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HomeMy WebLinkAboutAppendix D Stormwater Design Report 04-16-2021 INTRODUCTION The Palisade at 130 Village Crossing Way site plan consists of a multi-story residential building with a footprint of approximately 33,500 sq. ft. and associated improvements located on Lot 5 of Minor Subdivision 344 in Bozeman, Montana. A combination of site grading, curb and gutter, at- grade retention ponds and underground Stormtech MC-3500 storage chambers will be used to manage stormwater runoff for the development of the site. The proposed underground Stormtech MC-3500 stormwater storage system and retention ponds were sized for the 10-year, 2-hour storm and checked for the half-inch requirement. Supporting stormwater calculations are attached to this report. Drainage Area Maps for pre and post-development are included in Appendix A. Calculations for each individual drainage area are included in Appendix B. A Stormwater Facilities Inspection and Maintenance Plan is included in Appendix C. Excerpts from the Parklands at the Village Downtown Subdivision stormwater report are included in Appendix D. POST-DEVELOPMENT DRAINAGE AREAS Drainage Area 1 Drainage Area 1 consists of the south half of the parking lot and adjacent landscaping. Runoff from this drainage area flows via curb and gutter to Retention Pond #1. The proposed retention 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 required 585 cubic feet of storage. 620 cubic feet of storage is provided in Retention Pond #1. Drainage Area 2 Drainage Area 2 consists of the north half of the parking lot and adjacent landscaping. Runoff from this drainage area flows via curb and gutter to Retention Pond #2. The proposed retention 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 required 582 cubic feet of storage. 614 cubic feet of storage is provided in Retention Pond #2. Drainage Area 3 Drainage Area 3 consists of the entire building and courtyard. Runoff from this drainage area flows via a system of downspouts and pipes designed by others to the proposed Stormtech MC-3500 infiltration chambers located to the east of the building above the steep slope. The chambers are designed to retain stormwater runoff using the arch-shaped chambers and void space in the surrounding washed rock while the runoff infiltrates into the ground. The chambers were 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 required 2,130 cubic feet of storage. 2,382 cubic feet of storage is provided in the Underground Stormwater System including the surrounding washed rock using a stone porosity of 40%. The system is equipped with an overflow pipe that daylights to the adjacent hillside to the east. This overflow is set to the top of the chambers so it only overflows in the unlikely event that the system is full. Drainage Area 4 Drainage Area 4 consists of the landscaped and concrete areas located between the building and existing Right of Ways (ROW) of Village Crossing Way and Village Downtown Blvd. Runoff from this drainage area flows into said ROWs and subsequently into an existing detention pond at the end of Village Downtown Blvd. Said pond was recently re-designed and re-shaped for the new Parklands Subdivision. The pond was designed with a volume of 7,726 cubic feet, when only 7,522 cubic feet was required. An excess of 204 cubic feet of storage exists in said pond. DA #4 contributes 229 cubic feet of stormwater during the design storm event (10-yr, 2-hr). Given the approximate nature of the Rational Method and the relatively small excess stormwater volume of 25 cubic feet it is deemed negligible. Therefore, it is acceptable not to provide any stormwater improvements for DA #4 and to allow this area to naturally drain into said ROWs. Drainage Area 5 Drainage Area 5 consists primarily of the area of the site that is undisturbed. Runoff from this drainage area flows to the adjacent lots to the east. Historically, a large portion of the lot’s runoff has flowed in a similar fashion. See Drainage Area Map – Pre-Development in Appendix A for a visual reference. With the development of the lot, the runoff leaving the property is reduced from 1,205 cubic feet pre-development to 1,066 cubic feet post-development. Therefore, it is acceptable not to provide any stormwater improvements for DA #5 and to allow this area to naturally drain into adjacent lots to the east as it has done historically. DEPTH TO GROUNDWATER Groundwater monitoring has not been conducted on the subject property. Given that the wetland area on the far east end of the lot is approximately 15 feet lower in elevation than the bottom of the Stormtech chambers gravel retention volume, it is assumed that the underlying groundwater table will not interfere with the function of the subsurface stormwater system. The developer is planning to dig test pits on site on April 16, 2021 and will install groundwater monitoring wells to verify this assumption. If our assumption regarding groundwater is incorrect, we will submit a MOD application to rectify the issue. G:\C&H\20\201192\Design Reports\Storm\201192_Stormwater Design Report.Docx APPENDIX A DRAINAGE AREA MAPS 1235758596061ADAADA APPENDIX B DRAINAGE AREA, INFILTRATION/RETENTION SYSTEM CALCULATIONS DRAINAGE AREA #1 RETENTION POND #1 1. Calculate Area and Weighted C Factor Contributing Area DA #C Area (ft 2 )C * Area Hardscape 1 0.95 7012 6661 Landscape 1 0.20 10118 2024 Total 17130 8685 A = Area (acres)0.39 C = Weighted C Factor 0.51 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.51 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres)0.39 Q = runoff (cfs)0.08 V = REQUIRED VOL (ft3)585 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 7012 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 0.42 0.05 + 0.9*I I = Percent impervious cover (decimal)0.41 decimal A = Entire drainage area 0.39 acres RRV = Runoff Reduction Volume 0.007 acre-ft RRV = Runoff Reduction Volume 299 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 (585 cf). DRAINAGE AREA #2 RETENTION POND #2 1. Calculate Area and Weighted C Factor Contributing Area DA #C Area (ft 2 )C * Area Hardscape 2 0.95 8279 7865 Landscape 2 0.20 3821 764 Total 12100 8630 A = Area (acres)0.28 C = Weighted C Factor 0.71 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.71 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres)0.28 Q = runoff (cfs)0.08 V = REQUIRED VOL (ft3)582 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 8279 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 0.67 0.05 + 0.9*I I = Percent impervious cover (decimal)0.68 decimal A = Entire drainage area 0.28 acres RRV = Runoff Reduction Volume 0.008 acre-ft RRV = Runoff Reduction Volume 336 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 (582 cf). DRAINAGE AREA #3 UNDERGROUND STORMWATER SYSTEM 1. Calculate Area and Weighted C Factor Contributing Area DA #C Area (ft 2 )C * Area Hardscape 3 0.95 33254 31591 Landscape 3 0.20 0 0 Total 33254 31591 A = Area (acres)0.76 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)0.76 Q = runoff (cfs)0.30 V = REQUIRED VOL (ft3)2130 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 33254 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 0.95 0.05 + 0.9*I I = Percent impervious cover (decimal)1.00 decimal A = Entire drainage area 0.76 acres RRV = Runoff Reduction Volume 0.030 acre-ft RRV = Runoff Reduction Volume 1316 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 Underground Stormwater System is sized to handle the larger volume (2,130 cf). DRAINAGE AREA #3 ROOF DOWNSPOUT PIPE ROUTED TO STORMTECH AND OVERFLOW PIPE 1. Summary of Roof Area and C Factor Contributing Area DA #C Area (ft2 )C * Area Hardscape 3 0.95 33254 31591 Total 33254 31591 A = Area (acres)0.76 C = Weighted C Factor 0.95 2. Calculate Tc (Time to Concentration) Tc Overland Flow (on roof with pitch of 1/4" to 12") Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%)2.1%Return (yrs)CfC = Rational Method Runoff Coefficient 0.95 2 to 10 1 Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1 D = Length of Basin (ft)70 26 to 50 1.251 to 100 1.25 Tc Overland Flow (minutes)1.2 Tc Total =5.0 (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)0.76 (calculated above) Q 25-yr Flow Rate (cfs)=2.77 MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)Pipe: 1 Location:Roof Downspout pipe INPUT D=12 inchesd=11.26 inchesMannings Formula n=0.013 mannings θ=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 0.77 2.64 0.29 5.21 3.99 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 θ D User Inputs Chamber Model: MC-3500 Outlet Control Structure: No Project Name: Lot 5 - Palisade Engineer: Tim Staub Project Location: Measurement Type: Imperial Required Storage Volume: 2127 cubic ft. Stone Porosity: 40% Stone Foundation Depth: 48 in. Stone Above Chambers: 12 in. Average Cover Over Chambers: 48 in. Design Constraint Dimensions:(25 ft. x 40 ft.) Results System Volume and Bed Size Installed Storage Volume: 2381.85 cubic ft. Storage Volume Per Chamber: 109.90 cubic ft. Number Of Chambers Required: 7 Number Of End Caps Required: 4 Chamber Rows: 2 Maximum Length:38.35 ft. Maximum Width: 15.33 ft. Approx. Bed Size Required: 538.43 square ft. System Components Amount Of Stone Required: 143.79 cubic yards Volume Of Excavation (Not Including Fill): 174.49 cubic yards DRAINAGE AREA #4 EXISTING SUBDIVISION POND 1. Calculate Area and Weighted C Factor Contributing Area DA #C Area (ft2)C * Area Hardscape 4 0.95 2391 2271 Landscape 4 0.20 5628 1126Total80203397 A = Area (acres)0.18 C = Weighted C Factor 0.42 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.42 I = intensity (in/hr) 0.41 (10 yr, 2hr storm)A = Area (acres)0.18 Q = runoff (cfs)0.03 V = REQUIRED VOL (ft3)229 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 2391 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 0.32 0.05 + 0.9*I I = Percent impervious cover (decimal)0.30 decimal A = Entire drainage area 0.18 acres RRV = Runoff Reduction Volume 0.002 acre-ft RRV = Runoff Reduction Volume 106 cubic feet Because the runoff volume from the 10-yr, 2-hr storm (for flood control) is greater thanthe runoff volume produced by the half inch rainfall (for water quality) the capacity of the existing subdivision pond was checked for the larger volume (229 cf). The capacity of the existing subdivision detention pond located at the end of the Village Downtown Blvd (and recently redesigned for the Parklands Subdivision) is 7,522 cf. The pond was designed with a volume of 7,726 cf leaving a remainder of 204 cf. Given the approximate nature of the Rational Method and the relatively small excess stormwater volume of 25 cf it is deemed negligible. Therefore, it is acceptable not to provide any stormwater improvements for DA #4 and to allow this area to naturally drain into the ROW of Village Crossing Way and Village Downtown Blvd. DRAINAGE AREA #5 VERIFY RUNOFF TO ADJACENT PROPERTIES DOES NOT EXCEED HISTORICAL 1. Calculate Area and Weighted C Factor Contributing Area DA #C Area (ft 2 )C * Area Hardscape 5 0.95 272 258 Landscape 5 0.20 77734 15547Total7800615805 A = Area (acres)1.79 C = Weighted C Factor 0.203 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)1.79 Q = runoff (cfs)0.15 V = REQUIRED VOL (ft3)1066 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 272 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 0.05 0.05 + 0.9*I I = Percent impervious cover (decimal)0.00 decimal A = Entire drainage area 1.79 acres RRV = Runoff Reduction Volume 0.004 acre-ft RRV = Runoff Reduction Volume 173 cubic feet Because the runoff volume from the 10-yr, 2-hr storm (for flood control) is greater thanthe runoff volume produced by the half inch rainfall (for water quality) the historical lot drainage was checked for the larger volume (1066 cf). The existing lot's historical drainage pattern is delineated on the 'Drainage Area Map - Pre Development' located in Appendix A. Said map shows that the existing lot naturally drains to adjacent parcels to the east and the Village Downtown Blvd ROW due to the existing grade. A volume of 1,295 cf is produced during the 10-yr, 2-hr storm and flows off site to adjacent lots to the east. After the lot is developed the runoff to the adjacent lots is reduced to 1,066 cf; therefore, no stormwater improvements are proposed within DA #5. APPENDIX C STORMWATER FACILITIES INSPECTION AND MAINTENANCE PLAN INSPECTION AND MAINTENANCE FOR STORMWATER MANAGEMENT FACILITIES The owner shall be responsible for the maintenance of the stormwater drainage facilities within The Palisade at 130 Village Crossing Way development on Lot 5, Minor Subdivision No. 344. Storm Water Facilities: 1. Underground Stormtech MC-3500 Infiltration System collect storm water runoff and store the water until it infiltrates into the ground. 2. Retention Ponds collect storm water runoff and store the water until it evaporates and/or infiltrates into the ground. 3. Pipe Networks convey storm water to different discharge locations underground. Post Construction Inspection: 1. Use the attached Stormtech Isolator Row Operation & Maintenance manual to determine if maintenance is required on the system after construction is completed. 2. 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). Semi-Annual Inspection: 1. Use the attached Stormtech Isolator Row Operation & Maintenance manual to determine if maintenance is required on the system semi-annually. 2. Check for grass clippings, litter, and debris in retention ponds. Flush and/or vacuum storm water pipes if excessive material is observed in the facilities. Standard Maintenance: 1. Inspect and remove debris from catch basins. Use a trailer-mounter Vermeer VX50-800 vacuum excavator or similar (see Figure 1 below) to clean Stormtech pre-treatment manhole with sump. To accomplish the vacuuming procedure, the trailer-mounted vac system should be pulled into the parking garage to access the pre-treatment manhole with sump. There will need to be coordination with the building management to ensure no cars are parked in the parking spaces on the east wall of the parking garage to allow for the trailer-mounted vac system and truck to pull into the area to access said manhole. 2. 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. 3. Monitor health of vegetation and revegetate as necessary to maintain full vegetative cover. 4. 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. Figure 1 Trailer-mounted Vermeer VX50-800 vacuum excavator. Sediment accumulation: In most cases, sediment in a catch basin or a retention system 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. An company 2 THE MOST ADVANCED NAME IN WATER MANAGEMENT SOLUTIONS TM ECCENTRICHEADER MANHOLEWITHOVERFLOWWEIR STORMTECHISOLATOR ROW OPTIONAL PRE-TREATMENT OPTIONAL ACCESS STORMTECH CHAMBERS  )( APPENDIX D EXCERPTS FROM PARKLANDS SUBDIVISION STORMWATER REPORT DETENTION POND #1 REQUIRED VOLUME 1. Calculate Weighted C Factor for Right-of-Way Component Width C ROW Hardscape 64 0.95 ROW Landscape 16 0.2 Weighted C Factor = 0.80 2. Calculate Area and Weighted C Factor (Post-Development) Contributing Area C Area (ft 2 )C * Area Composite ROW 0.80 186132 148906 OS 0.2 8601 1720 Low-Med Residential 0.35 7888 2761 Dense Residential 0.5 52799 26400 Total 255421 179787 A = Area (acres) 5.8637 C = Weighted C Factor 0.70 3. Calculate Tc (Pre-Development) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 3.50% 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) 532 26 to 50 1.2 51 to 100 1.25 Tc (Pre-Development) (minutes) 26 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) 0.43 (Tc Pre-Development) i = rainfall intensity (in./hr.) 1.11 5. Calculate Runoff Rate (Pre-Development) Q = CiA C = Rational Method Runoff Coefficient 0.2 (open land) i = rainfall intensity (in./hr.) 1.11 (calculated above) A = Area (acres) 5.86 (calculated above) Q = Runoff Rate (Pre-Development) (cfs) 1.31 6. Calculate Required Pond Volume Total Area (acres) = 5.86 acres Weighted C = 0.70 Discharge Rate (cfs) = 1.31 cfs (Equal to Pre-Development Runoff Rate) Duration(min) Duration(hrs) Intensity (in/hr)Qin (cfs)Runoff Volume Release Volume Required Storage (ft3) 30 0.50 1.00 4.14 7461 0 7461 30.5 0.51 0.99 4.10 7504 0 7504 31 0.52 0.98 4.06 7547 39 7508 32 0.53 0.96 3.97 7631 118 7514 33 0.55 0.94 3.90 7714 196 7518 34 0.57 0.93 3.82 7795 274 7521 35 0.58 0.91 3.75 7875 353 7522 36 0.60 0.89 3.68 7953 431 7521 37 0.62 0.88 3.62 8029 510 7520 38 0.63 0.86 3.55 8104 588 7516 OUTLET STRUCTURE SLOT Q=CLH3/2 Q = Discharge (cfs) 1.31 (calculated above) C = Weir Coefficient 3.33 (per COB Design Standards) H = Head (ft) 1 L = Horizontal Length (ft) 0.39 L = Slot Width (inches) 4.7