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HomeMy WebLinkAboutAmbientAirStormwaterDesignReport_06242021 INTRODUCTION The Ambient Air Solutions site plan consists of a 7,835 sq. ft. building and associated improvements located on Lot 7, Block 2 of Nelson Meadows Subdivision in Bozeman, Montana. A combination of site grading, curb and gutter, and retention ponds will be used to manage stormwater runoff for the development of the site. The proposed retention ponds were sized for the 10-year, 2-hour storm and checked for the half inch requirement. Given the 8’-10’ thick layer of native silt/clay above the native gravel, the ponds were designed with a 36-inch HDPE pipe oriented vertically that provides a hydraulic connection from the pond to the underlying native gravel well-draining layer to ensure infiltration. The vertical 36-inch pipe will be backfilled with well-draining material and covered with landscape fabric, then landscaped over top to provide a maintenance-free design. The existing stormwater report from Morrison Maierle (MM) for the Nelson Meadows Subdivision was utilized to determine the stormwater management plan for the proposed site. It was determined using AutoCAD that the approximate area of Lot 7, Block 2 that was accounted for in the MM stormwater report as existing pasture land contributing to the NR-11 subbasin was 0.88 acres. Using 0.88 acres and a C coefficient of 0.23, the runoff rate for the 10-year 2-hour storm prior to development produced a runoff volume of 596 cubic feet. As explained in the drainage area section below, DA #1 only generates 278 cubic feet of runoff and will not impact the existing subdivision stormwater infrastructure. Supporting stormwater calculations are attached to this report. A Drainage Area Map is included in Appendix A. Calculations for each individual drainage area are included in Appendix B. Groundwater depth information is included in Appendix C. Excerpts from the original Nelson Meadows Subdivision Stormwater Report are included in Appendix D. A stormwater facilities maintenance plan is included in Appendix E. DRAINAGE AREAS Drainage Area 1 Drainage Area 1 consists of the access drive aisle and perimeter landscaped area which drain into Prince Lane and Nelson Road and eventually to a curb inlet (I.D: I.K52.00004) located on the south side of Prince Lane about 125 feet from Nelson Road. The runoff volume from DA#1 was calculated to be 278 cubic feet. This runoff is accounted for as a portion of the site’s runoff that previously drained into the NR-11 subbasin from the MM stormwater report. Drainage Area 2 Drainage Area 2 consists of a portion of the proposed building as well as the sidewalks and the north parking lot. Runoff from this drainage area flows via sheet flow, curb & gutter, and downspouts to the proposed Retention Pond #1. This system was sized for the 10-year, 2-hour storm and was checked for the half inch requirement. It was found that the 10-year, 2-hour storm governed the design and requires 765 cubic feet of storage. The retention pond was designed to have a storage volume of 787 cubic feet with 4:1 side slopes and a max. depth of 1.5 feet. Drainage Area 3 Drainage Area 3 consists of a southern portion of the fenced area, the southern part of the building’s roof, and the landscaped areas on the southern part of the site. Runoff from this drainage area flows from the roof downspouts and sheet flows across part of the fenced paved area to Retention Pond #2 south of the building. This pond was sized for the 10-year, 2-hour storm and was checked for the half inch requirement. It was found that the 10-year, 2-hour storm governed the design and requires 647 cubic feet of storage. The retention pond was designed to have a storage volume of 804 cubic feet with 4:1 side slopes and a max. depth of 1.5 feet. Drainage Area 4 Drainage Area 4 consists of the west end of the building roof, the landscaped areas at the west and northwest sides of the site, and a portion of the parking lot west of the building. Runoff from this drainage area flows from the roof downspouts and sheet flows across the parking lot to Retention Pond #3 at the northwest corner of the lot. This pond was sized for the 10-year, 2-hour storm and was checked for the half inch requirement. It was found that the 10-year, 2-hour storm governed the design and requires 577 cubic feet of storage. The retention pond was designed to have a storage volume of 649 cubic feet with 4:1 side slopes and a max. depth of 1.5 feet. The DA also includes stormwater chase #1 and is designed to handle a 25-year storm event. DEPTH TO GROUNDWATER Groundwater monitoring was performed by Morrison Maierle in 2018 for Nelson Meadows Subdivision. Four monitoring wells surrounding Lot 7, Block 2 were used to verify that the proposed stormwater infrastructure for Ambient Air Solutions is above the high groundwater table. An average groundwater level was used for designing all to retention systems. The average high groundwater recorded (Monitoring Wells 2,4,5, &6) was approximately 5.5 feet below ground surface in April 2018. Approximately 3.6 feet of separation from groundwater is provided for Retention Ponds #1&2. Approximately 3.1 feet of separation is provided for Retention Pond #3. See Appendix C for more groundwater depth information. APPENDIX A DRAINAGE AREA MAP UPUP APPENDIX B DRAINAGE AREA, RETENTION SYSTEM, AND CHASE SIZING CALCULATIONS Pre Development Site Runoff NR-11 PORTION OF LOT 7 RUNOFF INCLUDED IN NR-11 SUBBASIN 1. Calculate Area of portion of lot 7 contributing runoff the the NR-11 Subbasin Contributing Area DA#C Area (ft2 )C * Area Existing Pasture Land NR-11 0.23 38221 8791 TOTAL 38221 8790.83 A = Area (acres)0.8774 C w = Weighted C 0.23 2. Calculate Runoff Flow and Volume Q=CIA V=7200Q C = Weighted C Factor 0.23 (calculated above) I = Intenisty (in/hr)0.41 (10yr, 2hr Storm) A = Area (acres)0.88 (calculated above) Q=Flow (cfs)0.08 V=Allocated Volume (ft 3)596 DRAINAGE AREA #1 RUNOFF VOLUME FROM DA #1 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Landscape 0.2 6903 1381 Hardscape 0.95 2345 2227 Total 9247 3608 A = Area (acres)0.21 C = Weighted C Factor 0.39 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.39 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres)0.21 Q = RUNOFF (cfs)0.03 V = REQUIRED VOL (ft3)243 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 2345 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 0.72 0.05 + 0.9*I I = Percent impervious cover (decimal)0.75 decimal A = Entire drainage area 0.21 acres RRV = Runoff Reduction Volume 0.0064 acre-ft RRV = Runoff Reduction Volume 278 cubic feet Because the runoff volume from the 10-yr, 2-hr storm (for flood control) is less than the runoff volume produced by the half inch rainfall (for water quality) DA #1's runoff volume is considered to be the larger volume (278 cf). DA #1 runs off site into the surrounding R.O.W. which is within the Morrison Maierle basin NR-11 for the entire subdivision. See caluclation on previous page for Lot 7's contribution to NR-11. Because the original subdivision design accounted for 595 cf to runoff Lot 7 and into the R.O.W. our design is acceptable because we are only contributing 278 cf to the R.O.W. DRAINAGE AREA #2 To Retention Pond #1 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Landscape 0.2 2604 521 Hardscape 0.95 11398 10828 Total 14003 11349 A = Area (acres)0.32 C = Weighted C Factor 0.81 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.81 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres)0.32 Q = RUNOFF (cfs)0.11 V = REQUIRED VOL (ft3)765 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 11398 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 0.22 0.05 + 0.9*I I = Percent impervious cover (decimal)0.19 decimal A = Entire drainage area 0.32 acres RRV = Runoff Reduction Volume 0.003 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 Pond #1 is sized to handle the larger volume (765 cf). RETENTION POND # 1 VOLUME 3. Calculate Retention Pond Volume V Req.= 765 cf. V Provided=787 cf. DRAINAGE AREA #3 To Retention Pond #2 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Landscape 0.2 6757 1351 Hardscape 0.95 8684 8250 Total 15441 9601 A = Area (acres)0.35 C = Weighted C Factor 0.62 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.62 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres)0.35 Q = RUNOFF (cfs)0.09 V = REQUIRED VOL (ft3)647 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 8684 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 0.44 0.05 + 0.9*I I = Percent impervious cover (decimal)0.44 decimal A = Entire drainage area 0.35 acres RRV = Runoff Reduction Volume 0.007 acre-ft RRV = Runoff Reduction Volume 286 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 Pond #2 is sized to handle the larger volume (647 cf). RETENTION POND # 2 VOLUME 3. Calculate Retention Pond Volume V Req.= 647 cf. V Provided=804 cf. DRAINAGE AREA #4 To Retention Pond #3 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft2 )C * Area Landscape 0.2 3404 681 Hardscape 0.95 8243 7831 Total 11647 8512 A = Area (acres)0.27 C = Weighted C Factor 0.73 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.73 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres)0.27 Q = RUNOFF (cfs)0.08 V = REQUIRED VOL (ft3)577 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 8243 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 0.31 0.05 + 0.9*I I = Percent impervious cover (decimal)0.29 decimal A = Entire drainage area 0.27 acres RRV = Runoff Reduction Volume 0.003 acre-ft RRV = Runoff Reduction Volume 152 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 Pond #3 is sized to handle the larger volume (577 cf). RETENTION POND # 3 VOLUME 3. Calculate Retention Pond Volume V Req.= 577 cf. V Provided=649 cf. DRAINAGE AREA #4 25-YR STORM 1. Summary of Drainage Area and C Factor Contributing Area C Area (ft 2)C * Area Landscape 0.2 3404 681 Hardscape 0.95 8243 7831 Total 11647 8512 A = Area (acres)0.27 C = Weighted C Factor 0.73 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm C = Rational Method Runoff Coefficient 0.73 Return (yrs)Cf S = Slope of Basin (%)1.5%2 to 10 1 Cf = Frequency Adjustment Factor 1.1 (25-yr storm)11 to 25 1.1 D = Length of Basin (ft)162 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)6.2 Tc Total =6.2 (5 minute minimum) 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.73 (calculated above) I = 0.78 Tc-0.64 (in/hr)3.35 (25-yr storm) A = area (acres)0.27 (calculated above) Q 25-yr Flow Rate (cfs)=0.65 MANNING'S EQUATION for OPEN CHANNEL FLOW Storm Chase #1 Project:Ambient Air Solutions Location:Bozeman, MT By: JG Date:1/28/2021 Chk By:0 Date:0 INPUT z (sideslope)=0 Mannings Formula z (sideslope)=0 w (btm width, ft)=1 Q = (1.486/n)ARh2/3S1/2 d (depth, ft)=0.33 R = A/P S (slope, ft/ft)0.019 A = cross sectional area n low =0.013 P= wetted perimeter n high =0.013 S = slope of channel V = (1.49/n)Rh2/3S1/2 n = Manning's roughness coefficient Q = V x A Depth, ft Area, sf Wetted Perimete r, ft Hydraulic Radius, ft Velocity, fps Flow, cfs Velocity, fps Flow, cfs 0.33 0.33 1.66 0.20 5.37 1.77 5.37 1.77 T =1.00 Dm =0.330 Sc low =0.0070 Sc high =0.0070 sc =critical slope ft / ft T =top width of the stream .7 Sc 1.3 Sc .7 Sc 1.3 Sc dm =a/T = mean depth of flow 0.0049 0.0091 0.0049 0.0091 Low N High N d w z 11 z T APPENDIX C GROUNDWATER DEPTH INFORMATION N INTERSTATE 90 235 INTERSTATE 90 205 205 D D D D D D D D D D D D D INSTALL NEW 54" x 88" ARCP FESINVERT IN = 4593.6528 INSTALL NEW 54" x 88" ARCP FESINVERT OUT = 4592.0877 INSTALL ±156.0000 LF (TOTAL - 140 LF BID QTY)54" x 88" ARCP CULVERT @ ±1.0033% SLOPE INSTALL NEW 54" x 88" ARCP FESINVERT IN = 4587.1132 INSTALL NEW 54" x 88" ARCP FESINVERT OUT = 4585.9345INSTALL ±120.0000 LF (TOTAL - 104 LF BID QTY)54" x 88" ARCP CULVERT @ ±0.9822% SLOPE DD D D D D D NelsonRdV a l l e y C e n t e r E R d Fr o n t a g e R d WELL #4 WELL #3 WELL #5 WELL #11 WELL #12 WELL #6 WELL #2 WELL #7 WELL #1 WELL #8 WELL #9 WELL #10 engineerssurveyorsplannersscientists 2880 Technology Boulevard West Bozeman, MT 59718 Phone: (406) 587-0721 Fax: (406) 922-6702 STORM DRAIN FIG. 4:GROUNDWATER MONITORING WELLS SCALE:1" = 500' SUNSET MEMORI A L PARK MONTAN A DEPART M E N T OF TRANSP O R T A T I O N APPENDIX D EXCERPTS FROM NELSON MEADOWS SUBDIVISION STORMWATER DESIGN REPORT (2019) APPENDIX E STORMWATER FACILITIES INSPECTION AND MAINTENANCE PLAN INSPECTION AND MAINTENANCE FOR STORMWATER MANAGEMENT FACILITIES The Property Owner shall be responsible for the maintenance of the stormwater drainage facilities within The Ambient Air Solutions located on Lot 7, Block 2 of Nelson Meadows Subdivision. Storm Water Facilities: 1. Retention Ponds collect storm water runoff and store the water until it evaporates and/or infiltrates into the ground. 2. Pipe Networks convey storm water to different discharge locations underground. 3. Inlets are facilities where storm water runoff enters a pipe network. Inlets include storm water manholes and drains. 4. Catch Basins are sumps typically located directly below storm water inlets and allow sediment to settle before storm water enters the pipe network. Post Construction Inspection: 1. Observe drain time in retention 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 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 ponds 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, and retention ponds. Flush and/or vacuum storm water pipes if excessive material is observed in the facilities. Standard Maintenance: 1. Remove sediment and oil/grease from retention ponds. 2. Inspect and remove debris from drainage swales, catch basins, and retention 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 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.