HomeMy WebLinkAboutRS_Drainage Assessment_171114_v3Drainage Assessment Valley Vue Short Plat
LAND DEVELOPMENT ADVISORS, LLC. i September, 2017
TABLE OF CONTENTS
SECTION A PROJECT OVERVIEW .............................................................................. 2
SECTION B CONDITIONS AND REQUIREMENTS SUMMARY ................................... 2
LARGE LOT BMP REQUIREMENTS EVALUATION .............................................................................. 2
CONVEYANCE CALCULATIONS ......................................................................................................... 4
SECTION C SMALL SITE ESC PLAN & REPORT ........................................................ 4
SECTION D DECLARATION OF COVENANT ............................................................... 6
SECTION E OPERATIONS & MAINTENANCE MANUAL ............................................. 6
APPENDIX
1. Geotechnical Report
2. Geotechnical Infiltration Testing Report
3. Seattle IDF Curve
4. Conveyance Calculations
5. Construction and TESC Plans
6. Declaration of Covenant
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LAND DEVELOPMENT ADVISORS, LLC. 2 August, 2017
SECTION A
PROJECT OVERVIEW
The proposed project is to subdivide an existing parcel into two residential lots, leaving both of
the existing houses. Onsite construction will be limited to the addition of an emergency vehicle
turnaround, addition of a fire hydrant, two new water service lines and one new sewer service for
the easterly home.
The parcel is located at 3106 Talbot Road South and is approximately 2.3 acres in size with the
dimensions of 100’x1000’. Current zoning is R8, which requires a minimum lot size of 5,445 sf
per lot. Additionally, an NGPA will be dedicated along the eastern 100’ of the site to encompass
an existing wetland.
The required emergency vehicle turnaround will be located onsite between Lots 1 and 2, and will
connect to South 32nd Place via and existing dedicated ingress/egress easement area (Tract H)
within the Winsper Division No. 1 Subdivision.
SECTION B
CONDITIONS AND REQUIREMENTS SUMMARY
The drainage design will be in accordance with the 2017 Renton Surface Water Design Manual
(“RSWDM”). Since the project adds more than 1,999 sf but less than 5,000 sf of new plus
replaced impervious surface, and less than ¾ acres of new pervious surface, this project will
follow the Simplified Drainage Review Requirements as described in Appendix C of the RSWDM.
The proposed construction associated with the project will add 4,163 sf in new impervious surface
and is proposed on a project site/lot larger than 22,000 square feet. Therefore, per Table C.1.1.A,
the Large Lot BMP requirements per Section C.1.3 and Implementation Requirements in Section
C.1.3.4 will be followed. Additionally, the ESC and SWPPS measures in accordance with Section
C.1.4 and Simplified submittal requirements of C.1.5 will be followed.
LARGE LOT BMP REQUIREMENTS EVALUATION
Full Dispersion
As described in C.1.3.2 (1), the feasibility and applicability of full dispersion must be first evaluated
for all targeted impervious surfaces.
This BMP is not feasible on this site due to the existing slope that is greater than 15% downslope
(west) of the proposed targeted impervious surface. Additionally, a 100-foot flow path meeting
the requirements of Section C.2.1, cannot be provided.
Full Infiltration
Full infiltration per Section C.2.2, means the use of BMPs that can “fully and reliably” infiltrate
runoff into the ground. Per C.2.2.2-1.A.the existing soils must be coarse sands or cobbles or
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LAND DEVELOPMENT ADVISORS, LLC. 3 August, 2017
medium sands. The soils report by Geotech Consultants Inc., dated May 27, 2014, found onsite
soils near the proposed improvements to be mottled gray silty sand with gravel. A second onsite
soils investigation and infiltration testing analysis performed by Geotech Consultants Inc., dated
August 7, 2017 found the soils in Test Pit A to be grayish-brown, mottled with orange, slightly
gravelly, silty sand, over brown mottled with orange, slightly gravelly, silty sand, which became
very dense, which is consistent with glacial till. Therefore, the onsite soils do not meet the
requirements of C.2.2.2-1.A).
Additionally, a small-scale pilot infiltration test (PIT) at location A, yield an infiltration rate of 0.08
inches/hour. The geotechnical engineer therefore does not recommend the infiltration of
stormwater on this site. The geotechnical report and infiltration test report are included in the
Appendix.
Limited Infiltration
The infeasibility criteria for limited infiltration contained in Section C.2.3.2, is the same as full
infiltration except onsite soils may be fine sands, loamy sands, sandy loams, or loams. The last
sentence in Section C.2.3.2 says, “Silt and clay loams, and cemented till (hardpan) are not
suitable for limited infiltration systems. As the geotech noted, the existing soils are glacially
compressed (dense to very dense) and consistent with till. Therefore, the onsite soils do not
meet these requirements and therefore limited infiltration is not considered feasible. Additionally,
the very limited infiltration ability of the soils and geotechnical recommendation, make this an
infeasible BMP.
Bioretention
Per item 1), in the bioretention infeasibility criteria list in Section C.2.6, this BMP is considered
infeasible, “where professional geotechnical evaluation recommends infiltration not be used due
to reasonable concerns about erosion, slope failure, or down gradient flooding”. The onsite
infiltration testing performed by Geotech Consultants, Inc. on August 2017, found the infiltration
rate using a small-scale pilot infiltration test (PIT) to be 0.08 inches/hour. Due to the extremely
limited infiltration ability of the existing till soils, the geotech reiterated their recommendation, from
their 2014 geotechnical report, that the site conditions make infiltration of stormwater infeasible.
Permeable Pavement
Per item 1), in the permeable pavement infeasibility criteria list in Section C.2.7, this BMP is
considered infeasible, “where professional geotechnical evaluation recommends infiltration not
be used due to reasonable concerns about erosion, slope failure, or down gradient flooding”. The
onsite infiltration testing performed by Geotech Consultants, Inc. on August 2017, found the
infiltration rate using a small-scale pilot infiltration test (PIT) to be 0.08 inches/hour. Additionally,
they reiterated their recommendation, from their 2014 geotechnical report, that the site conditions
make infiltration of stormwater infeasible.
Basic Dispersion
Per C.1.3.2-Large Lot BMP Requirements, #4, all impervious surfaces not mitigated using items
1,2, or 3, must be mitigated to the maximum extent feasible utilizing basic dispersion per C.2.4.
Basic dispersion consisting of sheet flow per C.2.4.5 can be provided for approximately 55% of
the new impervious surface, meeting the requirements of item #5, a). This will be accomplished
by utilizing a gravel strip on the westerly side of the new impervious area that is located within the
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LAND DEVELOPMENT ADVISORS, LLC. 4 August, 2017
project site. The portion of impervious that is located within the existing Tract H will be collected
in a catch basin and conveyed to the existing City stormwater system located in S. 32nd Place,
which is where runoff from the area currently flows.
Per design criteria 3 and 4 in Section C.2.4.5, 10-feet of vegetated flow path is to be provided for
up to a 20 foot strip of impervious surface. An additional 10-feet of flow path is required for every
additional 20-feet of impervious surface. The widest portion of the turnaround is 53’, therefore,
26.5-feet of flow path will be provided west of the targeted surface, which will provide adequate
flow path for entire turnaround.
Summary: Basic dispersion per Section C.2.4.5 and Figure C.2.4.D will be provided to mitigate
runoff from approximately 2,306 sf (55%) of the new impervious surface. The remaining area
that cannot be mitigated (1,857 sf) will be collected and conveyed to the City’s existing storm
drainage system. Please see the attached drawing 3 which depicts the proposed stormwater
improvements.
CONVEYANCE CALCULATIONS
The onsite stormwater conveyance system will be constructed of 12" diameter PVC or N-12, at
0.5% slope which has a capacity of 2.7 cubic feet per second ("cfs"). See the conveyance
calculations in the Appendix.
Assuming a "C" value of 0.9 in the rational equation and a tc of 10 minutes, the area required to
fill the pipe (25 year storm) would be: Q=ciA or A=Q/ci (i = 2 inches/hour, Renton-Seattle IDF
Curve, Appendix), A = 2.7/0.9(2) = 1.5 acres or 65,340 sf. Therefore, a 12-inch pipe at 0.5% will
easily convey the flows from the 1,857 sf of new impervious tributary to the proposed catch basin
in Tract H.
SECTION C
SMALL SITE ESC PLAN & REPORT
Erosion and Sediment Control
Development of the project site will require earth disturbance. As a result, special attention needs
to be given to the construction stages of development. This section of the report, along with the
Temporary Erosion and Sedimentation Control (TESC) plans, serve as a guide to minimize
the effects of erosion onsite and off-site. The temporary erosion and sediment control plan is in
conformance with the 2017 Renton Surface Water Design Manual. The following elements
discuss how the project site will conform to County erosion and sedimentation control standards.
Element #1: Mark Clearing Limits
Clearing Limits will be flagged by the surveyor and/or fenced by the contractor prior to
commencement of construction activity. In addition, tree protection fencing will be installed.
Element #2: Establish Construction Access
A rock-construction entrance (or equivalent) shall be installed in conformance with City erosion
control standards.
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LAND DEVELOPMENT ADVISORS, LLC. 5 August, 2017
Element #3: Detain Flows
All surface runoff generated during construction will be controlled by perimeter filter fabric fence
prior to release.
Element #4: Install Sediment Controls
Sediment controls for the project site shall consist of catch basin protection and filter fabric
fencing installed around the site perimeter.
Element #5: Stabilize Soils
The temporary erosion and sediment control plan calls for the stabilization of exposed soils
through mulching and/or hydroseeding when the soils are not to be worked for a significant period
of time. The plan also calls for the establishment of permanent vegetation through hydroseeding
once the site has reached final grade.
Element #6: Protect Slopes
The temporary erosion and sediment control plan calls for the installation of filter fabric fencing
along the downstream limits of construction. Side slopes of soil stockpiles shall be temporarily
covered with plastic until the stockpile materials are used or exported from the site.
Element #7: Protect Drain Inlets
The Temporary Erosion and Sediment Control Plan calls for a filter fabric socks to be installed at
all newly constructed catch basin inlets and all existing inlets which lie immediately downstream
of areas under construction.
Element #8: Stabilize Channels and Outlets
There are no nearby channels.
Element #9: Control Pollutants
All waste materials shall be disposed of in an approved location, in accordance with City
Standards. In order to reasonably prevent a contamination event (such as a fuel spill), vehicle
maintenance shall occur off-site to the greatest extent practicable. Any contamination event that
occurs should be immediately contained and remedied to the satisfaction of the City.
Element #10: Control De-Watering
No significant amount of dewatering is expected to occur during this project.
Element #11: Maintain BMPs
All BMPs should be monitored and maintained regularly to insure adequate operation. A TESC
supervisor shall be identified at the beginning of the project, to provide monitoring and direct
appropriate maintenance activities. As site conditions change, all BMPs shall be updated as
necessary to maintain compliance with City standards.
Element #12: Manage the Project
The project will begin with a pre-construction conference, in which an onsite TESC supervisor
shall be identified. The onsite supervisor shall monitor all TESC facilities regularly and maintain
a log of inspections and improvements to demonstrate compliance with City standards. It is
important to ensure that the entire site is in conformance with City erosion control standards at
all times. The TESC supervisor shall notify Land Development Advisors, LLC of any problems
with the proposed erosion control elements, or if any revisions to the plans need to be made.
Drainage Assessment Valley Vue Short Plat
LAND DEVELOPMENT ADVISORS, LLC. 6 August, 2017
Additional erosion control materials, such as filter fabric fencing, plastic cover and straw bales,
shall be kept onsite at all times in the event that an erosion control feature needs to be replaced
or installed.
The TESC Plans are included in the Appendix.
SECTION D
DECLARATION OF COVENANT
A declaration of covenant will be executed and a draft is included in the attached
documents at the end of this report.
SECTION E
OPERATIONS & MAINTENANCE MANUAL
Operations and Maintenance
Operations and maintenance guidelines for the onsite storm system are covered in the RSWDM
Manual.
Drainage Assessment Valley Vue Short Plat
LAND DEVELOPMENT ADVISORS, LLC. 7 August, 2017
APPENDIX
Pipe
Input Output
Q (cfs)0.00 2.73
n 0.012 0.012
d (ft)1.00 1.00 1 0 /16 inches
y (ft)1.00 1.00
S (ft/ft)0.005 0.005
A (sf)0.785
Pw (ft)3.142 V (ft/s)3.475
R (ft)0.250
Critical y (ft) =
Qmax @ y (ft) =0.9382
Vmax @ y (ft) =0.8128
Job:Bothell Motor Crest Description:Conveyance with 25-year storm
By:JJJ Date:7/21/2017
D
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Page 1
RECORDING REQUESTED BY AND
WHEN RECORDED MAIL TO:
CITY CLERK’S OFFICE
CITY OF RENTON
1055 SOUTH GRADY WAY
RENTON, WA 98057
DECLARATION OF COVENANT
FOR MAINTENANCE AND INSPECTION OF FLOW CONTROL BMPS
Grantor:
Grantee: City of Renton
Legal Description:
Additional Legal(s) on:
Assessor's Tax Parcel ID#:
IN CONSIDERATION of the approved City of Renton(check one of the following) residential
building permit, commercial building permit, clearing and grading permit, subdivision permit, or
short subdivision permit for Application File No. LUA/SWP ____________________ relating to the
real property ("Property") described above, the Grantor(s), the owner(s) in fee of that Property, hereby
covenants(covenant) with City or Renton, a political subdivision of the state of Washington, 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:
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. City or Renton 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 the City of
Renton to enter for the sole purpose of inspecting the BMPs. In lieu of inspection by the City, 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 City of Renton within fifteen
days of receiving the City'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 the City of Renton. If the report
is not provided in a timely manner as specified above, the City of Renton may inspect the BMPs without
further notice.
4. If the City 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,
The City shall notify the Owners of the specific maintenance, repair, restoration, and/or mitigation work
(Work) required under RMC 4-6-030. The City 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 City access to re-inspect the BMPs unless an engineer's report has
been provided verifying completion of the Work. If the work is not completed properly within the time
frame set by the City, the City may initiate an enforcement action. Failure to properly maintain the BMPs
is a violation of RMC 4-6-030 and may subject the Owners to enforcement under the RMC 1-3, including
fines and penalties.
5. Apart from performing routine landscape maintenance, the Owners are hereby required to
obtain written approval from the City or Renton 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 the City of
Renton and its 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 the City of Renton that is recorded by King County in its real property records.
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