HomeMy WebLinkAbout007 L18 Stormwater Report
Stormwater Report:
for
NELSON MEADOWS LOT 18
Gallatin County, Montana
June, 2022
Prepared For:
Vista, LLC
Prepared By:
Hyalite Engineers, PLLC
Brett Megaard, PE
Page 1
NELSON MEADOWS LOT 18
ENGINEER’S STORM DRAINAGE DESIGN REPORT
Owner: Vista, LLC
1276 N 15th Ave Suite 103
Bozeman, MT 59715-3289
Introduction
This report provides a design basis and hydrologic calculations for sizing stormwater
infrastructure for Nelson Meadows Lot 18. The project site is located 0.08 miles south of the
Royal Wolf Way and Prince Lane intersection. This project in total is 0.95 acres of
commercial development constructed in one proposed phase. The owner would like to
construct one unit. The existing site has topography sloping at approximately -5.0% to the
north. There is currently no existing stormwater infrastructure on the lot. All precipitation is
currently either contained on site through absorption and infiltration or runs offsite to the
north.
The City of Bozeman Design Standards and Specifications (March 2004) was used as the
primary guideline for this stormwater drainage design.
General Design
The proposed development of the site will be 47% impervious area containing a building,
asphalt driveways, and concrete walkways and 53% pervious area of undisturbed land.
Groundwater observed 0.5 miles to the southwest of this site was found at depths ranging
from 3’ or deeper, while monitoring wells 1 mile to the northeast ranged from 5’ or deeper.
The proposed building will have roof drains that will capture rainwater which will then be
routed to a surface retention pond. The site is graded so that runoff from all the areas will
runoff into the proposed surface retention ponds. All stormwater will be captured and
retained within the site. No stormwater will be drained into any existing systems adding no
demands to the surrounding area.
Hydrologic Methodology
The modified rational method was used to calculate retention volumes. The modified rational
method is used to compute runoff for storm durations equal to or greater than time of
concentration. Total runoff volume is computed by multiplying the duration of the storm by
the runoff rate. Retention volumes were calculated using the following formulas:
Q = CIA
V = DQ
Where:
C = Weighted C Factor
I = Storm Intensity (in/hr)
A = Area (acres)
Q = Runoff (cfs)
D= Duration (s)
V = Volume (cf)
Retention volumes were analyzed for the 10-year 2-hour and 100-year 2-hour and 24-hour
storms using Figure I-3 from The City of Bozeman design Standards (March 2004).
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Runoff coefficients and watershed flow calculations are included in Appendix A. The storm
coefficients A and B are from the City of Bozeman Design Standards and the infiltration rates
are from DEQ 8.
Stormwater Retention Ponds
The stormwater retention ponds were designed to be sized for the development of Nelson
Meadows Lot 18. The proposed 20’x15’ retention pond will be installed to account for the
runoff from the southern quarter of the proposed commercial building and the unimproved
area around it. The proposed 80’x18’ retention pond will be installed to account for the runoff
from the driveway/parking areas and the unimproved areas surrounding them, along with the
northern three-quarters of the building. The water will flow away from the proposed
development and into the proposed retention ponds. Some of the water will be captured in a
swale that will direct the runoff to the ponds. At these locations where the swale meets the
pond, riprap will be installed to dissipate the water’s velocity. The stormwater calculations for
these areas are included in Appendix A and the drainage areas are shown in Appendix B.
Flooding
Excessive runoff from a large storm event will be routed such that runoff does not inundate
home sites, drain fields or over top the roadway. Finished floor elevations of buildings will be
set higher than the surrounding ground with finished ground sloping away from them. In the
event that the ponds overtop, stormwater will be diverted away from buildings on the site.
Appendix A – Stormwater Retention Pond Calculations
Appendix B – Drainage Area Exhibit
N E L S O N M E A D O W S L O T 1 8
J U N E 2 0 2 2
A P P E N D I X
APPENDIX A
STORMWATER RETENTION POND CALCULATIONS
NELSON MEADOWS LOT 18
Stormwater Basin 1
Page 1 of 1
2/11/2022
By Cody Kerkaert
RETENTION POND
Design Storm: 10-Year, 2 Hour (City of Bozeman)
Basin Area (ac): 0.09
Post-Dev C (weighted): 0.51
Storm Duration (min)120.00
Weighted Runoff Co-efficent:
Area Runoff
(acres) Coefficient Calculations
Impervious 0.04 0.90 0.04
Undisturbed 0.05 0.20 0.01
Totals 0.09 0.05
Cw =0.51
Runoff Rate (Rational Method):
Storm Event (yr): 2-yr, 2-hr 10-yr, 2-hr 25-yr, 2-hr 100-yr, 2-hr 100-yr, 24-hr
*Storm Coeff. A: 0.36 0.64 0.78 1.01 1.01
*Storm Coeff. B:0.60 0.65 0.64 0.67 0.67
Composite C: 0.51 0.51 0.51 0.51 0.51
Storm Intensity (in/hr): 0.24 0.41 0.50 0.63 0.12
Post-Dev Qp (cfs): 0.01 0.02 0.02 0.03 0.01
Pond Size:
10-Year, 2 Hour Storm
Duration Time: 7200.00 sec
V10YR/2HR : 135 cf
*Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes
NELSON MEADOWS LOT 18
Stormwater Basin 2
Page 1 of 1
2/11/2022
By Cody Kerkaert
RETENTION POND
Design Storm: 10-Year, 2 Hour (City of Bozeman)
Basin Area (ac): 0.86
Post-Dev C (weighted): 0.53
Storm Duration (min)120.00
Weighted Runoff Co-efficent:
Area Runoff
(acres) Coefficient Calculations
Impervious 0.41 0.90 0.37
Undisturbed 0.45 0.20 0.09
Totals 0.86 0.46
Cw =0.53
Runoff Rate (Rational Method):
Storm Event (yr): 2-yr, 2-hr 10-yr, 2-hr 25-yr, 2-hr 100-yr, 2-hr 100-yr, 24-hr
*Storm Coeff. A: 0.36 0.64 0.78 1.01 1.01
*Storm Coeff. B:0.60 0.65 0.64 0.67 0.67
Composite C: 0.53 0.53 0.53 0.53 0.53
Storm Intensity (in/hr): 0.24 0.41 0.50 0.63 0.12
Post-Dev Qp (cfs): 0.11 0.19 0.23 0.29 0.06
Pond Size:
10-Year, 2 Hour Storm
Duration Time: 7200.00 sec
V10YR/2HR : 1348 cf
*Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes
N E L S O N M E A D O W S L O T 1 8
J U N E 2 0 2 2
A P P E N D I X
APPENDIX B
DRAINAGE AREA EXHIBIT