HomeMy WebLinkAbout201397_Stormwater Design _2022.02.15DESIGN REPORT
FOR
STORMWATER MANAGEMENT
LOT4,
BAXTER LANE SUBDIVISION
Prepared for:
ABS,LLC
389 Ferguson Avenue
Bozeman, MT 59718
Prepared by:
Project Number: 201397
February 2022
INTRODUCTION
The Addison Place Condominium Site Plan project proposes to develop Lot 4 of the Baxter Lane
Subdivision. The proposed layout consists of one 20-plex condominium building, parking lot for
the development, and related stormwater and landscaping areas. The property is located within the
City of Bozeman limits and is currently zoned R-4. A combination of site grading, curb and gutter,
and retention ponds will be used to manage stormwater runoff on the site. Supporting stormwater
calculations are enclosed in Appendix B for reference.
BAXTER LANE SUBDIVISION – STORMWATER BACKGROUND INFORMATION
Currently the section of Thomas Drive from Sartain south to Baxter Lane is flowing to the north
to an existing pond in the northeast corner of the Thomas and Sartain intersection. The runoff is
currently conveyed to the pond by a 12” PVC pipe buried under Thomas Drive. The proposed
project will require Thomas Drive to be built out north of Sartain. The runoff from the existing
section of Thomas drive will be collected in a proposed curb inlet that is connected to the existing
12” PVC pipe and flow to the existing pond.
STORMWATER DESIGN
Stormwater runoff from the proposed improvements will be conveyed via curb and gutter
channel flow to a series of curb chases and storm sewer inlets. It will then be routed through
storm drainage pipes and chases to retention facilities for storage. The time of concentration for
all of the drainage areas were assumed to be less than 5 mins. A post development Drainage Area
Map is included in Appendix A, and calculations for pond sizing for each of the post
development drainage areas (total area, weighted C factor, required and provided storage
volumes, and discharge rates) are included in Appendix B. Retention Pond #1 will retain runoff
from the Thomas Drive ROW (Drainage Area 1) and Drainage Areas 2. Storm sewer facilities
were sized for the 25-yr storm using Manning’s Equation, and for each inlet, the contributing
area, gutter capacity, 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 curb gutters were designed to maintain 0.15’ freeboard per
C.O.B. Design Manual Section IV.C.5.
DRAINAGE AREAS
The site was divided into 3 drainage areas as shown on the attached Drainage Area Map in
Appendix A.
Drainage Area 1
Drainage Area 1 consists of the section of Thomas Drive that will be built out in this project.
Runoff from Drainage Area 1 flows via sheet flow and curb & gutter. The runoff is carried north
via curb and gutter to a curb inlet in Thomas Drive and piped into Retention Pond #1 in the
northeast corner of lot 4. It was found that the 10-year, 2-hour storm generated 164 cubic feet of
runoff. The retention pond was sized per the City of Bozeman Design Standards to retain the
volume of a 10-year, 2-hour storm. Stormwater and pond sizing calculations are included in
Appendix B.
Drainage Area 2
Drainage Area 2 consists of the building and surrounding landscaped areas. Runoff from Drainage
Area 2 flows via sheet flow, shallow stormwater swales into Retention Pond #1. The pond was
sized according to City of Bozeman Design Standards to retain the volume from the 10-year, 2-
hour storm event. Drainage Area 2 will generate 492 cubic feet of runoff, the proposed Retention
Pond will provide 686 cubic feet of storage. This is adequate storage to accommodate both DA
1&2. Calculations are included in Appendix B.
Drainage Area 3
Drainage Area 3 consists of the western portion of the lot, including the parking lot as well as the
surrounding sidewalks and landscaped areas. Runoff from Drainage Area 3 flows via sheet flow,
and curb & gutter into the proposed Retention Pond #2. The pond was sized according to City of
Bozeman Design Standards to retain the volume from the 10-year, 2-hour storm event. Drainage
Area 3 will generate 792 cubic feet of runoff, the proposed Retention Pond #3 will provide 1,133
cubic feet of storage. Calculations are included in Appendix B.
G:\c&h\20\201397\design reports\storm
APPENDIX A
DRAINAGE AREA FIGURE
APPENDIX B
STORMWATER CALCULATIONS
DRAINAGE AREA #1
RUNOFF VOLUME FROM DA#1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft
2) C * Area
Landscape 0.2 0 0
Hardscape 0.95 0 0
Composite ROW 0.71 3437 2440
Total 3437 2440
A = Area (acres)0.08
C = Weighted C Factor 0.71
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.71
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.08
Q = RUNOFF (cfs)0.02
V = REQUIRED VOL (ft3)164
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #1
Contributing Area Area (ft
2)
Hardscape 3437
2. Calculate 1/2" runoff volume over hardscape
(aka Runoff Reduction Volume [RRV] as calculated in Montana Post-
Construction Storwater BMP Manual - Equation 3-1)
RRV = [P*Rv*A]/12
P = Water quality rainfall depth 0.50 inches
Rv = Dimensionless runoff coefficient 0.95 0.05 + 0.9*I
I = Percent impervious cover (decimal)1.00 decimal
A = Entire drainage area 0.08 acres
RRV = Runoff Reduction Volume 0.0031 acre-ft
RRV = Runoff Reduction Volume 136 cubic feet
Because the runoff volume from the 10‐yr, 2‐hr storm (for flood control) is
greater than the runoff volume produced by the half inch rainfall (for water quality)
the proposed retention facility #1 is sized to handle the larger volume (164 cf).
1. Check Pond Volume
Vreq=164.49 cubic feet
Vpro=25.39 cyd
686 cubic feet
MANNING'S EQUATION FOR PIPE FLOW
Project: Addison Place Location: INLET # 2 PIPE
By: JG Date: 0
Chk. By: 0 Date: 0
INPUT
D= 12 inches
d= 11.25529 inches
Mannings Formula n= 0.01 mannings coeff
57.7 degrees
Q=(1.486/n)ARh2/3S1/2 S= 0.025 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 0.01
0.77 2.64 0.29 10.29 7.88 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
Check the quantity of stormwater allocated from DA 1 (Tc Method)
1. Calculate Tc (Time to Concentration) of DA 1
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 3.0% Return (yrsCf
C = Rational Method Runoff Coefficient 0.71 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 23.5 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)2.0
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) 86
V = mean velocity (ft/s) 3.00
Tc Gutter Flow (minutes) =0.5
Tc Total = 5.0 (5 minute minimum)
2. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.71 (calculated above)
I = .64 Tc-0.65 (in/hr)3.22 (10-yr storm)
A = area (acres) 0.08 (calculated above)
Q = Pro Flow (cfs) 0.18 (assuming no carry flow)
DRAINAGE AREA #2
RUNOFF VOLUME FROM DA#2
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft
2) C * Area
Landscape 0.2 5481 1096
Hardscape 0.95 6533 6206
Composite ROW 0.71 0 0
Total 12014 7303
A = Area (acres)0.28
C = Weighted C Factor 0.61
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.61
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.28
Q = RUNOFF (cfs)0.07
V = REQUIRED VOL (ft3)492
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #2
Contributing Area Area (ft
2)
Hardscape 0
2. Calculate 1/2" runoff volume over hardscape
(aka Runoff Reduction Volume [RRV] as calculated in Montana Post-
Construction Storwater BMP Manual - Equation 3-1)
RRV = [P*Rv*A]/12
P = Water quality rainfall depth 0.50 inches
Rv = Dimensionless runoff coefficient 0.54 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.54 decimal
A = Entire drainage area 0.28 acres
RRV = Runoff Reduction Volume 0.0062 acre-ft
RRV = Runoff Reduction Volume 270 cubic feet
Because the runoff volume from the 10‐yr, 2‐hr storm (for flood control) is
greater than the runoff volume produced by the half inch rainfall (for water quality)
the proposed retention facility # is sized to handle the larger volume (492 cf).
1. Check Pond Volume
Vreq=492 cubic feet
Vpro=25.39 cyd
686 cubic feet
DRAINAGE AREA #3
RUNOFF VOLUME FROM DA#3
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft
2) C * Area
Landscape 0.2 5271 1054
Hardscape 0.95 11264 10701
Composite ROW 0.71 0 0
Total 16535 11755
A = Area (acres)0.38
C = Weighted C Factor 0.71
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.71
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.38
Q = RUNOFF (cfs)0.11
V = REQUIRED VOL (ft3)792
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #3
Contributing Area Area (ft
2)
Hardscape 0
2. Calculate 1/2" runoff volume over hardscape
(aka Runoff Reduction Volume [RRV] as calculated in Montana Post-
Construction Storwater BMP Manual - Equation 3-1)
RRV = [P*Rv*A]/12
P = Water quality rainfall depth 0.50 inches
Rv = Dimensionless runoff coefficient 0.66 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.68 decimal
A = Entire drainage area 0.38 acres
RRV = Runoff Reduction Volume 0.0105 acre-ft
RRV = Runoff Reduction Volume 457 cubic feet
Because the runoff volume from the 10‐yr, 2‐hr storm (for flood control) is
greater than the runoff volume produced by the half inch rainfall (for water quality)
the proposed retention facility # is sized to handle the larger volume (792 cf).
1. Check Pond Volume
Vreq=792 cubic feet
Vpro=41.95 cyd
1133 cubic feet
MANNING'S EQUATION for OPEN CHANNEL FLOW Storm Swale #1
Project: 201397 Location: Lot 4 Baxter Lane Subdivision
By: JG Date: 1/13/2022
Chk By: DK Date: 1/13/2022
INPUT
z (sideslope)= 0
Mannings Formula z (sideslope)= 0
w (btm width, ft)= 1
Q = (1.486/n)ARh2/3S1/2 d (depth, ft)= 0.5
R = A/P S (slope, ft/ft) 0.015
A = cross sectional area n low =0.035
P= wetted perimeter n high =0.035
S = slope of channel V = (1.49/n)Rh2/3S1/2
n = Manning's roughness coefficient Q = V x A
Depth, ft Area, sf
Wetted
Perimete
r, ft
Hydraulic
Radius, ft Velocity, fps Flow, cfs
Velocity,
fps Flow, cfs
0.5 0.50 2.00 0.25 2.06 1.03 2.06 1.03 T = 1.00
Dm = 0.500
Sc low = 0.0566 Sc high = 0.0566
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 0.0396 0.0736 0.0396 0.0736
Low N High N
d
w
z
11
z
T
Clear Data
Entry Cells