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Home My WebLink AboutMillbridge Phase 7D - Millbridge Phase 7D Calculations STORMWATER NARRATIVE Millbridge Subdivision Phase 7D Waxhaw, NC July 18, 2018 W. K. Dickson & Co., Inc. 616 Colonnade Drive Charlotte, North Carolina 28205 (704) 334-5348 NC License No. F-0374 GENERAL Millbridge Subdivision is an existing single-family subdivision located in Waxhaw, North Carolina. Phase 7D is a proposed phase of the greater Millbridge subdivision located on the southwest side of the subdivision and is a total of 30.98 acres. Phase 7D consist of a total of 72 proposed single-family homes with a lot width of 60’ & 70’. The subdivision currently has entrances off Waxhaw-Marvin Road and Kensington Drive. Phase 7D is also being submitted to Town of Waxhaw for preliminary plat & Erosion Control and Union County Public Works for utility permitting. The estimated start of construction is Fall of 2018. PROPOSED IMPROVEMENTS The project will collect stormwater with valley gutter and storm drainage inlets. The stormwater will be conveyed in concrete storm pipes. Per the Town of Waxhaw’s stormwater detention requirements and NCDEQ Water Quality requirements, the stormwater will be conveyed from the storm drainage system to one of two wetpond facilities. The stormwater, once treated for the first 1” of rainfall and detained per the Town’s requirements will be discharged from the wetponds to a rip rap apron and vegetative filter strip. The site has two drainage points of interest. Point of interest #1 (POI#1) is located on the Northwest side of Phase 7D where Phase 7B and 7D join. Point of Interest #2 (POI#2) is located at the Southwest corner of the site where the Existing Pond Drains into South Carolina. The table below represents the pre-development and post development discharge. The wetponds provide stormwater quality for the first inch of rainfall (2-5 Day Drawdown) and detention for the peak stormwater runoff for the 2 &10-year storm events to meet the pre-development rates. Also provided is a table that shows the available freeboard in the two wetponds. 2 YEAR 10 YEAR PRE (CFS) POST (CFS) DELTA PRE (CFS) POST (CFS) DELTA POI#1 1.47 1.04 -0.43 12.85 11.05 --1.80 POI#2 1.43 1.02 -0.41 12.39 11.84 -0.55 Free Board Check Peak Elv. (50YR) Top of Berm Freeboad (ft) BMP 7D-1 548.82 550.00 1.18 BMP 7D-2 562.92 565.00 2.08 Millbridge Phase 7D PROJECT NO.:20170280.00.CL BY:CJH DATE:5/10/2018 T.M.:WGB REVISED:7/13/2018 P.M.:WGB CURVE NUMBER: Predeveloped CN BASIN #1 Cover Type Area (SF)%CN Wtd. CN Woods 771354 1.00 55 55.00 Total 771354 55.00 Composite CN 55.00 Use CN =55 Predeveloped CN BASIN #2 Cover Type Area (SF)%CN Wtd. CN Woods 782932 1.00 55 55.00 Total 782932 55.00 Composite CN 55.00 Use CN =55 Postdeveloped CN BMP #1 Cover Type Area (SF)%CN Wtd. CN Pavement/Concrete 53941 0.10 98 9.81 Buildings 177250 0.33 98 32.24 Grass 287811 0.53 61 32.58 Permanent Pool 19820 0.04 98 3.60 Total 538822 1.00 78.24 Composite CN 78.24 Use CN =79 Postdeveloped CN BMP #2 Cover Type Area (SF)%CN Wtd. CN Pavement/Concrete 93669 0.18 98 17.77 Buildings 129450 0.25 98 24.56 Grass/Woods 271774 0.53 61 32.09 Permanent Pool 21675 0.04 98 4.11 Total 516568 1.00 78.53 Composite CN 78.53 Use CN =79 Postdeveloped CN BYPASS POI #1 Cover Type Area (SF)%CN Wtd. CN Grass 42302 0.19 61 11.36 Woods 184817 0.81 55 44.76 Total 227119 1.00 56.12 Composite CN 56.12 Use CN =57 Postdeveloped CN BYPASS POI #2 Cover Type Area (SF)%CN Wtd. CN Grass 46101 0.17 61 10.30 Woods 226873 0.83 55 45.71 Total 272974 1.00 56.01 Composite CN 56.01 Use CN =56 PRE/POST DEVELOPMENT CURVE NUMBERS Millbridge Phase 7D PROJECT NO.:170280.00.CL BY:CJH DATE:5/10/2018 T.M.:WGB REVISED:43294 P.M.:WGB BMP 7D-1 Drainage Area 12.37 acres Impervious Areas Right of Way (paving, curb, sidewalk)=53941 sf # Lots sf/lot Lots (building, sidewalk, concrete patios, driveways, etc.)=177250 sf 23.75 4000 60 Total Impervious=231191 sf 17.5 4700 70 Percent Impervious=42.91% BMP 7D-2 Drainage Area 11.86 acres Impervious Areas Right of Way (paving, curb, sidewalk)=72169 sf # Lots sf/lot Lots (building, sidewalk, concrete patios, driveways, etc.)=129450 sf 16.5 4000 60 Amenity Center (Pool + Tennis Only)=21500 13.5 4700 70 Total Impervious=223119 sf Percent Impervious=43.19% IMPERVIOUS CALCULATIONS Impervious Calculations for Basins Millbridge Phase 7D PROJECT NO.:20170280.00.CL BY:CJH DATE:05/10/18 T.M:WGB REVISED:-P.M.:WGB STORM WATER PRE-DEVELOPMENT/POST-DEVELOPMENT TABLE PRE (CFS)POST (CFS)DELTA PRE (CFS)POST (CFS)DELTA BMP #1 1.47 1.04 -0.43 12.85 11.05 -1.80 BMP #2 1.43 1.02 -0.41 12.39 11.84 -0.55 Freeboad (ft) 1.18 2.08 * Per Town of Waxhaw Stormwater Design Manual a minimum of 0.50' freeboard required * Per NCDEQ Stormwater Design Manual a minimum of 1.00' freeboard required DRAINAGE BASIN AREA (AC.) Pre #1 17.71 Pre #2 17.97 DRAINAGE BASIN AREA (AC.) BMP #1 12.37 By-Pass #1 5.21 BMP #2 11.86 By-Pass #2 6.27 TOTAL 35.71 2 YEAR 10 YEAR Top of Berm BMP 1 548.82 550.00 POST-DEVELOPMENT AREA PRE-DEVELOPMENT AREA DRAINAGE AREA & WETPOND SUMMARY BMP 2 562.92 565.00 Free Board Check Peak Elv. (50YR) Millbridge Phase 7D PROJECT NO.:20170280.00.CL BY:CJH DATE:05/10/18 T.M.:WGB REVISED:07/13/18 P.M.:WGB WATER QUALITY REQUIREMENTS To meet the Water Quality requirements, the procedure outlined in the "Stormwater Best Management Practices" published by NCDENR will be used. This guide gives the required pond surface area and storage volume to meet an 85% pollutant removal efficiency. Also, the pond must detain the first 1" of rainfall and release this volume over a period of 2 to 5 days. Calculations for these requirements are as follows: %PERMANENT POOL DEPTH Surface area requirements Impervious 3 4 5 6 7 8 Drainage area to pond (Da)= 12.37 Ac 10 0.51 0.43 0.37 0.3 0.27 0.25 Post-development impervious percentage = 43%20 0.84 0.69 0.61 0.51 0.44 0.4 Average pool depth = 4.5 ft 30 1.17 0.81 0.84 0.72 0.61 0.56 Designed permanent pool elevation = 545.5 40 1.51 1.22 1.09 0.91 0.78 0.71 50 1.79 1.47 1.31 1.13 0.95 0.87 60 2.09 1.73 1.49 1.31 1.12 1.03 Interpolation to find required SA/DA ratio for non whole number 70 2.51 2.04 1.8 1.56 1.34 1.17 Millbridge Phase 7D Subdivision 80 2.92 2.36 2.07 1.82 1.62 1.4 Depth volume 90 3.25 2.64 2.31 2.04 1.84 1.59 4 1.29 4.5 1.22 5 1.15 From table I.1 in the BMP guidelines the required Surface Area to Drainage Area Ratio (SA/DA) for a permanent pool depth = 1.22 Required surface area = SA/DA / 100 x Drainage area=0.15 Ac 6591.456 19820 Surface area of pond as designed at permanent pool elevation = 0.5423 Ac Water Quality Requirements Using the runoff volume calculations in the "Simple Method" : Rv = 0.05 + 0.9 (I) Rv=0.436160673 in/in Volume = (Design rainfall) (Rv) (Drainage Area) Volume = 1" rainfall x Rv in/in x 1/12 in/ft x Da = or 0.45 ac-ft This volume must be released over a period of 2-5 days 19584 ft^3 As designed pond areas: Elevation Forebay Forebay FB Volume FB Volume Pond Pond Pond Volume Pond Volume Storage Volumes (msl)(sf)(ac)(cu-ft)(ac-ft)(sf)(ac)(cu-ft)(ac-ft) Cumulative 540 0 0.0000 0 0.0000 0 0.0000 0 0 Elevation Area(ac)Volume(ac-ft)541 1643 0.0377 0 0.0000 10698 0.2456 0 0.0000 545.5 0.455004591 0.00 542 1977 0.0454 1806 0.0415 11655 0.2676 11162 0.2562 546 0.49520202 0.24 543 2339 0.0537 3959 0.0909 12642 0.2902 23295 0.5348 547 0.542332415 0.76 544 2727 0.0626 6487 0.1489 15310 0.3515 37236 0.8548 548 0.590725436 1.32 545 3143 0.0722 9417 0.2162 14745 0.3385 52247 1.1994 549 0.640449954 1.94 545.5 3503 0.0804 11076 0.2543 16317 0.3746 60002 1.3774 546 0.0000 11659 0.2676 21571 0.4952 69434 1.5940 547 0.0000 11659 0.2676 23624 0.5423 92001 2.1120 Forbay Volume (CF)11076 548 25732 0.5907 116647 2.6778 Total Volume (CF)60002 549 27898 0.6404 143428 3.2926 Forebay % of Pond 18.46%550 31985 0.7343 173316 3.9788 0.0000 -5684737 -130.5036 Interpolation to find elevation at which required storage volume is reached Storage Storage Storage Elev volume area (ac.)area (sf.) 546 0.24 0.50 21571 Elev 0.45 0.51 22413 547 0.76 0.54 23624 Elev=546.41 Storage elevation = 0.91 feet above permanent pool elevation Assume 2 day minimum Determination of Orifice size for 2-5 day drawdown a=2A x sqrt(h)*Falling head equation Ct(sqrt(2g)) a=Area of orifice a sqft t=Time to drawdown 259200 sec A=Average pond surface area 23018 sf (3 day drawdown) g=Gravity constant 32.2 ft/s^2 h=Change in head 0.91 ft C=Orifice constant 0.6 Maximum orifice area =0.04 sqft Maximum orifice size =2.54 inches Selected orifice size =2.50 inches WATER QUALITY BASIN CALCULATIONS - WETPOND 1 Permanent Pond Storage Volume Millbridge Phase 7D PROJECT NO.20170280.00.CL BY:CJH DATE:05/10/18 T.M:WGB REVISED:07/13/18 P.M.:WGB FLOATATION CALCULATIONS - BMP #1 CONSTRAINTS: Weight of Concrete 150 lbs/ft^3 Weight of Water 62.4 lbs/ft^4 Safety Factor (S.F.)1.1 Height of structure 7.17 ft Structure thickness 6 in Outlet box width (interior)4 ft Weight of outlet structure(Ws) Ws = (width + thickness x 4) x thickness x height x weight of concrete =9680 lbs Weight of water displaced Ww = ((width + 2 x thickness)^2) x Weight of water x height = 11185.2 lbs Size Base Wb = ( S.F. x Ww) - Ws =2624 lbs Footing size Required concrete = Wb / (Weight of concrete-weight of water) =30 cuft *Assume base is 6" wider than exterior box width and minimum thickness is 8". Thickness of base =0.99 ft Length =6.00 ft Width =6.00 ft Thickness =1.00 ft Millbridge Phase 7D PROJECT NO.:20170280.00.CL BY:CJH DATE:05/10/18 T.M:WGB REVISED:07/13/18 P.M.:WGB ANTI-SEEP COLLARS BMP #1 36 in 33 ft 2 Anti-Seep Collar Sizes Length:5.30 ft Width:5.30 ft Note: Anti-seep collars are centered on pipe PRELIMINARY CALCULATIONS Pipe Diameter Length of Covered Pipe Number of Collars Millbridge Phase 7D PROJECT NO.:20170280.00.CL BY:CJH DATE:05/10/18 T.M.:WGB REVISED:07/13/18 P.M.:WGB WATER QUALITY REQUIREMENTS To meet the Water Quality requirements, the procedure outlined in the "Stormwater Best Management Practices" published by NCDENR will be used. This guide gives the required pond surface area and storage volume to meet an 85% pollutant removal efficiency. Also, the pond must detain the first 1" of rainfall and release this volume over a period of 2 to 5 days. Calculations for these requirements are as follows: %PERMANENT POOL DEPTH Surface area requirements Impervious 3 4 5 6 7 8 Drainage area to pond (Da)= 11.86 Ac 10 0.51 0.43 0.37 0.3 0.27 0.25 Post-development impervious percentage = 43%20 0.84 0.69 0.61 0.51 0.44 0.4 Average pool depth = 4.5 ft 30 1.17 0.81 0.84 0.72 0.61 0.56 Designed permanent pool elevation = 559.5 40 1.51 1.22 1.09 0.91 0.78 0.71 50 1.79 1.47 1.31 1.13 0.95 0.87 60 2.09 1.73 1.49 1.31 1.12 1.03 Interpolation to find required SA/DA ratio for non whole number 70 2.51 2.04 1.8 1.56 1.34 1.17 Millbridge Phase 7D Subdivision 80 2.92 2.36 2.07 1.82 1.62 1.4 Depth volume 90 3.25 2.64 2.31 2.04 1.84 1.59 4 1.30 4.5 1.23 5 1.16 From table I.1 in the BMP guidelines the required Surface Area to Drainage Area Ratio (SA/DA) for a permanent pool depth = 1.23 Required surface area = SA/DA / 100 x Drainage area=0.15 Ac 6353.918 21279 Surface area of pond as designed at permanent pool elevation = 0.4522 Ac Water Quality Requirements Using the runoff volume calculations in the "Simple Method" : Rv = 0.05 + 0.9 (I) Rv=0.438733089 in/in Volume = (Design rainfall) (Rv) (Drainage Area) Volume = 1" rainfall x Rv in/in x 1/12 in/ft x Da = or 0.43 ac-ft This volume must be released over a period of 2-5 days 18886 ft^3 As designed pond areas: Elevation Forebay Forebay FB Volume FB Volume Pond Pond Pond Volume Pond Volume Storage Volumes (msl)(sf)(ac)(cu-ft)(ac-ft)(sf)(ac)(cu-ft)(ac-ft) Cumulative 555 1059 0.0243 0 0.0000 9260 0.2126 0 0 Elevation Area(ac)Volume(ac-ft)556 1350 0.0310 1200 0.0276 10226 0.2348 9729 0.2234 559.5 0.408057851 0.00 557 1677 0.0385 2709 0.0622 11219 0.2576 20437 0.4692 560 0.4521809 0.21 558 2038 0.0468 4562 0.1047 12240 0.2810 32151 0.7381 561 0.497589532 0.69 559 2433 0.0559 6792 0.1559 13289 0.3051 44899 1.0307 562 0.544306703 1.21 559.5 2862 0.0657 8113 0.1863 14913 0.3424 51939 1.1924 563 0.592355372 1.78 560 0 0.0000 8590 0.1972 19697 0.4522 60555 1.3902 564 0.641689624 2.39 561 0 0.0000 8590 0.1972 21675 0.4976 81213 1.8644 562 0 0.0000 8590 0.1972 23710 0.5443 103875 2.3846 Forbay Volume (CF)8113 563 0 0.0000 8590 0.1972 25803 0.5924 128599 2.9522 Total Volume (CF)51939 564 27952 0.6417 155443 3.5685 Forebay % of Pond 15.62%565 30158 0.6923 184462 4.2347 566 32420 0.7443 215713 4.9521 Interpolation to find elevation at which required storage volume is reached Storage Storage Storage Elev volume area (ac.)area (sf.) 560 0.21 0.45 19697 Elev 0.43 0.47 20610 561 0.69 0.50 21675 Elev=560.46 Storage elevation = 0.96 feet above permanent pool elevation Assume 2 day minimum Determination of Orifice size for 2-5 day drawdown a=2A x sqrt(h)*Falling head equation Ct(sqrt(2g)) a=Area of orifice a sqft t=Time to drawdown 259200 sec A=Average pond surface area 20153 sf (3 day drawdown) g=Gravity constant 32.2 ft/s^2 h=Change in head 0.96 ft C=Orifice constant 0.6 Maximum orifice area =0.03 sqft Maximum orifice size =2.41 inches Selected orifice size =2.30 inches WATER QUALITY BASIN CALCULATIONS - WETPOND 2 Permanent Pond Storage Volume Millbridge Phase 7D PROJECT NO.20170280.00.CL BY:CJH DATE:05/10/18 T.M:WGB REVISED:07/13/18 P.M.:WGB FLOATATION CALCULATIONS - BMP #2 CONSTRAINTS: Weight of Concrete 150 lbs/ft^3 Weight of Water 62.4 lbs/ft^4 Safety Factor (S.F.)1.1 Height of structure 7.4 ft Structure thickness 6 in Outlet box width (interior)4.5 ft Weight of outlet structure(Ws) Ws = (width + thickness x 4) x thickness x height x weight of concrete =11100 lbs Weight of water displaced Ww = ((width + 2 x thickness)^2) x Weight of water x height = 13968.24 lbs Size Base Wb = ( S.F. x Ww) - Ws =4405 lbs Footing size Required concrete = Wb / (Weight of concrete-weight of water) =50 cuft *Assume base is 6" wider than exterior box width and minimum thickness is 8". Thickness of base =1.40 ft Length =6.50 ft Width =6.50 ft Thickness =1.40 ft Millbridge Phase 7D PROJECT NO.:20170280.00.CL BY:CJH DATE:05/10/18 T.M:WGB REVISED:07/13/18 P.M.:WGB ANTI-SEEP COLLARS BMP #2 42 in 126 ft 5 Anti-Seep Collar Sizes Length:7.54 ft Width:7.54 ft Note: Anti-seep collars are centered on pipe PRELIMINARY CALCULATIONS Pipe Diameter Length of Covered Pipe Number of Collars Millbridge Phase 7D PROJECT NO.20170280.00.CL BY:CJH DATE:05/10/18 T.M:WGB REVISED:07/13/18 P.M.:WGB PRELIMINARY CALCULATIONS DRAINAGE AREA #1 - TIME OF CONCENTRATION PRE-DEVELOPMENT Pre-development Type:Sheet Flow Flow Phase One Length:100 ft "N" Value:0.4 from tables Slope:0.045 ft/ft Computed 'T'15.7 Minutes Pre-development Type: Shallow Concentrated Flow Flow Phase Two Surface:u Length:468 ft u=unpaved Slope:0.053 ft/ft p=paved Velocity:3.7 ft/sec Computed 'T':0.035 hr, or 2.1 Minutes Pre-development Type:Open Channel Flow Flow Phase Three Length:1100 ft Velocity:5 ft/sec (assumed) Computed 'T':0.061 hr, or 3.7 Minutes Pre-development Time of Concentration Total Flow Total T c =21.5 Minutes Summary Lag Time T LAG =12.9 Minutes T LAG =0.6 x T c T LAG =0.22 Hours Millbridge Phase 7D PROJECT NO.170280.00.CL BY:CJH DATE:05/10/18 T.M:WGB REVISED: P.M.:WGB PRELIMINARY CALCULATIONS DRAINAGE AREA #1 BYPASS - TIME OF CONCENTRATION POST DEVELOPMENT Pre-development Type:Sheet Flow Flow Phase One Length:100 ft "N" Value:0.4 from tables Slope:0.08 ft/ft Computed 'T'12.5 Minutes Pre-development Type: Shallow Concentrated Flow Flow Phase Two Surface:u Length:103 ft u=unpaved Slope:0.117 ft/ft p=paved Velocity:5.51 ft/sec Computed 'T':0.005 hr, or 0.3 Minutes Pre-development Type:Open Channel Flow Flow Phase Three Length:883 ft Velocity:5 ft/sec (assumed) Computed 'T':0.049 hr, or 2.9 Minutes Pre-development Time of Concentration Total Flow Total T c =15.7 Minutes Summary Lag Time T LAG =9.4 Minutes T LAG =0.6 x T c T LAG =0.16 Hours Millbridge Phase 7D PROJECT NO.170280.00.CL BY:CJH DATE:05/10/18 T.M:WGB REVISED:07/13/18 P.M.:WGB PRELIMINARY CALCULATIONS DRAINAGE AREA #2 - TIME OF CONCENTRATION PRE-DEVELOPMENT Pre-development Type:Sheet Flow Flow Phase One Length:100 ft "N" Value:0.4 from tables Slope:0.035 ft/ft Computed 'T'17.4 Minutes Pre-development Type: Shallow Concentrated Flow Flow Phase Two Surface:u Length:135 ft u=unpaved Slope:0.067 ft/ft p=paved Velocity:4.17 ft/sec Computed 'T':0.009 hr, or 0.54 Minutes Pre-development Type:Open Channel Flow Flow Phase Three Length:1592 ft Velocity:5 ft/sec (assumed) Computed 'T':0.088 hr, or 5.3 Minutes Pre-development Time of Concentration Total Flow Total T c =23.2 Minutes Summary Lag Time T LAG =13.9 Minutes T LAG =0.6 x T c T LAG =0.23 Hours Millbridge Phase 7D PROJECT NO.20170280.00.CL BY:CJH DATE:05/10/18 T.M:WGB REVISED:07/13/18 P.M.:WGB PRELIMINARY CALCULATIONS DRAINAGE AREA #2 BYPASS - TIME OF CONCENTRATION POST-DEVELOPMENT Post-development Type:Sheet Flow Flow Phase One Length:100 ft "N" Value:0.4 from tables Slope:0.085 ft/ft Computed 'T'12.2 Minutes Post-development Type: Shallow Concentrated Flow Flow Phase Two Surface:u Length:104 ft u=unpaved Slope:0.154 ft/ft p=paved Velocity:6.33 ft/sec Computed 'T':0.005 hr, or 0.3 Minutes Post-development Type:Open Channel Flow Flow Phase Three Length:497 ft Velocity:5 ft/sec (assumed) Computed 'T':0.028 hr, or 1.7 Minutes Post-development Time of Concentration Total Flow Total T c =14.2 Minutes Summary Lag Time T LAG =8.5 Minutes T LAG =0.6 x T c T LAG =0.14 Hours Millbridge Phase 7D PROJECT NO.170280.00.CL BY:CJH DATE:05/10/18 T.M:WGB REVISED:07/13/18 P.M.:WGB PRELIMINARY CALCULATIONS BASIN #1 TIME OF CONCENTRATION PRE-DEVELOPMENT Post-development Type:Sheet Flow Flow Phase One Length:100 ft "N" Value:0.4 from tables Slope:0.04 ft/ft Computed 'T'16.5 Minutes Post-development Type: Shallow Concentrated Flow Flow Phase Two Surface:u Length:411 ft u=unpaved Slope:0.061 ft/ft p=paved Velocity:3.98 ft/sec Computed 'T':0.029 hr, or 1.74 Minutes Post-development Type:Open Channel Flow Flow Phase Three Length:4517 ft Velocity:5 ft/sec (assumed) Computed 'T':0.251 hr, or 15.1 Minutes Post-development Time of Concentration Total Flow Total T c =33.3 Minutes Summary Lag Time T LAG =20.0 Minutes T LAG =0.6 x T c T LAG =0.33 Hours Millbridge Phase 7D PROJECT NO.:20170280.00.CL BY:CJH DATE:05/10/18 T.M.:WGB REVISED:07/13/18 P.M.:WGB FES #Do (Pipe ø)Tw Tw<0.5ø Q V d50 La Apron width Apron width Max Stone Apron (ft)(ft)(cfs)(ft/s)(ft)(ft)at pipe (ft)at outlet (ft)ø (ft)Thickness(ft) FES-BMP#1 3 0 OKAY 4.27 5 0.25 20 9 23 0.38 0.56 FES-BMP#2 3 0 OKAY 6.22 5 0.25 22 9 25 0.38 0.56 Design of outlet protection from a round pipe flowing full, minimum tailwater condition Tw<0.5 diameter RIPRAP APRON DESIGN (NCDENR METHOD) Millbridge Phase 7D PROJECT NO.:20170280.00.CL BY:CJH DATE:05/10/18 T.M.:WGB REVISED:07/13/18 P.M.:WGB FES #Do(Pipe ø)Tw Tw≥0.5ø Q V d50 La Apron width Apron width Max Stone Apron (ft)(ft)(cfs)(ft/s)(ft)(ft)at pipe (ft)at outlet (ft)ø (ft)Thickness(ft) FES-100 3 3 OKAY 28.06 3.97 0.25 15 9 9 0.38 0.56 FES-134 1.25 1.25 OKAY 2.50 2.04 0.25 8 3.75 4 0.38 0.56 FES-200 3 3.5 OKAY 32.16 3.34 0.25 15 9 9 0.38 0.56 RIPRAP APRON DESIGN (NCDENR METHOD) Design of outlet protection from a round pipe flowing full, maximum tailwater condition Tw≥0.5 diameter