HomeMy WebLinkAbout938100427 Stormwater Mgmt
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STRATUM GROUP
1451 Grant Street, Bellingham, W A 98225
Phone (360) 714-9409
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APR 0 9 20D7
April 5, 2007
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Mary De V eneau
141 Sunset Blvd,
Port Townsend, W A 98368
Re: Review of Drainage Plan
Block 4, Lot 30 and 27, Sunset Blvd, Cape George
Port Townsend, Washington
Dear Ms De V eneau: .
As per your request I have reviewed the site drainage and storm water plan prepared by Low
Impact Engineering. The foundation drainage is consistent with the recommendations I made in
my March 7, 2005 Engineering Geology Evaluation.
The location of the trapezoidal stormwater infiltration area and infiltration of water at the
proposed location between the home site and Sunset Blvd. will not cause any slope stability
problems on or off of the site. Patrick Herbig, P .E. indicated that the trapezoidal area should be
lined if glacial till is not present. Glacial till is not likely underlying the site; however, silt/clay
layers associated with early advance outwash or transitional alluvial beds likely underlie the site
and this compact silt/clay units will perform as well or better than till if they are present. If
silt/clay is not present then a liner per the engineer's recommendation should be installed.
Stratum Group appreciates the opportunity to be of service to you. Should you have any
questions regarding our reconnaissance please contact our office at (360) 714-9409.
Sincerely yours,
Dan McShane, L.E.G., M.Sc.
Licensed Engineering Geologist
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LOW IMPACT CIVIL ENGINEERING Pc.
PO BOX 65229, Port Ludlow, W A 98365
360-581-9497
JEFFERSON COUNTY
DEPT. OF COMMUNITY DEVELOPMENT
November 26,2006
Mary & Michael Deveneau
141 Sunset Blvd.
Port Townsend, W A 98368
RE: Engineered Drainage Report for
Block 4, Lot 30 & 27
(341 Sunset Blvd. Port Townsend, W A 98368)
Dear Mr. & Mrs. Deveneau:
On September 28, 2006 we met at your proposed building site (341 Sunset Blvd). I
observed very healthy wetland like vegetation throughout most ofthe site as well as
approximately 1.5 gallons per minute of groundwater exiting to the adjacent roadside
drainage ditch. I was informed that Jefferson County has requested an engineered
drainage report to be reviewed by Jefferson County, your geologist, and your onsite
septic designer.
DRAINAGE FOR HOUSE; Attached are details for a drainage system to keep the
proposed construction site free from ground and surface water. See the attached
subsurface drainage details. As was mentioned in the Stratum Group report, the entire
foundation & footing should be drained using a network of 4 inch perforated PVC pipe.
The 4-inch perforated pipe should have several clean-outs at the upstream portion of the
drain pipes. It is my recommendation to bed the pipe in 12-inches of 1.5-3 inch clean
crushed rock and then bridge this layer with 12-inches of 3-inch minus structural fill. See
attached calculations for drain pipe cover.
DRAINAGE MODEL; The Western Washington Hydrology Model 3 (WWHM3)
software program was used to determine the size of the trapezoidal stormwater
infiltration area necessary to treat the surface water and ground water from the site.
APPROVED
STORMW A TER PLAN
APR 1 7 2007
.
TRAPEZOIDAL STORMWATERINFILTRATION AREA; The dimensions of the
bottom of the area are 8-feet by 8-feet; the depth of the area should be 5-feet. With side
slopes of the area at 3 to 1 the dimensions at the top of the area will be 38-feet by 38-feet.
The outlet riser height should be 4-feet and the orifice diameter should be lf4 -inch (1/2-
inch is acceptable if lf4 -inch cannot be installed), Also, there should be a rock lined
overflow spillway at a height of 4-feet. All these dimensions should be reflected in the
site plan and/or details for the stormwater infiltration area. The area set-backs need to be
at least 10-feet and the area should be lined (if glacial-till is not present) with compacted
glacial-till. WWHM3 calculations are attached and I have attached a sample stormwater
infiltration area with details.
SITE PLAN; The site plan should show the 4-inch perforated pipe and the 4-inch house
downspout tight lines coming together in a small catch basin well below the grade of the
subsurface drains as to prevent any back-up into the perforated drain pipe. The drainage
should be conveyed from the catch basin via a 6-inch tight line, or a rock lined ditch, to
the trapezoidal stormwater infiltration area. The trench for the 6-inch tight line should
also act as a French drain to pick up drainage form the driveway; this should also be
indicated on the site plan. All other drainage on the site should be directed to the
trapezoidal stonnwater infiltration area via ditch and/or pipe.
I hope this will provide the necessary information to complete your site plan and obtain
the necessary building permits. Should you have any question about this report or any
other matters please contact me directly at 360-581-9497.
Sincerely,
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Patrick Herbig, P.E.
Low Impact Civil Engineering, Pc.
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