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HomeMy WebLinkAbout19 - Design Report - Silver Creek Apartments - Stormwater DESIGN REPORT STORMWATER MANAGEMENT SILVER CREEK APARTMENTS Prepared for: Crow River Management Exchange, LLC 122 12th Avenue North St. Cloud, Minnesota 56303 Prepared by: C&H Engineering and Surveying, Inc. 1091 Stoneridge Drive, Bozeman, MT 59718 (406) 587-1115 Project Number: 180662 October 2018 INTRODUCTION The proposed Silver Creek Apartments is a 118 dwelling unit residential development located on a 4.24-acre parcel in Section 2, Township 2 South, Range 5 East of P.M.M., Gallatin County, City of Bozeman. A combination of site grading, curb and gutter, and swales will be used to manage stormwater runoff on the site. Supporting stormwater calculations are attached to this report. A Drainage Area Map is included in Appendix A. Calculations for each individual drainage area (total area, weighted C factor, and time of concentration) are included in Appendix B. RETENTION POND DESIGN All ponds have been sized according to City of Bozeman Design Standards. Retention facilities are sized to capture the entire volume of the 10-year 2-hour storm event. The retention ponds are designed with an effective water depth of 1.5 feet, and maximum side slope of 4:1. Calculations used for sizing each pond can be found in Appendix C. Design pond capacities were calculated using volume surfaces in AutoCAD Civil3D. STORM SEWER FACILITIES DESIGN Storm sewer facilities (storm chases) were sized for the 25-yr storm using Manning’s Equation. For the purposes of this report and to maintain a conservative approach, one calculation was performed for the whole drainage area which is far more runoff than any one chase will encounter. Additionally, the calculation assumed the chase is at minimum slope (1%). The calculated peak flow contributing to the chase is 4.85 cfs. The 2’ wide chase has a minimum capacity of 5.50 cfs and can therefore adequately handle the runoff from this site. Storm chase calculations are included in Appendix D. Proposed Retention Pond The proposed retention pond is located along the eastern border of the site, just west of 25th Avenue. It receives runoff from Drainage Area 1, totaling 3.88 acres. Runoff from Drainage Area 1 is conveyed via surface flow and gutters to three proposed curb cuts. All curb cuts drain to swales which drain to the proposed pond. The required pond volume was calculated to be 7,687 cubic feet. The proposed volume of the pond is 9,507 cubic feet and has an effective depth of 1.5’. Proposed Drywell The proposed drywell is located in the southern half of the site, just east of 25th Avenue. It receives runoff from Drainage Area 2, totaling 0.35 acres. Runoff from Drainage Area 2 is conveying via surface flow to the proposed swale that runs along the south side of the proposed building. The swale then conveys the runoff into the proposed drywell. The required drywell volume was calculated to be 312 cubic feet. The proposed drywell volume is 524 cubic feet with a gravel depth below the structure of 2’ and a gravel offset of 3’. Excavation for the drywell will go down to native gravels and will be backfilled with a well-draining pit-run. APPENDIX A DRAINAGE AREA MAP APPENDIX B DRAINAGE AREA CALCULATIONS DRAINAGE AREA #1 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Hardscape 0.95 106950 101603 Landscape 0.20 62082 12416 Total 169032 114019 A = Area (acres) 3.8804 C = Weighted C Factor 0.67 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%) 2.00% Return (yrs)Cf C = Rational Method Runoff Coefficient 0.35 2 to 10 1 Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1 D = Length of Basin (ft) 150 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)13.0 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 (%) 1.20% L = length of gutter (ft) 500 V = mean velocity (ft/s) 3.28 Tc Gutter Flow (minutes) =2.5 Tc Total = 15.5 (5 minute minimum) DRAINAGE AREA #2 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Hardscape 0.95 2054 1951 Landscape 0.20 13397 2679 Total 15451 4631 A = Area (acres) 0.3547 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) 0.35 Q = Runoff (cfs) 0.04 V = REQUIRED VOL (ft3)312 3. Calculate Drywell Volume Existing Soil Condition Gravel Percolation Rate (min/in) 3 (see Circular DEQ 4, Percolation Rate (ft/hr) 1.67 Table 2.1-1) Porous Media in Drywell Gravel Void Ratio of Media 30.00% Gravel Offset Dist. From Drywell (ft) 3 Infiltration Drywell Gravel Area (ft2)92.1 Infilitration Volume (ft3)307.06 Gravel Void Volume Gravel Bed Depth (below MH) 2.00 Gravel Volume (ft3)479.42 Gravel Storage Volume (ft3)143.83 Manhole Volume Manhole Depth (ft) 4.00 Manhole Volume (ft3)73.29 Provided Volume Inc. Perc. (ft3)524 APPENDIX C POND SIZING CALCULATIONS RETENTION POND # 1 REQUIRED VOLUME 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Hardscape 0.95 106950 101603 Landscape 0.20 62082 12416 Total 169032 114019 C=Weighted C Factor 0.67 2. Calculate Additional Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.67 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 3.88 Q = runoff (cfs) 1.07 V = REQUIRED VOL (ft3)7687 APPENDIX D STORM SEWER FACILITIES CALCULATIONS CURB CHASES REQUIRED CAPACITY 1. Calculate Area and Weighted C Factor (Post-Development) Contributing Area C Area (ft 2 )C * Area Hardscape 0.95 106950 101603 Landscape 0.20 62082 12416 Total 169032 114019 A = Area (acres) 3.8804 C = Weighted C Factor 0.67 2. Calculate Rainfall Intensity (Duration = Max Tc from Contributing Drainage Areas) i = 0.78x-0.64 (10-yr Storm, Fig. I-3, COB Design Standards) x = storm duration (hrs) 0.26 (DA #1) i = rainfall intensity (in./hr.) 1.85 3. Calculate 25-yr Pond Outflow Rate Q = CiA C = Rational Method Runoff Coefficient 0.67 (calculated above) i = rainfall intensity (in./hr.) 1.85 (calculated above) A = Area (acres) 3.88 (calculated above) Q = 25-yr Flow Rate (cfs) 4.85 MANNING'S EQUATION for OPEN CHANNEL FLOW Project: Silver Creek Apartments Location: Bozeman, MT By: LRS Date: 10/18/2018 Chk By: Date: INPUT z (sideslope)= 0 Mannings Formula z (sideslope)= 0 w (btm width, ft)= 2 Q = (1.486/n)ARh2/3S1/2 d (depth, ft)= 0.5 R = A/P S (slope, ft/ft) 0.01 A = cross sectional area n low =0.010 P= wetted perimeter n high =0.013 S = slope of channel V = (1.49/n)Rh2/3S1/2 n = Manning's roughness coeffiQ = V x A Depth, ft Area, sf Wetted Perimete r, ft Hydraulic Radius, ft Velocity, fps Flow, cfs Velocity, fps Flow, cfs 0.5 1.00 3.00 0.33 7.14 7.14 5.50 5.50 T = 2.00 Dm = 0.500 Sc low = 0.0031 Sc high = 0.0053 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 0.0022 0.0041 0.0037 0.0069 Low N High N d w z 11 z T Clear Data Entry Cells INSPECTION AND MAINTENANCE FOR STORMWATER MANAGEMENT FACILITIES The Property Owners Association shall be responsible for the maintenance of the stormwater drainage facilities within the Silver Creek Apartments 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 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.