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DESIGN REPORT
STORMWATER MANAGEMENT
THE PARKLANDS AT VILLAGE DOWNTOWN
MAJOR SUBDIVISION OF
LOT 4A, MINOR SUBDIVISION NO. 344B
Prepared for:
Village Investment Group
101 E. Main Street, Suite D
Bozeman, MT 59715
Prepared by:
Project Number: 170090
July 2020
INTRODUCTION
The Parklands Subdivision project proposes to develop Lot 4A, Minor Subdivision 344B into 9
single-household lots, 1 multi-family lot, 1 park, 1 common open space and 1 remainder lot. The
property is located in the City of Bozeman and is zoned as R-4. There is an existing detention pond
at the east side of the cul-de-sac in existing Common Open Space #2. This pond will be reformed
within Common Open Space A provided to the east of the cul-de-sac of Village Downtown
Boulevard. The reformed detention pond provides 7,552 cubic feet of runoff storage and
discharges into the existing wetland area. Runoff overland flows from here into an existing
drainage ditch located east of Minor Subdivision 344B which drains into Story Ditch and
continues to flow to the northeast under the Montana Rail Link railroad bridge. Supporting
stormwater calculations are attached to this report.
STORM SEWER FACILITIES DESIGN
Storm sewer facilities were sized for the 25-yr storm using Manning’s Equation. For the inlet, the
contributing area, weighted C factor, and time of concentration were calculated. These values were
input into Manning’s Equation to check capacity and flow characteristics for inlets, storm drain
pipes, and curb gutters. All curbs are designed to maintain 0.15’ freeboard per C.O.B. Design
Manual Section IV.C.5. For the purposes of this report, each pipe section was named to match the
associated upstream structure. Pipe sizing calculations are included in Appendix C.
RETENTION/DETENTION POND DESIGN
All ponds have been sized according to City of Bozeman Design Standards. Retention ponds are
sized to capture the entire volume of the 10-year 2-hour storm event. They are designed with a
maximum depth of 1.5 feet (retention) or 2.5 feet (detention), and maximum side slope of 4:1.
Detention Ponds are sized to limit discharge to pre-development rates for the 10-year storm event.
Calculations used for sizing each pond can be found in Appendix D.
Reformed Detention Pond
The site was divided into twelve drainage basins as shown on the attached Drainage Area Map
included in Appendix A. Detention Pond #1 is located in the north end of the site within Common
Open Space A. This detention pond will be replacing the existing detention pond installed with
Village Downtown Boulevard. The Reformed Detention Pond receives runoff from Drainage
Areas 1 & 2, totaling 5.86 acres. The pre-development time to concentration for the pond was
calculated to be 26 minutes and the pre-development runoff rate for the 10-yr storm event was
calculated to be 1.31 cfs. In order to limit discharge from the detention pond to pre-development
runoff rates the proposed outlet structure for the pond will have a 4.7” weir installed. The outlet
pipe (Pipe #Outlet Pipe) was sized for the 25-yr storm event using the maximum time of
concentration of the contributing drainage areas (13.9 minutes for DA #1). The pipe will discharge
into the same channel that the existing detention pond discharges into. The required pond volume
was calculated to be 7,522 cubic feet. The provided pond volume is 7,726 cubic feet at an effective
water depth of 2.5’. The first 1.5’ of the pond will be utilized to retain the first 0.5” of rainfall,
which results in 7,491 cubic feet of runoff. The weir will not begin discharging until this 1.5’ of
water is retained. There will be a weep hole in the bottom of the outlet structure to ensure the pond
drains between storm events. In the case of a storm exceeding the 10-yr design storm, runoff will
overflow the outlet structure top grate into the outlet pipe and flow into the existing channel.
Supporting calculations for the pond sizing can be found in Appendix D.
Rear Yard Retention Areas
The site was divided into twelve drainage basins as shown on the attached Drainage Area Map
included in Appendix A. The single family homes located on Block 2 will drain to the south.
Because of this, rear yard retention areas have been proposed to be installed with the home
construction. The largest single family lot is Lot 1, Block 2 (Drainage Area #3) and because of this,
it is this drainage area that was used to size the rear yard retention areas. This ensures a
conservative approach to the pond sizing. The required pond volume was calculated to be 202
cubic feet. The proposed volume of the pond is 207 cubic feet and has an effective depth of 1.5’.
Please refer to Appendix D for pond sizing calculations.
APPENDIX A
DRAINAGE AREA MAP
APPENDIX B
DRAINAGE AREA CALCULATIONS
DRAINAGE AREA #1
1. Calculate Weighted C Factor for Right-of-Way
Component Width C
ROW Hardscape 64 0.95
ROW Landscape 16 0.2
Weighted C Factor = 0.80
2. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Composite ROW 0.80 139391 111513
OS 0.2 8601 1720
Low-Med Residential 0.35 0 0
Dense Residential 0.5 0 0
Total 147992 113233
A = Area (acres) 3.3974
C = Weighted C Factor 0.77
3. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 2.00% Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.35 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 100 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)10.6
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 3.35%
L = length of gutter (ft) 1097
V = mean velocity (ft/s) 5.49
Tc Gutter Flow (minutes) =3.3
Tc Total = 13.9 (5 minute minimum)
4. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.77 (calculated above)
I = 0.78 Tc-0.64 (in/hr)1.98 (25-yr storm)
A = area (acres) 3.40 (calculated above)
Q = REQUIRED GUTTER CAPACITY (cfs) 5.16 (assuming no carry flow)
PROVIDED GUTTER CAPACITY
1. Calculate Gutter Capacity @ 0.15' Below Top of Curb
Q = (1.486/n)AR2/3 S1/2
n = Mannings Coefficient 0.013
A = Area (ft2)1.24 (0.15' below top of curb)
P = Wetted perimeter (ft) 9.23 (0.15' below top of curb)
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 3.35%
Q = PROVIDED GUTTER CAPACITY (cfs) 6.80
DRAINAGE AREA #2
1. Calculate Weighted C Factor for Right-of-Way
Component Width C
ROW Hardscape 64 0.95
ROW Landscape 16 0.2
Weighted C Factor = 0.80
2. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Composite ROW 0.80 46742 37393
OS 0.2 0 0
Low-Med Residential 0.35 7888 2761
Dense Residential 0.5 52799 26400
Total 107429 66554
A = Area (acres) 2.4662
C = Weighted C Factor 0.62
3. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 2.00% Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.35 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 100 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)10.6
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 1.00%
L = length of gutter (ft) 305
V = mean velocity (ft/s) 3.00
Tc Gutter Flow (minutes) =1.7
Tc Total = 12.3 (5 minute minimum)
4. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.62 (calculated above)
I = 0.78 Tc-0.64 (in/hr)2.15 (25-yr storm)
A = area (acres) 2.47 (calculated above)
Q = REQUIRED GUTTER CAPACITY (cfs) 3.28 (assuming no carry flow)
PROVIDED GUTTER CAPACITY
1. Calculate Gutter Capacity @ 0.15' Below Top of Curb
Q = (1.486/n)AR2/3 S1/2
n = Mannings Coefficient 0.013
A = Area (ft2)1.24 (0.15' below top of curb)
P = Wetted perimeter (ft) 9.23 (0.15' below top of curb)
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 1.00%
Q = PROVIDED GUTTER CAPACITY (cfs) 3.72
DRAINAGE AREA #3
1. Calculate Weighted C Factor for Right-of-Way
Component Width C
ROW Hardscape 64 0.95
ROW Landscape 16 0.2
Weighted C Factor = 0.80
2. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Composite ROW 0.80 0 0
OS 0.2 0 0
Low-Med Residential 0.35 8569 2999
Dense Residential 0.5 0 0
Total 8569 2999
A = Area (acres) 0.1967
C = Weighted C Factor 0.35
3. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 2.00% Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.35 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 100 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)10.6
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 1.00%
L = length of gutter (ft) 305
V = mean velocity (ft/s) 3.00
Tc Gutter Flow (minutes) =1.7
Tc Total = 12.3 (5 minute minimum)
APPENDIX C
STORM SEWER FACILITIES
CALCULATIONS
PIPE #1 25-YR OUTFLOW RATE
REQUIRED CAPACITY
1. Calculate Weighted C Factor for Right-of-Way
Component Width C
ROW Hardscape 64 0.95
ROW Landscape 16 0.2
Weighted C Factor = 0.80
2. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area C Area (ft 2 )C * Area
Composite ROW 0.80 46742 37393
OS 0.2 0 0
Low-Med Residential 0.35 7888 2761
Dense Residential 0.5 52799 26400
Total 107429 66554
A = Area (acres) 2.4662
C = Weighted C Factor 0.62
2. Calculate Rainfall Intensity (Duration = Max Tc from Contributing Drainage Areas)
i = 0.78x-0.64 (10-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs) 0.23 (DA #2)
i = rainfall intensity (in./hr.) 1.98
3. Calculate 25-yr Pond Outflow Rate
Q = CiA
C = Rational Method Runoff Coefficient 0.80 (calculated above)
i = rainfall intensity (in./hr.) 1.98 (calculated above)
A = Area (acres) 2.47 (calculated above)
Q = 25-yr Pond Outflow Rate (cfs) 3.92
MANNING'S EQUATION FOR PIPE FLOW
Pipe: Pipe 1 Location: Inlet Pipe to Detention Pond #1
INPUT
D= 15 inches
d= 14.07 inches
Mannings Formula n= 0.013 mannings coeff
57.7 degrees
Q=(1.486/n)ARh2/3S1/2 S= 0.005 slope in/in
R=A/P
A=cross sectional area
P=wetted perimeter V=(1.49/n)Rh2/3S1/2
S=slope of channel Q=V x A
n=Manning's roughness coefficient
Solution to Mannings Equation
Area,ft2
Wetted
Perimeter, ft
Hydraulic
Radius, ft velocity ft/s flow, cfs PVC 0.013
1.20 3.30 0.36 4.11 4.91 PE (<9"dia) 0.015
PE (>12"dia) 0.02
PE(9-12"dia) 0.017
CMP 0.025
ADS N12 0.012
HCMP 0.023
Conc 0.013
Manning's n-values
d
D
DETENTION POND #1 OUTLET PIPE 25-YR OUTFLOW RATE
REQUIRED CAPACITY
1. Calculate Weighted C Factor for Right-of-Way
Component Width C
ROW Hardscape 64 0.95
ROW Landscape 16 0.2
Weighted C Factor = 0.80
2. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area C Area (ft 2 )C * Area
Composite ROW 0.80 186132 148906
OS 0.2 8601 1720
Low-Med Residential 0.35 7888 2761
Dense Residential 0.5 52799 26400
Total 255421 179787
A = Area (acres) 5.8637
C = Weighted C Factor 0.70
2. Calculate Rainfall Intensity (Duration = Max Tc from Contributing Drainage Areas)
i = 0.78x-0.64 (10-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs) 0.23 (DA #1)
i = rainfall intensity (in./hr.) 1.98
3. Calculate 25-yr Pond Outflow Rate
Q = CiA
C = Rational Method Runoff Coefficient 0.80 (calculated above)
i = rainfall intensity (in./hr.) 1.98 (calculated above)
A = Area (acres) 5.86 (calculated above)
Q = 25-yr Pond Outflow Rate (cfs) 9.31
MANNING'S EQUATION FOR PIPE FLOW
Pipe: Detention Pond #1 Outlet Pipe Location: Detention Pond #1
INPUT
D= 21 inches
d= 19.70 inches
Mannings Formula n= 0.013 mannings coeff
57.7 degrees
Q=(1.486/n)ARh2/3S1/2 S= 0.006 slope in/in
R=A/P
A=cross sectional area
P=wetted perimeter V=(1.49/n)Rh2/3S1/2
S=slope of channel Q=V x A
n=Manning's roughness coefficient
Solution to Mannings Equation
Area,ft2
Wetted
Perimeter, ft
Hydraulic
Radius, ft velocity ft/s flow, cfs PVC 0.013
2.34 4.62 0.51 5.86 13.74 PE (<9"dia) 0.015
PE (>12"dia) 0.02
PE(9-12"dia) 0.017
CMP 0.025
ADS N12 0.012
HCMP 0.023
Conc 0.013
Manning's n-values
d
D
REAR YARD SWALE 25-YR OUTFLOW RATE
REQUIRED CAPACITY
1. Calculate Weighted C Factor for Right-of-Way
Component Width C
ROW Hardscape 64 0.95
ROW Landscape 16 0.2
Weighted C Factor = 0.80
2. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area C Area (ft 2 )C * Area
Composite ROW 0.80 0 0
OS 0.2 0 0
Low-Med Residential 0.35 8569 2999
Dense Residential 0.5 0 0
Total 8569 2999
A = Area (acres) 0.1967
C = Weighted C Factor 0.35
2. Calculate Rainfall Intensity (Duration = Max Tc from Contributing Drainage Areas)
i = 0.78x-0.64 (10-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs) 0.21 (DA #2)
i = rainfall intensity (in./hr.) 2.15
3. Calculate 25-yr Pond Outflow Rate
Q = CiA
C = Rational Method Runoff Coefficient 0.80 (calculated above)
i = rainfall intensity (in./hr.) 2.15 (calculated above)
A = Area (acres) 0.20 (calculated above)
Q = 25-yr Pond Outflow Rate (cfs) 0.34
MANNING'S EQUATION for OPEN CHANNEL FLOW
Project:Village Downtown Subdivision Location: Bozeman, MT
By: LRS Date: 7/28/2020
Chk By: Date:
INPUT
z (sideslope)= 4
Mannings Formula z (sideslope)= 4
w (btm width, ft)= 2
Q = (1.486/n)ARh2/3S1/2 d (depth, ft)= 0.5
R = A/P S (slope, ft/ft) 0.01
A = cross sectional area n low =0.013
P= wetted perimeter n high =0.013
S = slope of channel V = (1.49/n)Rh2/3S1/2
n = Manning's roughness coeffiQ = V x A
Depth, ft Area, sf
Wetted
Perimete
r, ft
Hydraulic
Radius, ft Velocity, fps Flow, cfs
Velocity,
fps Flow, cfs
0.5 2.00 6.12 0.33 5.42 10.84 5.42 10.84 T = 6.00
Dm = 0.333
Sc low = 0.0036 Sc high = 0.0036
sc =critical slope ft / ft
T = top width of the stream .7 Sc 1.3 Sc .7 Sc 1.3 Sc
dm =a/T = mean depth of flow 0.0026 0.0047 0.0026 0.0047
Low N High N
d
w
z
11
z
T
Clear Data
Entry Cells
APPENDIX D
POND SIZING CALCULATIONS
DETENTION POND #1
REQUIRED VOLUME
1. Calculate Weighted C Factor for Right-of-Way
Component Width C
ROW Hardscape 64 0.95
ROW Landscape 16 0.2
Weighted C Factor = 0.80
2. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area C Area (ft 2 )C * Area
Composite ROW 0.80 186132 148906
OS 0.2 8601 1720
Low-Med Residential 0.35 7888 2761
Dense Residential 0.5 52799 26400
Total 255421 179787
A = Area (acres) 5.8637
C = Weighted C Factor 0.70
3. Calculate Tc (Pre-Development)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 3.50% Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.2 2 to 10 1
Cf = Frequency Adjustment Factor 1 11 to 25 1.1
D = Length of Basin (ft) 532 26 to 50 1.2
51 to 100 1.25
Tc (Pre-Development) (minutes) 26
4. Calculate Rainfall Intensity (Duration = Pre-Development Tc)
i = 0.64x-0.65 (10-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs) 0.43 (Tc Pre-Development)
i = rainfall intensity (in./hr.) 1.11
5. Calculate Runoff Rate (Pre-Development)
Q = CiA
C = Rational Method Runoff Coefficient 0.2 (open land)
i = rainfall intensity (in./hr.) 1.11 (calculated above)
A = Area (acres) 5.86 (calculated above)
Q = Runoff Rate (Pre-Development) (cfs) 1.31
6. Calculate Required Pond Volume
Total Area (acres) = 5.86 acres
Weighted C = 0.70
Discharge Rate (cfs) = 1.31 cfs (Equal to Pre-Development Runoff Rate)
Duration(min) Duration(hrs)
Intensity
(in/hr)Qin (cfs)Runoff
Volume
Release
Volume
Required
Storage (ft3)
30 0.50 1.00 4.14 7461 0 7461
30.5 0.51 0.99 4.10 7504 0 7504
31 0.52 0.98 4.06 7547 39 7508
32 0.53 0.96 3.97 7631 118 7514
33 0.55 0.94 3.90 7714 196 7518
34 0.57 0.93 3.82 7795 274 7521
35 0.58 0.91 3.75 7875 353 7522
36 0.60 0.89 3.68 7953 431 7521
37 0.62 0.88 3.62 8029 510 7520
38 0.63 0.86 3.55 8104 588 7516
OUTLET STRUCTURE SLOT
Q=CLH3/2
Q = Discharge (cfs) 1.31 (calculated above)
C = Weir Coefficient 3.33 (per COB Design Standards)
H = Head (ft) 1
L = Horizontal Length (ft) 0.39
L = Slot Width (inches) 4.7
REAR YARD RETENTION AREAS
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Composite ROW 0.80 0 0
OS 0.2 0 0
Low-Med Residential 0.35 8569 2999
Dense Residential 0.5 0 0
TOTAL 8569 2999
C=Weighted C Factor 0.35
2. Calculate Additional Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.35
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.20
Q = runoff (cfs) 0.03
V = REQUIRED VOL (ft3)202