HomeMy WebLinkAbout107-Stormwater Report_03102022
Stormwater Report:
for
NELSON MEADOWS LOT 17
Gallatin County, Montana
March, 2022
Prepared For:
DGB Development, LLC
Prepared By:
Hyalite Engineers, PLLC
Brett Megaard, PE
Page 1
NELSON MEADOWS LOT 17
ENGINEER’S STORM DRAINAGE DESIGN REPORT
Owner: DGB Development, LLC
244 Cirque Dr.
Bozeman, MT 59718-9315
Introduction
This report provides a design basis and hydrologic calculations for sizing stormwater
infrastructure for Nelson Meadows Lot 17. The project site is located 0.05 miles south of the
Royal Wolf Way and Prince Lane intersection. This project in total is 0.97 acres of multifamily
housing and small office space development constructed in one proposed phase. The owner
would like to construct two buildings. The existing site has topography sloping at
approximately -2.5% to the northeast. 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 northeast.
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 68% impervious area containing buildings,
asphalt driveways, and concrete walkways and 32% 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.
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).
Runoff coefficients and watershed flow calculations are included in Appendix A.
Page 2
Stormwater Retention Ponds
The stormwater retention ponds were designed to be sized for the development of Nelson
Meadows Lot 17. The proposed 30’x15’ retention pond will be installed to account for the
runoff from the southern half of the proposed Building 1 and the area in front of it. The
proposed 60’x12’ retention pond will be installed to account for the runoff from the proposed
Building 2 and the area to the northeast of it. The irregular shaped retention pond
(70’x60’x45’) will be installed to account for the runoff from the driveways/parking areas and
the small areas surrounding them. A drainage area exhibit is included with this report in
Appendix B. The flow of water will also follow existing conditions and flow to the northwest
into swales to capture the stormwater and route them to the proposed ponds. The
stormwater calculations for these areas are included in Appendix A.
Flooding
Excessive runoff from a large storm event will be routed such that runoff does not inundate
home sites, drainfields 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 spill to the northeast away from the proposed
buildings.
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 7
M A R C H 2 0 2 2
A P P E N D I X
APPENDIX A
STORMWATER RETENTION POND CALCULATIONS
NELSON MEADOWS LOT 17
Stormwater Basin 1
Page 1 of 1
2/9/2022
By Cody Kerkaert
RETENTION CHAMBER
Design Storm: 10-Year, 2 Hour (City of Bozeman)
Basin Area (ac): 0.15
Post-Dev C (weighted): 0.57
Storm Duration (min)120.00
Weighted Runoff Co-efficent:
Area Runoff
(acres) Coefficient Calculations
Impervious 0.08 0.90 0.07
Undisturbed 0.07 0.20 0.01
Totals 0.15 0.09
Cw =0.57
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: 0.57 0.57 0.57 0.57 0.57
Storm Intensity (in/hr): 0.24 0.41 0.50 0.63 0.12
Post-Dev Qp (cfs): 0.02 0.04 0.04 0.05 0.01
Pond Size:
10-Year, 2 Hour Storm
Duration Time: 7200.00 sec
V10YR/2HR : 253 cf
*Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes
NELSON MEADOWS LOT 17
Stormwater Basin 2
Page 1 of 1
2/9/2022
By Cody Kerkaert
RETENTION CHAMBER
Design Storm: 10-Year, 2 Hour (City of Bozeman)
Basin Area (ac): 0.17
Post-Dev C (weighted): 0.65
Storm Duration (min)120.00
Weighted Runoff Co-efficent:
Area Runoff
(acres) Coefficient Calculations
Impervious 0.11 0.90 0.10
Undisturbed 0.06 0.20 0.01
Totals 0.17 0.11
Cw =0.65
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: 0.65 0.65 0.65 0.65 0.65
Storm Intensity (in/hr): 0.24 0.41 0.50 0.63 0.12
Post-Dev Qp (cfs): 0.03 0.05 0.06 0.07 0.01
Pond Size:
10-Year, 2 Hour Storm
Duration Time: 7200.00 sec
V10YR/2HR : 326 cf
*Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes
NELSON MEADOWS LOT 17
Stormwater Basin 3
Page 1 of 1
2/9/2022
By Cody Kerkaert
RETENTION CHAMBER
Design Storm: 10-Year, 2 Hour (City of Bozeman)
Basin Area (ac): 0.65
Post-Dev C (weighted): 0.71
Storm Duration (min)120.00
Weighted Runoff Co-efficent:
Area Runoff
(acres) Coefficient Calculations
Impervious 0.47 0.90 0.42
Undisturbed 0.18 0.20 0.04
Totals 0.65 0.46
Cw =0.71
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: 0.71 0.71 0.71 0.71 0.71
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 7
M A R C H 2 0 2 2
A P P E N D I X
APPENDIX B
DRAINAGE AREA EXHIBIT