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Les Schwab Tire Center
Tigard Promenade Store
Report
Prepared for:
City of Tigard
13125 SW Hall Boulevard
Tigard, Oregon 97223
Prepared by:
W &H Pacific
8405 SW Nimbus Avenue � PRO
Beaverton, Oregon 97008 � c `' 0%G IN
to
April 24, 1996 REGON
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DRAINAGE REPORT
INTRODUCTION
The purpose of this report is to analyze and discuss the hydraulic and hydrologic conditions for a +/ -1.2 acre
parcel, located within the City of Tigard. The subject land in this study is zoned general commercial. The
proposed use of the land is for a retail shopping center. Water quality and detention facilities have been approved
and constructed as a part of the Tigard Promenade project. This report deals only with onsite systems for the Les
Schwabfl'igard Promenade project.
STORM DRAINAGE SYSTEM
Due to a shallow outfall in Highway 99W and relatively flat grades on the project site, Uniform Plumbing Code
design standards for pipe slope could not be followed. The storm drainage system was designed using the rational
method to convey a ten year storm event at developed condition for each basin's respective runoff. Mannings
formula was used to calculate pipe capacity. An 'n' value of 0.014 was assigned to ductile iron pipe and an `n'
of 0.012 was used for ADS N -12 and PVC pipe.
The attached drainage basin map shows locations of the basins and structures that are tabulated in the
calculations.
STORM WATER QUANTITY DETENTION
The existing Tigard Promenade detention facility was designed, approved and constructed with adequate capacity
for the developed Les Schwab site.
STORM WATER QUALITY FACILITIES
The existing Tigard Promenade water quality facility was designed, approved and constructed with adequate
capacity for the developed Les Schwab Site.
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W&H PACIFIC
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