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June 29,2016 "l• `: k l'--. . ,
JUN 3 0 ?t1 (
Building Department
City of Tigard 1 yr OV"iF ;A`°
13125 SW Hall Blvd.Tigard,OR 97223 iltGiK � `
RE: Stormwater Lateral Analysis
Summit Ridge No. 5, Lots 144 through 149
This letter is intended to summarize our hydraulic analysis of two existing stormwater laterals that will be
utilized to collect runoff from Lots 144 through 149 of the Summit Ridge No.5 subdivision.
The homebuilder(DR Horton)is proposing/constructing a combination drainage system that utilizes both a fl-
inch perforated pipe and a 4-inch solid wall pipe with a 1'x2'burrito wrap within a drain rock trench. The
perforated and solid wall pipes will be connected to the existing 4-inch storm drain lateral for Lots 146&147.
The solid wall pipes will be utilized to collect roof runoff and yard surface area drains,while the perforated
pipe will collect any subsurface drainage. We understand that separate drainage easements will be recorded
along the rear of the lots to allow for this shared drainage system.
The combination drainage system will convey stormwater runoff from the backyard areas and the proposed
roof area for Lots 146&147(assumed to be approximately 2,640 square feet). The roof area for the
remainder of the lots(Lots 144,145,148,&149)will be directed towards their respective storm laterals to
the north.
The existing/proposed 4-inch storm drain laterals were evaluated assuming a minimum 2%slope. A
precipitation depth of 3.90-inches,per the Clean Water Service's Design and Construction Standards for
SanitarySewer and Surface Water Management(R&O 07-20),was utilized for our analysis(24-hr,25-year
design storm).
The stormwater analysis indicates that the existing 4-inch lateral pipe provided for Lots 146&147 have
sufficient capacity to convey the anticipated stormwater runoff from the proposed private storm drainage
system(see attached HydroCAD Stormwater Analysis).
Sincerely, P
)1a..44,9/4--41,4k :.. 6— -
Nye INfF`w
Paul Seilke PE,GE '
AKS ENGINEERING&FORESTRY,LLC
A�roa.�°
Attachments: 044E
A S ����.
Exhibit A—Private Storm Trench Detail 1101[rUls 30. 2011
4105 20160629 Lots 144-149 Storm Drain HydroCAD Report