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Sean Murray, P.E.
Engineering Manager OREGO
Macdonald- Miller of Oregon
5711 SW Hood
Portland, OR 97201
RE: Tigard Corporate Center — Storm Drainage System Alternate Design
12125 SW 69
PLM2000 -00131
Dear Sean,
We are in receipt of the proposed altemate method of construction for the storm
drainage at the above referenced address. We have reviewed your submittal and have
found that the alternate is at least equal to, or greater than, what the code prescribes.
Therefore, under Section 301.2 of the State of Oregon Plumbing Specialty Code and
Section 104.2.8 of the State of Oregon Structural Specialty Code, we are granting one
time approval of this system. This is specific to this project and other such requests will
be reviewed on a case by case basis.
If you have any questions regarding this, please do not hesitate to contact me at (503)
639 -4171 ext. 311.
Sincerely,
Gary Lampella
Building Official
c. Mike Sheehan, Senior. Plumbing Inspector
Bob Poskin, Senior Plans Examiner
Project File
13125 SW Hall Blvd., Tigard, OR 97223 (503) 639 -4171 TDD (503) 684 -2772
MacDonald - Miller of Oregon
5711 SW Hood
Portland. OR 97201
503 230 -8991, Fax 230 -9238
/V111/1k Vancouver 360 694-7294 f
MacDonald - Miller REGF47,TvED
a GroupMACcompany ' MAY 0 5 2000 a
May 3, 2000
Mr. Mike Sheehan
City of Tigard — Plumbing Plans Examiner
13125 SW Hall Blvd.
Tigard, OR 97223
RE: Tigard Corporate Center — Parking Ramp Storm Drainage System Alternate Design
Dear Mike,
This letter is presented to document the design basis for the storm drainage system at the Tigard
Corporate Center Parking Ramp.
As directed by your office the deck drains serving the areas open to the sky on the upper levels of the
ramp will be routed through an oil /water separator. This separator will be installed in a bypass
arrangement as detailed on our plans. The interior deck drains and trench drains at each entry will be
routed directly to the storm drainage system.
In calculating the required size of the separator we first looked at the full flow of the drainage system
based on the maximum predicted hourly rainfall for the Portland area as presented in UPC Table D -1
(Portland area 100 year storm = 1.3" per hour) This calculation yielded a flow from the upper deck of
340 GPM. Routing this flow through oil water separators we would require (2) large tank -type
separators each sized to retain 2,100 gallons of water with dimensions of 6'x 12'x8' deep.
We felt that designing to this level of capacity was physically excessive for the application. For this
reason we proposed using a single separator tank installed in a bypass arrangement as shown on the
plans submitted for permit review. This separator will accommodate a flow of 207 GPM with a 3,100
gallon retention capacity. The full flow from the upper level deck drains will be routed through the
separator for up to 60% of the predicted maximum hourly rainfall (.78 "/hour) Any storm flow in excess
of the 60% flow rate will bypass the separator and go directly to the storm sewer system.
We feel that the bypass system will provide excellent protection for the storm system for the majority of
rainfall that will be encountered. More importantly the systems ability to handle 60% of the rainfall in a
peak event will still permit the critical initial portion of the flow, which will wash most of the
contaminants off the deck, to be processed through the separator.
As you are aware we have worked closely with your office to arrive at and detail this alternative design
approach. We appreciate the time and guidance that you have offered thus far, and look forward to
working with you to successfully install this system.
Sincerely,
Sean Murray, P.E. • lip
Engineering Manager
MacDonald - Miller of Oregon
503/736 -0570
GCB 137340