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Home My WebLink AboutSW3230301_Calculations_20230612 Kerns Ridge 08433-0001 Stormwater and Erosion Control Measures DATE:09/19/22 REV:06/12/23 1 Cover1 Table of Contents2 Introduction3 Drainage Areas and Curve Numbers3 Impervious Calculations4 Riprap Aprons5 Wet Pond #16 Wet Pond #28 HydroCAD Routing for 100-Year Storm11 JCB JCB JCB Kerns Ridge is a proposed residential development located in East Spencer, Rowan County, North Carolina. The site is approximately 73 acres and consists of 128 single-family lots. The site will have 2 entrances along Bringle Ferry road. The site is currently vacant and wooded. A 100-year floodplain is located on the southern portion of the site along Crane Creek. Stormwater runoff will be conveyed through a system of curb and gutter, and storm sewers to two permanent stormwater control measures (SCM). Both SCMs will be wet ponds that will be used to treat runoff from the development. The SCMs are designed to provide water quality treatment in accordance with the NCDEQ Stormwater Design Manual. Streets/buildings171150B98 Wet Pond Water Surface14764B98 Open Space (>75% grass)193815B61 Open Space (>75% grass)37140C74 Streets/buildings570490B98 Future Amenity Impervious29500B98 Wet Pond Water Surface39518B98 Open Space (>75% grass)474119B61 Open Space (>75% grass)147435C74 3 60950 11020029 194290 99 376200 29500 4 SCM 1 Outfall2.0049.015.41.015.06.0 SCM 2 Outfall3.50115.011.01.022.010.5 A13.0033.07.20.516.09.0 B13.5057.46.00.822.010.5 5 C13.5048.65.10.822.010.5 DESIGN OBJECTIVE A wet pond shall be designed to capture the design storm and release it slowly over a period of two to five days via a properly designed outlet structure. The first outlet in the structure is designed to be above the bottom of the pond, thus creating a permanent pool of water. Stormwater shall have an adequate flow path to bring about removal of TSS through dilution and settling. The pond shall be designed in a manner that protects the device, the areas around the device and the receiving stream from erosion. The pond also must be maintained properly to prevent the resuspension of captured sediments. SITE CHARACTERISTICS 2 Drainage Area416,869 ft 2 Post-development impervious area171,150 ft Post-development impervious percentage41% Design rainfall depth1.0in DESIGN VOLUME 3 ft Minimum volume required14,5563630 x rainfall depth x drainage area x (0.05 + 0.9 x impervious percentage) 3 ft Volume provided15,534See Stage-Storage table below ELEVATIONS 713.50fmsl Temporary pool elevation 712.50fmsl Permanent pool elevation 713.00fmsl Top of 6ft vegetated shelf elevation 712.00fmsl Bottom of 6ft vegetated shelf elevation 708.00fmsl Sediment cleanout, top elevation (bottom of pond) 707.50fmsl Sediment cleanout, bottom elevation 0.5ft Sediment storage depth provided SURFACE AREAS 2 Temporary pool area16,998See Stage-Storage table below ft 2 7,462SA/DA ratio x Drainage Area Main pool area REQUIRED ft 2 Main pool area PROVIDED11,360See Stage-Storage table below ft SA/DA ratio 1.79From SA/DA table for Piedmont/Mountains (50% impervious, 3.0 feet average depth) 2 Bottom of vegetated shelf area10,332See Stage-Storage table below ft 2 5,412See Stage-Storage table below Bottom of main pool area ft VOLUMES 3 ft Temporary pool volume15,534See Stage-Storage table below 3 ft Main pool volume34,339See Stage-Storage table below 3 ft Forebay volume5,623See Stage-Storage table below 16.38%OK Forebay % of Main pool volume 6 MAIN POOL STAGE-STORAGE TABLE ElevationArea Volume Volume Area (sf) (msl)(ac)(cu-ft)(ac-ft) 708.0049370.113300.0000 709.0060840.139754950.1261 710.0072970.1675121700.2794 711.0085700.1967200870.4611 712.0095370.2189291270.6687 712.50113600.2608343390.7883 FOREBAY STAGE-STORAGE TABLE ElevationArea Volume Volume Area (sf) (msl)(ac)(cu-ft)(ac-ft) 709.005100.011700.0000 710.0014580.03359420.0216 711.0017830.040925590.0587 712.0021330.049045120.1036 712.5023170.053256230.1291 TEMPORARY POOL STAGE-STORAGE TABLE ElevationArea Volume Volume Area (sf) (msl)(ac)(cu-ft)(ac-ft) 712.50141890.325700.0000 713.00155170.356274170.1703 713.50169980.3902155340.3566 SA/DA TABLE DATA N Coastal SA/DA table used? Y Mountains/Piedmont SA/DA table used? 1.79From SA/DA table for Piedmont/Mountains (50% impervious, 3.0 feet average depth) SA/DA ratio 3.02Main pool volume / Main pool surface area Average depth DRAWDOWN CALCULATIONS Y Drawdown through orifice? 2.00in Diameter of orifice (if circular) 2 ft Area of orifice0.02 Coefficient of discharge (C) 0.60(unitless) D Driving head (H) 0.33ft o 2.96daysOK, draws down in 2-5 days. Design volume drawdown time 7 DESIGN OBJECTIVE A wet pond shall be designed to capture the design storm and release it slowly over a period of two to five days via a properly designed outlet structure. The first outlet in the structure is designed to be above the bottom of the pond, thus creating a permanent pool of water. Stormwater shall have an adequate flow path to bring about removal of TSS through dilution and settling. The pond shall be designed in a manner that protects the device, the areas around the device and the receiving stream from erosion. The pond also must be maintained properly to prevent the resuspension of captured sediments. SITE CHARACTERISTICS 2 Drainage Area1,261,062 ft 2 Post-development impervious area599,990 ft Post-development impervious percentage48% Design rainfall depth1.0in DESIGN VOLUME 3 ft Minimum volume required50,6533630 x rainfall depth x drainage area x (0.05 + 0.9 x impervious percentage) 3 ft Volume provided60,202See Stage-Storage table below ELEVATIONS 677.00fmsl Temporary pool elevation 675.50fmsl Permanent pool elevation 676.00fmsl Top of 6ft vegetated shelf elevation 675.00fmsl Bottom of 6ft vegetated shelf elevation 671.50fmsl Sediment cleanout, top elevation (bottom of main pool) 671.00fmsl Sediment cleanout, bottom elevation 0.5ft Sediment storage provided SURFACE AREAS 2 Temporary pool area43,257 ft 2 Main pool area REQUIRED22,573SA/DA ratio x Drainage Area ft 2 30,896OK Main pool area PROVIDED ft SA/DA ratio1.79From SA/DA table for Piedmont/Mountains (50% impervious, 3.0 feet average depth) 2 29,623 Bottom of vegetated shelf area ft 2 Bottom of main pool area19,747 ft VOLUMES 3 ft Temporary pool volume60,202See Stage-Storage table below 3 ft 100,396See Stage-Storage table below Main pool volume 3 ft Forebay volume15,891See Stage-Storage tables below 15.83%OK Forebay % of main pool volume 8 MAIN POOL STAGE-STORAGE TABLE ElevationArea Volume Volume Area (sf) (msl)(ac)(cu-ft)(ac-ft) 671.50208700.479100.0000 672.00217920.5003106540.2446 673.00238800.5482334590.7681 674.00260930.5990584131.3410 675.00283570.6510856031.9652 675.50308960.70931003962.3048 FOREBAY #1 STAGE-STORAGE TABLE ElevationArea Volume Volume Area (sf) (msl)(ac)(cu-ft)(ac-ft) 672.0010060.023100.0000 673.0021440.049215380.0353 674.0026270.060339170.0899 675.0031350.072067910.1559 675.5033990.078084230.1934 FOREBAY #2 STAGE-STORAGE TABLE ElevationArea Volume Volume Area (sf) (msl)(ac)(cu-ft)(ac-ft) 672.007080.016300.0000 673.0019690.045212850.0295 674.0023670.054334470.0791 675.0027900.064060200.1382 675.5030100.069174690.1715 TEMPORARY POOL STAGE-STORAGE TABLE ElevationArea Volume Volume Area (sf) (msl)(ac)(cu-ft)(ac-ft) 675.50357810.821400.0000 676.00396130.9094188220.4321 677.00432570.9930602021.3820 9 SA/DA TABLE DATA N Coastal SA/DA table used? Y Mountains/Piedmont SA/DA table used? 1.79From SA/DA table for Piedmont/Mountains (50% impervious, 3.0 feet average depth) SA/DA ratio 3.25Main pool volume / Main pool surface area Average depth of main pool DRAWDOWN CALCULATIONS Y Drawdown through orifice? 3.00in Diameter of orifice (if circular) 2 ft Area of orifice0.05 Coefficient of discharge (C) 0.60(unitless) D Driving head (H) 0.50ft o 4.17daysOK, draws down in 2-5 days. Design volume drawdown time 10 11 Type II 24-hr 100-Year Rainfall=7.17" 12 Type II 24-hr 100-Year Rainfall=7.17" 13 Type II 24-hr 100-Year Rainfall=7.17" 14 Type II 24-hr 100-Year Rainfall=7.17" 15 Type II 24-hr 100-Year Rainfall=7.17" 16 Type II 24-hr 100-Year Rainfall=7.17" 17 Type II 24-hr 100-Year Rainfall=7.17" 18 Type II 24-hr 100-Year Rainfall=7.17" 19 Type II 24-hr 100-Year Rainfall=7.17" 20 Type II 24-hr 100-Year Rainfall=7.17" 21 Type II 24-hr 100-Year Rainfall=7.17" 22 Type II 24-hr 100-Year Rainfall=7.17" 23 Type II 24-hr 100-Year Rainfall=7.17" 24 Type II 24-hr 100-Year Rainfall=7.17" 25 Type II 24-hr 100-Year Rainfall=7.17" 26 Type II 24-hr 100-Year Rainfall=7.17" 27 Type II 24-hr 100-Year Rainfall=7.17" 28 Type II 24-hr 100-Year Rainfall=7.17" 29 Type II 24-hr 100-Year Rainfall=7.17" 30 Type II 24-hr 100-Year Rainfall=7.17" 31 Type II 24-hr 100-Year Rainfall=7.17" 32 Type II 24-hr 100-Year Rainfall=7.17" 33 Type II 24-hr 100-Year Rainfall=7.17" 34 Type II 24-hr 100-Year Rainfall=7.17" 35 Type II 24-hr 100-Year Rainfall=7.17" 36 Type II 24-hr 100-Year Rainfall=7.17" 37 Type II 24-hr 100-Year Rainfall=7.17" 38 Type II 24-hr 100-Year Rainfall=7.17" 39 Type II 24-hr 100-Year Rainfall=7.17" 40 Type II 24-hr 100-Year Rainfall=7.17" 41 Type II 24-hr 100-Year Rainfall=7.17" 42 Type II 24-hr 100-Year Rainfall=7.17" 43 Type II 24-hr 100-Year Rainfall=7.17" 44 Type II 24-hr 100-Year Rainfall=7.17" 45 Type II 24-hr 100-Year Rainfall=7.17" 46 Type II 24-hr 100-Year Rainfall=7.17" 47 Type II 24-hr 100-Year Rainfall=7.17" 48 Type II 24-hr 100-Year Rainfall=7.17" 49 Type II 24-hr 100-Year Rainfall=7.17" 50 Type II 24-hr 100-Year Rainfall=7.17" 51 Type II 24-hr 100-Year Rainfall=7.17" 52 Type II 24-hr 100-Year Rainfall=7.17" 53 Type II 24-hr 100-Year Rainfall=7.17" 54 Type II 24-hr 100-Year Rainfall=7.17" 55 Type II 24-hr 100-Year Rainfall=7.17" 56 Type II 24-hr 100-Year Rainfall=7.17" 57 Type II 24-hr 100-Year Rainfall=7.17" 58 Type II 24-hr 100-Year Rainfall=7.17" 59 Type II 24-hr 100-Year Rainfall=7.17" 60 Type II 24-hr 100-Year Rainfall=7.17" 61