HomeMy WebLinkAboutTIR-3982Page 1
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Prepared by: September 15, 2017
Anstey Engineering
CIVIL ENGINEERING/CONSULTING
8627 NE 180th Street
Bothell, Washington 98011
Phone: (206) 303-7639
Fax: (425) 658-9203
E-mail: benansey@ansteyengineering.com
TARGETED STORM DRAINAGE ASSESSMENT
FOR
2800 NE Sunset Blvd., Renton WA 98056
RENTON, WA 98055
on
PARCEL # 7227801201
FOR:
JSH Properties Inc.
Phone:
DEVELOPMENT ENGINEERING
Justin Johnson 12/08/2017
SURFACE WATER UTILITY
rstraka 12/08/2017
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PROJECT OVERVIEW NARRATIVE:
This storm drainage assessment has been prepared to address the City of Renton site
development and storm drainage requirements for the proposed Shell Station and Convenience
Store (2,200 sf) located at 2800 NE Sunset Blvd. on Tax parcels # 7227801201 located at the
west side of NE Sunset Blvd. between NE 10th Street and NE Sunset Blvd in Renton, WA. The
Legal Description is: RENTON HIGHLANDS #2 CORRECT PLAT SWLY 125 FT OF SELY
90 FT. IN NW QTR STR 09-23-05
The proposed project is located at 2800 NE Sunset Blvd., Renton, WA 98056. The property is
11,250 square feet in area. The property is an Existing Gas Station fully paved. The site is flat
and currently used as a Convenience Store with Gas. The site slopes from northeast down to
southwest at 2% +/-. The proposed project involves re-constructing a new 2,200 sf Convenience
Store, with 3,772 sf of auto court. The total impervious area will amount to 10,423 square feet.
The runoff will be routed to the existing storm drainage system of NE 10th Street through a
perforated pipe connection trench.
There is an existing piped storm drainage system on NE 10th St. to the south of the site. There is
a developed site to the west of the parcel. Currently, the stormwater runoff sheetflows towards
the NE 10th Street ROW.
The soils on the site are Ur Urban land per Sheet 5 of the King County SCS Soil Survey.
Impervious Surfaces :
Proposed Shell Station & Convenience Store 2,020 sq.ft.
Sidewalk 624 sq.ft.
Auto Court 5,246 sq.ft.
Roof of Canopy 1,871 sq.ft.
Total Impervious Area 9,761 sq.ft.
Since this lot is smaller than 22,000 square feet, it is subject to the Simplified Drainage Review
and Small Lot BMP Requirements in Appendix C of the KC Surface Water Design Manual
(SWDM) which specify the target impervious surface is equivalent to the roof area and
driveway.
Target Impervious Surface = Building Footprint plus driveway = 9,761sq.ft.
As less than 5,000sf of impervious area is created the project does not require Full Drainage
Review per 1.1.2.3 of the KDSWDM. (The site is currently 100% impervious with pavement and
buildings covering the entire site) A site location and sensitive areas map can be seen in the
Appendix. This map shows that the site is not located near sensitive areas and displays that the
runoff will travel south and west towards NE 10th Street. The site is smaller than 22,000 sf so
BMP’s will be used to the extent possible to control stormwater runoff.
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Core Requirement 1 -Discharge at the Natural Location
The site drains toward NE 10th Street and will continue to do so. We will add a perforated pipe
connection which will collect the runoff from the roof of the new buildign and rout it to the
existing catch basin and storm drainage system on NE 10th Street on the west side of the site.
Core Requirement 2 – Offsite Analysis
This is an urban area and the properties are fully developed. There are no significant
onsite flows onto this property. Runoff from the site sheet flows to the northwest to NE 10th
Street. There is a piped downstream drainage course in NE 10th Street. No significant flows enter
the property.
Task 1. Study Area Definition and Maps
The properties to the north and west of this site are developed sites, and do not discharge
any significant amount of runoff onto this site. NE 10th Street to the southwest and NE Sunset
Blvd to the northeast do not drain much onto the site. No other significant flows enter the site.
We proposed to discharge our site runoff to a new catch basin at the south corner of the site on
the east side of NE Sunset Blvd. through a perforated pipe connection. So no surface water
runoff will be discharged off-site. There are no downstream effects anticipated from the
proposed development. The downstream flowpath of stormwater runoff during very large storms
is to flow southwest along NE 10th Street. A site map and topographical map with downstream
flow path can be found in the Appendix. The new drainage system will enter the NE 10th Street
drainage system as it does now. As we are proposing to reduce the amount of impervious area
the amount of runoff should decrease and should not be a problem for the existing drainage
system.
Task 2: Resource Review
Using the King County iMap interactive tool, it was found that there are no immediate
critical areas upstream or downstream of the property. The iMap also confirmed that there are no
mitigating rivers, floodplain issues, there are no adopted basin plans.
Task 3: Field Inspection
There are no reported problems to be investigated. The amount of space and soil types
present on the property clearly support the flow control BMPs being implemented. There will not
be any destruction of aquatic habitat on-site or downstream. The Site was visited on August 19,
2017 and a 1/4 mile downstream investigation was made. The drainage system is piped and no
problems were observed.
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From SCS Soil Survey of King County, Sheet 5, and Anstey Engineering on-site investigation
soil is Urban land (Ur).
Part A: Existing Site Hydrology
The lot is a developed site. The site is gently sloped with slopes of 2% +/- down to the
northwest. The storm water runoff from the site currently sheet flows across the site and drains
onto the catch basin and gutter of NE 10th Street. The current site is 100% impervious with
building and pavement. There is no evidence of any other on-site flows going off-site. The site
does not receive any significant off-site flows from the adjacent properties. To the west, NE 10th
Street has a concrete curb and gutter, and to the east, NE Sunset Blvd. has a concrete curb and
gutter.
Part B: Developed Site Hydrology
When the Convenience Store is constructed its roof runoff will be routed through a
Perforated Pipe Connection to the existing catch basin in NE 10th Street which connects to the
existing piped storm drainage system.
Part C: Performance Standards
Flow control BMPs are used on this site to control stormwater using a Perforated Pipe
Connection per Figure C.2.11.A. This is an existing Gas Station commercial site on a small lot
and as such does pose a significant threat for pollution and pollution generating activities are
planned. (Vehicle Re-fueling). This is a ‘High Use Site’, but pollution prevention measures are
in place including Canopy, and oil/water separator.
Part D: Flow Control System
The Erosion and Sediment Control Plans along with the Site Plans include the locations
of the flow control systems. In addition, cross-sections of the flow control BMPs can be found in
the Appendix.
BMPs were analyzed for their practicality and limited infiltration using a perforated pipe
connection to the storm drainage system in NE 10th Street was selected as the only practical
BMP.
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Part E: Water Quality System
The site will have more than 5,000sf of pollution generating surface. The site pavement is
existing therefore no further water quality treatment is required. This is a high use site but a
canopy and Oldcastle Precast 576-SA 1000 oil/water separator are already installed. No
additional facilities will be necessary.
Impervious Surfaces
Impervious Surfaces
Proposed Shell Station & Convenience Store 2,020 sq.ft.
Sidewalk 624 sq.ft.
Auto Court 5,246 sq.ft.
Roof of Canopy 1,871 sq.ft.
Total Impervious Area 9,761 sq.ft.
Since this lot is smaller than 22,000 square feet, it is subject to the Small Lot BMP Requirements
in Appendix C of the KC Surface Water Design Manual (SWDM) which specify the target
impervious surface is equivalent to the roof area and driveway.
Target Impervious Surface = Bldg Footprint plus driveway, canopy and walk = 9,761sf.
Flow control BMPs are used on this site to control stormwater. See discussion below:
Full Dispersion – Unable to achieve the minimum 100 feet of native vegetated flowpath.
Therefore, full dispersion is not feasible.
Full Infiltration – According to the attached soils information from the SCS
Soil Survey of King County Sheet 5, the soils on the site are listed as Urban Land (Ur).
Full infiltration is only allowed with medium sands, coarse sands, or cobbles. Therefore,
full infiltration is not applicable.
Since full dispersion or full infiltration of roof runoff is not feasible, one or more of the
alternate BMPs must be applied to (or used to mitigate for) the remaining impervious
area to the extent practical.
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Limited Infiltration – The soils on the site are specified as Urban Land (Ur). Due to the
indications of high winter water table limited infiltration is infeasible for this site. A
perforated pipe connection will be made to the piped Storm Drainage System in NE 10th
Street which will allow the water to infiltrate. There is not room for infiltration trenches
due to setback requirements, and existing paving.
Basic Dispersion – There is not the minimum 25 foot required vegetated flowpath to
implement basic roof downspout dispersion. Sheet flow dispersion will not be used as
runoff could flow onto the sidewalk.
Rain Garden – The use of a raingarden for stormwater mitigation is infeasible due to
constraints of the site, as open-space on the lot is limited.
Permeable Pavement – Permeable Pavement is not used as the pavement is existing.
Rainwater Harvesting – Rainwater harvesting methods would be fairly large and
therefore difficult to site on the lot due to space constraints. The landscaped area that
would receive irrigation is also fairly small. Considering the costs and benefits rainwater
harvesting is not practical for this site.
Vegetated Roof – A Vegetated roof is not proposed for this site.
Reduced Impervious Surface Credit – The site is not eligible for the Reduced Impervious
Surface Credit although we are reducing the existing impervious area by 1,489sf.
Since full dispersion, full infiltration, and additional BMPs have been ruled-out, a perforated
pipe connection per C.2.11.A is specified to allow limited infiltration from stormwater runoff
from the roof area. The perforated pipe connection will be in a 2’ by 10’ gravel trench. The
cross-section of the perforated pipe can be seen in the Appendix or Figure C.2.11.A. The pipe
will then connect to the existing Catch Basin in NE 10th Street. in front of the site. The catch
basin drains northwest to the existing piped drainage system.
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Core Requirement 4 – Conveyance
There will be little runoff from this site. Per the Rational Method Q=CIA =
0.9x3.5x0.26= 0.8cfs in a 100 year storm. The capacity of a 12 inch Ø pipe at 0.5% slope is
2.7cfs so there is plenty of capacity in the piped drainage system of NE 10th Street.
Core Requirement 5 – Erosion and Sediment Control
Temporary ESC measures will be required as there will be disturbance of the soil to
construct the Convenience Store, and clearing of the site. All of the flows from the Convenience
Store will flow onto the lot so no adjacent properties will be affected. A silt fence around the
construction area and construction entrance should be provided at a minimum. No special
requirements are needed
In order to prevent erosion and trap sediment within the project site, the following BMPs will be
used approximately as shown on the ESC plan:
Clearing limits will be marked by fencing or other means on the ground.
A rocked construction entrance will be placed at the location of the driveway throughout
construction. Street sweeping will be used if necessary to keep NE 10th Street clear of
tracked sediment.
Runoff will not be allowed to concentrate and no water will be allowed to point
discharge.
Silt fencing will be placed along slope contours at the downslope limit of clearing.
Mulch will be spread over all cleared areas of the site when they are not being worked.
Mulch will consist of air-dried straw and chipped site vegetation. Catch Basin Protection
will be provided for the existing catch basins.
The roof downspouts will be connected to the perforated pipe connection trench as soon
as the roof is installed.
No dewatering is expected for this project.
Sawcuting of pavement, concrete handling, and other potential pollution generating
activities will be controlled to prevent pollution.
When construction is complete the Erosion Control measures will be removed upon the
approval of the City of Renton inspector.
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Core Requirement 6 – Maintenance and Operations
There will be little maintenance required for this system. See attached maintenance
requirements for the Perforated Pipe Connection. The City will be responsible for maintaining
the existing pipe and catch basins in the ROW.
Perforated Pipe Connection Instructions:
Core Requirement 7- Financial Liability
JSH Properties Inc. will be responsible for providing the Financial Guarantee for the
work to be done on this project.
Special Requirement 4 - Source Control
This is a commercial site so may generate any significant pollution during or after
construction. As oil water separators and canopy are in place to prevent oil or gas drips from
entering the storm water system adequate pollution prevention measures are already in place to
prevent pollution. . BMPs will be used to limit pollution by silt laden water or dust and
sawcutting and cement handling during construction. After construction, there should be no
significant pollution generation. The Pollution Generating Surface is only 5,246SF which is all
existing. An OldCastle Precast 576-SA-1000 oil water separator is installed on the site to treat
runoff from the gas pumping area under the canopy.
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Special Requirement 6 – Aquifer Recharge
This site is not in an aquifer recharge area. The pervious pipe connection, will allow some
Stormwater runoff to infiltrate into the ground.
Appendix: IMap
SITE
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SCS Soil Map Sheet 5
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