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Home My WebLink AboutSW6230804_Stormwater Report_20230817 TABLE OF CONTENTS NARRATIVES Site Location and Description Project Development Erosion Control Stormwater Management Storm Drainage SUPPORT DOCUMENTS USGS Topography Map FEMA Floodplain Map NRCS Soils Information Stream Classification CALCULATIONS & ANALYSIS Storm Drainage Calculations Stormwater Management Calculations Ditch Calculations Sediment Basin Calculations Outlet Protection Calculations NARRATIVES Site Location & Description A&S Land, LLC plans to develop an outdoor storage facility on Oakridge Duncan Road in Fuquay-Varina, NC. The property is 13.13 acres and the site area is currently vacant. Dwellings (and wooded areas) are to the north, south, east, and west. Figure1-Vicinity Map–Courtesyof Google Earth SITE The property is not in a flood hazard as per FEMA map 3720066200J, dated October 3, 2006. The site is in the Cape Fear River basin. 3 Project Development The owner plans to construct an outdoor gravel storage facility on the property in question. The property is zoned GC (general commercial). The property will be graded as needed for the driveway, ditch sections, and storm drainage infrastructure. Domestic service for the building will come from a new well drilled on the northern side of the property. For sewer, there will be a septic tank draining to a drain field. Erosion Control The project will consist of clearing, grubbing, and general site development within the limits of disturbance on the erosion control plan. There will be associated silt fence and silt fence outlets around all limits of disturbance. The property is privately owned. See owner information on cover sheet C-00. Approximately 4.66 acres will be disturbed during construction. The maximum fill will be approx. 6 feet andmaximum cut will be approximately 4 feet. This project will involve removal of topsoil and associated fill toconstruct the storage area and ditch sections. An underground storm drainage system will be in- stalled to convey stormwater to a permanent stormwater management area. The project is scheduled to begin construction in late Fall 2023 with project completion and final stabilization by Winter 2024. The erosion and sediment control program for this project will include the installation of a suitable construction entrance, temporary silt fencing, silt fence outlets, inlet protection measures and sediment basin. The soils at this site are predominately loamy sands. Soils are well-drained with Ksat ranges from moderately low to high. Slopes are largely between 0 to 6%. On-site soils are primarily Fuquay Loamy Sand (FaB) which is a type B soil. Soils information has been obtained through the USDA & NRCS website: http://websoilsurvey.nrcs.usda.gov/app/ for Harnett County. 4 Stormwater Management The engineer has designed an surface sand filter system to help attenuate storm events per the NCDEQ Stormwater requirements that must be met. The project area was analyzed with points of analysis shown on all stormwater plans. This is documented in the enclosed plans. The dry detention stormwater facility was modeled using Hydraflow Hydrographs by AutoDesk. The SCS method was utilized for determining pre/post flow rates. The infiltration basin succeeds in reducing post- development runoff rates of the 10-year storm from 14.28 cfs to approx. 5.596 cfs. It is the engineer’s opinion that this reduction meets the intent of the stormwater requirements. Water Quantity Summary Table 5 Storm Sewer Hydraulic Gradeline Analysis The project will require the use of several curb inlets & yard inlets to capture stormwater runoff. Standard NCDOT grated type catch basins are being utilized within the parking lot. The storm drainage system for this project has been designed with the following criteria: ·Class III RCP with 15-in minimum pipe diameter. ·Start the HGL analysis at 80% full of the outlet pipe ·Maintain 25 yr. HGL inside system ·Minimum slope of 0.5%. ·Minimum Cover of 2-ft (top of subgrade to crown). Each inlet has a delineated drainage basin composite C value for land use (0.9 assumed for all parking lot drainage) and a minimum Tc of five (5) minutes for the enclosed pipe system sizing. The rational method was utilized for calculating peak flow. Refer to the calculations section for the pipe sizing and HGL output. 6 SITE A/D B A Stream Classification SITE Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023 Hyd.HydrographInflowPeak Outflow (cfs)Hydrograph No.typehyd(s)Description (origin)1-yr2-yr3-yr5-yr10-yr25-yr50-yr100-yr 1SCS Runoff------0.3470.641--------------1.6532.351--------------Pre-Development DA #1 2SCS Runoff------1.9843.566--------------8.93812.63--------------Pre-Development DA #2 3SCS Runoff------0.8621.594--------------4.1155.853--------------Pre-Development DA #3 4Combine1, 2, 33.1225.653--------------14.2820.22--------------Pre-Development Combination 5SCS Runoff------19.1623.30--------------34.4141.11--------------Post Development DA #2 - To BMP 6SCS Runoff------11.2013.62--------------20.1124.03--------------Post Development DA #3 - To BMP 7SCS Runoff------0.0530.162--------------0.5870.901--------------Post Development DA #1 OFFSITE 8SCS Runoff------0.5150.882--------------2.0762.884--------------Post Development DA #4 - OFFSITE 9Combine5, 6,30.2136.73--------------54.2364.79--------------To BMP Combination 10Reservoir91.7771.976--------------3.51010.62--------------Surface Sand Filter 11Combine7, 8, 102.3072.885--------------5.59613.18--------------Post-Development Combination Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Storage Indication method used. Exfiltration extracted from Outflow. Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Storage Indication method used. Exfiltration extracted from Outflow. Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Storage Indication method used. Exfiltration extracted from Outflow. Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 Storage Indication method used. Exfiltration extracted from Outflow. Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2023Tuesday, 06 / 27 / 2023 2-YR STORM 10-YR STORM 25-YR STORM h Sediment Basin Calculation Worksheet Project Name:Oakridge Duncan Sediment Basin #1 Area Disturbed =353050sf 8.10acres Peak Discharge Calculation: Drainage Area =353050sf 8.10acres Q = C x I x A 10 C =0.30 I = 7.24in/hr 10 A =8.10acres Q = 17.60cfs 10 Minimum Surface Area Calculation: = Q x 0.01 x 43500 sf/ac Surface Area (SF) 10 = 17.6 cfs x 0.01 x 43500 sf/ac Surface Area Req =7658sf Basin Volume Calculation: Volume of Sediment= (Disturbed Ac.) x (1800 cf sed./year) x (1 year) = 8.1 ac x 1800 x 1 yr Min Vol =14589cf Assumed basin depth =2ftmin D = 2' for "Temp Sediment Basin" Resultant Basin Area Req =7294sf Check Surface Area Criteria Surface Area Requirement =7658sf Basin Volume Calc Area =7294sf Therefore: Surface Area Governs Min Volume Req =14589cf Min Area Req =7658sf Assume L = 2xW, calculate basin dimensions Actual W =62ftMin W =60ft L =124ft D =2ft Actual Vol =15376cfVol Check:OK Actual Area =7688sfArea Check:OK Actual Basin Dimensions =62 ft W x 124 ft L x 2 ft D Weir Design and Velocity Check: 2/3 Calculate Flow DepthH = \[Q/CL\] Q = 17.60cfs 10 C =2.68constant L =10ftmin L = 4' H =0.76ftmax H = 0.5' Calculate Velocity Over WeirV = Q/A V =17.6 cfs / 10 ft x 0.76 ft V =2.33ft/secOK Skimmer Sizing: 3Skimmer Size (inches)Skimmer Size 0.25Head on Skimmer (feet)(Inches) 2.5Orifice Size (1/4 inch increments)1.5 2.02Dewatering Time (days)2 2.5 3 4 5 6 8