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Storm Drainage Calculations
Bozeman Gateway – Vance Thompson Vision
August 25, 2021
1 Introduction
This summary report provides hydrologic and hydraulic calculations for sizing of
retention storage facilities and other storm drainage structures for development of the
Vance Thompson Vision building, in accordance with the City of Bozeman Design
Standards. The proposed site consists of a new three-story medical office building
located north of Honor Lane. The purpose of the report and calculations is to
demonstrate the following:
- Post- development peak runoff does not exceed pre-development (historic)
conditions for the 10-year storm event
- The 25-year peak runoff is conveyed by underground storm drain facility with no
surface overflow
- Storm water treatment is provided to intercept and contain sediment and floating
debris on site, for eventual removal and disposal.
2 Drainage Basins
Catron Crossing Lots 1 & 2 have a drainage area of approximately 2.94 acres
(128,116.63 ft2). Pre-development topography is gradual, about 1 to 2 percent slope
from south to north. Existing groundcover is undeveloped.
The proposed property can be divided into flows that will be contained on-site (Basins 1
and 2). Refer to Figure 1 for pre-development drainage basins and Figure 2 for post-
development basins. See grading and drainage plans for more detail on retention and
conveyance design.
3 Approach
It will be shown that for the proposed development in Basins 1 & 2, the use of proposed
retention facilities will limit the post-development 10-year peak runoff rate to the pre-
development rate, and will provide effective treatment of storm runoff. All storm drain
pipes have been sized to convey the 25-year peak runoff.
Retention ponds shall be sized based on a 10-year, 2-hour storm intensity per the City of
Bozeman design standards. See Appendix A for rational method retention pond
calculations for Basins 1 & 2. The values used in each column of the table are as
follows:
C (Rational Method runoff coefficient):
Runoff coefficients were calculated using 0.10 for landscaped areas and 0.95 for
impervious areas (pavement, concrete and rooftops). Pro-rating by area resulted in the
weighted coefficients used in the calculations.
Area: Calculated in AutoCAD from Figures 1 and 2
Page 2 of 3
Intensity
The rainfall intensity value of I = 0.41in/hr is used per the City of Bozeman design
standards for a 10-year, 2-hour storm.
Retention Volume
Retention volume will be the stormwater runoff from the 10-year, 2-hour storm event or
the first 0.5 inch of rainfall over parking impervious surfaces, whichever is greater
3 Retention Ponds
As discussed previously, stormwater runoff from major Basins 1 & 2 is proposed to be
retained by surface storm water retention basins. In accordance with the Design
Standards and Specifications Policy, the retention facilities have been designed based
on the 10-year, 2-hour storm event. A summary of the design parameters and sizing of
the reservoirs is provided in Table 1 and the analysis are included in Appendix A.
Table 1 – Retention Facilities Sizing Summary
Drainage Basin(s)
Required 10-Year,
2-Hour Retention
Volume (ft3)
Design System
Storage Volume
(ft3) Additional Design Notes
1
Surface Retention Basin 2,184 2,571
Maximum Water Depth = 1 ft
Maximum Depth of Pond = 2.50 ft
Side and End Slopes = 4:1 (Horizontal : Vertical)
2
Surface Retention Basin 3,856 5,496
Maximum Water Depth = 1 ft
Maximum Depth of Pond = 2.50 ft
Side and End Slopes = 4:1 (Horizontal : Vertical)
4 Conveyance
On Site Conveyance
The storm drain piping system for the proposed development is designed to convey
flows from a 25-year storm. Provisions for overland flow of larger, less frequent storms
up to the 100-year storm will be incorporated into the site grading. Calculations are
provided for representative pipes as required to ensure pipe size is adequate for all
pipes; adequacy of other pipe sizes are evident based on the calculated basin areas and
flow rates for the representative pipes. A minimum pipe diameter of 12-inches at a
minimum slope of 0.5% was chosen for the Basin 1 conveyance design. A minimum pipe
diameter of 15-inches at a minimum slope of 0.5% was chosen for the Basin 2
conveyance design. See Appendix B for the conveyance calculations for the storm drain
pipe.
30” Relocated Pipe Conveyance
As a part of this project, the 30” storm drain pipe that bisected the site was relocated to
make room for the building. The original 30” storm pipe had a slope of 0.77% and 0.52%
through the site. With the realignment of the storm line, the storm line through the site
has been shortened making the new slope 1.2%. With the increase in slope, the 30” pipe
will have increased runoff capacity and improved conveyance. The proposed pipe will
enter the existing pond at the retention pond bottom elevation of 4638.9’. The proposed
Page 3 of 3
30” pipe will have an RCP flared end section as well as rip rap as to ensure that the
slope change will have no adverse effects on the receiving stormwater pond and outfall
to the north of the subject property.
5 Groundwater
There are three monitoring wells in Catron Crossing (see Appendix C) that were
measured for 7 months in 2017. Well #3 is the closest to the proposed development and
during this time period, April had the highest groundwater levels of -4.74 below ground
surface on 4/25/2017. Based on this information, it is assumed that around April, the
groundwater will be at its highest.
The geotech report by SK Geotechnical dated 4/02/2021 measured the groundwater
depth at several locations, with ST-6 being closest to Retention Pond 1 and ST-4 being
adjacent to Retention Pond 2. ST-6 has a high groundwater elevation of approximately
4640.5’ with Retention Pond 1 having a pond bottom of 4643.5’. ST-4 has a
groundwater elevation of 4639.5 with Retention Pond 2 having a pond bottom of 4641’.
Therefore, both of the storm retention systems are designed to be above the seasonal
high groundwater.
6 Maintenance Considerations
Storm Water Conveyance Facilities
Storm drain inlets, catch basins, and piping should be inspected at least once per year
and following large storm events. Any necessary repair or maintenance should be
prioritized and scheduled through the spring, summer, and fall. These items may include
inspecting for any damage, removing blockages, cleaning and flushing the length of pipes,
establishing vegetation on bare slopes at or near inlets, and sediment removal.
Storm Water Retention Facility
Maintenance of retention basin is also essential. General objectives of maintenance are
to prevent clogging, standing water and the growth of weeds and wetland plants. This
requires frequent unclogging of the outlets, inlets, and mowing. Cleaning out sediment
with earth-moving equipment may also be necessary in 10 to 20 years.
7 Conclusions
The included analyses and calculations show that the storm water management system
for the proposed development will adequately handle the design storm events. During
the 100-year design storm recurrence interval, ponding may occur at inlets due to their
capacities; however, depths are estimated to remain at or below the top-back of curb in
the lower areas of the site. If clogging of inlets or larger storm water runoff events occur
provisions have been included in the design for overflows without presenting a hazard to
the public or adjacent property. Based on the included analyses and calculations, the
proposed storm water management system meets the requirements of the City of
Bozeman Design Standards and Specifications Policy.
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FIG 1PRE DEVELOPMENT STORM
21012.00NOT FORCONSTRUCTIONVANCE THOMPSON VISION4/2/2021LEGENDBASIN BOUNDARYPRE-DEVELOPMENTSUB-BASIN DESIGNATIONSUB-BASIN AREA (ACRES)X.XX.XX Ac.FLOW DIRECTIONBASIN 1SUB-BASIN BOUNDARY
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issue dateproject #phasesheet
project
ownerrevisiondateSCHEMATIC DESIGN - NOT FOR CONSTRUCTION
FIG 2POST DEVELOPMENT STORM
21012.00NOT FORCONSTRUCTIONVANCE THOMPSON VISION4/2/2021LEGENDBASIN BOUNDARYPOST-DEVELOPMENTSUB-BASIN DESIGNATIONSUB-BASIN AREA (ACRES)X.XX.XX Ac.FLOW DIRECTIONBASIN 1BASIN 2SUB-BASIN BOUNDARY
APPENDIX A
Stormwater Retention Calculations
RATIONAL METHOD FOR ANALYSIS OF STORM WATER RETENTION
Vance Thompson Vision Center - Basin 1 | Post-Development - 10 Year Design Storm Frequency
Design Storm Frequency = Year
Input values for runoff coefficients from appropriate tables.
1Weighted runoff coefficient, Cwd = SCjAj / Saj where Cj is the adjusted runoff coefficient for surface type j and Aj is the area of surface type j
Calculation of Storm Water Runoff Flow Rate:
Q = Storm Water Runoff Flow Rate (cfs) i = Rainfall Intensity (in/hr)
Cwd = Weighted Runoff Coefficient A = Storm Drainage Basin Area (acres)
Storm Drainage Basin Weighted Runoff Coefficient, Cwd =
Rainfall Intensity, i =in/hr (10-year, 2-hour Design Storm)
Storm Drainage Basin Area, A = acres
Basin Design Peak Flow, Q p = cfs
Calculation of Required Retention Volume:
Q = Storm Water Runoff Flow Rate (cfs)
V = Required Retention Volume (cf)
Storm Drainage Basin Runoff Flow Rate, Q = cfs
Basin Required Retention Volume, V = cf
10
(Enter WQual, 2, 5, 10, 25, 50, or 100)
DRAINAGE BASIN CHARACTERISTICS
Weighted Adjusted Runoff
Surface Area, A Area, A Coefficient Coefficient1 Factor C' = Cwd x Cf
Runoff Runoff Frequency Coefficient
Description (ft2)(acres) C C x A Cwd Cf Cwd x Cf → Cwd x Cf ≤ 1.00 C' x A
Hard Surface (front parking area) 31,241 0.717 0.95 0.681
0.56 1.00 0.56 0.744
Landscaping 27,101 0.622 0.10 0.062
Undeveloped 0 0.000 0.20 0.000
0.744
BASIN RETENTION VOLUME ANALYSIS
0.56
0.41
1.339
0.30
0.56
Totals 58,342 1.339 0.744
0.30
2,183.52
=
=7200
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RATIONAL METHOD FOR ANALYSIS OF STORM WATER RETENTION
Vance Thompson Vision Center - Basin 2 | Post-Development - 10 Year Design Storm Frequency
Design Storm Frequency = Year
Input values for runoff coefficients from appropriate tables.
1Weighted runoff coefficient, Cwd = SCjAj / Saj where Cj is the adjusted runoff coefficient for surface type j and Aj is the area of surface type j
Calculation of Storm Water Runoff Flow Rate:
Q = Storm Water Runoff Flow Rate (cfs) i = Rainfall Intensity (in/hr)
Cwd = Weighted Runoff Coefficient A = Storm Drainage Basin Area (acres)
Storm Drainage Basin Weighted Runoff Coefficient, C wd =
Rainfall Intensity, i =in/hr (10-year, 2-hour Design Storm)
Storm Drainage Basin Area, A =acres
Basin Design Peak Flow, Q p = cfs
Calculation of Required Retention Volume:
Q = Storm Water Runoff Flow Rate (cfs)
V = Required Retention Volume (cf)
Storm Drainage Basin Runoff Flow Rate, Q = cfs
Basin Required Retention Volume, V = cf
10
(Enter WQual, 2, 5, 10, 25, 50, or 100)
DRAINAGE BASIN CHARACTERISTICS
Weighted Adjusted Runoff
Surface Area, A Area, A Coefficient Coefficient1 Factor C' = Cwd x Cf
Runoff Runoff Frequency Coefficient
Description (ft2)(acres) C C x A Cwd Cf Cwd x Cf → Cwd x Cf ≤ 1.00 C' x A
Hard Surface (back, sides parking) 52,967 1.216 0.95 1.155
0.75 1.00 0.75 1.194
Landscaping 16,808 0.386 0.10 0.039
Undeveloped 0 0.000 0.20 0.000
1.194
BASIN RETENTION VOLUME ANALYSIS
0.75
0.41
1.602
0.49
0.75
Totals 69,774 1.602 1.194
0.49
3,505.51
=
=7200
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APPENDIX B
Conveyance Calculations
Vance Thompson Vision Center
Conveyance Calculations
MODIFIED RATIONAL METHOD
i = A * (Tc/60) B
Qp = C i A Qp = peak runoff, cfs Design Coefficients
C = runoff coefficient Storm A B
i = A(Tc/60)B (Bozeman IDF curve)2-yr 0.36 -0.60
Tc = time of concentration, minutes 5-yr 0.52 -0.64
A = Area, acres 10-yr 0.64 -0.65
25-yr 0.78 -0.64
50-yr 0.92 -0.66
Retention Volume = 7,200*(C)*(i)*(A), where i = 0.41 in./hr 100-yr 1.01 -0.67
(Bozeman Design Standards, March 2004)
Retention
Sub-Basin Name Area C Tc Q2 Q5 Q10 Q25 Q50 Q100 Volume (ft.3)
Basin 1 1.34 0.56 5.00 1.19 1.90 2.39 2.85 3.53 3.97 2,184
Basin 2 1.60 0.75 5.00 1.91 3.05 3.84 4.57 5.66 6.37 3,506
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Thursday, Jun 10 2021
Basin 1: 25-year Conveyance
Circular
Diameter (ft) = 1.00
Invert Elev (ft) = 100.00
Slope (%) = 0.50
N-Value = 0.011
Calculations
Compute by: Q vs Depth
No. Increments = 10
Highlighted
Depth (ft) = 0.90
Q (cfs) = 3.173
Area (sqft) = 0.74
Velocity (ft/s) = 4.26
Wetted Perim (ft) = 2.50
Crit Depth, Yc (ft) = 0.77
Top Width (ft) = 0.60
EGL (ft) = 1.18
0 1 2 3
Elev (ft)Depth (ft)Section
99.50 -0.50
100.00 0.00
100.50 0.50
101.00 1.00
101.50 1.50
102.00 2.00
Reach (ft)
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Thursday, Jun 10 2021
Basin 2: 25-year Conveyance
Circular
Diameter (ft) = 1.25
Invert Elev (ft) = 100.00
Slope (%) = 0.50
N-Value = 0.011
Calculations
Compute by: Q vs Depth
No. Increments = 10
Highlighted
Depth (ft) = 1.13
Q (cfs) = 5.753
Area (sqft) = 1.16
Velocity (ft/s) = 4.94
Wetted Perim (ft) = 3.13
Crit Depth, Yc (ft) = 0.97
Top Width (ft) = 0.75
EGL (ft) = 1.50
0 1 2 3
Elev (ft)Section
99.50
100.00
100.50
101.00
101.50
102.00
Reach (ft)
APPENDIX C
Groundwater Measurements
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CATAMOUNTST AAVEWELL
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#2
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#1
engineerssurveyorsplannersscientists
2880 Technology Boulevard West
Bozeman, MT 59718
Phone: (406) 587-0721
Fax: (406) 922-6702
FIG. 4:GROUNDWATER OBSERVATION WELL LOCATIONS
SCALE:1" = 250'
STORM
WATER
TRACT 1
1.39 ACRES
LOT 1
1.04 ACRES
LOT 2
2.63 ACRES
LOT 3
1.77 ACRES
LOT 4
2.18 ACRES
LOT 5
0.93 ACRES
LOT 6
1.59 ACRESLOT 7
2.40 ACRES
LOT 8
2.84 ACRES
LOT 9
3.00 ACRES
LOT 10
1.50 ACRES
PAD
0.70
ACRES
DATE WELL 1 WELL 2 WELL 3 POND EST.
4/21/2017 -3.00 -3.51 -5.59 -3.3
4/25/2017 -2.77 -3.25 -4.74 -3.0
5/4/2017 -3.22 -3.58 -5.32 -3.4
5/9/2017 -3.41 -3.81 -5.84 -3.6
5/19/2017 -2.97 -2.28 -5.24 -2.6
5/24/2017 -4.10 -3.83 -5.53 -4.0
6/2/2017 -4.41 -4.15 -6.39 -4.3
6/6/2017 -4.34 -4.22 -6.58 -4.3
6/16/2017 -3.33 -3.85 -6.39 -3.6
6/22/2017 -3.81 -4.13 -6.60 -4.0
6/28/2017 -4.08 -4.22 -6.79 -4.2
7/6/2017 -4.14 -4.27 -6.94 -4.2
7/13/2017 -4.37 -4.42 -7.19 -4.4
7/18/2017 -4.51 -4.46 -7.36 -4.5
7/28/2017 -4.73 -4.52 -7.64 -4.6
8/1/2017 -4.81 -4.62 -7.74 -4.7
8/11/2017 -5.02 -4.76 -7.96 -4.9
8/18/2017 -5.05 -4.66 -8.02 -4.9
8/25/2017 -5.11 -4.70 -8.06 -4.9
8/30/2017 -5.14 -4.72 -8.08 -4.9
9/7/2017 -5.23 -4.81 -8.16 -5.0
9/14/2017 -5.28 -4.87 -8.24 -5.1
9/22/2017 -4.56 -4.30 -7.74 -4.4
9/28/2017 -4.49 -4.40 -7.59 -4.5
10/4/2017 -4.35 -4.31 -7.45 -4.3
DEPTH OF GROUNDWATER
BELOW SURFACE
-9.00
-8.00
-7.00
-6.00
-5.00
-4.00
-3.00
-2.00
-1.00
0.00
3/26/2017 4/15/2017 5/5/2017 5/25/2017 6/14/2017 7/4/2017 7/24/2017 8/13/2017 9/2/2017 9/22/2017 10/12/2017 11/1/2017
Groundwater Depth Below Ground Surface in FeetDate of Groundwater Depth Observation
Well 1
Well 2
Well 3
10. Boring Location Sketch
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