HomeMy WebLinkAboutDrainage Plan for BLD2002-01578 - PLN General - 5/6/2004 MASON COUNTY
PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
Shelton,Washington 985"
DATE: May 6t', 2004
INTER-DEPARTMENTAL COMMUNICATIONS
TO: Rick Mraz, DCD -Planner
FROM: Alan A. Tahja, P/W- Co. Hydr. Engr.
SUBJ: Drainage Plan NAME: Howick SFR 2 Beard's Cove - Belfair
BLD2002-01578
Rick,
I've been in contact with Mr. Glen Coad, P.E., who's been developing a plan to alleviate a
stormwater runoff problem on the Rick Howick property at Beard's Cove. Re-routing drainage
around the Howick property was investigated, but the County's maintenance department could
not support the proposed route, so Mr. Coad developed the attached plan.
The applicant will need an access permit issued by Public Works for the access shown on the
attached plan, and he will also need a utility permit issued by Public Works for the proposed
connection to the County's road cross culvert.
A copy of the attached plan has been forwarded to my maintenance crews for their review, but I
do not expect any changes or objections to this proposed drainage work. If there are any major
changes resulting from the road departments review of the plans, I'll let you know.
Please feel free to contact me at County extension 461 if you have any questions regarding these
comments, or if you feel any features need further discussion or attention.
S' c rely,
t.
Al n A. Tahja
File: H: \WP\Strmwtr\Reviews\Howick.doc
W. Glen Coad, P.E.
6512 192°d Ave East
Bonney Lake, WA 98390
April 16,2004
Mr. Allen Tajah
Mason County
Department of Community Development
426 West Cedar
Shelton, WA 98584 -
RE: Howick Drainage System, Parcel 912-33-05-20-00-14, BLD2002-01578
Dear Allen,
Enclosed please find our presented solution for the existing culvert that drains into the Howick
property.
PROJECT DESCRIPTION
The project is located at 10 NE Schooner Loop in Mason County, WA. The legal description is
lot 14 of Plat of Beards Cove No. 5 and is assigned the tax parcel number of 12-33-05-20-00-14.
Currently the site is a vacant undeveloped lot,which is surrounded by developed properties. The
site drains from west to east and has slopes approximately 2:1 along the road on the west side,
which tapers to approximately 5%across the remainder of the lot.
CURRENT DRAINAGE CONDITIONS
Figure 1 represents the three basins of which basin"B1"drains to the culvert,which crosses NE
Schooner Loop and discharges into lot 14. The culvert is located about mid-way along the west
property line and discharges about 3' below the road surface and approx. 3' above the existing
ground of the project site. Field observations indicated that water had flown through the culvert
in the past with minor erosion occurring as a result of the flow.
Given the three foot drop from the invert of the culvert to the ground below and the relative lack
of erosion it would seem that while water has flown in the culvert it was unlikely that it has been
a large amount of water. From the culvert discharge point the water continues to travel to the
east in an intermittent shallow channelized flow condition. Under these conditions the flow
travels approximately 60' at which point it leaves the property as a shallow flow onto the
neighboring property(lot 18).
Based on the basin map, areas calculated and the number of lots peak flows were calculated
using the Department of Ecology's Western Washington Hydrologic Model, (WWHM)version
2.5c. Basin"B1"represents an area of approximately
9.62 acres with 35 lots. Inputting this data into the model generates peak flows ranging from
3.58cfs for the 2yr to 8.24cfs for the 100yr storm. This basin contributes to the existing culvert
which discharges onto the property. Although the peak flows are relatively high compared to
what the field visit would suggest they have been used in sizing the proposed conveyance system
for the sake of conservancy.
PROPOSED DRAINAGE CONDITIONS
As discussed with the county in previous correspondence it has been determined that
abandonment of the existing culvert and re-routing the flow and containing it within public right
of way is not a viable option.
As such we have shown a connection to the existing culvert via a storm drain manhole and
extending the culvert a further 40' to get under the proposed driveway where it discharges into a
rip-rap pad. The discharge point will be constructed in such a way as to provide a pool and
overflow across a 10 foot wide berm. This discharge condition is intended to restore the flow
back into it's original pre-developed condition.
The existing culvert is approximately 18" in diameter, the new pipe will also be 18"to maintain
consistency in the conveyance system. The capacity of an 18"culvert at the proposed slope (1%
min) is 10.5cfs based on full flowing and n=0.013. This is more than enough to flow the 100yr
storm.
CONCLUSION
In terms of the change in flow conditions there will be no significant change of peak flows as a
result of the proposal or in discharge conditions from the site from pre to post developed
conditions. The pre-developed flows as previously stated were reasonably well defined as a
shallow discharge condition. The proposed conditions will not alter the peak flows (other than
the increase from roof drainage) and the discharge conditions will be similar in form as a result
of being spread across the width of the rip-rap pad thereby restoring the pre-developed discharge
conditions from the site.
I trust that this and the provided drawings will be sufficient to meet the County needs and will
allow for the release of the building permit for the project site. If you have any questions
regarding the project please feel free to call me at (253)228-0513 to discuss or e-mail me back.
Sincerely
CE GI_F�
Glen Coad, P.E. f
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WESTERN WASHINGTON HYDROLOGY MODEL V2
PROJECT REPORT
Project Name: default
Site Address:
City Belfair
Report Date 4/16/04
Gage Quilcene
Data Start 1955
Data End 1999
Precip Scale: 1.22
PREDEVELOPED LAND USE
Basin Basin 1
Flows To Point of Compliance
GroundWater: No
Land Use Acres
TILL GRASS: 6.245335
IMPERVIOUS: 3.374665
Standard residential land areas:
Soil Type Lot Acres Number of Lots
Outwash 0.000 0
Till 9. 620 35
Saturated 0.000 0
DEVELOPED LAND USE
RCHRES (POND) INFORMATION
ANALYSIS RESULTS
Flow Frequency Return Periods for Predeveloped
Return Period Flow(cfs)
2 year 3.575683
5 year 4.677784
10 year 5.463017
25 year 6.52069
50 year 7.357641
100 year 8.237853