HomeMy WebLinkAboutStormwater Plan - GRD Engineering / Geo-Tech Reports - 11/9/1994 MASON COUNTY
PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
Shelton, Washington 98584
November 9, 1994
INTER-DEPARTMENTAL COMMUNICATIONS DATE
TO: JASON NANASSEE, DCD - Planner
FROM: AT-AN A. TARTA, P/W - Hydraulic Engineer
SUBJ: CHET'S TRUSSES - Cammercial Stormwater Site Plan Review
Jason,
I have reviewed the proposed stormwater drainage plan prepared for "Chet's
Trusses" by CONSOLIDATED ENGINEERING, and do not feel that the proposal meets
minimum County requirements for commercial site developments.
Commercial sites are required in Mason County Parking Standards (Ordinance No.
815) to retain surface water runoff on site or dispose of it into an area
approved by the Mason County Engineer. Recognizing that many areas of Mason
County do not readily lend themselves to infiltrating stormwater runoff, Mason
County Public Works has allowed discharges from commercial sites to occur if
the rates of discharge from the site conform to Puget Sound Water Quality
Authority (PSWQA) downstream erosion control standards. These standards
underlie the requirements contained in the Stormwater Management Guidelines
distributed by the Mason County Department of Community Development.
The PSWQA Technical Manual (III-1.3 Minimum Computational Standards) requires;
1. For all computations, except for basin plans or equivalent, the
minimum standard shall be the Soil Conservation Service Unit Hydrograph
(SCSUH) method (1) with the level pool routing method, or equivalent
hydrograph techniques, subject to approval of the local government. The
Santa Barbara Urban Hydrograph (SBUH) method is also an acceptable
method.
A computer simulation using the information submitted by Consolidated
Engineering input into the SBUH method, indicates that the proposed facility
exceeds predevelopment discharge rates by appreciable amounts.
Other concerns include;
1) In calculating the pre-development runoff, the engineer appears to
only address the runoff from the front half of the property, then
applies a 5 year event rain intensity to the site. It would be to the
proponents advantage to use the full site in determining pre-development
discharge rates. The employment of a 5 year storm event has little
relevance in this site's analysis.
File: H:\WP\LETTERS\C-TRSS.IDC
A.
2) Catch Basin #4 discharges stormwater from the site to the north
without any control feature.
3) The extent of material to be excavated/graded cannot be determined
from the plan sheets submitted. Sheet 2 of the drainage plans shows
three elevation contour lines and indicate that a 25 foot cut will be
required for the building. Where this borrow material is to be deposited
is not indicated on the plans. If all the material is to be removed off
site, then the receiving site should be identified and permitted if
necessary. There is a remote possibility that the three contour lines
fully describe the surface topography, but there is no indication of the
frequency of the contours or their relative elevations. The contour
lines need labeling. If the entire area contained within the contour
line shown in the northeast corner of the property were excavated 25
feet deep, the volume would be approximately (2)x(70x8Ox25)/27=2,590
cubic yards.
In stumnary, I feel the stormwater plan needs further designing and information
needs to be submitted to facilitate calculation of earthwork quantities.
l 47,M �
A. TAIi7A
File: H:\WP\LETTERS\C-TRSS.IDC