HomeMy WebLinkAbout11--Stormwater Design ReportStormwater Design Report
Bozeman Pilates Site Plan
WGM Project Number: 210624
Client: Christopher Yeley – Local Element Architects, LLC
08.02.2022
109 East Main Street Suite B, Bozeman, MT 59715 I OFFICE 406.728.4611 I EMAIL wgm@wgmgroup.com
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REPORT DATE:
08.02.2022
AUTHOR:
Luke Stein, PE
Project Engineer
WGM Group, Inc.
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PROJECT INFORMATION
The proposed Bozeman Pilates project is a mixed-use development located on a 0.31 acre parcel in
Section 34, Township 01 South, Range 05 East of P.M.M., Gallatin County, City of Bozeman. A
combination of site grading, curb and gutter, storm drain lines, and retention chambers will be used to
manage stormwater runoff from the site. Supporting stormwater calculations are attached to this report.
A drainage area map is included in Appendix A. Calculations for each individual drainage area (total area,
weighted C-factor, and time of concentration) are included in Appendix B.
UNDERGROUND STORAGE DESIGN
The proposed underground storage chambers have been sized according to the City of Bozeman Design
Standards. Retention facilities are sized to capture the entire volume of the 10-year, 2-hour storm event
calculated using the Rational Equation. Calculations used for sizing the chambers can be found in
Appendix C.
STORM SEWER FACILITIES DESIGN
Storm sewer facilities (storm pipes and storm chases) were sized for the 25-year storm calculated using
Manning’s Equation. For the purposes of this report and to maintain a conservative approach, one
calculation was performed for the whole drainage area which is more runoff that any one pipe will
encounter. Calculation used for sizing the storm sewer facilities can be found in Appendix D.
PROPOSED STORMWATER SYSTEM
The proposed site consists of a building with roof drainage, a parking lot, sidewalks for pedestrian travel,
and landscaped areas. The overall site is 76% covered with hardscaped surfaces, which will produce
more surface runoff than what was originally allocated for this site during the Baxter Meadows Phase 2A
subdivision process. Because of this, underground retention chambers (Contech 18” perforated pipe) are
proposed to retain and infiltrate most of the runoff from this development. Due to design constraints, a
small portion of roof drainage (334 square feet) is proposed to flow into Equestrian Lane, however, this
runoff volume will not inundate any of the existing stormwater infrastructure installed with Baxter
Meadows Phase 2A.
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UNDERGROUND STORAGE CHAMBERS
The proposed underground retention chambers are located within the proposed parking lot, just south of
the existing alley. The proposed chambers are manufactured by Contech. They receive runoff from
Drainage Area 1, totaling 0.30 acres. Runoff from Drainage Area 1 is conveyed via surface flow, gutters,
and storm piping from roof downspouts to the proposed chambers. The required storage volume was
calculated to be 697 cubic feet. The proposed chamber design provides 711 cubic feet of runoff and can
therefore adequately handle the runoff from this site.
EQUESTRIAN LANE RUNOFF
There is a small portion of roof drainage that is not feasible to route to the proposed retention chambers.
This runoff is from Drainage Area 2, totaling 0.01 acres. Runoff from Drainage Area 2 is conveyed via
roof surface and gutter flow to a proposed downspout. A curb chase is proposed to convey the runoff
from the downspout into Equestrian Lane. The runoff entering the street/public storm system for Baxter
Meadows from this Drainage Area totals 21 cubic feet, which is well below what was allocated for this lot
during the Baxter Meadows Phase 2A subdivision review.
GROUNDWATER
The proposed retention system was designed with the seasonal-high groundwater level (SHGWL) in
mind. According to the geotechnical investigation report prepared by C&H Engineering and Surveying,
Inc. (included with the site plan submittal), groundwater was observed approximately 4’ below ground
surface. The existing grade where the underground retention chambers are proposed is approximately
4709’ which puts the SHGWL at 4705’. The bottom of the proposed chambers is set 15” above this level
at 4706.25. Several options are available to raise this bottom elevation if it is determined that the
groundwater level is higher than reported as no official groundwater monitoring was conducted. These
options include going to a smaller chamber pipe diameter and increasing the overall footprint of the
system. Due to the compact nature of this site and the desire to maintain all separation requirements, the
proposed system shown on the plans is the most desirable.
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APPENDIX A
DRAINAGE AREA MAP
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APPENDIX B
DRAINAGE AREA CALCULATIONS
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APPENDIX C
RUNOFF CALCULATIONS
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APPENDIX D
STORM SEWER FACILITIES CALCULATIONS
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