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HomeMy WebLinkAbout13_Stormwater Design ReportDESIGN REPORT STORMWATER MANAGEMENT FALLON APARTMENTS Prepared for: Norton Properties, LLC 63020 NE Lower Meadow Road, Suite #200 Bend, OR 97702 Prepared by: C&H Engineering and Surveying, Inc. 1091 Stoneridge Drive, Bozeman, MT 59718 (406) 587-1115 Project Number:200859 May 2021 INTRODUCTION The proposed Fallon Apartments is a 36 dwelling unit residential development located on a 1.4534- acre parcel in Section 2, Township 2 South, Range 5 East of P.M.M., Gallatin County,City of Bozeman. A combination of site grading, curb and gutter,and underground Rainstore retention system will be used to manage stormwater runoff on the site.Supporting stormwater calculations are attached to this report.A Drainage Area Map is included in Appendix A.Calculations for each individual drainage area (total area, weighted C factor,time of concentration, and conveyances) are included in Appendix B, while the underground Rainstore retention system calculations are included in Appendix C. RETENTION DESIGN The underground Rainstore retention system has been sized according to City of Bozeman Design Standards. Retention facilities are sized to capture the entire volume of the 10-year 2-hour storm event.Calculations used for sizing each pond can be found in Appendix C. EXISTING STORMWATER INFRASTURCTURE –NORTON RANCH SUBD. PH. 4 Existing Detention Ponds #1 and #2. There are existing Detention Ponds numbered #1 and #2, which treat stormwater from the underlying Norton Ranch Subdivision and are located in the Norton Ranch Park, just north of May Fly Street.These two detention ponds receive runoff from Lots 2-3, Block 13 via surface flow and gutters within May Fly Street. With the Norton Phase 4 development, these ponds were expanded to account for the proposed development within Phase 4. A C-factor of 0.5 (dense residential) was applied to the entirety Lots 2-3, Block 13. Using 1.4534 acres and a C coefficient of 0.50, the runoff for the 10-year 2-hour storm was calculated to be 2,145 cubic feet.Using the Drainage Area Exhibit of the Norton Ranch Subdivision Phase 4 Stormwater Design Report (February 2018),an analysis carried out in AutoCAD determined that pre-development approximately 55% of the site drained to the existing pond # 1 while 45%drained to pond 2.Therefore, it would be acceptable to allow 1,180 cf of stormwater runoff to discharge to pond 1 and 965 cf to pond 2.As shown later in the report, only 889 cubic feet of runoff is proposed to discharge off site into the adjacent street right of ways.As such,the existing Detention Ponds numbered #1 and #2 have more than adequate capacity to handle the off-site discharge from this development. Drainage Area #1 As shown on the drainage area exhibit, included herein, drainage area #1 consists of the perimeter of the of the property, including the half of building footprints.Stormwater from drainage area #1 is directed northward and eastward into the surrounding roadways via a combination of sheet flow and gutter conveyances. The volume of the 10-year 2-hour storm event results in 889 cubic feet of stormwater, which is allowed to be discharged into the underlying subdivision roads, as previously discussed.Of the 889 cf of runoff from DA 1 approximately 489 cf (55%) will flow into pond 1 and 400 cf (45%)will flow into pond 2. Drainage Area #2 Drainage area #2 consists of the central portion of the property, including the majority of the paved parking area as well and the internal landscape island. Stormwater from drainage area #2 is directed into an underground Rainstore retention system via a combination of sheet flow and gutter conveyances.The volume of the 10-year 2-hour storm event results in 1,293 cubic feet of stormwater. The proposed underground Rainstore retention system was designed with a total volume of 1,262 cubic feet and is therefore adequately sized to retain the 10-year 2-hour storm event.The inlet pipes for the rain store system were designed as 8-in PVC and can carry a flow of 1.19 cfs, this is more than adequate to handle the 25-year storm runoff of 0.64 cfs for the entire DA. Pipe sizing calculations can be found in Appendix C. Drainage Area #3 Drainage area #3 consists of half the western building footprint and the northwest portion of the parking walk with adjacent sidewalks. Stormwater from drainage area #3 is directed into drywell #1 via a combination of sheet flow, downspouts, and gutter conveyances. The volume of the 10- year 2-hour storm event results in 456 cubic feet of stormwater. The proposed drywell was designed with a total volume of 463 cubic feet and is therefore adequately sized to retain the 10- year 2-hour storm event. Drainage Area #4 Drainage area #4 consists of half the eastern building footprint and the northeast portion of the parking walk with adjacent sidewalks. Stormwater from drainage area #4 is directed into drywell #2 via a combination of sheet flow, downspouts, and gutter conveyances. The volume of the 10- year 2-hour storm event results in 457 cubic feet of stormwater. The proposed drywell was designed with a total volume of 463 cubic feet and is therefore adequately sized to retain the 10- year 2-hour storm event. DEPTH TO GROUNDWATER Groundwater monitoring has been conducted by C&H Engineering during the 2018 calendar year and the results are included in Appendix D.The monitoring wells nearest to Lots 2 and 3, Block 13 were used to verify that the proposed stormwater infrastructure for is above the high groundwater table.The property is located halfway between Monitoring Well (MW) #1 and #2. MW#1 and MW#2 had a measured high groundwater recording of approximately 6.1 feet below ground surface and 5.1 feet below ground surface, respectively.The average measured high groundwater between MW#1 and MW#2 is 5.6 feet below ground surface, which is most representative of the groundwater conditions of the property. The bottom of the proposed underground Rainstore retention system is approximately 4.5 feet below grade accounting for the specified profile design. Therefore, the anticipated seasonal-high groundwater depth is not expected to affect the proposed underground Rainstore retention system. Additionally,the bottom of the proposed underground drywells are approximately 5.0 feet below the proposed grade. It should be noted that the proposed grade is about one foot higher than existing grade, in which the previous groundwater depths were measured by. Therefore, the anticipated seasonal-high groundwater depth is not expected to affect the proposed drywells. OFF-SITE RUN-ON CONSIDERATIONS Stormwater run-on from adjacent properties is not expected to adversely impact this development. The surrounding properties have a slight downgradient slope to the north and, as a result, the only run-on would be from the adjacent property to the south, which is currently an undeveloped vegetated pasture. As such, the current conditions would not result is a measurable amount of run-on to this property. Any future development of the adjacent property to the south would be required to contain stormwater runoff within their property and not be allowed to discharge stormwater onto this property per City code. Given the existing and potential future conditions at adjacent properties, off-site run-on would have a negligible effect on this development. APPENDIX A DRAINAGE AREA MAP APPENDIX B DRAINAGE AREA CALCULATIONS RUNOFF VOLUME FROM DA#1 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Landscape 0.2 15901 3180 Hardscape 0.95 10539 10012 Total 26439 13192 A = Area (acres)0.61 C = Weighted C Factor 0.50 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.50 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres)0.61 Q = RUNOFF (cfs)0.12 V = REQUIRED VOL (ft3)889 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 10539 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 0.59 0.05 + 0.9*I I = Percent impervious cover (decimal)0.60 decimal A = Entire drainage area 0.61 acres RRV = Runoff Reduction Volume 0.0150 acre-ft RRV = Runoff Reduction Volume 651 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 larger runoff volume is used (2000 cf). RUNOFF VOLUME FROM DA#2 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Landscape 0.2 2161 432 Hardscape 0.95 19737 18750 Total 21898 19182 A = Area (acres)0.50 C = Weighted C Factor 0.88 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.88 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres)0.50 Q = RUNOFF (cfs)0.18 V = REQUIRED VOL (ft3)1293 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 19737 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 0.14 0.05 + 0.9*I I = Percent impervious cover (decimal)0.10 decimal A = Entire drainage area 0.50 acres RRV = Runoff Reduction Volume 0.0029 acre-ft RRV = Runoff Reduction Volume 127 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 Rainstore 3 retention facility is sized to handle the larger volume (1,077 cf). RUNOFF VOLUME FROM DA#3 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Landscape 0.2 453 91 Hardscape 0.95 7022 6671 Total 7475 6762 A = Area (acres)0.17 C = Weighted C Factor 0.90 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.90 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres)0.17 Q = RUNOFF (cfs)0.06 V = REQUIRED VOL (ft3)456 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 7022 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 0.10 0.05 + 0.9*I I = Percent impervious cover (decimal)0.06 decimal A = Entire drainage area 0.17 acres RRV = Runoff Reduction Volume 0.0007 acre-ft RRV = Runoff Reduction Volume 33 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 drywell #1 is sized to handle the larger volume (456 cf). RUNOFF VOLUME FROM DA#4 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Landscape 0.2 452 90 Hardscape 0.95 7047 6694 Total 7498 6785 A = Area (acres)0.17 C = Weighted C Factor 0.90 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.90 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres)0.17 Q = RUNOFF (cfs)0.06 V = REQUIRED VOL (ft3)457 Check the half inch requirement (per DSSP II.A.4) 1. Determine Area of Hardscape within Drainage Area #4 Contributing Area Area (ft 2 ) Hardscape 7047 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 0.10 0.05 + 0.9*I I = Percent impervious cover (decimal)0.06 decimal A = Entire drainage area 0.17 acres RRV = Runoff Reduction Volume 0.0007 acre-ft RRV = Runoff Reduction Volume 33 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 drywell #2 is sized to handle the larger volume (457 cf). MANNING'S EQUATION FOR PIPE FLOW Project: 0 Location: 0 By: 0 Date: 0 Chk. By: 0 Date: 0 INPUT D= 8 inches d= 7.503546 inches Mannings Formula n= 0.01 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 0.01 0.34 1.76 0.19 3.51 1.19 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 DRAINAGE AREA # 2 25-YR OUTFLOW RATE REQUIRED CAPACITY 1. Calculate Weighted C Factor for Right-of-Way Component Width C ROW Hardscape 41 0.95 ROW Landscape 19 0.2 Weighted C Factor = 0.71 1. Calculate Area and Weighted C Factor (Post-Development) Contributing Area C Area (ft 2 )C * Area Composite ROW 0.71 0 0 Landscape 0.2 2161 432 Hardscape 0.35 19737 6908 Dense Residential 0.5 0 0 Commercial Neighborhood 0.6 0 0 Commercial Downtown 0.8 0 0 Industrial 0.8 0 0 Total 21898 7340 A = Area (acres) 0.5027 C = Weighted C Factor 0.34 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) 0.08 (DA #2) i = rainfall intensity (in./hr.) 3.83 3. Calculate 25-yr Pond Outflow Rate Q = CiA C = Rational Method Runoff Coefficient 0.34 (calculated above) i = rainfall intensity (in./hr.) 3.83 (calculated above) A = Area (acres) 0.50 (calculated above) Q = 25-yr Flow Rate (cfs) 0.64 RUNOFF VOLUME FROM DA#2 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2) C * Area Landscape 0.2 2161 432 Hardscape 0.95 19737 18750 Total 21898 19182 A = Area (acres)0.50 C = Weighted C Factor 0.88 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.88 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.50 Q = RUNOFF (cfs)0.18 V = REQUIRED VOL (ft3)1293 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 19737 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 0.14 0.05 + 0.9*I I = Percent impervious cover (decimal)0.10 decimal A = Entire drainage area 0.50 acres RRV = Runoff Reduction Volume 0.0029 acre-ft RRV = Runoff Reduction Volume 127 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 (1,293 cf). CALCULATE RAINSTORE VOLUME # Rows= 6 3.28' x 3.28' x 4" Stackable Units (TYP.) # Columns = 8 System Depth= 2.60 ft Standard depths (0.7, 1.0, 1.3, 2.0, 2.6, 4.0, and 7.9) Total System Volume = 1343 cf Void Space = 94% *assumed no fill Pro Retention Volume=1262 cf Req Vol =1293 cf APPENDIX D GROUNDWATER MONITORING DATA Approximate Project Location Project Engineer:Matt H Project: Well Information:bgs = below ground surface ags = above ground surface MW-1 MW-2 MW-3 MW-4 10'10'10'10' 0.58 1.71 0.71 0.92 Groundwater Information: MW-1 MW-2 MW-3 MW-4 02/14/18 6.81 6.29 4.20 4.38 03/13/18 6.94 6.39 4.20 4.34 04/13/18 6.10 5.10 3.12 3.30 05/11/18 6.87 6.36 4.20 4.35 05/25/18 6.78 6.18 4.14 4.31 06/08/18 6.99 6.62 4.37 4.54 06/22/18 6.30 6.51 4.48 4.60 07/06/18 8.64 8.21 5.91 6.10 07/20/18 7.49 7.32 4.85 5.10 08/03/18 7.47 7.32 4.85 5.21 08/17/18 7.57 7.10 5.00 5.21 08/30/18 7.15 6.82 4.50 4.70 09/14/18 7.25 6.85 4.65 4.80 Monitor Well Data Project Number:161140 Norton East Ranch Subdivision, Phase 4 Project Location:Bozeman Well ID Well Depth (feet-bgs) Top of Well (feet-ags) Date Depth to Ground Water (feet-bgs) BB+z z2:1 Δσ EtBqH DCompressible soil layerRigid soil layer APPENDIX E NORTON PHASE 4 STORMWATER DESIGN REPORT DESIGN REPORT STORMWATER MANAGEMENT NORTON RANCH SUBDIVISION, PHASE 4 Prepared for: Norton Properties, LLC 63020 NE Lower Meadow Road, Suite A, Bend, OR 97702 Prepared by: C&H Engineering and Surveying, Inc. 1091 Stoneridge Drive, Bozeman, MT 59718 (406) 587-1115 Project Number: 161140 February 2018 INTRODUCTION The proposed Norton Ranch Subdivision, Phase 4 is a 21-lot subdivision located on a 15.58-acre parcel in the East Half and the West Half of Section 9, Township 2 South, Range 5 East of P.M.M., Gallatin County, City of Bozeman. A combination of site grading, curb and gutter, storm inlets, and piping will be used to manage stormwater runoff on the site. The stormwater infrastructure used in this phase is all existing and was installed with Norton Ranch Subdivision, Phase 1. Supporting stormwater calculations are attached to this report. A Drainage Area Map is included in Appendix A. Calculations for each individual drainage area (total area, weighted C factor, and time of concentration) are included in Appendix B. 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 An effective depth of 1.5 feet, and maximum side slope of 4:1. Existing Detention Pond 1 and Existing Detention Pond 2 are sized to limit discharge to pre-development rates for the 10-year storm event. Both ponds have been sized to retain the first 0.5 inches of rain before the outlet structure begins to discharge. Calculations used for sizing each pond can be found in Appendix C. Design pond capacities were calculated using volume surfaces in AutoCAD Civil3D. Existing Detention Pond #1 Existing Detention Pond #1 is located north of May Fly Street, between Bull Frog Drive and Laurel Parkway. It receives runoff from Drainage Areas 1, 2, 3, and 4, totaling 8.66 acres. The pond currently receives runoff from Norton Ranch Subdivision, Phase 1. The existing pond volume is 2,904 cubic feet at an effective water depth of 1.5’. For Norton Ranch Subdivision, Phase 4, the pre-development time to concentration was calculated to be 40 minutes and the pre-development runoff rate for the 10-yr storm was calculated to be 1.43 cfs. The existing outlet structure discharges into the existing wetland area in the park to the north. The outlet structure will need to be raised to retain the first 0.5 inches of stormwater runoff. The new flow out elevation will be set to 4804.25’ and the slot will be expanded to a width of 2.8”. The new required pond volume was calculated to be 8,964 cubic feet. The new provided pond volume is 9,146 cubic feet at an effective water depth of 1.5’ above the seasonal high groundwater level (see Appendix D and the Geotechnical Investigation Report for Phase 4). See construction plans for the expansion of the pond and modifications to the outlet structure. 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 wetland area. Supporting calculations for the pond sizing can be found in Appendix C. Existing Detention Pond #2 Existing Detention Pond #2 is located north of May Fly Street, between Bull Frog Drive and Laurel Parkway. It receives runoff from Drainage Areas 5, 6, 7, and 8, totaling 14.58 acres. The pond currently receives runoff from Norton Ranch Subdivision, Phase 1. The existing pond volume is 6,072 cubic feet at an effective water depth of 1.5’. For Norton Ranch Subdivision, Phase 4, the pre-development time to concentration was calculated to be 58 minutes and the pre-development runoff rate for the 10-yr storm was calculated to be 1.91 cfs. The existing outlet structure discharges into the existing wetland area in the park to the north. The outlet structure will need to be raised to retain the first 0.5 inches of stormwater runoff. The new flow out elevation will be set to 4803.50’ and the slot will be expanded to a width of 3.7”. The new required pond volume was calculated to be 14,321 cubic feet. The new provided pond volume is 14,915 cubic feet at an effective water depth of 1.5’ above the seasonal high groundwater level (see Appendix D and the Geotechnical Investigation Report for Phase 4). See construction plans for the expansion of the pond and modifications to the outlet structure. 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 wetland area. Supporting calculations for the pond sizing can be found in Appendix C. APPENDIX A DRAINAGE AREA MAP APPENDIX B DRAINAGE AREA CALCULATIONS DRAINAGE AREA #1 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft2 )C * Area Composite ROW 0.74 12158 8967 Park 0.2 0 0 Low-Med Density Residential 0.35 0 0 Dense Residential 0.5 72612 36306 Existing Development 0.5 0 0 Total 84771 45273 A = Area (acres) 1.95 C = Weighted C Factor 0.53 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 1.46 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) 249 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)18.57 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 (%) 0.52% L = length of gutter (ft) 214 V = mean velocity (ft/s) 2.17 Tc Gutter Flow (minutes) =1.64 Tc Total = 20.21 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.53 (calculated above) I = 0.78 Tc-0.64 (in/hr)1.56 (25-yr storm) A = area (acres) 1.95 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs) 1.63 (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 (%) 0.52% Q = PROVIDED GUTTER CAPACITY (cfs) 2.69 DRAINAGE AREA #2 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft2 )C * Area Composite ROW 0.74 11868 8752 Park 0.2 0 0 Low-Med Density Residential 0.35 0 0 Dense Residential 0.5 0 0 Existing Development 0.5 0 0 Total 11868 8752 A = Area (acres) 0.27 C = Weighted C Factor 0.74 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 2.00 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) 23 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)5.11 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 (%) 0.52% L = length of gutter (ft) 285 V = mean velocity (ft/s) 2.17 Tc Gutter Flow (minutes) =2.20 Tc Total = 7.31 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.74 (calculated above) I = 0.78 Tc-0.64 (in/hr)3.00 (25-yr storm) A = area (acres) 0.27 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs) 0.60 (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 (%) 0.52% Q = PROVIDED GUTTER CAPACITY (cfs) 2.69 DRAINAGE AREA #3 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft2 )C * Area Composite ROW 0.74 60231 44420 Park 0.2 0 0 Low-Med Density Residential 0.35 0 0 Dense Residential 0.5 189509 94755 Existing Development 0.5 0 0 Total 249740 139175 A = Area (acres) 5.73 C = Weighted C Factor 0.56 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 0.86 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) 350 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)26.30 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 (%) 1.07% L = length of gutter (ft) 784 V = mean velocity (ft/s) 3.10 Tc Gutter Flow (minutes) =4.21 Tc Total = 30.51 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.56 (calculated above) I = 0.78 Tc-0.64 (in/hr)1.20 (25-yr storm) A = area (acres) 5.73 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs) 3.84 (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 (%) 1.07% Q = PROVIDED GUTTER CAPACITY (cfs) 3.85 DRAINAGE AREA #4 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft2 )C * Area Composite ROW 0.74 30818 22728 Park 0.2 0 0 Low-Med Density Residential 0.35 0 0 Dense Residential 0.5 0 0 Existing Development 0.5 0 0 Total 30818 22728 A = Area (acres) 0.71 C = Weighted C Factor 0.74 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 1.18 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) 25 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)6.31 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 (%) 1.27% L = length of gutter (ft) 663 V = mean velocity (ft/s) 3.38 Tc Gutter Flow (minutes) =3.27 Tc Total = 9.59 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.74 (calculated above) I = 0.78 Tc-0.64 (in/hr)2.52 (25-yr storm) A = area (acres) 0.71 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs) 1.32 (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 (%) 1.27% Q = PROVIDED GUTTER CAPACITY (cfs) 4.19 DRAINAGE AREA #5 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft2 )C * Area Composite ROW 0.74 6870 5067 Park 0.2 0 0 Low-Med Density Residential 0.35 0 0 Dense Residential 0.5 18920 9460 Existing Development 0.5 0 0 Total 25790 14527 A = Area (acres) 0.59 C = Weighted C Factor 0.56 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 1.73 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) 206 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)16.00 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 (%) 0.50% L = length of gutter (ft) 82 V = mean velocity (ft/s) 2.12 Tc Gutter Flow (minutes) =0.64 Tc Total = 16.65 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.56 (calculated above) I = 0.78 Tc-0.64 (in/hr)1.77 (25-yr storm) A = area (acres) 0.59 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs) 0.59 (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 (%) 0.50% Q = PROVIDED GUTTER CAPACITY (cfs) 2.63 DRAINAGE AREA #6 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft2 )C * Area Composite ROW 0.74 56228 41468 Park 0.2 0 0 Low-Med Density Residential 0.35 0 0 Dense Residential 0.5 120620 60310 Existing Development 0.5 0 0 Total 176848 101778 A = Area (acres) 4.06 C = Weighted C Factor 0.58 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 0.90 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) 170 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)18.05 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 (%) 1.56% L = length of gutter (ft) 569 V = mean velocity (ft/s) 3.74 Tc Gutter Flow (minutes) =2.53 Tc Total = 20.58 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.58 (calculated above) I = 0.78 Tc-0.64 (in/hr)1.55 (25-yr storm) A = area (acres) 4.06 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs) 3.61 (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 (%) 1.56% Q = PROVIDED GUTTER CAPACITY (cfs) 4.64 DRAINAGE AREA #7 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft2 )C * Area Composite ROW 0.74 9297 6856 Park 0.2 0 0 Low-Med Density Residential 0.35 0 0 Dense Residential 0.5 0 0 Existing Development 0.5 0 0 Total 9297 6856 A = Area (acres) 0.21 C = Weighted C Factor 0.74 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 2.00 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) 26 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)5.42 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 (%) 0.31% L = length of gutter (ft) 165 V = mean velocity (ft/s) 1.66 Tc Gutter Flow (minutes) =1.66 Tc Total = 7.08 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.74 (calculated above) I = 0.78 Tc-0.64 (in/hr)3.06 (25-yr storm) A = area (acres) 0.21 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs) 0.48 (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 (%) 0.31% Q = PROVIDED GUTTER CAPACITY (cfs) 2.06 DRAINAGE AREA #8 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft2 )C * Area Composite ROW 0.74 54523 40211 Park 0.2 35811 7162 Low-Med Density Residential 0.35 0 0 Dense Residential 0.5 91442 45721 Existing Development 0.5 241552 120776 *See Norton Ranch Phase Total 423328 213870 1 Stormwater Design Report for C Factor A = Area (acres) 9.72 C = Weighted C Factor 0.51 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 1.17 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) 160 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)16.05 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 (%) 0.91% L = length of gutter (ft) 845 V = mean velocity (ft/s) 2.86 Tc Gutter Flow (minutes) =4.93 Tc Total = 20.98 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.51 (calculated above) I = 0.78 Tc-0.64 (in/hr)1.53 (25-yr storm) A = area (acres) 4.17 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs) 3.27 (assuming no carry flow) *Note: The existing development was removed for gutter capacity calculations due to a different time of concentration. 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 (%) 0.91% Q = PROVIDED GUTTER CAPACITY (cfs) 3.55 APPENDIX C POND SIZING CALCULATIONS EXISTING DETENTION POND # 1 REQUIRED VOLUME 1. Calculate Area and Weighted C Factor (Post-Development) Contributing Area C Area (ft 2 )C * Area Composite ROW 0.74 115074 84867 OS/Park 0.20 0 0 Low-Med Density Residential 0.35 0 0 Dense Residential 0.50 262121 131061 Existing Development 0.50 0 0 Total 377196 215928 A = Area (acres) 8.66 Storm C = Weighted C Factor 0.57 Return (yrs)Cf 2 to 10 1 2. Calculate T c (Pre-Development)11 to 25 1.1 Tc Overland Flow 26 to 50 1.2 Tc = 1.87 (1.1-CCf)D1/2/S1/3 51 to 100 1.25 S = Slope of Basin (%) 1.47 C = Rational Method Runoff Coefficient 0.2 Cf = Frequency Adjustment Factor 1.1 D = Length of Basin (ft) 780 Tc (Pre-Development) (minutes) 40 3. 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) 0.67 (Tc Pre-Development) i = rainfall intensity (in./hr.) 0.83 4. Calculate Runoff Rate (Pre-Development) Q = CiA C = Rational Method Runoff Coefficient 0.2 (open land) i = rainfall intensity (in./hr.) 0.83 (calculated above) A = Area (acres) 8.66 (calculated above) Q = Runoff Rate (Pre-Development) (cfs) 1.43 5. Calculate Required Pond Volume Total Area (acres) = 8.66 acres Weighted C = 0.57 Discharge Rate (cfs) = 1.43 cfs (Equal to Pre-Development Runoff Rate) Duration(min) Duration(hrs) Intensity (in/hr)Qin (cfs)Runoff Volume Release Volume Required Storage (ft3) 35 0.58 0.91 4.50 9457 516 8942 36 0.60 0.89 4.42 9551 602 8949 37 0.62 0.88 4.34 9643 688 8955 38 0.63 0.86 4.27 9734 774 8960 39 0.65 0.85 4.20 9823 860 8963 40 0.67 0.83 4.13 9910 946 8964 41 0.68 0.82 4.06 9996 1032 8964 42 0.70 0.81 4.00 10081 1118 8963 43 0.72 0.79 3.94 10164 1204 8960 44 0.73 0.78 3.88 10246 1290 8956 *Note: The outlet structure will not release any water until the initial 8,923 CF of runoff from the first 0.5"of rain has been retained. This occurs within the first 29 minutes of the storm event. PROVIDED VOLUME (ft3)9,146 OUTLET STRUCTURE SLOT Q=CLH3/2 Q = Discharge (cfs) 1.43 C = Weir Coefficient 3.33 (per COB Design Standards) H = Head (ft) 1.5 L = Horizontal Length (ft) 0.23 L = Slot Width (inches) 2.8 DETENTION POND #1 REQUIRED STORAGE FOR FIRST 0.5 INCHES OF RAIN FROM 24-HOUR STORM EVENT REQUIRED VOLUME 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Composite ROW 0.74 115074 84867 Park 0.20 0 0 Low-Med Density Residential 0.35 0 0 Dense Residential 0.50 262121 131061 Existing Development 0.50 0 0 Total 377196 215928 C=Weighted C Factor 0.57 2. Calculate Required Volume Q = CIA V=22194Q C = Weighted C Factor 0.57 I = intensity (in/hr) 0.08 (10 yr, 24 hr storm) A = Area (acres) 8.66 Q = runoff (cfs) 0.40 V = REQUIRED VOL (ft3)8923 *A 10-year, 24-hour storm event produces 0.5" of rain after 6.165 hours. (I=0.0811in/hr --> 0.5in / 0.0811in/hr = 6.165 hours) **The required volume was calculated by using the 6.165 hours it takes to produce the 0.5" of rain. (6.165hr * 60min/hr * 60sec/min * Q = 22194Q) EXISTING DETENTION POND # 2 REQUIRED VOLUME 1. Calculate Area and Weighted C Factor (Post-Development) Contributing Area C Area (ft 2 )C * Area Composite ROW 0.74 126918 93602 OS/Park 0.20 35811 7162 Low-Med Density Residential 0.35 0 0 Dense Residential 0.50 230982 115491 Existing Development 0.50 241552 120776 *See Norton Ranch Phase Total 635263 337031 1 Stormwater Design Report for C Factor A = Area (acres) 14.58 Storm C = Weighted C Factor 0.53 Return (yrs)Cf 2 to 10 1 2. Calculate T c (Pre-Development)11 to 25 1.1 Tc Overland Flow 26 to 50 1.2 Tc = 1.87 (1.1-CCf)D1/2/S1/3 51 to 100 1.25 S = Slope of Basin (%) 1.02 C = Rational Method Runoff Coefficient 0.2 Cf = Frequency Adjustment Factor 1.1 D = Length of Basin (ft) 1261 Tc (Pre-Development) (minutes) 58 3. 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) 0.97 (Tc Pre-Development) i = rainfall intensity (in./hr.) 0.65 4. Calculate Runoff Rate (Pre-Development) Q = CiA C = Rational Method Runoff Coefficient 0.2 (open land) i = rainfall intensity (in./hr.) 0.65 (calculated above) A = Area (acres) 14.58 (calculated above) Q = Runoff Rate (Pre-Development) (cfs) 1.91 5. Calculate Required Pond Volume Total Area (acres) = 14.58 acres Weighted C = 0.53 Discharge Rate (cfs) = 1.91 cfs (Equal to Pre-Development Runoff Rate) Duration(min) Duration(hrs) Intensity (in/hr)Qin (cfs)Runoff Volume Release Volume Required Storage (ft3) 45 0.75 0.77 5.97 16119 1833 14286 46 0.77 0.76 5.89 16243 1947 14296 47 0.78 0.75 5.80 16366 2062 14305 48 0.80 0.74 5.72 16487 2176 14311 49 0.82 0.73 5.65 16607 2291 14316 50 0.83 0.72 5.57 16724 2405 14319 51 0.85 0.71 5.50 16841 2520 14321 52 0.87 0.70 5.43 16956 2634 14321 53 0.88 0.69 5.37 17069 2749 14320 54 0.90 0.69 5.30 17181 2863 14318 *Note: The outlet structure will not release any water until the initial 13,927 CF of runoff from the first 0.5"of rain has been retained. This occurs within the first 29 minutes of the storm event. PROVIDED VOLUME (ft3)14,915 OUTLET STRUCTURE SLOT Q=CLH3/2 Q = Discharge (cfs) 1.91 C = Weir Coefficient 3.33 (per COB Design Standards) H = Head (ft) 1.5 L = Horizontal Length (ft) 0.31 L = Slot Width (inches) 3.7 DETENTION POND #2 REQUIRED STORAGE FOR FIRST 0.5 INCHES OF RAIN FROM 24-HOUR STORM EVENT REQUIRED VOLUME 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft2 )C * Area Composite ROW 0.74 126918 93602 OS/Park 0.20 35811 7162 Low-Med Density Residential 0.35 0 0 Dense Residential 0.50 230982 115491 Existing Development 0.50 241552 120776 Total 635263 337031 C=Weighted C Factor 0.53 2. Calculate Required Volume Q = CIA V=22194Q C = Weighted C Factor 0.53 I = intensity (in/hr) 0.08 (10 yr, 24 hr storm) A = Area (acres) 14.58 Q = runoff (cfs) 0.63 V = REQUIRED VOL (ft3)13927 *A 10-year, 24-hour storm event produces 0.5" of rain after 6.165 hours. (I=0.0811in/hr --> 0.5in / 0.0811in/hr = 6.165 hours) **The required volume was calculated by using the 6.165 hours it takes to produce the 0.5" of rain. (6.165hr * 60min/hr * 60sec/min * Q = 22194Q) APPENDIX D POND PROFILES