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TECHNICAL INFORMATION REPORT
FOR
Nguyen’s Family Short Plat
Located at 2309 Aberdeen Avenue NE
Renton, Washington
April, 2021
4/16/2021
EXHIBIT 6
RECEIVED
06/02/2021 jding
PLANNING DIVISION
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TABLE OF CONTENTS
1.0 Project Overview………………………………………………………..…….…...4
1.1 General……………………………………………………………………………4
1.2 Pre-Developed Condition……………………………………………………...…4
1.3 Proposed Development…………………………………………………………...4
1.4 Proposed Storm Drainage Facilities………………………………………………5
1.5 Soils………………………………………………………………………………5
2.0 Conditions and Requirements Summary…………………………………..…….7
2.1 Conditions and Requirements…………………………………………………….7
2.1.1 Core Requirements……………………………………………………………7
2.1.2 Special Requirements…………………………………………………………8
3.0 Offsite Analysis…………………………………………………………………....10
3.1 Standard Requirements…………………………………………………..…..….10
3.2 Scope of Analysis…………………………………………………………….....10
3.2.1 Resource Review……………………………………………………………10
3.2.2 Field Inspection…………………………………………………………..…10
3.2.3 Drainage System Description and Problem Descriptions………………..…10
3.2.4 Mitigation of Existing or Potential Problems………………………………10
4.0 Flow Control and Water Quality Facility Analysis and Design………..………11
4.1 Existing Site Hydrology…………………………………………………………11
4.2 Proposed Site Hydrology………………………………………………………..11
4.3 Performance Standards………………………………………………………….11
4.4 Flow Control Facilities………………………………………………………….12
4.5 Water Quality Facilities…………………………………………………………12
5.0 Conveyance System Analysis and Design……………………………….……….13
5.1 Standards Requirements…………………………………………………………13
5.2 Proposed Conveyance System…………………………………………….….…13
6.0 Special Reports and Studies……………………………………………………...14
6.1 Geotechnical…………………………………………………………………….14
7.0 Other Permits……………………..……………………………………………….15
7.1 NPDES Permit…………………………………………………………………..15
8.0 CSWPPP Analysis and Design…………………………………………….……..16
8.1 ESC Plan………………………………………………………………………...16
9.0 Bond Quantities, Facility Summaries, and
Declaration of Covenant……………………………………………………….…17
9.1 Bond Quantities Plan………………………………………………………........17
9.2 Declaration of Covenant for Privately Maintained Flow Control BMPs……….17
10.0 Operations and Maintenance Manual…………………………………...……...18
10.1 Maintenance Recommendations……………………………………………….18
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
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LIST OF FIGURES
Figure 1 – Vicinity Map
Figure 2 – Existing Conditions (Existing Drainage)
Figure 3 – Developed Conditions (Proposed Drainage)
Figure 4 – Flow Chart for Determining the Type of Drainage Review
Required
Figure 5 – Flow Control /Water Quality Application Map
Figure 6 – Groundwater Protection and Environmental Sensitive Areas
Figure 7 – Downstream Analysis Map
Figure 8 – Bond Quantities Worksheet
Figure 9 – Maintenance and Operations Manual
LIST OF TABLES
Table 1 – Pre-Developed Conditions Surface Areas
Table 2 – Developed Conditions Surface Areas
LIST OF APPENDICES
APPENDIX A – Geotechnical Report
APPENDIX B – Drainage, Cross-sections and Detail Plans
APPENDIX C – BMPs Design Criteria
APPENDIX D – Declaration of Covenant for Maintenance and Inspection of
On-Site BMPs
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1.0 PROJECT OVERVIEW
1.1 General
The Nguyen single-family homes Project proposes to construct three new single-
family residences located at 2309 Aberdeen Avenue NE, in Renton, WA (See
Figure 1 – Vicinity Map). The development will include the construction of
three, two story, single-family residences, pathways, landscaping, On-site BMPs
and associated right of way improvements along the Project frontage.
1.2 Pre-developed Condition
The general location of the project site is shown on Figure 1 (Vicinity Map). The
site consists of a roughly square-shaped covering approximately 0.84 acres. The
site is currently vacant but was previously occupied by a single-family residence
within the western portion of the site. Topography within the western and central
portion of the slopes gently down from the western property line to the eastern
portion of the property. The eastern portion of the property is generally level.
The site is generally vegetated by grass-covered yard areas, young to mature trees,
and landscaping. A horseshoe-shaped gravel driveway is located within the
eastern portion of the property. The property is bound to the north, south, and
west by single-family residences, and to the east by Aberdeen Avenue NE. See
Figure 2 for existing site conditions. See Table #1 below for Pre-Developed
Condition Surface Areas.
Table 1 – Pre-Developed Condition Surface Areas
Type of Land Covers
Area (ac)
Ex. PGIS
0.0775
Ex. NPGIS
0.0420
Ex. Pervious
0.7205
Total Lot Area
0.8400
1.3 Proposed Developed Condition
The development proposes to construct three new single-family residences
located at 2309 Aberdeen Avenue NE, in Renton, WA (See Figure 1 – Vicinity
Map). The development will include the construction of three, two story, single-
family residences, pathways, landscaping, On-site BMPs and associated right of
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
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way improvements along the Project frontage. See Table #2 below for proposed
developed conditions surface areas and Figure 3 for developed site conditions.
Table 2 – Developed Condition Surface Areas
Type of Land Covers
Area (ac)
Dev. PGIS
0.1045
Dev. NPGIS
0.2755
Dev. Pervious
0.4600
Total Area
0.840
1.4 Proposed Storm Drainage Facilities
The roof surface runoff for each of the residence will be convey to a roof
downspout. Each of roof downspout will convey to a perforated pipe connection
for each lot. The overflow from the perforated pipe connection systems will
directly convey to a new catch basin, then convey to the existing drainage system
under Aberdeen Avenue NE. The access driveway and residences driveway will
be convey to the proposed bioretention cell. The overflow of the bioretention
cell will connected to the existing drainage system under Aberdeen Avenue NE.
See Figure 3 for the proposed BMPs location, cross-sections, and details.
1.5 Soils
The geologic units for this site are shown on Preliminary Geologic Map of Seattle
and Vicinity, Washington, by Waldron, H.H., Mulineaux, D.R., and Crandell,
D.r., (USGS, 1961). This site is mapped as younger gravel (Qyg) with younger
sand (Qys) and alluvium (Qa) mapped nearby. The younger gravel and sand
deposits are described as fine to coarse sand that contains varying amounts of
gravel. The alluvium is described as silt with varying amounts of sand and
organic material. Our explorations generally encountered fine to coarse sand with
varying amounts of sand and organic material. The explorations generally
encountered fine to coarse sand with varying amounts of gravel and silt within the
western and central portion of the property that we interpreted as younger
outwash soils. The explorations within the very eastern portion of the property
generally encountered silt with sand and silty fine to medium sand that we
interpreted as native alluvium and younger sand deposits at depth. See
Geotechnical Report in Appendix A for more detail of soil information.
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
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2.0 CONDITIONS AND REQUIREMENTS SUMMARY
2.1 Conditions and Requirements
Stormwater management for this project will be provided in accordance with the
2017 City of Renton Surface Water Design Manual / 2016 King County Surface
Water Design Manual (KCSWDM) requirements. The flow chart in Figure 5 –
Flow Chart for Determining the Type of Drainage Review Required, provides the
framework to determine which core Requirements apply to the Project. The
Project will add/replace more than 2,000 square feet of impervious surface;
therefore, the Project must comply with Core Requirements #1 - #9 and Special
Requirements #1 - #5.
2.1.1 Core Requirements
Core Requirement #1: Discharge at Natural Location
• Stormwater plans have been prepared; a copy is in Appendix B. 2017 City
of Renton Surface Water Design Manual / 2016 King County Surface
Water Design Manual (KCSWDM) requirements for flow control and
water quality treatment through the use of perforated pipe connections and
bioretention cell. Any runoff not detain by the perforated pipe
connections and bioretention cell will be disperse and infiltrate into
ground by yard grass like “natural discharge location”.
Core Requirement #2: Offsite Analysis
• All projects are required to perform an offsite analysis. A Level 1 analysis
has been performed and presented in Section 3.0 of this report.
Core Requirement #3: Flow Control
• The project is located in none of the flow control area based on the King
County Flow Control Application Map, See Figure 5.
Core Requirement #4: Conveyance System
• Runoff from the residence roofs will sheet flows to the downspout and
piped to each individual perforated pipe connection. Runoff from
residence driveways and access roadway flows to the bioretention cell.
See figure 3 for perforated pipe connections and bioretention cell
locations.
Core Requirement #5: Erosion and Sediment Control
• Erosion and Sediment Control (ESC) Plans has been prepared and will be
submitted with the Stormwater plans. Additionally, the Contractor will be
required to prepare a Stormwater Pollution Prevention and Spill (WSPPS)
Plan to comply with County requirements. Applicable Best Management
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Practices (BMPs) will be selected and maintained by the Project Owner to
control pollution.
Core Requirement #6: Maintenance and Operations
• A maintenance and operations manual had been prepared and is provided
in Section 10.0 of this report.
Core Requirement #7: Financial Guarantees and Liability
• All required bonds will be paid by the Owner prior to permit approval, per
City of Renton/ King County Manual. See Section 9.0 of this report for
Bond Quantities.
Core Requirement #8: Water Quality
• Runoff Treatment is not required for this project pollution generating
impervious surfaces (PGIS). PGIS is less than 5,000 SF threshold.
Core Requirement #9: On-Site BMPs
• Flow control is required for this project impervious areas. Impervious
surfaces for this project will be mitigate by perforated pipe connections
and bioretention cell. Each residence roof surface runoff will be mitigate
by perforated pipe connection. Residence driveways and roadway access
will be mitigate by bioretention cell.
2.1.2 Special Requirements
Special Requirement #1: Other Adopted Requirements
• No other area-specific requirements apply to this Project.
Special Requirement #2: Flood Hazard Delineation
• Not applicable. The Project is not adjacent to a flood hazard area.
Special Requirement #3: Flood Protection Facilities
• Not applicable. There are no flood protection facilities on or adjacent to
the site.
Special Requirement #4: Source Control
• Not applicable. The Project does not require a commercial building or
commercial site development permit.
Special Requirement #5: Oil Control
• Not Applicable. This site is not classified as high-use roadway.
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3.0 OFF-SITE ANALYSIS
3.1 Standard Requirements
Offsite analysis is required for all projects per KCWSDM. This Project is not
exempt from this requirement, since the project will add more than 2,000 square
feet of impervious area.
3.2 Scope of Analysis
As shown in Figure 7, the offsite analysis study area extends approximately ¼
mile downstream from the Project site.
3.2.1 Resource Review
• City of Renton COR Map and King County IMap.
3.2.2 Field Inspections
The study area was observed on March 9, 2021. Atmospheric conditions were
chill and semi sunny. No evidence of downstream conveyance issues was
identified. All of downstream conveyance is storm sewer pipe system
underground. From the proposed development to May Creek is all underground
drainage pipe system. From May Creek to Lake Washington open Creek channel
flow to Lake Washington.
3.2.3 Drainage System Description and Problem Descriptions
Figure 7 shows a map of the downstream system. No downstream drainage issues
were identified by the Level 1 downstream analysis.
3.2.4 Mitigation of Existing or Potential Problems
Since no downstream issues were identified no further downstream analysis will
be required. No mitigation measures, in addition to the On-site BMP’s are
proposed.
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
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4.0 FLOW CONTROL AND WATER QUALITY FACILITY
ANALYSIS AND DESIGN
4.1 Existing Site Hydrology
The general location of the project site is shown on Figure 1 – Vicinity Map. The
site consists of a roughly square-shaped covering approximately 0.84 acres. The
site is currently vacant but was previously occupied by a single-family residence
withing the western portion of the site. Topography within the western and
central portion of the slopes gently down from the western property line to the
eastern portion of the property. The eastern portion of the property is generally
level. The site is generally vegetated by grass-covered yard areas, young to
mature trees, and landscaping. A horseshoe-shaped gravel driveway is located
within the eastern portion of the property. The site does not have any stormwater
management. Stormwater runoff either infiltrate into the ground and/or sheet
flows toward Aberdeen Avenue NE. See Figure 2 for an existing site conditions
and photos.
4.2 Proposed Site Hydrology
All of the residence roofs surface runoff sheet flow to downspouts. The
downspout will be tightlined to individual perforated pipe connection. Other
impervious surfaces onsite, including residence driveways and access roadway
has been graded to sheets flows to bioretention cell. The overflow from the
BMPs will flow to the existing drainage system under Aberdeen Avenue NE. See
Figure 3 for developed condition flow patterns.
4.3 Performance Standards
The project is located in none of the flow control area based on the King County
Flow Control Application Map, See Figure 5.
4.4 Flow Control Facilities
Flow control for the non-pollution generating surfaces (roof surface areas) will be
provided by perforated pipe connections for each residence. Total of three, one
for each residence. Flow control for residence driveways and roadway access
impervious surfaces will provided by bioretention cell. This project will provide
a 3 ft x 126 ft bioretention cell for retain of 1 foot of ponding. The overflow will
be directly connected the existing drainage system under Aberdeen Avenue NE.
See Appendix C for On-site BMPs for design criteria and Appendix B for BMPs
location, cross sections and details.
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4.5 Water Quality Facilities
This project contains PGIS within the residence driveways and access roadway to
the houses. The Project new plus replace PGIS is less 5,000 SF. Therefore, basic
treatment for this project is not required. But this project is proposing a
bioretention cell for the new plus replaced PGIS for treatment. See Appendix C
for details.
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
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5.0 CONVEYANCE SYSTEMS ANALYSIS AND DESIGN
5.1 Standard Requirements
New pipe systems shall be designed with sufficient capacity to convey and
contain (at minimum) the 25 years peak flow.” All proposed conveyance systems
will meet or exceed this requirement.
5.2 Proposed Conveyance System
Roof surface runoff from the three residences will be collected in downspout
tightlines and piped to a 20 ft long perforated pipe connection for each lot.
In general, runoff from non-building areas will either infiltrate into the ground
through lawn area. Residence driveways and access roadway sheet flows to a
bioretention cell.
The project will utilize a 6-inch dia. PVC at 1.0% slope to tightline the residence
roofs runoff to a perforated pipe connection. Perforated pipe connections will
directly pipe to an existing drainage system under Aberdeen Avenue NE. See
Appendix B for drainage plans.
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
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6.0 SPECIAL REPORTS AND STUDIES
6.1 Geotechnical
A copy of the report titled, “Geotechnical Investigation,” dated February 26, 2021
by Nelson Geotechnical Associates, Inc. is provided in Appendix A.
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7.0 OTHER PERMITS
7.1 NPDES Permit
A Construction Stormwater General Permit is not required since the Project will
be disturbing less than one acre with construction activities such as clearing,
grading, or excavation.
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8.0 CSWPPP ANALYSIS AND DESIGN
8.1 ESC Plan
A preliminary ESC plan has been provided in Appendix B. A final ESC plan
will be prepared by the Contractor according to The Contractor’s means and
methods and construction sequencing. The ESC plan will be prepared in
accordance with the City’s requirements. Erosion control measures indicated on
the preliminary plan includes: marking clearing limits and the use of straw
wattles. Sediment control measures indicated on the preliminary plan include a
stabilized construction entrance and catch basin/inlet insert protection.
SWPPS Plan
A SWPPS plan will be prepared by the Contractor according to the Contractor’s
means and methods. The SWPPS plan will be prepared in accordance with
City’s requirements.
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9.0 BOND QUANTITIES, FACILITY SUMMARIES, AND
DECLARATION OF COVENANT
9.1 Bond Quantities Plan
A Bond Quantities Worksheet is provided in Figure 10.
9.2 Declaration of Covenant for Privately Maintained Flow Control BMPs
A copy of the unsigned Declaration of Covenant is provided in Appendix D. A
furnished copy of the Declaration of Covenant will be provided by the Owner at
a later time for the perforated pipe connections, bioretention call and catch basins.
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
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10.0 MAINTENANCE AND OPERATIONS MANUAL
10.1 Maintenance Recommendations
Operation and maintenance requirements for the proposed drainage elements and
On-site BMPs have been provided for reference in Figure 9.
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Figure 1
Site Location
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Figure 2
Existing Conditions (Existing Drainage)
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
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Figure 3
Developed Conditions (Proposed Drainage)
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
DATE:
FIELDBOOK:
DRAWING NO:
PAGE:
SHEET: OF:
SCALE:
DESIGNED:
DRAWN:
CHECKED:
APPROVED:NO.REVISION BY DATE APPR
FILENAME:SURVEYED:
VERTICAL: NAVD 1988
IF NOT ONE INCH
ONE INCH
AT FULL SCALE
HORIZONTAL: NAD 1983/1991
SCALE ACCORDINGLY
DATUM
Planning/Building/Public Works Dept.
CITY OF
RENTON
IN COMPLIANCE WITH CITY OF RENTON STANDARDS
IE=273.60
6" PVC
1
2
3
4
5
1
2
2
3
4 5
6" STORM WATER, L= 8.5', MIN. 2.0% GRADE (ASTM D-3034 PVC)
6" STORM WATER, L=8.5', GRADE MIN. 2.0% (ASTM D-3034 PVC)
6",STORM WATER, L=20', S=2.5%. (ASTM D-3034 PVC)
6", STORM WATER, L=182', S=8.2% (ASTM D-3034 PVC)
6" STORM WATER, L=95', S=6.7% GRADE (ASTM D-3034 PVC)
SCALE IN FEET
02010 20 40
STORM WATER NOTE
11'
5'
SEE C-2 FOR DETAILS
TOP WIDTH 11'
3' X 149' BIORETENTION CELL
SEE C-2 FOR DETAILS
BOTTOM ELE.=273.0
TOP ELE.=275.0
2' X 20' PERFORATED STUB OUT TRENCH
SEE C-2 FOR DTAILS
BOTTOM ELE.=270.0
TOP ELE.=272.0
2' X 20' PERFORATED STUB OUT TRENCH
SEE C-2 FOR DETAILS
BOTTOM ELE.=279.0
TOP ELE.=281.0
2' X 20' PERFORATED STUB OUT TRENCH
SPROUT
ROOF DOWN
IE=269.40
RIM=270.90
CATCH BASIN TYPE 1
4 6
4/16/2021
PRIOR CONNECTION
CONTRACTOR SHALL FIELD VERIFY IE.
IE=268.90
6" PVC CONNECT WITH EXISTING PIPE
PRIOR CONNECTION
CONTRACTOR SHALL FIELD VERIFY IE.
IE=268.15
6" PVC CONNECT WITH EXISTING PIPE
PAVEMENT AREA FLOWS TO BIORETENTION CELL
DRAINAGE PLAN
TRAN-NGUYEN DEVELOPMENTABERDEEN AVE NE 272
274
276
282
276
274
13,242.10
LOT # A 10,679.50
LOT # B
13,300.70
LOT # C147.8190'80'
20'
50'
DS18DF30DF24DF26DF34
0+001+001+491+49
278
82.6'97.81'147.81147.81N0°30'04"ES89°10'27"W252.60'N0°29'37"E252.62'N89°08'52"E
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
DATE:
FIELDBOOK:
DRAWING NO:
PAGE:
SHEET: OF:
SCALE:
DESIGNED:
DRAWN:
CHECKED:
APPROVED:NO.REVISION BY DATE APPR
FILENAME:SURVEYED:
VERTICAL: NAVD 1988
IF NOT ONE INCH
ONE INCH
AT FULL SCALE
HORIZONTAL: NAD 1983/1991
SCALE ACCORDINGLY
DATUM
Planning/Building/Public Works Dept.
CITY OF
RENTON
IN COMPLIANCE WITH CITY OF RENTON STANDARDS
5 6
4/16/2021
DRAINAGE DETAILS
TRAN-NGUYEN DEVELOPMENT
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Figure 4
Flow Chart for Determining the Type of Drainage Review
Required
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SECTION 1.1 DRAINAGE REVIEW
12/12/2016 2017 City of Renton Surface Water Design Manual
1-14
FIGURE 1.1.2.A FLOW CHART FOR DETERMINING TYPE OF DRAINAGE REVIEW REQUIRED
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Figure 5
Flow Control /Water Quality
Application Map
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Figure 6
Groundwater Protection Area/
Environmental Sensitive Areas
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Figure 7
Downstream Analysis Map
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Figure 8
Bond Quantities worksheet
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
Site Improvement Bond Quantity WorksheetS15 Web date: 04/03/2015Project Name: Date: Location: Project No.: Activity No.: Note: All prices include labor, equipment, materials, overhead and Clearing greater than or equal to 5,000 board feet of timber? profit. Prices are from RS Means data adjusted for the Seattle area or from local sources if not included in the RS Means database.yesXno If yes, Forest Practice Permit Number: (RCW 76.09)Page 1 of 9Tran-Nguyen Short Plat2309 Aberdeen Ave NE, Renton WA 98056For alternate formats, call 206-296-6600. __________________4/16/2021BOND.xlsxUnit prices updated: 3/2/2021 Report Date: 4/17/2021DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
Site Improvement Bond Quantity WorksheetS15 Web date: 04/03/2015Unit# ofReference #Price Unit Quantity Applications CostEROSION/SEDIMENT CONTROLNumberBackfill & compaction-embankmentESC-16.00$ CY 50 1300Check dams, 4" minus rockESC-2SWDM 5.4.6.3 80.00$ EachCrushed surfacing 1 1/4" minusESC-3WSDOT 9-03.9(3) 95.00$ CYDitchingESC-49.00$ CY 75 1675Excavation-bulkESC-52.00$ CY 300 1600Fence, siltESC-6SWDM 5.4.3.1 1.50$ LFFence, Temporary (NGPE)ESC-71.50$ LFHydroseedingESC-8SWDM 5.4.2.40.80$ SY200011600Jute MeshESC-9SWDM 5.4.2.2 3.50$ SY 50 1175Mulch, by hand, straw, 3" deepESC-10SWDM 5.4.2.1 2.50$ SYMulch, by machine, straw, 2" deepESC-11SWDM 5.4.2.1 2.00$ SYPiping, temporary, CPP, 6"ESC-1212.00$ LFPiping, temporary, CPP, 8"ESC-1314.00$ LFPiping, temporary, CPP, 12"ESC-1418.00$ LFPlastic covering, 6mm thick, sandbaggedESC-15SWDM 5.4.2.3 4.00$ SY 300 22400Rip Rap, machine placed; slopesESC-16WSDOT 9-13.1(2) 45.00$ CYRock Construction Entrance, 50'x15'x1'ESC-17SWDM 5.4.4.1 1,800.00$ Each 1 11800Rock Construction Entrance, 100'x15'x1'ESC-18SWDM 5.4.4.1 3,200.00$ Each 1 13200Sediment pond riser assemblyESC-19SWDM 5.4.5.2 2,200.00$ EachSediment trap, 5' high berm ESC-20SWDM 5.4.5.1 19.00$ LFSed. trap, 5' high, riprapped spillway berm section ESC-21SWDM 5.4.5.1 70.00$ LFSeeding, by handESC-22SWDM 5.4.2.4 1.00$ SYSodding, 1" deep, level groundESC-23SWDM 5.4.2.5 8.00$ SYSodding, 1" deep, sloped groundESC-24SWDM 5.4.2.5 10.00$ SYTESC SupervisorESC-25110.00$ HR 50 211000Water truck, dust controlESC-26SWDM 5.4.7 140.00$ HRWRITE-IN-ITEMS **** (see page 9)EachESC SUBTOTAL:21,750.00$ 30% CONTINGENCY & MOBILIZATION:6,525.00$ ESC TOTAL:28,275.00$ COLUMN:APage 2 of 9BOND.xlsxUnit prices updated: 3/2/2021 Report Date: 4/17/2021DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
Site Improvement Bond Quantity WorksheetWeb date: 04/03/2015ExistingFuture PublicPrivateRight-of-WayRight of WayImprovementsUnit Price Unit Quant.CostQuant.CostQuant.CostGENERAL ITEMSNo.Backfill & Compaction- embankment GI - 16.00$ CY 80480.00195311,718.00Backfill & Compaction- trenchGI - 29.00$ CY 20180.0040360.00Clear/Remove Brush, by handGI - 31.00$ SYClearing/Grubbing/Tree Removal GI - 4 10,000.00$ Acre 0.01100.000.03300.00Excavation - bulkGI - 52.00$ CY16853,370.00Excavation - TrenchGI - 65.00$ CY 40200.0030150.00Fencing, cedar, 6' highGI - 7 20.00$ LFFencing, chain link, vinyl coated, 6' high GI - 8 20.00$ LFFencing, chain link, gate, vinyl coated, 20GI - 9 1,400.00$ EachFencing, split rail, 3' high GI - 10 15.00$ LFFill & compact - common barrow GI - 11 25.00$ CY 30 750.00 450 11,250.00Fill & compact - gravel base GI - 12 27.00$ CY 50 1,350.00 60 1,620.00Fill & compact - screened topsoil GI - 13 39.00$ CYGabion, 12" deep, stone filled mesh GI - 14 65.00$ SYGabion, 18" deep, stone filled mesh GI - 15 90.00$ SYGabion, 36" deep, stone filled mesh GI - 16 150.00$ SYGrading, fine, by hand GI - 17 2.50$ SYGrading, fine, with grader GI - 18 2.00$ SYMonuments, 3' long GI - 19 250.00$ EachSensitive Areas Sign GI - 20 7.00$ EachSodding, 1" deep, sloped ground GI - 21 8.00$ SYSurveying, line & grade GI - 22 850.00$ Day 6 5,100.00Surveying, lot location/lines GI - 23 1,800.00$ AcreTraffic control crew ( 2 flaggers ) GI - 24 120.00$ HR 12 1,440.00Trail, 4" chipped wood GI - 25 8.00$ SYTrail, 4" crushed cinder GI - 26 9.00$ SYTrail, 4" top course GI - 27 12.00$ SYWall, retaining, concrete GI - 28 55.00$ SFWall, rockery GI - 29 15.00$ SFPage 3 of 9SUBTOTAL9,600.0028,768.00 & Drainage Facilities*KCC 27A authorizes only one bond reduction.BOND.xlsxUnit prices updated: 03/02/2021 Report Date: 4/17/2021DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
Site Improvement Bond Quantity WorksheetWeb date: 04/03/2015ExistingFuture PublicRight-of-wayRight of WayImprovementsUnit Price Unit Quant.CostQuant.CostQuant.CostROAD IMPROVEMENTNo.AC Grinding, 4' wide machine < 1000sy RI - 1 30.00$ SYAC Grinding, 4' wide machine 1000-2000 RI - 2 16.00$ SYAC Grinding, 4' wide machine > 2000sy RI - 3 10.00$ SYAC Removal/Disposal RI - 4 35.00$ SYBarricade, type III ( Permanent ) RI - 6 56.00$ LFCurb & Gutter, rolled RI - 7 17.00$ LFCurb & Gutter, vertical RI - 8 12.50$ LF 140 1,750.00Curb and Gutter, demolition and disposal RI - 9 18.00$ LFCurb, extruded asphalt RI - 10 5.50$ LFCurb, extruded concrete RI - 11 7.00$ LFSawcut, asphalt, 3" depth RI - 12 1.85$ LF 200 370.00Sawcut, concrete, per 1" depth RI - 13 3.00$ LFSealant, asphalt RI - 14 2.00$ LF 100 200.00Shoulder, AC, ( see AC road unit price ) RI - 15 SYShoulder, gravel, 4" thick RI - 16 15.00$ SY 50 750.00Sidewalk, 4" thick RI - 17 38.00$ SY 585 22,230.00Sidewalk, 4" thick, demolition and disposaRI - 18 32.00$ SYSidewalk, 5" thick RI - 19 41.00$ SYSidewalk, 5" thick, demolition and disposaRI - 20 40.00$ SYSign, handicap RI - 21 85.00$ EachStriping, per stall RI - 22 7.00$ EachStriping, thermoplastic, ( for crosswalk ) RI - 23 3.00$ SFStriping, 4" reflectorized line RI - 24 0.50$ LFPage 4 of 9 SUBTOTAL 25,300.00Private & Drainage Facilities*KCC 27A authorizes only one bond reduction.BOND.xlsxUnit prices updated: 03/02/2021 Report Date: 4/17/2021DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
Site Improvement Bond Quantity WorksheetWeb date: 04/03/2015ExistingFuture PublicPrivateRight-of-wayRight of WayImprovementsUnit Price Unit Quant.CostQuant.CostQuant.CostROAD SURFACING No. (4" Rock = 2.5 base & 1.5" top course) 9 1/2" Rock= 8" base & 1.5" top course)Additional 2.5" Crushed Surfacing RS - 13.60$ SYHMA 1/2" Overlay, 1.5" RS - 2 14.00$ SY 2002,800.00HMA 1/2" Overlay 2"RS - 3 18.00$ SYHMA Road, 2", 4" rock, First 2500 SY RS - 4 28.00$ SYHMA Road, 2", 4" rock, Qty. over 2500 SYRS - 5 21.00$ SYHMA Road, 3", 9 1/2" Rock, First 2500 SYRS - 6 42.00$ SYHMA Road, 3", 9 1/2" Rock, Qty Over 250RS - 7 35.00$ SYNot Used RS - 8Not Used RS - 9HMA Road, 6" Depth, First 2500 SY RS - 10 33.10$ SYHMA Road, 6" Depth, Qty. Over 2500 SYRS - 11 30.00$ SY 145 4,350.00 80 2,400.00HMA 3/4" or 1", 4" Depth RS - 12 20.00$ SY 70 1,400.00Gravel Road, 4" rock, First 2500 SY RS - 13 15.00$ SYGravel Road, 4" rock, Qty. over 2500 SY RS - 14 10.00$ SYPCC Road (Add Under Write-Ins w/DesigRS - 15Thickened Edge RS - 17 8.60$ LFPage 5 of 9 SUBTOTAL 8,550.00 2,400.00 & Drainage Facilities*KCC 27A authorizes only one bond reduction.BOND.xlsxUnit prices updated: 03/02/2021 Report Date: 4/17/2021DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
Site Improvement Bond Quantity WorksheetWeb date: 04/03/2015ExistingFuture PublicPrivateRight-of-wayRight of WayImprovementsUnit Price Unit Quant.CostQuant.CostQuant.CostDRAINAGE (CPP = Corrugated Plastic Pipe, N12 or Equivalent)Access Road, R/D D - 1 21.00$ SYBollards - fixed D - 2 240.74$ EachBollards - removable D - 3 452.34$ Each* (CBs include frame and lid)CB Type ID - 4 1,500.00$ Each 2 3,000.00 1 1,500.00CB Type IL D - 5 1,750.00$ EachCB Type II, 48" diameter D - 6 2,300.00$ Each for additional depth over 4' D - 7 480.00$ FTCB Type II, 54" diameter D - 8 2,500.00$ Each for additional depth over 4' D - 9 495.00$ FTCB Type II, 60" diameter D - 10 2,800.00$ Each for additional depth over 4' D - 11 600.00$ FTCB Type II, 72" diameter D - 12 3,600.00$ Each for additional depth over 4' D - 13 850.00$ FTThrough-curb Inlet Framework (Add) D - 14 400.00$ EachCleanout, PVC, 4" D - 15 150.00$ EachCleanout, PVC, 6" D - 16 170.00$ EachCleanout, PVC, 8" D - 17 200.00$ EachCulvert, PVC, 4" D - 18 10.00$ LF200 2,000.00Culvert, PVC, 6" D - 19 13.00$ LF300 3,900.00Culvert, PVC, 8" D - 20 15.00$ LF220 3,300.00Culvert, PVC, 12" D - 21 23.00$ LF 20 460.00Culvert, CMP, 8" D - 22 19.00$ LFCulvert, CMP, 12" D - 23 29.00$ LFCulvert, CMP, 15" D - 24 35.00$ LFCulvert, CMP, 18" D - 25 41.00$ LFCulvert, CMP, 24" D - 26 56.00$ LFCulvert, CMP, 30" D - 27 78.00$ LFCulvert, CMP, 36" D - 28 130.00$ LFCulvert, CMP, 48" D - 29 190.00$ LFCulvert, CMP, 60" D - 30 270.00$ LFCulvert, CMP, 72" D - 31 350.00$ LFPage 6 of 9 SUBTOTAL 3,460.00 10,700.00 & Drainage Facilities For Culvert prices, Average of 4' cover was assumed. Assume perforated PVC is same price as solid pipe.*KCC 27A authorizes only one bond reduction.BOND.xlsxUnit prices updated: 03/02/2021 Report Date: 4/17/2021DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
Site Improvement Bond Quantity WorksheetWeb date: 04/03/2015ExistingFuture PublicPrivateRight-of-wayRight of WayImprovementsDRAINAGE CONTINUEDNo. Unit Price Unit Quant.CostQuant.CostQuant.CostCulvert, Concrete, 8"D - 32 25.00$ LFCulvert, Concrete, 12"D - 33 36.00$ LFCulvert, Concrete, 15"D - 34 42.00$ LFCulvert, Concrete, 18"D - 35 48.00$ LFCulvert, Concrete, 24"D - 36 78.00$ LFCulvert, Concrete, 30"D - 37 125.00$ LFCulvert, Concrete, 36"D - 38 150.00$ LFCulvert, Concrete, 42"D - 39 175.00$ LFCulvert, Concrete, 48"D - 40 205.00$ LFCulvert, CPP, 6"D - 41 14.00$ LFCulvert, CPP, 8"D - 42 16.00$ LFCulvert, CPP, 12"D - 43 24.00$ LFCulvert, CPP, 15"D - 44 35.00$ LFCulvert, CPP, 18"D - 45 41.00$ LFCulvert, CPP, 24"D - 46 56.00$ LFCulvert, CPP, 30"D - 47 78.00$ LFCulvert, CPP, 36"D - 48 130.00$ LFDitching D - 499.50$ CYFlow Dispersal Trench (1,436 base+) D - 50 28.00$ LFFrench Drain (3' depth)D - 51 26.00$ LFGeotextile, laid in trench, polypropylene D - 523.00$ SYMid-tank Access Riser, 48" dia, 6' deep D - 54 2,000.00$ EachPond Overflow SpillwayD - 55 16.00$ SYRestrictor/Oil Separator, 12"D - 56 1,150.00$ EachRestrictor/Oil Separator, 15"D - 57 1,350.00$ EachRestrictor/Oil Separator, 18"D - 58 1,700.00$ EachRiprap, placedD - 59 42.00$ CYTank End Reducer (36" diameter) D - 60 1,200.00$ EachTrash Rack, 12"D - 61 350.00$ EachTrash Rack, 15"D - 62 410.00$ EachTrash Rack, 18"D - 63 480.00$ EachTrash Rack, 21"D - 64 550.00$ EachPage 7 of 9SUBTOTAL & Drainage Facilities*KCC 27A authorizes only one bond reduction.BOND.xlsxUnit prices updated: 03/02/2021 Report Date: 4/17/2021DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
Site Improvement Bond Quantity WorksheetWeb date: 04/03/2015ExistingFuture PublicPrivateRight-of-wayRight of WayImprovementsUnit Price Unit Quant.PriceQuant.CostQuant.CostPARKING LOT SURFACINGNo.2" AC, 2" top course rock & 4" borrow PL - 1 21.00$ SY NA NA2" AC, 1.5" top course & 2.5" base coursPL - 2 28.00$ SY NA NA4" select borrow PL - 3 5.00$ SY NA NA1.5" top course rock & 2.5" base course PL - 4 14.00$ SY NA NAUTILITY POLES & STREET LIGHTINGUtility pole relocation costs must be accompanied by Franchise Utility's Cost EstimateUtility Pole(s) RelocationUP-1Street Light Poles w/Luminaires UP-2 7,500.00$ Each 17,500.00$ WRITE-IN-ITEMS(Such as detention/water quality vaults.)No.BIORETENTION CELLWI - 1 25,000.00$ Each125,000.00RETAINING WALL WI - 2 250.00$ SY1490372,500.00Yard DrainWI - 3 225.00$ CYWI - 4LFWI - 5FT1.00WI - 6WI - 7WI - 8WI - 9WI - 10SUBTOTAL7,500.00SUBTOTAL (SUM ALL PAGES):54,410.00439,369.0030% CONTINGENCY & MOBILIZATION:16,323.00131,810.70 GRANDTOTAL: 70,733.00571,179.70COLUMN:BCDPage 8 of 9Lump Sum & Drainage FacilitiesNot To Be Used For Roads Or Shoulders*KCC 27A authorizes only one bond reduction.BOND.xlsxUnit prices updated: 03/02/2021 Report Date: 4/17/2021DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
Site Improvement Bond Quantity WorksheetWeb date: 04/03/2015Original bond computations prepared by:Name:Date:PE Registration Number:Tel. #:Firm Name:Address:Project No:Stabilization/Erosion Sediment Control (ESC)(A)Existing Right-of-Way Improvements(B)Future Public Right of Way & Drainage Facilities (C)Private Improvements (D)Calculated Quantity CompletedTotal Right-of Way and/or Site Restoration Bond*/** (A+B)(First $7,500 of bond* shall be cash.Performance Bond* Amount (A+B+C+D) = TOTAL (T)T x 0.30Minimum is $2000.Minimum is $2000.Maintenance/Defect Bond* TotalMinimum is $2000.NAME OF PERSON PREPARING BOND* REDUCTION:Date:* NOTE:The word "bond" as used in this document means a financial guarantee acceptable to King County.** NOTE: KCC 27A authorizes right of way and site restoration bonds to be combined when both are required.The restoration requirement shall include the total cost for all TESC as a minimum, not a maximum. In addition, corrective work, both on- and off-site needs to be included. Quantities shall reflect worse case scenarios not just minimum requirements. For example, if a salmonid stream may be damaged, some estimated costs for restoration needs to be reflected in this amount. The 30% contingency and mobilization costs are computed in this quantity.*** NOTE: Per KCC 27A, total bond amounts remaining after reduction shall not be less than 30% of the original amount (T) or as revised by major design changes.REQUIRED BOND* AMOUNTS ARE SUBJECT TO REVIEW AND MODIFICATION BY KING COUNTY2067235782 FINANCIAL GUARANTEE REQUIREMENTSPUBLIC ROAD & DRAINAGE MAINTENANCE/DEFECT BOND*STEVE WUTEC INC8822 NE 178TH ST BOTHELL, WA 98011PERFORMANCE BOND*AMOUNT38852MINIMUM BOND* AMOUNT REQUIRED FOR RECORDING ORTEMPORARY OCCUPANCY AT SUBSTANTIAL COMPLETION ***17,683.3$ 28,275.0$ 70,733.0$ -$ 201,056.3$ (B+C) x 0.25 =99,008.0$ 571,179.7$ 670,187.7$ Page 9 of 9BOND.xlsxCheck out the DDES Web site at www.kingcounty.gov/permitsUnit prices updated: 03/02/2021 Report Date: 4/17/2021DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
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Figure 9
Maintenance and Operations Manual
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
MAINTENANCE INSTRUCTIONS FOR A BIORETENTION CELL
Your property contains an on-site BMP (best management practice) called “bioretention,” which was
installed to mitigate the stormwater quantity and quality impacts on both the impervious (paved or
roof) and pervious surfaces (lawn or landscape) on your property.
Bioretention cells, like rain gardens, are vegetated closed depressions or ponds that retain and filter
stormwater from an area of impervious surface or nonnative pervious surface. Bioretention cells rely
on effective infiltration performance more so than rain gardens. The soil in the bioretention cell has
been enhanced to encourage and support vigorous plant growth that serves to filter the water and
sustain a minimum infiltration capacity. Depending on soil conditions, bioretention cells may have
water in them throughout the wet season and may overflow during major storm events. However,
standing water can also be an indicator that periodic maintenance is required to sustain infiltrative
performance. This on-site BMP shall be maintained per Appendix A of the City of Renton’s Surface
Water Design Manual.
MAINTENANCE RESTRICTIONS
The size, placement, and design of the rain garden as depicted by the site plan and design details must
be maintained and may not be changed without written approval from the City of Renton or through a
future development permit from the City of Renton. Chemical fertilizers and pesticides must not be
used.
INSPECTION FREQUENCY AND MAINTENANCE GUIDELINES
• Bioretention cells must be inspected annually for physical defects and sediment accumulation.
• Bioretention cells have inflow and overflow inlets and outlets. These need to be maintained to
ensure that water is moving into and out of the bioretention area. Check inlets/outlets for
debris/sediment blockage, bare spots (exposed soil), or other signs of erosion damage (soil
movement). Remove debris and obstructions as necessary.
• After major storm events, the bioretention cell should be checked to see that the overflow
system is working properly and sedimentation is not occurring at the inlet. If erosion damage
or bare spots are evident, they should be stabilized with soil, plant material, mulch, or
landscape rock. Sediment deposits should be carefully removed and the sediment source
eliminated.
• Plants must be adapted to wet winter conditions and dry summer conditions. Vegetation is to
be watered and pruned as needed.
• Frequent watering is required to keep the plants healthy:
o Year 1: weekly,
o Year 2: bimonthly,
o Year 3: bimonthly,
o Year 4 and beyond: as needed for established plantings and dry periods.
• Chemical fertilizers and pesticides must not be used.
• Bioretention soil must be replaced in areas where sediment accumulation is preventing
adequate infiltration of water through the soil.
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
• Compacted soil should be decompacted.
• Trash and debris must be removed often from the bioretention depression.
• Mulch must be applied to bare soil at a minimum of 2 inches to maintain healthy growth.
• Compost may be added if soil nutrients are no longer adequate to support plant growth.
• Plant materials may be changed to suit tastes.
• Vegetation should be maintained as follows:
1) Replace all dead vegetation as soon as possible;
2) Remove fallen leaves and debris as needed;
3) Remove all noxious vegetation when discovered;
4) Manually weed without herbicides or pesticides;
5) To protect infiltration performance, do not compact soils in the bioretention cell with heavy
maintenance equipment and/or excessive foot traffic;
6) During drought conditions, use mulch to prevent excess solar damage and water loss.
RECORDING REQUIREMENT
These bioretention on-site BMP maintenance and operation instructions must be recorded as an
attachment to the required declaration of covenant and grant of easement per Requirement 3 of
Section C.1.3.4 of the City of Renton Surface Water Design Manual. The intent of these instructions is
to explain to future property owners, the purpose of the BMP and how it must be maintained and
operated. These instructions are intended to be a minimum; the City of Renton may require additional
instructions based on site-specific conditions. See the City of Renton’s Surface Water Design Manual
website for additional information and updates.
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DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
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APPENDIX A
Geotechnical Report
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APPENDIX B
Drainage, Cross-sections and Details Plans
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C.2.6 BIORETENTION
2017 City of Renton Surface Water Design Manual 12/12/2016
C-75
FIGURE C.2.6.B SCHEMATIC REPRESENTATION OF A TYPICAL BIORETENTION
WITH CONTAINMENT BERM
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
SECTION C.2 ON-SITE BMPS
12/12/2016 2017 City of Renton Surface Water Design Manual
C-96
Future updates to the instructions will be posted on City of Renton’s Surface Water Design Manual
website.
A reproducible copy of the instructions, prepared for inclusion with the declaration of covenant, is located
in Reference Section 8-M.
FIGURE C.2.11.A SCHEMATIC REPRESENTATION OF A PERFORATED PIPE CONNECTION FOR A
SINGLE FAMILY RESIDENCE
PLAN VIEW OF ROOF
NTS
24" MIN.
TRENCH X-SECTION
NTS
1 12" - 34" WASHED ROCK
FILTER FABRIC18" MIN.
6" MIN.
RANDOM FILL
6" PERFORATED PIPE
2' x 10' LEVEL TRENCH
w/PERFORATED PIPE
TO ROAD
DRAINAGE SYSTEM
SLOPE
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APPENDIX C
On-site BMPs Design Criteria
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C.2.11 PERFORATED PIPE CONNECTION
2017 City of Renton Surface Water Design Manual 12/12/2016
C-95
C.2.11 PERFORATED PIPE CONNECTION
Perforated pipe connections are intended to convey concentrated runoff from impervious surfaces (usually
roof runoff) directly to a local drainage system while providing some infiltration of that runoff in the
process. They are required for any pipe connection of roof downspouts to the local drainage system
regardless of the extent to which on-site BMPs are required or being used onsite. Perforated pipe
connections are intended to provide for some infiltration during drier periods (late spring through early
fall), which may help dampen the flashiness of stream flows in developed areas and provide some
groundwater recharge. During the wet winter months, however, this BMP likely provides little or no flow
control benefits. Therefore, it is not credited with mitigating target impervious surface.
Applicable Surfaces
Subject to the minimum design requirements below, the perforated pipe connection may be applied to
concentrated runoff from any impervious surface or nonnative pervious surface.
Operation and Maintenance
See Section C.2.11.2.
C.2.11.1 MINIMUM DESIGN REQUIREMENTS
Figure C.2.11.A illustrates a perforated pipe connection for a typical single family residence. Impervious
areas larger than 10,000 square feet and nonnative pervious areas larger than 35,000 square feet may
require larger pipe to adequately convey flows and should be designed by a civil engineer. Perforated pipe
connections must be installed according to the following requirements:
1. Where possible, the perforated pipe connection must be placed in native soil to maximize infiltration
of water, and must not be located under impervious surfaces, except as a last resort.
2. The gravel filled trench must be at least 10-feet in length for every 5,000 square feet of impervious
surface or 35,000 square feet of nonnative pervious surface from which runoff is conveyed.
3. The perforated portion of the system may not be placed in a critical area buffer or on slopes steeper
than 25%. Any proposed placement of the perforated portion on slopes steeper than 15% or within
50 feet of a steep slope hazard area or landslide hazard must be approved by a geotechnical engineer
or engineering geologist unless otherwise approved by CED.
4. For sites with septic systems, the perforated portion of the system must be down slope of the
drainfield primary and reserve areas. CED review staff can waive this requirement if site topography
clearly prohibits subsurface flows from intersecting the drainfield.
5. The perforated pipe connection must not create flooding or erosion impacts as determined by CED.
If the system discharges toward or is near a landslide hazard, erosion hazard area, steep slope hazard
area, or a slope steeper than 15%, CED may require evaluation and approval of the proposal by a
geotechnical engineer or engineering geologist.
6. A minimum of a 5 foot setback is required between any part of the perforated pipe trench and any
property line.
C.2.11.2 MAINTENANCE INSTRUCTIONS
If the perforated pipe connection on-site BMP is required for a project, maintenance and operation
instructions must be recorded as an attachment to the required declaration of covenant and grant of
easement per Requirement 3 of Section C.1.3.4. The intent of these instructions is to explain to future
property owners, the purpose of the BMP and how it must be maintained and operated. These instructions
are intended to be a minimum; CED may require additional instructions based on site-specific conditions.
DocuSign Envelope ID: B278C209-6F8A-4B0C-AAC8-9AB13CB715FC
SECTION C.2 ON-SITE BMPS
12/12/2016 2017 City of Renton Surface Water Design Manual
C-96
Future updates to the instructions will be posted on City of Renton’s Surface Water Design Manual
website.
A reproducible copy of the instructions, prepared for inclusion with the declaration of covenant, is located
in Reference Section 8-M.
FIGURE C.2.11.A SCHEMATIC REPRESENTATION OF A PERFORATED PIPE CONNECTION FOR A
SINGLE FAMILY RESIDENCE
PLAN VIEW OF ROOF
NTS
24" MIN.
TRENCH X-SECTION
NTS
1 12" - 34" WASHED ROCK
FILTER FABRIC18" MIN.
6" MIN.
RANDOM FILL
6" PERFORATED PIPE
2' x 10' LEVEL TRENCH
w/PERFORATED PIPE
TO ROAD
DRAINAGE SYSTEM
SLOPE
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C.2.6 BIORETENTION
C.2.6 BIORETENTION
Bioretention areas are shallow landscaped depressions, with a designed soil mix and plants adapted to the
local climate and soil moisture conditions that receive stormwater from a contributing area. Four types of
bioretention designs are discussed in this section: bioretention cells, bioretention swales, bioretention
planters, and a road-side ditch bioretention alternative design. These are briefly described below:
Bioretention cells:
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. (See Figures 2.6.A and 2.6.B)
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:
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.
Road-side bioretention ditch:
This feature uses a designed soil mix underlain by drain rock for increased storage, low maintenance
vegetation typical of road-side ditches, and requires no permanent ponding-- allowing for road side
conveyance to function per current design and maintenance standards without need for weirs or excess
standing water along the road. Where a bioretention cell or swale would be deemed infeasible for a
project, the road side bioretention ditch is considered as optional, not required.
Applicable Surfaces
Subject to the minimum design requirements and specifications in this section, bioretention may be
applied to any impervious surface such as a roof, driveway, parking area, road, or sidewalk, and to any
non-native pervious surface such as a lawn, landscaped area, or pasture. Bioretention proposed in road
right-of-ways must be per the King County Road Design and Construction Standards (KCRDCS) or as
otherwise approved by the King County Road Engineer.
Infeasibility Criteria
Bioretention is considered infeasible and not required for projects that cannot meet the minimum design
requirements for bioretention or via the bioretention infeasibility criteria list that follows in this section.
Where the minimum design requirements call for geotechnical evaluation and approval for bioretention
that proposes to discharge towards or is within described setbacks of a steep slope hazard area, erosion
hazard area, landslide hazard area, or slopes greater than 15%, bioretention is considered infeasible and
not required. Though not required in these circumstances, a project proponent may still opt to use
bioretention as long as the geotechnical evaluation and approval requirement is met.
Additional Requirements for Large Bioretention Facilities:
Bioretention that receives runoff from:
• impervious surfaces totaling 10,000 square feet or more; or
• new pervious surfaces totaling ¾ acre or more, or
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• a combination of impervious and pervious surfaces that results in a 0.15 cfs increase (using 15
minute time steps) or 0.1 cfs (using 1 hour time steps) in the 100 year peak flow when comparing
predeveloped (historic) and developed conditions,
must meet the following additional requirements to address their larger size, risk, and maintenance needs:
Bioretention Cells:
• SWDM Section 5.2: 100 year overflow conveyance (if applicable); Spill Control Device;
Presettling, Protection from Upstream Erosion; Facility Construction Guidelines; Offsite
Groundwater Impacts; Groundwater Protection; Infiltration near Water Supply Wells; and
Infiltration near Steep Slopes and Landslide Hazard Areas.
• SWDM Section 5.2.2.1 Infiltration Ponds Design Criteria: General, Setbacks.
Bioretention Swales:
• SWDM Section 5.2: 100 year overflow conveyance (if applicable); Protection from Upstream
Erosion; Facility Construction Guidelines; Offsite Groundwater Impacts; Groundwater Protection;
Infiltration near Water Supply Wells; and Infiltration near Steep Slopes and Landslide Hazard
Areas.
• SWDM Section 6.3: Vegetated Flowpath Facility Designs: Access; Construction Considerations;
Flow Velocity, Energy Dissipation, and Flow Spreading (#2, 3, 4 and 5)
Other Site Suitability Factors:
Utility conflicts: Consult local jurisdiction requirements for horizontal and vertical separation required for
publicly-owned utilities, such as water and sewer. Consult the appropriate franchise utility owners for
separation requirements from their utilities, which may include communications and gas. When separation
requirements cannot be met, designs should include appropriate mitigation measures, such as impermeable
liners over the utility, sleeving utilities, fixing known leaky joints or cracked conduits, and/or adding an
underdrain to the bioretention.
Transportation safety: The design configuration and selected plant types should provide adequate sight
distances, clear zones, and appropriate setbacks for roadway applications in accordance with local
jurisdiction requirements.
Ponding depth and surface water draw-down: Flow control needs, as well as location in the
development, and mosquito breeding cycles will determine draw-down timing. For example, front yards
and entrances to residential or commercial developments may require rapid surface dewatering for
aesthetics.
Impacts of surrounding activities: Human activity influences the location of the facility in the
development. For example, locate bioretention areas away from traveled areas on individual lots to prevent
soil compaction and damage to vegetation or provide elevated or bermed pathways in areas where foot
traffic is inevitable, and provide barriers, such as wheel stops, to restrict vehicle access in roadside
applications.
Visual buffering: Bioretention facilities can be used to buffer structures from roads, enhance privacy
among residences, and for an aesthetic site feature.
Site growing characteristics and plant selection: Appropriate plants should be selected for sun
exposure, soil moisture, and adjacent plant communities. Native species or hardy cultivars are
recommended and can flourish in the properly designed and placed Bioretention Soil Mix with no nutrient
or pesticide inputs and 2-3 years irrigation for establishment. Invasive species control may be necessary.
Required Soils Report
Many locations in King County have soils that are underlain by a compacted layer of soil (i.e., glacial till
or hardpan) which severely limits soaking capacity and causes water to perch on the relatively impervious
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survivability in the bioretention system. Thus, a soils report is necessary to identify soil types, depth to
impermeable layers (hardpan) or the maximum wet season groundwater level, and infiltration rates.
Detailed requirements for the soils report are found under “Required Soils Report” in Section C.1.3.
Operation and Maintenance
See Section 2.6.3
Bioretention Infeasibility Criteria List
These are conditions that make bioretention not required to be implemented as part of the prescriptive
BMP lists detailed in Core Requirement #9 of the SWDM and Section 1.3 of Appendix C. The lists
require BMPs to be evaluated and installed to the maximum extent feasible. Where determined infeasible
by these criteria, an applicant has the option to propose a functional design via the adjustment process
described in Section 1.4 of the SWDM in order to: (a) use the FCBMP facility modeling credits described
in SWDM Section 1.2.9.4; (b) to use in achieving the LID Performance Standard (where required or
optional);or (c) to meet the minimum FCBMP implementation requirements ensconced in the “Small Lot,
Large Lot, and Large Rural Lot requirements” lists located in SWDM Section 1.2.9.2 and Appendix C,
Section 1.3.
Note: Criteria with setback distances are as measured from the bottom edge of the bioretention soil mix.
Citation of any of the following infeasibility criteria (#1-7) must be based on an evaluation of site-
specific conditions and a written recommendation from an appropriate licensed professional (e.g.,
engineer, geologist, hydrogeologist):
1. Where professional geotechnical evaluation recommends infiltration not be used due to reasonable
concerns about erosion, slope failure, or down gradient flooding.
2. Within an area whose ground water drains into an erosion hazard, or landslide hazard area.
3. Where the only area available for siting would threaten the safety or reliability of pre-existing
underground utilities, pre-existing underground storage tanks, pre-existing structures, or pre-existing
road or parking lot surfaces.
4. Where the only area available for siting does not allow for a safe overflow pathway to the municipal
separate storm sewer system or private storm sewer system.
5. Where there is a lack of usable space for bioretention facilities at re-development sites, or where there
is insufficient space within the existing public right-of-way on public road projects.
6. Where infiltrating water would threaten existing below grade basements.
7. Where infiltrating water would threaten shoreline structures such as bulkheads.
The following criteria can be cited as reasons for a finding of infeasibility without further
justification (though some require professional services):
8. Within setbacks from structures as established by the local government with jurisdiction.
9. Where they are not compatible with surrounding drainage system as determined by the local
government with jurisdiction (e.g., project drains to an existing stormwater collection system whose
elevation or location precludes connection to a properly functioning bioretention facility).
10. Where land for bioretention is within area designated as an erosion hazard, or landslide hazard.
11. Where the site cannot be reasonably designed to locate bioretention facilities on slopes less than 8%.
12. Within 50 feet from the top of slopes that are greater than 20% and over 10 feet of vertical relief.
13. For properties with known soil or ground water contamination (typically federal Superfund sites or
state cleanup sites under the Model Toxics Control Act (MTCA)):
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a) Within 100 feet of an area known to have deep soil contamination;
b) Where ground water modeling indicates infiltration will likely increase or change the direction of
the migration of pollutants in the ground water;
c) Wherever surface soils have been found to be contaminated unless those soils are removed within
10 horizontal feet from the infiltration area;
d) Any area where these facilities are prohibited by an approved cleanup plan under the state Model
Toxics Control Act or Federal Superfund Law, or an environmental covenant under Chapter 64.70
RCW.
14. Within 100 feet of a closed or active landfill.
15. Within 100 feet of a drinking water well, or a spring used for drinking water supply.
16. Within 10 feet of small on-site sewage disposal drainfield, including reserve areas, and grey water
reuse systems. For setbacks from a “large on-site sewage disposal system”, see Chapter 246-272B
WAC.
17. Within 10 feet of an underground storage tank and connecting underground pipes when the capacity
of the tank and pipe system is 1100 gallons or less. (As used in these criteria, an underground storage
tank means any tank used to store petroleum products, chemicals, or liquid hazardous wastes of which
10% or more of the storage volume (including volume in the connecting piping system) is beneath the
ground surface.
18. Within 100 feet of an underground storage tank and connecting underground pipes when the capacity
of the tank and pipe system is greater than 1100 gallons.
19. Where the minimum vertical separation of 1 foot to the seasonal high water table, bedrock, or other
impervious layer would not be achieved below bioretention that would serve a drainage area that is: 1)
less than 5,000 sq. ft. of pollution-generating impervious surface, and 2) less than 10,000 sq. ft. of
impervious surface; and, 3) less than ¾ acres of pervious surface.
20. Where the a minimum vertical separation of 3 feet to the seasonal high water table, bedrock or other
impervious layer would not be achieved below bioretention that: 1) would serve a drainage area that
meets or exceeds: a) 5,000 square feet of pollution-generating impervious surface, or b) 10,000 square
feet of impervious surface, or c) three-quarter (3/4) acres of pervious surfaces; and 2) cannot
reasonably be broken down into amounts smaller than indicated in (1).
21. Where the field testing indicates potential bioretention sites have a measured (a.k.a., initial) native soil
saturated hydraulic conductivity less than 0.30 inches per hour.
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C.2.6.1 MINIMUM DESIGN REQUIREMENTS (CELLS, SWALES, AND PLANTERS)
All of the following requirements must be met in order for bioretention to be applicable to a target
impervious or pervious surface:
1. A minimum water storage volume shall be provided in the bioretention BMP that is calculated by
multiplying the equivalent storage depth by the square footage of the target surface area served.
For target impervious areas:
• Inside the UGA (Rainfall region SeaTac 1.0 and less ): 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
• Inside the UGA (Rainfall regions greater than SeaTac 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,
• Outside the UGA: In till soils, provide bioretention volume based on 1.9 inches of equivalent
storage depth; in outwash soils provide bioretention volume based on 1.0 inches of equivalent
storage depth
For target pervious areas: In till soils, provide bioretention volume based on 0.7 inches of equivalent
storage depth; in outwash soils, provide bioretention volume based on 0.006 inches of equivalent
storage depth.
2. The water storage area, containing the minimum required storage volume, shall be 6-12 inches deep
at overflow and have side slopes no steeper than 3 horizontal to 1 vertical (except where the storage
area is structurally contained, e.g. a planter configuration). The overflow point of the water storage
area 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.
3. The bioretention footprint as measured at the overflow elevation shall be a minimum of 5% of the
impervious surface directed to the bioretention BMP.
4. 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 depth of ponding (6” 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 to the minimum 6” in order to meet the drawdown criteria).
As an example, a 6” deep pool with an initial measured rate of .5”/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).
5. Bioretention with underdrains is not allowed unless approved via a SWDM adjustment per Section 1.4
that, at minimum, addresses comparable hydraulic performance, water quality concerns, and
maintainability.
6. Bioretention areas should have a minimum shoulder of 12 inches between the road edge and
beginning of the bioretention side slope where flush curbs are used.
7. Flow entrance techniques for energy dissipation shall be utilized and may include: flow spreaders
described in Section 6.2.6 of SWDM, gravel flow spreaders described in Section 6.3.4.2 of SWDM,
rock pads for pipe flow entrances, and/or catch basins preceding bioretention where high sediment
loads are anticipated. Other equivalent options may be considered.
8. If a containment berm is used to form the water storage area, the berm must be at least 2 feet wide
and 6 inches above the 12 inches of water depth. A catch basin or rock pad must be provided to
release water when the pond's water level exceeds the 12 inches of water depth. The catch basin may
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discharge to the local drainage system or other acceptable discharge location via a 4-inch rigid pipe.
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.
9. An 18” thick bioretention soil mix liner extending up slopes to maximum storage depth is
required in the bioretention cell, swale, or planter. The bioretention soil mix shall be per Reference
11-C. Compost shall meet Specification 1 described in Reference 11-C.
10. Do not use filter fabrics between the subgrade and the Bioretention Soil Mix. The gradation between
existing soils and Bioretention Soil Mix is 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.
11. On-site 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.
12. 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.
13. 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.
14. 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.
15. If using the default bioretention soil mix described in Reference 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.
16. Custom bioretention soil mixes may be considered under the adjustment process described in Section
1.4.
17. Water tolerant plants such as those in Table C.2.6.A shall be planted in the pond bottom. Plants
native to Western Washington are preferred.
18. A minimum 5-foot setback shall be maintained between any part of a bioretention cell, swale, or
planter and any structure or property line.
19. 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.
20. 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.
21. Bioretention swales shall have a minimum 2- foot bottom width. Swales shall be flat in cross section
to promote even flow across the width of the swale. See KCRDCS for design details for bioretention
swales in the ROW.
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22. Bioretetention swales shall meet the conveyance requirements described in Section 1.2.4.1 of the
SWDM. Maximum 100 year peak flow velocity through bioretention swales is 3 feet per second.
23. Maximum longitudinal (along direction of flow) slope of bioretention swales shall be 6%.
24. For sites with septic systems, bioretention must be located downgradient of the primary and reserve
drainfield areas. DPER permit review staff can waive this requirement if site topography clearly
prohibits subsurface flows from intersecting the drainfield.
25. Bioretention is not allowed in critical area buffers or on slopes steeper than 20%.
26. Bioretention is not allowed within 50 feet of a steep slope hazard area, erosion hazard area, or
landslide hazard area.
27. Bioretention proposed on slopes steeper than 15% must be approved by a geotechnical engineer or
engineering geologist unless otherwise approved by the DPER staff geologist. The geotechnical
analysis must consider cumulative impacts from the project and surrounding areas under full built-out
conditions.
28. 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 the DPER staff
geologist. The geotechnical analysis must consider cumulative impacts from the project and
surrounding areas under full built-out conditions.
29. 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 DPER.
The geotechnical analysis must consider cumulative impacts from the project and surrounding areas
under full built-out conditions.
30. Bioretention proposed within 200 feet of a steep slope hazard area, erosion hazard area, or landslide
hazard area must be approved by a geotechnical engineer or engineering geologist unless otherwise
approved by the DPER staff geologist. The geotechnical analysis must consider cumulative impacts
from the project and surrounding areas under full built-out conditions.
31. Bioretention must not create flooding or erosion impacts as determined by DPER. If bioretention is
proposed near or directs overflows towards a landslide hazard area, erosion hazard area, or a steep
slope hazard area, DPER 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.
C.2.6.2 MINIMUM DESIGN REQUIREMENTS (ROADSIDE BIORETENTION
DITCH)
All of the following requirements must be met in order for bioretention to be applicable to a target
impervious surface:
1. The roadside bioretention ditch is only allowed to serve road and sidewalk improvements.
2. The roadside bioretention ditch is considered optional only and not required for purposes of
complying with the prescriptive BMP list approach from Core Requirement #9 where a standard
bioretention cell is deemed infeasible.
3. The longitudinal slope of the road side bioretention ditch shall be consistent with KCRDCS 7.02.A
standards for grass-lined ditches which allow a maximum slope of 6%. Grades between 3% and 6%
may require check dams to reduce potential erosion.
4. The roadside bioretention ditch shall meet the conveyance requirements described in Section 1.2.4.1
of the SWDM.
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5. Flow entrance techniques for energy dissipation shall be utilized and may include where applicable:
flow spreaders described in Section 6.2.6 of SWDM, gravel flow spreaders described in Section
6.3.4.2 of SWDM, rock pads for pipe flow entrances, and/or catch basins preceding bioretention
where high sediment loads are anticipated. Other equivalent options may be considered.
Consideration should be given as to whether the design details specified in Section 4.2.2 (Outfalls) are
applicable to a given design.
6. Side slopes shall be per KCRDCS standards for roadside ditches.
7. Vegetation in the roadside bioretention ditch shall conform to the KCRDCS for grass lined road
ditches.
8. A minimum 2 ft. wide, 18” thick bioretention soil mix liner is required along the full length of the
roadside bioretention ditch. The bioretention soil mix shall be per Reference 11-C. Compost shall
meet Specification 1 described in Reference 11-C.
9. A linear trench of washed rock that is 1.5 ft. deep x 2 ft. wide (typical ditch bottom width) shall be
placed underneath the bioretention soil mix liner. The length of the trench shall be that specified in
Section C.2.3.3 “Use of Gravel Filled Trenches for Limited Infiltration” based on the tributary area to
the roadside bioretention ditch. Where this length is not achievable, the width of the gravel trench
may be widened as allowed by ROW or tract width to provide an equivalent area footprint. To slow
flows and encourage infiltration, the gravel filled trench shall be broken up along every 50 feet its
length by a minimum 4 foot long plug of native, unexcavated soil or native material (or equivalent)
free of wood waste, debris, clods or rocks greater than 6” in any dimension, compacted to 95%
maximum density (ASTM D1557).
10. Do not use filter fabrics between the subgrade and the Bioretention Soil Mix. The gradation between
existing soils and Bioretention Soil Mix is 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.
11. On-site 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.
12. 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.
13. 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.
14. 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.
15. If using the default bioretention soil mix described in Reference 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.
16. Custom bioretention soil mixes may be considered under the adjustment process described in Section
1.4.
17. Bioretention with underdrains is not allowed unless approved via a SWDM adjustment per Section 1.4
that, at minimum, addresses comparable hydraulic performance, water quality concerns, and
maintainability.
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18. 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.
19. 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.
20. For sites with septic systems, bioretention must be located downgradient of the primary and reserve
drainfield areas. DPER permit review staff can waive this requirement if site topography clearly
prohibits subsurface flows from intersecting the drainfield.
21. Bioretention is not allowed in critical area buffers or on slopes steeper than 20%.
22. Bioretention is not allowed within 50 feet of a steep slope hazard area, erosion hazard area, or
landslide hazard area.
23. Bioretention proposed on slopes steeper than 15% must be approved by a geotechnical engineer or
engineering geologist unless otherwise approved by the DPER staff geologist. The geotechnical
analysis must consider cumulative impacts from the project and surrounding areas under full built-out
conditions.
24. 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 the DPER staff
geologist. The geotechnical analysis must consider cumulative impacts from the project and
surrounding areas under full built-out conditions.
25. 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 DPER.
26. Bioretention proposed within 200 feet of a steep slope hazard area, erosion hazard area, or landslide
hazard area must be approved by a geotechnical engineer or engineering geologist unless otherwise
approved by the DPER staff geologist. The geotechnical analysis must consider cumulative impacts
from the project and surrounding areas under full built-out conditions.
27. Bioretention must not create flooding or erosion impacts as determined by DPER. If bioretention is
proposed near or directs overflow towards a landslide hazard area, erosion hazard area, or a steep
slope hazard area, DPER 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.
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SECTION C.2 FLOW CONTROL BMPs
C.2.6.3 MAINTENANCE INSTRUCTIONS FOR BIORETENTION
If a bioretention flow control BMP is proposed for a project, the following maintenance and operation
instructions must be recorded as an attachment to the required declaration of covenant and grant of
easement per Requirement 3 of Section C.1.3.4 (p. C-23). The intent of these instructions is to explain to
future property owners, the purpose of the BMP and how it must be maintained and operated. These
instructions are intended to be a minimum; DPER may require additional instructions based on site-
specific conditions. Also, as the County gains more experience with the maintenance and operation of
these BMPs, future updates to the instructions will be posted on King County's Surface Water Design
Manual website.
A reproducible copy of the instructions, prepared for inclusion with the declaration of covenant, is located
in Reference M.
TEXT OF INSTRUCTIONS
Your property contains a stormwater management flow control BMP (best management practice) called
"bioretention," which was installed to mitigate the stormwater quantity and quality impacts of some or all of
the impervious or non-native pervious surfaces on your property. Bioretention includes vegetated closed
depressions (ponds), swales, and/or planters that retain and filter stormwater from an area of impervious
surface or non-native pervious surface. The soil in the bioretention area has been enhanced to encourage
and support vigorous plant growth that serves to filter the water and sustain infiltration capacity.
Depending on soil conditions, the bioretention area may have water in them throughout the wet season
and may overflow during major storm events.
The size, placement, and design of the bioretention BMP as depicted by the flow control BMP site plan and
design details must be maintained and may not be changed without written approval either from the King
County Water and Land Resources Division or through a future development permit from King County.
Plant materials may be changed to suit tastes, but chemical fertilizers and pesticides must not be used.
Mulch may be added and additional compost should be worked into the soil over time.
Bioretention BMPs must be inspected annually for physical defects. After major storm events, the system
should be checked to see that the overflow system is working properly. If erosion channels or bare spots
are evident, they should be stabilized with soil, plant material, mulch, or landscape rock. A supplemental
watering program may be needed the first year to ensure the long-term survival of the bioretention BMP.
Vegetation should be maintained as follows: 1) replace all dead vegetation as soon as possible; 2) remove
fallen leaves and debris as needed; 3) remove all noxious vegetation when discovered; 4) manually weed
without herbicides or pesticides; 5) during drought conditions, use mulch to prevent excess solar damage
and water loss.
4/24/2016 2016 Surface Water Design Manual – Appendix C
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C.2.6 BIORETENTION
TABLE C.2.6.A WATER TOLERANT PLANTS
Common Name Scientific Name Spacing (on center)
Western manna grass Glyceria occidentalis seed
Velvetgrass Holcus mollis seed
Shortawn foxtail Alopecurus aequalis seed
Water foxtail Alopecurus geniculatus 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
Slender rush Juncus tenuis 6 inches
Water parsley Oenanthe sarmentosa 6 inches
Hardstem bulrush Scirpus acutus 6 inches
Watercress Rorippa nasturtium-aquaticum 12 inches
Small-fruited bulrush Scirpus microcarpus 12 inches
C.2.6.4 EXAMPLE SIZING CALCULATION
Runoff Source: 20-foot by 20-foot driveway
Equivalent Storage Depth = 1.0 inches (0.083 feet)
Bioretention Storage Volume Needed = 20 feet x 20 feet x 0.083 feet = 33 cubic feet
Note that if the design soil percolation rate is 0.5 inches per hour, a pond 1-foot deep will take 24 hours to
drain when filled.
2016 Surface Water Design Manual – Appendix C 4/24/2016
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C.2.6 BIORETENTION
FIGURE C.2.6.B TYPICAL BIORETENTION WITH CONTAINMENT BERM
NOTES:
•WATER STORAGE VOLUME PER SECTION C.2.6.1.
•WATER SURFACE AREA AT OVERFLOW ELEVATION MUST BE EQUAL TO
AT LEAST 5% OF THE IMPERVIOUS AREA TRIBUTARY TO THE FACILITY.
•BIORETENTION THAT RECEIVES DRAINAGE FROM IMPERVIOUS AND/OR
PERVIOUS AREA EXCEEDING THE THRESHOLDS IN SECTION C.2.6 MUST
MEET "ADDITIONAL REQUIREMENTS FOR LARGE BIORETENTION
FACILITIES" DESCRIBED IN THAT SECTION.
TREES, SHRUBS
GROUND COVER
BIORETENTION AREA
PLAN VIEW
NTS
SECTION A-A
NTS
BIORETENTION AREA
GROUND
COVER
TREES TREES
SHRUBS
6" MIN TO 12" MAX
WATER DEPTH
2' MIN TOP WIDTH
COMPACTED EARTH
BERM (AS NEEDED)
VEGETATED
COMPACTED
EARTH BERM,
TOP WIDTH
2' MIN
6" MIN FREEBOARD
ABOVE OVERFLOW WS
TO TOP OF BERM OR
ADJACENT ROADWAY
BIORETENTION SOIL
MIX PER REFERENCE
11-C, MIN.18" DEPTH
MAX SLOPE 3H:1V
BELOW OVERFLOW
WATER SURFACE
ELEVATION (TYP.)
4" RIGID PIPE OUTLET
TO STORM SYSTEM
CATCH BASIN
w/GRATE
CATCH BASIN
w/GRATE
4" (MIN) RIGID PIPE
OUTLET TO STORM
SYSTEM
2016 Surface Water Design Manual – Appendix C 4/24/2016
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C.2.6 BIORETENTION
2017 City of Renton Surface Water Design Manual 12/12/2016
C-75
FIGURE C.2.6.B SCHEMATIC REPRESENTATION OF A TYPICAL BIORETENTION
WITH CONTAINMENT BERM
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APPENDIX D
Declaration of Covenant for Maintenance and Inspection of
On-site BMPs
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RECORDING REQUESTED BY AND
WHEN RECORDED MAIL TO:
DECLARATION OF COVENANT
FOR MAINTENANCE AND INSPECTION OF FLOW CONTROL BMPS
Grantor:
Grantee: King County
Legal Description:
Additional Legal(s) on:
Assessor's Tax Parcel ID#:
IN CONSIDERATION of the approved King County (check one of the following) residential
building permit, commercial building permit, clearing and grading permit, subdivision permit, or
short subdivision permit for Application No. ____________________ relating to the real property
("Property") described above, the Grantor(s), the owner(s) in fee of that Property, hereby
covenants(covenant) with King County, a political subdivision of the state of Washington, and its
municipal successors in interest and assigns ("King County" and "the County", or "its municipal
successor"), that he/she(they) will observe, consent to, and abide by the conditions and obligations set
forth and described in Paragraphs 1 through 8 below with regard to the Property. Grantor(s) hereby
grants(grant), covenants(covenant), and agrees(agree) as follows:
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1. Grantor(s) or his/her(their) successors in interest and assigns ("Owners") shall retain, uphold,
and protect the stormwater management devices, features, pathways, limits, and restrictions, known as
flow control best management practices ("BMPs"), shown on the approved Flow Control BMP Site Plan
for the Property attached hereto and incorporated herein as Exhibit A.
2. The Owners shall at their own cost, operate, maintain, and keep in good repair, the Property's
BMPs as described in the approved Design and Maintenance Details for each BMP attached hereto and
incorporated herein as Exhibit B.
3. King County shall provide at least 30 days written notice to the Owners that entry on the
Property is planned for the inspection of the BMPs. After the 30 days, the Owners shall allow King
County to enter for the sole purpose of inspecting the BMPs. In lieu of inspection by the County, the
Owners may elect to engage a licensed civil engineer registered in the state of Washington who has
expertise in drainage to inspect the BMPs and provide a written report describing their condition. If the
engineer option is chosen, the Owners shall provide written notice to the Director of the Water and Land
Resources Division or its municipal successor in interest ("WLR") within fifteen days of receiving the
County's notice of inspection. Within 30 days of giving this notice, the Owners, or the engineer on
behalf of the Owners, shall provide the engineer's report to WLR. If the report is not provided in a timely
manner as specified above, the County may inspect the BMPs without further notice.
4. If King County determines from its inspection, or from an engineer's report provided in
accordance with Paragraph 3, that maintenance, repair, restoration, and/or mitigation work is required for
the BMPs, WLR shall notify the Owners of the specific maintenance, repair, restoration, and/or
mitigation work (Work) required under Title 9 of the King County Code ("KCC"). WLR shall also set a
reasonable deadline for completing the Work or providing an engineer's report that verifies completion of
the Work. After the deadline has passed, the Owners shall allow the County access to re-inspect the
BMPs unless an engineer's report has been provided verifying completion of the Work. If the work is not
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completed properly within the time frame set by WLR, King County may initiate an enforcement action.
Failure to properly maintain the BMPs is a violation of KCC Chapter 9.04 and may subject the Owners to
enforcement under the KCC, including fines and penalties.
5. Apart from performing routine landscape maintenance, the Owners are hereby required to
obtain written approval from WLR before performing any alterations or modifications to the BMPs.
6. Any notice or approval required to be given by one party to the other under the provisions of
this Declaration of Covenant shall be effective upon personal delivery to the other party, or after three (3)
days from the date that the notice or approval is mailed with delivery confirmation to the current address
on record with each Party. The parties shall notify each other of any change to their addresses.
7. This Declaration of Covenant is intended to promote the efficient and effective management
of surface water drainage on the Property, and it shall inure to the benefit of all the citizens of King
County and its municipal successors and assigns. This Declaration of Covenant shall run with the land
and be binding upon Grantor(s), and Grantor's(s') successors in interest and assigns.
8. This Declaration of Covenant may be terminated by execution of a written agreement by the
Owners and King County that is recorded by King County in its real property records.
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IN WITNESS WHEREOF, this Declaration of Covenant for the Maintenance and Inspection of
Flow Control BMPs is executed this _____ day of ____________________, 20_____.
GRANTOR, owner of the Property
GRANTOR, owner of the Property
STATE OF WASHINGTON ) COUNTY OF KING )ss.
On this day personally appeared before me:
, to me known to be the individual(s) described in
and who executed the within and foregoing instrument and acknowledged that they signed the same as
their free and voluntary act and deed, for the uses and purposes therein stated.
Given under my hand and official seal this _____ day of ____________________, 20_____.
Printed name
Notary Public in and for the State of Washington,
residing at
My appointment expires
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