Infiltration System Design For Koopai Residence.pdf
Infiltration System Design For Koopai Residence
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20216 76 Avenue W
Review Comments:
1.Need to have a signed and dated infiltration report to be completed by a qualified
technician. The geotechnical report needs to include the soil profile.
2.Per the DOE 1992 Stormwater Management Manual III-3.3.2 General Limitations 2 (GL-
2) Depth to Bedrock, Water Table, or Impermeable Layer: The base of all facilities shall
be located at least three feet above the seasonal high water mark, bedrock (or hardpan),
and/or impermeable layer. This could result in the BMP not functioning as designed.
The silt/hardpan layer needs to be located and included in a geotechnical report. The
geotechnical report shall include soil types, composition, and depth of silt/hardpan layer.
3.Per the DOE 1992 Stormwater Management Manual Figure III-3.17 Roof Downspout
System. Roof Downspout System overflow with splash block is needed as shown in the
figure. See notes on plan details.
4.The infiltration system needs to be sized to handle the 100-yr storm event.
Conclusions:
There are several concerns related to this type of system at this location. 1) The
Alderwood soils usually have an impermeable silt layer about 20 to 40 inches below the surface.
If there is an impermeable silt layer within 7 to 8 feet of the surface the infiltration system may
not be feasible. (See item # 2 above)
Recommendations:
Items #1 through #4 need to be addressed. If the depth to Bedrock, Water
Table, or Impermeable Layer is not over 7 to 8 feet then infiltration may not be feasible and a
more conventional detention system as outlined in the City of Edmonds Stormwater Handout #
E72 should be considered.