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HomeMy WebLinkAbout01 SW KOHD Drainage Report 05062021-JUNEDESIGN REPORT STORMWATER MANAGEMENT KOHD BUILDING LOT 16, BLOCK 4 NELSON MEADOWS SUBDIVISION Prepared for: Plum Design Lab 3141 Foxtail Street Bozeman, MT 59718 Prepared by: C&H Engineering and Surveying, Inc. 1091 Stoneridge Drive, Bozeman, MT 59718 (406) 587-1115 Project Number:201196 June 2021 INTRODUCTION The Kaufmann’s Overhead Door (KOHD)site plan consists of a 5,964 sq. ft.building and associated improvements located on Lot 16, Block 4 of Nelson Meadows Subdivision in Bozeman, Montana. A combination of site grading, curb and gutter,curb inlet,and a retention pond will be used to manage stormwater runoff for the development of the site.The proposed stormwater features were sized for the 10-year, 2-hour storm and checked for the half inch requirement.The proposed development consists of six (6) Drainage Areas, which are shown on the Drainage Area Map included herein.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 Inspection and Maintenance Plan is included in Appendix E. NELSON MEADOWS SUBDIVISION –STORMWATER BACKGROUND INFORMATION Nelson Meadows Subdivision included the construction of regional retention basins that handle the subdivision runoff by curb and gutter and underground stormwater infrastructure designed by Morrison Maierle, Inc.This Nelson Meadows Subdivision Stormwater Design Report did not account for the future development of the lots within the subdivision.Instead, the subdivision stormwater infrastructure was sized for the R.O.W.’s and portions of the existing un-developed lots that naturally drain into the R.O.Ws.A C coefficient for existing pasture land of 0.23 was used in the original calculations for the portions of the lots that naturally drain into the R.O.W.s. It was determined using AutoCAD that the approximate area of Lot 16, Block 4 that was accounted for as existing pasture land was 1.00 acres. Using 1.00 acres and a C coefficient of 0.23, the runoff for the 10-year 2-hour storm was calculated to be 679 cubic feet. Therefore, it is acceptable to allow 679 cubic feet of stormwater runoff to discharge off site.The remaining stormwater generated from this development will be retained on site. A C coefficient of 0.95 was applied to all hardscaped areas including the paved drive aisles, parking area, sidewalks, curbing and the building. The remaining area is landscaped with wither proposed landscaping improvements around the buildable area as well as retaining the native vegetation (or pasture land) located outside of the buildable area. A C coefficient of 0.20 was applied to all landscaped areas. The hardscaped and landscaped areas for each drainage area were determined using AutoCAD Civil 3D area calculations.Each surface type is depicted with hatching as shown on the civil site plans.It should be noted that the eastern portion of the property beyond the 50-foot watercourse setback was not included in the drainage areas because this area will remain unimproved. DRAINAGE AREAS Drainage Area 1 Drainage Area 1 consists of the northwest portion of the site including the west portion of the parking lot,landscaping,and roof drainage footprint of the proposed building, as shown on the Drainage Area Exhibit included herein. Runoff from this drainage area both sheet flows and flows via curb and gutter to a proposed drywell located in the northwest portion of the site.The drywell was sized by applying an infiltration rate for native gravel subgrades and compared to the 10-year 2-hour storm event to determine the required volume. It was found that the 10-year, 2-hour storm governed the design and required 637 cubic feet of storage.680 cubic feet of storage is provided in Drywell #1 including the surrounding washed rock using a void ratio of 0.3. Drainage Area 2 Drainage Area 2 consists of the portion of the parking lot and landscaping south and southwest of the proposed building.Runoff from this drainage area flows via curb and gutter to proposed Retention Pond #1 located on the eastern portion of the property.A proposed curb cut allows the water to flow into the retention pond.It was found that the 10-year, 2-hour storm governed the design and required 1,175 cubic feet of storage.1,624 cubic feet of storage is provided in Retention Pond #1. Drainage Area 3 Drainage Area 3 consists of the proposed loading dock area.Runoff from this drainage area flows from the loading dock ramps and collects within a proposed “Durslot” trench drain.The stormwater discharges into a french drain, which consists on a subsurface 8-in perforated PVC pipe surrounded with washed rock. Details for the Durslot drain and french drain are shown on civil sheet, C3.2. It was found that the 10-year, 2-hour storm governed the design and required 165 cubic feet of storage. 187 cubic feet of storage is provided in this french drain system. Drainage Areas 4 Drainage Area 4 consists of the northern portion of the proposed building.It was found that the 10-year, 2-hour storm resulted in 451 cubic feet for Drainage Areas 4.This area is allowed to drain to Prince Lane based on the 679 cubic feet of runoff that is allowed to be discharged off site. Drainage Area 5 Drainage Area 5 consists of the undeveloped area on the southern and southeastern portions of the property.Runoff from this drainage area sheet flows eastward toward the existing drainage ditch located along the eastern property boundary.It was found that the 10-year, 2-hour storm resulted in 611 cubic feet for Drainage Area 6.Drainage Area 6 will fully contain landscaping with no increase of impervious area. Therefore, the existing drainage pattern of the pasture land into the existing drainage ditch will be maintained. DEPTH TO GROUNDWATER Groundwater monitoring was performed by Morrison Maierle in 2018 for Nelson Meadows Subdivision. The monitoring wells nearest to Lot 16, Block 4 were used to verify that the proposed stormwater infrastructure for is above the high groundwater table.Monitoring Well (MW)#9 had a measured high groundwater recording of approximately 6.5 feet below ground surface in April 2018. The existing surface at the location of the proposed drywell is about 4,595 feet, while the drywell rim elevation is about 4594 feet. Therefore, the seasonal high groundwater will be about 5.5 feet below the rim elevation. The profile of the proposed drywell is shown on civil sheet, C3.0 and depicts that the structure, including the surrounding washed rock, is about 5 feet deep.As such, the proposed underground stormwater storage systems would not be negatively impacted by groundwater.See Appendix C for more groundwater depth information. OFF-SITE RUN-ON CONSIDERATIONS Stormwater run-on from adjacent properties is not expected to adversely impact this development. The surrounding properties have a slight downgradient slope to the north and, as a result, the only run-on would be from the adjacent properties to the south, which are currently an undeveloped vegetated pasture. As such, the current conditions would not result is a measurable amount of run-on to this property. Any future development of the adjacent property to the south would be required to contain stormwater runoff within their property and not be allowed to discharge stormwater onto this property per City code. Given the existing and potential future conditions at adjacent properties, off-site run-on would have a negligible effect on this development. APPENDIX A DRAINAGE AREA MAP APPENDIX B DRAINAGE AREA RETENTION SYSTEM CALCULATIONS Lot 16, Block 4 - Nelson Meadows Subdivision Stormwater Design Calculations Calculate required volume Hardscape 31095Q = CiA Landscape 36596 V=7200Q Gravel 10230 Drainage Area Area (acre)C FACTOR Hardscape 0.71 0.95 Landscape 0.84 0.20 Gravel 0.23 0.5 Total 1.79 Weighted C=0.54 ***Note - The area of the 25-foot trail easement per plat (along the eastern property boundary) has been excluded from the toal site area for the purposes of the stormwater calculations. ALL STORMWATER GENERATED ON SITETotal Area =1.79 acres Weighted C =0.54 Rainfall intensity =0.41 in./hr.(C.O.B Design Standards 10-yr, 2-hr storm) Runoff, Q =0.40 cfs Volume, V =2845 ft3 Proportion of allowable C directed towards sub-division 0.23 C Factor from Morrison Maierle Stormwater Report (March 2019) VOLUME OF STORMWATER ALLOWED TO BE DISCARGED OFF SITE INTO NELSON MEADOWS SUBDIVISION Total Area =1.00 acres Weighted C =0.23Rainfall intensity =0.41 in./hr.(C.O.B Design Standards 10-yr, 2-hr storm) Runoff, Q =0.09 cfs Volume, V =679 ft3 TOTAL VOLUME TO BE RETAINED ON SITE 2166 ft3 DRAINAGE AREA # 2 RETENTION POND 1 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft2)C * Area Hardscape 0.95 17262 16399 Landscape 0.20 5117 1023 Total 22379 17422 A = Area (acres) 0.5138 C = Weighted C Factor 0.78 2. Calculate Required Volume Q=CIA V=7200Q C = Weighted C Factor 0.78 I = Intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.51 Q = Runoff (cfs) 0.16 V = REQUIRED VOL (ft3) 1175 3. Calculate Retention Pond Volume Modeled In AutoCAD Civil 3D 1624 (ft3) Provided Volume Inc. Perc. (ft3)1,624 Check the half inch requirement (per DSSP II.A.4) 1. Determine Area of Hardscape within Drainage Area #2 Contributing Area Area (ft2) Hardscape 17262 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 0.74 0.05 + 0.9*I I = Percent impervious cover (decimal)0.77 decimal A = Entire drainage area 0.51 acres RRV = Runoff Reduction Volume 0.0159 acre-ft RRV = Runoff Reduction Volume 694 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 larger runoff volume is used  (1175 cf). DRAINAGE AREA # 4 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Hardscape 0.95 5355 5087 Landscape 0.20 8021 1604 Total 13376 6691 A = Area (acres) 0.3071 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) 0.31 Q = Runoff (cfs) 0.06 V = REQUIRED VOL (ft3) 451 VOLUME OF STORMWATER ALLOWED TO BE DISCARGED OFF SITE INTO NELSON MEADOWS SUBDIVISION 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 5355 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 0.41 0.05 + 0.9*I I = Percent impervious cover (decimal)0.40 decimal A = Entire drainage area 0.31 acres RRV = Runoff Reduction Volume 0.0052 acre-ft RRV = Runoff Reduction Volume 229 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 larger runoff volume is used  (451 cf). DRAINAGE AREA # 1 DRYWELL 1 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft2 )C * Area Hardscape 0.95 9861 9368 Landscape 0.20 384 77 Total 10245 9445 A = Area (acres) 0.2352 C = Weighted C Factor 0.92 2. Calculate Required Volume Q=CIA V=7200Q C = Weighted C Factor 0.92 I = Intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.24 Q = Runoff (cfs) 0.09 V = REQUIRED VOL (ft3) 637 3. Calculate Drywell Volume Void Ratio of Media 30.00% Gravel Offset Dist. From Drywell (ft) 10.5 Gravel Void Volume Gravel Bed Depth (below MH) 0.00 Gravel Volume (ft3)2022.75 Gravel Storage Volume (ft3)606.82 Manhole Volume Manhole Depth (ft) 4.00 Manhole Volume (ft3)73.29 Provided Volume (ft3)680 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 9861 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 0.92 0.05 + 0.9*I I = Percent impervious cover (decimal)0.96 decimal A = Entire drainage area 0.24 acres RRV = Runoff Reduction Volume 0.0090 acre-ft RRV = Runoff Reduction Volume 391 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 larger runoff volume is used  (637 cf). DRAINAGE AREA # 3 FRENCH DRAIN 1 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Hardscape 0.95 2570 2442 Landscape 0.20 0 0 Total 2570 2442 A = Area (acres) 0.0590 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) 0.06 Q = Runoff (cfs) 0.02 V = REQUIRED VOL (ft3) 165 3. Calculate French Drain Volume Porous Media in French Drain Gravel Void Ratio of Media 30.00% Gravel Offset Dist. From French Drain (ft) 6 Length of Media 26 Depth of Media 4 Gravel Void Volume Gravel Volume (ft3)624 Gravel Storage Volume (ft3)187 Provided Volume (ft3)187 Check the half inch requirement (per DSSP II.A.4) 1. Determine Area of Hardscape within Drainage Area #3 Contributing Area Area (ft2 ) Hardscape 2570 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 0.06 acres RRV = Runoff Reduction Volume 0.0023 acre-ft RRV = Runoff Reduction Volume 102 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 larger runoff volume is used  (165 cf). DRAINAGE AREA # 5 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Hardscape 0.95 0 0 Landscape 0.20 19736 3947 Gravel 0.50 10230 5115 Total 29966 9062 A = Area (acres) 0.6879 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) 0.69 Q = Runoff (cfs) 0.08 V = REQUIRED VOL (ft3) 611 VOLUME OF STORMWATER ALLOWED TO BE DISCARGED OFF SITE INTO NELSON MEADOWS SUBDIVISION Check the half inch requirement (per DSSP II.A.4) 1. Determine Area of Hardscape within Drainage Area #5 Contributing Area Area (ft 2 ) Hardscape 10230 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 0.36 0.05 + 0.9*I I = Percent impervious cover (decimal)0.34 decimal A = Entire drainage area 0.69 acres RRV = Runoff Reduction Volume 0.0102 acre-ft RRV = Runoff Reduction Volume 446 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 larger runoff volume is used  (611 cf). APPENDIX C GROUNDWATER DEPTH INFORMATION 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 Lot 16, Block 4 Nelson Meadows development. Storm Water Facilities: 1.Drainage swales slope toward retention and detention ponds to collect storm water runoff and channel it to the retention or detention pond. 2.Retention Ponds collect storm water runoff and store the water until it evaporates and/or infiltrates into the ground. 3.Detention ponds collect storm water runoff while allowing some water to drain to another location. 4.Culverts are pipes which channel storm water from ditches or swales under roads. 5.Pipe Networks convey storm water to different discharge locations underground. 6.Inlets are facilities where storm water runoff enters a pipe network. Inlets include storm water manholes and drains. 7.Catch Basins are sumps typically located directly below storm water inlets and allow sediment to settle before storm water enters the pipe network. 8.Outlets are points where storm water exits a pipe network. 9.Drywells are underground storm water collection facilities that collect and temporarily store runoff from roof tops and landscaped areas before allowing storm water to infiltrate into the ground. 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 drywells,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 and dry wells 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, dry wells, culverts and retention ponds.Flush and/or vacuum drywells or storm water pipes if excessive material is observed in the facilities. Standard Maintenance: 1.Remove sediment and oil/grease from retention ponds and detention ponds. 2.Inspect and remove debris from drainage swales, catch basins, dry wells,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. Equipment Type/Access: All drywells will be cleaned using vacuum trucks. All vacuum trucks will access the site through the proposed alley and parking lot. Cost Estimate: Depending on the amount of rainfall in the given year, the cost to maintain the stormwater infrastructure will vary. It is estimated that the drywells will need to be vacuumed out once per year, with an estimated cost of $1,500 to do so. The applicant will be responsible for financing the maintenance of the stormwater infrastructure.