20170254 Stormwater Peer Rev Cmt 07262017� �
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SCJ ALLIANCE
CONSULTING SERVICES
July 26, 2017
Karen Bennett
City of Yelm Community Development Department
105 Yelm Ave West
Yelm WA 98597
RE: Ministerial Review of Preliminary Stormwater Plan
City of Yelm Project No 20170554 — ICE Storage Yard
SCJ Alliance Project No. 0605.05 -37
Dear Karen:
The City of Yelm has requested SCJ Alliance to conduct a stormwater peer review of the above
referenced project. We have performed the review based on the City of Yelm Unified Code,
Development Guidelines and known City policies. The review consisted of the following documents:
• Preliminary Site Plan CIA — dated June 30, 2017 (date of PE stamp)
• Stormwater Report —dated July 3, 2017 (date of PE stamp)
Our comments and observations from the review are:
Preliminary Site Plan
1. Parking lot surfacing is required to be an all- weather surface. Acceptable all- weather surfaces
are asphalt and concrete, gravel is not allowed. (City of Yelm Development Guidelines, Sec
4G.110)
Preliminary Storm Drainage Report
This review was completed utilizing the BMP's provided by 2014 Stormwater Manual for Western
Washington as adopted by the City of Yelm. This project does trigger Minimum Requirements 1 -10.
Below we have provided comments that do not appear to meet requirements 1 -10.
The applicant is proposing to treat the stormwater runoff from the gravel area using bio-
retention swales. The soils present in this area are not sufficient for this use. They have an
infiltration rate that is above the recommended rate specified in the "Site Suitability Criteria"
(SSC -4). From SSC -4, "For infiltration facilities used for treatment purposes, the measured
(initial) soil infiltration rate should be 9 in. /hour, or less. Design (long -term) infiltration rates up
to 3.0 inches /hour can also be considered, if the infiltration receptor is not a sole - source aquifer,
and in the judgment of the site professional, the treatment soil has characteristics comparable to
those specified in SSC -6 to adequately control the target pollutants."
This land use will require and Oil Control Facility /Device. The stormwater manual states that "a
Construction Business (paving, heavy equipment storage and maintenance...) is considered a
high use site ". This proposal does not address this requirement.
212 N Tower Ave • Centralia, WA 98531 • Office 360.669.0700 • scjalliance.com
Karen Bennett
July 26, 2017
Page 2 of 2
4. The geotechnical report recommends a corrected infiltration rate of 40 and 31 in /hr. and the
applicant has used 20 in /hr. in the flow control design. This is acceptable as flow control if a
different treatment method is used.
5. The applicant has demonstrated that flow control mitigation can be accomplished on this site.
6. No stormwater facilities have been proposed from frontage improvements based on the request
for a deferment. If this deferment is not granted additional stormwater mitigation will be
required for those impervious surfaces.
It is our recommendation, that the applicant has demonstrated that this site could accommodate this
land use with some modifications to the proposal. Paving of the storage area and driveway would
prevent the heavy pollutants from infiltrating into the underlying critical aquafer and would direct the
stormwater to a central location for an oil control devise.
Please do not hesitate to contact me at 360.352.1465 with questions on concerns.
Respectfully,
SG Alliance _
Gibson PE
Project Manager
c: Brandon Johnson
Enclosure(s):
c: \users\jim.gibson \desktop \0605.05 -37 stormwater peer review 2017 0726.docx