HomeMy WebLinkAbout008 Stormwater Design Report - Block 2DESIGN REPORT
STORMWATER MANAGEMENT
FERGUSON FARM SUBDIVSION
BLOCK 2
Prepared for:
Delaney & Company, Inc.
101 E. Main Street D, Bozeman, MT 59705
Prepared by:
C&H Engineering and Surveying, Inc.
1091 Stoneridge Drive, Bozeman, MT 59718
(406)587-1115
Project Number: 170520
October 2017
INTRODUCTION
Block 2 of The Ferguson Farm Subdivision is an existing commercial development bordered by
Boardwalk Avenue on the east, Valley Commons Drive on the north, a commercial lot to the west,
and Huffine Lane to the south. It was approved and constructed in 2017-2018. Drainage
calculations for the entire subdivision were provided during the Master Site Plan review process.
This report summarizes the drainage within Block 2 with a total area of 3.55 acres. Stormwater
runoff is conveyed via surface flow and gutters to internal retention ponds. A drainage area map
is included in Appendix A. Calculations for each individual drainage area are included in Appendix
B.Please note the roof drainage from the two future buildings at the south side of Block 2 (Lots 2
and 3) each have a drywell connected, and are not included in this updated analysis.
STORM SEWER FACILITIES DESIGN
Storm sewer facilities were sized for the 25-yr storm using Manning’s Equation. For each inlet,
the contributing area, weighted C factor, and time to concentration were calculated (see Appendix
B). These values were input into Manning’s Equation to check capacity and flow characteristics
for inlets and storm drain pipes. The Storm Sewer Summary Report is included in Appendix C.
RETENTION POND DESIGN
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. Pond sizing calculations can be
found in Appendix D.
Retention Pond #1 (No change from original design)
Retention Pond #1 is located just west of the existing parking lot. It receives runoff from Drainage
Areas 2 and 3, totaling 0.52 acres. Runoff from Drainage Area 2 is captured in the existing storm
inlet 1, located on the south side of the middle island curb in the west half of the existing parking
lot. From this point, the runoff flows into a 10” PVC pipe that conveys runoff to storm
inlet/manhole combo 2, located along the west curb of the existing parking lot. Runoff from
Drainage Area 3 also is captured in the existing storm inlet/manhole combo 2. From this point, the
runoff flows into a 15” PVC pipe that conveys the runoff to the existing retention pond #1. The
required pond volume was calculated to be 1,130 cubic feet. The existing pond is designed to store
3,093 cubic feet of stormwater. In the case of larger storms beyond the pond’s capacity to capture
and infiltrate, the pond will overtop and flow north into the proposed retention pond #2. Supporting
calculations for the required pond volume can be found in Appendix D.
Retention Pond #2
Retention Pond #2 is located just west of the existing parking lot and just north of the existing
retention pond #1. It receives runoff from Drainage Area 4 (originally part of Drainage Area #9,
see Appendix E), totaling 0.50 acres. Runoff from Drainage Area 4 is conveyed via surface flow
and gutters to an existing curb cut and drainage swale located to the north of the existing pond.
From this point, the runoff flows into the drainage swale into the existing retention pond #2. The
required pond volume was calculated to be 1,336 cubic feet. The existing pond was designed to
store 1,500 cubic feet of stormwater. In the case of larger storms beyond the pond’s capacity to
capture and infiltrate, the pond will overtop and flow north into the landscape area. Supporting
calculations for the required pond volume can be found in Appendix D.
Subdivision Storm System
Drainage Area #1 (Drainage Area #6 and part of Drainage Area #9 in original stormwater design
report for Lot 4, Spring Creek Village Resort Subdivision; see Appendix E) consists of the east
half of the existing Block 2 parking lot and the north half of the northwestern drive aisle, totaling
1.10 acres. Runoff from Drainage Area 1 flows to the existing stormwater infrastructure
in Boardwalk Avenue, which ultimately discharges to the existing Retention Pond 1A.
Retention Pond 1A is located in the northwest corner of the subdivision and receives runoff from
the existing Boardwalk Avenue, Valley Commons Drive, and Field Street. The pond is
sized to receive drainage from these streets as well as runoff from the parking lots in Blocks 1,
2, 3, 4, and 5. The original master drainage plan allocated 2,840 cubic feet of runoff volume for
Block 2 (Drainage Area #6. Drainage Area #9 originally went to proposed retention pond
2). The proposed contribution from Block 2 (Drainage Area #1) is now 2,701 cubic feet;
therefore, Retention Pond 1A will have adequate capacity. In the case of a storm exceeding the
design storm, runoff will overflow the pond banks and flow north onto Fallon Street. See
Appendix D for supporting calculations that the existing stormwater infrastructure can
accommodate this runoff.
APPENDIX A
DRAINAGE AREA MAP
BLOC
K
2
APPROX. EXTENT OFUTILITY EASMENT LINE PERCIVIL DRAWINGS
APPENDIX B
DRAINAGE AREA CALULATIONS
DRAINAGE AREA #1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
ROW Hardscape 0.95 41211 39151
ROW Landscape 0.2 7493 1499
OS/Park 0.2 0 0
Low-Med Residential 0.35 0 0
Dense Residential 0.5 0 0
Commercial Neighborhood 0.6 0 0
Commercial Downtown 0.8 0 0
Industrial 0.8 0 0
Total 48704 40649
A = Area (acres)1.12
C = Weighted C Factor 0.83
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 2.36 Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.95 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 332 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)2.56
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 (%)2.20%
L = length of gutter (ft)0
V = mean velocity (ft/s)4.45
Tc Gutter Flow (minutes) =0.00
Tc Total =2.56
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.83 (calculated above)
I = 0.78 Tc-0.64 (in/hr)5.87 (25-yr storm)
A = area (acres) 1.12 (calculated above)
Q = REQUIRED GUTTER CAPACITY (cfs) 5.48 (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 (%)2.36%
Q = PROVIDED GUTTER CAPACITY (cfs)5.51
DRAINAGE AREA #2
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
ROW Hardscape 0.95 8909 8464
ROW Landscape 0.2 884 177
OS/Park 0.2 0 0
Low-Med Residential 0.35 0 0
Dense Residential 0.5 0 0
Commercial Neighborhood 0.6 0 0
Commercial Downtown 0.8 0 0
Industrial 0.8 0 0
Total 9793 8640
A = Area (acres)0.22
C = Weighted C Factor 0.88
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 3.52 Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.95 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 145 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)1.48
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 (%)2.00%
L = length of gutter (ft)0
V = mean velocity (ft/s)4.24
Tc Gutter Flow (minutes) =0.00
Tc Total =1.48
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.88 (calculated above)
I = 0.78 Tc-0.64 (in/hr)8.34 (25-yr storm)
A = area (acres) 0.22 (calculated above)
Q = REQUIRED GUTTER CAPACITY (cfs) 1.65 (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 (%) 2.00%
Q = PROVIDED GUTTER CAPACITY (cfs) 5.26
DRAINAGE AREA #3
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
ROW Hardscape 0.95 7271 6907
ROW Landscape 0.2 5611 1122
OS/Park 0.2 0 0
Low-Med Residential 0.35 0 0
Dense Residential 0.5 0 0
Commercial Neighborhood 0.6 0 0
Commercial Downtown 0.8 0 0
Industrial 0.8 0 0
Total 12882 8030
A = Area (acres)0.30
C = Weighted C Factor 0.62
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 3.82 Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.95 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 142 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)1.43
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 (%)2.38%
L = length of gutter (ft)32
V = mean velocity (ft/s)4.62
Tc Gutter Flow (minutes) =0.12
Tc Total =1.54
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.62 (calculated above)
I = 0.78 Tc-0.64 (in/hr)8.12 (25-yr storm)
A = area (acres) 0.30 (calculated above)
Q = REQUIRED GUTTER CAPACITY (cfs) 1.50 (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 (%) 2.38%
Q = PROVIDED GUTTER CAPACITY (cfs) 5.73
DRAINAGE AREA #4
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
ROW Hardscape 0.95 18271 17358
ROW Landscape 0.2 3543 709
OS/Park 0.2 0 0
Low-Med Residential 0.35 0 0
Dense Residential 0.5 0 0
Commercial Neighborhood 0.6 0 0
Commercial Downtown 0.8 0 0
Industrial 0.8 0 0
Total 21814 18066
A = Area (acres)0.50
C = Weighted C Factor 0.83
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 2.44 Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.95 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 229 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)2.10
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 (%)2.00%
L = length of gutter (ft)0
V = mean velocity (ft/s)4.24
Tc Gutter Flow (minutes) =0.00
Tc Total =2.10
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.83 (calculated above)
I = 0.78 Tc-0.64 (in/hr)6.66 (25-yr storm)
A = area (acres) 0.50 (calculated above)
Q = REQUIRED GUTTER CAPACITY (cfs) 2.76 (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 (%) 2.00%
Q = PROVIDED GUTTER CAPACITY (cfs) 5.26
APPENDIX C
STORM SEWER FACILITIES
CALCULATIONS
PIPE # 1 (ST Inlet 1 Outlet Pipe) 25-YR OUTFLOW RATE
REQUIRED CAPACITY
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
ROW Hardscape 0.95 8909 8464
ROW Landscape 0.2 884 177
OS/Park 0.2 0 0
Low-Med Residential 0.35 0 0
Dense Residential 0.5 0 0
Commercial Neighborhood 0.6 0 0
Commercial Downtown 0.8 0 0
Industrial 0.8 0 0
Total 9793 8640
A = Area (acres) 0.22
C = Weighted C Factor 0.88
2. Calculate Rainfall Intensity (Duration = Max Tc from Contributing Drainage Areas)
i = 0.78x-0.64 (25-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs) 0.02 (DA #2)
i = rainfall intensity (in./hr.) 8.34
3. Calculate 25-yr Pond Outflow Rate
Q = CiA
C = Rational Method Runoff Coefficient 0.88 (calculated above)
i = rainfall intensity (in./hr.) 8.34 (calculated above)
A = Area (acres) 0.22 (calculated above)
Q = 25-yr Pipe Flow Rate (cfs) 1.65
MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)
Pipe: 1 Location: ST Inlet 1 Outlet Pipe
INPUT
D= 10 inches
d= 9.38 inches
Mannings Formula n= 0.013 mannings
57.7 degrees
Q=(1.486/n)ARh2/3S1/2 S= 0.0085 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
0.53 2.20 0.24 4.09 2.17 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
PIPE # 2 (ST Inlet/MH Combo 2 Outlet Pipe) 25-YR OUTFLOW RATE
REQUIRED CAPACITY
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
ROW Hardscape 0.95 16180 15371
ROW Landscape 0.2 6495 1299
OS/Park 0.2 0 0
Low-Med Residential 0.35 0 0
Dense Residential 0.5 0 0
Commercial Neighborhood 0.6 0 0
Commercial Downtown 0.8 0 0
Industrial 0.8 0 0
Total 22675 16670
A = Area (acres) 0.52
C = Weighted C Factor 0.74
2. Calculate Rainfall Intensity (Duration = Max Tc from Contributing Drainage Areas)
i = 0.78x-0.64 (25-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs) 0.03 (DA #3)
i = rainfall intensity (in./hr.) 8.12
3. Calculate 25-yr Pond Outflow Rate
Q = CiA
C = Rational Method Runoff Coefficient 0.74 (calculated above)
i = rainfall intensity (in./hr.) 8.12 (calculated above)
A = Area (acres) 0.52 (calculated above)
Q = 25-yr Pipe Flow Rate (cfs) 3.11
MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)
Pipe: 2 Location: ST Inlet/MH Combo 2 Outlet Pipe
INPUT
D= 15 inches
d= 14.07 inches
Mannings Formula n= 0.013 mannings
57.7 degrees
Q=(1.486/n)ARh2/3S1/2 S= 0.0058 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 4.43 5.29 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
APPENDIX D
POND SIZING CALCULATIONS
RETENTION POND # 1
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
ROW Hardscape 0.95 16180 15371
ROW Landscape 0.2 6495 1299
OS/Park 0.2 0 0
Low-Med Residential 0.35 0 0
Dense Residential 0.5 0 0
Commercial Neighborhood 0.6 0 0
Commercial Downtown 0.8 0 0
Industrial 0.8 0 0
Total 22675 16670
C=Weighted C Factor 0.74
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.74
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.52
Q = runoff (cfs) 0.16
V = REQUIRED VOL (ft3)1130
PROVIDED VOLUME (ft3)3,093
RETENTION POND # 2
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
ROW Hardscape 0.95 18271 17358
ROW Landscape 0.2 3543 709
OS/Park 0.2 0 0
Low-Med Residential 0.35 0 0
Dense Residential 0.5 0 0
Commercial Neighborhood 0.6 0 0
Commercial Downtown 0.8 0 0
Industrial 0.8 0 0
Total 21814 18066
C=Weighted C Factor 0.83
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.83
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.50
Q = runoff (cfs) 0.17
V = REQUIRED VOL (ft3)1224
PROVIDED VOLUME 1,500
EX. RETENTION POND # 1A CONTRIBUTIONS FROM PROPOSED DA #1
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
ROW Hardscape 0.95 41211 39151
ROW Landscape 0.2 7493 1499
OS/Park 0.2 0 0
Low-Med Residential 0.35 0 0
Dense Residential 0.5 0 0
Commercial Neighborhood 0.6 0 0
Commercial Downtown 0.8 0 0
Industrial 0.8 0 0
Total 48704 40649
C=Weighted C Factor 0.83
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.83
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 1.12
Q = runoff (cfs) 0.38
V = REQUIRED VOL (ft3)2,755
PREVIOUSLY ALLOCATED VOLUME (ft3)2,840
APPENDIX E
EXISTING STORMWATER SYSTEM
Page 16 of 25
C = 0.75
Runoff Volume V = 0.068x0.75x29,964 ft2 = 1,534 ft3
Drainage Area 32:
Roof Area = 0 ft2
Hardscape Area = 4,877 ft2
Landscape Area = 748 ft2
__________
TOTAL = 5,625 ft2 Weighted Runoff Coefficient:
C = (0 ft2 x 0.85)+(4,877 ft2x0.90)+(748 ft2x0.20)
5,625 ft2 C = 0.81
Runoff Volume
V = 0.068x0.81x5,625 ft2 = 309 ft3
STORMWATER ROUTING (POST DEVELOPMENT)
Stormwater runoff from the drainage areas will be conveyed, via overland, pipe and gutter flow, to
retention ponds. The drainage area distribution is detailed below:
Retention Ponds 1A and 1B:
Drainage Areas flowing into Ponds 1A and 1B include: 1, 6, 12, 21, 21A, 21B, and 21C. Runoff from Resort Drive and Fallon Street (portions of Drainage Area 21) will follow existing patterns
and discharge to the existing storm drain inlet on the south side of Fallon Street, directly north of
Retention Pond 1A. The existing retention pond will be re-shaped as shown on the associated
construction plans.
Drainage Areas 1, 6, and 21A will discharge to a storm drain inlet (Inlet 1) on the south side of the
intersection of Valley Commons Drive and Boardwalk Avenue. A 15-inch PVC pipe will convey
water from Inlet 1 to Inlet 2, located on the north side of the same intersection. Inlet 2 will collect
all runoff from Valley Commons Drive (Drainage Area 21B). The combined flows from Inlets 1
and 2 will then flow in an 18-inch PVC pipe to Inlet 3, located on the south side of the Field Street and Boardwalk Avenue intersection. The combined flows from all inlets will then flow via a
21-inch PVC pipe to discharge at the bottom of Retention Pond 1B. The rest of Drainage Area 21
Page 17 of 25
will discharge to the ponds via surface flow. Retention Pond 1B will overflow to Retention Pond
1A if runoff volumes ever exceed Pond 1B’s capacity.
Total Volume Required: 25,347 ft3 (sum of runoff volumes from Drainage Areas: 1, 6, 12, 21, 21A, 21B, and 21C)
Total Volume Supplied: 20,660 ft3
Deficient Volume: 4,687 ft3
A concrete overflow structure with a 21-inch PVC outlet pipe will be constructed in Pond 1A to convey any overflow from Pond 1A to the existing retention pond in Lot 3A on the north side of Fallon Street. The current capacity of the existing pond on the north side of Fallon Street is
1,350 ft3. The proposed pond expansion will result in a total capacity of 6,387 ft3 – enough to
manage existing drainage and the addition of 4,687 ft3.
Retention Ponds 2A and 2B: Drainage areas flowing into Ponds 2A and 2B include: 16, 17, 27, and 31. Runoff from Drainage
Areas 16, 17, a portion of 27, and 31 will discharge to a new stormwater chase (Stormwater Chase
1 – sizing calculations below) located north of the Field Street and Resort Drive intersection. The
chase will route runoff into Retention Pond 2B. Pond 2B will overflow to Pond 2A if runoff volumes ever exceed Pond 2B’s capacity.
Volume Required: 6,607 ft3 (sum of runoff volumes from Drainage
Areas: 16, 17, 27, and 31)
Volume Supplied: 11,642 ft3
Surplus Volume: 5,017 ft3
Retention Pond 3: 803 ft3 of Drainage Area 8.
Volume Required: 803 ft3 Volume Supplied: 887 ft3
Retention Pond 4: 1,137 ft3 of Drainage Area 8.
Volume Required: 1,137 ft3 Volume Supplied: 2,720 ft3
Retention Pond 5: Drainage Area 9.
Volume Required: 1,361 ft3 Volume Supplied: 2,457 ft3
Retention Pond 6: Drainage Area 11.
Volume Required: 2,536 ft3 Volume Supplied: 2,720 ft3