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TRADITIONS
5-PLEX
July 2021
Strom Drainage Engineering Report
Prepared For:
The City of Bozeman
121 N Rouse Ave
Bozeman, MT 59715
Prepared By:
Woith Engineering, Inc.
405 3rd Street NW, Suite 206
Great Falls, Montana 59404
Traditions 5-Plex
Storm Drainage Engineering Report
Page 3
ENGINEERโS REPORT
GENERAL INFORMATION
a. (see water design report for introductory development information)
The proposed stormwater drainage system for the development will be a simple
swale stretching from the southeast end of the property, facing Durston Rd (front
end of the 5-plex) at a slope of approximately 1%.
b. Owner contact information: Montana Land Project, LLC
PO Box 2163
Great Falls, MT 59403
c. Stamp included on front Cover Page
RATIONAL METHOD
Phase 1 of the Traditions subdivision is a smaller portion of sub-area #1-2 as described
in the Rosa Subdivision (Traditions) stormwater report by C & H Engineering and
Surveying, Inc. C & H shows that the total projected post-developed outflow from the
entire, 3.1770-acre, sub-area for a 25-year storm event is 1.56 cfs.
Phase 1 of the Traditions subdivision is approximately 0.421 acres. Therefore, the post-
developed storm outflow from that area was calculated as follows:
๐๐ด=1.56 cfs * 0.421 acres
3.1770 acres =0.206 cfs
Post-developed time of concentration, as well as rainfall intensity, and storm outflow for
a 25-year storm event was calculated as required by the City of Bozemanโs Design
Standards, once using a runoff coefficient of 0.35 (the parcel is zoned as โmedium
density residentialโ) and again using a composite runoff coefficient of 0.51, as
determined from the site layout:
C=0.35
๐๐25(249.9 ft @ 1.0%, C =0.35) = 98.12 min
๐25(X=98.12
60 hours)=0.78(X)โ0.64 =0.569 in/hr
Traditions 5-Plex
Storm Drainage Engineering Report
Page 4
๐๐ด1 =๐ถ๐๐ด=0.35(0.569 in/hr)(0.421 acres)=0.084 cfs
C=0.51
๐๐25(249.9 ft @ 1.0%, C =0.51) = 73.96 min
๐25(X=73.96
60 hours)=0.78(X)โ0.64 =0.682 in/hr
๐๐ด2 =๐ถ๐๐ด=0.51(0.682 in/hr)(0.421 acres)=0.146 cfs
Therefore, the post-development storm outflow for Phase 1 of the Traditions subdivision
will be less than the pre-development outflow.
REFERENCES
C & H Engineering and Surveying, Inc., โDrainage Area #1,โ Storm Water Management
Design Report โ Rosa Subdivision
City of Bozeman, Montana, โII. Drainage Policy E. Estimation of Runoffโ, Design
Standards and Specifications Policy