Infiltration System Design Review DOC.pdf
Infiltration System Design For Eaglewood Homes
901 Cedar Street
Review Comments:
1.During the site visit on 4-20-2007 there was no evidence of test pits at the locations
marked on the attached map. A signed and dated infiltration report to be completed by a
qualified technician. An infiltration rate of 11.2 inches per hour seems too optimistic for
the site and soil types.
2.Per the DOE 1992 Stormwater Management Manual III-3.6.6 BMP RI.15 Roof
Downspout System: System shall be a minimum of 10 feet from any structure, property
line, or NGPE. (There does not appear to be enough room for this kind of system.)
3.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.
4.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.
5.Per the DOE 1992 Stormwater Management Manual Figure III-3.17 Roof Downspout
System Section A-A. Surround system with washed drain rock and wrap trench
entirely with filter fabric. See notes on plan details.
6.Show the location of the observation port on the Infiltrator Chamber detail. See notes on
plan details.
Conclusions: Infiltration does not appear to be feasible at this location. 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. On the west side of the property
there has been some filling so the impermeable layer may be slightly deeper. If there is an
impermeable silt layer within 7 to 8 feet of the surface the infiltration system may not be feasible.
(See item # 3 above) 2) There is not enough space for the minimum required setbacks (See item
# 2 above)
Recommendations: Consider going to a more conventional detention system as outlined in the
City of Edmonds Stormwater Handout # E72.