HomeMy WebLinkAboutRS_Frontage_Improvement_TIR_181116_v1TARGETED STORM DRAINAGE ASSESSMENT
FOR
FRONTAGE IMPROVEMENTS
FOR
FUPC – 3811 NE 21
st Street
RENTON, WA 98055
PARCEL # 0423059237
FOR:
FUPC
Prepared by: Feb 14, 2018
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
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 frontage improvements
located adjacent to 3811 NE 21st Street, Renton, WA on Tax parcel # 0423059237
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located on the south side of NE 21st Street in Renton, WA. The Legal Description is:
POR OF E 1/2 OF E 1/2 OF NW 1/4 OF NE 1/4 OF SE 1/4 LY N OF S LN OF LOT 4 OF ROSE
HAVEN PROD E in King County Washington.
The proposed project is located at 3811 NE 21st Street, Renton, WA. The existing ROW
is developed with paved roads. The Shoulders of the roads are grass and landscaping. The
site is an existing Church. The site slopes from south to north toward NE 21st Street at
1.6% +/-.
The proposed project involves improving NE 21st Street Frontage improvements to
include new sidewalk will be done under this permit . As part of frontage improvement, 23’
of the north side of the site will be dedicated as ROW to provide a 53 foot ROW for NE
21st Street. The total ROW dedication will equal 163.25’ x 23’ = 3,754.75sf
There is existing catch basin on North of site, which is connected to storm drainage
system via 12” drain under street 3811 NE 21st St, drains toward west . Major portion of
existing development on North and East section of site (parcels #0423059237 and 307)
drains to the existing North CB. NE 21st Street has an existing piped drainage system with
catch basins at the NE and NW corners of the site and 12” CP connecting them. NE 21st
Street will be improved with 23 feet new ROW with a pavement width of 34-36 feet and
new concrete curb and gutter, 14’ planter, and 5’co ncrete sidewalk. The Existing iron
fence will be relocated to the new property line. Street trees will be provided in the
planter.
There will be no increase in impervious area as the area for the sidewalk is already paved
with asphalt. The 175lf of new 5’ sidewalk will co ver 875sf.
There is an existing piped storm drainage system on 102nd Ave. to the south of the site.
There is a single family residence to the east of the parcel. Currently, the stormwater
runoff sheetflows towards the 102nd Avenue SE ROW
The soils on the site are Glacial Till per the Geotechnical Report by Cobalt Geosciences
dated January 19, 2018. This till soil is not suitable for infiltration.
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Impervious Surfaces :
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 Renton Surface Water
Design Manual (RSWDM) which specify the target impervious surface is equivalent to the
paved area. As the new impervious area is less than 2,000sf drainage review is not
required.
Target Impervious Surface = New Pavement
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New Sidewalk = 875 sf
Total New Impervious Surface = 875sf
Land Disturbing Activity = 3,754.75sf
As less than 2,000sf of impervious area is created and less than 7,000sf of land disturbing
activity is planned the project does not require Drainage Review per 1.1.2.3 of the
RentonSWDM. 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
north towards NE 21st Street and then flow west in the gutter or piped drainage system of NE
21st Street. The site is smaller than 22,000 sf so BMP’s will be used to the extent possible to
control stormwater runoff.
Core Requirement 1 -Discharge at the Natural Location
The site drains toward NE 21st Street and will continue to do so.
CORE REQUIREMENT #2 OFFSITE ANALYSIS
This is an urban area and the properties are well developed. There are no significant on
site flows onto this property. Runoff from the site sheet flows north to the gutter or piped
drainage system in NE 21st Street and then west in the piped drainage system of NE 21st Street.
No significant flows enter the property or leave the property.
Core Requirement No. 3 Flow Control
This sites smaller than 22,000 square feet, it is not subject to the Small Lot BMP
Requirements in Appendix C of the Renton Surface Water Design Manual (RSWDM)
which specify the target impervious surface is equivalent to the paved area as the
new/replaced impervious area is less than 2,000sf
Target Impervious Surface = Target Impervious Surface = New Pavement
New Sidewalk = 875 sf
Total New Impervious Surface = 875sf
Total Land Disturbing Activity = 4,330sf
As the site creates less than 5,000sf of new impervious surface and less than ¾
acre of new pervious surface it is exempt from flow control per Section 1.2.3
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Core Requirement 4 – Conveyance
There will be little runoff from this site. The Basin includes half of the ROW fronting 3811 NE
21st Street. The area is Per the Rational Method Q=CIA = 0.55x3.5x0.087= 0.167cfs 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.
Core Requirement 5 – Construction Stormwater Pollution Prevention Plan
A CSWPPP has been prepared. See below. Temporary ESC measures will be required as
there will be disturbance of the soil to construct the additional pavement, curbs gutters and
sidewalks, and clearing of the site. All of the flows from the ROW stay in the ROW so no adjacent
properties will be affected. No special requirements are needed on this small project.
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:
⦁ Runoff will not be allowed to concentrate and no water will be allowed to point
discharge.
⦁ 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 new catch basins.
⦁ 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.
⦁ The Clearing limits will be the Center of the sawcut line on the existing pavement
and the edge of the ROW.
⦁ Exposed soil will be covered with mulch as needed.
⦁ No Perimeter Protection is needed as the adjacent lot is vacant.
⦁ Traffic Area Stabilization will not be required as the traffic areas will be covered
with gravel as soon as they are graded.
⦁ Sediment retention will be provided by covering the exposed earth with gravel or
mulch.
⦁ Surface water ccollection will not be needed on this small project. By using mulch
and gravel ground cover there should be no significant surface water.
⦁ Dewatering Control will not be needed on this project
⦁ Dust control should not be needed on this small project
⦁ Flow control is not required on this small project
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⦁ Control of Pollutants will include proper handling of saw-cutting runoff and proper
storage of materials
There should be no materials stored on this site
There should be no spills on the site but if there are they can be cleaned up by
the workers.
There should not be an over-application of fertilizer on this small project
Saw-Cutting waste will not be allowed to drain into the storm drainage system.
⦁ Protection of BMPs is not requires as there are no BMPs proposed
⦁ Maintain BMPs is not required as there are no BMPs proposed
⦁ Managing the Project will be done by the General Contractor.
Core Requirement 6 – Maintenance and Operations
There will be little maintenance required for this system. See attached maintenance
requirements for the Perforated Pipe Connection and Basic Dispersion. The City will be
responsible for maintaining the new pipe and catch basins in the ROW.
Core Requirement 7- Financial Liability
The FUPC will be responsible for providing the Financial Guarantee for the work to be
done on this project. A Bond Quantity Worksheet has been prepared to calculate the Bond
Amount needed.
Core Requirement 8: Water Quality
This project creates less than 5,000sf of new Pollution Generating surface therefore no
special water quality treatment is required per section 1.2.8
9. On-site BMPS
On-site BMPs are analyzed for their feasibility as listed 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 Cobalt
GeoSciences report dated Jan 19, 201, the soils on the site are listed as TILL. Full
infiltration is only allowed with medium sands, coarse sands, or cobbles.
Since full dispersion or full infiltration of runo ff is not feasible, one or more of the
alternate BMPs must be applied to (or used to mitigate for) the remaining
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impervious area to the extent practical.
Limited Infiltration – The soils on the site are specified as Glacial Till. Due to the
indications of high winter water table limited infiltration is infeasible for this site.
Basic Dispersion – There is not the minimum 10 foot required flowpath to
implement sheet flow dispersion.
Rain Garden – The use of a raingarden for stormwater mitigation is infeasible due
to constraints of the site, as open-space in the ROW is limited and the City Road
X-section does not support bioretention. The Till soils and high winter
groundwater make bio-retention infeasible.
Permeable Pavement – Permeable Pavement is not used as it would not hold up
under the expected traffic and would not function well due to the high winter
water table.
Rainwater Harvesting – Rainwater harvesting methods are not practical for ROW
Improvements.
Since full dispersion, full infiltration, and addit ional BMPs have been ruled-out, a
direct connection to the piped storm drainage system in NE 21st Street is proposed.
As no BMPs are found to be feasible a direct connection to the storm drainage system in NE
21st Street is proposed.
Appendix: Imap
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