Infiltration Sizing.pdf
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:
()
054
PA
100
T
=A4
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 Edmonds);
P
100
2
= Tributary area served by infiltration facility(ft);
A
T
= Design infiltration rate determined by percolation tests (in/min cannot exceed 0.333
I
D
in/min).
Infiltration facilities must be designed to provide the area required for infiltration, as determined
by the above formula.
Page 1 of 2 Infiltration Sizing.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 test
2.
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 required for
3.
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 infiltration
4.
rate (I). This process is repeated three times, and the results of the three tests are
m
averaged to compute the maximum infiltration rate (I).
m
Page 2 of 2 Infiltration Sizing.DOC