HomeMy WebLinkAboutStorm Drainage Plan Review for BLD93-01056 - COM Letters / Memos - 2/14/1994 Y
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PPLKAT/ONS,
Febuary 4, 1994
Alan Tahja
Drainage Engineer
Mason County Department of Public Works
P. O. Box 1850
Shelton, Washington 98584
Dear Alan, -
As I have indicated in our recent discussions, Bob Lenhart has retained me
to revise the site design for the proposed AMPM Little Deli No. 3 in Belfair. The
previous consultant was Micheal Wnek of Bovay Northwest. The major improve-
ments that I made to the site consisted of moving the drainfield to a more suitable
location up on the hill to the South of the property. Removing the drainfield from
the back of the property provided the opportunity to construct a drive around the
building and to increase the number of parking spaces from 11 to 19. The in-
crease in paved area created an increase in the storm water runoff from the site
so it was necessary to revise the design of the storm water control system. I have
enclosed the following for your review and comment:
1: A set of existing and proposed watershed maps and calculations for the
2, 10 and 100 year storms.
2. A proposed strategy for improving water quality treatment for the site.
3. A revised drawing for the site drainage.
When I began the project I did a field review of the existing site drainage
and the topography. The topography map of the site was expanded to include
more the area off the site on the South and West boundaries. Careful analysis of
this new data resulted in the mapping of the existing site drainages show on the
maps included with the calculations. I did not consider it desirable to divert all the
surface water from this site that flows into the watershed that is drained by the
small unnamed creek to the South of the site. Therefor, I separated the clean roof
runoff from existing liquor store and the proposed AMPM Market and the canopy
over the gas pumps from the rest of the site and drained it to Southerly side of the
Engineering Software Applications,Inc. 210 N.E.Cherokee Beach,Belfair,Washington•98628 (200)276.0578
site. I then designed an infiltration system using two, 60 foot runs of H-10
graveless infiltration units developed by the Infiltrator Company in Old Saybrook,
Conneticut to both store the peak runoff flows and soak the water into the ground.
The outlet from these units will be controlled by two orifices located in a plate in
the outlet catch basin. The lower orifice will be 1.5 inches in Dia. and at an Elev. of
59.0 and the upper orifice will be 4 inphes in diameter and located at Elev. 60.4.
The infiltrators will serve to lower the ground water table in the wetter months so
that infiltration of the storm water into the ground will be possible during wet peri-
ods.
The current calculations demonstrate that the site meets the stated design
criteria by not exceeding the existing storm runoff flow rates for the 10 and 100
year storms and not exceeding 1/2 the storm water flow rate for the-2 year storm.
.,Please review the hydrographs I have, included to for a visual demonstration'of the
calculation results.
Please note that I have continued to use the drawings previously approved
by your office that were prepared by Mike Wnek for the storm water detention
facility and the oil/water separator. The drawings remain unchanged except that
the upper orifice in the distribution vault changed based on the new calculations.
increased the outlet elevation from Elev. 54.01 to 54.41 and increased the orifice
size from 5 inches from 5.5 inches to decrease the two year storm flows for the
entire site. Based on lower storm water flows to the oil/water separator, resulting
from moving the roof drainage to the South side of the site, I was able to reduce
- the oil/water separator size from 577-CPS to 660-CPS.
am very pleased with the proposed redesign of this system. It exceeds your
criteria for the site yet keeps the clean water from the three roofs flowing into the
stream system in a controlled manor. The average flow to the 660-CPS (Utility
Vault Co.) is approximately 100 to 120 gpm which is less then the recommended
125 gpm process rate allowed through the unit. The unit can pass a maximum
flow rate of 415 gpm.
Thank you for your visit to the site and your interest in this project. Your coopera-
tion has helped me to prepare this design.
Your truly,
h A
14Z
Pa McCullough, P.E.
President, ESA
MASON COUNTY
PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
Shelton, Washington 98584
February 14, 1994
INTER-DEPARTMENTAL COMMUNICATIONS DATE
TO: JASON MANASSEE, Planner - DCD
FROM: ALAN A. TAHJA, Office Tech. - P/W
SUBS: Belfair AM/PM - BOVAY - ESA, McCullough Proposal
Jason,
I have reviewed the proposed storm drainage plan prepared by Pat
McCullough, P.E. of ESA and feel his proposal provides adequate stormwater
provisions. The plans submitted allude to an oil/water separator to be
utilized that is detailed in the original plans submitted by Bovay Northwest.
The plans submitted by Bovay only say that an oil/water separator will be
sized to accommodate the water quality storm event (6 month storm event) . In
the absence of more specific specifications, I would request that
specifications for the oil/water separator to be used be submitted prior to
its installation to ensure it provides adequate capacity. McCullough alludes
to a Utility Vault Company 660-CPS with a recommended capacity less than 125
gpm. McCullough's calculations show that the water quality storm would deliver
just under 100 gpm, so I feel comfortable with his proposal . I would like to
make myself available to accompany whoever does the final inspection and
acceptance of the stormwater drainage facility.
ALAO A. TAHJA
File: H:\WP\LETTERS\BEL-AMPM.IDC