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HomeMy WebLinkAboutStormwater Design Report (msp) 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 installed with the Phase 6 infrastructure improvements. 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. The lot was intended to drain to Detention Pond A. 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 allocated for Lot 5 is 2,138 ft3 (see Appendix A for calculations). Drainage Areas 1 & 2 will be retained in underground storage systems. Drainage Areas 3 & 4 and a majority of the landscaping are anticipated to infiltrate into the respective landscaping beds. The runoff coefficient for Drainage Area 1 is 0.86 over an area of 28,114 sf which translates to 1,642 cf of runoff. Runoff form Drainage Area 1 will drain to a catch basin located in the middle curb island in the center 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 26’ long, 36” dia. perforated CMP. The system provides a storage capacity of 1,673 cf. Details for the Drainage Area 1 infiltration system are included in Appendix D. Drainage Area 2 has a runoff coefficient of 0.90 over an area of 25,317 sf which translates to 1,549 ft3 of runoff. Similar to Drainage Area 1, Drainage Area 2 will drain to a catch basin at the north end of the parking lot. The runoff will be conveyed to another Contech Stormwater Infiltration Chamber System beneath the north row of parking stalls. The design will consist of a double manifold system with four rows of 52’ long, 30” dia. perforated CMP. The system provides a storage capacity of 1,559 cf. Details for the Drainage Area 2 infiltration system are included in Appendix D. Drainage Area 3 consists of the landscaping along the eastern edge and a small area of sidewalk. Drainage Area 3 has a weighted C value of 0.30 over an area of 5,098 sf which translates to a runoff volume of 102 cf. Drainage Area 4 has a weighted C value of 0.20 over an area of 2,611 sf which translates to a runoff volume of 35 cf. Since nearly 95% of Drainage Area 3 and Drainage Area 4 are landscaping it is anticipated that the small amount of runoff will infiltrate into the landscape beds along the northern, eastern, and southern edges of the property. SEASONAL HIGH GROUNDWATER Three groundwater monitoring wells were installed on the property on April 12, 2019. Readings were taken weekly from April 12 to June 21 to determine the seasonal high levels. Two of the wells were installed directly where the underground retention chambers are proposed. The third monitoring well was installed in the low point of the existing property. Monitoring Well 1 was installed where the Drainage Area 1 system is proposed. The peak groundwater level was measured at 6.42 feet below ground surface (bgs). The existing grade at Monitoring Well 1 is 4702.18’; which makes the seasonal high-water elevation 4695.76’. The underground retention system for Drainage Area 1 has a bottom elevation of 4698.31’. Monitoring Well 3 was installed where the Drainage Area 2 system is proposed. The peak groundwater level was measured at 6.55 feet below ground surface (bgs). The existing grade at Monitoring Well 3 is 4700.92’; which makes the seasonal high-water elevation 4694.37’. The underground retention system for Drainage Area 2 has a bottom elevation of 4696.42’. Based on this information, groundwater is not anticipated to decrease the systems storage capacity. As an additional measure to promote infiltration into the water table, the proposed design includes excavation to the native gravels layer and backfilling with well-draining pit run. Monitoring well data is included in Appendix E of this report. 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 24814 23573 Landscape 0.2 3300 660 Total 28114 24233 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) 0.65 Q = runoff (cfs) 0.23 V = REQUIRED VOL (ft3)1642 DRAINAGE AREA 2 REQUIRED VOLUME 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Hardscape 0.95 23718 22532 Landscape 0.2 1599 320 Total 25317 22852 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) 0.58 Q = runoff (cfs) 0.22 V = REQUIRED VOL (ft3)1549 DRAINAGE AREA 3 REQUIRED VOLUME 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Hardscape 0.95 655 622 Landscape 0.2 4443 889 Total 5098 1511 C=Weighted C Factor 0.30 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.30 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.12 Q = runoff (cfs) 0.01 V = RUNOFF VOL. (ft3)102 DRAINAGE AREA 4 REQUIRED VOLUME 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Hardscape 0.95 0 0 Landscape 0.2 2611 522 Total 2611 522 C=Weighted C Factor 0.20 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.20 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.06 Q = runoff (cfs) 0.00 V = RUNOFF VOL. (ft3)35 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 24814 23573 Total 28114 24233 A = Area (acres) 0.6454 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) 0.65 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs) 1.30 (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 23718 22532 Total 25317 22852 A = Area (acres) 0.5812 C = Weighted C Factor 0.90 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 0.90 (calculated above) I = 0.78 Tc-0.64 (in/hr)2.14 (25-yr storm) A = area (acres) 0.58 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs) 1.12 (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 1.30 PE(9-12"dia) 0.017 DA 2 1.12 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 Detention or Infiltration System Designer Design - CMP System Selection Structural Backfill Detail DYODS® #9302 System Description:Drainage Area 1 Project:Westland Lofts Master Site Plan Application:Infiltration (Perforated CMP) Created:11/01/2018 Modified:06/26/2019 Application:Infiltration System:CMP - Perforated w/ Stone Backfill Stone Porosity:40 % Width At Ends:18 in Above Pipe:6 in Width At Sides:18 in Below Pipe:6 in Design Parameters Pipe Detail Storage Volume:1,642 cf Limiting Length:40 ft Limiting Width:25 ft Pavement Type:No Pavement Pavement Depth:0 in Freeboard Depth:0 in Invert Depth:5 ft Material:Steel Shape:Round Size:36 in Corrugation:2-2/3" x 1/2" Gage:16 Pipe Spacing:18 in Coating:Aluminized Steel Type 2 (ALT2) Layout:Double Manifold Calculations Status:OK Barrel Count:4 CMP Footprint (LxW):32 x 16.5 ft Length Per Barrel:26 ft Length Per Header:16.50 ft Pipe Storage:968 cf Structural Backfill Zone Storage:704 cf Total Storage Provided:1,673 cf Percent Of Required Storage:101.89 % Total CMP Footage:137 ft Approximate CMP Piece Count:10 Approximate Coupling Bands:12 Approximate Truckloads:1 Total Excavation:140 cy Structural Backfill:65 cy Pavement Volume:0 cy Remaining Backfill To Pavement:38 cy Draftboard Detention or Infiltration System Designer Design - CMP System Selection Structural Backfill Detail DYODS® #9314 System Description:Drainage Area 2 Project:Westland Lofts Master Site Plan Application:Infiltration (Perforated CMP) Created:11/05/2018 Modified:06/26/2019 Application:Infiltration System:CMP - Perforated w/ Stone Backfill Stone Porosity:40 % Width At Ends:15 in Above Pipe:6 in Width At Sides:15 in Below Pipe:6 in Design Parameters Pipe Detail Storage Volume:1,549 cf Limiting Length:75 ft Limiting Width:15 ft Pavement Type:Flexible Pavement Pavement Depth:3 in Freeboard Depth:0 in Invert Depth:4 ft Material:Steel Shape:Round Size:30 in Corrugation:2-2/3" x 1/2" Gage:16 Pipe Spacing:15 in Coating:Aluminized Steel Type 2 (ALT2) Layout:Double Manifold Calculations Status:OK Barrel Count:3 CMP Footprint (LxW):57 x 10 ft Length Per Barrel:52 ft Length Per Header:10 ft Pipe Storage:863 cf Structural Backfill Zone Storage:695 cf Total Storage Provided:1,559 cf Percent Of Required Storage:100.69 % Total CMP Footage:176 ft Approximate CMP Piece Count:11 Approximate Coupling Bands:12 Approximate Truckloads:1 Total Excavation:124 cy Structural Backfill:64 cy Pavement Volume:7 cy Remaining Backfill To Pavement:20 cy Draftboard APPENDIX E GROUNDWATER MONITORING WELL DATA Project Engineer: Project: Gallatin Green, Gallatin County, MT Well Information:bgs = below ground surface ags = above ground surface MW-1 MW-2 MW-3 2.00 Groundwater Information: MW-1 MW-2 MW-3 7.22 3.05 7.40 6.42 2.25 6.55 6.72 2.58 6.83 6.85 2.70 6.98 7.11 3.00 7.26 7.29 3.17 7.45 7.27 3.16 7.42 7.36 3.21 7.49 7.59 3.46 7.75 7.30 3.29 7.46 7.21 3.23 7.45 Date 4.23.19 5.17.19 5.24.19 5.31.19 6.7.19 6.14.19 6.21.19 4.30.19 5.3.19 5.10.19 4.12.19 Project Number: 171281.1 Gallatin Green Groundwater Monitoring Project Location: Monitor Well Data Well ID Well Depth (feet-bgs) Top of Well (feet-ags) Ground Elevation Depth to Ground Water (feet-bgs)