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Home My WebLink AboutSW6210602_Design Calculations_20210901Per ject Data Revisions Project # 2018-002 No Date Description Project Name Kathryn's Retreat 1 8/30/2021 NCDENR Revisions Date 8/31/2021 Wet Detention Pond #3 1) Calculate percent impervious draining to pond. Impervious Drainage Area = 6.61 acres Road/Parking 0.42 ac / sf 1& Impervious Area = 3.13 acres Sidewalk 0.13 ac / sf 5E Lots 2.09 ac / sf 911 % Imperv. = 47.42 % Driveways 0.19 ac / sf 8C Ex. Street 0.31 ac / sf 13, 2) Find the volume of the 1.0" storm using the "Simple Method" (Schueler 1987): 1 The volume of the 1" storm must be detained in the pond above the permanent pool and must be drawn down over a period of two to five days. Rv = 0.05 + 0.009 (I) Rv = runoff coefficient (ratio of runoff to rainfall in inches) I = percent impervious Rv = 0.477 in/in Volume of 1.0" storm (must exceed 3630 cf): WQv = 3630 x RD x Rv X A WQv = volume of runoof gnerated from design storm Ro = Design storm = 1 inch Rv= Runoff Coef= 0.477 inch/inch A= Drainage Area = 6.61 acre WQrr= 11,440 ft 3) Determine Surface Area to Drainage Area Ratio & Surface of Permanent Pool (Water Quality) Table 1: Piedmont and Mountain SAIDA Table (Adapted from Driscoll, 1986) 10% 0s1 0.43 0.37 0.30 0.27 0.25 20% 0.84 IN 0.61 0.51 0.44 0.40 30% 1.17 381 0.84 072 061 0.56 40% 1.51 1.22 1.n9 0-91 n.79 071 50% 1.70 1 47 1.31 1.13 0.95 087 60% 2.09 1.73 1.40 191 1.12 1.03 70% 2.51 2.04 180 1-66 1.34 1.17 W% 2.92 2.36 2.07 1.82 1.62 14C 90% 3.25 2.64 2.31 2.04 1.84 1.59 Desired Depth = 3 ft Imp. % Ratio Lower Limit = 40.00 1.51 Upper Limit = 50.00 1.79 Minimum SA Required = 4,946 ft 2 Acutal = 47.42 1.72 Project Data Revisions Project #k 2018-002 No Date Description Project Name Kathryn's Retreat 1 8/30/2021 NCDENR Revisions Date 8/31/2021 Wet Detention Pond 03 Total Main Pool Stage Level Elevation (ft) Area (so StageVolume (co Cumulative Storage (co 0 252.5 2412 - - 0.5 253 2658 1,268 1,268 1.5 254 3186 2,922 4,190 2.5 255 3762 3,474 7,664 3.5 256 4385 4,074 11,737 4.5 257 5057 4,721 16,458 5 257.5 5412 2,617 19,075 5.5 258 6536 2,987 22,062 Total Forebay Volume Stage Level Elevation (ft) Area (sf) Stage Volume (co Uumulative Storage (co 0 255 389 - - 1 256 804 597 597 2 257 1414 1,109 1,706 3 258 2030 1,722 3,428 Temporary Pool Volume Stage Level Elevation (ft) Area (so rage olume (co Cumulative Storage (co 0 258 8566 - - 0.5 258.5 10052 4,655 4,655 1 259 10995 5,262 9,916 1.25 259.25 12018 2,877 12,793 Average Depth Calculation: VPP—Vsheif D.O= Abottomofsheif Where: D„,, = Average depth in feet V- = Total volume of permanent pod (feet') V,n,n = Volume over the shelf only (feel') — see below A.—.~ = Area of wet pond at the bottom of the shelf (feet') V.—= 0.5' Depth_a.,r—r' Perimeterp,,,,, * Wldth„e,.,,,� , Where: D,,,,,,,,,,,,,,, = Depth of water at the deep side of the shelf as measured at permanent pool (feet) Petimeterp.,.,, - = Penmeter of permanent pool at the bottom of the shelf (feet) wAth,,,o,,--p,n a,na = Width from the deep side to the dry side of the shelf as measured at permanent pool (feet) Vsheif = 257 cf Dma,—shelf = 0.5 ft Pei meter = 343 ft Width = 3 ft Vpp = 22062 cf Abe .f sh0f = 5412 sf D®g = 4.03 ft Per ject Data Revisions oject # 2018-002 No Date Description oject Name E Kathryn's Retreat te 8/31/2021 Wet Detention Pond #3 1) Input Orifice and Spillway Information Orifice Diameter = 1.50 in Office Coefficent = 0.60 Volume Provided = 12,793 cf Number Orifices = 1.00 Inv Office = 258.00 Inv Primary Spillway = 259.25 2) Calculate Area, Head and Discharge Flow Q = Co A 2gh g= 32.2 ft/s2 h= use h/3 to simulate decreasing head A= cross section area of orifice Q= discharge cfs Q= discharge cfs Orfice Cross Section Area = 0.012 sf Average head = 0.417 feet Discharge = 0.038 cfs 3) Calculate Drawn Down Time Time= Volume/Flowrate Drawdown Time = 3.88 days Check = OK Per jest Data Revisions Project # 2018-002 No Date Description Project Name Kathryn's Retreat Date 8/31/2021 Wet Detention Pond #3 DETERMINE BOUYANT FORCES Volume = Depth * Area Depth = Riser Top Elev - Riser Bottom Elev Riser Top Elev. _ Bottom of Riser EIev. _ Riser Wall Thickness = Diameter of Manhole = Area = pi * diameter^2 / 4 Area (@ outside diam)= Volume = times wt of water Bouyant Force = 2. DETERMINE COUNTERWEIGHT REQUIREMENTS Riser Top Top Elevation = Thickness = Outside Diam = Volume = Riser Walls Inside Diam = Wall Thickness = Height = Volume = Riser Bottom - Extended Base Bottom Elevation = Diameter = Thickness= Volume = Total Volume = x unit weight of reinforced conc. Counterweight = Factor of Safety Provided (FSP) _ Additional Concrete Required (Y/N) _ Amount of Concrete Required = 3. ADD CONCRETE BELOW INVERT IN RISER Area of Riser (@inside diam) _ Volume Required = Calculated Depth of Concrete = Actual Depth Used = Backcheck Volume = Backcheck Weight of Added Conc (150-62.4) _ Factor of Safety = Factor of Safety > 1.30 (Y/N) _ 261 ft 257.50 ft 6.00 in 4.00 ft 19.64 sf 68.72 cf 62.4 Ibs/cf 4,288 Ibs 261 0 inches 5.00 ft 0.00 cf 4.00 ft 6.00 in 3.50 ft 24.74 cf 257.50 0.00 ft 6.00 in Summary 0.00 cf Add minimum 24 inches below invert of pond 24.74 cf 150 Ibs/cf 3,711 Ibs 0.87 Y 1,864 Ibs 12.57 sf 21.28 cf 1.69 ft 24 in 25.13 cf 2202 lb 1.38 Y