Stormwater Review for 9123 196th Street SW.pdf
Review Comments for:
th
9123 196 Street SW
1) Any additions to the infiltration system should be sized per the City of Edmonds
infiltration sizing criteria. An accurate soils report and percolation test is needed
for sizing of the infiltrations system. (see the following City of Edmonds,
Development Information, Infiltration Facility Sizing handout.)
Infiltration Facility Sizing
In select cases, the City of Edmonds will allow infiltration of drainage for proposed
developments or redevelopments. In all cases, properties proposing to install infiltration
facilities must be located close to Type A soils as indicated on the SCS Soil Survey of
Snohomish County Area Washington, and percolation tests performed on the property
must indicate a minimum infiltration rate of 0.0333 inches/minute. In all cases, the final
decision to allow infiltration will rest with the City Engineer.
In cases where infiltration has been allowed, the City of Edmonds allows infiltration
facility sizing based on the following formula:
()
0.054
PA
100
T
=
A
RI()
10
I
D
where:
2
= Area required for infiltration (ft);
A
RI
= 100-year, 24-hour design storm total precipitation (3.0 inches in City of
P
100
Edmonds);
2
= Tributary area served by infiltration facility(ft);
A
T
= Design infiltration rate determined by percolation tests (in/min cannot exceed
I
D
0.333 in/min).
Infiltration facilities must be designed to provide the area required for infiltration, as
determined by the above formula.
9123_196thStSW_Review_Comments.doc
Sub-Surface Maximum Infiltration Test
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:
1.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.
The pipe is filled and kept to a minimum depth of one foot of water above the
2.
test elevation at the bottom inside of the pipe, for a period of not less than 4
hours (the saturation period).
Following the saturation period, the pipe is filled to the top and the time
3.
required for the water to fall every inch, down to 6 inches below the top of the
pipe, is recorded.
The rates for all on-inch times are then averaged to estimate the maximum
4.
infiltration rate (I). This process is repeated three times, and the results of
m
the three tests are averaged to compute the maximum infiltration rate (I).
m
9123_196thStSW_Review_Comments.doc