HomeMy WebLinkAboutStormwater Report Review for GRD99-0016 - GRD Engineering / Geo-Tech Reports - 3/6/2000 MASON COUNTY
PUBLIC WORKS DIRECTORICOUNTY ROAD ENGINEER
Shelton,Wuhington 98584
DATE: March 6, 2000
INTER-DEPARTMENTAL COMMUNICATIONS
TO: Allan Borden, DCD - Planner
FROM: Alan A. Tahja, P/W- Co. Hydr. Engr. DCD WO#00-009
SUBJ: Stormwater Report Review NAME: Jack Johnson- Sweetwater
GRD99-0016
Allan,
Additional information has been received from Mike Wnek, P.E.,pertaining to Jack Johnson's
SWEETWATER CENTER proposed in Belfair, Washington. Also I discussed this project with
Mike when we met with him and Jack at Jack's DETROIT TOWNSITE development.
Mike has addressed the issues I questioned in my February 151h IDC regarding this proposal, and
I am persuaded to agree with the proposal as submitted with one exception. Mike has
recalculated the flows expected to be delivered from off-site sources to the subject property, and
has upsized the proposed tightline culvert through the property from a 12 inch to an 18 inch
culvert.
With this change to the proposed development, I feel this project should be allowed to proceed
with permits and development.
An Operation and Maintenance (O&M) Covenant is required to be created to cover the approved,
constructed stormwater control facilities. Jack Johnson has been given a copy of the form
required to be used, and I request that final approval of the facilities for occupancy not be granted
until the O&M requirement has been satisfied, and I have been given an opportunity to inspect
and approve the completed stormwater control facilities.
Please feel free to contact me at County extension 461 if you have any questions regarding these
comments, or if you feel additional concerns need further attention.
Sincerely,
r
Alan A. ahja
Encl: new submittals
File: H:\WP\STRMWTR\REVIEWS\JJ-Sweet-3.doc
MASON COUNTY
PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
Shelton,Washington 98584
DATE: Feb. 15. 2000
INTER-DEPARTMENTAL COMMUNICATIONS
TO: Allan Borden, DCD - Planner
FROM: Alan A. Tahja, P/W- Co. Hydr. Engr. DCD WO#00-009
SUBJ: Stormwater Report Review NAME: Jack Johnson - Sweetwater
GRD99-0016
A Stormwater Site Plan prepared for the proposed Jack Johnson SWEETWATER MALL has
been received and reviewed by Public Works, prompting the following observations;
(1) A french drain is proposed along the westerly edge of the proposed infiltration pond. The
engineer has promoted the soils in the infiltration pond area as being excessively porous.
This subterranean gravel pipe this close to the infiltration facility can be expected to
provide a short circuit route for water escaping out from the infiltration pond to daylight
out directly to Sweetwater Creek. Even with the installation of the curtain drain, the water
profile cannot be expected to be at a constant elevation below the infiltration pond floor,
and the 3 feet minimum requirement for unsaturated/permeable soil between the pond
floor and an underlying restrictive layer(water table) is not likely to be realized. The
engineer has typified the soil as an A soil with an infiltration rate of 20 inches per hour (3
minutes per inch). I can entertain a reasonably high infiltration rate, but a soil classified as
an XXX-XXX-XXX-LOAM, is typically not coarse sands or cobbles, but is a sandy loam
or gravelly loam. Table 111-3.1 which appears to be the basis for the infiltration rate
ascribes sandy loam with an infiltration rate of 1.02 inches per hour. This pond will
probably not work as designed, and will either remain full of water, or will short circuit to
the curtain drain.
(2) The bioswale function has been checked with a computer simulation employing most of the
engineer's input parameters. The Manning "n "numbers employed in the bioswale
modeling by the engineer are inconsistent and their retardance effect unachievable. It can
be entertained that the i► number would change as the flow through the bioswale increased
(grass bends over during high flows resulting in lower n numbers), but the n number used
for the 6 month storm is too large to be defendable. The design of the bioswale needs to
be reconsidered.
(3) The analysis of the upstream basin employs the lowest runoff coefficient available, a
runoff number representing an undisturbed forest (42). A runoff curve number for second
growth forest land in a A soil is 55. I've modeled the basin using a CN of 55 and
developed a 100 year flow nearly double the peak rate indicated in the engineer's
File: H:\WP\STRMWTR\REVIEWS\JJ-Sweet-2.doc
, f
simulation. The engineer has routed his 100 year flow through a culvert with a friction
coefficient representing a high capacity smooth bore plastic pipe. My modeling indicates
that the culvert will be overwhelmed. The highway culvert is shown to be a concrete pipe
with a relatively steep slope (5.84%). It's capacity should at a minimum be matched, if not
exceeded in anticipation of its upsizing in answer to almost certain increases in runoff due
to development of upland properties.
With these issues in mind, I don't feel this design meets minimum requirements for runoff
treatment and attenuation, and that the issues I've identified need to be revisited by the engineer.
When these issues have been revisited and an acceptable design developed, an Operation and
Maintenance Covenant will need to be created to cover the approved constructed stormwater
control facilities.
Please feel free to contact me at County extension 461 if you have any questions regarding these
comments, or if you feel additional concerns need further attention.
Sincerely,
I——la-417N.
Alan A. Ta ja
File: H:\WP\STRMWTR\REVIEWS\JJ-Sweet-2.doc
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SITEPLAN Prepared By SCALE:1" = 20' Revisions: Prepared for:
LAMPLIGHT HOMES DISPLAY MICHAEL F.VMEK,P.E. DATE: 08-30-01 JACK JOHNSON
�A CONSUL"NG aW ENGINEER CHECKED BY: MFW P.O. BOX 1119
SWEETWATER CENTER 1665 NWDESIGN BY: MFW BELFAIR, WA 98528
BREMERTON, W WA_ 96311
HWY 3, BELFAIR SHE DRIVE DRAWN BY: MFW
REA.
MASON COUNTY WASHINGTON 1 (360)692-3802
(360)-275-5400 ((Oz)-275-6400