HEX Rolling Sunrise Engineer Rsp Part 2
241 ERICKSEN AVENUE NE, BAINBRIDGE ISLAND, WA 98110 206.842.0605 info@BrowneWheeler.com
November 6, 2014
Michel Girard
BGH Development, LLC
Michel@BGHdevelopment.com
206.335.1649
Re: Rolling Sunrise Subdivision Soil Infiltration
Dear Michel:
This letter is in response to the request from the City of Bainbridge Island for information regarding the
determination of infiltration rates used for sizing of infiltration facilities for mitigation of the Rolling
Sunrise Preliminary Plat on Bainbridge Island.
Dave’s Septic Services Inc. was hired by you to perform soil logs for the proposed plat. This included
seven soil logs across the site including information on the soil depth, soil description, soil type and
general slope of the area the samples were taken (attached). We used Table 3.7 (attached) in
Volume III of the 2005 Stormwater Management Manual for Western Washington to determine the
short term infiltration rate of 1 inch/hour based on Dave’s Septic Services soil descriptions. Soil
descriptions were visually verified by us in the field. Each stormwater facility was preliminarily sized
with an infiltration rate of 0.5-inches/hour after a correction factor of 2 was applied to 1 inch/hour rate.
To evaluate feasibility, we laid out preliminary infiltration trenches and rain garden areas sized to
infiltrate 100% of the runoff from impervious areas proposed for the access driveways and areas on
each lot considering setbacks from septic drainfields. Preliminary sizing was performed through the
use of an approved continuous runoff model, Western Washington Hydrology Model 3 (WWHM3).
Please do not hesitate to contact us if you have any questions.
Very truly yours,
BROWNE WHEELER ENGINEERS, INC.
11/6/2014 11/6/2014
for homogeneous soils. These rates not consider the effects of site
variability and long-term clogging due to siltation and biomass buildup in
the infiltration facility.
Table 3.7 -- Recommended Infiltration Rates
based on USDA Soil Textural Classification.
*Short-Term
Infiltration
Rate (in./hr)
Correction
Factor, CF
Estimated Long-
Term (Design)
Infiltration Rate
(in./hr)
Clean sandy gravels and
gravelly sands (i.e., 90% of
the total soil sample is
retained in the #10 sieve)
20 2 10
**
Sand 8 4 2***
Loamy Sand 2 4 0.5
Sandy Loam 1 4 0.25
Loam 0.5 4 0.13
*From WEF/ASCE, 1998.
**Not recommended for treatment
*** Refer to SSC-4 and SSC-6 for treatment acceptability criteria
Based on experience with long-term full-scale infiltration pond
performance, Ecology’s Technical Advisory Committee (TAC)
recommends that the short-term infiltration rates be reduced as shown in
Table 3.7, dividing by a correction factor of 2 to 4, depending on the soil
textural classification. The correction factors provided in Table 3.7
represent an average degree of long-term facility maintenance, TSS
reduction through pretreatment, and site variability in the subsurface
conditions. These conditions might include deposits of ancient landslide
debris, buried stream channels, lateral grain size variability, and other
factors that affect homogeneity).
These correction factors could be reduced, subject to the approval of the
local jurisdiction, under the following conditions:
x For sites with little soil variability,
x Where there will be a high degree of long-term facility maintenance,
x Where specific, reliable pretreatment is employed to reduce TSS
entering the infiltration facility
In no case shall a correction factor less than 2.0 be used.
3-76 Volume III – Hydrologic Analysis and Flow Control BMPs February 2005