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INTRODUCTION
The proposed building with one residential unit, one warehouse unit and one warehouse/retail unit
is located on Lot 5, Block 2 of Glen Lake Commerce Subdivision [Plat J-642] in Bozeman,
Montana. A combination of site grading, drywells, and existing subdivision stormwater
infrastructure will be used to manage stormwater runoff for the development of the site. A
Drainage Area Map is included in Appendix A. Calculations for each individual drainage area are
included in Appendix B. Photos of the existing retention pond for the subdivision are included in
Appendix E.
DRAINAGE AREAS
Drainage Area 1
Drainage Area 1 consists of the proposed drive approach, the building, a portion of the sidewalk
and two landscaped areas. Runoff from Drainage Area 1 flows via sheet flow and downspouts into
Iron horse Road. The runoff is then carried north into the existing subdivision retention pond in
Lot 9. It was found that the 10-year, 2-hour storm generated 247 cubic feet of runoff.
Drainage Area 2
Drainage Area 2 consists of a portion of the sidewalk and the parking lot. Runoff from Drainage
Area 2 flows via sheet flow into the proposed Drywell # 1. The drywell was sized according to
City of Bozeman Design Standards to retain the volume from the 10-year, 2-hour storm event. DA
2 will generate 151 cubic feet of runoff, the proposed Drywell #1 will provide 172 cubic feet of
storage. Storage calculations are included in Appendix C.
EXISTING RETENTION POND ADEQUACY CHECK
The existing retention pond for the entire subdivision is located on Lot 9, Block 2 and was sized
according to City of Bozeman Design Standards to retain the volume from the 10-year, 2-hour
storm event. The subdivision stormwater design report by Madison Engineering calculated the
required volume for the entire subdivision as 17,364 cubic feet using a C value of 0.65 for all
future developable lots, including Lot 5, Block 2. A retention pond with a volume of 19,275 cubic
feet was installed providing an excess of 1,911 cubic feet of storage capacity. Using a C value of
0.65 it was determined that Lot 5, Block 2 generated 330 cubic feet of runoff prior to development.
The actual C value for the proposed development of Lot5, Block 2 was calculated as .78, which
results in an excess of 68 cubic feet of runoff. As described earlier in the report only DA 1 will
contribute runoff to the subdivision retention pond. The site will send 247 cubic feet to the existing
retention pond which is approx. 75% of the volume Lot 5 contributed prior to development.
APPENDIX A
DRAINAGE AREA MAP
APPENDIX B
DRAINAGE AREA CALCULATIONS
APPENDIX C
DRYWELL SIZING CALCULATIONS
APPENDIX D
EXCERPTS FROM GLEN LAKE COMMERCE
SUBDIVISION STORMWATER DESIGN
REPORT, 2018 (MADISON ENGINEERING)
APPENDIX E
PHOTOS OF EXISTING RETENTION POND