Loading...
HomeMy WebLinkAboutSW3190301_Apprentice Academy - SWM Report_20190513STORMWATER MANAGEMENT CALCULATIONS FOR THE APPRENTICE ACADEMY HIGH SCHOOL LOCATED IN UNION COUNTY, NORTH CAROLINA BCCG March 2019 Revised (April 29, 2019) Revised (May, 13, 2019) Comprehensive-Stormwater Pollution Prevention Plan (C-S WPPP) Apprentice Academy High School TABLE OF CONTENTS 1.0 Project Overview.............................................................................................................4 1.1 Narrative.....................................................................................................................................................4 1.2 Stormwater Management and Sediment Control........................................................................................6 1.3 Sequence of Construction.........................................................................................................................11 1.4 Non -Numeric Effluent Limits..................................................................................................................12 1.5 Management of Non-Stormwater Discharge............................................................................................12 1.6 Buffer Zone Management.........................................................................................................................12 1.7 Certification Statement.............................................................................................................................13 2.0 Site Features and Sensitive Areas.................................................................................15 2.1 Sources of Pollution.................................................................................................................................15 2.2 Surface Waters..........................................................................................................................................16 3.0 Compliance Requirements............................................................................................16 3.1 SWPPP Availability.................................................................................................................................16 3.2 Pre -Construction Conference...................................................................................................................16 3.3 Inspection Requirements..........................................................................................................................17 3.4 Maintenance Policies................................................................................................................................17 3.5 Record Keeping........................................................................................................................................18 3.6 Final Stabilization.....................................................................................................................................18 EXHIBITS Exhibit A Erosion and Sediment Control Plans.............................Error! Bookmark not defined. Alliance Consulting Engineers, Inc. March 2019 (Revised May 13, 2019) ALLIANCE CONSULTING ENGINEERS 11 Comprehensive-Stormwater Pollution Prevention Plan (C-S WPPP) Apprentice Academy High School LIST OF TABLES Table 1- Post Developed Sand Filter Peak Discharges and Storage Elevations................................6 Table 2 - Locations of Potential Sources of Stormwater Contaminations.................................................10 Table 6 - Potential Construction Site Stormwater Pollutants....................................................................15 APPENDICES APPENDIXA Site Maps.......................................................................................................................19 SiteLocation Map......................................................................................................................................20 AerialMap.......................................................................................................................................21 TopographicMap.......................................................................................................................................22 SoilsMap.......................................................................................................................................23 FEMAFlood Map......................................................................................................................................24 National Wetlands Inventory (NWI) Map.................................................................................................25 APPENDIX B Storm Pipe, Sediment Basin, Diversion Ditch Drainage Area Maps ............................26 APPENDIX C Rainfall Data..................................................................................................................27 APPENDIX D Hydraulic Engineering Report .......................................................................................28 APPENDIX E Erosion Control Calculations........................................................................................29 APPENDIX F Sand Filter Calculations................................................................................................30 Alliance Consulting Engineers, Inc. March 2019 (Revised May 13, 2019) f'OLLUANCE CON5VLTING ENGINEERS III Comprehensive-Stormwater Pollution Prevention Plan (C-S WPPP) Apprentice Academy High School 1.0 Proiect Overview 1.1 Narrative Apprentice Academy High School is proposing a new school, which consists of a new approximately 32,513 SF, 2-story classroom building located at 2505 Weddington Road (NC-84) in unincorporated Union County, North Carolina. The property (TMS # 09316009C) is located approximately at the southeast corner of the intersection of Weddington Road (NC-84) and Clark Road as illustrated in the Site Location Map in Appendix A. The proposed development will include approximately 16.90 acres of land disturbance on the 30.32-acre site. The Mecklenburg County Digital Ortho Quarter Quadrangle (DOQQ) imagery provided by the United States Depart of Agriculture Farm Service Agency (USDA-FSA) (See Aerial Map in Appendix A) was utilized to provide a general overview of the site and its surroundings. The United States Geological Survey's (USGS) 7.5 Minute Union County Quad Map (See Topographic Map in Appendix A) was utilized to provide a general overview of the site and surrounding area drainage patterns. Utilizing a topographic survey conducted by R.B. Pharr & Associates, P.A. dated January 9, 2019, it appears that the stormwater runoff drains generally south -to -north. Most of the stormwater runs into a stream along the western side of the property. The remainder of the on -site stormwater runoff drains to the northeast. Based on a review of the North Carolina Soil Survey for Union County, it is apparent Badin channery silty clay loam (BdC2) (HSG C), Cid channery silt loam (CmB) (HSG D), and Tarrus gravelly silty clay loam (TbB2) (HSG B) soils comprise the project site (See Soils Map in Appendix A). The Federal Emergency Management Agency (FEMA) Flood Map Panels 3710542500J (See FEMA Flood Map in Appendix A) identifies that the project site is located outside of any floodzone, which indicates the site is in an area of minimal flooding and is outside of the 100-year Flood Plain. Based on the provided base flood elevations shown on the FEMA Map, our project site should not disturb this flood zone area. The National Wetlands Inventory (NWI) Map provided by the US Fish and Wildlife Service (See NWI Map in Appendix A), indicates there is an approximately 1.7-acre wetland in the northwest corner of the site. However, Wetland Delineation performed by ECS Southeast, LLC indicates potential wetlands exist along the bottom of the existing drainage feature draining the site. These wetlands have been delineated and are shown on the Construction Plans. The project will consist of the construction of the approximately 32,513 SF, 2-story building, one (1) roadway loop, parking lot, and associated facilities. The post development impervious coverage of the lot will only be 9.5% (14.02% for future expected expansion), below the 24% maximum coverage limit for low density development, however, the parking lot and drives located in front of the school building will create a point discharge so a sand filter is proposed to treat run-off from the discharge of the storm drain system. Areas upstream of the storm drain system are still designed to sheet flow into a vegetative swale before draining into the storm drain system which is consistent with low density development. The remaining developed areas of Alliance Consulting Engineers, Inc. March 2019 (Revised May 13, 2019) ,ALLIANCE CONSULTING ENGINEERS M Comprehensive-Stormwater Pollution Prevention Plan (C-SWPPP) Apprentice Academy High School the site are proposed to sheet flow into lawn and wooded areas which is also consistent with low density development. All earth moving activities will tie into existing grade and will maintain the existing flow patterns of stormwater on -site. The remaining portion of the site will be left wooded except portions necessary for the erosion control measures during construction and installation of utilities and facilities. The following are the calculations for the proposed Sand Filter. Sand Filter Drainage Area: 3.96 Acres Design Volume Required: 3.96 Acres x 43,560 sf / acre x (1 in / 12in/ft) = 14,375 cf Discounted Design Volume: 14,375 cf x 0.75 = 10,782 cf Design Volume Provided: 16,405 cf at elevation 639.5 Surface Area Required: 10,782 cf / 2'ponding depth = 5,391 sf Sand Chamber Surface Area Required: 2,696 sf Surface Area Provided: 6,640 SF at elevation 637.5 Sand Chamber Surface Area Provided: 2,777 sf at elevation 637.5 Sediment Chamber Surface Area Provided: 3,863 sf at elevation 637.5 The sand filter is designed with a 12" sand filter media depth with a 6" underdrain encased in stone underneath that. The bottom of the detention is proposed at 637.5 with a sediment chamber provided to elevation 639.5 with a volume of 9,538 and a sand chamber to elevation 639.5 and volume of 6,867 as shown on the attached pond reports contained in the Sand Filter Appendix. The area draining to the sand filter is 3.96 acres containing 2.16 acres of impervious coverage (CN = 61) which includes 0.47 acres of future parking and 1.80 acres of grass, good condition (CN = 61), see attached drainage map (Appendix B). Resulting in a composite CN of 81. The area draining to the sand filter contains soils with Hydrologic Soil Group B. The assumed time of concentration is assumed the minimum of 6 minutes. For the modeling of the system the media filtration rate is conservatively assumed 2 in/hr. This will result in a flow rate input in the attached Hydraflow Model (Appendix F) of: 2 in/hr x 1 ft/12in x 3,600 s/hr x 2,777 sf (sand chamber bottom area) = 0.129 cfs For comparison to pre -development the same area (3.96 acres) is calculated as woods, good condition (CN = 55) with a time of concentration calculated to be 18.7 minutes (see attached drainage map - Appendix B). The following are the peak flow rates to the sand filter, routed thru the sand filter with storage elevations and the predevelopment rates calculated utilizing Autodesk ® Hydraflow Hydrographs. As shown below the sand filter will attenuated the 2-yr and 10-yr storm events to pre -development peak flow rates and pass the 100-yr storm event thru the emergency spillway with 0.70' of freeboard. Alliance Consulting Engineers, Inc. March 2019 (Revised May 13, 2019) Ail LIANCE CONSULTING ENGINEERS 5 14 Hydrograph Report Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020 Monday, 05 / 13 / 2019 Hyd. No. 1 To Sand Filter Hydrograph type = SCS Runoff Peak discharge = 36.90 cfs Storm frequency = 100 yrs Time to peak = 716 min Time interval = 2 min Hyd. volume = 77,512 cuft Drainage area = 3.960 ac Curve number = 81 * Basin Slope = 0.0 % Hydraulic length = 0 ft Tc method = User Time of conc. (Tc) = 6.00 min Total precip. = 8.01 in Distribution = Type II Storm duration = 24 hrs Shape factor = 484 Composite (Area/CN) = [(2.160 x 98) + (1.800 x 61)] / 3.960 Q (cfs) 40.00 30.00 20.00 10.00 0.00 0 120 240 — Hyd No. 1 To Sand Filter Hyd. No. 1 -- 100 Year 360 480 600 720 840 960 Q (cfs) 40.00 30.00 20.00 10.00 0.00 1080 1200 1320 Time (min) Hydrograph Report 15 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020 Monday, 05 / 13 / 2019 Hyd. No. 2 Routed thru Sand Filte Hydrograph type = Reservoir Peak discharge = 34.27 cfs Storm frequency = 100 yrs Time to peak = 718 min Time interval = 2 min Hyd. volume = 77,509 cuft Inflow hyd. No. = 1 - To Sand Filter Max. Elevation = 640.30 ft Reservoir name = Sand Filter Max. Storage = 24,994 cuft Storage Indication method used Q (cfs) 40.00 30.00 20.00 10.00 Routed thru Sand Filte Hyd. No. 2 -- 100 Year 0.00 1 0 120 240 360 480 600 720 840 960 1080 1200 1320 — Hyd No. 2 Hyd No. 1 Total storage used = 24,994 cuft Q (cfs) 40.00 30.00 20.00 10.00 — 0.00 1440 Time (min) 16 Hydrograph Report Hydraflow Hydrographs Extension for Autodesk® Civil 3D® 2019 by Autodesk, Inc. v2020 Monday, 05 / 13 / 2019 Hyd. No. 4 Pre -Development Hydrograph type = SCS Runoff Peak discharge = 12.33 cfs Storm frequency = 100 yrs Time to peak = 726 min Time interval = 2 min Hyd. volume = 40,119 cuft Drainage area = 3.960 ac Curve number = 55 Basin Slope = 0.0 % Hydraulic length = 0 ft Tc method = TR55 Time of conc. (Tc) = 18.70 min Total precip. = 8.01 in Distribution = Type II Storm duration = 24 hrs Shape factor = 484 Q (cfs) 14.00 12.00 10.00 MIX N. We 4.00 2.00 Pre -Development Hyd. No. 4 -- 100 Year Q (cfs) 14.00 12.00 10.00 01 4.00 2.00 0.00 120 240 360 480 600 720 840 960 1080 1200 1320 1440 1560 Hyd No. 4 Time (min) Hydraflow Rainfall Report 17 Hydraflow Hydrographs Extension for Autodesk0 Civil 3DO 2019 by Autodesk, Inc. v2020 Return Period Intensity -Duration -Frequency Equation Coefficients (FHA) (Yrs) B D E (N/A) 1 0.0000 0.0000 0.0000 -------- 2 54.6434 10.3000 0.8092 -------- 3 0.0000 0.0000 0.0000 -------- 5 52.5793 10.3000 0.7554 -------- 10 53.4352 10.3000 0.7315 -------- 25 57.1938 10.5000 0.7117 -------- 50 60.4231 10.6000 0.7000 -------- 100 63.8015 10.7000 0.6906 -------- File name: OF - Union Co. NC.IDF Intensity = B / (Tc + D)^E Monday, 05 / 13 / 2019 Return Period Intensity Values (in/hr) (Yrs) 6 min 10 16 20 26 30 36 40 46 60 66 60 1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2 6.01 4.78 4.00 3.46 3.06 2.75 2.50 2.29 2.13 1.98 1.86 1.75 3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 5 6.70 5.41 4.58 4.00 3.56 3.22 2.95 2.73 2.54 2.38 2.24 2.12 10 7.26 5.91 5.03 4.41 3.94 3.58 3.28 3.04 2.84 2.66 2.51 2.38 25 8.13 6.67 5.71 5.02 4.51 4.11 3.78 3.51 3.28 3.09 2.92 2.77 50 8.83 7.27 6.24 5.51 4.96 4.52 4.17 3.88 3.63 3.42 3.23 3.07 100 9.53 7.87 6.78 6.00 5.40 4.93 4.56 4.24 3.97 3.74 3.55 3.37 Tc = time in minutes. Values may exceed 60. 9-1090 New Apprentice Academv Hiah School Weddinaton Rd Union Co NC\Enaineerina Calculations\Monroe-NC.DCD Storm Rainfall Precipitation Table (in) Distribution 1-yr 2-yr 3-yr 6-yr 10-yr 26-yr 60-yr 100-yr SCS 24-hour 2.99 3.61 0.00 4.54 5.28 6.31 7.14 8.01 SCS 6-Hr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Huff-1 st 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Huff-2nd 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Huff-3rd 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Huff-4th 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Huff-Indy 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Custom 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00