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HomeMy WebLinkAbout18 - Design Report - The Nest PUD - Stormwater DESIGN REPORT STORMWATER MANAGEMENT THE NEST P.U.D. SUBDIVISION Prepared for: Center Arrow Partners, LLC P.O. Box #1633, Bozeman, Montana 59771 Prepared by: C&H Engineering and Surveying, Inc. 1091 Stoneridge Drive, Bozeman, MT 59718 (406) 587-1115 Project Number: 180805 October 2019 Page 1 of 21 INTRODUCTION The proposed Nest PUD Subdivision is a 44-lot townhome and condominium development located on a 5.41-acre parcel in Section 4, 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, piping, and swales 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, 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. They are designed with an effective water depth of 1.5 feet, and maximum side slope of 4:1. Detention ponds are sized to limit discharge to pre-development rates for the 10-year storm event. Detention ponds are also sized to retain the first 0.5 inches of rain from a 10-year, 2-hour storm event 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. 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 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 D. Existing Retention Pond #1 Existing Retention Pond #1 is located north of Durston Road along the southern property line Lot 4. This pond will be relocated with the proposed development and will be upgraded to a detention pond in order to reduce the required footprint. The pre-development time to concentration was calculated to be 9.3 minutes and the pre-development runoff rate for the 10-yr storm was calculated Page 2 of 21 to be 0.51 cfs. There is a 1” weir in the outlet structure that limits runoff to the pre-development runoff rate of 0.51 cfs. The weir is to be located 1’ above the bottom of pond so that the first 0.5 inches of rain is retained before the stormwater is discharged into the existing wetland area. It receives runoff from Durston Road (Drainage Area #EX1). The new required pond volume was calculated to be 1,167 cubic feet from Drainage Area #EX1, totaling 1.20 acres. The proposed pond volume is 2,014 cubic feet. In the case of larger storms beyond the pond’s capacity to capture and infiltrate, the pond will overtop and flow west into Baxter Creek. Supporting calculations for the required pond volume can be found in Appendix C. Existing Retention Pond #2 Existing Retention Pond #2 is located south of Annie Street and west of proposed Lot 4. It receives runoff from Drainage Areas 1 and EX2, totaling 6.22 acres. The existing retention pond will be upgraded to a detention pond by installing an outlet structure in order to reduce the required footprint of the pond. The pre-development time to concentration was calculated to be 52 minutes and the pre-development runoff rate for the 10-yr storm was calculated to be 0.87 cfs. There is a 1.7” weir in the outlet structure that limits runoff to the pre-development runoff rate of 0.87 cfs. The weir is to be located 1’ above the bottom of pond so that the first 0.5 inches of rain is retained before the stormwater is discharged into the existing wetland area. The pond currently receives runoff from Annie Street (Drainage Area #EX2). The existing pond volume is 6,445 cubic feet at an effective water depth of 1.5’. The water depth has the ability to exceed 1.5’ as it is currently a 2.5’ deep pond. There is an existing chain link fence installed around the pond to provide a safety barrier to pedestrians. The existing required pond volume is 2,669 cubic feet (retention) from the Westbrook Subdivision and the new required pond volume including the Nest development was calculated to be 6,151 cubic feet (detention) from Drainage Areas #1 and #EX2, totaling 6.22 acres. In the case of larger storms beyond the pond’s capacity to capture and infiltrate, the pond will overtop and flow west into Baxter Creek. Supporting calculations for the required pond volume can be found in Appendix C. Page 3 of 21 APPENDIX A DRAINAGE AREA MAP Page 4 of 21 Page 5 of 21 APPENDIX B DRAINAGE AREA CALCULATIONS Page 6 of 21 DRAINAGE AREA #1 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Hardscape 0.95 126546 120219 Landscape 0.2 109161 21832 Total 235707 142051 A = Area (acres)5.4111 C = Weighted C Factor 0.60 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 1.3% 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)139 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)14.4 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 (%)0.72% L = length of gutter (ft)1000 V = mean velocity (ft/s)2.54 Tc Gutter Flow (minutes) =6.6 Tc Total =21.0 (5 minute minimum) Page 7 of 21 DRAINAGE AREA #EX1 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Hardscape 0.95 52151 49544 Landscape 0.2 0 0 Total 52151 49544 A = Area (acres)1.1972 C = Weighted C Factor 0.95 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 1.0% 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)15 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)5.2 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 (%)0.65% L = length of gutter (ft)600 V = mean velocity (ft/s)2.42 Tc Gutter Flow (minutes) =4.1 Tc Total =9.3 (5 minute minimum) Page 8 of 21 DRAINAGE AREA #EX2 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Hardscape 0.95 35155 33397 Landscape 0.2 0 0 Total 35155 33397 A = Area (acres)0.8070 C = Weighted C Factor 0.95 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 1.0% 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)15 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)5.2 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 (%)0.72% L = length of gutter (ft)400 V = mean velocity (ft/s)2.54 Tc Gutter Flow (minutes) =2.6 Tc Total =7.8 (5 minute minimum) Page 9 of 21 APPENDIX C POND SIZING CALCULATIONS Page 10 of 21 DETENTION POND #1 REQUIRED VOLUME 1. Calculate Area and Weighted C Factor (Post-Development) Contributing Area C Area (ft 2 )C * Area Hardscape 0.95 52151 49544 Landscape 0.2 0 0 Total 52151 49544 A = Area (acres)1.1972 C = Weighted C Factor 0.95 3. Calculate Tc (Pre-Development) Tc (Pre-Development) (minutes)9.3 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)0.16 (Tc Pre-Development - DA EX1) i = rainfall intensity (in./hr.)2.15 5. Calculate Runoff Rate (Pre-Development) Q = CiA C = Rational Method Runoff Coefficient 0.2 (open land) i = rainfall intensity (in./hr.) 2.15 (calculated above) A = Area (acres) 1.20 (calculated above) Q = Runoff Rate (Pre-Development) (cfs) 0.51 Page 11 of 21 6. Calculate Required Pond Volume Total Area (acres) = 1.20 acres Weighted C = 0.95 Discharge Rate (cfs) = 0.51 cfs (Equal to Pre-Development Runoff Rate) Duration(min) Duration(hrs) Intensity (in/hr)Qin (cfs)Runoff Volume Release Volume Required Storage (ft3) 16 0.27 1.51 1.72 1650 494 1156 17 0.28 1.45 1.65 1685 525 1161 18 0.30 1.40 1.59 1719 555 1164 19 0.32 1.35 1.54 1752 586 1166 20 0.33 1.31 1.49 1784 617 1167 21 0.35 1.27 1.44 1815 648 1167 22 0.37 1.23 1.40 1845 679 1166 23 0.38 1.19 1.36 1873 710 1164 24 0.40 1.16 1.32 1902 741 1161 25 0.42 1.13 1.29 1929 771 1157 OUTLET STRUCTURE SLOT Q=CLH3/2 Q = Discharge (cfs)0.51 (calculated above) C = Weir Coefficient 3.33 (per COB Design Standards) H = Head (ft)1.5 L = Horizontal Length (ft)0.08 L = Slot Width (inches)1.0 Page 12 of 21 DETENTION POND #2 REQUIRED VOLUME 1. Calculate Area and Weighted C Factor (Post-Development) Contributing Area C Area (ft 2 )C * Area Hardscape 0.95 161701 153616 Landscape 0.2 109161 21832 Total 270862 175448 A = Area (acres)6.2181 C = Weighted C Factor 0.65 3. Calculate Tc (Pre-Development) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 1.24% 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)1115 26 to 50 1.2 51 to 100 1.25 Tc (Pre-Development) (minutes)52 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)0.87 (Tc Pre-Development) i = rainfall intensity (in./hr.)0.70 5. Calculate Runoff Rate (Pre-Development) Q = CiA C = Rational Method Runoff Coefficient 0.2 (open land) i = rainfall intensity (in./hr.) 0.70 (calculated above) A = Area (acres) 6.22 (calculated above) Q = Runoff Rate (Pre-Development) (cfs) 0.87 Page 13 of 21 6. Calculate Required Pond Volume Total Area (acres) = 6.22 acres Weighted C = 0.65 Discharge Rate (cfs) = 0.87 cfs (Equal to Pre-Development Runoff Rate) Duration(min) Duration(hrs) Intensity (in/hr)Qin (cfs)Runoff Volume Release Volume Required Storage (ft3) 59 0.98 0.65 2.61 9225 3080 6145 60 1.00 0.64 2.58 9280 3133 6147 61 1.02 0.63 2.55 9334 3185 6149 62 1.03 0.63 2.52 9387 3237 6150 63 1.05 0.62 2.50 9440 3289 6151 64 1.07 0.61 2.47 9492 3341 6151 65 1.08 0.61 2.45 9544 3394 6150 66 1.10 0.60 2.42 9595 3446 6149 67 1.12 0.60 2.40 9645 3498 6147 68 1.13 0.59 2.38 9695 3550 6145 OUTLET STRUCTURE SLOT Q=CLH3/2 Q = Discharge (cfs)0.87 (calculated above) C = Weir Coefficient 3.33 (per COB Design Standards) H = Head (ft)1.5 L = Horizontal Length (ft)0.14 L = Slot Width (inches)1.7 Page 14 of 21 APPENDIX D STORM SEWER FACILITIES DESIGN Page 15 of 21 PIPE #1 25-YR OUTFLOW RATE REQUIRED CAPACITY 1. Calculate Area and Weighted C Factor (Post-Development) Contributing Area C Area (ft 2 )C * Area Hardscape 0.95 52151 49544 Landscape 0.2 0 0 Total 52151 49544 A = Area (acres)1.1972 C = Weighted C Factor 0.95 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) 0.16 DA #EX1 i = rainfall intensity (in./hr.) 2.57 3. Calculate 25-yr Pond Outflow Rate Q = CiA C = Rational Method Runoff Coefficient 0.95 (calculated above) i = rainfall intensity (in./hr.) 2.57 (calculated above) A = Area (acres) 1.20 (calculated above) Q = 25-yr Flow Rate (cfs) 2.92 Page 16 of 21 MANNING'S EQUATION FOR PIPE FLOW Project: Pipe 1 Location: Detention Pond 1 Outlet Pipe INPUT D= 15 inches d= 14.07 inches Mannings Formula n= 0.013 mannings c 57.7 degrees Q=(1.486/n)ARh2/3S1/2 S= 0.004 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 1.20 3.30 0.36 3.68 4.39 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 Page 17 of 21 PIPE #2 25-YR OUTFLOW RATE REQUIRED CAPACITY 1. Calculate Area and Weighted C Factor (Post-Development) Contributing Area C Area (ft 2 )C * Area Hardscape 0.95 161701 153616 Landscape 0.2 109161 21832 Total 270862 175448 A = Area (acres)6.2181 C = Weighted C Factor 0.65 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) 0.35 DA #1 i = rainfall intensity (in./hr.) 1.53 3. Calculate 25-yr Pond Outflow Rate Q = CiA C = Rational Method Runoff Coefficient 0.65 (calculated above) i = rainfall intensity (in./hr.) 1.53 (calculated above) A = Area (acres) 6.22 (calculated above) Q = 25-yr Flow Rate (cfs) 6.15 Page 18 of 21 MANNING'S EQUATION FOR PIPE FLOW Pipe: Pipe 2 Location: Detention Pond 2 Outlet Pipe INPUT D= 15 inches d= 14.07 inches Mannings Formula n= 0.013 mannings C 57.7 degrees Q=(1.486/n)ARh2/3S1/2 S= 0.01 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 5.81 6.95 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 Page 19 of 21 APPENDIX E BAXTER CREEK DOWNSTREAM INFRASTRUCTURE Page 20 of 21 Page 21 of 21 INSPECTION AND MAINTENANCE FOR STORMWATER MANAGEMENT FACILITIES The Property Owners Association shall be responsible for the maintenance of the stormwater drainage facilities within The Nest PUD Subdivision development. Storm Water Facilities: 1. Drainage swales slope toward retention and detention ponds to collect storm water runoff and channel it to the retention or detention pond. 2. Retention/Detention Ponds collect storm water runoff and store the water until it evaporates and/or infiltrates into the ground. 3. Detention ponds collect storm water runoff while allowing some water to drain to another location. 4. Culverts are pipes which channel storm water from ditches or swales under roads. 5. Pipe Networks convey storm water to different discharge locations underground. 6. Inlets are facilities where storm water runoff enters a pipe network. Inlets include storm water manholes and drains. 7. Catch Basins are sumps typically located directly below storm water inlets and allow sediment to settle before storm water enters the pipe network. 8. Outlets are points where storm water exits a pipe network. 9. Drywells are underground storm water collection facilities that collect and temporarily store runoff from roof tops and landscaped areas before allowing storm water to infiltrate into the ground. Post Construction Inspection: 1. Observe drain time in retention/detention ponds for a storm event after completion of the facility to confirm that the desired drain time has been obtained. If excessively slow infiltration rates are observed then excavate a minimum 5 ft by 5 ft drain to native gravels (or native well-draining material) and backfill with well-draining material (pit-run). 2. Observe that drywells, catch basins, and outlet structures are clear of any material or obstructions in the drainage slots. Inspect these structures to insure proper drainage following a storm event. Immediately identify and remove objects responsible for clogging if not draining properly. Semi-Annual Inspection: 1. Check retention/detention ponds and dry wells three days following a storm event exceeding ¼ inch of precipitation. Failure for water to percolate within this time period indicates clogging or poor-draining soils. Clear any clogs and replace any poor-draining soils with well-draining gravely soils. 2. Check for grass clippings, litter, and debris in drainage swales, catch basins, dry wells, culverts and retention/detention ponds. Flush and/or vacuum drywells or storm water pipes if excessive material is observed in the facilities. Standard Maintenance: 1. Remove sediment and oil/grease from retention ponds and detention ponds. 2. Inspect and remove debris from drainage swales, catch basins, dry wells, and retention/detention ponds. Use a vacuum truck to clean catch basins and dry wells. 3. Monitor health of vegetation and revegetate as necessary to maintain full vegetative cover. 4. Inspect for the following issues: differential accumulation of sediment, drain time, signs of petroleum hydrocarbon contamination (odors, oil sheen in pond water), standing water, trash and debris. Sediment accumulation: In most cases, sediment from a retention/detention pond does not contain toxins at levels posing a hazardous concern. However, sediments should be tested for toxicants in compliance with current disposal requirements and if land uses in the drainage area include commercial or industrial zones, or if visual or olfactory indications of pollution are noticed. Sediments containing high levels of pollutants should be disposed of in accordance with applicable regulations and the potential sources of contamination should be investigated and contamination practices terminated.