HomeMy WebLinkAboutStormwater Report
Stormwater Report:
for
145 Simmental Way
Gallatin County, Montana
February, 2022
Prepared For:
915 Building Partners, LLC
Prepared By:
Hyalite Engineers, PLLC
Brett Megaard, PE
Page 1
145 Simmental Way
ENGINEER’S STORM DRAINAGE DESIGN REPORT
Owner: 915 Building Partners, LLC
1891 Boot Hill Court
Bozeman, MT 59715-7245
Introduction
This report provides a design basis and hydrologic calculations for sizing stormwater
infrastructure for 145 Simmental Way. The project site is located in the northwest corner of
the intersection of Boot Hill Court and Simmental Way. This project in total is 1.14 acres of
multifamily housing development constructed in one proposed phase. The owner would like
to construct three units. The existing site has two storage structures with topography sloping
at approximately -2.5% to the west. 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 west.
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 57% impervious area containing buildings and
concrete driveways/walkways and 43% pervious area of landscaping. Groundwater observed
to the north of this site was found at depths ranging from 4.5’ or deeper, while the nearby
monitoring well as the MDOT rest area (approximately ½ mile north) fluctuated around 3.5-
4.0’ deep. 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 proposed surface retention ponds, with the exception of the loading
docks which will have individual collection systems. Also, the existing parking area will
continue to flow into Lot 4A to the north of the property. The changes to the areas and
impervious areas flowing to the north will not increase the runoff and will therefore have no
negative affects to the drainage system. 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)
Page 2
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. 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 at the corner
of Simmental Way and Boot Hill Court. The proposed 40’x15’ retention pond will be installed
to account for the runoff from the new proposed parking area and its surrounding area. The
proposed 150’x7’ retention pond will be installed to account for the runoff from the proposed
building area. The 90’x12’ retention pond will be installed to account for the runoff from the
Boot Hill Court drainage area. No other ponds will be required because the post-
development area has less runoff than the pre-development area, outside of the new building
and parking lot. The flow of water will also follow existing conditions and flow to the
northwest, away from the proposed development. The two proposed loading docks will also
not contribute to any runoff for the retention ponds. 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, 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 be diverted away from buildings on the site.
Appendix A – Stormwater Retention Pond Calculations
Appendix B – Drainage Area Exhibit
1 4 5 S I M M E N T A L W A Y
F E B R U A R Y 2 0 2 2
A P P E N D I X
APPENDIX A
STORMWATER RETENTION POND CALCULATIONS
145 SIMMENTAL WAY
Retention Pond Sizing - Boot Hill Court
Page 1 of 1
2/23/2022
By Cody Kerkaert
RETENTION CHAMBER
Design Storm: 10-Year, 2 Hour (City of Bozeman)
Basin Area (ac): 0.41
Post-Dev C (weighted): 0.55
Storm Duration (min)120.00
Weighted Runoff Co-efficent:
Area Runoff
(acres) Coefficient Calculations
Impervious 0.23 0.90 0.21
Undisturbed 0.18 0.10 0.02
Totals 0.41 0.23
Cw =0.55
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: 0.55 0.55 0.55 0.55 0.55
Storm Intensity (in/hr): 0.24 0.41 0.50 0.63 0.12
Post-Dev Qp (cfs): 0.05 0.09 0.11 0.14 0.03
Pond Size:
10-Year, 2 Hour Storm
Duration Time: 7200.00 sec
V10YR/2HR : 661 cf
*Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes
145 SIMMENTAL WAY
Retention Pond Sizing - Proposed Building Area
Page 1 of 1
2/23/2022
By Cody Kerkaert
RETENTION CHAMBER
Design Storm: 10-Year, 2 Hour (City of Bozeman)
Basin Area (ac): 0.20
Post-Dev C (weighted): 0.58
Storm Duration (min)120.00
Weighted Runoff Co-efficent:
Area Runoff
(acres) Coefficient Calculations
Impervious 0.12 0.90 0.11
Undisturbed 0.08 0.10 0.01
Totals 0.20 0.12
Cw =0.58
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: 0.58 0.58 0.58 0.58 0.58
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 : 341 cf
*Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes
145 SIMMENTAL WAY
Retention Pond Sizing - Existing Parking Area
Post Develoment
Page 1 of 1
2/23/2022
By Cody Kerkaert
RETENTION CHAMBER
Design Storm: 10-Year, 2 Hour (City of Bozeman)
Basin Area (ac): 0.41
Post-Dev C (weighted): 0.74
Storm Duration (min)120.00
Weighted Runoff Co-efficent:
Area Runoff
(acres) Coefficient Calculations
Impervious 0.33 0.90 0.30
Undisturbed 0.08 0.10 0.01
Totals 0.41 0.31
Cw =0.74
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: 0.74 0.74 0.74 0.74 0.74
Storm Intensity (in/hr): 0.24 0.41 0.50 0.63 0.12
Post-Dev Qp (cfs): 0.07 0.12 0.15 0.19 0.04
Pond Size:
10-Year, 2 Hour Storm
Duration Time: 7200.00 sec
V10YR/2HR : 896 cf
*Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes
145 SIMMENTAL WAY
Retention Pond Sizing - Existing Parking Area
Pre Develoment
Page 1 of 1
2/23/2022
By Cody Kerkaert
RETENTION CHAMBER
Design Storm: 10-Year, 2 Hour (City of Bozeman)
Basin Area (ac): 0.45
Post-Dev C (weighted): 0.69
Storm Duration (min)120.00
Weighted Runoff Co-efficent:
Area Runoff
(acres) Coefficient Calculations
Impervious 0.33 0.90 0.30
Undisturbed 0.12 0.10 0.01
Totals 0.45 0.31
Cw =0.69
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: 0.69 0.69 0.69 0.69 0.69
Storm Intensity (in/hr): 0.24 0.41 0.50 0.63 0.12
Post-Dev Qp (cfs): 0.07 0.13 0.15 0.20 0.04
Pond Size:
10-Year, 2 Hour Storm
Duration Time: 7200.00 sec
V10YR/2HR : 907 cf
*Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes
145 SIMMENTAL WAY
Retention Pond Sizing - Proposed Parking Area
Page 1 of 1
2/23/2022
By Cody Kerkaert
RETENTION CHAMBER
Design Storm: 10-Year, 2 Hour (City of Bozeman)
Basin Area (ac): 0.29
Post-Dev C (weighted): 0.62
Storm Duration (min)120.00
Weighted Runoff Co-efficent:
Area Runoff
(acres) Coefficient Calculations
Impervious 0.19 0.90 0.17
Undisturbed 0.10 0.10 0.01
Totals 0.29 0.18
Cw =0.62
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: 0.62 0.62 0.62 0.62 0.62
Storm Intensity (in/hr): 0.24 0.41 0.50 0.63 0.12
Post-Dev Qp (cfs): 0.04 0.07 0.09 0.11 0.02
Pond Size:
10-Year, 2 Hour Storm
Duration Time: 7200.00 sec
V10YR/2HR : 532 cf
*Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes
145 SIMMENTAL WAY
Retention Pond Sizing - North Loading Dock
Page 1 of 1
2/23/2022
By Cody Kerkaert
RETENTION CHAMBER
Design Storm: 10-Year, 2 Hour (City of Bozeman)
Basin Area (ac): 0.01
Post-Dev C (weighted): 0.90
Storm Duration (min)120.00
Weighted Runoff Co-efficent:
Area Runoff
(acres) Coefficient Calculations
Impervious 0.01 0.90 0.01
Undisturbed 0.00 0.10 0.00
Totals 0.01 0.01
Cw =0.90
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: 0.90 0.90 0.90 0.90 0.90
Storm Intensity (in/hr): 0.24 0.41 0.50 0.63 0.12
Post-Dev Qp (cfs): 0.00 0.00 0.00 0.01 0.00
Pond Size:
10-Year, 2 Hour Storm
Duration Time: 7200.00 sec
V10YR/2HR : 26 cf
*Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes
145 SIMMENTAL WAY
Retention Pond Sizing - South Loading Dock
Page 1 of 1
2/23/2022
By Cody Kerkaert
RETENTION CHAMBER
Design Storm: 10-Year, 2 Hour (City of Bozeman)
Basin Area (ac): 0.01
Post-Dev C (weighted): 0.90
Storm Duration (min)120.00
Weighted Runoff Co-efficent:
Area Runoff
(acres) Coefficient Calculations
Impervious 0.01 0.90 0.01
Undisturbed 0.00 0.10 0.00
Totals 0.01 0.01
Cw =0.90
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: 0.90 0.90 0.90 0.90 0.90
Storm Intensity (in/hr): 0.24 0.41 0.50 0.63 0.12
Post-Dev Qp (cfs): 0.00 0.00 0.00 0.01 0.00
Pond Size:
10-Year, 2 Hour Storm
Duration Time: 7200.00 sec
V10YR/2HR : 26 cf
*Values are from COB Design Standards Figure 2 - Rainfall Intensity Druation in Minutes
1 4 5 S I M M E N T A L W A Y
F E B R U A R Y 2 0 2 2
A P P E N D I X
APPENDIX B
DRAINAGE AREA EXHIBIT
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SIMMENTAL WAY
145 SIMMENTAL WAYDRAINAGE AREASPROPOSED BUILDING AREA - 0.20 ACRESPROPOSED PARKING AREA - 0.29 ACRESNORTH LOADING DOCK - 0.01 ACRESSOUTH LOADING DOCK - 0.01 ACRESBOOT HILL COURT - 0.41 ACRES