HomeMy WebLinkAboutStormwater Design Report DESIGN REPORT
STORMWATER MANAGEMENT
LOTS 5 & 6 GALLATIN CENTER P.U.D PHASE I
Prepared for:
Stefan Associates
1209.5 East Main Street
Bozeman, MT 59715
Prepared by:
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Engineering and Surveying Inc.
1091 Stoneridge Drive • Bozeman, MT 59718
Phone (406) 587-1 1 15 • Fax (406) 587-9768
www.chengineers,com • info@chengineers.com
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o�ect Number. 190179 14AR C A �
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INTRODUCTION
The proposed development on Lots 5 & 6 of Gallatin Center Subdivision P.U.D. Phase 1 consists
of a new carwash business with a drive-thru carwash, parking area, office space, car wash detail
bays, and associated site landscaping and stormwater management features. The property is
located within the Bozeman city limits and is currently zoned B-2 commercial. A combination of
site grading, curb and gutter, drainage swales, and retention and detention ponds will be used to
manage stormwater runoff on the site.
EXISTING SITE & STORMWATER
The existing vacant lots have no on-site stormwater management facilities so any runoff generated
from Lots 5 and 6 currently sheet flows to East Catron Creek along the northwest side of the
property and west onto the adjacent Lot 7. The undeveloped site currently has a weighted C value
of 0.20 per the Bozeman Design Standards and Specifications Policy (DSSP). The NW side of the
property contains an 80' wide stream corridor along Catron Street while the NE side of the
property has a 50' wide green way corridor. Since neither of these corridors will be developed,
only the portion of the site that is proposed for development was included in this drainage design.
PROPOSED STORMWATER DESIGN
The site was divided into 4 drainage areas as shown on the Proposed Drainage Area Map included
in Appendix B. Each drainage area was assessed using the 10-year 2-hour storm event. Drainage
Area 1 is located in the SW corner of the site and conveys runoff from the southern portion of the
driveway into Retention Pond 1. The runoff coefficient for Drainage Area 1 is 0.61 over an area of
11,924 sf, which generates 493 cf of runoff during the 10-year 2-hour storm event. Retention Pond
1 has a proposed volume of 690 cf, a design depth of 1.0', and 4:1 (H:V) side slopes.
Drainage Area 2 encompasses the interior of the site and will capture runoff in Detention Pond 2
located in the landscape island in the center of the site. Drainage Area 2 has a runoff coefficient of
0.79 over an area of 36,056 sf. Detention Pond 2 will release runoff through an outlet structure at
the pre-development flow rate into East Catron Creek. The pre-development time of concentration
for Drainage Area 2 was found to be 19 minutes. The pre-development runoff rate for Drainage
Area 2 was found to be 0.22 cfs (see Appendix A). To maintain this pre-development runoff rate
for Drainage Area 2, the outlet structure will have an internal weir with a slot width of 0.4". The
pond volume needed to detain the design storm and maintain pre-development runoff rates is 777
cf. Detention Pond 2 has a volume of 810 cf, a design depth of 1.5', and 4:1 (H:V) side slopes.
Supporting calculations for all drainage areas are included in Appendix A.
Drainage Area 3 is located along the perimeter of the development and has a runoff coefficient of
0.68 over an area of 30,395 sf. Runoff in Drainage Area 3 will be captured in Detention Pond 3 at
the north end of the site, which will release runoff through an outlet structure at the
pre-development flow rate into East Catron Creek. The pre-development time of concentration for
Drainage Area 3 was found to be 15 minutes. The pre-development runoff rate for Drainage Area 3
was found to be 0.22 cfs (see Appendix A). To maintain this pre-development runoff rate for
Drainage Area 3 the outlet structure will have a weir with a slot width of 0.4". The pond volume
needed to detain the design storm and maintain pre-development runoff rates is 471 cf. Detention
Pond 1 has a volume of 582 cf, a design depth of 1.5', and 4:1 (H:V) side slopes. Supporting
calculations for all drainage areas are included in Appendix A.
Drainage Area 4 includes the 18' wide landscaped area along the west side of the site as well as the
western half of the car wash roof Drainage Area 4 has a runoff coefficient of 0.41 over an area of
9,505 sf and generates 267 cf of runoff during the 10-year 2-hour storm event. Since over 70%of
this drainage area is comprised of landscaping, it is anticipated that runoff from the western
portion of the car wash roof will flow into this 18' wide landscape strip and infiltrate.
GROUNDWATER AND INFILTRATION CONSIDERATIONS
From previous geotechnical investigations in the vicinity of the site, native gravels are expected to
be approximately 3'-4' below existing grade, with a silt/clay layer above the gravel. Groundwater
is typically encountered at the top of native gravel elevation, but during periods of high
groundwater it could rise as much as a foot above this gravel elevation. The bottoms of Pond 1
and Pond 3 are less than 2' below existing grade and the bottom of Pond 2 is approximately 2.5'
below existing grade, so groundwater inundation of these ponds is not anticipated.
Since a poorly draining silt/clay layer likely exists above the native gravel elevation, the proposed
design for Retention Pond 1 includes over-excavating the pond footprint down to the native gravel
depth. This excavation will be backfilled with a well-draining pit run gravel to provide a conduit
for the stormwater to infiltrate through. Above the gravel drain,the pond will be finish graded with
6"of topsoil and seeded.
APPENDIX A
STORMWATER CALCULATIONS
APPENDIX B
DRAINAGE AREA MAP