20100129 LandUseResponse letter-2 11 12 10 (2)SOUND ENGINEERING, INC. CIVIL ENGINEERS LAND PLANNERS
File No.: 1010410
November 12, 2010
Tami Merriman, Associate Planner
Community Development Department
City of Yelm
105 Yelm Avenue West
Yelm, WA 98597
Project: America's Credit Union-Yelm
Subject: Letter response
Dear Tami:
This letter is in regards to your October 25, 20101etter for the America's Credit Union in which
you stated:
Please haveyour engineer submit a rerised prelimiuary draiuage plau that shows how he intends to meet the
requirements for conveyance and treatment of stormzvater from Yelm Avenue ~a~ith the addz~tion of the second driveway
access and bus landing pad no later than November 99, 2090. Speczfically calculations which show there will be
adequate retention, treatment and infiltration of stormwater as needed in order to proceed with processing this pr ject.
Following is a summarization addressing your concerns:
To begin, I would like to point out the area associated with the "second" driveway access and the
5' wide bus stop "landing pad" adds an additional 174 sq.ft of impervious surface. It is important
to note that the addition of this area to the original sizing calculations, is negligible and does not
that affect the outcome of our preliminary water quality treatment or infiltration system sizing.
With being said, I've taken a look at the following items to confirm exactly how the landscape
strip/swale, along the Yelm Avenue frontage is currently functioning, and, to confirm if the
intent of the original design was to provide treatment or conveyance of stormwater runoff, or a
combination of both.
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1102 Commerce Street, Suite 300, Tacoma, WA 98402 • (253) 573-0040 • FAX (253) 573-0142
November 12, 2010
Page 2
Following is the list of items I've reviewed:
Topographic survey of the Yelm Avenue Frontage
WSDOT engineering design documents
In addition, and, more importantly I recently visited the site and saw "first hand" the existing
function of the swale.
The topographic survey completed for the subject site, as well as my recent visit, confirmed there
are two (2) curb breaks along the, subject sites, Yelm Avenue frontage. The curb breaks allow
stormwater runoff, from Yelm Avenue, to enter the swale.
More specifically, the northwesterly curb break (Curb break #1) drains into the westerly portion
of the swale where stormwater then flows to an existing catch basin approximately 5 feet to the
west (see picture 1).
The southeasterly curb break (Curb break #2) flows to the easterly portion of the swale where it
ponds, infiltrates and eventually flows to the west (see picture 2).
The survey indicates that the landscape swale generally flows from the southeast to the
northwest. Contrary to the WSDOT engineering plans and the subject site topographic survey,
the pictures clearly show that the swale functions, generally, for conveyance only, with very
little to no treatment occurring. Even if one assumed that the swale was properly seeded and
maintained, the length of the treatment area is effectively less than 8 feet at both curb break
locations.
I did note that the WSDOT drawings call this area out, generally, as biofiltration swales (see
exhibit "A"), but the reality is, there is no treatment occurring presently, nor could occur with the
correct maintenance.
As a result, the final engineering drawings will propose relocation of both curb breaks and the
addition of a new catch basin 5' downstream of relocated curb break #1. In summary, both curb
breaks will be relocated to allow the swales to continue to function as they do today.
Further, due to this design approach, the on-site preliminary design will remain exactly the same
as originally proposed, hence, no change in calculations or design are needed as mentioned in
your October 25th letter.
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November 12, 2010
Page 3
Picture 1.
Note:
Even if the swale was completely vegetated there would be less than 8 feet of effective treatment
area.
The yellow arrow indicates the approximate location of the westerly driveway. As part of the final
engineering design, the curb break will be relocated approximately 8 feet to the east along with the
addition of a new catch basin in the location shown above.
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November 12, 2010
Page 4
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Picture 2.
Curb break will be relocated to a position approximately 3' to the west.
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Curb break #2 located at southeast
end of Yelm frontage approximately
30' west of the northeast property
corner.
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November 12, 2010
Page 5
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Approximate location of new
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Approximate location of new
driveway.
Picture 3.
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This concludes my response at this time. Should you have questions or comments regarding this
matter please feel free to call me at 253-573-0040.
Sincerely,
Timothy D. Holderman, P.E.
Principal
Cc: File
S:~PROJECTS\2010\10104\DOCUMENTS\LETTERS\2010~LandUseResponse letter11.0110.docx
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