, bottom width, side slope, required hydraulic residence time (i.e., the time
required for flow to travel the full length of the swale), and design flow depth. Swales sized and built using
the method of analysis outlined in this section and the required design criteria presented in Section 6.3.1.2
are expected to meet the Basic Water Quality menu goal of 80% TSS removal. Procedures for sizing
bioswales are summarized below.
Step 1: Calculate design flows. The swale design is based on the water quality design flow Qwq (see
Section 6.2.1, for a definition of water quality design flow). If a bioswale is used for conveyance, the
capacity requirements of Core Requirement #4 must be met. These flows must be estimated using the
hydrologic analysis procedures described in detail in Chapter 3 and applying the flow rate modifications
described in Section 6.2.1.
If the bioswale is located upstream of an onsite detention facility, or if a detention facility is not required,
the bioswale design flow shall be the on-line or off-line (as applicable) water quality flow rate determined
from the approved continuous model, modified by a factor k, the on-line or off-line ratio determined from
Table 6.2.1. This modified design flow rate is an estimate of the design flow rate determined by using
SBUH procedures.
Guidance for Bypassing Off-Line Facilities
Most bioswales are currently designed to be on-line facilities. However, an off-line design is possible.
Bioswales designed in an off-line mode should not engage a bypass until the flow rate exceeds the
modified off-line water quality design flow rate.
If the bioswale is located downstream of an onsite detention facility, the swale design flow shall be the
2-year release rate from the detention facility.
AGENDA ITEM # 8. a)
6.3.1 BASIC BIOSWALES
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Step 2: Calculate swale bottom width. The swale bottom width is calculated based on Manning’s
equation for open-channel flow. This equation can be used to calculate discharges as follows:
Q = (6-3)
where Q = flow rate (cfs)
n = Manning’s roughness coefficient (unitless)
A = cross-sectional area of flow (sf)
R = hydraulic radius (ft) = area divided by wetted perimeter
s = longitudinal slope (along direction of flow) (ft/ft)
For shallow flow depths in swales, channel side slopes are ignored in the calculation of bottom width. Use
the following equation (a simplified form of Manning’s formula) to estimate the required swale bottom
width:
b =
(6-4)
where b = bottom width of swale (ft)
Qwq = the modified water quality design flow, k(Q, modeled on-line or off-line rate), (cfs)
where k = correlation ratio determined from Table 6.2.1.A
nwq = Manning’s roughness coefficient for shallow flow conditions = 0.20 (unitless)
y = design flow depth (ft)
S = longitudinal slope (along direction of flow) (ft/ft)
See “Water Depth and Base Flow” to determine the allowable design water depth. Proceed to Step 3 if the
bottom width is calculated to be between 2 and 10 feet.
A minimum 2-foot bottom width is required. Therefore, if the calculated bottom width is less than 2 feet,
increase the width to 2 feet and recalculate the design flow depth y using Equation 6-5 as follows:
y =
(6-5)
where Qwq, nwq, and s are the same values as used in Equation 6-4, but b = 2 feet.
The maximum bottom width is 10 feet; therefore if the calculated bottom width exceeds 10 feet, then one
of the following steps is necessary to reduce the design bottom width:
Increase the longitudinal slope S to a maximum of 6 feet in 100 feet (0.06 feet per foot).
Increase the design flow depth y to a maximum of 4 inches (0.333 feet).
Reduce the design flow rate by rearranging the swale location with respect to detention facilities; a
swale located downstream of a detention facility may have a lower flow rate due to flow attenuation in
the detention facility. However, if a swale is located downstream of a detention facility providing
Flow Control Duration Standard or Flood Problem Flow Control Standard, and it is located in till
soils (according to the soil groups in Table 3.2.2.A), then the swale must be designed as a wet
bioswale (see Section 6.3.2).
Place a divider lengthwise along the swale bottom (cross section) at least three-quarters of the swale
length (beginning at the inlet), without compromising the design flow depth and swale lateral slope
requirements. See “Design Criteria” for swale divider requirements. A flow spreader must be provided
5.067.049.1 sARn
5.067.149.1 sy
nQ wqwq
53
5.049.1
bs
nQwqwq
AGENDA ITEM # 8. a)
SECTION 6.3 VEGETATED FLOW PATH FACILITY DESIGNS
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at the inlet to evenly divide flows into each half of the swale cross section. See Section 6.2.6 for
details on flow spreaders.
Step 3: Determine design flow velocity. To calculate the design flow velocity through the swale, use the
flow continuity equation:
Vwq =
(6-6)
where Vwq= design flow velocity (fps)
Awq= by + Zy2 = cross-sectional area (sf) of flow at design depth
Z = side slope length per unit height (e.g., Z = 3 if side slopes are 3H:1V)
If the design flow velocity exceeds 1 foot per second, go back to Step 2 and modify one or more of the
design parameters (longitudinal slope, bottom width, or flow depth) to reduce the design flow velocity to 1
foot per second or less. If the design flow velocity is calculated to be less than 1 foot per second, proceed
to Step 4.
Note: It is desirable to have the design velocity as low as possible, both to improve treatment effectiveness
and to reduce swale length requirements.
Step 4: Calculate swale length. Use the following equation to determine the necessary swale length to
achieve a hydraulic residence time of at least 9 minutes (540 seconds):
L = 540Vwq (6-7)
where L = minimum allowable swale length (ft)
Vwq= design flow velocity (fps)
The minimum swale length is 100 feet; therefore, if the swale length is calculated to be less than 100 feet,
increase the length to a minimum of 100 feet, leaving the bottom width unchanged. If a larger swale could
be fitted on the site, consider using a greater length to increase the hydraulic residence time and improve
the swale’s pollutant removal capability. If the calculated length is too long for the site, or if it would
cause layout problems, such as encroachment into shaded areas, proceed to Step 5 to further modify the
layout. If the swale length can be accommodated on the site, proceed to Step 6.
Step 5: Adjust swale layout to fit on site. If the swale length calculated in Step 4 is too long for the site,
the length may be reduced (to a minimum of 100 feet) by increasing the bottom width up to a maximum of
16 feet, as long as the 9 minute retention time is retained. However, the length cannot be increased in
order to reduce the bottom width because Manning’s depth-velocity-flow rate relationships would not be
preserved. If the bottom width is increased to greater than 10 feet, a low dividing berm is needed to split
the swale cross section in half.
Length can be adjusted by finding the top area of the swale and providing an equivalent top area with the
adjusted dimensions.
1. Calculate the swale treatment top area based on the swale length calculated in Step 4:
Atop = (bi + bslope) Li (6-8)
where Atop = top area (sf) at the design treatment depth
bi = bottom width (ft) calculated in Step 2
bslope = the additional top width (ft) above the side slope for the design water depth (for
3:1 side slopes and a 4-inch water depth, bslope = 2 feet)
Li = initial length (ft) calculated in Step 4.
wq
wq
A
Q
AGENDA ITEM # 8. a)
6.3.1 BASIC BIOSWALES
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2. Use the swale top area and a reduced swale length Lf to increase the bottom width, using the following
equation:
Lf =
(6-9)
where Lf = reduced swale length (ft)
bf = increased bottom width (ft).
3. Recalculate Vwq according to Step 3 using the revised cross-sectional area Awq based on the increased
bottom width bf. Revise the design as necessary if the design flow velocity exceeds 1 foot per second.
4. Recalculate to ensure that the 9 minute retention time is retained.
Step 6: Provide conveyance capacity for flows higher than Qwq. Bioswales may be designed as flow-
through channels that convey flows higher than the water quality design flow rate, or they may be
designed to incorporate a high-flow bypass upstream of the swale inlet. A high-flow bypass usually results
in a smaller swale size (see flow splitter options, Section 6.2.5, for more information on designing
bypasses). If a high-flow bypass is provided, this step is not needed. If no high-flow bypass is provided,
proceed with the procedure below.
1. Check the swale sized using Steps 2 through 5 above to determine whether the swale can convey the
25-year and 100-year peak flows consistent with the conveyance requirements of Core Requirement
#4 in Chapter 1. The roughness coefficient n in Manning’s equation shall be selected to reflect the
deeper flow conditions with less resistance provided by grass during these high-flow events. The
bottom width (Step 2) should be calculated as per Section 4.4.1.2, “Methods of Analysis” for open
channels.
2. The 100-year peak flow velocity (V100 = Q100/A100) based on the 100-year flow depth must be less than
3.0 feet per second. If V100 exceeds 3.0 feet per second, return to Step 2 and increase the bottom width
or flatten the longitudinal slope as necessary to reduce the 100-year peak flow velocity to 3.0 feet per
second or less. If the longitudinal slope is flattened, the swale bottom width must be recalculated (Step
2) and meet all design criteria.
3. The conveyance requirements in Core Requirement #4 (see Section 1.2.4) must be met.
6.3.1.2 DESIGN CRITERIA
An effective bioswale achieves uniform sheet flow over and through a densely vegetated area for a period
of several minutes. Figure 6.3.1.A shows a typical bioswale schematic. Basic design requirements for
achieving proper flow conditions through a bioswale are described below.
Swale Geometry
1. Swale bottom width shall be between 2 and 16 feet.24
a) Minimum bottom width is 2 feet to allow for ease of mowing.
b) If the bottom width exceeds 10 feet, a length-wise divider shall be provided. The divider shall
extend from the flow spreader at the inlet for at least three-quarters of the swale length.
c) Maximum bottom width is 16 feet, excluding the width of the divider.
24 Experience with biofiltration swales shows that when the width exceeds about 10 feet it is difficult to keep the water from
forming low-flow channels. It is also difficult to construct the bottom level and without sloping to one side. Biofilters are best
constructed by leveling the bottom after excavating, and after the soil is amended. A single-width pass with a front-end loader
produces a better result than a multiple-width pass.
)+ ( slopef
top
bb
A
AGENDA ITEM # 8. a)
SECTION 6.3 VEGETATED FLOW PATH FACILITY DESIGNS
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Note: Multiple swales may be placed side by side provided the flow to each swale is split at the inlet
and spread separately for each swale. Adjacent swales may be separated with a vertical wall, but a
low berm is preferred for easier maintenance and better landscape integration.
2. The longitudinal slope (along the direction of flow) should be between 1.5 percent and 6 percent.
a) If the longitudinal slope is less than 1.5 percent, underdrains must be provided (see next page and
Figure 6.3.1.C, for underdrain specifications) or the swale must be designed according to the
criteria presented in Section 6.3.2 for wet bioswales.
b) Wet bioswales in outwash soils and low groundwater conditions are discouraged as plant survival
may be compromised.
c) If the longitudinal slope exceeds 6 percent, check dams with vertical drops of 12 inches or less
shall be provided to achieve a bottom slope of 6 percent or less between the drop sections.
3. The swale bottom shall be flat in cross section (perpendicular to the flow direction) to promote even
flow across the whole width of the swale.
4. The minimum swale length shall be 100 feet; no maximum length is set.
5. The swale treatment area (below the WQ design water depth) shall be trapezoidal in cross-section.
Side slopes within the treatment area should be 3H:1V or flatter whenever possible, but shall not
steeper than 2H:1V.
6. Side slope sections above the treatment area may be steeper than 3H:1V, subject to the following
provisions:
a) If there is an interior side slope between 1H:1V and 2H:1V outside the treatment area, the slope
shall be reinforced with erosion control netting or matting during construction.
b) Any interior slope steeper than 1H:1V shall be constructed as a rockery or structural retaining
wall25 to prevent the swale slope from sloughing. To ensure that adequate sunlight reaches the
swale bottom, only one wall can be taller than 2 feet. If possible, the higher wall should be on
the northern or eastern side of the swale to maximize the amount of light reaching the swale
bottom.
7. Curved swales are encouraged for aesthetic reasons, but curves must be gentle to prevent erosion and
allow for vehicle access to remove sediment. Criteria for maintenance access road curves shall also
be applied for swale curves (see Section 5.1.1.1 for design of access roads).
Water Depth and Base Flow
1. A swale that will be frequently mowed, as in commercial or landscaped areas, shall have a design
water depth of no more than 2 inches (0.17 feet) under the water quality design flow conditions.
2. A swale that will not be frequently mowed, such as along roadsides or in rural areas, shall have a
design water depth of no more than 4 inches (0.33 feet) under the water quality design flow
conditions.
3. If a swale is located downstream of a detention facility providing Flow Control Duration Standard
or Flood Problem Flow Control Standard, and it is located in till soils (according to the soil groups
in Table 3.2.2.B, Chapter 3), then the swale must be designed as a wet bioswale (see Section 6.3.2).
4. If a swale will receive base flows because of seeps and springs onsite, then either a low-flow drain
shall be provided or a wet bioswale shall be used. Low-flow drains are narrow surface drains filled
with pea gravel that run lengthwise through the swale to bleed off base flows; they should not be
confused with underdrains. In general, base flows less than 0.01 cfs per acre can be handled with a
low-flow drain. If flows are likely to be in excess of this level, a wet bioswale shall be used.
25 Soil bioengineering techniques may be used as an alternative to a rockery or structural retaining wall.
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5. If a low-flow drain is used, it shall extend the entire length of the swale. The drain shall be a
minimum of 6 inches deep, and its width shall be no greater than 5 percent of the calculated swale
bottom width; the width of the drain shall be in addition to the required bottom width. If an anchored
plate or concrete sump is used for flow spreading at the swale inlet, the plate or sump wall shall have a
v-notch (maximum top width = 5% of swale width) or holes to allow preferential exit of low flows
into the drain. See Figure 6.3.1.D for low-flow drain specifications and details.
Flow Velocity, Energy Dissipation, and Flow Spreading
1. The maximum flow velocity through the swale under the water quality design flow conditions shall
not exceed 1.0 foot per second.
2. The maximum flow velocity through the swale under the peak 100-year flow conditions shall not
exceed 3.0 feet per second.
3. A flow spreader shall be used at the inlet of a swale to dissipate energy and evenly spread runoff as
sheet flow over the swale bottom. Flow spreaders are recommended but not required at mid-length.
For details on various types of flow spreaders, see Section 6.2.6.
4. If check dams are used to reduce the longitudinal slope of the swale, a flow spreader shall be
provided at the toe of each vertical drop. The spreader must span the width of the swale. An energy
dissipater shall also be provided if flows leaving the spreader could be erosive.
5. If a swale discharges flows to a slope rather than to a piped system or confined channel, an energy
dissipater shall be provided at the swale outlet. This requirement also applies to discharges from
swale underdrains. The outlet energy dissipater may be a riprap pad sized according to the
specifications described in Table 4.2.2.A for conveyance system outfalls.
Underdrains
If underdrains are required by Criterion 2 under “Swale Geometry,” they must meet the following criteria:
1. Underdrains must be made of PVC perforated pipe (SDR 35), laid parallel to the swale bottom and
backfilled and bedded as shown in Figure 6.3.1.C.
2. For facilities to be maintained by the City, the underdrain pipe must be 6 inches or greater in
diameter. (Six inches is the smallest diameter pipe that can be cleaned without damage to the pipe.)
3. Six inches of clean drain rock (5/8-inch minus) must be above the top of the pipe.
4. The drain rock must be wrapped in geotextile. See WSDOT Standard Specifications (2014), 9-33.2(1)
Geotextile Properties/Table 1/Moderate Survivability/Woven, and Table 2, Class A
5. The underdrain must drain freely to an acceptable discharge point.
Swale Divider
1. If a swale divider is used (such as when swale bottom widths are greater than 10 feet), the divider
shall be constructed of a firm material that will resist weathering and not erode, such as concrete,
compacted soil seeded with grass, untreated heartwood cedar, or untreated whole de-barked cedar
logs. Selection of divider material shall take into consideration swale maintenance, especially
mowing.
2. The divider shall have a minimum height of one inch higher than the water quality design water
depth.
3. Earthen berms shall be no steeper than 2H:1V.
4. Materials other than earth (e.g., concrete, untreated heartwood cedar lumber, etc.) shall be embedded
to a depth sufficient to be stable.
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Access
1. For swales to be maintained by the City, an access road shall be provided to the swale inlet and along
one side of the swale according to the schedule shown in Table 6.3.1.B below.
Note: City streets and paved parking areas adjacent to the top of slope may be counted as access.
TABLE 6.3.1.B REQUIREMENTS FOR BIOSWALE ACCESS ROAD
Swale Bottom Area*: L x w (sf) Access Road Length
200–1000 1/2 swale length L
1000–1600 2/3 swale length L
Over 1600 entire swale length L
* The swale area used for computing access road length may be the bottom area.
2. In areas outside critical area buffers, wheel strips made of modular grid pavement may be built into
the swale bottom for maintenance vehicle access instead of an access road. The subgrade for the
strips must be engineered to support a vehicle weight of 16,000 pounds and installed according to the
manufacturer’s recommendations on firm native soil or structural fill, not on the amended topsoils.
Each strip shall be 18 inches wide and spaced as shown in Figure 6.3.1.E. The strip lattice should be
filled or covered with native soil (no amendments required) and overseeded with grass. If a low-flow
drain is also needed (see “Water Depth and Base Flow” in Section 6.3.1.2), a portion of the wheel
strip may be filled with pea gravel as appropriate to form the drain. Continuous vehicle access shall be
provided to the wheel strips from the access road. If access to the wheel strips is over the flow-
spreader, then a grate (or other CED approved method) shall be placed over the flow-spreader for
vehicle access. Wheel strips shall not be counted as treatment area; therefore, the swale bottom width
must be increased accordingly.
Soil Amendment
1. If the soil has an organic content of 1 percent or greater, do not amend. If the soil has an organic
content of less than 1%, two inches of mature, stable compost shall be tilled into the entire swale
treatment area. This applies to both till soils as well as sandy soils. In very coarse soils (gravels or
coarser), top soil must be imported and amended to the required organic content.
a) Compost must be tilled into the underlying native soil to a depth of 6 inches to prevent the
compost from being washed out and to avoid creating a defined layer of different soil types that
can prevent downward percolation of water.
b) Compost must meet Specification 1 described in Reference Section 11-C.
2. Soil or sod with a clay content of greater than 10 percent should be avoided. If there is concern for
contamination of the underlying groundwater, the swale bottom shall be lined with a treatment liner to
prevent groundwater contamination. See “Facility Liners,” Section 6.2.4, for details on treatment liner
options.
Planting Requirements
1. Vegetation shall be established throughout the entire treatment area of the swale subject to the
following provisions:
a) Seeding is best performed in fall (late September to October) or in spring (mid-March to June).
For summer seeding or seeding during dry conditions, sprinkler systems or other measures for
watering the seed must be provided. Soil temperatures should be between 50 and 65 degrees to
allow for seed germination of cool season grasses.
b) Seed may be applied via hydroseeding or broadcast application.
c) Irrigation is required during the first summer following installation if seeding occurs in spring or
summer or during prolonged dry times of year. Swales seeded in the fall may not need irrigation.
However, the maintenance and defect financial guarantee will not be released unless a healthy
grass cover is established. Therefore, site planning should address the need for sprinklers or other
means of irrigation.
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2. Swale treatment areas are subject to both dry and wet conditions, as well as accumulation of sediment
and debris. A mixture of dry-area and wet-area grass, rush, and sedge species that can continue to
grow through silt deposits is most effective. Two acceptable grass seed mixes for the City of Renton
are listed in Table 6.3.1.C. The mixes shall be applied throughout the swale in the treatment area at a
rate of 120 to 140 seeds per square foot. As an alternative to these mixes, a horticultural or erosion
control specialist may develop a seed specification tailored to the site. Table 6.3.1.D lists grasses or
other plants particularly tolerant of wet conditions. Some of these seed types, however, may not be
commercially available.
3. A newly constructed swale shall be protected from stormwater flows until vegetation has been
established. This may be done by diverting flows or by placing an erosion control blanket over the
freshly applied seed mix until the grass is well rooted. See Appendix D, ESC Standards, for details on
erosion control blankets.
4. Above the design treatment elevation, either a typical lawn seed mix or landscape plants may be
used. However, for swales also used to convey high flows, consideration shall be given to the soil
binding capacity of the vegetation. Acceptable grasses and groundcovers are presented in
Table 6.3.1.E. Plant material other than that given in the table may be used if the swale is privately
maintained and the plants selected will not spread into the swale treatment area. Ivy shall not be used
because of its tendency to spread. Native plant species (e.g., kinnikinnick) are preferred.
5. Sod may be used as a temporary cover during the wet season, but sodded areas must be reseeded with
a suitable grass seed mix as soon as the weather is conducive to seed germination, unless the sod is
grown from a seed mix suitable for the wetter conditions of a bioswale. Sod must be removed or
rototilled into the underlying soil before reseeding. Criteria #1 and 2 above for seeding shall then be
followed.
TABLE 6.3.1.C GRASS SEED MIXES SUITABLE
FOR BIOSWALE TREATMENT AREAS
MIX 1 STANDARD SEED MIX MIX 2 LOW GROWING SEED MIX
Species
Composition Latin Name
Common
Name
Species
Composition Latin Name
Common
Name
15% Beckmannia
syzigachne
American
sloughgrass
15% Bromus carinatus California
brome
20% Deschampsia
cespitosa
Tufted
hairgrass
18% Bromus vulgaris Columbia
brome
18% Elymus glaucus Blue wildrye 15% Deschampsia
cespitosa
Tufted
hairgrass
20% Festuca rubra
var. rubra
Native red
fescue
15% Danthonia
californica
California
oatgrass
12% Hordeum
brachyantherum
Meadow
barley
17% Festuca rubra
var. rubra
Native red
fescue
15% Glyceria
occidentalis
Northwestern
mannagrass
10% Glyceria
occidentalis
Western
manna
grass
10% Hordeum
brachyantherum
Meadow
barley
Notes:
All percentages are targeted species composition of seed.
Mixes are comprised of species native to King County and are not considered turf grass mixes. Mowing, if necessary,
is best done after mature seeds have dispersed to continue self-propagation of plant community.
Sow Mix 1 at a rate of 31 pounds of pure live seed (PLS) per acre.
Sow Mix 2 at a rate of 39 pounds of pure live seed (PLS) per acre.
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TABLE 6.3.1.C FINELY-TEXTURED PLANTS TOLERANT OF
FREQUENT SATURATED SOIL CONDITIONS OR STANDING WATER
Grasses Wetland Plants
Latin Name Common Name Latin Name Common Name
Alopecurus aequalis Shortawn Foxtail Carex deweyana Dewey Sedge
Agrosits spp. Bentgrass Carex stipata Sawbeak Sedge
A. exarata Spike Bentgrass Carex pachystachya Thick Headed Sedge
A. alba or gigantea Redtop Eleocharis palustris Spike Rush
Glyceria spp. Mannagrass Juncus tenuis Slender Rush
G. occidentalis Western Juncus ensifolius Swordleaf Rush
G. borealis Northern
G. leptostachya Slender-Spiked
Poa palustris Fowl Bluegrass
Deschampsia cespitosa Tufted hairgrass
Holcus mollis Velvet Grass
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TABLE 6.3.1.D GROUNDCOVERS AND GRASSES
SUITABLE FOR THE UPPER SIDE SLOPES OF A BIOSWALE
Groundcovers
Common Name Latin Name
Kinnikinnick* Arctostaphylos uva-ursi
Alumroot* Heuchera micrantha
Fringecup Tellima grandiflora
Strawberry* Fragaria chiloensis
Broadleaf Lupine* Lupinus latifolius
Dull Oregon grape* Mahonia nervosa
Creeping raspberry Rubus calycinoides
Creeping snowberry* Symphoricarpos mollis
Yarrow* Achillea millifolium
Youth on age Tolmiea menziesii
Grasses (drought-tolerant, minimum mowing)
California brome* Bromus carinatus
California oatgrass* Danthonia californica
Blue wildrye* Elymus glaucus
Tufted Fescue Festuca amethystina
Hard Fescue Festuca ovina duriuscula (e.g., Reliant, Aurora)
Red Fescue* Festuca rubra var. rubra
Blue Oatgrass Helictotrichon sempervirens
Low-growing turf mix (% species composition):
Hard fescue/Festuca brevipila (25%)
Sheep fescue/Festuca ovina (30%)
Red fescue/Festuca rubra var. rubra (25%)
Prairie junegrass/Koeleria macrantha (20%)
*Native species.
Notes:
Many other ornamental grasses which require only annual mowing are suitable.
Ivy is not permitted.
Recommended Design Features
The following features should be incorporated into bioswale designs where site conditions allow.
Swale Layout and Grading
1. If the longitudinal slope is less than 1.5 percent, and an underdrain is used per Section 6.3.1.2,
“Design Criteria,” the subgrade should contain 10 percent or more of sand to promote infiltration of
standing water. If sand is added to promote drainage, the soil or sand substrate must still be amended
with compost. Compost must meet Specification 1 described in Reference Section 11-C.
2. Underdrains may be necessary for swales greater than 1.5 percent longitudinal slope on poorly
drained till soils, especially if it is likely that the swale will intercept groundwater.
3. Bioswales should be aligned to avoid sharp bends where erosion of the swale side slope can occur.
However, gradual meandering bends in the swale are desirable for aesthetic purposes and to promote
slower flow.
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Location and Landscaping
1. During seeding, slow-release fertilizers may be applied to speed the growth of grass. If the swale is
located in a sensitive lake watershed, low phosphorus fertilizers (such as formulations in the
proportion 3:1:3 N-P-K or less) or slow-release phosphorus formulations such as rock phosphate
should be used, and at no more than the minimum necessary agronomic rate. A typical fertilizer
application rate should be no more than 2 pounds per 1,000 square feet. Regardless of location, the
fertilizer must meet the requirements of Chapter 15.54.500 RCW limiting the use of fertilizer
containing phosphorus.
2. Consultation with a landscape or erosion control specialist is recommended for project-specific
recommendations on grass seed, fertilizer, and mulching applications to ensure healthy grass growth.
The grass mix should be capable of surviving and remaining healthy under both dry and wet
conditions with limited maintenance.
3. A grassy swale should be incorporated into the project site landscape design. Shrubs may be planted
along the edges of a swale (above the WQ treatment level) provided that exposure of the swale bottom
to sunlight and maintenance accessibility are not compromised.
Note: For swales used to convey high flows, the plant material selected must bind the soil adequately
to prevent erosion.
4. Swales should not be located in areas where trees will drop leaves or needles that can smother the
grass or clog part of the swale flow path. Likewise, landscaping plans should take into consideration
the problems that falling leaves and needles can cause for swale performance and maintenance.
Landscape planter beds should be designed and located so that soil does not erode from the beds and
enter a nearby bioswale.
Construction Considerations
1. If a bioswale is put into operation before all construction in the drainage area of the swale is complete,
the swale must be cleaned of sediment and reseeded prior to acceptance by the City. The City will not
release financial guarantees if swales are not restored and vigorous grass growth established.
2. It is preferable to provide good erosion control before runoff enters a bioswale. Swales are designed to
handle only modest sediment loads from stabilized sites.
Maintenance Considerations
The design criteria given previously have incorporated maintenance concerns into swale design.
However, the designer should know the type and frequency of maintenance anticipated so that alternative
proposals can incorporate maintenance activity.
Typical swale maintenance includes routine mowing, sediment and debris removal, and repair of eroded
or scoured channel sections as described below.
1. Grass should be mowed to maintain an average grass height between 4 inches and 9 inches,
depending on the site situation. Annual mowing after seed fall is recommended to maintain grass
vigor.
2. Mulch mowing is allowed to replenish soil nutrients. Grass clippings may also be removed and
disposed of properly offsite.
3. Sediment deposited at the head of the swale should be removed if grass growth is being inhibited for
more than 10 percent of the swale length or if the sediment is blocking the even spreading or entry of
water to the rest of the swale. Annual sediment removal and spot reseeding may be necessary.
4. If flow channelization or erosion has occurred, the swale should be regraded to produce a flat bottom
width, and then reseeded as necessary. If the channel results from constant base flow, it may be better
to install a low-flow drain rather than to regrade. Regrading should not be required every year.
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5. For swales with underdrains, vehicular access to the swale bottom (other than grass mowing
equipment) should be avoided because the drainpipe cannot support vehicle weight. Consideration
should be given to providing wheel strips in the swale bottom if access is needed.
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FIGURE 6.3.1.A SCHEMATIC REPRESENTATION OF A BIOSWALE
MID-SWALE FLOW
SPREADER
(RECOMMENDED)
BIOSWALE BOTTOM
( MIN. SWALE LENGTH=100' )
FLOW SPREADER
OUTLET
NOTE:
PREFERRED LONGITUDINAL SLOPE 1.5% TO 6%, SEE SECTION 6.3.1.2.
FOR SLOPE < 1.5%, PROVIDE UNDERDRAIN OR WET BIOSWALE.
SLOPE > 6% REQUIRES CHECK DAMS AND VERTICAL DROPS TO REDUCE EFFECTIVE SLOPE.
PLAN VIEW
NTS
1
2
'
(
1
5
' ONCURVE
S
)
MAINTENANCE ACCESS ROAD
PER SECTION 5.1.1.1 FOR
VEHICLE ACCESS
(MODULAR GRID PAVEMENT
POROUS PAVEMENT, ASPHALT,
CONCRETE OR GRAVEL)
ROADWAY LENGTH DEPENDS
ON SWALE AREA, SEE TABLE
6.3.1.B
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FIGURE 6.3.1.B SCHEMATIC REPRESENTATION OF A BIOSWALE CROSS-SECTION
FIGURE 6.3.1.C SCHEMATIC REPRESENTATION OF A BIOSWALE UNDERDRAIN
BOTTOM WIDTH (b) MAX. = (16 FT + DIVIDER WIDTH)
BOTTOM WIDTH (b) MIN. = 2 FT
TYPICAL SWALE SECTION
NTS
SWALE DIVIDER FOR WIDTH >10 FT
WATER QUALITY DESIGN DEPTH (Y) = 4" MAX.
(2" FOR FREQUENTLY MOWED AREAS)
Y + 1"
2" COMPOST
TILLED INTO
6" NATIVE SOIL
BOTTOM
WIDTH (b)
DETAIL A
SECTION
NTS
UNDERDRAIN FOR SLOPES < 1.5%
NOTE:
UNDERDRAIN MUST INFILTRATE OR DRAIN
FREELY TO AN ACCEPTABLE DISCHARGE POINT.
PERFORATED UNDERDRAIN PIPE
CENTERED BENEATH SWALE
FILTER FABRIC WRAP
OF TOP, SIDES AND BOTTOM
NATIVE SOIL
6" MIN. AMENDED SOIL
SWALE
BOTTOM
SOIL AMENDED WITH COMPOST
PERFORATED UNDERDRAIN
PIPE CENTERED BENEATH SWALE
5/8" MINUS CLEAN DRAIN ROCK
FILTER FABRIC
6" MIN. OVER PIPE
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FIGURE 6.3.1.D SCHEMATIC REPRESENTATION OF A BIOSWALE LOW-FLOW DRAIN
FIGURE 6.3.1.E SCHEMATIC REPRESENTATION OF BIOSWALE WHEEL STRIPS
PLAN VIEW
NTS
SECTION A
NTS
TOP NOTCH OPENING NO
MORE THAN 5% OF SWALE
BOTTOM WIDTH OR USE
WEEP HOLES
2"
SWALE GRADE
A
6" MIN. DEEP PEA GRAVEL
TRENCH LENGTH OF SWALE
(SEE TEXT FOR APPLICATION)
NOTCH OR
WEEP HOLES
LONGITUDINAL
SLOPE 1.5-6%
CONCRETE
SUMP
SECTION
NTS
6' O.C.
COMPOST
AMENDED SOILMODULAR GRID PAVERS ON
NATIVE SOIL OR ENGINEERED
FILL PER MANUFACTURER'S
RECOMMENDATIONS 8" MIN.
b1 18" b2 18" b3
DESIGN BOTTOM WIDTH
(b) = b1 + b2 + b3
AGENDA ITEM # 8. a)
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6.3.2 WET BIOSWALES
A wet bioswale is a variation of a basic bioswale for use where the longitudinal slope is slight, water tables
are high, or continuous low base flow is likely to result in saturated soil conditions. Where saturation
exceeds about 2 weeks, typical grasses will die. Thus, vegetation specifically adapted to saturated soil
conditions is needed. Different vegetation in turn requires modification of several of the design parameters
for the basic bioswale detailed in Section 6.3.1.
Applications
Wet bioswales are applied where a basic bioswale is desired but not allowed or advisable because one or
more of the following conditions exist:
The swale is on till soils and is downstream of a detention facility providing Flow Control Duration
Standard or Flood Problem Flow Control Standard.
Saturated soil conditions are likely because of seeps or base flows on the project site.
Longitudinal slopes are slight (generally less than 1.5 percent).
Consult the water quality menus in Section 6.1 for information on how this facility may be used to meet
Core Requirement #8.
6.3.2.1 METHODS OF ANALYSIS
Wet bioswales use the same methods of analysis as basic bioswales (see Section 6.3.1.1) except the
following step is added:
Step 7: Adjust for extended wet season flow. If the swale will be downstream of a detention facility
providing Flow Control Duration Standard or Flood Problem Flow Control Standard, multiply the
treatment area (bottom width times length) of the swale by 2, and readjust the swale length, if desired.
Maintain a 5:1 length to width ratio (see criteria under “Swale Geometry” below).
Intent: An increase in the treatment area of swales following Flow Control Duration Standard or
Flood Problem Flow Control detention facilities is required because of the differences in vegetation
established in a constant flow environment. Although flows following Flow Control Duration
Standard or Flood Problem Flow Control detention facilities are small, and swales are likewise much
smaller than those sized for upstream flows, they are much more protracted. These protracted flows
result in more stream-like conditions than are typical for other wet bioswale situations. Since
vegetation growing in streams is often less dense, this increase in treatment area is needed to ensure
that equivalent pollutant removal is achieved in extended flow situations.
6.3.2.2 DESIGN CRITERIA
Swale Geometry
Same as specified for basic bioswales (see Section 6.3.1.2) except for the following modifications:
1. Criterion 1: The maximum bottom width may be increased to 25 feet, but a length-to-width ratio of
5:1 must be provided. No longitudinal dividing berm is needed. Note: The minimum swale length is
still 100 feet.
2. Criterion 2: If longitudinal slopes are greater than 2 percent, the wet swale must be stepped so that
the slope within the stepped sections averages 2 percent. Steps may be made of retaining walls, log
check dams, or short riprap sections. No underdrain or low-flow drain is required.
3. Criterion 3: Curved swales are allowed and the application of criteria for maintenance access road
curves are not required.
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High-Flow Bypass
A high-flow bypass is required for flows greater than the water quality design flow to protect wetland
vegetation from damage.26 The bypass may be an open channel parallel to the wet bioswale.
Water Depth and Base Flow
Same as for basic bioswales (see Section 6.3.1.2), except the design water depth shall be 4 inches or less
for all wetland vegetation selections, and no underdrains or low-flow drains are required.
Flow Velocity, Energy Dissipation, and Flow Spreading
Same as for basic bioswales (see Section 6.3.1.2), except no flow spreader is needed.
Access
Same as for basic bioswales (see Section 6.3.1.2) except access is only required to the inflow and the
outflow of the swale; access along the length of the swale is not required. Also, wheel strips may not be
used for access in the swale.
Intent: An access road is not required along the length of a wet swale because of infrequent access needs.
Frequent mowing or harvesting is not desirable. In addition, wetland plants are fairly resilient to sediment-
induced changes in water depth, so the need for access should be infrequent.
Soil Amendment
Same as for basic bioswales (see Section 6.3.1.2).
Planting Requirements
Same as for basic bioswales (see Section 6.3.1.2) except for the following modifications:
1. A list of acceptable plants with recommended spacing is given in Table 6.3.2.A. In general, it is best
to plant several species to increase the likelihood that at least some of the selected species will find
growing conditions favorable.
2. A wetland seed mix may be applied by hydroseeding, but if coverage is poor, planting of rootstock or
nursery stock is required. Poor coverage is considered to be more than 30 percent bare area through
the upper 2/3 of the swale after four weeks.
Recommended Design Features
Same as for basic bioswales (see Section 6.3.1.2).
Construction Considerations
Same as for basic bioswales (see Section 6.3.1.2).
Maintenance Considerations
Same as for basic bioswales (see Section 6.3.1.2), except mowing of wetland vegetation is not required.
However, harvesting of very dense vegetation may be desirable in the fall after plant die-back to prevent
the sloughing of excess organic material into receiving waters. Many native Juncus species remain green
throughout the winter; therefore, fall harvesting of Juncus species is not recommended.
26 Unlike grass, wetland vegetation will not quickly regain an upright attitude after being laid down by high flows. New growth,
usually from the base of the plant, often taking several weeks, is required to regain its upright form.
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TABLE 6.3.2.A RECOMMENDED PLANTS FOR WET BIOSWALE
Common Name Scientific Name Spacing (on center)
Shortawn foxtail Alopecurus aequalis seed
Spike rush Eleocharis spp. 4 inches
Slough sedge* Carex obnupta 6 inches or seed
Sawbeak sedge Carex stipata 6 inches
Sedge Carex spp. 6 inches
Western mannagrass Glyceria occidentalis seed
Slender rush Juncus tenuis 6 inches
Watercress* Rorippa nasturtium-aquaticum 12 inches
Water parsley* Oenanthe sarmentosa 6 inches
Hardstem bulrush Scirpus acutus 6 inches
Small-fruited bulrush Scirpus microcarpus 12 inches
* Good choices for swales with significant periods of flow, such as those downstream of a detention facility.
Note: Cattail (Typha latifolia) is not appropriate for most wet swales because of its very dense and clumping growth habit,
which prevents water from filtering through the clump.
6.3.3 LATERAL INFLOW BIOSWALES
In situations where water enters a bioswale along the side rather than discretely at the head, a different
design approach – the lateral inflow bioswale – is needed. The basic swale design (see Section 6.3.1) is
modified by increasing swale length to achieve an equivalent average residence time.
Applications
A lateral inflow bioswale is to be used when inflows are not concentrated, such as locations along the
shoulder of a road without curbs. This design may also be used where frequent, small point flows enter
a swale, such as through curb inlet ports spaced at intervals along a road, or from a parking lot with
frequent curb cuts. In general, no inlet port should carry more than about 10 percent of the flow.
A lateral inflow swale is not appropriate for a situation in which significant lateral flows enter a swale at
some point downstream from the head of the swale. In this situation, the swale width and length must be
recalculated from the point of confluence to the discharge point in order to provide adequate treatment for
the increased flows.
Consult the water quality menus in Section 6.1 for information on how this facility may be used to meet
Core Requirement #8.
6.3.3.1 METHODS OF ANALYSIS
The design flow for lateral inflow swales must include runoff from the pervious side slopes draining to the
swale along the entire swale length.
The method of analysis for lateral inflow swales is the same as for basic bioswales (see Section 6.3.1.1)
except for the following clarification of Step 1 and modification to Step 4:
Step 1: The WQ design flow may be variable to reflect the increase in flows along the swale length. If
only a single design flow is used, the flow at the outlet shall be used.
Step 4: Double the hydraulic residence time so that it is a minimum of 18 minutes (1,080 seconds).
Equation 6-7 becomes:
L = 1080Vwq (6-10)
where L = minimum allowable swale length (ft)
Vwq = design flow velocity calculated in Step 3 (fps).
Note: Although bottom widths may be increased to reduce length, bottom width cannot be reduced
because Manning’s depth-velocity-flow rate relationships would not be preserved.
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6.3.3.2 DESIGN CRITERIA
Same as specified for basic bioswales (in Section 6.3.1.2) except for the following modification:
Planting Requirements, Criterion 4: For lateral inflow bioswales, interior side slopes above the WQ
design treatment elevation shall be planted in grass. A typical lawn seed mix or the bioswale seed
mixes are acceptable. Landscape plants or groundcovers other than grass shall not be used anywhere
between the runoff inflow elevation and the bottom of the swale.
Intent: The use of grass on interior side slopes reduces the chance of soil erosion and transfer of
pollutants from landscape areas to the bioswale treatment area.
6.3.4 STANDARD FILTER STRIPS
A filter strip is a grassy slope located adjacent and parallel to an impervious area such as a parking lot,
driveway, or roadway (see the detail in Figure 6.3.4.A). A filter strip is graded to maintain sheet flow of
stormwater runoff over the entire width of the strip. Pollutants are removed primarily by means of
sedimentation, which is enhanced as a consequence of the resistance that the grass blades present to
flowing water. To a much lesser degree, pollutants may adhere or sorb to grass and thatch. Some dissolved
pollutants may also be sorbed by the underlying soil when infiltration occurs, but the extent of infiltration
depends on the type of soil, the density of the grass, and the slope of the strip. The primary pollutant
removal mechanism is particle settling.
Applications and Limitations
Filter strip design is based on the expectation that water will flow fairly evenly across the entire width and
length of the strip area. Thus, paved areas without underground stormwater collection systems, gutters, or
other runoff control features are good candidates for filter strips.
Filter strips are suitable for areas that meet the following conditions:
Stormwater runoff from the area requiring treatment shall be uniformly distributed along the top of the
entire filter strip. If stormwater runoff from the entire area cannot be spread evenly along the top of
the filter strip, the filter strip shall be applied only to flows that can be uniformly distributed. A
different stormwater treatment facility, such as a swale, should be used for areas of the project site
with concentrated flow (for instance, at road intersections).
The flowpath draining to the filter strip shall not exceed 150 feet. Runoff flows traveling greater
distances tend to concentrate before entering the filter strip.
The lateral slope of the drainage area contributing flows to the filter strip (parallel to the edge of
pavement) shall be less than 2 percent. A stepped series of flow spreaders installed at the head of the
strip could compensate for slightly steeper slopes (see “Flow Spreading and Energy Dissipation”).
The longitudinal slope of the contributing drainage area (parallel to the direction of flow entering the
filter strip) should be less than 5 percent. Contributing drainage areas with slopes steeper than 5
percent shall either use a different WQ facility or must provide energy dissipation and flow spreading
mechanisms upslope of the upper edge of the filter strip.
A filter strip generally requires more land area than a bioswale because the flow depth through the filter is
shallower than through a swale. Although the space requirements may be greater, the filter strip is a viable
water quality treatment option in locations where grassy slopes already exist, or where a slope can be
incorporated easily into the landscape design for the project site. Other limitations that shall be considered
are listed below:
1. Filter strips are susceptible to short-circuiting via flow channelization because they rely on a large
smoothly graded area. If rills, gullies, or channels occur in the filter strip area, inflows will travel too
quickly through the filter strip, reducing contact time and pollutant removal performance. A filter strip
slope with uneven grading perpendicular to the sheet flow path will develop flow channels over time.
These problems can be overcome with careful site planning, good soil compaction, skillful grading,
and periodic maintenance.
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2. Filter strip areas shall not be used for material storage or any activities that could cause disturbance of
the ground surface in a manner that could create or promote preferential flowpaths (rills or channels)
in the filter strip.
3. Filter strips shall not be located in shaded areas, for filter strips require exposure to sunlight to ensure
healthy grass growth.
Consult the water quality menus in Section 6.1 for information on how this facility may be used to meet
Core Requirement #8.
6.3.4.1 METHODS OF ANALYSIS
In this manual, filter strip length is defined as the length of the flowpath through the strip. Strip width is
typically the same as the extent of pavement along the upstream edge of the strip. Thus, in sizing filter
strips, the length is normally the dimension to be sized (see Figure 6.3.4.A).
FIGURE 6.3.4.A FILTER STRIP TERMINOLOGY
The procedure for filter strip design (described below) relies on Manning’s equation to calculate some
design variables. It is recognized that there are problems in this application.27 The filter strip sizing method
will be modified as new research results become available.
Filter strips sized and built using the method of analysis outlined below and the required design criteria
presented in Section 6.3.4.2 are expected to meet the Basic Water Quality menu goal of 80% TSS removal.
Step 1: Calculate design flow. Determine the on-line water quality design flow Qwq (see Section 6.2.1)
using the hydrologic analysis procedures described in Chapter 3 and applying the modification described
in Table 6.2.1.A.
Step 2: Calculate design flow depth. The design flow depth is calculated based on the width of the filter
strip (typically equivalent to the length of the edge of impervious surface contributing flow to the filter
strip) and the longitudinal slope of the filter strip (parallel to the direction of flow) using a form of
Manning’s equation as follows:
27 Ree, W.O., F.L. Wimberley, and F.R. Crow. 1977. Manning n and the overland flow equation. Transactions of the American
Society of Agricultural Engineers 20 (89).
lateral slope
contributing
drainage area
direction
of flow
filter strip length (L)
filter strip width (W)longitudinal slopeAGENDA ITEM # 8. a)
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Qwq = (6-11)
where Qwq = water quality design flow, k(Q, modeled on-line rate), (cfs)
where k = correlation ratio determined from Table 6.2.1.A
nwq = Manning’s roughness coefficient (either 0.35 or 0.45; see the criteria under “Filter
Strip Geometry and Flow Resistance”)
W = width of filter strip perpendicular to the direction of flow (ft) ( length of impervious
surface contributing flow)
df = design depth of flow (ft), which is also assumed to be the hydraulic radius (maximum
1 inch, or 0.083 feet; see the criteria under “Water Depth and Velocity”)
s = longitudinal slope of filter strip parallel to the direction of flow (ft/ft) (averaged over
the width of the filter strip; all portions averaged must also meet the slope design
criteria).
Rearranging the above equation, the design depth of flow can be calculated using the following equation:
df = (6-12)
If the calculated flow depth exceeds 1 inch (0.083 feet), the design flow rate routed through the strip must
be reduced. If this is not feasible, it is not possible to use a filter strip.
Step 3: Calculate design flow velocity through filter strip. The design flow velocity Vwq is based on the
water quality design flow rate, the width of the filter strip, and the calculated design flow depth from Step
2 using the following equation:
Vwq = (6-13)
where Vwq = design flow velocity (fps)
W = strip width (ft) (parallel to the edge of pavement)
df = water depth (ft).
If Vwq exceeds 0.5 feet per second, a filter strip shall not be used. Either redesign the area to provide a
gentler longitudinal slope for the strip, or select a different WQ facility.
Step 4: Calculate required length of filter strip. Determine the required length L of the filter strip to
achieve a desired hydraulic residence time of at least 9 minutes (540 seconds) using the following
equation:
L = 540Vwq (6-14)
where L = filter strip length (ft)
Vwq = design flow velocity from Step 3 (fps)
5.067.149.1 sWdnf
wq
6.0
5.049.1
Ws
nQwqwq
f
wq
Wd
Q
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6.3.4.2 DESIGN CRITERIA
Figure 6.3.4.B shows typical filter strip details. The most effective filter strips achieve uniform sheet flow
under all runoff flow conditions. To achieve proper flow conditions, the following basic design
requirements apply.
Drainage Area Restrictions
1. The longest flowpath from the area contributing sheet flow to the filter strip shall not exceed 150 feet.
2. The lateral slope of the contributing drainage (parallel to the edge of pavement) shall be 2 percent
or less.
3. A stepped series of flow spreaders installed at the head of the strip may be used to compensate for
drainage areas having lateral slopes of up to 4 percent (see Section 6.2.6 for information on flow
spreader designs).
4. The longitudinal slope of the contributing drainage area (parallel to the direction of flow entering
the filter strip) should be 5 percent or less.
5. Contributing drainage areas with longitudinal slopes steeper than 5 percent shall either use a different
WQ facility or provide energy dissipation and flow spreading options upslope of the upper edge of the
filter strip to achieve flow characteristics equivalent to those meeting the criteria in items 2 and 4
above.
Filter Strip Geometry and Flow Resistance
1. The longitudinal slope of a filter strip (along the direction of flow) shall be between 1 percent
minimum and 15 percent maximum.
2. The lateral slope of a strip (parallel to the edge of pavement, perpendicular to the direction of flow)
shall be less than 2 percent.
3. The ground surface at the upper edge of a filter strip (adjacent to the contributing drainage area) shall
be at least 1 inch lower than the edge of the impervious area contributing flows.
4. Manning’s roughness coefficient (nwq) for flow depth calculations shall be 0.35. An exception to this
requirement may be made for situations where the filter strip will be mowed weekly in the growing
season to consistently provide a grass height of less than 4 inches; in this case, the value of nwq in
Equation 6-12 may be set to 0.45.
Note: In filter strip design, a larger n value results in a smaller strip size.
Water Depth and Velocity
1. The maximum depth of flow through a filter strip for the WQ design flow shall be 1.0 inch.
2. The maximum allowable flow velocity for the water quality design flow Vwq shall be 0.5 feet per
second.
Flow Spreading and Energy Dissipation
1. Runoff entering a filter strip must not be concentrated. A flow spreader shall be installed at the edge
of the pavement to uniformly distribute the flow along the entire width of the filter strip.
2. At a minimum, a gravel flow spreader (gravel-filled trench) shall be placed between the impervious
area contributing flows and the filter strip, and meet the following requirements:
a) The gravel flow spreader shall be a minimum of 6 inches deep and shall be 18 inches wide for
every 50 feet of contributing flowpath.
b) The gravel shall be a minimum of 1 inch below the pavement surface.
c) Intent: This allows sediment from the paved surface to be accommodated without blocking
drainage onto the strip.
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d) For contributing flowpaths less than 50 feet, the spreader width may be reduced to a minimum of
12 inches.
e) Where the ground surface is not level, the gravel spreader must be installed so that the bottom of
the gravel trench and the outlet lip are level.
f) Along roadways, gravel flow spreaders must meet the specification for shoulder ballast given in
Section 9-03.9(2) of the current WSDOT/APWA Standard Specifications for Road, Bridge and
Municipal Construction. The ballast shall be compacted to 90 percent standard proctor.
Intent: This specification was chosen to meet traffic safety concerns as well as to limit fines to
less than 2 percent passing the No. 100 sieve.
3. Other flow spreaders (see Section 6.2.6) may also be used. For filter strip applications, the notched
curb spreader and through-curb port spreaders shall not be used without also adding a gravel spreader
to better ensure that water sheet-flows onto the strip.
4. Energy dissipaters are needed in a filter strip if sudden slope drops occur, such as locations where
flows in a filter strip pass over a rockery or retaining wall aligned perpendicular to the direction of
flow. Adequate energy dissipation at the base of a drop section can be provided by a riprap pad (see
Chapter 4, Table 4.2.2.A, for guidance).
Access
Access shall be provided at the upper edge of a filter strip to enable maintenance of the inflow spreader
throughout the strip width and allow access for mowing equipment.
Soil Quality
1. Native topsoil six inches deep with no less than 1% organic matter (OM) does not require soil
amendment, except where grading has occurred and topsoil meeting that OM standard has not been
replaced.
2. Where topsoil has been removed or if native soil OM is less than 1%, Two inches (minimum) of well-
rotted compost shall be provided for the entire filter strip treatment area to amend the topsoil. The
compost must be tilled into the underlying native soil to a depth of 6 inches to prevent washing out the
compost and avoid creating a defined layer of different soil types that can prevent downward
percolation of water. Compost shall meet Specification 1 described in Reference Section 11-C.
3. Soil or sod with a clay content of greater than 10 percent should be avoided. If there is potential for
contamination of the underlying groundwater, the filter strip shall be lined with a treatment liner to
prevent groundwater contamination. See Section 6.2.4, for details on soil liner options.
Planting Requirements
1. Grass shall be established throughout the entire treatment area of the filter strip.
2. Sod may be used instead of grass seed as long as the entire filter strip area is completely covered with
no gaps between sod pieces.
3. Filter strips are subject to drier conditions than bioswales and also may be more vulnerable to erosion
than swales. For these reasons, the following permanent erosion-control grass seed mix shall be
applied at a rate of 39 pounds per acre in filter strips (percentages are by weight):
a) 6 percent spiked bentgrass (Agrostis exarata)
b) 15 percent California brome (Bromus carinatus)
c) 15 percent tufted hairgrass (Deschampsia cespitosa)
d) 18 percent blue wildrye (Elymus glaucus)
e) 18 percent California oatgrass (Danthonia californica)
f) 18 percent red fescue (Festuca rubra var. rubra)
g) 10 percent Meadow barley (Hordeum brachyantherum)
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4. Alternate seed mixes may be used if a horticultural or erosion-control specialist recommends a
different mix and if erosion prevention is adequately addressed by other erosion-control measures.
5. Seed may be applied by hydroseeding or broadcast application.
6. Seeding is best performed in fall (late September to October) or in spring (mid-March to June). For
summer seeding or seeding during dry conditions, sprinkler systems or other measures for watering
the seed must be provided. Soil temperatures should be between 50 and 65 degrees to allow for seed
germination of cool season grasses.
7. Runoff shall be diverted around a filter strip until the grass is established, or an erosion control
blanket shall be placed over the freshly applied seed mix. See ESC Standards (Appendix D) for
information on erosion control blankets.
Recommended Design Features
Where conditions allow, the following features should be incorporated into a filter strip’s design and its
corresponding site configuration.
Site Layout and Landscaping
1. Filter strips should be incorporated into the landscape design of the site; however, the treatment areas
(i.e., grassy areas) should not be fertilized unless needed for healthy grass growth.
2. Curbs should be avoided, if possible, at the downslope edge of the contributing area. If curbing is
needed, through-curb ports shall be provided (see Section 6.2.6).
3. If parking lot wheel stops are necessary, individual wheel stops should have gaps for water to pass
through. The shorter the wheel stops, the better for sheet flow purposes. See Section 6.2.6 for
requirements.
4. During seeding, slow-release fertilizers may be applied to speed the growth of grass. If the filter strip
is located in a sensitive lake watershed, low phosphorus fertilizers (such as formulations in the
proportion 3:1:3 N-P-K or less) or slow-release phosphorus formulations should be used, and at no
more than the minimum necessary agronomic rate. Regardless of location, the fertilizer must meet the
requirements of Chapter 15.54 RCW limiting the use of fertilizer containing phosphorus.
5. Filter strips should be well defined on a site and marked with signs to prevent future destruction or
alteration of the treatment areas. Small at-grade signage is preferred.
Maintenance Features
1. Irrigation may be required in the summer months following initial filter strip construction to prevent
the filter strip grass from wilting or dying. Site planning should address the need for sprinklers or
other means of irrigation.
2. Flatter slopes are preferred for filter strips to make grass mowing easier.
Use with Oil Control Facilities
A project providing oil control (see the high-use definition in Chapter 1) may employ a filter strip for
runoff treatment if a linear sand filter (see Section 6.5.4) is used for oil control preceding the filter strip.
In this situation, the sand filter should be designed so that flows exit the underdrain gravel along the whole
length of the trench directly to the filter strip.
Construction Considerations
1. If a filter strip is put into operation before all construction in the contributing drainage catchment has
been completed, the strip must be cleaned of sediment and reseeded prior to acceptance by the City.
The City will not release financial guarantees if the filter strip is not restored and vigorous grass
growth re-established.
2. It is preferable to provide erosion control before construction-phase sediment enters the filter strip.
Filter strips are designed to handle only modest sediment loads without frequent maintenance.
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Maintenance Considerations
Maintenance considerations, including mowing frequency and sediment removal, are similar to those for
bioswales (see Section 6.3.1.2).
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FIGURE 6.3.4.B SCHEMATIC REPRESENTATION OF A TYPICAL FILTER STRIP
2% max. slope
(lateral)
contributing
drainage area
edge of pavement
or roadway shoulder longitudinal slope5% max150' max. flow path
18" for each
50' of contributing
flow path (12" min.)
length (L)
(4' min.)
flow spreader extending entire
length of pavement
2% max. slope
filter strip
width (W)
PLAN VIEW
NTS
SECTION A-A
NTS
4' minimum length filter strip
(1% - 15% long. slope)
6"
2" compost tilled into
6" of native soil
flow spreader or gravel
filled trench (see note)
1" drop
5% max.
pavement surface
NOTE: Invert of flow
spreader must be level.
Roadway shoulders
must use shoulder ballast.
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6.3.5 NARROW AREA FILTER STRIPS
This BMP is not allowed in the City for Basic WQ. Designers should refer to the Standard Filter Strip.F
28
29F
29
28
28 Footnote 29 does not apply.
29 Footnote 30 does not apply.
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6.4 WETPOOL FACILITY DESIGNS
This section presents the methods, criteria, and details for analysis and design of wetponds, wetvaults, and
stormwater wetlands. These facilities have as a common element a permanent pool of water, the wetpool.
Each of the wetpool facilities may be combined with a detention or flow control pond in a combined
facility. Included are the following specific facility designs:
“Wetponds — Basic and Large,” Section 6.4.1
“Wetvaults,” Section 6.4.2
“Stormwater Wetlands,” Section 6.4.3
“Combined Detention and Wetpool Facilities,” Section 6.4.4.
The information presented for each facility is organized into the following two categories:
1. Methods of Analysis: Contains a step-by-step procedure for designing and sizing each facility.
2. Design Criteria: Contains the details, specifications, and material requirements for each facility.
6.4.1 WETPONDS — BASIC AND LARGE
A wetpond is a constructed stormwater pond that retains a permanent pool of water (a “wetpool”) at least
during the wet season (see the schematic representation in Figure 6.4.1.A and Figure 6.4.1.B). The volume
of the wetpool is related to the effectiveness of the pond in settling particulate pollutants. The following
design procedures, requirements, and recommendations cover two wetpond applications, the basic
wetpond and the large wetpond. The two sizes are designed for two different levels of pollutant removal.
Applications and Limitations
A wetpond requires a larger area than a bioswale or a sand filter, but it can be integrated to the contours of
a site fairly easily. In till soils, the wetpond holds a permanent pool of water that provides an attractive
aesthetic feature. In more porous soils, wetponds may still be used, but water seepage from unlined cells
could result in a dry pond, particularly in the summer months. Lining with impervious material is one way
to deal with this situation.
Wetponds may be single-purpose facilities, providing only water quality treatment, or they may be
combined with a detention pond to also provide flow control. If combined, the wetpond can often be
stacked under the detention pond with little further loss of development area. See Section 6.4.4 for a
description of combined WQ and detention facilities.
Wetponds treat water primarily by gravity settling and to some degree by biological uptake by algae and
transformation and degradation by microorganisms. Wetponds can remove some dissolved pollutants such
as soluble phosphorus (phosphate) by uptake, and phosphate may react and combine with cations in
solution, forming solid particulates. Wetponds are therefore used in the Sensitive Lake Protection menu
for phosphorus control in addition to the Basic WQ menu for solids removal. Wetponds work best when
the water already in the pond is moved out en masse by incoming flows, a phenomena called plug flow.
Because treatment works on this displacement principle, the dead storage pool of wetponds may be
provided below the groundwater level without interfering unduly with treatment effectiveness. However, if
combined with a detention function, the live storage must be above the seasonal high groundwater level.
Consult the water quality menus in Section 6.1 for information on how basic and large wetponds may be
used to meet Core Requirement #8.
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6.4.1.1 METHODS OF ANALYSIS
This section describes methods of analysis for the following two wetpond sizes:
Basic wetpond
Large wetpond.
BASIC WETPOND
The primary design factor that determines a wetpond’s particulate removal efficiency is the volume of
the wetpool in relation to the volume of stormwater runoff. The larger the wetpond volume in relation to
the volume of runoff, the greater the potential for pollutant removal. Also important are the avoidance of
short-circuiting and the promotion of plug flow. Plug flow describes the hypothetical condition of
stormwater moving through the pond as a unit, displacing the “old” water in the pond with incoming
flows. To prevent short-circuiting, water is forced to flow, to the extent practical, to all potentially
available flow routes, avoiding “dead zones,” and maximizing the time water stays in the pond during the
active part of a storm.
Design features that encourage plug flow and avoid dead zones are as follows:
Dissipating energy at the inlet
Providing a large length-to-width ratio
Providing a broad surface for water exchange across cells rather than a constricted area.
Maximizing the flowpath between inlet and outlet, including the vertical path, also enhances treatment by
increasing residence time.
The basic wetpond volume is equal to the 91% water quality treatment volume (see Section 6.2.1),
calculated with the approved model or by using the Natural Resources Conservation Service (NRCS,
formerly Soil Conservation Service [SCS]) curve number method described in Urban Hydrology for Small
Watersheds, Technical Release 55 (TR-55), June 1986, published by the NRCS. Wetponds designed with
the basic wetpond volume using the method below, and the required design criteria in Section 6.4.1.2 are
expected to meet the Basic WQ menu goal of 80% TSS removal. The actual performance of a wetpond
may vary, however, due to a number of factors, including but not limited to design features, maintenance
frequency, storm characteristics, pond algae dynamics, and waterfowl use.
Procedures for determining a wetpond’s dimensions and volume are outlined below.
Step 1: Identify the required wetpool volume. A basic wetpond requires a volume equal to the 91%
treatment volume, calculated with the approved model or by using the NRCS curve number method. When
using the water quality treatment volume reported by the approved model, skip Steps 2 through 4.
Step 2: Determine the weighted NRCS curve number for the developed tributary area.
Table 6.4.1.1.A shows the CNs, by land use description, for the four hydrologic soil groups. These
numbers are for a 24-hour duration storm and typical antecedent soil moisture condition preceding 24-hour
storms.
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TABLE 6.4.1.1.A RUNOFF CURVE NUMBERS FOR SELECTED AGRICULTURAL,
SUBURBAN, AND URBAN AREAS
(Sources: TR 55, 1986, and Stormwater Management Manual (SWMMWW), 1992.
See SWMMWW Section 2.1.1 for explanation)
COVER TYPE AND HYDROLOGIC CONDITION
CNs for Hydrologic
Soil Group
A B C D
CURVE NUMBERS FOR PRE-DEVELOPMENT CONDITIONS
Pasture, grassland, or range-continuous forage for grazing:
Fair condition (ground cover 50% to 75% and not heavily grazed) 49 69 79 84
Good condition (ground cover >75% and lightly or only occasionally grazed) 39 61 74 80
Woods:
Fair (woods are grazed but not burned, and some forest litter covers the soil) 36 60 73 79
Good (woods are protected from grazing, and litter and brush adequately cover
the soil)
30 55 70 77
CURVE NUMBERS FOR POST-DEVELOPMENT CONDITIONS
Open Space (lawns, parks, golf courses, cemeteries, landscaping, etc.)1
Fair condition (grass cover on 50%–75% of the area) 77 85 90 92
Good condition (grass cover on >75% of the area) 68 80 86 90
Impervious Areas
Open water bodies: lakes, wetlands, ponds etc. 100 100 100 100
Paved parking lots, roofs,2 driveways, etc. (excluding right-of-way) 98 98 98 98
Permeable Pavement
(see SWDM 5.2.2 and Appendix C to decide which condition to use)
Landscaped area 77 85 90 92
50% landscaped area/50% impervious 87 91 94 96
100% impervious area 98 98 98 98
Paved 98 98 98 98
Gravel (including right-of-way) 76 85 89 91
Dirt (including right-of-way) 72 82 87 89
Pasture, Grassland, or Range-Continuous Forage for Grazing
Poor condition (ground cover <50% or heavily grazed with no mulch) 68 79 86 89
Fair condition (ground cover 50% to 75% and not heavily grazed) 49 69 79 84
Good condition (ground cover >75% and lightly or only occasionally grazed) 39 61 74 80
Woods:
Poor (Forest litter, small trees, and brush are destroyed by heavy grazing or
regular burning)
45 66 77 83
Fair (woods are grazed but not burned, and some forest litter covers the soil) 36 60 73 79
Good (woods are protected from grazing, and litter and brush adequately cover
the soil)
30 55 70 77
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TABLE 6.4.1.1.A RUNOFF CURVE NUMBERS FOR SELECTED AGRICULTURAL,
SUBURBAN, AND URBAN AREAS
Single Family
Residential:3
Dwelling Unit/Gross
Acre
Should only be used for
subdivisions > 50 acres
Average Percent
impervious
area3,4
1.0 DU/GA 15 Separate curve
number shall be
selected for pervious
and impervious
portions of the site or
basin
1.5 DU/GA 20
2.0 DU/GA 25
2.5 DU/GA 30
3.0 DU/GA 34
3.5 DU/GA 38
4.0 DU/GA 42
4.5 DU/GA 46
5.0 DU/GA 48
5.5 DU/GA 50
6.0 DU/GA 52
6.5 DU/GA 54
7.0 DU/GA 56
7.5 DU/GA 58
PUDs, condos, apartments,
commercial businesses, industrial
areas, and subdivisions < 50 acres
% impervious must be
computed5
Separate curve numbers shall be
selected for pervious and impervious
portions of the site
For a more detailed and complete description of land use curve numbers refer to Chapter 2, NRCS
<Technical Release No. 55 (June 1986)>.
1 Composite CNs may be computed for other combinations of open space cover type.
2 Where roof runoff and driveway runoff are infiltrated or dispersed according to the requirements in Chapter 5 and
Appendix C, the average percent impervious area may be adjusted in accordance with the procedure described under
Section 5.2.2.
3 Assumes roof and driveway runoff is directed into street/storm system.
4 All the remaining pervious areas (lawn) are considered to be in good condition for these curve numbers.
5 See Section 5.2 and Table 3.2.2.E for application of effective impervious area in percentage calculation.
The following are important criteria/considerations for selection of CN values:
Many factors may affect the CN value for a given land use. For example, the movement of heavy
equipment over bare ground may compact the soil so that it has a lesser infiltration rate and greater runoff
potential than would be indicated by strict application of the CN value to developed site conditions.
CN values can be area weighted when they apply to pervious areas of similar CNs (within 20 CN points).
However, high CN areas should not be combined with low CN areas. In this case, separate estimates of S
(potential maximum natural detention) and Qd (runoff depth) should be generated and summed to obtain
the cumulative runoff volume unless the low CN areas are less than 15 percent of the subbasin.
Separate CN values must be selected for the pervious and impervious areas of an urban basin or subbasin.
For residential districts, for subdivisions larger than 50 acres, the percent impervious area given in
Table 6.4.1.1.A must be used to compute the respective pervious and impervious areas; for subdivisions of
50 acres or less, the percentage must be computed. For proposed commercial areas, planned unit
developments, etc., the percent impervious area must be computed from the site plan. For all other land
uses the percent impervious area must be estimated from best available aerial topography and/or field
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reconnaissance. The pervious area CN value must be a weighted average of all the pervious area CNs
within the subbasin. The impervious area CN value shall be 98.
Cover categories are based on existing U.S. Department of Agriculture soil survey data or site-specific
data where available.
Example: The following is an example of how CN values are selected for a sample project.
Select CNs for the following development:
Existing Land Use – forest (undisturbed)
Future Land Use – residential plat (3.6 DU/GA)
Basin Size – 60 acres
Soil Type – 80 percent Alderwood, 20 percent Ragnor
Table 3.2.2.B shows that Alderwood soil belongs to the “C” hydrologic soil group and Ragnor soil belongs
to the “B” group. Therefore, for the existing condition, CNs of 70 and 55 are read from Table 6.4.1.1.A
and areal weighted to obtain a CN value of 67. For the developed condition with 3.6 DU/GA the percent
impervious of 39 percent is interpolated from Table 6.4.1.1.A and used to compute pervious and
impervious areas of 36.6 acres and 23.4 acres, respectively. The 36.6 acres of pervious area is assumed to
be in Fair condition (for a conservative design) with residential yards and lawns covering the same
proportions of Alderwood and Ragnor soil (80 percent and 20 percent respectively). Therefore, CNs of 90
and 85 are read from Table 6.4.1.1.A and areal weighted to obtain a pervious area CN value of 89. The
impervious area CN value is 98. The result of this example is summarized below:
Onsite Condition Existing Developed
Land use Forest Residential
Pervious area 60 ac. 36.6 ac.
CN of pervious area 67 89
Impervious area 0 ac. 23.4 ac.
CN of impervious area – 98
Step 3: Calculate runoff depth for the developed tributary area. The rainfall-runoff equations of the
NRCS curve number method relate a land area’s runoff depth (precipitation excess) to the precipitation it
receives and to its natural storage capacity, as follows:
Qd = (P - 0.2S)² /(P + 0.8S) for P ≥ 0.2S (6-15)
Qd = 0 for P < 0.2S (6-16)
Where:
Qd = runoff depth in inches over the area,
P = precipitation depth in inches over the area, and
S = potential maximum natural detention, in inches over the area, due to infiltration, storage, etc.
The area’s potential maximum detention, S, is related to its curve number, CN:
S = (1000 /CN) - 10 (6-17)
The combination of the above equations allows for estimation of the total runoff volume by computing
total runoff depth, Qd, given the total precipitation depth, P. For example, if the curve number of the area
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is 70, then the value of S is 4.29. With a total precipitation for the design event of 2.0 inches, the total
runoff depth would be:
Qd = [2.0 - 0.2 (4.29)]² /[2.0 + 0.8 (4.29)] = 0.24 inches
This computed runoff represents inches over the tributary area.
Step 4: Calculate the design wetpool volume. The total volume of runoff is found by multiplying Qd by
the area (with necessary conversions):
Total runoff volume = 3,630 x Qd x A
(cu. ft.) (cu. ft./ac. in.) (in) (ac)
If the area is 10 acres, the total runoff volume is:
3,630 cu. ft./ac. in. x 0.24 in. x 10 ac. = 8,712 cu. ft.
This is the design volume for treatment facilities for which the design criterion is based on the volume of
runoff.
Step 5: Determine wetpool dimensions. Determine the wetpool dimensions satisfying the design criteria
outlined below. A simple way to check the volume of each wetpool cell is to use the following equation:
Vb = (6-18)
where Vb = wetpool volume (cf) (from Step 4 or as determined from the approved model)
h = wetpool depth (ft)
A1 = water quality design surface area of wetpool (sf)
A2 = bottom area of wetpool (sf)
Step 6: Design pond outlet pipe and determine primary overflow water surface. The design criteria
for wetponds (see Section 6.4.1.2) calls for a pond outlet pipe to be placed on a reverse grade from the
pond’s wetpool to the outlet structure. Use the following procedure to design the pond outlet pipe and
determine the primary overflow water surface elevation:
1. Use the nomographs in Section 4.3 (Figures 4.3.1.B and 4.3.1.C) to select a trial size for the pond
outlet pipe sufficient to pass the WQ design flow Qwq.
2. Use Figure 4.3.1.F to determine the critical depth dc at the outflow end of the pipe for Qwq.
3. Use Figure 4.2.1.G to determine the flow area Ac at critical depth.
4. Calculate the flow velocity at critical depth using continuity equation (Vc = Qwq /Ac).
5. Calculate the velocity head VH (VH =Vc2/2g), where g is the gravitational constant, 32.2 feet per
second).
6. Determine the primary overflow water surface elevation by adding the velocity head and critical depth
to the invert elevation at the outflow end of the pond outlet pipe (i.e., overflow water surface elevation
= outflow invert + dc + VH)
7. Adjust outlet pipe diameter as needed and repeat Steps (a) through (e).
LARGE WETPOND
Large wetponds are expected to meet the Sensitive Lake Protection menu goal of 50% total phosphorus
removal. The actual performance of a wetpond may vary, however, due to a number of factors.
2
21 )+(AAh
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The methods of analysis presented above for basic wetponds apply to large wetponds, except that a large
wetpond requires an increased volume of 1.5 times the volume reported by the approved model, or
calculated per the NRCS hand method.
6.4.1.2 DESIGN CRITERIA
This section sets forth design criteria for the following:
Basic wetpond
Large wetpond
General wetpond design criteria and concepts are shown in Figure 6.4.1.A.
BASIC WETPOND
Wetpool Geometry
1. The wetpool shall be divided into two cells separated by a baffle or berm.30 The first cell shall contain
between 25 to 35 percent of the total wetpool volume. The baffle or berm volume shall not count as
part of the total wetpool volume.
Intent: The full-length berm or baffle promotes plug flow and enhances quiescence and laminar flow
through as much of the entire water volume as possible. Use of a pipe and full-width manifold system
to introduce water into the second cell is possible on a case-by-case basis if approved by CED.
2. Wetponds with wetpool volumes less than or equal to 4,000 cubic feet may be single celled (i.e., no
baffle or berm is required).
3. Both cells of a two-cell wetpond and the single cell of a one cell wetpond must retain a permanent
pool of water throughout the wet season. A wetpond is considered non-compliant if the pond level
drops more than 12″ in any 7-day measurement period. A low permeability liner per Section 6.2.4
will be required to achieve this standard in infiltrative soils.
4. Sediment storage shall be provided in the first cell. The sediment storage shall have a minimum
depth of 1 foot.
5. The minimum depth of the first cell shall be 4 feet, exclusive of sediment storage requirements. The
depth of the first cell may be greater than the depth of the second cell. If the wetpool is a single cell,
the volume equivalent to the first cell shall have a minimum depth of 4 feet.
6. The maximum depth of each cell shall not exceed 8 feet (exclusive of sediment storage in the first
cell). Pool depths of 3 feet or shallower (second cell) shall be planted with emergent wetland
vegetation (see Planting Requirements).
7. Inlets and outlets shall be placed to maximize the flowpath through the facility. The ratio of flowpath
length to width from the inlet to the outlet shall be at least 3:1. The flowpath length is defined as the
distance from the inlet to the outlet, as measured at mid-depth. The width at mid-depth can be found as
follows: width = (average top width + average bottom width)/2.
8. All inlets shall enter the first cell. If there are multiple inlets, the length-to-width ratio shall be based
on the average flowpath length for all inlets.
Berms, Baffles, and Slopes
1. A berm or baffle shall extend across the full width of the wetpool, and tie into the wetpond side
slopes. If the berm embankments are greater than 4 feet in height, the berm must be constructed by
30 As used here, the term baffle means a vertical divider placed across the entire width of the pond, stopping short of the pond
bottom. A berm is a vertical divider typically built up from the bottom, or if in a vault, connects all the way to the bottom.
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excavating a key equal to 50% of the embankment cross-sectional height and width. This requirement
may be waived if recommended by a geotechnical engineer for specific site conditions.31
2. The top of the berm shall extend to the WQ design water surface or be one foot below the WQ
design water surface. If at the WQ design water surface, berm side slopes must be 3H:1V. Berm side
slopes may be steeper (up to 2:1) if the berm is submerged one foot.
3. Intent: Submerging the berm is intended to enhance safety by discouraging pedestrian access when
side slopes are steeper than 3H:1V.
4. If good vegetation cover is not established on the berm, erosion control measures shall be used to
prevent erosion of the berm back-slope when the pond is initially filled.
5. The interior berm or baffle may be a retaining wall provided that the design is prepared and stamped
by a civil engineer. If a retaining wall is used, it shall be submerged one foot below the design water
surface to discourage access by pedestrians.
6. Criteria for wetpond side slopes and fencing are given under “General Requirements for WQ
Facilities,” Section 6.2.3.
7. Berm embankments shall be the same as for detention ponds (see Section 5.1.1).
8. Internal berms to lengthen the flow path or allow the inlet and outlet to be at the same side of the
pond may be used if an adjustment is granted. An adjustment may be granted only if physical site
constraints prevent the standard configuration and design features promote water quality treatment.
Required design features to approve an adjustment include minimizing dead spaces, minimizing
turbulence, and promoting plug flow. Internal berms must extend to the 2-year water elevation, a
minimum of 10 feet must be between the berms, and a distance equal to the width between the internal
berms must be provided between the internal berm and the pond side at the point that the flow turns
around the berm.
Inlet and Outlet
See Figure 6.4.1.A for details on the following requirements:
1. The inlet to the wetpond shall be submerged with the inlet pipe invert a minimum of two feet from
the pond bottom (not including sediment storage). The top of the inlet pipe shall be submerged at least
1 foot.
Intent: The inlet is submerged to dissipate energy of the incoming flow. The distance from the bottom
is set to minimize resuspension of settled sediments. Alternative inlet designs that accomplish these
objectives are acceptable.
2. An outlet structure shall be provided. Either a Type 2 catch basin with a grated opening (jail house
window) or a manhole with a cone grate (birdcage) may be used (see Section 5.1.1.1). No sump is
required in the outlet structure for wetponds not providing detention storage. The outlet structure
receives flow from the pond outlet pipe. The grate or birdcage openings provide an overflow route
should the pond outlet pipe become clogged. Criterion 5 below specifies the sizing and position of the
grate opening.
3. The pond outlet pipe (as opposed to the structure outlet) shall be back-sloped or have a turn-down
elbow, and extend 1 foot below the WQ design water surface.
Note: A floating outlet, set to draw water from 1 foot below the water surface, is also acceptable if
vandalism concerns are adequately addressed.
Intent: The inverted outlet pipe provides for trapping of oils and floatables in the wetpond.
31 The geotechnical analysis must address situations in which one of the two cells is empty while the other remains full of water.
These situations can occur, for example, during pump down of either cell for sediment removal, or when water from the second
unlined cell percolates into the ground.
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4. The pond outlet pipe shall be sized, at a minimum, to pass the WQ design flow.
Note: The highest invert of the outlet pipe sets the WQ design water surface elevation.
5. The overflow criteria for single-purpose wetponds are as follows:
a) The requirement for primary overflow as described for flow control ponds is satisfied by either
the grated inlet to the outlet structure or by a birdcage above the pond outlet structure as shown
in Figure 5.1.1.C.
b) The bottom of the grate opening in the outlet structure shall be set at or above the height needed
to pass the WQ design flow through the pond outlet pipe (see Section 6.4.1.1 for sizing details).
Note: The grate invert elevation sets the overflow water surface elevation.
c) In flow-through ponds, the grated opening shall be sized to pass the 100-year design flow.
6. An emergency spillway shall be provided and designed according to the requirements for detention
ponds (see Section 5.1.1).
7. A gravity drain for maintenance shall be provided if grade allows.
Intent: It is anticipated that sediment removal will only be needed for the first cell in the majority of
cases. The gravity drain is intended to allow water from the first cell to be drained to the second cell
when the first cell is pumped dry for cleaning.
a) The drain invert shall be at least 6 inches below the top elevation of the dividing berm or baffle.
Deeper drains are encouraged where feasible, but must be no deeper than 18 inches above the
pond bottom.
b) Intent: to prevent highly sediment-laden water from escaping the pond when drained for
maintenance.
c) The drain shall be at least 8 inches (minimum) diameter and shall be controlled by a valve. Use of
a shear gate is allowed only at the inlet end of a pipe located within an approved structure.
Intent: Shear gates often leak if water pressure pushes on the side of the gate opposite the seal.
The gate should be situated so that water pressure pushes toward the seal.
8. Operational access to the valve shall be provided to the finished ground surface.
a) The valve location shall be accessible and well-marked with one foot of paving placed around the
box. It must also be protected from damage and unauthorized operation.
b) A valve box is allowed to a maximum depth of 5 feet without an access manhole. If over 5 feet
deep, an access manhole or vault is required.
9. All metal parts shall be corrosion-resistant. Galvanized materials are discouraged where substitutes are
available.
Access and Setbacks
1. The location of the pond relative to site constraints (e.g., buildings, property lines, etc.) shall be the
same as for detention ponds (see Section 5.1.1). See Section 6.2.3 for typical setback requirements for
WQ facilities.
2. Access and maintenance roads shall be provided and designed according to the requirements for
detention ponds (see Section 5.1.1). Access and maintenance roads shall extend to both the wetpond
inlet and outlet structures. An access ramp shall be provided to the bottom of the first cell unless all
portions of the cell can be reached and sediment loaded from the top of the pond. Also see Section
5.1.1, “Access Requirements” for more information on access alternatives.
3. If the dividing berm is also used for access, it must be built to sustain loads of up to 80,000 pounds.
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Signage
General signage shall be provided according to the requirements for detention ponds (see Section 5.1.1).
Planting Requirements
1. Planting requirements for detention ponds (see Section 5.1.1.1) also apply to wetponds.
If the second cell of the wetpond is 3 feet or shallower, the bottom area shall be planted with emergent
wetland vegetation. See Table 6.4.1.A for recommended emergent wetland plant species for wetponds.
Intent: Planting of shallow pond areas helps to stabilize settled sediment and prevent resuspension.
Note: The recommendations in Table 6.4.1.A are for western Washington only. Local knowledge
should be used to adapt this information if used in other areas.
2. Cattails (Typha latifolia) are not allowed because they tend to crowd out other species, and the dead
shoots need to be removed to prevent oxygen depletion in the wetpool.
3. If the wetpond is in a sensitive lake or sphagnum bog protection area, shrubs that form a dense cover
shall be planted on slopes above the WQ design water surface on at least three sides. For banks that
are berms, no planting is allowed if the berm is regulated by dam safety requirements (see
Section 5.1.1). The purpose of planting is to discourage waterfowl use of the pond and to provide
shading.32 Some suitable trees and shrubs include vine maple (Acer circinatum), wild cherry (Prunus
emarginata), red osier dogwood (Cornus stolonifera), California myrtle (Myrica californica), Indian
plum (Oemleria cerasiformis), and Pacific yew (Taxus brevifolia) as well as numerous ornamental
species.
Recommended Design Features
The following design features should be incorporated into the wetpond design where site conditions allow:
1. For wetpool depths in excess of 6 feet, it is recommended that some form of recirculation be
provided in the summer, such as a fountain or aerator, to prevent stagnation and low dissolved oxygen
conditions. A special use permit is needed for a pump or fountain in a City maintained pond.
2. A flow length-to-width ratio greater than the 3:1 minimum is desirable. If the ratio is 4:1 or greater,
then the dividing berm is not required, and the pond may consist of one cell rather than two.
3. A tear-drop shape, with the inlet at the narrow end, rather than a rectangular pond is preferred since
it minimizes dead zones caused by corners.
4. A small amount of base flow may maintain circulation and reduce the potential for low oxygen
conditions during late summer.
5. Evergreen or columnar deciduous trees along the west and south sides of ponds are recommended to
reduce thermal heating, except that no trees or shrubs shall be planted on berms meeting the criteria of
dams regulated for safety (see “Dam Safety Compliance” in Section 5.1.1). In addition to shade, trees
and shrubs also discourage waterfowl use and the attendant phosphorus enrichment problems they
cause. Trees should be set back so that the branches will not extend over the pond.
Intent: Evergreen trees or shrubs are preferred to avoid problems associated with leaf drop. Columnar
deciduous trees (e.g., hornbeam, Lombardy poplar, etc.) typically have fewer leaves than other
deciduous trees.
6. The number of inlets to the facility should be limited; ideally there should be only one inlet. The
flowpath length should be maximized from inlet to outlet for all inlets to the facility.
32 Waterfowl are believed to limit use of areas where their view of predator approach paths is blocked. Some suitable native
shrubs include vine maple, Indian plum, bitter cherry, red osier dogwood, cascara, and red elderberry. Ornamental hedge
plants such as English laurel, privet and barberry are also good choices.
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7. The access and maintenance road could be extended along the full length of the wetpond and could
double as playcourts or picnic areas. Placing finely ground bark or other natural material over the road
surface would render it more pedestrian friendly.
8. Stormwater facilities may be incorporated within the open space, common space or recreation space
on a case by case basis if:
a) The stormwater facility utilizes the techniques and landscape requirements set forth in The
Integrated Pond, King County Water and Land Resources Division, or an equivalent manual, or
b) The surface water feature serves areas outside of the planned urban development and is
appropriate in size and creates a benefit.
9. The following design features should be incorporated to enhance aesthetics where possible:
a) Subject to dam safety restrictions (WAC 175-175), provide visual enhancement with clusters of
trees and shrubs around the wetpond, above the emergency overflow water surface elevation. In
most pond areas, it is important to amend the soil with compost before planting since ponds are
typically placed well below the native soil horizon in very poor soils. Compost must meet quality
criteria in Reference Section 11-C.
b) Orient the pond length along the direction of prevailing summer winds (typically west or
southwest) to enhance aeration.33 This is beneficial for both aesthetics and treatment.
Construction Considerations
1. Sediment that has accumulated in the pond must be removed after construction in the drainage area of
the pond is complete (unless used for a liner—see Criteria 2 below). If no more than 12 inches of
sediment have accumulated after plat construction, cleaning may be left until after building
construction is complete. In general, sediment accumulation from stabilized drainage areas is not
expected to exceed an average of 4 inches per year in the first cell. If sediment accumulation is greater
than this amount, it will be assumed to be from construction unless it can be shown otherwise. The
City will not release maintenance and defect financial guarantees or assume maintenance
responsibility for a facility unless it has been cleaned of construction phase sediments.
2. Sediment that has accumulated in the pond at the end of construction may be used as a liner in
excessively drained soils if the sediment meets the criteria for low permeability or treatment liners
defined in Section 6.24 and in keeping with guidance given in Table 6.2.4.A. Sediment used for a soil
liner must be graded to provide uniform coverage and thickness.
Maintenance Considerations
1. The pond should be inspected annually. Floating debris and accumulated petroleum products should
be removed as needed, but at least annually.
2. Nearby vegetation should be trimmed as necessary to keep the pond free of leaves and to maintain the
aesthetic appearance of the area. Slope areas that have become bare should be revegetated and eroded
areas should be regraded prior to being revegetated.
3. Sediment should be removed when the 1-foot sediment zone is full plus 6 inches. Sediments should be
tested for toxicants in compliance with current disposal requirements if land uses in the catchment
include commercial or industrial zones, or if visual or olfactory indications of pollution are noticed.
4. Water drained or pumped from ponds prior to sediment removal may be discharged to storm drains if
it is not excessively turbid (i.e., if water appears translucent when held to light) and if floatable debris
33 Wind moving over the surface of standing water can often induce some mixing of surface and near-surface water, replenishing
oxygen and reducing stagnant conditions. If the pond is aligned with the prevailing wind direction, this effect can be maximized.
See Bentzen et al. 2009. Predictions of Resuspension of Highway Detention Pond Deposits in Interrain Event Periods due to
Wind-Induced Currents and Waves. Journal of Environmental Engineering 135 (12):1286-1293
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and visual petroleum sheens are removed. Excessively turbid water (i.e., water appears opaque when
held to light) should be discharged only after the solids have been settled and removed.
5. Pumping rates should be slow enough so that downstream channel erosion problems do not develop.
LARGE WETPOND
All design criteria for basic wetponds shall apply to large wetponds, with the following modifications:
1. The wetpool for a large wetpond shall have a volume equal to 1.5 times the Basic wetpond volume
described above.
2. If the project is subject to the Sensitive Lake Protection menu or the Sphagnum Bog Protection menu,
the following shall apply:
a) Shrubs that form a dense cover shall be planted along the top of the wetpond bank on cut slopes.
Planting is recommended for bermed slopes, except for berms meeting the criteria of dams
regulated for safety (see “Dam Safety Compliance” in Section 5.1.1). Evergreen trees and shrubs
are preferred.
Intent: Trees and shrubs discourage waterfowl use. Waterfowl tend to avoid areas that are not
visually open.
b) Measures to enhance waterfowl habitat value (e.g., nesting structures) are not allowed.
TABLE 6.4.1.A EMERGENT WETLAND PLANT SPECIES RECOMMENDED FOR WETPONDS
Species Common Name Notes Maximum Depth
INUNDATION TO 6 INCHES
Carex amplifolia Bigleaf sedge Pond margins, prefers steady water
levels rather than large water
elevation fluctuations
Carex lenticularis var.
lipocarpa
Kellogg’s sedge Wet, sunny, or partially shaded sites
along stream banks, lakeshores, wet
meadows, and bogs.
Carex stipata Sawbeak sedge Wet ground
Glyceria occidentalis Western mannagrass Marshes, pond margins
Juncus effusus var.
pacificus
Soft rush Wet meadows, pastures, wetland
margins
Juncus tenuis Slender rush Wet soils, wetland margins
Oenanthe sarmentosa Water parsley Shallow water along stream and pond
margins; needs saturated soils all
summer
Scirpus atrocinctus
(formerly S. cyperinus)
Woolgrass Tolerates shallow water; tall clumps
Sagittaria latifolia Arrowhead
Beckmania
syzigachne(1)
Western sloughgrass Wet prairie to pond margins
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TABLE 6.4.1.A EMERGENT WETLAND PLANT SPECIES RECOMMENDED FOR WETPONDS
Species Common Name Notes Maximum Depth
INUNDATION TO 2 FEET
Agrostis exarata(1) Spike bent grass Prairie to coast
Alisma plantago-
aquatica
Water plantain
Eleocharis palustris Spike rush Margins of ponds, wet meadows
Glyceria grandis Reed mannagrass Rhizomatous grass in freshwater
habitats, sun or shade
Scirpus microcarpus Small-fruited bulrush Wet ground 18 inches
Sparganium
emmersum
Bur reed Shallow standing water, saturated
soils
INUNDATION TO 3 FEET
Carex aquatilis* Watersedge Wet and boggy meadows, stream
banks, pond, and lake margins.
Tolerates 1 to 2 months of
submersion.
Carex obnupta Slough sedge Wet ground or standing water
Schoenoplectus
acutus(2)
Hardstem bulrush Single tall stems, not clumping
Schoenoplectus
tabernaemontani(2)
Softstem bulrush
INUNDATION GREATER THAN 3 FEET
Nuphar polysepalum Spatterdock Deep water 3 to 7.5 feet
Nymphaea odorata(1) White waterlily Shallow to deep ponds to 6 feet
Notes:
(1) Nonnative species. Beckmania syzigachne is native to Oregon. Native species are preferred. Carex aquatilis is native to both
Washington and Oregon, but not documented within the USDA Plants Database in King County.
(2) Scirpus tubers must be planted shallower for establishment, and protected from foraging waterfowl until established. Emerging aerial
stems should project above water surface to allow oxygen transport to the roots.
Primary sources: Municipality of Metropolitan Seattle, Water Pollution Control Aspects of Aquatic Plants, 1990. Hortus Northwest,
Wetland Plants for Western Oregon, Issue 2, 1991. Hitchcock and Cronquist, Flora of the Pacific Northwest, 1973.
AGENDA ITEM # 8. a)
SECTION 6.4 WETPOOL FACILITY DESIGNS
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FIGURE 6.4.1.A SCHEMATIC REPRESENTATION OF A WETPOND PLAN VIEW
PLANTINGS REQUIRED
ON SLOPES ABOVE WQ
DESIGN WS FOR LAKE OR
BOG PROTECTION
FACILITIES
ACCESS ROAD TO INLET STRUCTURE
NOTE:
BERM NOT REQUIRED FOR PONDS
WITH LENGTH TO WIDTH RATIO 4:1
OR IF VOLUME LESS THAN 4000 C.F.
wetpool
width
PLAN VIEW
NTS
ACCESS ROAD TO
OUTLET STRUCTURE
PLACE BERM OR BAFFLE AT
DESIGN WS OR SUBMERGED
1' BELOW DESIGN WS.
EXTEND BERM ACROSS
ENTIRE WETPOOL WIDTH
CATCH BASIN & OUTLET PIPE
SIZED TO PASS PEAK FLOW PER
CONVEYANCE REQUIREMENTS OUTLET EROSION CONTROL &
ENERGY DISSIPATION PER
DETENTION FACILITY REQUIREMENTS
EMERGENCY SPILLWAY
PER DETENTION FACILITY
REQUIREMENTS
EMERGENCY
OVERFLOW WS
OVERFLOW WS
WQ DESIGN WS
SECOND WETPOOL CELL
BERM TOP WIDTH 5' MIN.
(IF EARTHEN)
FIRST WETPOOL CELL
APPROX. 1
3 OF TOTAL
WETPOOL VOLUME
EXCLUDING ACCESS
RAMP
ACCESS RAMP TO
BOTTOM OF FIRST
WETPOOL CELL
(7H:1V) (SEE TEXT)
INLET PIPE & CATCH BASIN
PER DETENTION FACILITY
REQUIREMENTS
AGENDA ITEM # 8. a)
6.4.1 WETPONDS — BASIC AND LARGE
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FIGURE 6.4.1.B SCHEMATIC REPRESENTATION OF A WETPOND PROFILE
EMERGENCY OVERFLOW WS
OVERFLOW WS
WQ DESIGN WS
SECTION A-A
NTS
INLET
SECTION B-B
NTS
EMERGENCY OVERFLOW WS
OVERFLOW WS
WQ DESIGN WS
NOTE:
SEE DETENTION FACILITY REQUIREMENTS FOR
LOCATION AND SETBACK REQUIREMENTS.
12" MIN.
18" MIN.
FENCE REQUIRED FOR SIDE SLOPES
STEEPER THAN 3(H):1(V)
ACCESS ROAD
CAPACITY OF OUTLET SYSTEM
SIZED TO PASS PEAK FLOW
FOR CONVEYANCE
REQUIREMENT
EXTERIOR BERMS DESIGNED PER
DAM SAFETY REQUIREMENTS
IF APPLICABLE
MANHOLE OR
TYPE 2
CATCH BASIN
OUTLET PIPE INVERT AT
WETPOOL WS ELEVATION
GRAVITY DRAIN
(IF GRADE ALLOWS)
8" MIN. DIAMETER
VALVE
(MAY BE LOCATED INSIDE MH
OR OUTSIDE WITH APPROVED
OPERATIONAL ACCESS)
INVERT 6" MIN.
BELOW TOP
OF INTERNAL
BERM. LOWER
PLACEMENT
IS DESIRABLE
EMERGENT VEGETATION
REQUIRED FOR WETPOOL
DEPTHS 3' OR LESS
KEYED BERM
NOTE: BERM SLOPE MAY BE
2:1 WHEN TOP SUBMERGED
1' BELOW WQ DESIGN WS
SLOPE VEGETATION
PER DETENTION
FACILITY REQUIREMENTS
WETPOOL FLOW LENGTH (INLET TO OUTLET) = 3 (MIN.) x WIDTH
INLET EROSION CONTROL/
SLOPE PROTECTION PER
DETENTION FACILITY
REQUIREMENTS
SEDIMENT STORAGE
DEPTH = 1' MIN.
2' MIN.
FIRST CELL DEPTH
4' TO 8' MAX.WETPOOL DEPTH 8' MAX.
RECIRCULATION RECOMMENDED
FOR DEPTH >6'
AGENDA ITEM # 8. a)
SECTION 6.4 WETPOOL FACILITY DESIGNS
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6.4.2 WETVAULTS
A wetvault is an underground structure similar in appearance to a detention vault, except that a wetvault
has a permanent pool of water that dissipates energy and improves the settling of particulate pollutants
(see the schematic representation in Figure 6.4.2.A). Being underground, the wetvault lacks the biological
pollutant removal mechanisms, such as algae uptake, present in surface wetponds.
Applications and Limitations
A wetvault may be used in any type or size of development. However, it is most practical in relatively
small catchments (less than 10 acres of impervious surface) with high land values because vaults are
relatively expensive. Combined detention and wetvaults are allowed; see Section 6.4.4.
A wetvault is believed to be ineffective in removing dissolved pollutants such as soluble phosphorus or
metals such as copper. There is also concern that oxygen levels will decline, especially in warm summer
months, because of limited contact with air and wind. However, the extent to which this potential problem
occurs has not been documented.
If oil control is required for a project, the wetvault may be combined with the baffle oil/water separator
facility (see Section 6.6.2) to fulfill Special Requirement #5, “Oil Control” (see Option 5, Section 6.1.5).
Consult the water quality menus in Section 6.1 for information on how this facility may be used to meet
Core Requirement #8 and Special Requirement #5.
6.4.2.1 METHODS OF ANALYSIS
As with wetponds, the primary design factor that determines the removal efficiency of a wetvault is the
volume of the wetpool in relationship to the volume of runoff. The larger the volume, the higher the
potential for pollutant removal. Performance is also improved by avoiding dead zones (like corners) where
little exchange occurs, using large length-to-width ratios, dissipating energy at the inlet, and ensuring that
flow rates are uniform to the extent possible and not increased between cells.
Wetvaults sized using the design methodology below (with a volume equal to the 91% treatment volume
per Section 6.2.1) and following the required design criteria in Section 6.4.2.2 are expected to meet the
Basic WQ menu goal of 80% TSS removal.
The methods of analysis for a wetvault are identical to the methods of analysis for the wetpond . Follow
the procedure specified in Section 6.4.1.1 to determine the wetpool volume for a wetvault.
6.4.2.2 DESIGN CRITERIA
A schematic representation of a wetvault is shown in Figure 6.4.2.A.
Wetpool Geometry
Same as specified for wetponds (see Section 6.4.1.2) except for the following two modifications:
1. Criterion 3: The sediment storage in the first cell shall be an average of 1 foot. Because of the v-
shaped bottom, the depth of sediment storage needed above the bottom of the side wall is roughly
proportional to vault width according to the schedule below:
Vault Width
15′
20′
40′
60′
Sediment Depth (from bottom of side wall)
10″
9″
6″
4″
2. Criterion 5: The second cell shall be a minimum of 3 feet deep since planting cannot be used to
prevent resuspension of sediment in shallow water as it can in open ponds.
AGENDA ITEM # 8. a)
6.4.2 WETVAULTS
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Vault Structure
1. Wetvaults shall be designed as flow-through systems.
2. The vault shall be separated into two cells by a wall or a removable baffle.34 If a wall or non-
removable baffle is used, a 5-foot by 10-foot removable maintenance access must be provided for
both cells. If a removable baffle is used, the following criteria apply:
a) The baffle shall extend from a minimum of 1-foot above the WQ design water surface to a
minimum of 1 foot below the invert elevation of the inlet pipe.
b) The lowest point of the baffle shall be a minimum of 2 feet from the bottom of the vault, and
greater if feasible.
3. If the vault is less than 2,000 cubic feet (inside dimensions) or if the length-to-width ratio of the vault
pool is 5:1 or greater, the baffle or wall may be omitted and the vault may be one-celled.
4. The two cells of a wetvault shall not be divided into additional subcells by internal walls. If internal
structural support is needed, post and pier construction may be used to support the vault lid rather than
walls. Any walls used within cells must be positioned so as to lengthen, rather than divide, the
flowpath.
Intent: Treatment effectiveness in wetpool facilities is related to the extent to which plug flow is
achieved and short-circuiting and dead zones are avoided. Structural walls placed within the cells can
interfere with plug flow and create significant dead zones, reducing treatment effectiveness.
5. Internal walls to lengthen the flow path or allow the inlet and outlet to be at the same side of the vault
may be used if an adjustment is granted, or if the requirements for the below exception are met. An
adjustment may be granted only if physical site constraints prevent the standard configuration and
design features promote water quality treatment. Required design features to approve an adjustment
include minimizing dead spaces, minimizing turbulence, and promoting plug flow. Internal walls must
extend to the 2-year water elevation, a minimum of 10 feet must be between the walls, and a distance
equal to the width between the internal walls must be provided between the internal wall and the vault
wall at the point that the flow turns around the wall. All vault requirements apply to each
length/segment. Exception: If the above requirements are met, internal walls are not used to form
more than two u-turns, and these internal walls extend from floor to ceiling, then no adjustment is
required.
Intent: Confined movement around the internal walls creates turbulence, creates dead zones and
decreases treatment effectiveness.
6. The bottom of the first cell shall be sloped toward the access opening. Slope shall be between
0.5 percent (minimum) and 2 percent (maximum). The second cell may be level (longitudinally)
sloped toward the outlet, with a high point between the first and second cells.
7. The vault bottom shall slope laterally a minimum of 5% from each side towards the center, forming a
broad “v” to facilitate sediment removal.
Note: More than one “v” may be used to minimize vault depth.
Exception: The vault bottom may be flat if removable panels are provided over the entire vault.
Removable panels shall be at grade, have stainless steel lifting eyes, and weigh no more than 5 tons
per panel.
8. The highest point of a vault bottom must be at least 6 inches below the outlet elevation to provide for
sediment storage over the entire bottom.
9. Provision for passage of flows should the outlet plug shall be provided.
34 As used here, the term baffle means a divider that does not extend all the way to the bottom of the vault, or if a bottom baffle,
does not extend all the way to the top of the water surface. A wall is used here to mean a divider that extends all the way from
near the water surface to the bottom of the vault.
AGENDA ITEM # 8. a)
SECTION 6.4 WETPOOL FACILITY DESIGNS
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10. Wetvaults may be constructed using arch culvert sections provided the top area at the WQ design
water surface is, at a minimum, equal to that of a vault with vertical walls designed with an average
depth of 6 feet. If arched culverts are used, the manufacturer must certify that they are water-tight.
Intent: To prevent decreasing the surface area available for oxygen exchange.
11. Wetvaults shall conform to the “Materials” and “Structural Stability” criteria specified for
detention vaults in Section 5.1.3.
12. Where pipes enter and leave the vault below the WQ design water surface, they shall be sealed using a
non-porous, non-shrinking grout.
Inlet and Outlet
1. The inlet to the wetvault shall be submerged with the inlet pipe invert a minimum of 3 feet from the
vault bottom (not including sediment storage). The top of the inlet pipe shall be submerged at least
1 foot.
Note: These dimensional requirements may increase the minimum 4 foot depth of the first cell,
depending on the size of the inlet pipe.
Intent: The submerged inlet is to dissipate energy of the incoming flow. The distance from the bottom
is to minimize resuspension of settled sediments. Alternative inlet designs that accomplish these
objectives are acceptable.
2. Unless designed as an off-line facility, the capacity of the outlet pipe and available head above the
outlet pipe shall be designed to convey the 100-year design flow for developed site conditions (as
described in Section 5.1.4.2) without overtopping the vault. The available head above the outlet pipe
must be a minimum of 6 inches.
3. The outlet pipe shall be back-sloped or have tee section, the lower arm of which shall extend 1 foot
below the WQ design water surface to provide for trapping of oils and floatables in the vault.
4. A gravity drain for maintenance shall be provided if grade allows.
a) The gravity drain should be as low as the site situation allows; however, the invert shall be no
lower than the average sediment storage depth. At a minimum, the invert shall be 6 inches above
the base elevation of the vault side walls.
Intent: This placement prevents highly sediment-laden water from escaping when the vault is
drained for maintenance. A lower placement is allowed than for wetponds since the v-shaped
vault bottom will capture and retain additional sediments.
b) The drain shall be 8 inches (minimum) diameter and shall be controlled by a valve. Use of a shear
gate is allowed only at the inlet end of a pipe located within an approved structure.
Intent: Shear gates often leak if water pressure pushes on the side of the gate opposite the seal.
The gate should be situated so that water pressure pushes toward the seal.
c) Operational access to the valve shall be provided to the finished ground surface. The valve
location shall be accessible and well-marked with one foot of paving placed around the box. It
must also be protected from damage and unauthorized operation.
d) If not located in the vault, a valve box is allowed to a maximum depth of 5 feet without an access
manhole. If over 5 feet deep, an access manhole is required.
Access Requirements
Same as for detention vaults (see Section 5.1.3).
Note: If the 5-foot by 10-foot removable maintenance access also provides inlet/outlet access, then a
3-foot by 3-foot inspection port must be provided at the inlet pipe and outlet structure.
AGENDA ITEM # 8. a)
6.4.2 WETVAULTS
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Ventilation Requirements
A minimum of 50 square feet of grate shall be provided over the second cell. For vaults in which the
surface area of the second cell is greater than 1,250 square feet, 4% of the total surface area shall be
grated. This requirement may be met by one grate or by many smaller grates distributed over the second
cell area. If the vault is a single cell, ventilation shall be provided over the second half of the vault.
Note: a grated access door may be used to meet this requirement.
Intent: The grate allows air contact with the wetpool in order to minimize stagnant conditions that can
result in oxygen depletion, especially in warm weather.
Access Roads, Right of Way, and Setbacks
Same as for detention vaults (see Section 5.1.3).
Recommended Design Features
The following design features should be incorporated into wetvaults where feasible, but they are not
specifically required:
1. The floor of the second cell should slope toward the outlet for ease of cleaning.
2. The inlet and outlet should be at opposing corners of the vault to increase the flowpath.
3. A flow length-to-width ratio greater than 3:1 minimum is desirable.
4. Lockable grates instead of solid manhole covers are recommended to increase air contact with the
wetpool.
5. Galvanized materials should be avoided whenever possible.
6. The number of inlets to the wetvault should be limited, and the flowpath length should be maximized
from inlet to outlet for all inlets to the vault.
Construction Considerations
Sediment that has accumulated in the vault must be removed after construction in the drainage area is
complete. If no more than 12 inches of sediment have accumulated after the infrastructure is built,
cleaning may be left until after building construction is complete. In general, sediment accumulation from
stabilized drainage areas is not expected to exceed an average of 4 inches per year in the first cell. If
sediment accumulation is greater than this amount, it will be assumed to be from construction unless it can
be shown otherwise. The City will not release maintenance and defect financial guarantees or assume
maintenance responsibility for a facility unless it has been cleaned of construction phase sediments.
Maintenance Considerations
1. Accumulated sediment and stagnant conditions may cause noxious gases to form and accumulate in
the vault.
2. Facilities should be inspected annually. Floating debris and accumulated petroleum products shall be
removed as needed, but at least annually. The floating oil shall be removed from wetvaults used as
oil/water separators when oil accumulation exceeds one inch.
3. Sediment should be removed when the 1-foot (average) sediment zone is full thus 6 inches. Sediments
should be tested for toxicants in compliance with current disposal requirements if land uses in the
catchment include commercial or industrial zones, or if visual or olfactory indications of pollution are
noticed.
4. Water drained or pumped from the vault prior to removing accumulated sediments may be discharged
to storm drains if it is not excessively turbid (i.e., if water appears translucent when held to light) and
if all floatable debris and visual petroleum sheens are removed. Excessively turbid water (i.e., water
appears opaque when held to light) should be discharged only after the settleable solids have been
removed.
AGENDA ITEM # 8. a)
SECTION 6.4 WETPOOL FACILITY DESIGNS
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MODIFICATIONS FOR COMBINING WITH A BAFFLE OIL/WATER SEPARATOR
If the project site is a high-use site and a wetvault is proposed to meet the Basic WQ menu criteria, the
vault may be combined with a baffle oil/water separator (see Section 6.6.2) to meet the requirements of
Special Requirement #5 with one facility rather than two. Structural modifications and added design
criteria are given below. However, the maintenance requirements for baffle oil/water separators must be
adhered to, in addition to those for a wetvault. This will result in more frequent inspection and cleaning
than for a wetvault used only for TSS removal. See Section 6.6.2.2 for information on maintenance of
baffle oil/water separators.
1. The sizing procedures for the baffle oil/water separator should be run as a check to ensure the vault is
large enough. If the oil/water separator sizing procedures result in a larger vault size, increase the
wetvault size to match.
2. An oil retaining baffle shall be provided in the second cell near the vault outlet. The baffle should not
contain a high-flow overflow, or else the retained oil will be washed out of the vault during large
storms.
3. The vault shall have a minimum length-to-width ratio of 5:1.
4. The vault shall have a design water depth-to-width ratio of between 1:3 to 1:2.
5. The vault shall be watertight and shall be coated to protect from corrosion.
6. Separator vaults shall have a shutoff mechanism on the outlet pipe to prevent oil discharges during
maintenance and to provide emergency shut-off capability in case of a spill. A valve box and riser
shall also be provided.
7. Wetvaults used as oil/water separators must be off-line and must bypass flows greater than the off-line
WQ design flow described in Section 6.2.1
Intent: This design minimizes the entrainment and/or emulsification of previously captured oil during
very high flow events.
AGENDA ITEM # 8. a)
6.4.2 WETVAULTS
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FIGURE 6.4.2.A SCHEMATIC REPRESENTATION OF A WETVAULT
SECTION B-B
NTS
SECTION A-A
NTS
PLAN VIEW
NTS
ACCESS COVER OR DOORS REQUIRED.
SEE KCRDCS FOR SPECIFICATIONS FOR
MANHOLES AND LADDERS.
SIZE TO MEET CONVEYANCE
REQUIREMENTS (SEE CH.1)
NOTE: CAPACITY OF OUTLET
PIPE DESIGNED TO PEAK FLOW
FOR CONVEYANCE
WQ DESIGN WS
DETENTION OPTIONAL
1' FOR WQ VAULTS
2' FOR COMBINED W.Q.
AND DETENTION VAULTS
INLET
FIRST CELL SIZED FOR
25% TO 35% OF
WETPOOL VOLUME
WQ DESIGN WS
1'
7' MIN.4' MIN.
AVERAGE 1' SEDIMENT
STORAGE (FIRST CELL)
ACCESS DOORS OR
REMOVAL PANEL
5% (MIN.) SLOPE
TROWEL FINISH
GRAVITY DRAIN (IF GRADE ALLOWS)
PLACE AS LOW AS GRADE ALLOWS BUT
MUST BE 6" MIN. ABOVE THE BASE
ELEVATION OF VAULT WALLS OR ABOVE
SEDIMENT STORAGE AREA
BOTTOM SLOPE 0.5-2%
TOWARDS OUTLET
END OF SECOND CELL
(RECOMMENDED)
BOTTOM SLOPE 0.5%-2%
TOWARD INLET END
AVERAGE SEDIMENT
STORAGE 1' MIN. (FIRST
CELL)
3' MIN.
FRAME, GRATE AND
LOCKING COVER MARKED
"DRAIN" (TYP.)
WETPOOL
WIDTH
WETPOOL LENGTH
VENTILATION PIPE
(12" MIN.)
LADDER
FOR VAULTS > 1250 S.F. PROVIDE A
5' X 10' ACCESS DOOR OR REMOVABLE
PANEL OVER LOWEST PORTION OF VAULT
"V" SHAPED
BOTTOM
OUTLET
OPEN PIPE FOR WETVAULT ONLY.
SEE DETENTION VAULT FOR
COMBINED WATER QUALITY/
DETENTION VAULT OUTLET
5' X 10' GRATE OVER SECOND CELL
(MAY BE PROVIDED BY A GRATED
5' X 10' ACCESS DOOR OR PANEL)
LADDER
WETPOOL DEPTH8' MAX.2'MIN.1'
REMOVABLE
BAFFLE
REMOVABLE
BAFFLE
ACCESS DOORS OR REMOVABLE PANEL
(AS REQUIRED)
1'
AGENDA ITEM # 8. a)
SECTION 6.4 WETPOOL FACILITY DESIGNS
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6.4.3 STORMWATER WETLANDS
In land development situations, wetlands are usually constructed for two main reasons: to replace or
mitigate impacts when natural wetlands are filled or impacted by development (mitigation wetlands), and
to treat stormwater runoff (stormwater treatment wetlands). Stormwater wetlands are shallow man-made
ponds that are designed to treat stormwater through the biological processes associated with emergent
aquatic plants (see the schematic representations in Figure 6.4.3.A and Figure 6.4.3.B).
In the City, wetlands created to mitigate disturbance impacts, such as filling, shall not also be used as
stormwater treatment facilities. This is because of the different, incompatible functions of the two kinds of
wetlands. Mitigation wetlands are intended to function as full replacement habitat for fish and wildlife,
providing the same functions and harboring the same species diversity and biotic richness as the wetlands
they replace. Stormwater treatment wetlands are used to capture and transform pollutants, just as
wetponds are, and over time the sediment will concentrate pollutants. This is not a healthy environment for
aquatic life. Stormwater treatment wetlands are used to capture pollutants in a managed environment so
that they will not reach natural wetlands and other ecologically important habitats. In addition,
vegetation must be harvested and sediment dredged in stormwater treatment wetlands, further interfering
with use for wildlife habitat.
In general, stormwater wetlands perform well to remove sediment, metals, and pollutants which bind to
humic or organic acids. Phosphorus removal in stormwater wetlands is highly variable.35
Applications and Limitations
This stormwater wetland design occupies about the same surface area as wetponds, but has the potential to
be better integrated aesthetically into a site because of the abundance of emergent aquatic vegetation. The
most critical factor for a successful design is the provision of an adequate supply of water for most of the
year to replace any lost by infiltration or evapotranspiration. Careful planning is needed to be sure
sufficient water will be retained to sustain good wetland plant growth. Since water depths are shallower
than in wetponds, water loss by evapotranspiration is an important concern, especially during the relatively
warm dry season. Stormwater wetlands may be a good WQ facility choice in areas with high winter
groundwater levels, if there is also some pond intrusion of summer base flow.
Consult the water quality menus in Section 6.1 for information on how this facility may be used to meet
Core Requirement #8.
6.4.3.1 METHODS OF ANALYSIS
When used for stormwater treatment, stormwater wetlands employ some of the same design features as
wetponds. However, in addition to gravity settling, some degree of pollutant removal is mediated by
aquatic vegetation and the microbiological community associated with that vegetation. When designing
wetlands, water volume and factors which affect plant vigor and biomass are all concerns.
Stormwater wetlands designed and constructed using the criteria below are expected to meet both the
Basic and Enhanced Basic water quality treatment goals.
Steps 1 through 5: Determine the volume of a basic wetpond. Follow Steps 1 through 5 for wetponds
(see Section 6.4.1.1). The volume of a basic wetpond is used as a template for sizing the stormwater
wetland.
Step 6: Calculate the surface area of the stormwater wetland. The surface area of the wetland shall be
the same as the top area of a wetpond sized for the same site conditions. Calculate the surface area of the
stormwater wetland by using the volume from Step 5 and dividing by the average water depth (use 3 feet).
Step 7: Determine the surface area of the first cell of the stormwater wetland. Use the volume
determined from Criterion 2 under “Wetland Geometry,” and the actual depth of cell 1.
35 Richardson, C. 1987. "Mechanisms controlling phosphorus retention capacity in freshwater wetlands," Science, 228: 1424.
AGENDA ITEM # 8. a)
6.4.3 STORMWATER WETLANDS
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Step 8: Determine the surface area of the wetland cell. Subtract the surface area of the first cell
(Step 7) from the total surface area (Step 6).
Step 9: Determine water depth distribution in the second cell. Decide if the top of the dividing berm
will be at the surface or submerged (designer’s choice). Adjust the distribution of water depths in the
second cell according to Criterion 8 under “Wetland Geometry” below. Note: This will result in a facility
that holds less volume than that determined in Step 5 above. This is acceptable.
Intent: The surface area of the stormwater wetland is set to be roughly equivalent to that of a wetpond
designed for the same project site so as not to discourage use of this option.
Step 10: Choose plants. See Table 6.4.1.A for a list of plants recommended for wetpond water depth
zones, or consult a wetland scientist.
6.4.3.2 DESIGN CRITERIA
Typical details for a stormwater wetland are shown in Figure 6.4.3.A and Figure 6.4.3.B.
Wetland Geometry
1. Stormwater wetlands shall consist of two cells, a presettling cell and a wetland cell.
2. The presettling cell shall contain a volume equal to approximately one-third of the wetpool volume
calculated in Steps 1 through 5 of “Methods of Analysis,” Section 6.4.3.1.
3. The depth of the presettling cell shall be between 4 feet (minimum) and 8 feet (maximum).
4. One foot of sediment storage shall be provided in the presettling cell.
5. The wetland cell shall have an average water depth of about 1.5 feet (plus or minus 3 inches).
6. The “berm” separating the two cells shall be shaped such that its downstream side gradually slopes to
form the second shallow wetland cell (see the section view in Figure 6.4.3.A). Alternatively, the
second cell may be graded naturalistically from the top of the dividing berm (see Criterion 8 below).
7. The top of berm shall be either at the WQ design water surface or submerged 1 foot below the WQ
design water surface, as with wetponds. Correspondingly, the side slopes of the berm must meet the
following criteria:
a) If the top of berm is at the WQ design water surface, the berm side slopes shall be no steeper than
3H:1V.
b) If the top of berm is submerged 1 foot, the upstream side slope may be up to 2H:1V.36
8. Two options (A and B) are provided for grading the bottom of the wetland cell. Option A is a
shallow, evenly graded slope from the upstream to the downstream edge of the wetland cell (see
Figure 6.4.3.A).
Option B is a “naturalistic” alternative, with the specified range of depths intermixed throughout the
second cell (see Figure 6.4.3.B). A distribution of depths shall be provided in the wetland cell
depending on whether the dividing berm is at the water surface or submerged (see Table 6.4.3.A).
The maximum depth is 2.5 feet in either configuration.
36 If the berm is at the water surface, then for safety reasons, its slope must be no greater than 3:1, just as the pond banks must
be 3:1 if the pond is not fenced. A steeper slope (2:1 rather than 3:1) is allowed if the berm is submerged in 1 foot of water. If
submerged, the berm it is not considered accessible, and the steeper slope is allowed.
AGENDA ITEM # 8. a)
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TABLE 6.4.3.A DISTRIBUTION OF DEPTHS IN WETLAND CELL (OPTION B)
DIVIDING BERM AT
WQ DESIGN WATER SURFACE DIVIDING BERM SUBMERGED 1 FOOT
Depth Range (feet)
Percent of Cell 2
Surface Area Depth Range (feet)
Percent of Cell 2
Surface Area
0.1 to 1 25 1 to 1.5 40
1 to 2 55 1.5 to 2 40
2 to 2.5 20 2 to 2.5 20
Lining Requirements
1. In infiltrative soils, both cells of the stormwater wetland shall be lined. To determine whether a low-
permeability liner or a treatment liner is required, determine whether the following conditions will be
met. If low soil permeability will ensure sufficient water retention, lining may be waived.
The first cell of a treatment wetland must retain a permanent pool of water throughout the wet
season. It is considered non-compliant if the pond level drops more than 12″ in any 7-day
measurement period. A low permeability liner, per Section 6.2.4 will be required to achieve this
standard in infiltrative soils.
The second cell must retain water for at least 10 months of the year.
The complete historical precipitation record should be used in the approved model when
establishing these conditions.
Intent: Many wetland plants can adapt to periods of summer drought, so a limited drought period is
allowed in the second cell. This may allow a treatment liner rather than a low permeability liner to be
used for the second cell. The first cell must retain a permanent pool of water throughout the wet
season in order for the presettling function to be effective.
2. If a low permeability liner is used, a minimum of 18 inches of native soil amended with good topsoil
or compost (one part compost mixed with 3 parts native soil) must be placed over the liner. Compost
must be Specification 1 Compost detailed in Reference Section 11-C. For geomembrane or
geotextile liner, a soil depth of 3 feet covering the liner is required to prevent damage to the liner
during planting. Hydric soils are not required.
3. The criteria for liners given in Section 6.2.4 must be observed.
Inlet and Outlet
Same as for basic wetponds (see Section 6.4.1.2) but with the added requirement that spill control be
provided as detailed in Section 4.2.1.1 prior to discharge of runoff from non-roof-top pollution generating
impervious surface into the stormwater wetland.
Access and Setbacks
1. Location of the stormwater wetland relative to site constraints (e.g., buildings, property lines, etc.)
shall be the same as for detention ponds (see Section 5.1.1). See Section 6.2.3 for typical setback
requirements for WQ facilities.
2. Access and maintenance roads shall be provided and designed according to the requirements for
detention ponds (see Section 5.1.1). Access and maintenance roads shall extend to both the wetland
inlet and outlet structures. An access ramp shall be provided to the bottom of the first cell unless all
portions of the cell can be reached and sediment loaded from the top of the wetland side slopes. Also
see “Access Requirements” in Section 5.1.1, for more information on access alternatives.
3. If the dividing berm is also used for access, it must be built to sustain loads of up to 80,000 pounds.
AGENDA ITEM # 8. a)
6.4.3 STORMWATER WETLANDS
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Signage
General signage shall be provided according to the requirements for detention ponds (see Section 5.1.1).
Planting Requirements
1. The wetland cell shall be planted with emergent wetland plants following the recommendations given
in Table 6.4.1.A or the recommendations of a wetland specialist.
Note: Cattails (Typha latifolia) are not allowed. They tend to escape to natural wetlands and crowd
out other species. In addition, the shoots die back each fall and will result in oxygen depletion in the
wetpool unless they are removed.
2. If the stormwater wetland is in a sensitive lake or sphagnum bog protection area, shrubs that form a
dense cover shall be planted on slopes above the WQ design water surface on at least three sides of the
presettling cell. For banks that are berms, no planting is allowed if the berm is regulated by dam safety
requirements (see Section 5.1.1). The purpose of planting is to discourage waterfowl use of the pond
and to provide shading.37 Some suitable trees and shrubs include vine maple (Acer circinatum), wild
cherry (Prunus emarginata), red osier dogwood (Cornus stolonifera), California myrtle (Myrica
californica), Indian plum (Oemleria cerasiformis), and Pacific yew (Taxus brevifolia) as well as
numerous ornamental species.
Construction and Maintenance Considerations
Construction and maintenance considerations are the same as for basic wetponds. Construction of the
naturalistic alternative (Option B) can be easily done by first excavating the entire area to the 1.5-foot
average depth. Then soil subsequently excavated to form deeper areas can be deposited to raise other areas
until the distribution of depths indicated in the design is achieved.
37 Waterfowl are believed to limit use of areas where their view of predator approach paths is blocked. Some suitable native
shrubs include vine maple, Indian plum, bitter cherry, red osier dogwood, cascara, and red elderberry. Ornamental hedge
plants such as English laurel, privet and barberry are also good choices.
AGENDA ITEM # 8. a)
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FIGURE 6.4.3.A SCHEMATIC REPRESENTATION OF A STORMWATER WETLAND — OPTION A
SECTION VIEW Option A
NTS
PLAN VIEW Option A
NTS
PLANT WITH
WETLAND
PLANTS
(SEE TEXT)ACCESS ROADACCESS ROAD
SPILLWAY
WETLAND CELL
FIRST CELL
(FOREBAY)
INFLOW
PROVIDE SPILL
CONTROL AT
INLET PER
SECTION 4.2.1.1
NOTE:
SEE DETENTION FACILITY REQUIREMENTS FOR
LOCATION AND SETBACK REQUIREMENTS.
WQ DESIGN WS1'
2 min.
1
OUTLET STRUCTURE
(SEE DETAIL
FIGURE 6.4.1.B)
INLET AND OUTLET SUBMERGED
1' OVER PIPE CROWN
FIRST CELL DEPTH
4' MIN. TO 8' MAX.2.5' MAX.
IF REQUIRED, PLACE LINER IN
SECOND CELL TO HOLD WATER
SLOPE MAY BE 2:1 WHEN
TOP SUBMERGED
1 FT BELOW DESIGN WS
INLET EROSION CONTROL /
SLOPE PROTECTION PER
DETENTION FACILITY
REQUIREMENTS
SEDIMENT STORAGE
DEPTH = 1' MIN.
2' MIN.
SUBMERGED OUTLET
AGENDA ITEM # 8. a)
6.4.3 STORMWATER WETLANDS
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FIGURE 6.4.3.B SCHEMATIC REPRESENTATION OF A STORMWATER WETLAND — OPTION B
PLAN VIEW Option B
NTS
Pond Depth
2.5 ft
2.0 ft
1.0 ft
0.5 ft
BERM OR BAFFLE
AT DESIGN WS
OR SUBMERGED 1'
BELOW DESIGN WS
EXTEND BERM
ACROSS ENTIRE
WIDTH
INLET PIPE &
CATCH BASIN
PER DETENTION
FACILITY
REQUIREMENTS
OUTLET EROSION CONTROL
ENERGY DISSIPATION PER
DETENTION FACILITY
REQUIREMENTS
CATCH BASIN & OUTLET PIPE,
DESIGNED TO PEAK FLOW
FOR CONVEYANCE
EMERGENCY SPILLWAY
PER DETENTION FACILITY
REQUIREMENTS
BERM TOP WIDTH
5' MIN. (IF EARTHEN)
PLANTINGS REQUIRED ON
FIRST CELL SLOPES FOR
LAKE AND BOG PROTECTION
FACILITIES
EMERGENCY
OVERFLOW WS
OVERFLOW WS
DESIGN WS
FIRST WETPOOL CELL
VOLUME (EXCLUDING
ACCESS RAMP) = APPROX.
1
3 OF TOTAL WQ VOLUME
ACCESS RAMP TO
BOTTOM OF FIRST
CELL (7H:1V)
ACCESS ROAD TO
INLET STRUCTURE
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6.4.4 COMBINED DETENTION AND WETPOOL FACILITIES
Combined detention and WQ wetpool facilities have the appearance of a detention facility but contain a
permanent pool of water as well. The following design procedures, requirements, and recommendations
cover differences in the design of the stand-alone WQ facility when combined with detention storage. The
following combined facilities are addressed:
Detention/wetpond (basic and large)
Detention/wetvault
Detention/stormwater wetland.
There are two sizes of the combined wetpond, a basic and a large, but only a basic size for the combined
wetvault and combined stormwater wetland. The facility sizes (basic and large) are related to the pollutant
removal goals stated in the WQ menus. See Section 6.1 for more information on the WQ menus and
treatment goals.
Applications and Limitations
Combined detention and water quality facilities are very efficient for sites that also have detention
requirements. The water quality facility may often be placed beneath the detention facility without
increasing the facility surface area. However, the fluctuating water surface of the live storage will create
unique challenges for plant growth and for aesthetics alike.
The basis for pollutant removal in combined facilities is the same as in the stand-alone WQ facilities.
However, in the combined facility, the detention function creates fluctuating water levels and added
turbulence. For simplicity, the positive effect of the extra live storage volume and the negative effect of
increased turbulence are assumed to balance, and are thus ignored when sizing the wetpool volume.38 For
the combined detention/stormwater wetland, criteria that limit the extent of water level fluctuation are
specified to better ensure survival of the wetland plants.
Unlike the wetpool volume, the live storage component of the facility should be provided above the
seasonal high water table.
Consult the water quality menus in Section 6.1 for information on how these combined facilities may be
used to meet Core Requirement #8.
6.4.4.1 METHODS OF ANALYSIS
COMBINED DETENTION AND WETPOND (BASIC AND LARGE)
The methods of analysis for combined detention and wetponds are identical to those outlined for wetponds
and for detention facilities. Follow the procedure specified in Section 6.4.1.1 to determine the wetpool
volume for a combined facility. Follow the standard procedure specified in Chapter 5 to size the detention
portion of the pond.
COMBINED DETENTION AND WETVAULT
The methods of analysis for combined detention and wetvaults are identical to those outlined for wetvaults
and for detention facilities. Follow the procedure specified in Section 6.4.2 to determine the wetvault
volume for a combined facility. Follow the standard procedure specified in Chapter 5 to size the detention
portion of the vault.
COMBINED DETENTION AND STORMWATER WETLAND
The methods of analysis for combined detention and stormwater wetlands are identical to those outlined
for stormwater wetlands and for detention facilities. Follow the procedure specified in Section 6.4.3.1 to
38 Many of the ponds studied in the Nationwide Urban Runoff Program were combined ponds.
AGENDA ITEM # 8. a)
6.4.4 COMBINED DETENTION AND WETPOOL FACILITIES
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determine the stormwater wetland size. Follow the standard procedure specified in Chapter 5 to size the
detention portion of the wetland.
6.4.4.2 DESIGN CRITERIA
COMBINED DETENTION AND WETPOND (BASIC AND LARGE)
Schematic representations of a combined detention and wetpond are shown in Figure 6.4.4.A and Figure
6.4.4.B. The detention portion of the facility shall meet the design criteria set forth in Chapter 5 and
sizing procedures in Chapter 3.
Detention and Wetpool Geometry
1. The wetpool and sediment storage volumes shall not be included in the required detention volume.
2. The “Wetpool Geometry” criteria for wetponds (see Section 6.4.1.2) shall apply with the following
modification:
Criterion 4: The minimum sediment storage depth in the first cell is 1 foot. The 6 inches of
sediment storage required for detention ponds does not need to be added to this, but 6 inches of
sediment storage must be added to the second cell to comply with the detention sediment storage
requirement.
Berms, Baffles, and Slopes
Same as for wetponds (see Section 6.4.1.2).
Inlet and Outlet
The “Inlet and Outlet” criteria for wetponds (see Section 6.4.1.2) shall apply with the following
modifications:
1. Criterion 2: A sump must be provided in the outlet structure of combined ponds.
2. The detention flow restrictor and its outlet pipe shall be designed according to the requirements for
detention ponds (see Section 5.1.4.2).
Access and Setbacks
Same as for wetponds (see Section 6.4.1.2).
Signage
Signage shall be provided according to the requirements for detention ponds (see Section 5.1.1).
Planting Requirements
Same as for wetponds (see Section 6.4.1.2).
COMBINED DETENTION AND WETVAULT
The design criteria for detention vaults and wetvaults must both be met, except for the following
modifications or clarifications:
1. The minimum sediment storage depth in the first cell shall average 1 foot. The 6 inches of sediment
storage required for detention vaults does not need to be added to this, but 6 inches of sediment
storage must be added to the second cell to comply with detention vault sediment storage
requirements.
2. The oil retaining baffle shall extend a minimum of 2 feet below the WQ design water surface.
Intent: The greater depth of the baffle in relation to the WQ design water surface compensates for the
greater water level fluctuations experienced in the combined vault. The greater depth is deemed
prudent to better ensure that separated oils remain within the vault, even during storm events.
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Note: If a vault is used for detention as well as water quality control, the facility shall not be modified
to function as a baffle oil/water separator as allowed for wetvaults in Section 6.4.2.2. This is because
the added pool fluctuation in the combined vault does not allow for the quiescent conditions needed
for oil separation.
COMBINED DETENTION AND STORMWATER WETLAND
The design criteria for detention ponds and stormwater wetlands must both be met, except for the
following modifications or clarifications:
1. The “Wetland Geometry” criteria for stormwater wetlands (see Section 6.4.3.2) are modified as
follows:
Criterion 4: The minimum sediment storage depth in the first cell is 1 foot. The 6 inches of
sediment storage required for detention ponds does not need to be added to this, nor does the 6 inches
of sediment storage in the second cell of detention ponds need to be added.
Intent: Since emergent plants are limited to shallower water depths, the deeper water created before
sediments accumulate is considered detrimental to robust emergent growth. Therefore, sediment
storage is confined to the first cell which functions as a presettling cell.
2. The “Inlet and Outlet” criteria for wetponds (see Section 6.4.1.2) shall apply with the following
modifications:
a) Criterion 2: A sump must be provided in the outlet structure of combined facilities.
b) The detention flow restrictor and its outlet pipe shall be designed according to the requirements
for detention ponds (see Section 5.1.4.2).
3. The “Planting Requirements” for stormwater wetlands (see Section 6.4.3.2) are modified to use the
following plants which are better adapted to water level fluctuations:
Scirpus acutus (hardstem bulrush) 2′ to 6′ depth
Scirpus microcarpus (small-fruited bulrush) 1′ to 2.5′ depth
Sparganium emersum (burreed) 1′ to 2′ depth
Sparganium eurycarpum (burreed) 1′ to 2′ depth
Veronica sp. (marsh speedwell) 0′ to 1′ depth
In addition, the shrub Spirea douglasii (Douglas spirea) may be used in combined facilities.
Water Level Fluctuation Restrictions: The difference between the WQ design water surface and the
maximum water surface associated with the 2-year runoff shall not be greater than 3 feet. If this restriction
cannot be met, the size of the stormwater wetland must be increased. The additional area may be placed
in the first cell, second cell, or both. If placed in the second cell, the additional area need not be planted
with wetland vegetation or counted in calculating the average depth.
Intent: This criterion is designed to dampen the most extreme water level fluctuations expected in
combined facilities to better ensure that fluctuation-tolerant wetland plants will be able to survive in the
facility. It is not intended to protect native wetland plant communities and is not to be applied to natural
wetlands.
AGENDA ITEM # 8. a)
6.4.4 COMBINED DETENTION AND WETPOOL FACILITIES
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FIGURE 6.4.4.A SCHEMATIC REPRESENTATION OF A COMBINED DETENTION AND WETPOND
PLAN VIEW
ACCESS ROAD TO INLET STRUCTURE
PLAN VIEW
NTS
ACCESS ROAD
TO OUTLET
STRUCTURE
PLANTINGS
REQUIRED ON
SLOPES ABOVE
WQ DESIGN WS
FOR LAKE OR BOG
PROTECTION
FACILITIES
OUTLET EROSION CONTROL
& ENERGY DISSIPATION
PER DETENTION FACILITY
REQUIREMENTSCONTROL STRUCTURE & OUTLET
PIPE SIZED PER DETENTION
FACILITY REQUIREMENTS
SECOND WETPOOL CELL
BERM OR BAFFLE AT WQ
DESIGN WS OR SUBMERGED
1' BELOW WQ DESIGN WS
EXTEND BERM ACROSS
ENTIRE
WETPOOL.
FIRST WETPOOL CELL
APPROX. 1
3 OF TOTAL
WETPOOL VOLUME,
EXCLUDING ACCESS RAMP
INLET PIPE & CATCH BASIN
PER WETPOND FACILITY
REQUIREMENTS
EMERGENCY SPILLWAY
PER DETENTION FACILITY
REQUIREMENTS
WETPOOL
WIDTH
ACCESS RAMP TO
BOTTOM OF FIRST
WETPOOL CELL
(7H:1V) (SEE TEXT)
BERM TOP WIDTH 5'
MIN. (IF EARTHEN)
WQ DESIGN WS
DETENTION WS
OVERFLOW WS
EMERGENCY
OVERFLOW WS
AGENDA ITEM # 8. a)
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FIGURE 6.4.4.B SCHEMATIC REPRESENTATION OF A
COMBINED DETENTION AND WETPOND PROFILE VIEW
SECTION A-A
NTS
SECTION B-B
NTSNOTE:
SEE DETENTION FACILITY REQUIREMENTS
FOR LOCATION, INTERIOR & EXTERIOR SIDE
SLOPES, AND SETBACK REQUIREMENTS.
3 1
EMERGENCY OVERFLOW WS
DETENTION OVERFLOW WS
DETENTION DESIGN WS
WQ DESIGN WS
WQ DESIGN WS
WETPOOL LENGTH (INLET TO OUTLET) = 3 (MIN.) x WIDTH
EMERGENCY OVERFLOW WS
DETENTION OVERFLOW WSDETENTION DESIGN WS
DETENTION PER
DETENTION FACILITY
REQUIREMENTS
EMERGENT VEGETATION
REQUIRED FOR WETPOOL
DEPTHS 3' OR LESS.NOTE: BERM SLOPE
MAY BE 2:1 WHEN TOP
OF BERM SUBMERGED
1' BELOW WQ DESIGN WS
WETPOOL DEPTH 8' MAX.
RECIRCULATION RECOMMENDED
FOR DEPTH > 6'.
KEYED
BERM
FIRST CELL DEPTH
4' MIN. TO 8' MAX.
TOP OF BERM OR BAFFLE LEVEL
AND AT WETPOOL DESIGN
ELEVATION (FLOW EXITS FIRST
CELL OVER BERM) OR AS NOTED
ACCESS ROAD
PER DETENTION
FACILITY
REQUIREMENTS
SEDIMENT STORAGE
DEPTH = 1' MIN
INLET EROSION CONTROL/
SLOPE PROTECTION PER
DETENTION FACILITY
REQUIREMENTS
SUBMERGED INLET (WQ
DESIGN WS 1' ABOVE CROWN)
SLOPE VEGETATION
PER DETENTION
FACILITY
REQUIREMENTS
2' MIN.
1'
MIN.
OUTLET PIPE INVERT OUT AT
WETPOOL WS ELEVATION
1' MIN.
INVERT 6" MIN.
BELOW TOP
OF INTERNAL
BERM. LOWER
PLACEMENT
IS DESIRABLE
GRAVITY DRAIN
(IF GRADE ALLOWS)
8" MIN. DIAMETER
VALVE TYPE 2 CATCH
BASIN w/SUMP
FENCE REQUIRED FOR
INTERIOR SIDE SLOPES
STEEPER THAN 3(H):1 (V)ACCESS
ROAD
CAPACITY OF OUTLET
SYSTEM PER DETENTION
FACILITY REQUIREMENTS
EXTERIOR BERMS
DESIGNED PER DAM
SAFETY REQUIREMENTS
IF APPLICABLE
18" MIN.
AGENDA ITEM # 8. a)
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6.5 FILTRATION FACILITY DESIGNS
This section presents the methods, criteria, and details for analysis and design of sand filters and generic
information for proprietary cartridge filters. The following specific facility designs are included in this
section:
“Sand Filters — Basic and Large,” Section 6.5.2
“Sand Filter Vaults,” Section 6.5.3
“Linear Sand Filters,” Section 6.5.4
The information presented for each filtration facility is organized into the following categories:
1. Methods of Analysis: Contains a step-by-step procedure for designing and sizing each facility.
2. Design Criteria: Contains the details, specifications, and material requirements for each facility.
6.5.1 GENERAL REQUIREMENTS FOR FILTRATION FACILITIES
Presettling Requirement
Filtration facilities are particularly susceptible to clogging. Presettling must therefore be provided before
stormwater enters a filtration facility to prolong the periods between required maintenance activities. The
presettling treatment goal is to remove 50 percent of the total suspended solids (TSS). This requirement
may be met by any of the following:
1. A water quality facility from the Basic WQ Menu (Section 6.1.1), except for Basic WQ options 7
(sand filter) and 8 (proprietary media/membrane filter), which shall not be used to meet the presettling
requirement.
2. A presettling pond or vault, which may be integrated as the first cell of the filtration facility, with a
treatment volume equal to 0.25 times the basic water quality treatment volume (see Section 6.2.1)
calculated by the approved model or by using the NRCS curve number method (see Section 6.4.1.1).
See design requirements below.
Note: For the linear sand filter, use the sediment cell sizing given in the design instead of the above sizing.
3. A detention facility sized to meet the Flow Control Duration Standard.
4. An alternative City approved pretreatment technology (see New Facility Designs in Section 6.2,
Section 6.7, “Proprietary Facility Designs,” and Reference Section 14-A and 14-B).
Other Pretreatment Requirements, Limitations, and Notes
1. Sand filters not preceded by a facility that captures floatables, such as a spill control tee, must provide
additional pretreatment to remove floatable trash and debris before flows reach the sand bed. This
requirement may be met by providing a catch basin with a capped riser on the inlet to the sand filter
(see Figure 6.5.2.C).
2. For high-use sites, sand filters must be preceded by an oil control option from the High-Use menu,
Section 6.1.5.
3. The presettling requirement (with 50% TSS treatment goal) for filtration facilities is in addition to and
exclusive of the treatment requirement for the filtration facility (80% TSS treatment goal).
Design Criteria for Presettling Cells
1. If water in the presettling cell or upstream WQ facility will be in direct contact with the soil, the cell
or WQ facility must be lined per the liner requirements in Section 6.2.4.
Intent: to prevent groundwater contamination from untreated stormwater runoff in areas of
excessively drained soils.
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2. The presettling cell shall conform to the following:
a) The length-to-width ratio shall be 2:1, at minimum. A 3:1 ratio is recommended. Berms or
baffles may be used to lengthen the flowpath.
b) The minimum depth shall be 3 feet; the maximum depth shall be 6 feet.
c) One foot of sediment storage shall be provided.
3. Inlets and outlets shall be designed to minimize velocity and reduce turbulence. The top of the inlet
pipe shall be submerged at least 1 foot. The bottom of the inlet pipe shall be at least 1 foot above
sediment storage.
4. If the presettling cell is in a sensitive lake or sphagnum bog protection area, shrubs that form a dense
cover shall be planted on slopes above the WQ design water surface on at least three sides (see the
wetpond planting requirements in Section 6.4.1.2).
5. See Section 6.5.3.2 for details of presettling vault structures.
6.5.2 SAND FILTERS — BASIC AND LARGE
A sand filter operates much like an infiltration pond (see schematic representations in Figure 6.5.2.A and
Figure 6.5.2.B). However, instead of infiltrating into native soils, stormwater filters through a constructed
sand bed with an underdrain system. Runoff enters the pond and spreads over the surface of the filter. As
flows increase, water backs up in the pond where it is held until it can percolate through the sand. The
treatment pathway is vertical (downward through the sand). High flows in excess of the WQ treatment
goal simply spill out over the top of the pond. Water that percolates through the sand is collected in an
underdrain system of drain rock and pipes which directs the treated runoff to the downstream drainage
system.
A sand filter removes pollutants primarily by physical filtration. As stormwater passes through the sand,
pollutants are trapped in the small spaces between sand grains or adhere to the sand surface. Over time, silt
will build up on the surface and soil organisms (bacteria, fungi, protozoa, nematodes, etc.) will populate
the slit layer and sand bed. The silt will enhance pollutant filtration while the organisms may be
responsible for some biological treatment and some filtration by formation of a biofilm. Over time, either
may decrease the sand filter infiltration rate sufficient to require removal and replacement of some to all of
the media. A large sand filter will treat more of the annual flow than will a basic sand filter and will
therefore remove more pollutant load on an annual basis. Increasing the sand thickness will not
appreciably improve performance.
The following design procedures, requirements, and recommendations cover two sand filter sizes: a basic
size and a large size. The basic sand filter is designed to meet the Basic WQ menu goal of 80% TSS
removal. The large sand filter is expected to meet the Enhanced Basic WQ menu goal of > 30% reduction
of dissolved copper and > 60% removal of dissolved zinc, and the Sensitive Lake Protection menu goal of
50% total phosphorus removal.
Applications and Limitations
A sand filter may be used in most residential, commercial, and industrial developments where site
topography and drainage provide adequate hydraulic head to operate the filter. An elevation difference of
about 4 feet between the inlet and outlet of the filter is usually needed to install a sand filter.
Landscaping may be somewhat constrained because the vegetation capable of surviving in sand and not
interfering with sand filter operation, maintenance, or longevity is limited. Trees and shrubs which
generate a large leaf fall shall be avoided in the immediate vicinity of the filter because leaves and other
debris can clog the surface of the filter.
Sand filters are designed to prevent water from backing up into the sand layer (the underdrain system must
drain freely). Therefore, a sand filter is more difficult to install, and may not be suitable, in areas with
high water tables where groundwater could potentially flood the underdrain system. Water standing in the
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underdrain system will also keep the sand saturated. Under these conditions, oxygen can be depleted,
releasing pollutants such as metals and phosphorus that are more mobile under anoxic conditions.
Sand filter discharge must be by gravity, and must not rely on a pump system. If the pump fails, the sand
will become saturated, create anoxic conditions, and release pollutants. Pumped inflow is only allowed for
privately maintained systems meeting the criteria in Section 4.2.3.
Because the surface of the sand filter will clog from sediment and other debris, this facility should not be
used in areas where heavy sediment loads are expected. A sand filter should not be used during
construction to control sediments unless the sand bed is replaced periodically during construction and after
the project site is stabilized.
Consult the water quality menus in Section 6.1 for information on how basic and large sand filters may be
used to meet Core Requirement #8.
6.5.2.1 METHODS OF ANALYSIS
This section presents the methods of analysis for both basic and large sand filters.
A sand filter is designed with two parts: (1) a temporary storage reservoir to store runoff, and (2) a sand
filter bed through which the stored runoff must percolate. Usually the storage reservoir is simply placed
directly above the filter, and the floor of the reservoir pond is the top of the sand bed. For this case, the
storage volume also determines the hydraulic head over the filter surface, which increases the rate of flow
through the sand.
The modeled routing method described below uses the approved continuous runoff computer model to
determine sand filter area and pond size based on individual site conditions. The method includes
parameters for sizing either a basic or a large sand filter.
Background
There are several variables used in sand filter design which are similar and often confused, even by well-
trained individuals. Use of these variables is explained below.
The sand filter design is based on Darcy’s law:
Q = KiA (6-19)
where Q = WQ design flow (cfs)
K = hydraulic conductivity (fps)
A = surface area perpendicular to the direction of flow (sf)
i = hydraulic gradient (ft/ft) for a constant head and constant media depth, computed as
follows:
i = (6-20)
where h = average depth of water above filter (ft), defined for this design as d/2
d = maximum storage depth above filter (ft)
l = thickness of sand media (ft)
Although it is not seen directly, Darcy’s law underlies the modeled routing design method. V is the direct
input in the sand filter design. The relationship between V and K is revealed by equating Darcy’s law and
the equation of continuity, Q = VA.
Note: When water is flowing into the ground, V is commonly called the filtration rate. It is ordinarily
measured in a percolation test.
l
lh
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Specifically:
Q = KiA and Q = VA
So,
VA = KiA or V = Ki (6-21)
Note that V K—that is, the filtration rate is not the same as the hydraulic conductivity, but they do have
the same units (distance per time). K can be equated to V by dividing V by the hydraulic gradient i, which
is defined above in Equation 6-20.
The hydraulic conductivity K does not change with head nor is it dependent on the thickness of the
media, only on the characteristics of the media and the fluid. The hydraulic conductivity of 1 inch per hour
(2.315 x 10-5 fps) used in this design is based on bench-scale tests of conditioned rather than clean sand.
This design hydraulic conductivity represents the average sand bed condition as silt is captured and held in
the filter bed.39
Unlike the hydraulic conductivity, the filtration rate V changes with head and media thickness, although
the media thickness is constant in the sand filter design.
Modeled Routing Method
The modeled routing method allows the designer to optimize filter geometry and sizing to meet specific
site conditions. The modeled method requires a trial and error solution using the approved model to route
the developed inflow runoff time series through various sand filter configurations until the amount of
runoff that passes through the filter media and is treated meets or exceeds the treatment objective defined
for the facility. Refer to the approved model’s computer software reference manual for specific
instructions on using the program. The general design process is described below.
Step 1: Determine whether a basic or large sand filter is required. Consult the water quality menus in
Section 6.1 to determine the size of filter needed. A basic sand filter is sized so that 91% of the runoff
volume will pass through the filter. A large sand filter is sized such that a minimum of 95% of the runoff
volume passes through the filter. See Section 6.2.1 for discussion of the WQ design volume.
Step 2: Prepare the inflow time series. The developed inflow time series is prepared using the approved
model as generally described in Chapter 3. Detailed instructions for preparing the time series can be found
in the approved model’s computer software reference manual. If the sand filter is upstream of detention,
the time series is that of the developed site. If the sand filter is downstream of detention, the time series is
the outflow time series leaving the detention facility.
Note: Sand filters located downstream from detention facilities are significantly smaller than those
treating runoff before it is detained. Likewise, sand filters receiving flows from Flow Control Duration
Standard detention facilities are smaller than those below Peak Rate Flow Control facilities.
Step 3: Determine whether the sand filter will be on-line or off-line. For most WQ facilities, the
designer may choose to design the facility as either on-line (all flow goes through the facility) or off-line
(flows above the WQ design flow bypass the facility). An off-line sand filter has a high-flow bypass with
an upstream flow splitter designed to bypass flows above the WQ design flow (see Section 6.2.5, for more
information on flow splitter design).
Note that the WQ design flow rate for the flow splitter is the rate required to pass the WQ volume (basic or
large). For the basic sand filter, the rate is reported directly by the approved model (i.e., not modified in
39 King County has tested various sand mixes conditioned with simulated stormwater to establish realistic design standards.
Tests were conducted under falling head conditions in columns containing 18 inches of sand underlain with a 2-inch layer of
washed drain gravel containing a section of 2-inch perforated PVC pipe to simulate the underdrain system. Details are given in
Koon, John, "Determination of infiltration rate and hydraulic conductivity for various sand filter media." January 1996.
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the manner for bioswales in Section 6.2.1); for the large sand filter, derive the rate from the ratio of the
basic and large water quality volumes. The basic sand filter uses the 91% runoff volume as the water
quality design volume, corresponding to a 2-year return interval peak flow from the approved continuous
model. The large sand filter design flow can be calculated by increasing the 2-year return interval peak
flow by the ratio of the 95% runoff volume (water quality design volume for the large sand filter) and the
91% runoff volume (water quality design volume for the basic sand filter). In equation form,
Design Flow
Rate for
Large Sand
Filter
=
(95% Runoff Volume)
x 2-year return
interval peak flow (6-22)
(91% Runoff Volume)
Step 4: Define sand filter modeling parameters. Sand filters can be sized in WWHM using the sand
filter element, or in MGS Flood using the infiltration pond element with the Sand Filter Data tab. Follow
the guidance in the approved model’s reference manual and apply the additional guidance below for the
parameters required for the analysis:
1. The surface area of the filter computed by the approved model using inputs of the bottom length and
width of the infiltration pond (ft).
2. Maximum water depth over filter: depth at which runoff begins to overflow the sand filter
3. Permeable surfaces: bottom only.
4. Riser and orifice information:
Riser head: same as the maximum water depth.
Number of orifices: zero. All runoff will either percolate through sand or overflow the riser.
Top of riser: flat.
5. Vertical infiltration: Assume a design filtration rate of 1 inch per hour. Though the sand specified
below will initially infiltrate at a much higher rate, that rate will slow as the filter accumulates
sediment. When the filtration rate falls to 1 inch per hour, removal of sediment is necessary to
maintain rates above the rate assumed for sizing purposes.
Step 5: Size the sand filter. Follow the facility sizing guidance in the approved model’s reference manual
to input the preliminary design configuration of the sand filter.
Step 6: Route the inflow time series through the sand filter and compare volumes. Compare the
volume percentage passing through the filter with the percentage required for the treatment volume (91%
or 95%). The approved model calculates the routed volume percentage for the comparison.
If the volume percentage of water passing through the filter exceeds the design treatment volume
percentage, decrease the bottom area of the facility. Repeat this step until the desired performance is
achieved.
If the volume percentage of water passing through the filter is less than the design treatment volume
percentage, increase the bottom area until the desired performance is achieved.
Step 7: Size the underdrain system. The underdrain system is sized to convey the peak filtered flows to
the outlet. For the basic sand filter, the central collector pipe(s) shall be sized to convey, at a minimum,
the 2-year return frequency flow into the facility using the KCBW program’s backwater analysis
techniques described in Chapter 4.
For large sand filter design, the design flows for the underdrain collector pipe(s) must be increased from
the basic sand filter, which uses the 91% runoff volume as the water quality design volume, corresponding
to a 2-year return interval peak flow from the approved continuous model. For the large sand filter, the
underdrain design flow can be calculated by increasing the 2 year return interval peak flow by the ratio of
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the 95% runoff volume (water quality design volume for the large sand filter) and the 91% runoff volume
(water quality design volume for the basic sand filter). In equation form:
Design Flow Rate
for Large Sand
Filter Underdrain
=
(95% Runoff Volume)
x 2-year return
interval peak flow (6-23)
(91% Runoff Volume)
To simplify the analysis, all flows for basic and large sand filters may be assumed to enter the collector
pipe at the upstream end. Typically, the collector pipe will not be inlet controlled, so a simple square inlet
type may be assumed. The full head of the facility may be utilized to convey flows through the pipe.
Feeder pipes may be sized using the design criteria in “Underdrain Systems” instead of analyzing the
conveyance capacity as described above.
Strip drains must be analyzed for conveyance per manufacturer’s specifications.
Intent: The underdrain must be able to remove standing water from beneath the sand. If standing water
remains, the sand will remain saturated. This could cause reducing conditions in the sand, allowing some
pollutants to become mobile and be released from the filter to downstream receiving waters.
6.5.2.2 DESIGN CRITERIA
Schematic representations of a sand filter are shown in Figure 6.5.2.A, Figure 6.5.2.B, Figure 6.5.2.C, and
Figure 6.5.2.D.
Sand Filter Geometry
1. Any shape sand bed may be used, including circular or free-form designs.
Note: The treatment process is governed by vertical flow, so short-circuiting is not a concern as it is
in wetponds.
2. Sand depth (l) shall be 18 inches (1.5 feet) minimum.
3. Depth of storage over the filter media (d) shall be 6 feet maximum.
Pretreatment, Flow Spreading, and Energy Dissipation
1. See general presettling and pretreatment requirements for filtration facilities in Section 6.5.1.
2. A flow spreader shall be installed at the inlet along one side of the filter to evenly distribute incoming
runoff across the filter and prevent erosion of the filter surface. See Section 6.2.6 for details on flow
spreaders.
a) If the sand filter is curved or an irregular shape, a flow spreader shall be provided for a
minimum of 20 percent of the filter perimeter.
b) If the length-to-width ratio of the filter is 2:1 or greater, a flow spreader must be located on the
longer side and for a minimum length of 20 percent of the facility perimeter.
c) In other situations, use good engineering judgment in positioning the spreader.
3. Erosion protection shall be provided along the first foot of the sand bed adjacent to the flow
spreader. Geotextile meeting the specifications in WSDOT Standard Specifications, 9-33.2(1)
Geotextile Properties/Table 1/Moderate Survivability/Woven, and Table 2, Class A, weighted with
sand bags at 15-foot intervals may be used. Quarry spalls may also be used.
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Overflow and Bypass Structures
1. On-line filters40 shall be equipped with overflows (primary, secondary, and emergency) in
accordance with the design criteria for detention ponds (see Section 5.1.1.1, criteria for “Overflow”
and “Emergency Overflow Spillway”).
Note: The primary overflow may be incorporated into the emergency spillway in cases where the
spillway discharges into a downstream detention facility, or where overflows can be safely controlled
and redirected into the downstream conveyance system.
2. For off-line filters, the outlet structure for the basic sand filter must be designed to pass the 2-yr peak
inflow rate, as determined using the approved model with 15-minute time steps calibrated to specific
site conditions.
For large sand filter design, the design flows for the overflow must be increased from the basic sand
filter, which uses the 91% runoff volume as the water quality design volume, corresponding to a 2-
year return interval peak flow from the approved continuous model. For the large sand filter, the
overflow design flow can be calculated by increasing the 2 year return interval peak flow by the ratio
of the 95% runoff volume (water quality design volume for the large sand filter) and the 91% runoff
volume (water quality design volume for the basic sand filter). In equation form:
Design Flow
Rate for Large
Sand Filter
Overflow
=
(95% Runoff Volume)
x 2-year return
interval peak flow (6-24)
(91% Runoff Volume)
Intent: Overflow capacity is required for low-flow, high-volume storms which may exceed the
storage capacity of the filter.
3. To the extent base flow conditions can be identified, base flow must be bypassed around the filter to
keep the sand from remaining saturated for extended periods of time.
Filter Composition
A sand filter consists of three or four layers:
Top layer (optional): grass seed or sod grown in sand
Second layer: sand
Third layer: geotextile fabric
Fourth layer: underdrain system.
Sand Specifications
The sand in a filter shall consist of a medium sand with few fines meeting the size gradation (by weight)
given in Table 6.5.2.A. The contractor must obtain a grain size analysis from the supplier to certify that the
No. 100 and No. 200 sieve requirements are met.
40 Whether a WQ facility is designed as on-line (all flow going through the facility) or off-line (high flows bypassing the facility) is a
choice made by the designer. Section 6.2.5 contains information on flow splitters for WQ facilities.
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Note: Many sand mixes supplied locally meet this specification. However, standard backfill for sand
drains (as specified in the Washington Standard Specifications 9-03.13) does not meet this specification
and shall not be used for sand filters.
TABLE 6.5.2.A SAND MEDIA SPECIFICATIONS
U.S. Sieve Size Percent Passing
U.S. No. 4 95 to 100 percent
U.S. No. 8 70 to 100 percent
U.S. No. 16 40 to 90 percent
U.S. No. 30 25 to 75 percent
U.S. No. 50 2 to 25 percent
U.S. No. 100 Less than 4 percent
U.S. No. 200 Less than 2 percent
Geotextile Materials
Geotextile material requirements are specified in WSDOT Standard Specifications, 9-33.2(1) Geotextile
Properties/Table 1/Moderate Survivability/Woven, and Table 2, Class A.
Underdrain Systems
1. Several underdrain systems are acceptable:
A central collector pipe with lateral feeder pipes in an 8-inch drain rock bed
A central collector pipe with a geotextile drain strip in an 8-inch drain rock bed
Longitudinal pipes in an 8-inch drain rock bed, with a collector pipe at the outlet end.
In smaller installations a single perforated pipe in 8 inches of drain rock may be adequate.
2. The maximum perpendicular distance between any two feeder pipes, or the edge of the filter and a
feeder pipe, shall be 15 feet.
Intent: This spacing is required to prevent the underdrain system from backing up into the sand filter
during the early life of the filter when high filtration rates exist.
3. All pipe shall be placed with a minimum slope of 0.5%.
4. The invert of the underdrain outlet shall be above the seasonal high groundwater level. The
seasonal high groundwater level is the highest elevation of groundwater observed.
Intent: The underdrain must be able to remove standing water from beneath the sand. If standing
water remains, the sand will remain saturated. This could cause depletion of dissolved oxygen and
reducing conditions in the sand, allowing some pollutants to become mobile and be released from the
filter to downstream receiving waters.
5. Cleanout wyes with caps or junction boxes shall be provided at both ends of all collector pipes.
Cleanouts shall extend to the surface of the filter.
a) A valve box must be provided for access to the cleanouts.
b) The cleanout assembly must be watertight to prevent short circuiting of the filter.
Intent: Caps are required on cleanout wyes to prevent short-circuiting of water into the underdrain
system when the pond fills with water.
6. If a drain strip is used for lateral drainage, the strip must be placed at the slope specified by the
manufacturer but at least at 0.5%. All drain strip must extend to the central collector pipe. Drain strips
installations must be analyzed for conveyance because manufactured products vary in the amount of
flow they are designed to handle.
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7. At least 8 inches of drain rock must be maintained over all underdrain piping or drain strip, and
6 inches must be maintained on either side to prevent damage by heavy equipment during
maintenance.
Note: If drain strip is used, it may be easier to install the central collector pipe in an 8-inch trench
filled with drain rock, making the cover over the drain strip and the collector pipe the same thickness.
In this case the pipe shall be wrapped with geotextile to prevent clogging. Use the same geotextile
specification as given in WSDOT Standard Specifications, 9-33.2(1) Geotextile
Properties/Table 1/Moderate Survivability/Woven, and Table 2, Class A.
8. A geotextile fabric shall be used between the sand layer and the drain rock and be placed so that one
inch of drain rock is above the fabric.
Intent: The position of the geotextile fabric provides a transition layer of mixed sand and drain rock.
A distinct layer of finely textured sand above a coarser one may cause water to pool at the interface
and not readily drain downward due to the greater capillary forces in the finer material.
9. Sand filters shall not be used in combination with a downstream pump system.
Intent: Sand filters are designed to prevent water from backing up into the sand layer; the underdrain
system must drain freely. If the pump fails, the sand will become saturated, create anoxic conditions,
and release pollutants.
Underdrain Materials
1. Underdrain pipe shall be minimum 6 inch diameter perforated PVC, SDR 35. One acceptable
specification for perforations is as follows: 2 rows of holes (1/2-inch diameter) spaced 6 inches apart
longitudinally (max), with rows 120 degrees apart (laid with holes downward). Other drain pipe may
be used if it adequately drains the filter.
2. Drain rock shall be 11/2- to 3/4-inch rock, washed and free from clay or organic material.
3. If a geotextile drain strip system is used, the attached geotextile fabric should not be used, or the
fabric side should be positioned away from the sand blanket. Geotextile is already required between
the sand and drain rock layers, and must meet the specifications in WSDOT Standard Specifications,
9-33.2(1) Geotextile Properties/Table 1/Moderate Survivability/Woven, and Table 2, Class A, to avoid
clogging the filter prematurely.
Access Roads and Setbacks
1. An access road shall be provided to the inlet and outlet of a sand filter for inspection and maintenance
purposes. Requirements for access roads are the same as for detention ponds (see Section 5.1.1.1,
“Design of Access Roads” and “Construction of Access Roads”).
2. The location of the facility relative to site constraints (e.g., buildings, property lines, etc.) shall be the
same as for detention ponds (see Section 5.1.1) except as noted in 3, below. See Section 6.2.3 for
typical setback requirements for WQ facilities.
3. For a sand filter that infiltrates to ground, setbacks shall be same as those for infiltration ponds, (see
Section 5.2.2).
Grass Cover
1. No top soil shall be added to sand filter beds because fine-grained materials (e.g., silt and clay) reduce
the hydraulic capacity of the filter.
2. Growing grass will require selecting species that can tolerate the demanding environment of the sand
bed. Sand filters experience long periods of saturation during the winter wet season, followed by
extended dry periods during the summer. Modeling predicts that sand filters will be dry about
60 percent of the time in a typical year. Consequently, vegetation must be capable of surviving
drought as well as wetness.
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The grasses and plants listed in Table 6.5.2.B are good choices for pond sides. They are
facultative (i.e., they can tolerate fluctuations in soil water). These species can generally survive
approximately 1 month of submersion while dormant in the winter (until about February 15), but
they can withstand only about 1 to 2 weeks of submersion after mid-February.
The lower portion of Table 6.5.2.B lists grass species that are good choices for the sand filter
bottom. They can withstand summer drying and are fairly tolerant of infertile soils. In general,
planting a mixture of 3 or more species is recommended. This ensures better coverage since
tolerance of the different species is somewhat different, and the best adapted grasses will spread
more rapidly than the others. Legumes, such as clover, fix nitrogen and hence can thrive in low-
fertility soils such as sands. This makes them particularly good choices for planting the sand filter
bed.
3. To prevent any use that could compact and potentially damage the filter surface, both permanent and
temporary structures (e.g., playground equipment or bleachers) are not permitted.
4. If the sand filter is located in a Sensitive Lake Protection Area, or discharges to a stream that is listed
as a Dissolved Oxygen (DO) Problem (Type 2) under “Downstream Water Quality Problems
Requiring Special Attention” (Section 1.2.2.1.2) and the problem cause has been identified as nutrient
loading, then low phosphorus fertilizers (such as formulations in the proportion 3:1:3 N-P-K or less)
or slow-release phosphorus formulations should be used, and at no more than the minimum agronomic
rate. Regardless of location, the fertilizer must meet the requirements of Chapter 15.54.500 RCW
limiting the use of fertilizer containing phosphorus.
TABLE 6.5.2.B RECOMMENDED PLANTS FOR SATURATED AREAS
RECOMMENDED PLANTS FOR POND SIDES
Scientific Name Common Name
Bromus carinatus California brome
Calamagrostis nutkaensis Pacific reed grass
Deschampsia caespitosa Tufted hairgrass
Distichlis spicata Saltgrass
Glyceria borealis Northern mannagrass
Poa palustris Fowl bluegrass
Juncus ensifolius Daggerleaf rush
Juncus patens Spreading rush
Juncus tenuis Poverty rush
RECOMMENDED PLANTS FOR POND BOTTOM (SAND SURFACE)
Agrostis tenuis Colonial bentgrass (Highland strain good)
Festuca brevipila Hard fescue
Festuca elatior “Many Mustang,”
“Silverado”
Dwarf tall fescues
Festuca ovina Sheep fescue
Festuca rubra var. rubra Red fescue
Koeleria macrantha Prairie junegrass
Lolium perenne Perennial ryegrass
Lupinus rivularis Riverbank lupine
Note: Other grasses may be used if recommended by a horticultural or erosion control specialist for the specific site.
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Recommended Design Features
The following design features should be incorporated into sand filter designs where site conditions allow:
1. A horticultural specialist should be consulted for advice on planting.
2. Seeding is best performed in fall (late September to October) or in spring (mid-March to June). For
summer seeding or seeding during dry conditions, sprinkler systems or other measures for watering
the seed must be provided. Soil temperatures should be between 50 and 65 degrees to allow for seed
germination of cool season grasses.
3. Seed should be applied at 80 to 100 seeds per square foot. Pounds of seed per acre will depend on
actual species composition as number of seeds vary dramatically by species per pound.
4. During seeding, Slow-release fertilizers may be applied to speed the growth of grass. If the sand filter
is located in a sensitive lake watershed or discharges to a stream that is listed as a Dissolved Oxygen
(DO) Problem (Type 2) under “Downstream Water Quality Problems Requiring Special Attention”
(Section 1.2.2.1.2) and the problem cause has been identified as nutrient loading, then low phosphorus
fertilizers (such as formulations in the proportion 3:1:3 N-P-K or less) or slow-release phosphorus
formulations should be used, and at no more than the minimum agronomic rate. Regardless of
location, the fertilizer must meet the requirements of Chapter 15.54.500 RCW limiting the use of
fertilizer containing phosphorus.
5. A sand filter can add landscape interest and may be incorporated into the project landscape design.
Interior side slopes may be stepped with flat areas for planting (Figure 6.5.2.E). Perennial beds may be
planted above the overflow water surface elevation. However, large shrubs and trees are not allowed
because falling leaves and needles can clog the filter surface, requiring more frequent maintenance,
and roots may damage the structure and/or function of the filter.
Note: Examples of areas with stepped side slopes can be found at the Ballard Locks in Seattle and at
Luther Burbank Park on Mercer Island.
6. Recreational use of the filter surface is not allowed as activity can disrupt the structure and function
of the filter media. Signage discouraging recreation is required. Signage shall be placed for maximum
visibility from adjacent streets, sidewalks, and paths. More than one sign may be required to be sure
the advisory will be noted by anyone approaching the facility.
Construction Considerations
1. If sand filters are put into service before construction of all parcels within the catchment is complete
and all disturbed soil in the sand filter catchment has been stabilized, the filter will very likely clog
prematurely. If individual lots are not stabilized, the options for protection from upstream erosion
given in Section 5.2.1 for infiltration ponds may be used.
An alternative is to install the sand filter pond including full excavation for the filter sand and
underdrain layers, delaying placement of the sand and underdrains until the project site is stabilized.
The partially complete sand filter will then function like a small wetpond. Later, the accumulated
sediment must be removed and the underdrain with gravel, geotextile separator, and sand layers
placed. A second alternative is to place only the gravel underdrain during the construction phase. Then
clean the gravel and place the geotextile separator and sand layer after the project site is stabilized.
The City will not assume maintenance responsibility or release financial guarantees unless the sand
filter is installed per design and functioning properly. If the final sand layer cannot be completed
before the typical two-year holding period for financial guarantees, the applicant may elect to pay the
City to clean and install the sand when the watershed is stabilized, or may arrange a smaller financial
guarantee specifically for completion of the sand filter.
2. Careful placement of the sand is necessary to avoid formation of voids within the sand that could
lead to short-circuiting, particularly around penetrations for underdrain cleanouts, as well as to prevent
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damage to the underlying geomembranes and underdrain system. Voids between the trench wall and
geotextile fabric should also be avoided.
3. Over compaction must be avoided to ensure adequate filtration capacity. Sand is best placed with a
low ground pressure tracked bulldozer (4.6 pounds per square inch or less ground pressure). The
number of passes over sand fill should be minimized during placement; using low ground-pressure
vehicles can minimize ground pressure and compaction.
4. After the sand layer is placed, water settling is recommended. Flood the sand with 10 to 15 gallons of
water per cubic foot of sand.
Maintenance Considerations
Sand filters are subject to clogging by fine sediment, oil and grease, and other debris (e.g., trash and
organic matter such as leaves). Filters and pretreatment facilities should be inspected every 6 months
during the first year of operation. Inspections should also occur immediately following a storm event to
assess the filtration capacity of the filter. Once the filter is performing as designed, the frequency of
inspection may be reduced to once per year.
During an inspection the following features should be evaluated and maintained as needed:
1. Remove debris and sediment from the pretreatment facility when depth exceeds 12 inches.
2. Remove debris and sediment from the surface of the filter when accumulations exceed 0.5 inches.
3. Observe operation of the overflow and drawdown time in the filter. Frequent overflow through the
grated “birdcage” or “jailhouse” window into the outlet structure or slow drawdown are indicators of
plugging problems. Under normal operating conditions, a sand filter should completely empty within
9 to 24 hours following a storm event (i.e., after the inflow of runoff to the filter ceases), depending on
pond depth. Generally, if the water level over the filter drops at a rate less than 1/2-inch per hour
(V < 1/2-inch per hour), corrective maintenance is needed. Recommendations for improving sand filter
performance are summarized below:
a) Remove thatch accumulation in grass.
b) Aerate the filter surface to improve permeability.
c) Till the filter surface. Two separate passes following a criss-cross pattern (i.e., second pass at right
angles to the first) are recommended.
d) Replace upper 4 to 6 inches of grass and sand.
4. Experience with sand filters used for stormwater treatment in Austin, Texas, has shown that the sand
becomes clogged and must be replaced every 4 to 10 years.
5. Rapid drawdown in the filter (i.e., greater than 12 inches per hour) indicates short-circuiting of the
filter media. Inspect the cleanouts on the underdrain pipes and along the base of the embankment for
leakage.
6. Formation of rills and gullies on the surface of the filter indicates improper function of the inlet flow
spreader or poor sand compaction. Check for accumulation of debris on or in the flow spreader, and
refill rills and gullies with sand.
Other maintenance practices that should be employed to ensure proper operation of the sand filter are
summarized below:
1. Avoid use of fertilizers along the bottom or sides of a landscape sand filter. Any fertilizer used must
meet the requirements of Chapter 15.54.500 RCW limiting the use of fertilizer containing
phosphorus.41
41 <http://apps.leg.wa.gov/billinfo/summary.aspx?bill=1489&year=2011>.
AGENDA ITEM # 8. a)
6.5.2 SAND FILTERS — BASIC AND LARGE
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2. Avoid driving heavy machinery or equipment on the sand filter to minimize compaction of the filter
media, prevent the formation of ruts in the surface of the filter that could concentrate or channelize
flow, and prevent damage to the underdrain system. Use only low ground pressure tracked equipment
(4.6 pounds per square inch or less ground pressure). The number of passes over sand fill should be
minimized to the greatest extent possible.
3. Mow grass as needed, and remove the cut grass from the sand filter.
4. If vegetation is present, water it periodically when needed, especially during the summer dry season.
5. Discourage use of the sand bed by pets by installing signs reminding residents of scoop laws,
providing scoop stations near the facilities, planting barriers such as barberry, and/or providing other
measures as appropriate.
MODIFICATIONS FOR COMBINING WITH AN INFILTRATION POND
Where an infiltration pond is proposed for flow control, a sand filter (basic or large) may be combined
with the infiltration pond by making the following modifications in design criteria:
1. The “100-year Overflow Conveyance” requirements for infiltration ponds (see Section 5.2.1) shall
apply in place of the “Overflow and Bypass” requirements for sand filters.
2. The “Filter Composition” criteria are changed to eliminate the requirement for an underdrain system.
The fourth layer of the filter becomes the native infiltrative soils.
3. The “Underdrain System” and “Underdrain Materials” criteria for sand filters are not applied.
Water infiltrating through the sand layer need not be collected but may simply continue infiltrating
downward into native soils.
4. The sides of the infiltration pond must be provided with a treatment liner up to the WQ design water
surface elevation, at a minimum. In a groundwater protection area, the liner must extend up to the
overflow water surface elevation of the pond. See Section 6.2.4 (Facility Liners) for information on
liners.
AGENDA ITEM # 8. a)
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FIGURE 6.5.2.A SCHEMATIC REPRESENTATION OF A SAND FILTER WITH LEVEL SPREADER
PLAN VIEW
PLAN VIEW
NTS
ACCESS ROAD
3:1
0.5%0.5%
0.5%
ACCESS ROAD
OUTLET STRUCTURE AND OVERFLOW
PER SECTION 5.1.1 DETENTION PONDS
LATERAL FEEDER
PIPE OR DRAIN STRIP
UNDERDRAIN
COLLECTOR
(PERF. PIPE)
CLEANOUT WYES W/CAP
IN VALVE BOX (BOTH
ENDS)
15' SPACING BETWEEN
FEEDER PIPES (MAX)
FLOW SPREADER PER
SECTION 6.2.6.1
(CONCRETE CHANNEL
OR OTHER) FOR
20% OF BOTTOM
PERIMETER (MIN)
EMERGENCY
SPILLWAY
EROSION
PROTECTION
GRATING OVER
FLOW SPREADER
(OPTIONAL)
INLET STRUCTURE
AGENDA ITEM # 8. a)
6.5.2 SAND FILTERS — BASIC AND LARGE
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FIGURE 6.5.2.B SCHEMATIC REPRESENTATION OF A SAND FILTER WITH LEVEL SPREADER
PROFILE VIEW
SECTION A-A
NTS
8"
min.
24" min.
TRENCH DETAIL
NTS
WQ DESIGN WS
INLET
SECTION B-B
NTS
3H:1V SLOPE
RECOMMENDED
INVERT OF UNDERDRAIN ABOVE
SEASONAL HIGH GROUND WATER LEVELTRENCH OPTIONAL, BUT 8" CRUSHED
GRAVEL REQUIRED OVER DRAIN PIPE
EROSION PROTECTION
GRASS (OPTIONAL)
NO TOPSOIL MAY BE ADDED
DESIGN WS
6' MAX.
FLOW SPREADER PER SECTION 6.2.6.1
SAND
1" COVER OF
DRAIN ROCK OVER
GEOTEXTILE
GEOTEXTILE
FABRIC, SEE
TABLE 6.5.2.D
DRAIN ROCK, 1-1
2" TO 34"
GRATING
(OPTIONAL)
UNDERDRAIN
COLLECTOR PIPE
(6" MIN.)
18" MIN.
8" MIN.
SPILL CONTROL PROVIDED BY TEE
SECTION IN TYPE II CATCH BASIN
(NOT REQUIRED IF FILTER PROCEEDED
BY FACILITY WITH SPILL CONTROL)
AGENDA ITEM # 8. a)
SECTION 6.5 FILTRATION FACILITY DESIGNS
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FIGURE 6.5.2.C SCHEMATIC REPRESENTATION OF A SAND FILTER WITH PRETREATMENT
CELL PLAN VIEW
ACCESS ROAD
NOTE:
SEE SECTION 6.2.6 FOR
OTHER TYPES OF FLOW
SPREADERS
0.5%0.5%
0.5%
PLAN VIEW
NTS
Lc (INLET TO OUTLET)
= 2 (MIN.) x (Wc AT
MID-DEPTH)
PRESETTLING CELL
(IF NO WQ OR
DETENTION
FACILITY UPSTREAM),
VOLUME PER
SECTION 6.5.1
Wc AT MID-DEPTHARMORED OUTLET
DITCH INTO
FLOW SPREADER
EROSION PROTECTION
INLET FLOW SPREADER
(SECTION 6.2.6.1) FOR
20% (MIN.) OF PERIMETER
OF POND BOTTOM
UNDERDRAIN
COLLECTOR
(PERF. PIPE)
EMERGENCY SPILLWAY
DRAIN STRIP
(SPACING PER
MANUFACTURER'S
RECOMMENDATIONS)
OR FEEDER PIPES
CLEANOUT WYES
W/WATERTIGHT
CAP IN VALVE
BOX (BOTH ENDS)
INFLOW
5' MIN. TOP WIDTH
A
A
AGENDA ITEM # 8. a)
6.5.2 SAND FILTERS — BASIC AND LARGE
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FIGURE 6.5.2.D SCHEMATIC REPRESENTATION OF A SAND FILTER WITH PRETREATMENT
CELL PROFILE VIEW
FIGURE 6.5.2.E SCHEMATIC REPRESENTATION OF STEPPED SIDE SLOPES
SECTION A-A
NTS
INLET
3
1
3
1
WQ DESIGN WS
OVERFLOW STRUCTURE SIZED TO CONVEY PEAK FLOW RATE THROUGH FILTER
(OFF-LINE SYSTEM) OR PEAK FLOW FOR DEVELOPED SITE (ON-LINE SYSTEM)
(SEE SECTION 5.1.1.1 DETENTION PONDS - DESIGN CRITERIA, OVERFLOW)
RECOMMENDED
SAND
GRAVEL OR DRAIN ROCK
UNDERDRAIN COLLECTOR
IN GRAVEL FILLED TRENCH
1' STORAGE
SEDIMENT
PROVIDE TEE FOR CONTROL
OF FLOATABLES IF NEEDED
(SEE SECTION 6.5.1)
GRASS (OPTIONAL)
NO SOIL MAY BE ADDED TO SAND
RECOMMENDED
6' MAX.
3' MIN.
6' MAX.
SECTION
NTS
3
1
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SECTION 6.5 FILTRATION FACILITY DESIGNS
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6.5.3 SAND FILTER VAULTS
A sand filter vault is similar to an open sand filter except that the sand layer and underdrains are installed
below grade in a vault. Like a sand filter, a sand filter vault may be sized as either a basic or a large facility
to meet different water quality objectives. The basic sand filter vault is designed to meet the Basic WQ
menu goal of 80% TSS removal for the water quality design flow. The large sand filter vault is expected
to meet the Sensitive Lake Protection menu goal of 50% total phosphorus removal.
Applications and Limitations
A sand filter vault may be used on sites where space limitations preclude the installation of above
ground facilities. In highly urbanized areas, particularly on redevelopment and infill projects, a vault is a
viable alternative to other treatment technologies that require more area to construct.
Like sand filters, sand filter vaults are not suitable for areas with high water tables where infiltration of
groundwater into the vault and underdrain system will interfere with the hydraulic operation of the filter.
Soil conditions in the vicinity of the vault installation should also be evaluated to identify special design or
construction requirements for the vault.
It is desirable to have an elevation difference of 4 feet between the inlet and outlet of the filter for
efficient operation. Therefore, site topography and drainage system hydraulics must be evaluated to
determine whether use of an underground filter is feasible.
Because the surface of a sand filter vault is prone to clogging from sediment and other debris, this facility
should not be used in areas where heavy sediment loads are expected .
Refer to the WQ menus, Section 6.1, for information on how sand filter vaults may be used to meet Core
Requirement #8.
6.5.3.1 METHODS OF ANALYSIS
The methods of analysis for basic and large sand filter vaults are identical to the methods described for
basic and large sand filters. Follow the procedures described in Section 6.5.2.1.
6.5.3.2 DESIGN CRITERIA
Schematic representations of sand filter vaults are shown in Figure 6.5.3.A and Figure 6.5.3.B.
Sand Filter Geometry
Same as for sand filters (see Section 6.5.2.2).
Pretreatment, Flow-Spreading, and Energy Dissipation
1. See general presettling and pretreatment requirements for filtration facilities, Section 6.5.1.
2. A flow spreader shall be installed at the inlet to the filter bed to evenly distribute incoming runoff
across the filter and prevent erosion of the filter surface.
3. For vaults with presettling cells, the presettling cells shall be constructed so that the divider wall
extends from the floor of the vault to the WQ design water surface and is water tight
4. The flow spreader shall be positioned so that the top of the spreader is no more than 8 inches above
the top of the sand bed (and at least 2 inches higher than the top of the inlet pipe if a pipe and manifold
distribution system is used). See Section 6.2.6 for details on flow spreaders. For vaults with
presettling cells, a concrete sump-type flow spreader (see Figure 6.2.6.B) shall be built into or
affixed to the divider wall. The sump shall be a minimum of 1 foot wide and extend the width of the
sand filter. The downstream lip of the sump shall be no more than 8 inches above the top of the sand
bed.
AGENDA ITEM # 8. a)
6.5.3 SAND FILTER VAULTS
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5. Flows shall enter the sand bed by spilling over the top of the wall into a flow spreader pad, or
alternatively a pipe and manifold system may be designed and approved at the discretion of CED to
deliver water through the wall to the flow spreader.
Note: Water in the first or presettling cell is dead storage. Any pipe and manifold system designed
must retain the required dead storage volume in the first cell, minimize turbulence, and be readily
maintainable.
6. If a pipe and manifold system is used, the minimum pipe size shall be 8 inches. Multiple inlets are
recommended to minimize turbulence and reduce local flow velocities.
7. Erosion protection shall be provided along the first foot of the sand bed adjacent to the spreader.
Geotextile weighted at the corners with sand bags, quarry spalls, or other suitable erosion control may
be used.
Overflow and Bypass Structures
Same as for sand filters (see Section 6.5.2.2).
Filter Composition
The filter bed shall consist of three layers as follows:
Top layer: sand
Second layer: geotextile fabric
Third layer: underdrain system.
Sand Specifications and Geotextile Materials
Same as for sand filters (see Section 6.5.2.2).
Underdrain Systems and Underdrain Materials
Same as for sand filters (see Section 6.5.2.2).
Vault Structure
1. Sand filter vaults are typically designed as on-line (flow-through) systems with a flat bottom under the
filter bed.
2. If a presettling cell is provided, the cell bottom may be longitudinally level or inclined toward the
inlet. To facilitate sediment removal, the bottom shall also slope from each side towards the center at a
minimum of 5%, forming a broad “v.”
Note: More than one “v” may be used to minimize cell depth.
Exception: The bottom of the presettling cell may be flat rather than v-shaped if removable panels
are provided over the entire presettling cell. Removable panels shall be at grade, have stainless steel
lifting eyes, and weigh no more than 5 tons per panel.
3. One foot (average) of sediment storage must be provided in the presettling cell.
4. Where pipes enter and leave the presettling cell below the WQ design water surface, they shall be
sealed using a non-porous, non-shrinking grout.
5. If an oil retaining baffle is used for control of floatables in the presettling cell, it must conform to the
following:
a) The baffle shall extend from 1 foot above to 1 foot below the WQ design water surface (minimum
requirements) and be spaced a minimum of 5 feet horizontally from the inlet and 4 feet
horizontally from the outlet.
b) Provision for passage of flows in the event of plugging shall be provided.
c) An access opening and ladder shall be provided on both sides of the baffle into the presettling
cell.
AGENDA ITEM # 8. a)
SECTION 6.5 FILTRATION FACILITY DESIGNS
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6. Sand filter vaults shall conform to the “Materials” and “Structural Stability” criteria specified for
detention vaults in Section 5.1.3.
7. The arch culvert sections allowed for wetvaults shall not be used for sand filter vaults. Free access to
the entire sand bed is needed for maintenance.
Access Requirements
Same as for detention vaults (see Section 5.1.3) except for the following modifications:
1. For facilities maintained by the City, removable panels must be provided over the entire sand bed.
Panels shall be at grade, have stainless steel lifting eyes, and weigh no more than 5 tons per panel.
Concrete bridge decking or industrial decking are options. If within the roadway and outside the travel
lane, the panels must meet traffic loading requirements.
2. A minimum of 24 square feet of ventilation grate must be provided for each 250 square feet of
sandbed surface area. Grates may be located in one area if the sand filter is small, but placement at
each end is preferred. Small grates may also be dispersed over the entire sand bed.
Intent: Grates are important to allow air exchange above the sand. Poor air exchange will hasten
anoxic conditions which may result in release of pollutants such as phosphorus and metals and cause
objectionable odors.
Access Roads, Right of Way, and Setbacks
Same as for detention vaults (see Section 5.1.3).
Recommended Design Features
The following design features should be incorporated into sand filter vaults where feasible but are not
specifically required:
1. The floor of the presettling cell should be sloped toward the inlet to allow for sediment
accumulation and ease of cleaning.
2. A geotextile fabric is recommended over the sand bed to make sand bed maintenance easier. If used,
the geotextile should be a flexible, high-permeability, three-dimensional matrix of the kind commonly
used for erosion control. Sand bags should be used at 10 to 15 foot intervals to hold the geotextile in
place.
3. Additional grates are recommended instead of solid panels to increase air contact with the sand bed.
Construction Considerations
Same as for sand filters (see Section 6.5.2.2) plus, upon completion of installation, the vault shall be
thoroughly cleaned and flushed prior to placement of sand and drain rock.
Maintenance Considerations
Maintenance considerations for sand filter vaults are similar to those described for sand. Maintenance
practices need to be modified somewhat due to the sand filter being in a vault, including the use of safe
confined space entry procedures.
MODIFICATIONS FOR COMBINING WITH AN INFILTRATION VAULT
Where an infiltration vault is proposed for flow control, a sand filter vault (basic or large) may be
combined with the infiltration facility by making the following modifications in design criteria:
1. The “100-year Overflow Conveyance” requirements for infiltration ponds (see Section 5.2.1) shall
apply in place of the “Overflow and Bypass” requirements for sand filter vaults.
2. The “Filter Composition” criteria are changed to eliminate the requirement for an underdrain system.
The third layer of the filter becomes the native infiltrative soils.
AGENDA ITEM # 8. a)
6.5.3 SAND FILTER VAULTS
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3. The “Underdrain System” and “Underdrain Materials” criteria for sand filter vaults are not
applied. Water infiltrating through the sand layer need not be collected but may simply continue
infiltrating downward into native soils.
4. “Access requirements” for grating may be reduced at the discretion of the design and review
engineers.
Intent: when water infiltrates into the soil directly without being collected by an underdrain system,
the concern for pollutant release diminishes. Ventilation for odor control is, then, the only concern .
FIGURE 6.5.3.A SCHEMATIC REPRESENTATION OF A SAND FILTER VAULT PLAN VIEW
PLAN VIEW
NTS
INLET
B
B
AA
UNDERDRAIN SLOPE
0.5% (MIN.)
UNDERDRAIN
COLLECTOR
NOTE:
PROVIDE 24 S.F. OF GRATE FOR
EACH 250 S.F. OF SAND AREA
VENTILATION
GRATESLOPE FLOOR
TOWARDS CENTER
AT 5% SLOPE (MIN.)FIRST CHAMBER FOR
ENERGY DISSIPATION
AND PRETREATMENT
PROVIDE REMOVABLE
ACCESS PANELS
OVER ENTIRE SAND AREA
CONCRETE SUMP
W/LIP USED AS
FLOW SPREADER
OVERFLOW
WEIR
OIL RETAINING BAFFLE
FOR RETENTION OF
FLOATABLES
(OPTIONAL)
VENTILATION
PIPE (12" MIN.)
ACCESS
COVER
"V" SHAPED
BOTTOM
5' MIN.4' MIN.
EROSION
PROTECTIONCLEANOUT WYES
WITH CAPS (BOTH ENDS)
MUST BE WATERTIGHT
AGENDA ITEM # 8. a)
SECTION 6.5 FILTRATION FACILITY DESIGNS
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FIGURE 6.5.3.B SCHEMATIC REPRESENTATION OF A SAND FILTER VAULT PROFILE VIEW
SECTION A-A
NTS
WS ELEV. (MAX.)
6" MIN.
8" MAX.
OVERFLOW SIZED TO CONVEY
DESIGN FLOW RATE THROUGH
FILTER (OFF-LINE SYSTEM) OR
PEAK FLOW AS DEFINED IN
CHAPTER 5 (ON-LINE SYSTEM)
PROVIDE REMOVABLE PANELS
OVER THE ENTIRE SAND AREA.
A 4' X 6' AREA (MIN.) MUST BE
GRATED FOR EACH 250 SF
OF SAND BED
STEPS
2' MIN.BOTTOM
SLOPE 0.5-2%
TOWARD
INLET
(RECOM-
MENDED)1' AVG. SEDIMENT
STORAGE
PRE-SETTLING CELL
VOLUME PER SECTION 6.5.1
SAND
GEOTEXTILE FABRIC
DRAIN PIPE
DRAIN ROCK, 1-1/2"
TO 3
4" WASHED
45°
MAX.
FLOW SPREADER
EROSION PROTECTION1' (MIN.)
SUMP
WIDTH
PIPE
SUPPORT
18" (MIN.)
GEOTEXTILE FABRIC
w/1" DRAIN ROCK COVER
WATERTIGHT
CAP SAND
UNDERDRAIN
COLLECTOR PIPE
6" MIN.
18" MIN.
DRAIN ROCK
(8" MIN. DEPTH)
8" MIN. COVER
OVER PIPE
BAFFLE
ACCESS
COVER
1'
1'
SECTION B-B
NTS
AGENDA ITEM # 8. a)
6.5.4 LINEAR SAND FILTERS
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6.5.4 LINEAR SAND FILTERS
Linear sand filters are typically long, shallow, rectangular vaults. The vaults consist of two cells or
chambers, one for settling coarse sediment from the runoff and the other containing sand. Stormwater
flows into the second cell via a weir section that also functions as a flow spreader to distribute the flow
over the sand. The outlet consists of an underdrain pipe system that connects to the storm drain system. As
with other sand filters, linear filters come in two sizes, basic and large. The basic linear sand filter is
designed to meet the Basic WQ menu goal of 80% TSS removal for the water quality design flow. The
large linear sand filter is expected to meet the Sensitive Lake Protection menu goal of 50% total
phosphorus removal.
Applications and Limitations
The linear sand filter is used for stormwater flows for two different treatment purposes:
1. To provide basic or second-tier water quality treatment, and
2. To treat runoff from high-use sites (i.e., sites generating higher than typical concentrations of oil and
grease).
The presettling cell in a linear sand filter does not meet standard presettling cell requirements, so it is not
expected to achieve the presettling goal of 50% TSS removal. Sediment storage capacity will also be more
limited. These factors will necessitate more frequent maintenance than for a standard sand filter, and are
likely to result in poorer net pollutant removal overall. Therefore, linear sand filters are discouraged where
a different facility can be used. Linear sand filters are best suited for treating small drainages (less than
two acres), particularly long, narrow areas. A linear sand filter may be located along the perimeter of a
paved impervious surface or may be installed downstream of a filter strip where additional treatment is
needed. If used for oil control, the filter should be located upstream from the main water quality treatment
facility (i.e., wetpond, bioswale, or combined detention and wetpond).
Consult the water quality menus in Section 6.1 for information on how linear sand filters may be used to
meet Core Requirement #8 or Special Requirement #5.
6.5.4.1 METHODS OF ANALYSIS
Size the sand filter bed. A linear sand filter is sized based on the infiltration rate of the sand and the
amount of runoff draining to the facility. The filter is sized to infiltrate the sand filter design flow without
significant ponding above the sand. The sand filter bed for linear sand filters, basic and large, is sized
using the modeled routing procedure of Section 6.5.2.1.
Size the sediment cell. The sediment cell width should be set after the sand filter width is determined. Use
Table 6.5.4.A below to set the width of the sediment cell. If another WQ facility precedes the sand filter,
the sediment cell may be waived.
TABLE 6.5.4.A SEDIMENT CELL WIDTH, LINEAR SAND FILTER
If Sand Filter Width Is: Width of Sediment Cell Shall Be:
1 to 2 feet 12 inches
2 to 4 feet 18 inches
4 to 6 feet 24 inches
Over 6 feet One-third of sand cell width
AGENDA ITEM # 8. a)
SECTION 6.5 FILTRATION FACILITY DESIGNS
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6.5.4.2 DESIGN CRITERIA
A schematic representation is shown in Figure 6.5.4.A.
Geometry, Sizing, and Overflow
1. A linear sand filter shall consist of two cells or chambers, a sediment cell and a sand bed cell, divided
by a low divider wall. If the sand filter is preceded by another WQ facility, and the flow enters the
sand filter along the side as sheet flow, the sediment cell may be waived.
2. Stormwater may enter the sediment cell by sheet flow or via a piped inlet.
3. Minimum inside width of the sand filter cell shall be 1 foot. Maximum width shall be 15 feet.
4. The two cells must be separated by a divider wall that is level and extends a minimum of 6 inches and
a maximum of 12 inches above the sand bed. The riser overflow elevation must be adjusted for the
wall height.
5. The sand filter bed shall be 18 inches deep, reducible to no less than 12 inches deep if grade
limitations show a greater depth is not feasible. An 8-inch layer of drain rock with perforated
drainpipe shall be installed beneath the sand layer.
6. The drainpipe shall have a minimum diameter of 6 inches and be wrapped in geotextile and sloped
0.5 % (min) to drain.
7. For design, the maximum depth of ponding over the sand shall be 1 foot.
8. If separated from traffic areas, a linear sand filter may be covered or open, but if covered, the cover
must be removable for the entire length of the filter. Covers must be grated if flow to the filter is from
sheet flow.
9. A linear sand filter shall have an emergency overflow route, either surface overland, tightline, or
other structure for safely controlling the overflow, and shall meet the conveyance requirements
specified in Chapter 1.
Structure Specifications
1. A linear sand filter vault shall be concrete (precast/prefabricated or cast-in-place). The concrete must
conform to the “Material” requirements for detention vaults in Section 5.1.3.
2. Where linear sand filters are located in traffic areas, they must meet the “Structural Stability”
requirements specified for detention vaults in Section 5.1.3. The sediment cell shall have a
removable grated cover that meets HS-25 traffic loading requirements. The cover over the sand filter
cell may be either solid or grated.
3. A minimum of 24 square feet of ventilation grate must be provided for each 250 square feet of
sandbed surface area. Grates located over the sediment chamber are preferred. Grates may be in one
central location or dispersed over the entire sand bed. Vertical grates may also be used such as at a
curb inlet. If a sediment chamber is not required, ventilation shall be provided over the sandbed.
Intent: Grates are important to allow air exchange above the sand. Poor air exchange will hasten
anoxic conditions which may result in release of pollutants such as phosphorus and metals and cause
objectionable odors.
Sand Specifications
Same as for sand filters (see Table 6.5.2.A).
Geotextile Materials
Same as for sand filters (see WSDOT Standard Specifications (2014), 9-33.2(1) Geotextile
Properties/Table 1/Moderate Survivability/Woven, and Table 2, Class A).
AGENDA ITEM # 8. a)
6.5.4 LINEAR SAND FILTERS
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Underdrain Materials
Same as for sand filters (see Section 6.5.2.2).
Access Roads, Right of Way, and Setbacks
Same as for detention vaults (see Section 5.1.3).
Construction Considerations
If put into service before the project site is stabilized, placement of the sand layer should be delayed, and
the linear sand filter may be used with the gravel layer only. The gravel layer must be replaced and the
vault cleaned when the project site is stabilized and the sand bed installed. The City will not assume
maintenance responsibility or release financial guarantees until the final installation is complete.
Maintenance Considerations
Maintenance considerations for linear sand filters are similar to those for basic sand filters (see
Section 6.5.2.2) except sediment should be removed from the sediment cell when the sediment depth
exceeds 6 inches.
AGENDA ITEM # 8. a)
SECTION 6.5 FILTRATION FACILITY DESIGNS
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FIGURE 6.5.4.A SCHEMATIC REPRESENTATION OF A LINEAR SAND FILTER
PLAN VIEW
NTS
SECTION A-A
NTS
SECTION B-B
NTS
NOTES
1. WITH PIPED INLET,
COVER MAY BE SOLID
2. SAND DEPTH MAY BE
REDUCED TO 12", SEE
SECTION 6.5.4.2
3. SEE TABLE 6.5.4.A
INLET PIPE
FLOW DIRECTIONB REMOVABLE GRATED COVER
(OPTIONAL, SEE NOTE 1) (MUST BEAR
TRAFFIC LOADS IF IN A ROAD OR
PARKING AREA)
PERF. PIPE, 0.5% SLOPE
TOWARD OUTLET
(OR STRIP DRAIN)
OUTLET PIPE
CLEANOUT WYE
GRAVEL DRAIN ROCK
SAND LAYER
BAFFLE
6" PERF. PIPE
w/GEOTEXTILE
FABRIC WRAP
OUTLET PIPE3'-6" MIN.(SEE NOTE 2)18" (SEE
NOTE 2)
12"
8"
SAND FILTER CHAMBER
REMOVABLE COVER
(OPTIONAL, SEE NOTE 1)
GRATED COVER
SAND LAYER (18"
DEPTH, SEE NOTE 2)
GRAVEL DRAIN ROCK
6" PERF. PIPE
w/GEOTEXTILE
FABRIC WRAPFILTER
WIDTH
SEDIMENT CHAMBER
OPTIONAL INLET PIPE
12" MIN.
12" MIN.
15" MAX.
8" MIN.
SEE NOTE 3
AGENDA ITEM # 8. a)
CITY OF RENTON SURFACE WATER DESIGN MANUAL
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6.6 OIL CONTROL FACILITY DESIGNS
This section presents the methods, criteria, and details for oil control facilities that are not discussed in
other sections. Included are the following facility designs:
“Oil/Water Separators,” Section 6.6.2.
Other oil control facilities include wetvaults, with minor modifications (see Section 6.4.2), and linear sand
filters (see Section 6.5.4). Non-facility options include parking lot washing with proper disposal of wash
water and compliance with a NPDES permit that already addresses oil control. More information on non-
structural options can be found in the High-Use menu, Section 6.1.5.
The information presented for each facility is organized into the following two categories:
1. Methods of Analysis: Contains a step-by-step procedure for designing and sizing each facility.
2. Design Criteria: Contains the details, specifications, and material requirements for each facility.
6.6.1 CATCH BASIN INSERTS
A catch basin insert is a device installed underneath a catch basin inlet that treats stormwater through
filtration, settling, absorption, adsorption, or a combination of these mechanisms. This BMP is not allowed
in the City for oil control for compliance with Special Requirement #5.42,43
6.6.2 OIL/WATER SEPARATORS
Oil/water separators rely on passive mechanisms that take advantage of oil being lighter than water. Oil
rises to the surface and can be periodically removed. The two types of oil/water separators typically used
for stormwater treatment are the baffle type or API (American Petroleum Institute) oil/water separator and
the coalescing plate oil/water separator.
Baffle oil/water separators use vaults that have multiple cells separated by baffles extending down from
the top of the vault (see Figure 6.6.2.D for schematic representation). The baffles block oil flow out of the
vault. Baffles are also commonly installed at the bottom of the vault to trap solids and sludge that
accumulate over time. In many situations, simple floating or more sophisticated mechanical oil skimmers
are installed to remove the oil once it has separated from the water.
Coalescing plate separators are typically manufactured units consisting of a baffled vault containing
several inclined corrugated plates stacked and bundled together (see Figure 6.6.2.E for schematic
representation). The plates are equally spaced (typical plate spacing ranges from 1/4-inch to 1 inch) and are
made of a variety of materials, the most common being fiberglass and polypropylene. Efficient separation
results because the plates reduce the vertical distance oil droplets must rise in order to separate from the
stormwater. Once they reach a plate, oil droplets form a film on the plate surface. The film builds up over
time until it becomes thick enough to migrate upward because of oil’s lower density relative to water.
When the film reaches the edge of the plate, oil is released as large droplets which rise rapidly to the
surface, where the oil accumulates until the unit is maintained. Because the plate pack increases treatment
effectiveness significantly, coalescing plate separators can achieve a specified treatment level with a
smaller vault size than a simple baffle separator.
Oil/water separators are meant to treat stormwater runoff from more intensive land uses, such as high-use
sites, and facilities that produce relatively high concentrations of oil and grease. Although baffle separators
historically have been used to remove larger oil droplets (150 microns or larger), they may also be sized to
42 Footnote 43 is not used.
43 Footnote 44 is not used.
AGENDA ITEM # 8. a)
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remove smaller oil droplets. Both separators may be used to meet a performance goal of 10 to 15 mg/L
by designing the unit to removal oil particles 60 microns and larger.
Applications and Limitations
Oil/water separators are designed to remove free oil and are not generally effective in separating oil that
has become either chemically or mechanically emulsified and dissolved in water. Therefore, it is desirable
for separators be installed upstream of facilities and conveyance structures that introduce
turbulence and consequently promote emulsification. Emulsification of oil can also result if surfactants
or detergents are used to wash parking areas that drain to the separator. Detergents shall not be used to
clean parking areas unless the wash water is collected and disposed of properly (usually to the sanitary
sewer).
There is concern that oil/water separators used for stormwater treatment have not performed to
expectations.44 Therefore, emphasis should be given to proper application, design, operations and
maintenance – particularly sludge and oil removal, and prevention of coalescing-plate fouling and
plugging.45
Oil/water separators are best located in areas where the tributary drainage area is nearly all
impervious, and a fairly high load of petroleum hydrocarbons is likely to be generated . Oil/water
separators are not recommended for areas with very dilute concentrations of petroleum hydrocarbons since
their performance is not effective at low concentrations. Excluding unpaved areas helps to minimize the
amount of sediment entering the vault, reducing the need for maintenance. A unit that fails and ceases to
function can release previously trapped oil to the downstream receiving water, both in release from the
oily sediments and from entrainment of surface oils.
Wetvaults may also be modified to function as baffle oil/water separators (see design criteria for
wetvaults, Section 6.4.2.2).
Consult the water quality menus in Section 6.1 for information on how baffle and coalescing plate
oil/water separators may be used to meet Special Requirement # 5.
6.6.2.1 METHODS OF ANALYSIS
Background
Generally speaking, in most oil and water mixtures the degree of oil/water separation that occurs is
dependent on both the time the water is detained in the separator and the oil droplet size. The sizing
methods in this section are based on Stokes’ law:
VT = (6-25)
where VT = rise velocity of oil droplet
g = gravitational constant
dp = density of oil droplet to be removed
dc = density of carrier fluid
Do = diameter of oil droplet
= absolute viscosity of carrier fluid
44 WA Ecology 2014, SWMMWW, citing: Schueler, Thomas R., “Water Quality inlets/Oil Grit Separators,” BMP Fact Sheet #11,
Current Assessment of Urban Best Management Practices, March 1992.; Watershed Protection Techniques, “Hydrocarbon
Hotspots in the Urban Landscape: Can They be Controlled?,” February 1994.
45 WA Ecology 2014, SWMMWW, citing: U.S. Army Corps of Engineers, “Selection and Design of Oil and Water Separators,”
August 26, 1994.
18
2
ocpDddg
AGENDA ITEM # 8. a)
6.6.2 OIL/WATER SEPARATORS
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The basic assumptions inherent in Stokes’ law are: (1) flow is laminar, and (2) the oil droplets are
spherical.
Traditional baffle separators are designed to provide sufficient hydraulic residence time to permit oil
droplets to rise to the surface. The residence time Tr is mathematically expressed as follows:
Tr = (6-26)
where V = effective volume of the unit or container, or As x H, where
As = surface area of the separator unit, and
H = height of water column in the unit Q = hydraulic capacity or flow through the separator
The time required for the oil droplet to rise to the surface within the unit is found by the relation:
TT = (6-27)
where VT = rise velocity of the oil droplet
The oil droplet rises to the water surface if the residence time in the separator is at least equal to the oil
droplet rise time. This can be expressed as follows:
Tr = TT
By substituting terms and simplifying:
VT = (6-28)
where As = surface area of the separator unit
The ratio in Equation 6-28 is designated as the surface overflow rate or loading rate. It is this rate that
governs the removal efficiency of the process and predicts whether an oil droplet will be removed by the
separator.
Method for Baffle Separators
Design steps for the baffle separator are summarized below:
Step 1: Determine the WQ design flow (Q). The facility is sized based on the WQ design flow (see
Section 6.2.1). The separator must be designed as an off-line facility. That is, flows higher than the WQ
design flow (i.e., the modified off-line flow rate) must bypass the separator.
Step 2: Calculate the minimum vertical cross-sectional area. Use the following equation:
Ac = (6-29)
where Ac = minimum cross-sectional area (sf)
Q = modified off-line water quality design flow per Section 6.2.1 (cfs)
VH = design horizontal velocity (fps)
Q
V
TV
H
sA
Q
HV
Q
AGENDA ITEM # 8. a)
SECTION 6.6 OIL CONTROL FACILITY DESIGNS
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Set the horizontal velocity VH equal to 15 times the oil droplet’s rise rate VT. A design rise rate of 0.033
feet per minute shall be used unless it is demonstrated that conditions of the influent or performance
function warrant the use of an alternative value. Using the 0.033 feet per minute rise rate results in VH =
0.008 fps (= 0.495 fpm).
Step 3: Calculate the width and depth of the vault. Use the following equation:
D = (6-30)
where D = maximum depth (ft)
W = width of vault (ft)
and where Ac is from Step 2 above.
The computed depth D must meet a depth-to-width ratio r of between 0.3 and 0.5 (i.e., 0.3 D/W 0.5).
Note: D = (r Ac)0.5 and
W = D/r and
r = the depth-to-width ratio
Step 4: Calculate the length of the vault. Use the following equation:
L = FD (6-31)
where L = length of vault (ft)
F = turbulence and short-circuiting factor (unitless, see Figure 6.6.2.A)
VH = horizontal velocity (ft/min)
VT = oil droplet rise rate (ft/min)
D = depth (ft)
The turbulence factor F shall be selected using a VH /VT ratio of 15, so F = 1.64.
Therefore Equation 6-31 becomes: L = 1.64 15 D
W
Ac
T
H
V
V
AGENDA ITEM # 8. a)
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FIGURE 6.6.2.A TURBULENCE FACTOR PLOT
Step 5: Check the separator’s length-to-width ratio. The length L of the vault must be at least 5 times
its width in order to minimize effects from inlet and outlet disturbances. The length of the forebay shall be
approximately L/3.
Step 6: Compute and check that the minimum horizontal surface area (AH) criterion is satisfied. This
criterion is expressed by the following equation:
AH = LW (6-32)
Step 7: Compute and check that the horizontal surface area of the vault forebay. This area must be
greater than 20 square feet per 10,000 square feet of tributary impervious area. The length of the forebay
(L/3) may be increased to meet this criterion without having to increase the overall length of the vault.
Step 8: Design the flow splitter and high-flow bypass. See Section 6.2.5 for information on flow splitter
design.
Method for Coalescing Plate Separators
Coalescing plate separators are designed using the same basic principles as baffle separators. The major
difference is that in the baffle separator, horizontal separation is related only to water surface area, while in
the coalescing plate separator, horizontal separation is related to the sum of the plan-areas of the plates.
The treatment area is increased by the sum of the horizontal projections of the plates being added, and is
referred to as the plate effective separation area.
The basic procedure for designing a coalescing plate separator is to determine the effective separation area
required for a given design flow. The specific vault sizing then depends on the manufacturer’s plate
design. The specific design, analysis, configuration, and specifications for coalescing plates are
empirically based and variable. Manufacturers’ recommendations may be used to vary the
recommendations given below.
165
000055
.
.
Q
TURBULENCE FACTOR PLOT
1.2
1.3
1.4
1.5
1.6
1.7
1.8
0 2 4 6 8 10 12 14 16 18 20
V H/V TTurbulence Factor, FAGENDA ITEM # 8. a)
SECTION 6.6 OIL CONTROL FACILITY DESIGNS
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Step 1: Determine the WQ design flow. The coalescing plate oil/water separator must be sized based on
the WQ design flow (see Section 6.2.1). The separator must be designed as an off-line facility; flows
higher than the WQ design flow (i.e., the modified off-line flow rate) must bypass the separator.
Step 2: Calculate the plate minimum effective separation area (Ah). Ah is found using the following
equation:
ow
h SS
QA
00386.0
60 (6-33)
where Sw = specific gravity of water = 1.0
So = specific gravity of oil = 0.85
= absolute viscosity of water (poises); use 0.015674 for temp = 39°F
Q = modified off-line water quality design flow rate per Section 6.2.1 (cfs)
Ah = required effective (horizontal) surface area of plate media (sf).
Equation 6-33 is based on an oil droplet diameter of 60 microns. A graphical relation of Equation 6-33 is
shown in Figure 6.6.2.B below. This graph may be used to determine the required effective separation
surface area of the plate media.
FIGURE 6.6.2.B EFFECTIVE SEPARATION SURFACE VS FLOW RATE
Step 3: Calculate the collective projected surface area (Ap). A key design step needed to ensure
adequate performance of the separator unit is to convert the physical plate area (the surface area of the
plates if laid flat) into the effective (horizontal) separation surface area Ah (calculated in step 2). The
effective separation surface area Ah is based on the collective projected horizontal surface area Ap of the
plates where the plates are inclined, rather than laid flat.
Ah = Ap = Aa (cos H) (6-34)
where Aa = actual collective plate area of the plate configuration (sf)
H = angle of the plates to the horizontal (degree)
This equation is represented graphically in Figure 6.6.2.C below. The designer shall make sure that the
manufacturer sizes the oil/water separator using the projected surface area rather than the actual plate area.
Note: For this method, only the lower plate surface may be counted as effective separation surface,
regardless of manufacturer’s claims.
0.00
500.00
1000.00
1500.00
2000.00
2500.00
3000.00
3500.00
4000.00
4500.00
0.00 0.50 1.00 1.50 2.00 2.50 3.00
FLOW RATE (CFS)REQUIRED EFFECTIVE SEPARATION AREA (SF).Operating Temp=40 F
Oil Droplet Size
= 60 microns
AGENDA ITEM # 8. a)
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FIGURE 6.6.2.C PROJECTED HORIZONTAL PLATE AREA FOR
COALESCING PLATE OIL/WATER SEPARATOR
Step 4: Check with specific separator manufacturers. Check with specific manufacturers to choose a
separator that provides the required actual collective plate area calculated in Step 3, and meets the other
design criteria given in the next section. The specific vault design will depend upon each manufacturer’s
design. The geometric configuration and dimensions of the plate pack as well as the vault design are
variable and flexible depending on each manufacturer’s product.
Table 6.6.2.A provides approximate vault sizes for rough planning purposes. In reality, various
manufacturers have quite different designs, both for the plate packs themselves as well as for forebay and
afterbays. In addition, standard pre-cast vault dimensions vary with each manufacturer. These various
factors can greatly affect the volume of vault needed to provide a given effective separation area. The
numbers in Table 6.6.2.A should therefore be considered “order of magnitude” estimates only.
TABLE 6.6.2.A
APPROXIMATE COALESCING PLATE OIL/WATER SEPARATOR VAULT DIMENSIONS*
Area of Effective Separation
(square feet)
Approximate Vault Volume Required (cubic feet) for Plates with 1/2
Inch Spacing and Inclined 60 Degrees from Horizontal (cubic feet)
100 150
200 240
300 330
600 530
1,200 890
2,400 1150
3,200 2090
4,800 2640
* Order of magnitude estimates for planning purposes only. Actual vault volumes vary considerably depending on
separator design features and pre-cast vault dimensions.
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6.6.2.2 DESIGN CRITERIA
A schematic representation of a baffle oil/water separator is shown in Figure 6.6.2.D. Other designs and
configurations of separator units and vaults are allowed, including above ground units. However, they
must produce equivalent treatment results and treat equivalent flows as conventional units.
General Siting
1. Oil/water separators must be installed off-line, bypassing flows greater than the WQ design flow
described in Step 1 above.
2. When a separator is required, it shall precede other water quality treatment facilities (except
wetvaults). It may be positioned either upstream or downstream from flow control facilities, since
there are both advantages and disadvantages with either placement.
3. In moderately pervious soils where seasonal groundwater may induce flotation, buoyancy tendencies
shall be balanced by ballasting or other methods as appropriate.
4. Any pumping devices shall be installed downstream of the separator to prevent oil emulsification in
stormwater.
Vault Structure — General
The following criteria apply to both baffle and coalescing plate separators:
1. Separator vaults shall be watertight. Where pipes enter and leave a vault below the WQ design water
surface, they shall be sealed using a non-porous, non-shrinking grout.
2. Separator vaults shall have a shutoff mechanism on the outlet pipe to prevent oil discharges during
maintenance and to provide emergency shut-off capability in case of a spill. A valve box and riser
shall also be provided according to the design criteria for wetponds (see “Inlet and Outlet Criteria,”
Section 6.4.1.2).
Vault Structure — Baffle Separators
In addition to the above general criteria, the following criteria apply specifically to baffle separators:
1. Baffle separators shall be divided into three compartments: a forebay, an oil separation cell, and an
afterbay. The forebay is primarily to trap and collect sediments, encourage plug flow, and reduce
turbulence. The oil separation cell traps and holds oil as it rises from the water column, and it serves
as a secondary sediment collection area. The afterbay provides a relatively oil-free cell before the
outlet, and it provides a secondary oil separation area and holds oil entrained by high flows.
2. The length of the forebay shall be approximately 1/3 to 1/2 of the length of the vault, L. In addition,
the surface area of the forebay must be at least 20 square feet per 10,000 square feet of tributary
impervious area draining to the separator.
3. A removable flow-spreading baffle, extending from the surface to a depth of up to 1/2 the vault depth
(D) is required to spread flows.
4. The removable bottom baffle (sediment-retaining baffle) shall be a minimum of 24 inches (see
Figure 6.6.2.D), and located at least 1 foot from the oil-retaining baffle. A “window wall” baffle may
be used, but the area of the window opening must be at least three times greater than the area of the
inflow pipe.
5. A removable oil retaining baffle shall be provided and located approximately 1/4 L from the outlet
wall or a minimum of 8 feet, whichever is greater (the 8-foot minimum is for maintenance purposes).
The oil-retaining baffle shall extend from the elevation of the water surface to a depth of at least 50%
of the design water depth. Various configurations are possible, but the baffle shall be designed to
minimize turbulence and entrainment of sediment.
6. Baffles may be fixed rather than removable if additional entry ports and ladders are provided so that
both sides of the baffle are accessible by maintenance crews.
AGENDA ITEM # 8. a)
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7. Baffle separator vaults shall have a minimum length-to-width ratio of 5.
8. The design water depth (D) shall be no deeper than 8 feet unless approved by CED.
9. Baffle separator vaults shall have a design water depth-to-width ratio of between 0.3 and 0.5.
Vault Structure — Coalescing Plate Separators
In addition to the above general criteria, the following criteria apply specifically to coalescing plate
separators:
1. Coalescing plate separators shall be divided by baffles or berms into three compartments: a forebay,
an oil separation cell which houses the plate pack, and an afterbay. The forebay controls turbulence
and traps and collects debris. The oil separation cell captures and holds oil. The afterbay provides a
relatively oil-free exit cell before the outlet.
2. The length of the forebay shall be a minimum of 1/3 the length of the vault, L (but 1/2 L is
recommended). In addition, it is recommended that the surface area of the forebay be at least
20 square feet per 10,000 square feet of tributary impervious area draining to the separator. In lieu of
an attached forebay, a separate grit chamber, sized to provide be at least 20 square feet per 10,000
square feet of tributary impervious area, may precede the oil/water separator.
3. An oil-retaining baffle shall be provided. If maintained by the City, the baffle must be a minimum of
8 feet from the outlet wall (for maintenance purposes). For large units, a baffle position of 0.25L from
the outlet wall is recommended. The oil-retaining baffle shall extend from the water surface to a depth
of at least 50% of the design water depth. Various configurations are possible, but the baffle shall be
designed to minimize turbulence and entrainment of sediment.
4. A bottom sediment-retaining baffle shall be provided upstream of the plate pack. The minimum
height of the sludge-retaining baffle shall be 18 inches. Window walls may be used, but the window
opening must be a minimum of three times greater than the area of the inflow pipe.
5. It is recommended that entire space between the sides of the plate pack and the vault wall be filled
with a solid but light-weight removable material such as a plastic or polyethylene foam to reduce
short-circuiting around the plate pack. Rubber flaps are not effective for this purpose.
6. If a separator will be maintained by the City, the separator plates shall meet the following
requirements:
a) Plates shall be inclined at 45° to 60° from the horizontal. This range of angles exceeds the angle
of repose of many solids and therefore provides more effective droplet separation while
minimizing the accumulation of solids on the individual plates.
b) Plates shall have a minimum plate spacing of 1/2-inch and have corrugations.
c) Plates shall be securely bundled in a plate pack so that they can be removed as a unit.
d) The plate pack shall be a minimum of 6 inches from the vault bottom.
e) There should be 1 foot of head space between the top of the plate pack and the bottom of the vault
cover.
Inlet and Outlet
1. The inlet shall be submerged. A tee section may be used to submerge the incoming flow and must be
at least 2 feet from the bottom of the tank and extend above the WQ design water surface.
Intent: The submerged inlet is to dissipate energy of the incoming flow. The distance from the bottom
is to minimize resuspension of settled sediments. Extending the tee to the surface allows air to escape
the flow, thus reducing turbulence. Alternative inlet designs that accomplish these objectives are
acceptable.
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2. The vault outlet pipe shall be sized to pass the modified off-line WQ design flow before overflow
(using the pipe sizing methods in Chapter 4). The vault outlet pipe shall be back-sloped or have a tee
extending 1 foot above and below the WQ design water surface to provide for secondary trapping of
oils and floatables in the wetvault.
Note: The invert of the outlet pipe sets the WQ design water surface elevation.
Material Requirements
1. All metal parts shall be corrosion-resistant. Zinc and galvanized materials shall not be used unless
there is no substitute, because of aquatic toxicity potential. Painting or other coating of metal parts for
corrosion resistance is not allowed due to lack of longevity and lack of standardization or assurance of
non-toxic coatings.
2. Vault baffles shall be concrete, stainless steel or other acceptable material and shall be securely
fastened to the vault.
3. Gate valves, if used, shall be designed for seating and unseating heads appropriate for the design
conditions.
4. For coalescing plate separators, plate packs shall be made of stainless steel or polypropylene.
Access Requirements
Same as for detention vaults (see Section 5.1.3) except for the following modifications:
1. Access to each compartment is required. If the length or width of any compartment exceeds 50 feet,
an additional access point for each 50 feet is required.
2. Access points for the forebay and afterbay shall be positioned partially over the inlet or outlet tee to
allow visual inspection as well as physical access to the bottom of the vault.
3. For coalescing plate separators, the following also apply:
a) Access to the compartment containing the plate pack shall be a removable panel or other
access able to be opened wide enough to remove the entire coalescing plate bundle from the cell
for cleaning or replacement. Doors or panels shall have stainless steel lifting eyes, and panels
shall weigh no more than 5 tons per panel.
b) A parking area or access pad (25-foot by 15-foot minimum) shall be provided near the
coalescing plate bundles to allow for their removal from the vault by a truck-mounted crane or
backhoe, and to allow for extracting accumulated solids and oils from the vault using a vactor
truck.
Access Roads, Right of Way, and Setbacks
Same as for detention vaults (see Section 5.1.3).
Recommended Design Features
1. A gravity drain for maintenance is recommended if grade allows. The drain invert should be at a
depth equal to the depth of the oil retaining baffle. Deeper drains are encouraged where feasible.
2. The recommended design features for wetvaults should be applied.
3. If large amounts of oil are likely to be captured, a bleed-off pipe and separate waste oil tank may be
located adjacent to the vault to channel separated oils into the tank. This improves the overall
effectiveness of the facility, especially if maintenance is only annually. It also improves the quality of
the waste oil recovered from the facility.
AGENDA ITEM # 8. a)
6.6.2 OIL/WATER SEPARATORS
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Construction Considerations
1. Construction of oil/water separators shall follow and conform to the manufacturer’s recommended
construction procedures and installation instructions as well as the City of Renton Standard Details.
Where the possibility of vault flotation exists, the vault shall be properly anchored in accordance with
the manufacturer’s recommendations or an engineer’s design and recommendations.
2. Particular care must be taken when inserting coalescing plate packs in the vault so as not to damage or
deform the plates.
3. Upon completion of installation, the oil/water separator shall be thoroughly cleaned and flushed prior
to operating.
Maintenance Considerations
1. Oil/water separators must be cleaned regularly to ensure that accumulated oil does not escape from the
separator. Separators should be cleaned by November 15 of each year to remove accumulation during
the dry season. They must also be cleaned after spills of polluting substances such as oil, chemicals, or
grease. Vaults must also be cleaned when inspection reveals any of the following conditions:
a) Oil accumulation in the oil separation compartment equals or exceeds 1 inch, unless otherwise
rated for greater oil accumulation depths recommended by the specific separator manufacturer.
b) Sediment deposits in the bottom of the vaults equals or exceeds 6 inches in depth.
2. For the first several years, oil/water separators should be checked on a quarterly basis for proper
functioning and to ensure that accumulations of oil, grease, and solids in the separator are at
acceptable levels. Effluent from the vault shall also be observed for an oil sheen to ensure that oil
concentrations are at acceptable levels and that expected treatment is occurring. Separators should also
be inspected after large storm events (about 2 inches in 24 hours).
3. Access to separators shall be maintained free of all obstructions, and units shall be readily accessible
at all times for inspection and maintenance.
4. Maintenance personnel entering oil/water separator vaults should follow the state regulations
pertaining to confined space entry, if applicable.
AGENDA ITEM # 8. a)
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FIGURE 6.6.2.D SCHEMATIC REPRESENTATION OF A BAFFLE OIL/WATER SEPARATOR
PLAN VIEW
NTS
SECTION VIEW
NTS
INFLOW
ACCESS COVER (TYP.) W/LADDER ACCESS
TO VAULT. IF > 1250 SF, PROVIDE 5 X 10
REMOVABLE PANEL OVER INLET/OUTLET PIPE.
VENTILATION PIPES
(12" MIN.) AT CORNERS
LADDER (TYP.)
(KCRDCS DWG 7-006)
INLET PIPE (8" MIN.)
TYPE II CATCH BASIN
HIGH FLOW BYPASS
OUTLET PIPE (8" MIN.)
SHUT OFF VALVE w/
RISER & VALVE BOX
20' MAX.
(RECOMMENDED)6' MIN.W5' MAX.
VARIES (CAN BE
CONSTRUCTED ON
GRADE WITHOUT
RISERS)
SLUDGE RETAINING BAFFLE
OIL RETAINING BAFFLE
EXISTING GRADE
TEE (8" MIN.)
GRAVITY DRAIN
(RECOMMENDED,
SEE CRITERIA
FOR WETVAULTS)
REMOVABLE TEE
(RECOMMENDED)
L = 5W
L/3 - L/2
(APPROX.)8' MIN.1' MIN.
FOREBAY OIL/WATER
SEPARATOR
CHAMBER20' MAX.H=7' MIN.2' MIN.
D*
D* = 3' MIN.
8' MAX.
1' MIN.24" MIN.
50%D
(MIN.)
1' MIN.
1' MIN.
FLOW SPREADING
BAFFLE
6" MIN.
OUTLET PIPE (8" MIN.)
VALVE BOX
AGENDA ITEM # 8. a)
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FIGURE 6.6.2.E SCHEMATIC REPRESENTATION OF A COALESCING PLATE OIL/WATER
SEPARATOR
PLAN VIEW
NTS
SECTION VIEW
NTS
WQ
WATER
SURFACE
SUBMERGED
INLET PIPE
AFTERBAYFOREBAY
LADDER (TYP.)
(KCRDCS DWG. 7-006)
8" TEE
OUTLET PIPE (8" MIN.)
SHUT OFF VALVE w/
RISER & VALVE BOXACCESS DOOR ALLOWING REMOVAL OF
PLATE PACK OR PROVIDE FULL LENGTH
REMOVABLE COVERS ACROSS ENTIRE CELL
ACCESS COVER
(OVER OUTLET)
ACCESS COVER
(OVER INLET)
VENTILATION PIPES
(12" MIN.) AT CORNERS
COALESCING
PLATE PACK
20' MAX.
(RECOMMENDED)
LADDER
AND
ACCESS
INLET
PIPE
(8" MIN.)
HIGH FLOW BYPASS
5' MAX.
VARIES (CAN BE CONSTRUCTED
ON GRADE WITHOUT RISERS)
OIL RETAINING BAFFLE
(50% D, MIN.)
COALESCING PLATE PACK
INLET WEIR-SOLIDS
RETAINING BAFFLE
OR WINDOW WALL
(SEE TEXT)20' MAX.7' MIN.18"
MIN.
D
1' MIN.
1' MIN.
6" MIN.
L
(L/2 RECOMM.)
L/3 MIN.8' MIN.
(L/4 RECOMM.)
1' MIN.6" MIN.
2' MIN.
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6.7 PROPRIETARY FACILITY DESIGNS
Traditional public domain water quality treatment facilities such as wetponds and bioswales may not be
feasible or appropriate in some situations due to size and space constraints or limited ability or inability to
remove target pollutants.46 Even where public domain facilities are feasible, development applicants may
seek to use proprietary manufactured alternatives for economic, aesthetic, or other reasons. This is a
narrower range of facilities than those referred to by Ecology as “Emerging Technologies,”47 which also
include some public domain facilities that are in process of or have been approved through Ecology’s
TAPE program, e.g., WSDOT’s Media Filter Drain. Proprietary designs have been and are continuing to
be developed by the stormwater treatment industry.
Approval by Ecology through TAPE, CTAPE or Ecology’s Approved as Equivalent process does not itself
constitute approval by the City. An adjustment is required for use of proprietary facilities approved by
Ecology but not yet approved by the City.
Proprietary facilities which have been approved by the City are listed in Reference Section 14-A.
6.7.1 ECOLOGY REQUIREMENTS
Ecology refers to proprietary facilities as emerging technologies, and more broadly includes under that
designation stormwater treatment devices and some public domain facilities for which Ecology has
required testing through its Technology Assessment Protocol – Ecology (TAPE) program. All proprietary
facilities are emerging technologies, but not all emerging technologies are proprietary.
Proprietary systems include both permanent and construction site treatment technologies. Many of these
have not undergone complete performance testing so their performance claims cannot be verified. Some
have been tested and approved by Ecology through its TAPE program or Chemical Technology
Assessment Protocol Ecology (CTAPE) protocols (see <https://ecology.wa.gov/Regulations-
Permits/Guidance-technical-assistance/Stormwater-permittee-guidance-resources/Emerging-stormwater-
treatment-technologies>).
In addition, Ecology also has a category designated Approved as Equivalent to Existing Technologies, with
the following description: These technologies …
“… have been approved by Ecology as equivalent to existing water quality treatment technologies that
are currently listed in the 2014 Stormwater Management Manual for Western Washington and/or the
2004 Stormwater Management Manual for Eastern Washington . These technologies did not pass
through the Technology Assessment Protocol – Ecology (TAPE) process.”
6.7.2 CITY OF RENTON REQUIREMENTS
Only water quality facilities listed in Chapter 6 of this manual, Reference Section 14-A or 14-B, or
approved via a Blanket Adjustment may be used for water quality treatment required per Core
Requirement #8.
46 “Traditional” target pollutants are TSS, heavy metals, phosphorus, and petroleum hydrocarbons (“high-use” oil, etc.). There are many
TMDLs for bacteria, but no facilities approved by Ecology for bacteria treatment. Other pollutants of concern for which there are no
designated facilities include but are not limited to e.g., nitrate, PAHs, and phthalates. The SWDM presents treatment trains for alkalinity
(sphagnum bog wetland menu), but there is evidence that at least one of the allowed treatment trains does not work for alkalinity, hence,
potential need for other options.
47 “Emerging” implies previously unknown, undeveloped, or unused. While some of these technologies are new, others are not, nor is their
application for stormwater management necessarily new. While performance demonstration through TAPE is required for use of all
proprietary facilities, it has also been required by Ecology for the public domain Ecology-approved Media Filter Drain and Compost
Amended Bioswales (CABS), but not for the Ecology-approved Compost Amended Vegetated Filter Strips (CAVFS), or any of the legacy
stormwater facilities, e.g., ponds, vaults, bioswales, or sand filters.
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6.7.2.1 GENERAL
The following requirements are expected to be applicable to any proprietary facility included in Reference
Section 14-A, and may be applicable to other proprietary facilities depending on the details of those
designs.
1. At a minimum, all proprietary facilities must meet design, construction, and maintenance
requirements required by Ecology, as documented at Ecology’s Emerging Technologies website.48
2. In addition, vaults shall conform to the “Materials” and “Structural Stability” requirements
specified for detention vaults (see Section 5.1.3).
Presettling
For any proprietary facilities included in Reference Section 14-A, presettling requirements will be
described in detail within the design criteria for the approved facility in Reference Section 14-A.
Note that where a proprietary facility is used as the second or third facility in a treatment train for
Enhanced Basic treatment, presettling is provided by the first facility. Use of a proprietary facility for
Basic treatment or as the first facility (Basic) in a treatment train may require presettling. See
Section 6.5.1, for general presettling requirements for filtration facilities.
Access Requirements for Vaults
1. Access must be provided by either removable panels or other City approved accesses to allow for
removal and replacement of the filter cartridges. Removable panels, if used, shall be at grade, have
stainless steel lifting eyes, and weight no more than 5 tons per panel.
2. Access to the inflow and outlet cells must also be provided.
3. Ladder access is required when vault height exceeds 4 feet.
4. Required clear space for ladder access is a minimum two foot diameter floor-to-ceiling space at the
ladder, and between the ladder and any cartridges or other vertical obstructions on the vault floor.
5. Locking lids shall be provided as specified for detention (see Section 5.1.3).
6. If removable panels are not used, corner ventilation pipes shall be provided, and the minimum
internal height and width and maximum depth shall be met (see Section 5.1.3).
Access Roads, Right of Way, and Setbacks for Vaults
Same as for detention vaults (see Section 5.1.3).
Construction Considerations
Installation of a proprietary facility shall follow the manufacturer’s recommended procedures.
Maintenance Requirements
Maintenance needs vary depending on the facility, and from site to site based on the type of land use
activity, implementation of source controls, and weather conditions. The facility shall be inspected
quarterly or at a frequency recommended by the supplier. Inspection and maintenance shall include the
following:
1. The operation and maintenance instructions from the manufacturer shall be kept along with an
inspection and maintenance log. The maintenance log shall be available for review by City
inspectors.
2. Routine maintenance criteria can be found in Appendix A and Reference Section 14-A.
48 <http://www.ecy.wa.gov/programs/wq/stormwater/newtech/technologies.html>.
AGENDA ITEM # 8. a)
SECTION 6.7.2 CITY OF RENTON REQUIREMENTS
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3. Media shall be disposed of in accordance with applicable regulations, including RMC Title VIII,
Chapter 1 and state dangerous waste regulations (WAC 173-303). In most cases, the media may be
disposed of as solid waste.
6.7.2.2 FACILITY APPROVAL
The City’s facility approval process is summarized as follows:
Ecology may assign General Use Level Designation (GULD) or Conditional Use Level Designation
(CULD) to a given facility.
Before the City will consider adding a proprietary facility to the list of water quality facilities
approved for use without adjustments, Ecology must grant GULD approval and the City must
determine that sufficient performance monitoring data satisfying all requirements of TAPE are met.
City approval may require that monitoring data satisfying requirements of TAPE be provided for 3 or
more sites and that qualified samples equal 12 or more at each site. The City’s evaluation for inclusion
of facilities in this manual will also consider maintenance, operation, and durability factors. For
facilities to be maintained by the City, regular maintenance frequency must be no more than once per
year.
During the permitting process with CED, an applicant for an alternative facility may apply for an
adjustment to use a device or system not listed in this manual. There is no guarantee that an
adjustment will be granted, but if one is, monitoring will be required. All TAPE monitoring
requirements and criteria are applicable. The City does not pay for this monitoring. The cost of
monitoring commercial products is covered by the applicant and/or the facility vendor according to
their agreement. The cost of testing public domain devices or systems for which an adjustment is
requested is borne by the applicant.
6.7.2.3 DIFFERENCES BETWEEN CITY MAINTAINED AND PRIVATELY
MAINTAINED PROPRIETARY FACILITIES
The City will not consider adoption of proprietary facilities for public maintenance which are likely
to require maintenance more frequently than annually. A privately maintained proprietary facility
may have an inspection/maintenance cycle as short as quarterly.
Where the City will be taking over maintenance responsibilities from a developer, the City may
consider maintenance costs in deciding which proprietary facilities to allow.
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6.8 BIORETENTION FACILITY DESIGNS
This section presents the methods, details of analysis, and design criteria for bioretention facilities.
Included in this section are the following specific facility designs:
“Bioretention cells”
“Bioretention swales”
“Bioretention planters”
6.8.1 BIORETENTION
Bioretention cells are shallow depressions with a designed planting soil mix and a variety of plant
material, including trees, shrubs, grasses, and/or other herbaceous plants. Bioretention cells are not
designed as a conveyance system.
Bioretention swales incorporate the same design features as bioretention cells; however, bioretention
swales are designed as part of a system that can convey stormwater when maximum ponding depth is
exceeded. Bioretention swales have relatively gentle side slopes and ponding depths that are typically
6 to 12 inches.
Bioretention planters include a designed soil mix and a variety of plant material including trees, shrubs,
grasses, and/or other herbaceous plants within a vertical walled container usually constructed from formed
concrete, but could include other materials. Planters have an open bottom and allow infiltration to the
subgrade. These designs are often used in ultra-urban settings.
Applications and Limitations
1. A minimum of 3 feet of clearance is necessary between the lowest elevation of the bioretention soil,
or any underlying gravel layer, and the seasonal high groundwater elevation or other impermeable
layer if the area tributary to the bioretention facility meets or exceeds any of the following limitations:
5,000 square feet of pollution-generating impervious surface; or
10,000 square feet of impervious area; or
¾ acres of lawn and landscape.
2. If the tributary area to an individual bioretention facility does not exceed the areal limitations above, a
minimum of 1 foot of clearance is adequate between the lowest elevation of the bioretention soil (or
any underlying gravel layer) and the seasonal high groundwater elevation or other impermeable layer.
Because bioretention facilities use an imported soil mix that has a moderate design infiltration rate, they
are best applied for small drainages, and near the source of the stormwater. Cells may be scattered
throughout a subdivision; a swale may run alongside the access road; or a series of planter boxes may
serve the road. In these situations, they can but are not required to fully meet the requirement to treat 91%
of the stormwater runoff file from pollution-generating surfaces. But the amount of stormwater that is
predicted to pass through the soil profile may be estimated and subtracted from the 91% volume that must
be treated. Downstream treatment facilities may be significantly smaller as a result.
When used in combination with other BMPs, they can also help achieve compliance with the 0.15 cfs
threshold for Core Requirement #3.
Applications with or without underdrains vary extensively and can be applied in new development,
redevelopment and retrofits. Typical applications include:
Individual lots for rooftop, driveway, and other on-lot impervious surface.
Shared facilities located in common areas for individual lots.
Areas within loop roads or cul-de-sacs.
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Landscaped parking lot islands.
Within right-of-ways along roads (often linear bioretention swales and cells).
Common landscaped areas in apartment complexes or other multifamily housing designs.
Planters on building roofs, patios, and as part of streetscapes.
Setbacks
Note: Criteria with setback distances are as measured from the outermost edge of the bioretention soil
mix.
1. Bioretention areas should have a minimum shoulder of 6 inches between the road edge and beginning
of the bioretention side slope where flush curbs are used.
2. A minimum 5-foot setback shall be maintained between the outermost edge of the bioretention soil
mix and any building structure or property line.
3. For sites with septic systems, bioretention must be located downgradient of the primary and reserve
drainfield areas. CED review staff can waive this requirement if site topography clearly prohibits
subsurface flows from intersecting the drainfield.
4. Bioretention is not allowed in critical area buffers or on slopes steeper than 20%.
5. Bioretention is not allowed within 50 feet of a steep slope hazard area, erosion hazard area, or
landslide hazard.
6. Bioretention proposed on slopes steeper than 15% must be approved by a geotechnical engineer or
engineering geologist unless otherwise approved by CED. The geotechnical analysis must consider
cumulative impacts from the project and surrounding areas under full built-out conditions.
7. Bioretention proposed near slopes steeper than 15% must be approved by a geotechnical engineer or
engineering geologist if the facility is located within a setback from the top of slope equal to the total
vertical height of the slope area that is steeper than 15% unless otherwise approved by CED. The
geotechnical analysis must consider cumulative impacts from the project and surrounding areas under
full built-out conditions.
8. Bioretention that directs overflow towards slopes steeper than 15% may require evaluation and
approval of the proposal by a geotechnical engineer or engineering geologist as determined by CED.
The geotechnical analysis must consider cumulative impacts from the project and surrounding areas
under full built-out conditions.
9. Bioretention proposed within 200 feet of a steep slope hazard area, erosion hazard area, or landslide
hazard must be approved by a geotechnical engineer or engineering geologist unless otherwise
approved by CED. The geotechnical analysis must consider cumulative impacts from the project and
surrounding areas under full built-out conditions.
10. Bioretention must not create flooding or erosion impacts as determined by CED. If bioretention is
proposed near or directs overflows towards a landslide hazard, erosion hazard area, or a steep slope
hazard area, CED may require evaluation and approval of the proposal by a geotechnical engineer or
engineering geologist. The geotechnical analysis must consider cumulative impacts from the project
and surrounding areas under full built-out conditions.
6.8.1.1 DESIGN CRITERIA
This section provides a description, recommendations, and requirements for the components of
bioretention facilities. Refer to Appendix C for additional infeasibility criteria for Core Requirement #9.
Design criteria are provided in this section for the following elements:
Contributing area
Flow entrance
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Presettling
Water storage area
Bioretention soil
Subgrade
Underdrain (if required)
Overflow
Liners (optional)
Plant material
Mulch layer
Check dams and weirs
UIC discharge
Contributing Area
Bioretention cells are small and distributed. The contributing area to a bioretention facility is limited as
follows:
No single cell may receive runoff from more than 5,000 square feet of impervious area, except as
noted below for a series of bioretention cells.
Runoff from more than 5,000 square feet of impervious area may be directed to an upstream cell in a
bioretention series (interconnected series of cells).
The bioretention facility should be sized for the contributing area routed to the facility. It is recommended
that facilities not be oversized because the vegetation in oversized facilities may not receive sufficient
stormwater runoff for irrigation, increasing maintenance.
Stormwater flows from other areas (beyond the area for which the facility is sized) should be bypassed
around the facility in order to reduce sediment loading to the cell and the potential for bioretention soil
clogging and increased maintenance needs. If bypass is not feasible, facilities shall be sized to treat runoff
from the entire area draining to the facility.
Additional flows may pass through a bioretention facility with the following limitations:
The maximum additional area (i.e., areas beyond the area for which the facility is sized) that may pass
through a bioretention facility shall not exceed twice the area for which it is sized due to sediment
loading concerns;
If additional area is routed to the bioretention facility, it shall be clearly noted on submitted plans;
The overflow infrastructure shall be sized for the full contributing area; and
Presettling calculations shall demonstrate that the water velocities in the vegetated areas of the
bioretention facility do not exceed 2 feet per second during peak flows with 4 percent annual
probability (the 25 year recurrence interval flow) (calculated through the narrowest vegetated cross
section of the facility).
Flow Entrance
Flow entrances shall be sized to capture flow from the drainage area and designed to both reduce the
potential for clogging at the inlet and prevent inflow from causing erosion in the facility. Four primary
types of flow entrances can be used for bioretention facilities: dispersed flow (e.g., vegetated buffer
strips), sheet flow, curb cuts, and concentrated flow (e.g., piped flow). Where feasible and appropriate
within the site context, vegetated buffer strips are the preferred entrance type because they slow incoming
flows and provide initial settling of particulates.
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Requirements associated with the flow entrance design include the following:
For facilities in the right-of-way, the flow entrance elevation shall be above the overflow elevation.
For sheet flow into a facility, a minimum 1 inch drop from the edge of a contributing hard surface to
the vegetated flow entrance is required. This drop is intended to allow for less frequent maintenance
by allowing some sediment/debris buildup at the edge where flow enters the facility.
The following requirements apply to roadway and parking lot curb cut flow entrances:
o The curb cut width shall be sized based on the drainage area, longitudinal slope along the curb,
and the cross slope at the inlet.
o The minimum curb cut opening shall be 12 inches; however, 18 inches is recommended.
o The curb cut shall have either a minimum of 8 percent slope from the outer curb face extending to
a minimum of 12 inches beyond the back of curb, or provide a minimum of a 2 inch vertical drop
from the back of curb to the vegetated surface of the facility.
If concentrated flows are entering the facility (e.g., pipe or curb cut), flow energy dissipation (e.g.,
rock/cobble pad or flow dispersion weir) shall be incorporated to reduce the potential for erosion at
the inlet.
Presettling
Presettling to capture debris and sediment load from contributing drainage areas is required at the flow
entrance for some bioretention facilities. By having a designated presettling zone, maintenance can be
targeted in this area to remove sediment build-up.
Requirements associated with the presettling design include the following:
Presettling requirements for bioretention facilities are provided in Table 6.8.1.A.
If the cell will receive flows from impervious areas beyond the area for which the facility is sized, the
presettling measures shall be designed for the entire area draining to the facility.
The area designated as the presettling zone shall not be included in the calculation of the bottom area of
the bioretention facility.
TABLE 6.8.1.A PRESETTLING REQUIREMENTS FOR BIORETENTION FACILITIES
IMPERVIOUS AREA (SQUARE FEET)
CONTRIBUTING RUNOFF TO A SINGLE FLOW
ENTRANCE PRESETTLING REQUIREMENTS
< 5,000 No presettling is required. Designer to determine if site
specific presettling is needed based on upstream area
conditions.
≥ 5,000 and < 10,000 The bottom of the first 2 to 3 feet of the upstream
bioretention cell (at the flow entrance) shall be
designated the presettling zone. This bottom area of
the cell shall be constructed of cobbles, concrete open
celled paving grids, plastic lattices filled with gravel or
groundcover vegetation, a roughened concrete pad, or
similar material for collection of sediment for
maintenance.
Alternatively, a catch basin with a minimum 2-foot
sump may be used as the presettling zone. Where the
pipe (from the catch basin) daylights into the
bioretention cell, provide energy dissipation within the
cell.
≥ 10,000 Presettling requirements are project specific, to be
determined by designer and approved by the City.
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Water Storage Area
The water storage area provides space for storm flows and the first stages of pollutant treatment within the
bioretention facility. Requirements for water storage area design for bioretention facilities with both side
slopes and vertical sides include:
The bottom area of an individual cell shall be no larger than 800 square feet (limitation is to ensure
that bioretention facilities are small-scale and distributed).
The bottom area of an individual cell shall be no less than 4 square feet.
The average ponding depth shall be no less than 2 inches.
The ponding depth shall be no more than 12 inches. In right-of-way areas with high pedestrian traffic,
the ponding depth may be restricted to 6 inches or less.
The maximum allowable drawdown time of the water storage area is 24 hours. A correction factor of
0.33 to 1 (no correction factor) as recommended by a licensed geotechnical professional should be
applied to initial measured infiltration rates of the in situ soils to determine the design rate for this
drawdown calculation. The designed water storage depth (2″ minimum to 12″ maximum) must be
considered in light of the drawdown time requirement (e.g., in slow draining soils, the designed
ponding depth may need to be decreased in order to meet the drawdown criteria). As an example, a
6″ deep pool with an initial measured rate of 0.5 in/hour and a correction factor of 0.5 applied will
achieve drawdown in exactly 24 hours (0.5 in/hour x 0.5 correction factor x 24 hours = 6 inches).
The bottom slope shall be no more than 6 percent.
Additional requirements for water storage area design specific to bioretention facilities with side slopes
include the following:
The maximum planted side slope is 2.5H:1V. In the ROW, if the facility is on a curbless street and
less than 50 feet of an intersection, the maximum planted sides slope is 3H:1V. If total facility depth
exceeds 3 feet, the maximum planted side slope is 3H:1V. If steeper sides are necessary, rockery,
concrete walls, or steeper soil wraps may be used.
If berming is used to achieve the minimum top facility elevation needed to meet ponding depth and
freeboard needs, the following requirements apply:
o Maximum berm slope is 2.5H:1V
o Minimum berm top width is 6 inches.
o Soil used for berming where the permanent restoration is landscape shall meet the bioretention
soil mix specification and be compacted to a minimum of 90 percent dry density.
o A catch basin or rock pad must be provided to release water when the water level exceeds the
12 inches of water depth. The catch basin may discharge to the local drainage system or other
acceptable discharge location via a 6-inch rigid pipe (private) or 8-inch rigid pipe (public). The
rock pad may be used with or without a constructed drainage system downstream. If a rock pad is
used, it must be composed of crushed or fractured rock, 6 inches deep and 2 feet wide
(perpendicular to flow) and must extend at least 4 feet or beyond the containment berm,
whichever is greater. The rock pad must be situated so that overflow does not cause erosion
damage or unplanned inundation
For trees planted alongside slopes of the bioretention cell, the maximum side slope around the tree is
1H:1V.
The average bottom width for the facility shall be no less than 18 inches.
Additional requirements for water storage area design specific to bioretention facilities with vertical sides
include the following:
The facility width (planted area between walls) shall be no less than 2 feet. For plant health, the
recommended minimum facility width is 4 feet.
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Additional requirements for bioretention swales:
Bioretention swales shall have a minimum 18-inch bottom width. Swales shall be flat in cross section
to promote event flow across the width of the swale. See Renton Standard Details for design details
for bioretention swales in the ROW.
Bioretention swales shall meet the conveyance requirements described in Section 1.2.4.1 of this
manual. Maximum 100-year peak flow velocity through bioretention swales is 3 feet per second.
Maximum longitudinal (along direction of flow) slope of bioretention swales shall be 6%.
To address traffic and pedestrian safety concerns, the following additional requirements apply to
bioretention facilities in the right-of-way:
The following minimum setbacks shall be provided for facilities with sloped sides:
o 2 feet minimum from face of curb to top of slope on non-major arterial streets
o 4 feet minimum from face of curb to top of slope for major arterial street
o 1 foot minimum from edge of sidewalk to top of slope
A minimum of one access path across planting strip shall be provided between the street and public
sidewalk for each parcel. Access paths shall be a minimum of 5 feet wide. It is preferred that the
access path is within 15 feet of the structure access point (such as path to doorway or stairs).
Bioretention cells shall not impact driveway/alley access. A 2-foot minimum setback shall be
provided from the pavement edge of the driveway curb cut wing to the top (top of slope) of
bioretention cell.
A 2-foot minimum setback shall be provided from the edge of paving for the public sidewalk/curb
ramp at the intersection to the top of slope of the bioretention cell. Curb ramp improvements are
required whenever the construction of bioretention cells and associated street improvements remove
pavement within the crosswalk area of the street or sidewalk, impact curbs, sidewalks, curb ramps,
curb returns or landings within the intersection area, or affect access to or use of a public facility.
Bioretention Soil Mix
Requirements for the bioretention soil mix include:
1. An 18″-thick bioretention soil mix liner extending up slopes to maximum water storage depth is
required in the bioretention cell, swale, or planter. The bioretention soil mix shall be per Reference
Section 11-C. Compost shall meet Specification 1 described in Reference Section 11-C.
2. Do not use filter fabrics between the subgrade and the Bioretention Soil Mix. The gradation between
existing soils and Bioretention Soil Mix is typically not great enough to allow significant migration of
fines into the Bioretention Soil Mix. Additionally, filter fabrics may clog with downward migration of
fines from the Bioretention Soil Mix.
3. Onsite soil mixing or placement shall not be performed if Bioretention Soil Mix or subgrade soil is
saturated. The bioretention soil mixture should be placed and graded by machinery operating adjacent
to the bioretention facility.
4. If machinery must operate in the bioretention cell for soil placement, use light weight equipment with
low ground-contact pressure. The soil mixture shall be placed in horizontal layers not to exceed
12 inches per lift for the entire area of the bioretention facility.
5. Compact the Bioretention Soil Mix to a relative compaction of 85 percent of modified maximum dry
density (ASTM D 1557). Compaction can be achieved by boot packing (simply walking over all areas
of each lift), and then apply 0.2 inches (0.5 cm) of water per 1 inch (2.5 cm) of Bioretention Soil Mix
depth. Water for settling should be applied by spraying or sprinkling.
6. Prior to placement of the BSM, the finished subgrade shall: (a) Be scarified to a minimum depth of
3 inches; (b) have any sediment deposited from construction runoff removed (to remove all introduced
sediment, subgrade soil should be removed to a depth of 3–6 inches and replaced with BSM); and
(c) be inspected by the responsible engineer to verify required subgrade condition.
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7. If using the default bioretention soil mix described in Reference Section 11-C, pre-placement
laboratory analysis for saturated hydraulic conductivity of the bioretention soil mix is not required.
Verification of the mineral aggregate gradation, compliance with the compost specifications, and the
mix ratio must be provided.
8. Custom bioretention soil mixes may be considered under the adjustment process described in
Section 1.4.
9. Bioretention constructed with imported compost materials are not allowed within one-quarter mile of
a sensitive lake if the underlying native soil does not meet the soil suitability criteria for treatment in
Section 5.2.1.
10. Bioretention constructed with imported compost materials are not allowed within ¼ mile of those
waterbodies listed as category 2, 4, or 5 for either nutrients or low DO determined to be caused by
nutrients. These waterbodies are found on Ecology’s combined 303(d)/305(b) Water Quality
Assessment list. The exception to this prohibition is where phosphorous is the identified nutrient and
the underlying native soil meets soil suitability criteria for treatment described in Section 5.2.1.
Subgrade
The minimum measured subgrade infiltration rate for bioretention facilities without underdrains is
0.3 inches per hour. For bioretention facilities with underdrains, there is no minimum subgrade infiltration
rate.
During construction, the subgrade soil surface can become smeared and sealed by excavation equipment.
The design shall require scarification or raking of the side walls and bottom of the bioretention facility
excavation to a minimum depth of 4 inches after excavation to restore infiltration rate.
Follow the process outlined in Section 5.2.1 for determining the design infiltration rate for the subgrade.
Underdrain (if required)
Underdrain systems must be installed if the bioretention facility is:
Located near sensitive infrastructure (e.g., unsealed basements) and potential for flooding is likely
Used for filtering stormwater flows from gas stations or other pollutant hotspots (requires an
impermeable liner)
Located above subgrade soils with a measured infiltration rate of less than 0.3 inches per hour.
In an area that does not provide a minimum of 3 feet of clearance between the lowest elevation of the
bioretention soil mix, or any underlying gravel layer, and the seasonal high groundwater elevation or
other impermeable layer
The underdrain pipe diameter will depend on hydraulic capacity required. The underdrain shall be
connected to an acceptable discharge point which can either be an enclosed drainage system (i.e., pipe
system, culvert, or tightline) or an open drainage feature (e.g., second bioretention cell, ditch, channel).
Requirements associated with the underdrain design include:
Slotted subsurface drain PVC per ASTM D1785 SCH 40.
Slots should be cut perpendicular to the long axis of the pipe and be 0.04 to 0.069 inches by 1 inch
long and be spaced 0.25 inches apart (spaced longitudinally). Slots should be arranged in four rows
spaced on 45-degree centers and cover ½ of the circumference of the pipe.
Underdrain pipe shall have a minimum diameter of 8 inches in the public ROW and 6 inches for
private property.
Underdrain pipe slope shall be no less than 0.5 percent unless otherwise specified by an engineer.
Pipe shall be placed in filter material and have a minimum cover depth of 12 inches and bedding depth
of 6 inches. Cover depth may be reduced up to 6 inches in order to discharge stormwater from the
facility under gravity flow conditions while meeting the applicable engineering standards, if approved
by the City.
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Filter material shall meet the specifications in Table 6.8.1.C.
TABLE 6.8.1.C. UNDERDRAIN AGGREGATE
Sieve Size Percent Passing
¾ inch 100 percent
¼ inch 30 to 60 percent
U.S. No. 8 20 to 50 percent
U.S. No. 50 3 to 12 percent
U.S. No. 200 0 to 1 percent
Underdrains shall have a maintenance access point (e.g., cleanout, observation port, overflow
structure) at each end of a facility and a minimum of every 100 feet along the pipe. Cleanouts and
observation ports shall have locking cast iron caps and shall be constructed of non-perforated pipe
(sized to match the underdrain diameter).
When bioretention facilities with underdrains drain to a retention or detention facility, the subsurface
gravel reservoir beneath the underdrain pipe shall be widened to extend across the entire facility
bottom.
If an orifice is included in the design, the minimum diameter shall be 0.5 inches to minimize clogging
and maintenance requirements.
Overflow
A bioretention facility overflow controls overtopping with a pipe, an earthen channel, a weir, or a curb cut
installed at the designed maximum ponding elevation and is connected to a downstream BMP or an
approved point of discharge.
The minimum requirements associated with the overflow design include the following:
Overflows shall convey any flow exceeding the capacity of the facility.
The overflow point of the water storage area (i.e., freeboard) shall be at least 6 inches below any
adjacent pavement area.
The overflow point must be situated so that overflow does not cause erosion damage or unplanned
inundation
The drain pipe, if used, shall have a minimum diameter of 8 inches in the public ROW and 6 inches
for private property.
Liners (optional)
Adjacent roads, foundations, slopes, utilities, or other infrastructure may require that certain infiltration
pathways are restricted to prevent excessive hydrologic loading. Two types of hydraulic restricting layers
can be incorporated into bioretention facility designs with underdrains:
Clay (bentonite) liners as low permeability liners
Geomembrane liners which completely block flow
Plants
In general, the predominant plantings used in bioretention facilities are species adapted to stresses
associated with wet and dry conditions. Soil moisture conditions will vary within the facility from
saturated (bottom of cell) to relatively dry (rim of cell). Accordingly, wetland plants may be planted in the
lower areas and drought-tolerant species planted on the perimeter of the facility or on mounded areas.
Trees outside of the saturated zone are allowed as part of bioretention facility designs. Trees installed in
the public ROW must also comply with the City’s Street Tree Standards (RMC 4-4-070).
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Requirements associated with the vegetation design include the following:
The design plans shall specify that vegetation coverage of plants will achieve 90 percent coverage
within 2 years. For this purpose, cover is defined as canopy cover and should be measured when
deciduous plants are in bloom.
For facilities receiving runoff from 5,000 square feet or more impervious surface, plant spacing and
plant size shall be designed by a licensed landscape architect to achieve specified coverage.
The plants shall be sited according to sun, soil, wind, and moisture requirements.
At a minimum, provisions shall be made for supplemental irrigation/watering during the first two
growing seasons following installation and in subsequent periods of drought.
Water tolerant plants shall be planted in the pond bottom.
Plants native to Western Washington are preferred.
Mulch
Properly selected organic mulch material reduces weed establishment, regulates soil temperatures and
moisture, and adds organic matter to the soil. Compost and arborist wood chip mulch are required for
different applications within the bioretention cell. Compost mulch is an excellent slow-release source of
plant nutrients and does not float, but compost does not suppress weed growth as well as bulkier, higher
carbon mulches like arborist wood chips. Arborist wood chips are superior to bark mulch in promoting
plant growth, feeding beneficial soil organisms, reducing plant water stress, and maintaining surface soil
porosity.
Requirements associated with organic mulch include:
Organic mulch in the bottom of the cell and up to the ponding elevation shall consist of coarse
compost. Coarse compost shall meet the requirements for fine compost provided in Reference Section
11-C and the following gradation by dry weight:
Sieve Size
Percent Passing
Minimum Maximum
3″ 100%
1″ 90% 100%
3/4″ 70% 100%
1/4″ 40% 6%
Organic mulch on cell slopes above the ponding elevation and the around the rim area shall consist of
arborist wood chip mulch. Arborist wood chip mulch shall meet the criteria below:
o Arborist wood chip mulch shall be coarse ground wood chips (approximately 0.5 inch to 6 inches
along the longest dimension) derived from the mechanical grinding or shredding of the
aboveground portions of trees. It may contain wood, wood fiber, bark, branches, and leaves; but
may not contain visible amounts of soil. It shall be free of weeds and weed seeds Including but
not limited to plants on the King County Noxious Weed list available at:
<www.kingcounty.gov/weeds>, and shall be free of invasive plant portions capable of
resprouting, including but not limited to horsetail, ivy, clematis, knotweed, etc. It may not contain
more than 0.5 percent by weight of manufactured inert material (plastic, concrete, ceramics,
metal, etc.).
o Arborist wood chip mulch, when tested, shall meet the following loose volume gradation:
Sieve Size
Percent Passing
Minimum Maximum
2″ 95 100
1″ 70 100
5/8″ 0 50
1/4″ 0 40
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No particles may be longer than eight inches.
A minimum of 2 inches and a maximum of 3 inches for both types of organic mulch
In bioretention areas where higher flow velocities are anticipated, an aggregate mulch may be used to
dissipate flow energy and protect underlying bioretention soil. Aggregate mulch varies in size and type,
but 1- to 1.5-inch gravel (rounded) decorative rock is typical. The aggregate mulch shall be washed rock
(free of fines) and the area covered with aggregate mulch shall not exceed one-fourth of the facility bottom
area.
As an alternative to mulch, a dense groundcover may be used. Mulch is required in conjunction with the
groundcover until groundcover is established.
Check Dams and Weirs
Check dams are necessary for reducing flow velocity and potential erosion, as well as increasing detention
time and infiltration capability on sloped sites. Typical materials include concrete, rock, compacted dense
soil covered with vegetation, and vegetated hedge rows. Design depends on flow control goals, local
regulations for structures within road right-of-ways and aesthetics. Optimum spacing is determined by
modeling and cost considerations.
UIC Discharge
Stormwater that has passed through the bioretention soil mix may also discharge to a gravel-filled dug or
drilled drain. Underground Injection Control (UIC) regulations are applicable and must be followed
(Chapter 173-218 WAC).
6.8.1.2 INSTALLATION
Excavation
Soil compaction can lead to facility failure; accordingly, minimizing compaction of the base and sidewalls
of the bioretention area is critical. Excavation should never be allowed during wet or saturated conditions
(compaction can reach depths of 2-3 feet during wet conditions and mitigation is likely not be possible).
Excavation should be performed by machinery operating adjacent to the bioretention facility and no heavy
equipment with narrow tracks, narrow tires, or large lugged, high pressure tires should be allowed on the
bottom of the bioretention facility. If machinery must operate in the bioretention cell for excavation, use
light weight, low ground-contact pressure equipment and rip the base at completion to refracture soil to a
minimum of 12 inches. If machinery operates in the facility, subgrade infiltration rates must be field tested
and compared to design rates. Failure to meet or exceed the design infiltration rate will require revised
engineering designs to verify achievement of treatment and flow control benefits that were estimated in the
Stormwater Site Plan.
Prior to placement of the bioretention soil mix, the finished subgrade shall:
Be scarified to a minimum depth of 3 inches.
Have any sediment deposited from construction runoff removed. To remove all introduced sediment,
subgrade soil should be removed to a depth of 3–6 inches and replaced with bioretention soil mix.
Be inspected by the responsible engineer to verify required subgrade condition.
Sidewalls of the facility, beneath the surface of the bioretention soil mix, can be vertical if soil stability is
adequate. Exposed sidewalls of the completed bioretention area with bioretention soil mix in place should
be no steeper than 3H:1V. The bottom of the facility should be flat.
Soil Placement
Onsite soil mixing or placement shall not be performed if bioretention soil mix or subgrade soil is
saturated. The bioretention soil mixture should be placed and graded by machinery operating adjacent to
the bioretention facility. If machinery must operate in the bioretention cell for soil placement, use light
weight equipment with low ground-contact pressure. If machinery operates in the facility, subgrade
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infiltration rates must be field tested and compared to design rates. Failure to meet or exceed the design
infiltration rate will require revised engineering designs to verify achievement of treatment and flow
control benefits that were estimated in the Stormwater Site Plan.
The soil mixture shall be placed in horizontal layers not to exceed 12 inches per lift for the entire area of
the bioretention facility.
Compact the bioretention soil mix to a relative compaction of 85 percent of modified maximum dry
density (ASTM D 1557). Compaction can be achieved by boot packing (simply walking over all areas of
each lift), and then apply 0.2 inches (0.5 cm) of water per 1 inch (2.5 cm) of bioretention soil mix depth.
Water for settling should be applied by spraying or sprinkling.
Temporary Erosion and Sediment Control (TESC)
Controlling erosion and sediment are most difficult during clearing, grading, and construction;
accordingly, minimizing site disturbance to the greatest extent practicable is the most effective sediment
management. During construction:
Bioretention facilities should not be used as sediment control facilities and all drainage should be
directed away from bioretention facilities after initial rough grading. Flow can be directed away from
the facility with temporary diversion swales or other approved protection. If introduction of
construction runoff cannot be avoided see below for guidelines.
Construction on bioretention facilities should not begin until all contributing drainage areas are
stabilized according to erosion and sediment control BMPs and to the satisfaction of the engineer.
If the design includes curb and gutter, the curb cuts and inlets should be blocked until bioretention
soil mix and mulch have been placed and planting completed (when possible), and dispersion pads are
in place.
Every effort during design, construction sequencing and construction should be made to prevent sediment
from entering bioretention facilities. However, bioretention areas are often distributed throughout the
project area and can present unique challenges during construction.
Erosion and sediment control practices must be inspected and maintained on a regular basis.
6.8.1.3 VERIFICATION
If using the default bioretention soil mix, pre-placement laboratory analysis for saturated hydraulic
conductivity of the bioretention soil mix is not required. Verification of the mineral aggregate gradation,
compliance with the compost specifications, and the mix ratio must be provided.
If using a custom bioretention soil media, verification of compliance with the minimum design criteria
cited above for such custom mixes must be provided. This will require laboratory testing of the material
that will be used in the installation. Testing shall be performed by a Seal of Testing Assurance, AASHTO,
ASTM or other standards organization accredited laboratory with current and maintained certification.
Samples for testing must be supplied from the bioretention soil mix that will be placed in the bioretention
areas.
If testing infiltration rates is necessary for post-construction verification use the Pilot Infiltration Test
(PIT) method or a double ring infiltrometer test (or other small-scale testing allowed by the local
government with jurisdiction). If using the PIT method, do not excavate bioretention soil mix (conduct
test at level of finished bioretention soil mix elevation), use a maximum of 6 inch ponding depth and
conduct test before plants are installed.
6.8.1.4 RUNOFF MODEL REPRESENTATION IN WWHM2012
Use new bioretention element for each type: cell, swale, or planter box.
The equations used by the elements are intended to simulate the wetting and drying of soil as well as how
the soils function once they are saturated. This group of LID elements uses the modified Green Ampt
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equation to compute the surface infiltration into the amended soil. The water then moves through the top
amended soil layer at the computed rate, determined by Darcy’s and Van Genuchten’s equations. As the
soil approaches field capacity (i.e., gravity head is greater than matric head), the model determines when
water will begin to infiltrate into the second soil layer (lower layer). This occurs when the matric head is
less than the gravity head in the first layer (top layer). The second layer is intended to prevent loss of the
amended soil layer. As the second layer approaches field capacity, the water begins to move into the third
layer – the gravel underlayer. For each layer, the user inputs the depth of the layer and the type of soil.
For the bioretention soil mix, the model will automatically assign pre-determined appropriate values for
parameters that determine water movement through that soil. These include: wilting point, minimum
hydraulic conductivity, maximum saturated hydraulic conductivity, and Van Genuchten number.
If a user opts to use soils that deviate from the bioretention soil mix specifications, the default parameter
values do not apply. The user will have to use the “Gravel trench/bed” element to represent the
bioretention facility and follow the procedures identified for WWHM3 in Section 6.8.1.5.
For bioretention facilities with underdrains, the only volume available for storage (and modeled as storage
as explained herein) is the void space within the aggregate bedding layer below the invert of the drain
pipe. Use 40% void space for the Type 26 mineral aggregate specified in Table 6.8.1.C.
Using one of the procedures explained in Section 5.2.1, estimate the initial measured (a.k.a., short-term)
infiltration rate of the native soils beneath the bioretention soil and any base materials. Because these soils
are protected from fouling, no correction factor will be applied.
6.8.1.5 RUNOFF MODEL REPRESENTATION IN WWHM3
Pothole design (bioretention cells)
Bioretention is represented by using the “Gravel trench/bed” element with a steady-state infiltration rate.
Proper infiltration rate selection is described in Section 5.2.1. The user inputs the dimensions of the gravel
trench. Layer 1 on the input screen is the bioretention soil layer. Enter the soil depth and a porosity of
40%. Layer 2 is the free standing water above the bioretention soil. Enter the maximum depth of free
standing water (i.e., up to the invert of an overflow pipe or a spillway, whatever engages first for surface
release of water), and 100% for porosity. Bioretention with underdrains can also be modeled as a gravel
trench/bed with a steady-state infiltration rate. However, the only volume available for storage (and
modeled as storage as explained herein) is the void space within the imported material (usually sand or
gravel) below the bioretention soil layer and below the invert of the drain pipe.
Using one of the procedures explained in Section 5.2.1, estimate the initial measured (a.k.a., short-term)
infiltration rate of the native soils beneath the bioretention soil and any base materials. Because these soils
are protected from fouling, no correction factor will be applied.
Facilities without an underdrain
If using the default bioretention soil mix, 12 inches per hour is the initial infiltration rate. The long-term
rate is either 3 inches per hour or 6 inches per hour depending upon the size of the drainage area and the
use of a pretreatment device for solids removal prior to the bioretention facility. See Section 5.2.1. If using
a custom imported soil mix other than the default, its saturated hydraulic conductivity (used as the
infiltration rate) must be determined using the procedures described in Section 5.2.1. The long-term
infiltration rate is one-fourth or one-half of that rate depending upon the size of the drainage area and the
use of a pretreatment device for solids removal. See Section 5.2.1.
Facilities with an elevated underdrain
Note that only the estimated void space of the aggregate bedding layer that is below the invert of the
underdrain pipe provides storage volume that provides a flow control benefit. Assume a 40% void volume
for the Type 26 mineral aggregate specified in Table 6.8.1.C.
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Linear Design: (bioretention swale or slopes)
Where a bioretention swale has a roadside slope and a back slope between which water can pond, and an
overflow/drainage pipe at the lower end of the swale, the swale may be modeled as a gravel trench/bed
with a steady state infiltration rate. This method does not apply to bioretention swales that are underlain
by a drainage pipe.
If the long-term infiltration rate through the imported bioretention soil is lower than the infiltration rate of
the underlying soil, the surface dimensions and slopes of the swale should be entered into WWHM3 as the
trench dimensions and slopes. The effective depth is the distance from the soil surface at the bottom of the
swale to the invert of the overflow/drainage pipe. If the infiltration rate through the underlying soil is
lower than the estimated long-term infiltration rate through the imported bioretention soil mix, the gravel
trench/bed dimensions entered into WWHM3 should be adjusted to account for the storage volume in the
void space of the bioretention soil. Use 40 percent porosity for the bioretention soil mix.
This procedure to estimate storage space should only be used on bioretention swales with a 1% slope or
less. Swales with higher slopes should more accurately compute the storage volume in the swale below the
drainage pipe invert.
For a bioretention swale with an underdrain, follow the directions provided above.
WWHM Routing and Runoff File Evaluation
In WWHM3, all infiltrating facilities must have an overflow riser to model overflows that occur should the
available storage be exceeded. In the Riser/Weir screen, for the Riser head enter a value slightly smaller
than the effective depth of the trench (e.g., 0.1 ft below the Effective Depth); and for the Riser diameter
enter a large number (e.g., 10,000 inches) to ensure that there is ample capacity for overflows.
Within the model, route the runoff into the gravel trench by placing the gravel trench/bed element below
the tributary “basin” area. Include the surface area of the bioretention area in the tributary “basin” area.
Run the model to produce the effluent runoff file from the theoretical gravel trench.
6.8.1.6 MODELING OF MULTIPLE BIORETENTION FACILITIES
Where multiple bioretention facilities are scattered throughout a development, it may be possible to
cumulatively represent a group of them that have similar characteristics as one large bioretention facility
serving the cumulative area tributary to those facilities. For this to be a reasonable representation, the
design of each bioretention facility in the group should be similar (e.g., same depth of soil, same depth of
surface ponded water, roughly the same ratio of impervious area to bioretention volume). In addition, the
group should have similar (0.5x to 1.5x the average) controlling infiltration rates (i.e., either the long-term
rate of the bioretention soil mix, or the initial rate of the underlying soil) that can be averaged as a single
infiltration rate.
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6.9 WSDOT WQ FACILITY DESIGNS
This section presents the methods, details of analysis, and design criteria for the following WSDOT WQ
facilities:
“Media Filter Drain (MFD),” Section 6.9.1
“Compost-Amended Vegetated Filter Strips (CAVFS),” Section 6.9.2
“Compost-amended biofiltration swales (CABS),” Section 6.9.3
6.9.1 MEDIA FILTER DRAIN
The media filter drain (MFD), previously referred to as the ecology embankment, is a linear flow-through
stormwater runoff treatment device that can be sited along highway side slopes (conventional design) and
medians (dual MFDs), borrow ditches, or other linear depressions. Cut-slope applications may also be
considered. The MFD can be used where available right of way is limited, sheet flow from the highway
surface is feasible, and lateral gradients are generally less than 25% (4H:1V).
MFDs have four basic components: a gravel no-vegetation zone, a grass strip, the MFD mix bed, and a
conveyance system for flows leaving the MFD mix. This conveyance system usually consists of a gravel-
filled underdrain trench or a layer of crushed surfacing base course (CSBC). This layer of CSBC must be
porous enough to allow treated flows to freely drain away from the MFD mix.
Typical MFD configurations are shown in Figures 6.9.1.A, 6.9.1.B, and 6.9.1.C.
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FIGURE 6.9.1.A MEDIA FILTER DRAIN: SIDE SLOPE APPLICATION WITH UNDERDRAIN
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FIGURE 6.9.1.B DUAL MEDIA FILTER DRAIN: MEDIAN APPLICATION
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FIGURE 6.9.1.C MEDIA FILTER DRAIN: SIDE SLOPE APPLICATION WITHOUT UNDERDRAIN
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Functional Description
The MFD removes suspended solids, phosphorus, and metals from highway runoff through physical
straining, ion exchange, carbonate precipitation, and biofiltration.
Stormwater runoff is conveyed to the MFD via sheet flow over a vegetation-free gravel zone to ensure
sheet dispersion and provide some pollutant trapping. Next, a grass strip, which may be amended with
composted material, is incorporated into the top of the fill slope to provide pretreatment, further enhancing
filtration and extending the life of the system. The runoff is then filtered through a bed of porous,
alkalinity-generating granular medium—the MFD mix. MFD mix is a fill material composed of crushed
rock (sized by screening), dolomite, gypsum, and perlite. The dolomite and gypsum additives serve to
buffer acidic pH conditions and exchange light metals for heavy metals. Perlite is incorporated to improve
moisture retention, which is critical for the formation of biomass epilithic biofilm to assist in the removal
of solids, metals, and nutrients. Treated water drains from the MFD mix bed into the conveyance system
below the MFD mix. Geotextile lines the underside of the MFD mix bed and the conveyance system.
The underdrain trench is an option for hydraulic conveyance of treated stormwater to a desired location,
such as a downstream flow control facility or stormwater outfall. The trench’s perforated underdrain pipe
is a protective measure to ensure free flow through the MFD mix and to prevent prolonged ponding. It
may be possible to omit the underdrain pipe if it can be demonstrated that the pipe is not necessary to
maintain free flow through the MFD mix and underdrain trench.
It is critical to note that water should sheet flow across the MFD. Channelized flows or ditch flows running
down the middle of the dual MFD (continuous off-site inflow) should be minimized.
Applications and Limitations
In many instances, conventional runoff treatment is not feasible due to right of way constraints (such as
adjoining wetlands and geotechnical considerations). The MFD and the dual MFD designs are runoff
treatment options that can be sited in most right of way confined situations. In many cases, a MFD or a
dual MFD can be sited without the acquisition of additional right of way needed for conventional
stormwater facilities or capital-intensive expenditures for underground wet vaults.
Media Filter Drains
The longest flow path from the contributing area delivering sheet flow to the MFD should not exceed
150 feet.
If there is sufficient roadway embankment width, the designer should consider placing the grass strip
and MFD mix downslope when feasible.
Steep slopes. Avoid construction on longitudinal slopes steeper than 5%. As side slopes approach
3H:1V, without design modifications, sloughing may become a problem due to friction limitations
between the separation geotextile and underlying soils. Avoid construction on 3H:1V lateral slopes,
and preferably use less than 4H:1V slopes. In areas where lateral slopes exceed 4H:1V, it may be
possible to construct terraces to create 4H:1V slopes or to otherwise stabilize up to 3H:1V slopes.
Wetlands. Do not construct in wetlands and wetland buffers. In many cases, a MFD (due to its small
lateral footprint) can fit within the highway fill slopes adjacent to a wetland buffer. In those situations
where the highway fill prism is located adjacent to wetlands, an interception trench/underdrain will
need to be incorporated as a design element in the MFD.
Shallow ground water. The designer should ensure the MFD does not intercept seeps, springs, or
ground water. Mean high water table levels at the project site need to be determined to ensure the
MFD mix bed and the underdrain (if needed) will not become saturated by shallow ground water.
Unstable slopes. In areas where slope stability may be problematic, consult a geotechnical engineer.
Areas of seasonal ground water inundations or basement flooding. Site-specific piezometer data
may be needed in areas of suspected seasonal high ground water inundations. The hydraulic and
runoff treatment performance of the dual MFD may be compromised due to backwater effects and
lack of sufficient hydraulic gradient.
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Narrow roadway shoulders. In areas where there is a narrow roadway shoulder that does not allow
enough room for a vehicle to fully stop or park, consider placing the MFD farther down the
embankment slope. This will reduce the amount of rutting in the MFD and decrease overall
maintenance repairs.
Dual Media Filter Drain for Medians
The dual MFD is fundamentally the same as the side-slope version. It differs in siting and is more
constrained with regard to drainage options. Prime locations for dual MFDs are medians, roadside
drainage or borrow ditches, or other linear depressions. It is especially critical for water to sheet flow
across the dual MFD. Channelized flows or ditch flows running down the middle of the dual MFD
(continuous off-site inflow) should be minimized.
6.9.1.1 METHODS OF ANALYSIS
Media Filter Drain Mix Bed Sizing Procedure
The basic design concept behind the MFD and dual MFD is to fully filter all runoff through the MFD mix.
Therefore, the infiltration capacity of the medium and drainage below needs to match or exceed the
hydraulic loading rate. The MFD mix should be a minimum of 12 inches deep, including the section on
top of the underdrain trench. Table 6.9.1.1.A was developed to simplify the design steps and should be
used to establish an appropriate width. In general, the length of a MFD or dual MFD is the same as the
contributing pavement. Any length is acceptable as long as the surface area MFD mix bed is sufficient to
fully infiltrate the runoff treatment design flow rate.
TABLE 6.9.1.1.A DESIGN WIDTHS FOR MEDIA FILTER DRAINS
Pavement Width that Contributes Runoff to the
Media Filter Drain Minimum Media Filter Drain Width*
≤ 20 feet 2 feet
≥ 20 and ≤ 35 feet 3 feet
> 35 feet 4 feet
*Width does not include the required 1- to 3-foot gravel vegetation-free zone or the 3-foot filter strip width
(see Figure 6.9.1.A).
Underdrain Design
Underdrain pipe can provide a protective measure to ensure free flow through the MFD mix and is sized
similar to storm drains. For MFD underdrain sizing, an additional step is required to determine the flow
rate that can reach the underdrain pipe. This is done by comparing the contributing basin flow rate to the
infiltration flow rate through the media filter mix and then using the smaller of the two to size the
underdrain. The analysis described below considers the flow rate per foot of MFD, which allows you the
flexibility of incrementally increasing the underdrain diameter where long lengths of underdrain are
required. When underdrain pipe connects to a storm drain system, place the invert of the underdrain pipe
above the 25-year water surface elevation in the storm drain to prevent backflow into the underdrain
system.
The following describes the procedure for sizing underdrains installed in combination with MFDs.
1. Calculate the flow rate per foot from the contributing basin to the MFD. The design storm event used
to determine the flow rate should be relevant to the purpose of the underdrain. For example, if the
underdrain will be used to convey treated runoff to a detention facility, size the underdrain for the
50-year storm event. (See Chapter 4, for conveyance flow rate determination.)
AGENDA ITEM # 8. a)
SECTION 6.9.1 MEDIA FILTER DRAIN
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𝑄௪
𝑓𝑡=𝑄௪௬
𝐿ெி
where:
ொೢೌ
௧ = contributing flow rate per foot (cfs/ft)
LMFD = length of MFD contributing runoff to the underdrain (ft)
2. Calculate the MFD flow rate of runoff per foot given an infiltration rate of 10 in/hr through the MFD
mix.
sec3600
1
12
11 hr
in
ft
ft
ftWfQ
ft
MFD
where:
= flow rate of runoff through MFD mix layer (cfs/ft)
W = width of underdrain trench (ft); the minimum width is 2 ft
f = infiltration rate though the MFD mix (in/hr) = 10 in/hr
Size the underdrain pipe to convey the runoff that can reach the underdrain trench. This is taken to be the
smaller of the contributing basin flow rate or the flow rate through the MFD mix layer.
𝑄
௧
= 𝑠𝑚𝑎𝑙𝑙𝑒𝑟 ቊ𝑄௪
௧
𝑜𝑟 𝑄ெி
௧
ቋ
where:
𝑄ೆವ
= underdrain design flow rate per foot (cfs/ft)
3. Determine the underdrain design flow rate using the length of the MFD and a factor of safety of 1.2.
MFD
ft
UDUD LWQQ2.1
where:
QUD = estimated flow rate to the underdrain (cfs)
W = width of the underdrain trench (ft); the minimum width is 2 ft
LMFD = length of MFD contributing runoff to the underdrain (ft)
ft
MFDQ
AGENDA ITEM # 8. a)
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4. Given the underdrain design flow rate, determine the underdrain diameter. Round pipe diameters to
the nearest standard pipe size and have a minimum diameter of 6 inches. For diameters that exceed
12 inches, contact the City.
8/3
5.0
)(16
s
nQDUD
where:
D = underdrain pipe diameter (inches)
n = Manning’s coefficient
s = slope of pipe (ft/ft)
6.9.1.2 DESIGN CRITERIA
Design criteria are provided in this section for the following elements:
Inflow
No-vegetation zone
Grass strip
Media filter drain mix bed
Conveyance system below media filter drain mix bed
Side slopes
Signage
Inflow
Runoff is conveyed to a MFD using sheet flow from the pavement area. The longitudinal pavement slope
contributing flow to a MFD should be less than 5%.
Although there is no lateral pavement slope restriction for flows going to a MFD, the designer should
ensure flows remain as sheet flow.
No-Vegetation Zone
The no-vegetation zone (vegetation-free zone) is a shallow gravel zone located directly adjacent to the
highway pavement. The no-vegetation zone is a crucial element in a properly functioning MFD or other
BMPs that use sheet flow to convey runoff from the roadway surface to the BMP. The no-vegetation zone
functions as a level spreader to promote sheet flow and a deposition area for coarse sediments. The no-
vegetation zone should be between 1 foot and 3 feet wide. Depth will be a function of how the roadway
section is built from subgrade to finish grade; the resultant cross section will typically be triangular to
trapezoidal. Within these bounds, width varies depending on maintenance spraying practices.
Grass Strip
The width of the grass strip is dependent on the availability of space within the side slope. The baseline
design criterion for the grass strip within the MFD is a 3-foot minimum width, but wider grass strips are
recommended if the additional space is available. The designer may consider adding aggregate to the soil
mix to help minimize rutting problems from errant vehicles. The soil mix should ensure grass growth for
the design life of the MFD. Composted material used in the grass strip shall meet the specifications for
compost in Reference Section 11-C. Landscaping for the grass strip is the same as for bioswales unless
otherwise specified in the special provisions for the project’s construction documents.
Media Filter Drain Mix Bed
The MFD mix is a mixture of crushed rock, dolomite, gypsum, and perlite as listed in Table 6.9.1.2.A. The
MFD mix has an estimated initial filtration rate of 50 inches per hour and a long-term filtration rate of
28 inches per hour due to siltation. With an additional safety factor, the rate used to size the length of the
AGENDA ITEM # 8. a)
SECTION 6.9.1 MEDIA FILTER DRAIN
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MFD should be 10 inches per hour. Mixing and transportation must occur in a manner that ensures the
materials are thoroughly mixed prior to placement and that separation does not occur during transportation
or construction operations.
Conveyance System Below Media Filter Drain Mix
The gravel underdrain trench provides hydraulic conveyance when treated runoff needs to be conveyed to
a desired location such as a downstream flow control facility or stormwater outfall.
In Group C and D soils, an underdrain pipe would help to ensure free flow of the treated runoff through
the MFD mix bed. In some Group A and B soils, an underdrain pipe may be unnecessary if most water
percolates into subsoil from the underdrain trench. The need for underdrain pipe should be evaluated in all
cases. The underdrain trench should be a minimum of 2 feet wide for either the conventional or the dual
MFD.
The gravel underdrain trench may be eliminated if there is evidence to support that flows can be conveyed
laterally to an adjacent ditch or onto a fill slope that is properly vegetated to protect against erosion. The
MFD mix should be kept free draining up to the 50-year storm event water surface elevation represented in
the downstream ditch.
Side Slopes
In profile, the surface of the MFD should preferably have a lateral slope less than 4H:1V (<25%). On
steeper terrain, it may be possible to construct terraces to create a 4H:1V slope, or other engineering may
be employed if approved by the City, to ensure slope stability up to 3H:1V. If sloughing is a concern on
steeper slopes, consideration should be given to incorporating permeable soil reinforcements, such as
geotextiles, open-graded/permeable pavements, or commercially available ring and grid reinforcement
structures, as top layer components to the MFD mix bed. Consultation with a geotechnical engineer is
required.
Signage
Nonreflective guideposts will delineate the MFD. This practice allows personnel to identify where the
system is installed and to make appropriate repairs should damage occur to the system. If the MFD is
located in an Aquifer Protection Area, signage prohibiting the use of pesticides must be provided.
AGENDA ITEM # 8. a)
SECTION 6.9 WSDOT WQ FACILITY DESIGNS
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TABLE 6.9.1.2.A MEDIA FILTER DRAIN MIX
Amendment Quantity
Mineral Aggregate: Aggregate for Media Filter Drain Mix
Aggregate for MFD Mix shall be manufactured from ledge rock,
talus, or gravel in accordance with the WSDOT Standard
Specifications for Road, Bridge, and Municipal Construction,
which meets the following test requirements for quality. The use
of recycled material is not permitted:
Los Angeles Wear, 500 Revolutions 35% max.
Degradation Factor 30 min.
Aggregate for the MFD Mix shall conform to the following
requirements for grading and quality:
3 cubic yards
Sieve Size Percent Passing (by weight)
1/2″ square
3/8″ square
U.S. No. 4
U.S. No. 1
U.S. No. 200
100
90–100
30–56
0 0–10
0–1.5
% fracture, by weight, min. 75
Static stripping test Pass
The fracture requirement shall be at least two fractured faces
and will apply to material retained on the U.S. No. 10.
Aggregate for the MFD shall be substantially free from adherent
coatings. The presence of a thin, firmly adhering film of
weathered rock shall not be considered as coating unless it
exists on more than 50% of the surface area of any size
between successive laboratory sieves.
Perlite:
Horticultural grade, free of any toxic materials)
0–30% passing US No. 18 Sieve
0–10% passing US No. 30 Sieve
1 cubic yard per 3 cubic yards of mineral
aggregate
Dolomite: CaMg(CO3)2 (calcium magnesium carbonate)
Agricultural grade, free of any toxic materials)
100% passing US No. 8 Sieve
0% passing US No. 16 Sieve
10 pounds per cubic yard of perlite
Gypsum: Noncalcined, agricultural gypsum CaSO4•2H2O
(hydrated calcium sulfate)
Agricultural grade, free of any toxic materials)
100% passing US No. 8 Sieve
0% passing US No. 16 Sieve
1.5 pounds per cubic yard of perlite
AGENDA ITEM # 8. a)
SECTION 6.9.2 COMPOST-AMENDED FILTER STRIPS
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6.9.2 COMPOST-AMENDED FILTER STRIPS
The CAVFS is a variation of the basic vegetated filter strip that adds soil amendments to the roadside
embankment (See Figure 6.9.2.A). The soil amendments improve infiltration characteristics, increase
surface roughness, and improve plant sustainability. Once permanent vegetation is established, the
advantages of the CAVFS are higher surface roughness; greater retention and infiltration capacity;
improved removal of soluble cationic contaminants through sorption; improved overall vegetative health;
and a reduction of invasive weeds. Compost-amended systems have somewhat higher construction costs
due to more expensive materials, but require less land area for runoff treatment, which can reduce overall
costs.
6.9.2.1 METHODS OF ANALYSIS
Use the “CAVFS” element in an approved continuous runoff model to determine the amount of water that
is treated by the CAVFS. To fully meet treatment requirements, 91 percent of the influent runoff file must
pass through the soil profile of the CAVFS. Water that merely flows over the surface is not considered
treated. Approved continuous runoff models should be able to report the amount of water that it estimates
will pass through the soil profile.
6.9.2.2 DESIGN CRITERIA
Soil Component
The texture for the soil component should be loamy sand (USDA Soil Textural Classification).
Compost Component
Compost shall be per the specifications in Reference Section 11-C.
Compost must not contain biosolids, manure, any street or highway sweepings, or any catch basin
solids.
Soil/Compost Mix
Presumptive approach: Place and rototill 1.75 inches of composted material into 6.25 inches of soil (a
total amended depth of about 9.5 inches), for a settled depth of 8 inches. Water or roll to compact soil
to 85% maximum. Plant grass.
Custom approach: Place and rototill the calculated amount of composted material into a depth of soil
needed to achieve 8 inches of settled soil at 5% organic content. Water or roll to compact soil to 85%
maximum. Plant grass. The amount of compost or other soil amendments used varies by soil type and
organic matter content. If there is a good possibility that site conditions may already contain a
relatively high organic content, then it may be possible to modify the pre-approved rate described
above and still be able to achieve the 5% organic content target.
The final soil mix (including compost and soil) should have an initial saturated hydraulic conductivity
less than 12 inches per hour, and a minimum long-term hydraulic conductivity of 1.0 inch/hour per
ASTM Designation D 2434 (Standard Test Method for Permeability of Granular Soils) at 85%
compaction per ASTM Designation D 1557 (Standard Test Method for Laboratory Compaction
Characteristics of Soil Using Modified Effort. Infiltration rate and hydraulic conductivity are assumed
to be approximately the same in a uniform mix soil. Note: Long term saturated hydraulic conductivity
is determined by applying the appropriate infiltration correction factors as explained in Section 5.2.1.
The final soil mixture should have a minimum organic content of 5% by dry weight per ASTM
Designation D 2974 (Standard Test Method for Moisture, Ash and Organic Matter of Peat and Other
Organic Soils).
Achieving the above recommendations will depend on the specific soil and compost characteristics. In
general, the recommendation can be achieved with 60% to 65% loamy sand mixed with 25% to 30%
compost or 30% sandy loam, 30% coarse sand, and 30% compost.
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The final soil mixture should be tested prior to installation for fertility, micronutrient analysis, and
organic material content.
Clay content for the final soil mix should be less than 5%.
The pH for the soil mix should be between 5.5 and 7.0. If the pH falls outside the acceptable range, it
may be modified with lime to increase the pH or iron sulfate plus sulfur to lower the pH. The lime or
iron sulfate must be mixed uniformly into the soil.
The soil mix should be uniform and free of stones, stumps, roots, or other similar material larger than
2 inches.
When placing topsoil, it is important that the first lift of topsoil is mixed into the top of the existing
soil. This allows the roots to penetrate the underlying soil easier and helps prevent the formation of a
slip plane between the two soil layers.
6.9.3 COMPOST-AMENDED BIOFILTRATION SWALES
The CABS is a variation of the basic biofiltration swale (bioswale) that adds soil amendments. The soil
amendments improve infiltration characteristics, increase surface roughness, and improve plant
sustainability. Once permanent vegetation is established, the advantages of the CABS are higher surface
roughness; greater retention and infiltration capacity; improved removal of soluble cationic contaminants
through sorption; improved overall vegetative health; and a reduction of invasive weeds. Compost-
amended systems have somewhat higher construction costs due to more expensive materials, but require
less land area for runoff treatment, which can reduce overall costs.
6.9.3.1 METHODS OF ANALYSIS
Follow the methods of analysis outlined in Section 6.3.1 for Basic Bioswales.
6.9.3.2 DESIGN CRITERIA
Follow the design criteria outlined in Section 6.3.1 for Basic Bioswales with the addition of a compost
blanket with the following requirements:
Compost Component
Compost depth shall be 3 inches
Compost shall be per the specifications in Reference Section 11-C.
Compost must not contain biosolids, manure, any street or highway sweepings, or any catch basin
solids.
AGENDA ITEM # 8. a)
CITY OF RENTON SURFACE WATER DESIGN MANUAL
2022 City of Renton Surface Water Design Manual 6/22/2022
1
DEFINITIONS
Note: The following terms are provided for reference and use with this manual. They shall be superseded
by any other definitions for these terms adopted by ordinance.
Acceptable discharge point means an enclosed drainage system (i.e., pipe system, culvert, or tightline) or
open drainage feature (e.g., ditch, channel, swale, stream, river, pond, lake, or wetland) where
concentrated runoff can be discharged without creating a significant adverse impact.
Adjustment means a department-approved variation in the application of the requirements of
RMC 4-6-030 and this manual.
Alkalinity means a measure of the acid neutralizing capacity of water; the ability of a solution to resist
changes in pH by neutralizing acidic input.
Alluvial soil means a soil found in valley bottoms that is generally fine-grained and often has a high
seasonal water table.
Anadromous fish means fish that live as adults in saltwater and migrate up freshwater streams and rivers
for spawning.
Applicant means a property owner or a public agency or public or private utility that owns a right-of-way
or other easement or has been adjudicated the right to such an easement under RCW 8.12.090, or any
person or entity designated or named in writing by the property or easement owner to be the applicant,
in an application for a development proposal, permit, or approval.
Appurtenances means machinery, appliances, or auxiliary structures attached to a main structure, but not
considered an integral part thereof, for the purpose of enabling it to function.
Aquatic area means any non-wetland water feature including all shorelines of the state, rivers, streams,
marine waters, inland bodies of open water including lakes and ponds, reservoirs and conveyance
systems and impoundments of these features if any portion of the feature is formed from a stream or
wetland and if any stream or wetland contributing flows is not created solely as a consequence of
stormwater pond construction. Aquatic area does not include water features that are entirely
artificially collected or conveyed storm or wastewater systems or entirely artificial channels, ponds,
pools or other similar constructed water features.
Aquifer means a geologic stratum containing groundwater that can be withdrawn and used for human
purposes.
Aquifer Protection Area (APA) means the portion of an aquifer within the zone of capture and recharge
area for a well or well field owned or operated by the City of Renton, as depicted in the Wellhead
Protection Area Zones layer of COR Maps
(<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>).
Area-specific flow control facility requirement means the requirement of an onsite flow control facility
or facilities designed in accordance with the performance criteria, target surfaces, and exceptions
specified for the mapped flow control area in which a proposed project is located.
Area-specific water quality facility requirement means the requirement of an onsite water quality
facility or facilities designed in accordance with the treatment menu, target surfaces, and exceptions
specified for the mapped water quality treatment area in which a proposed project is located.
Arterial – A high traffic-volume road or street primarily for through traffic. The term generally includes
roads or streets considered collectors. It does not include local access roads which are generally
limited to providing access to abutting property. Arterial streets are depicted in the Arterials layer of
COR Maps (<https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps>).
AGENDA ITEM # 8. a)
CHAPTER 1 DRAINAGE REVIEW AND REQUIREMENTS
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As-built drawings means engineering plans which have been revised to reflect all changes to the plans
which occurred during construction.
Back-up system means a retention/detention facility where inflows are routed through the control
structure before entering the facility; they are “backed up” into the facility by the flow restrictor.
Backwater means water upstream from an obstruction that is deeper than it would normally be without
the obstruction.
Bacteria problem means a stream reach, lake, or other waterbody of the state that is either (1) currently
designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern for
exceedance of the state’s numeric water quality standard for fecal coliform bacteria as documented in
the state’s latest Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA
Ecology’s electronic database and map viewers1 of these waterbodies, or (2) where subject to any
other local, state, or federal cleanup plan or contaminated site designation for fecal coliform.
Baffle means a device, usually a flow-directing or impeding panel, used to deflect, check or regulate flow.
Base flood means a flood having a one percent chance of being equaled or exceeded in any given year;
also referred to as the 100-year flood. The base flood is determined for existing conditions, unless a
basin plan including projected flows under future developed conditions has been completed and
adopted by the City, in which case these future flow projections shall be used. In areas where the
Flood Insurance Study includes detailed base flood calculations, those calculations may be used until
projections of future flows are completed and approved by the City.
Base flood elevation means the water surface elevation of the base flood. It shall be referenced to either
the North American Vertical Datum of 1988 (NAVD88) or the National Geodetic Vertical Datum of
1929 (NGVD), depending on the datum used in the relative FEMA flood insurance rate map (FIRM)2.
Basin means a geographic area that contains and drains to a stream or river named and noted on common
maps, such as the Cedar River, Sammamish River, Green River, Snoqualmie River, Skykomish River,
or White River, or a geographic area that drains to a non-flowing water body named and noted on
common maps, such as Lake Washington or Puget Sound.
Basin plan means a plan and all implementing regulations and procedures including, but not limited to,
capital projects, public education activities, land use management adopted by ordinance for managing
surface and storm water within the basin.
Berm means a constructed mound of earth or other material used to confine, control, spread, or filter
water.
Best management practice (BMP) means any schedule of activities, prohibition of practices,
maintenance procedure, or structural and/or managerial practice approved by the City that, when used
singly or in combination, prevents or reduces the release of pollutants and other adverse impacts to
surface water, stormwater and groundwater.
Bioswale means a long, gently sloped, vegetated ditch designed to remove pollutants from stormwater.
Grass is the most common vegetation, but wetland vegetation can be used if the soil is saturated.
Bioretention – An on-site and water quality treatment best management practice consisting of a shallow
landscaped depression designed to temporarily store and promote infiltration of stormwater runoff.
Standards for bioretention design, including soil mix, plants, storage volume and feasibility criteria,
are specified in Appendix C of this manual. Bioretention can be used to meet Core Requirement #3, 8,
and/or 9.
1 The link to the Query Tool is <https://apps.ecology.wa.gov/ApprovedWQA/ApprovedPages/ApprovedSearch.aspx>; select all
appropriate mediums. The Map Tool is at <https://apps.ecology.wa.gov/waterqualityatlas/wqa/map>.
2 See <http://www.fema.gov/media-library-data/e0431351fd0536694a66cef26268a694/440+NGVD-NAVD+5-09+508OK.pdf > for
discussion of the datum conversion from NGVD29 to NAVD88.
AGENDA ITEM # 8. a)
CHAPTER 1—KEY TERMS AND DEFINITIONS
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Blanket adjustment means an adjustment established by the City that can be applied routinely or globally
to all projects where appropriate. Blanket adjustments are usually based on a previously approved
adjustment and can be used to effect minor changes or corrections to the design requirements of this
manual, or to add new designs and methodologies to this manual.
Blind, blinding means to severely reduce the ability of a normally infiltrative media to pass water, usually
by plugging with sediment or debris.
BMP means best management practice.
Bollard means a post used to prevent vehicular access. A bollard may or may not be removable.
BSBL means building setback line.
Buffer means a designated area contiguous to a steep slope or landslide hazard area intended to protect
slope stability, attenuation of surface water flows, and landslide hazards, or a designated area
contiguous to and intended to protect and be an integral part of an aquatic area or wetland
Building setback line means a line measured parallel to a property, easement, drainage facility, or buffer
boundary that delineates the area (defined by the distance of separation) where buildings or other
obstructions are prohibited (including decks, patios, outbuildings, or overhangs beyond 18 inches).
Wooden or chain link fences and landscaping are allowable within a building setback line. In this
manual the minimum building setback line shall be 5 feet.
Catch basin means a chamber or well, usually built at the curb line of a street, for the admission of
surface water to a sewer or subdrain, having at its base a sediment sump designed to retain grit and
detritus below the point of overflow.
Catch basin insert means a device installed underneath a catch basin inlet that uses gravity, filtration, or
various sorbent materials to remove pollutants from stormwater. When used with sorbent material,
catch basin inserts are primarily for oil removal.
Catch line means the point where a severe slope intercepts a different, gentler slope.
Cation exchange means “The interchange between a cation in solution and another cation on the surface
of any surface-active material such as clay or organic matter.” (Buckman & Brady, 1969)
Cation exchange capacity (CEC) means the quantity of ammonium cations in a dry mass saturated with
ammonium acetate that can be displaced by a strong solution of NaCl, measured in milliequivalents
per gram or 100 grams. The test is usually performed at neutral pH (Freeze & Cherry, Groundwater,
1979).
CED means the Community and Economic Development Department.
Certified Erosion and Sediment Control Lead (CESCL) means an individual who has current
certification through an approved erosion and sediment control training program that meets the
minimum training standards established by the Washington State Department of Ecology (Ecology). A
CESCL is knowledgeable in the principles and practices of erosion and sediment control. The CESCL
must have the skills to assess site conditions and construction activities that could impact the quality
of stormwater and, the effectiveness of erosion and sediment control measures used to control the
quality of stormwater discharges. Certification is obtained through an Ecology approved erosion and
sediment control course.
Channel means a long, narrow excavation or surface feature that conveys surface water and is open to the
air.
Channel, constructed means a channel or ditch constructed to convey surface water; also includes
reconstructed natural channels.
Channel, natural means a channel that has occurred naturally due to the flow of surface waters or a
channel that, although originally constructed by human activity, has taken on the appearance of a
natural channel including a stable route and biological community.
AGENDA ITEM # 8. a)