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HomeMy WebLinkAboutApp. B_Stormwater Design Report_2020_07 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