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Home My WebLink About171281 Stormater Design Report Master Site Plan - 02 18 2019 DESIGN REPORT STORMWATER MANAGEMENT WESTLAND LOFTS MIXED USE MASTER SITE PLAN LOT 5, BLOCK 14 BAXTER MEADOWS SUBDIVISION, PHASE 6 Prepared for: Dallas Financial, LLC P.O. Box 12009 Bozeman, MT 59715 Prepared by: Project Number: 171281 February 2019 INTRODUCTION The Westland Lofts Mixed Use Master Site Plan proposes to develop Lot 5, Block 14 of the Baxter Meadows Subdivision, Phase 6. The proposed layout consists of a parking area centrally located on the lot with two approaches on the north and south side of the lot. Surrounding the parking lot are four proposed buildings, three of which will be exclusively residential and the fourth will have commercial and residential spaces. The property is located within the City of Bozeman limits and is within the community business district (B-2) zoning. A combination of site grading, curb and gutter, and underground infiltration chambers will be used to manage stormwater runoff on the site. Supporting stormwater calculations are enclosed in Appendix A for reference. BAXTER MEADOWS SUBDIVISION, PHASE 6 – STORMWATER BACKGROUND INFORMATION Baxter Meadows Subdivision included the construction of regional detention and retention ponds that handle the runoff generated by the subdivision through a system of stormwater conveyance structures and stormwater detention ponds. The pond that was constructed for Lot 5, Block 14 was constructed with Phases 2C & 2D then, resized during Phase 6 construction. Lot 5, Block 14 was assigned a Rational Method Runoff Coefficient of 0.5 per Bozeman Design Standards and Specifications Policy (DSSP) in the original design of the subdivision. See Appendix C for the Phase 6 stormwater design report by TD&H Engineering. A Runoff Coefficient of 0.5 does not provide adequate storm water detention for the proposed development of this lot. The actual Runoff Coefficient for the proposed project on Lot 5, Block 14 is 0.80 (see enclosed calculations). The original underestimation of the future development of this lot means that the regional detention ponds for the subdivision are not adequate to handle the entire runoff contribution from the proposed development. LOT 5, BLOCK 14 STORMWATER DESIGN The site was divided into 4 drainage areas as shown on the attached Drainage Area Map in Appendix B. Using the design C value of 0.5 and the DSSP 10-year, 2-hour storm, the runoff allotted for Lot 5 is 2,138 ft3 (see Appendix A for calculations). Drainage Area 3 will discharge directly to the detention pond in the open space parcel. Drainage Area 4 will runoff to the street and be conveyed to the detention pond. Drainage Area 3 has a weighted C value of 0.63 over an area of 10,332 sf which translates to a runoff volume of 439 ft3. Drainage Area 4 has a weighted C value of 0.67 over an area of 5,440 sf which translates to a runoff volume of 247 ft3. Combined, these drainage areas will discharge a total of 686 ft3 of runoff which is far less than what was originally allotted for the lot. The runoff coefficient for Drainage Area 1 is 0.86 over an area of 26,447 sf which translates to 1,535 ft3 of runoff. Runoff form Drainage Area 1 will drain to a catch basin located in the curb island in the south half of the parking lot. The runoff will then be conveyed and retained in Contech Stormwater Infiltration Chambers beneath the open space located in the southeast corner of the lot. The design will consist of a double manifold system with four rows of 24’ long, 36” perforated CMP. The system provides a storage capacity of 1,576 ft3. Details for the Drainage Area 1 infiltration system are included in Appendix D. Drainage Area 2 has a runoff coefficient of 0.88 over an area of 18,327 sf which translates to 1,099 ft3 of runoff. Similar to Drainage Area 1, Drainage Area 2 will drain to a catch basin in the north of the parking lot. The runoff will be conveyed to another Contech Stormwater Infiltration Chamber System beneath the northern row of parking stalls. The design will consist of a double manifold system with four rows of 25’ long, 42” perforated CMP. The system provides a storage capacity of 1,172 ft3. Details for the Drainage Area 2 infiltration system are included in Appendix D. APPENDIX A STORMWATER CALCULATIONS PRE-DEVELOPMENT RUNOFF VOLUME 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Total 0.5 63090 31545 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) 1.45 Q = runoff (cfs) 0.30 V = RUNOFF VOL (ft3)2138 OVERALL SITE REQUIRED VOLUME 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Hardscape 0.95 50709 48174 Landscape 0.2 12381 2476 Total 63090 50650 C=Weighted C Factor 0.80 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.80 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 1.45 Q = runoff (cfs) 0.48 V = RUNOFF VOL TO SUBDIVISION (ft3)3432 DRAINAGE AREA 1 REQUIRED VOLUME 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Hardscape 0.95 23147 21990 Landscape 0.2 3300 660 Total 26447 22650 C=Weighted C Factor 0.86 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.86 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.61 Q = runoff (cfs) 0.21 V = REQUIRED VOL (ft3)1535 DRAINAGE AREA 2 REQUIRED VOLUME 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Hardscape 0.95 16728 15892 Landscape 0.2 1599 320 Total 18327 16211 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) 0.42 Q = runoff (cfs) 0.15 V = REQUIRED VOL (ft3)1099 DRAINAGE AREA 3 REQUIRED VOLUME 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Hardscape 0.95 5889 5595 Landscape 0.2 4443 889 Total 10332 6483 C=Weighted C Factor 0.63 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.63 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.24 Q = runoff (cfs) 0.06 V = RUNOFF VOL TO SUBD. (ft3)439 DRAINAGE AREA 4 REQUIRED VOLUME 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Hardscape 0.95 3415 3244 Landscape 0.2 2025 405 Total 5440 3649 C=Weighted C Factor 0.67 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.67 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.12 Q = runoff (cfs) 0.03 V = RUNOFF VOL TO SUBD. (ft3)247 DRAINAGE AREA #1 INLET CAPACITY 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Landscape 0.2 3300 660 Hardscape 0.95 23147 21990 Total 26447 22650 A = Area (acres) 0.6071 C = Weighted C Factor 0.86 2. Calculate Tc (Time to Concentration) Tc Overland Flow (Roof and Parking Lot) Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 2.5 Return (yrs)Cf C = Rational Method Runoff Coefficient 0.20 2 to 10 1 Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1 D = Length of Basin (ft) 71 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)10.22 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) 81 V = mean velocity (ft/s) 2.12 Tc Gutter Flow (minutes) =0.64 Tc Total = 10.85 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.86 (calculated above) I = 0.78 Tc-0.64 (in/hr)2.33 (25-yr storm) A = area (acres) 0.61 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs) 1.21 (assuming no carry flow) 4. Calculate Inlet Capacity-Sag Location Q=3.3P(h)1.5 (Modified Weir Eq.)from Neenah Foundry P=Perimeter (ft) 5.9 1.03 (0.15' below top of curb) Q= capacity of Inlet (cfs) 20.21 Q=0.6A(2gh)0.5 (Orifice Equation)from Neenah Foundry A= Free Open Area of Grate (ft2) 2.10 1.03 (0.15' below top of curb) Q= capacity of inlet (cfs) 10.24 Q=True Capacity (lesser of the two) 10.24 Number of Inlets Required 1.0 h=depth of flow from above (ft) + (0.875in/12in for Depression of Grate) h=depth of flow from above (ft) + (0.875in/12in for Depression of Grate) DRAINAGE AREA #2 INLET CAPACITY 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Landscape 0.2 1599 320 Hardscape 0.95 16728 15892 Total 18327 16211 A = Area (acres) 0.4207 C = Weighted C Factor 0.88 2. Calculate Tc (Time to Concentration) Tc Overland Flow (Roof and Parking Lot) Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 2.5 Return (yrs)Cf C = Rational Method Runoff Coefficient 0.20 2 to 10 1 Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1 D = Length of Basin (ft) 90 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)11.50 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 (%) 0.75% L = length of gutter (ft) 139 V = mean velocity (ft/s) 2.60 Tc Gutter Flow (minutes) =0.89 Tc Total = 12.39 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.88 (calculated above) I = 0.78 Tc-0.64 (in/hr)2.14 (25-yr storm) A = area (acres) 0.42 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs) 0.80 (assuming no carry flow) 4. Calculate Inlet Capacity-Sag Location Q=3.3P(h)1.5 (Modified Weir Eq.)from Neenah Foundry P=Perimeter (ft) 5.9 1.03 (0.15' below top of curb) Q= capacity of Inlet (cfs) 20.21 Q=0.6A(2gh)0.5 (Orifice Equation)from Neenah Foundry A= Free Open Area of Grate (ft2) 2.10 1.03 (0.15' below top of curb) Q= capacity of inlet (cfs) 10.24 Q=True Capacity (lesser of the two) 10.24 Number of Inlets Required 1.0 h=depth of flow from above (ft) + (0.875in/12in for Depression of Grate) h=depth of flow from above (ft) + (0.875in/12in for Depression of Grate) MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY) Pipe: CONTECH INLETS Location:ST MH 2 & ST MH 1 INPUT D=12 inches d=11.26 inches Mannings Formula n=0.013 mannings q=57.5 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 0.77 2.64 0.29 3.54 2.71 PE (<9"dia) 0.015 PE (>12"dia) 0.02 flow required (cfs) DA 1 1.21 PE(9-12"dia) 0.017 DA 2 0.80 CMP 0.025 ADS N12 0.012 HCMP 0.023 Conc 0.013 Manning's n-values d q D APPENDIX B DRAINAGE AREA MAP APPENDIX C BAXTER MEADOWS ORIGINAL STORMWATER DESIGN REPORT APPENDIX D CONTECH STORMWATER INFILTRATION CHAMBERS DETAILS