SW_Review_Comments.pdf
City of Edmonds
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121 5 AVENUE NORTH
EDMONDS, WA 98020
(425) 771-0220
FAX(425) 771-0221
Website: www.ci.edmonds.wa.us
DEVELOPMENT SERVICES DEPARTMENT
Planning Building Engineering
Stormwater Review Comments
Permit : Date:
# PLN20060141 11/07/2007
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Project Name/Address:
Arnhold Short Plat 24310 101 Ave. W.
Contact Person/Address/Fax:
Jesse Jarrell, EIT./13000 Highway 99 South, Everett,
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WA, 98204/ (425)356-2708
Reviewer: Division:
Blaine Chesterfield Engineering
During review of the subject submittal it was found that the following information,
corrections, or clarifications will need to be addressed:
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1.
The proposed detention system drains to an infiltration system along 101
Avenue W. When the infiltration system overflows the water drains down to
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104 Ave W before entering the City of Shoreline. The drainage then enters a
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stream in the City of Shoreline along 244 Street SW (AKA 205 St. NW)
before draining into Puget Sound. Based on the above information there are
two design alternatives.
The first alternative is a designed infiltration system. Per the Southwest
Edmonds Drainage Plan infiltration systems should be used as the primary
method for the disposal of stormwater runoff in this area. A soil pit located
approximately 100 feet from the site indicated very good soils for infiltration.
The soil pit information can be made available if needed.
The second alternative is to design the proposed detention system to the
predeveloped condition peak flow rate for the 2-yr, 10-yr, and 100-yr, 24-hour
design storm.
The infiltration system is preferred alternative.
Please redesign and resubmit.
2.
An infiltration system would need to be designed for 100-year, 24-hour design
storm total precipitation (3.0 inches in City of Edmonds).
3.
If you decide to use infiltrate for the drainage, one of the acceptable methods
for testing to determine the percolation rate of soils on your property is the
sub-surface maximum infiltration test. This test is used to estimate the
maximum sub-surface vertical infiltration rate at a particular level in the soil
horizon that corresponds with the lowest finished grade elevation of a
proposed infiltration facility. The test is designed to simulate the physical
process that will occur during design storm event conditions; therefore, a
saturation period is required to approximate the soil moisture conditions that
would occur during a major storm event. A pipe is employed to allow only
the vertical infiltration rate to be measured so that the computed maximum
infiltration may be used to compute the rate of seepage over the area of
interest for varying head.
Testing Procedure:
a.A hole is dug to the finished grade elevation of a proposed infiltration
facility and of sufficient diameter to allow a 6-inch diameter section of
pipe to be placed in the hole and driven a depth of 6 inches into the soil at
the test elevation.
b.The pipe is filled and kept to a minimum depth of one foot of water above
the test elevation at the bottom inside of the pipe, for a period of not less
than 4 hours (the saturation period).
c.Following the saturation period, the pipe is filled to the top and the time
required for the water to fall every inch, down to 6 inches below the top of
the pipe, is recorded.
d.The rates for all on-inch times are then averaged to estimate the maximum
infiltration rate (I). This process is repeated three times, and the results
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of the three tests are averaged to compute the maximum infiltration rate
(I).
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Please resubmit three copies of the revised plans/documents to a
Development Services Coordinator. You may contact me at 425-771-0220
if you have specific questions regarding these plan corrections.