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Home My WebLink About007 Stormwater Design Report - 777 Boardwalk Building INTRODUCTION The 777 Boardwalk Building (Previously the Bozeman Design Center) proposes to develop an existing building pad site located on Lot 1, Block 2, Ferguson Farm Subdivision. The proposed development consists of a two-story commercial building, adjacent sidewalk, parking and landscaping improvements. Existing stormwater infrastructure was installed with the overall subdivision street improvements in 2015. Additional infrastructure was completed in 2018 with the Block 2 parking lot improvements. Both stormwater design reports were referenced in this analysis to ensure adequate capacity in the subdivision and parking lot exist. The Block 2 drainage report has been updated as well. It is included with this site plan application. Within Block 2, Drainage Area 1 will drain to the subdivision’s retention pond at the north end of the subdivision. Drainage Areas 2, 3 and 4 will drain to the Block 2 parking lot retention ponds. The proposed building’s roof will drain to a new underground Stormtech SC-740 chamber system under the proposed parking stalls to the south of the building. A proposed underground Stormtech SC-740 chamber system will also be installed to service the 2 dry wells that were installed with the Block 2 parking lot improvements and had collection lines stubbed out to the Lot 1 building pad. These existing dry wells and stubs are being abandoned with this project due to the influence of the high groundwater table in the lower 1 to 2 feet of the drywells. STORMWATER DESIGN The original Block 2 stormwater report calculated 2,755 cubic feet of runoff from the area outside of the proposed building pad that drains into the right of way (Drainage Area #1 in the original Block 2 stormwater report). With the revision of the Block 2 site plan for the Bozeman Design Center, the runoff from this drainage area decreased to 2,701 cubic feet, which is within the allowable limit of the previously allocated amount of 2,840 cubic feet. (See updated Block 2 stormwater report included with this site plan application). Additionally, the drainage from the area that contributes to Retention Pond #2 located on the west side of the parking lot (Drainage Area #4 in the original Block 2 stormwater report) increased slightly due to site modifications for the Bozeman Design Center. This increase in stormwater drainage was from 1,224 to 1,336 cubic feet and is still within the originally allocated volume of the pond, which is 1,500 cubic feet. The existing previously-installed drywells (approximately 8 feet deep) are going to be abandoned in place due to the influence of the high groundwater table. Therefore, a Stormtech SC-740 chamber system is proposed to be installed under the parking stalls along the south side of the Bozeman Design Center to capture the runoff from the building. The Stormtech chamber systems are sized according to City of Bozeman Design Standards to capture and retain or detain the volume of the 10-year 2-hour storm event. The proposed Stormtech infiltration chambers are designed to detain stormwater runoff using the arch-shaped chambers and void space in the surrounding washed rock, while the runoff infiltrates into the ground. The footprint of these chambers will be over-excavated down to native gravels to remove any existing non-draining material beneath the chambers. This excavation will be back-filled with a well-draining gravel to ensure infiltration. The chambers were sized using the 10-year 2-hour design storm per the City of Bozeman Design Standards and Specifications. The proposed underground infiltration chambers are designed to retain the design storm volume for the respective areas without accounting for infiltration of the runoff into the groundwater. Drainage Areas #1 and #2 are composed of the two halves of the proposed building’s roof. Runoff from the roof will be directed to collection points along the southern face of the building. The flow will then be directed through two 6” PVC pipes to the proposed Stormtech Chamber #2. The proposed chamber has a required retention volume of 926 cubic feet. Stormtech chamber #2 has a provided volume of 975 cubic feet. Drainage Area #3 consists of the new parking lot to the west of the building, as well as some adjacent sidewalks and landscaped areas. Runoff from Drainage Area #3 will be conveyed via sheet flow and curb and gutter to a proposed inlet located at the northeast corner of the parking lot. This inlet will flow directly to the proposed Stormtech Chamber #1. The required volume for this chamber is 433 cubic feet. The provided volume for Stormtech Chamber #1 is 535 cubic feet. GROUNDWATER A groundwater monitoring well exists within the building pad. Groundwater measurements were taken in November 2021. Given a lack of monitoring data during peak runoff season, two feet was added to the November measurements to estimate a high groundwater table elevation. (Based on groundwater monitoring data for the nearby proposed Ferguson Farm II Subdivision to the east, two feet was chosen as an average fluctuation in groundwater elevation for this site.) The approximate high groundwater table elevation in the footprint of the proposed Stormtech Chamber #1 system is 4,827.10 feet and the bottom of Stormtech gravel elevation is 4,828.53 feet. Approximately 1.4 feet of separation between high groundwater table and bottom of Stormtech Chamber #1. The approximate high groundwater table elevation in the footprint of the proposed Stormtech Chamber #2 system is 4,828.80 feet and the bottom of Stormtech gravel elevation is 4,831.13 feet. Approximately 2.3 feet of separation between high groundwater table and bottom of Stormtech Chamber #2 is provided. APPENDIX A BOZEMAN DESIGN CENTER DRAINAGE AREA MAP APPENDIX B SUPPORTING CALCULATIONS RUNOFF VOLUME FROM DA#1 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Landscape 0.2 0 0 HARDSCAPE 0.95 7226 6865 Total 7226 6865 A = Area (acres)0.17 C = Weighted C Factor 0.95 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.95 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.17 Q = RUNOFF (cfs)0.06 V = REQUIRED VOL (ft3)463 RUNOFF VOLUME FROM DA#2.0 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Landscape 0.2 0 0 Hardscape 0.95 7226 6865 Total 7226 6865 A = Area (acres)0.17 C = Weighted C Factor 0.95 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.95 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.17 Q = RUNOFF (cfs)0.06 V = REQUIRED VOL (ft3)463 RUNOFF VOLUME FROM DA#3.0 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Landscape 0.2 823 165 Hardscape 0.95 6595 6265 Total 7418 6430 A = Area (acres)0.17 C = Weighted C Factor 0.87 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.87 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.17 Q = RUNOFF (cfs)0.06 V = REQUIRED VOL (ft3)433 DRAINAGE AREA # 1 ROOF DOWNSPOUT PIPE ROUTED TO INFILTRATION SYSTEM #1 1. Summary of Roof Area and C Factor Contributing Area DA # C Area (ft 2)C * Area Hardscape 1 0.95 7226 6865 Total 7226 6865 A = Area (acres)0.17 C = Weighted C Factor 0.95 2. Calculate Tc (Time to Concentration) Tc Total = 5.0 (Assume 5 minute minimum) 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.95 (calculated above) I = 0.78 Tc-0.64 (in/hr)3.83 (25-yr storm) A = area (acres) 0.17 (calculated above) Q 25-yr Flow Rate (cfs)=0.60 MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY) Pipe: Downspout #1 INPUT D= 6 inches d= 5.63 inches Mannings Formula n= 0.013 mannings 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 0.19 1.32 0.15 3.28 0.63 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 ROOF DOWNSPOUT PIPE ROUTED TO INFILTRATION SYSTEM #1 1. Summary of Roof Area and C Factor Contributing Area DA # C Area (ft 2)C * Area Hardscape 2 0.95 7226 6865 Total 7226 6865 A = Area (acres)0.17 C = Weighted C Factor 0.95 2. Calculate Tc (Time to Concentration) Tc Total = 5.0 (Assume 5 minute minimum) 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.95 (calculated above) I = 0.78 Tc-0.64 (in/hr)3.83 (25-yr storm) A = area (acres) 0.17 (calculated above) Q 25-yr Flow Rate (cfs)=0.60 MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY) Pipe: Downspout #1 INPUT D= 6 inches d= 5.63 inches Mannings Formula n= 0.013 mannings 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 0.19 1.32 0.15 3.28 0.63 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 APPENDIX C STORMTECH MAINTENANCE PLAN An company 2 THE MOST ADVANCED NAME IN WATER MANAGEMENT SOLUTIONS TM ECCENTRICHEADER MANHOLEWITHOVERFLOWWEIR STORMTECHISOLATOR ROW OPTIONAL PRE-TREATMENT OPTIONAL ACCESS STORMTECH CHAMBERS )(