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Home My WebLink AboutWPO201800079 Calculations WPO VSMP 2018-11-30 Stormwater Pollution Prevention Plan Water Quality Calculations VA DCR SWPP LOC(SF) LOC(AC) Performance Based Water Quality Calculation Procedure LEFT 5004.00 0.11 RIGHT 3899.00 0.09 Project: OTV-Block Entry Feature TOTAL 8903.00 0.20 Locality: Albemarle Co. Date: 11/30/2018 10:53 Worksheet No.1 Step 1 Determine the applicable area(A)and the post-developed impervious cover(Ipost). Applicable areal 8903 s.f.or 0.204 acres Post-development impervious cover: structures = 170 s.f.or 0.004 acres parking lot = s.f.or 0.000 acres roadway = s.f.or 0.000 acres driveways&swlk= s.f.or 0.000 acres s.f.or 0.000 acres s.f.or 0.000 acres Total = 170 s.f.or 0.004 acres =(total post-development impervious cover-A)x 100= 2% Step 2 Determine the average land cover condition(t„ate1,hed)or the existing impervious cover(texigt;s). Average land cover condition(1,,,,,.,,,„,,): If the locality has determined land cover conditions for individual watersheds within its jurisdiction,use the watershed specific value determined by the locality as(watershed. ,.a,ershui 0% o Otherwise,use the Chesapeake Bay default value: l.,atershea 16% Later.,hed= 16", Existing impervious cover Determine the existing impervious cover of the development site if present. Pre-development impervious cover: structures = I 0 s.f.or 0.000 acres parking lot = 0 s.f.or 0.000 acres roadway = 0 s.f.or 0.000 acres other 0 s.f.or 0.000 acres Concrete 0 s.f.or 0.000 acres 0 s.f.or 0.000 acres Total = 0 s.f.or 0.000 acres leC1,,hR=(total existing impervious cover=A)x 100= 0% * The area should be the same as used in STEP 1. I Prepared by Riki VanNiekerk 11/30/2018 Page 1 Stormwater Pollution Prevention Plan Water Quality Calculations Step 3 Determine the appropriate development situation. The site information determined in STEP 1 and STEP 2 provide enough information to determine the appropriate development situation under which the performance criteria will apply. Check(_)the appropriate development situation as follows: yet' Situation I: This consists of land development where the existing percent impervious cover(le„1,;,,a)is less than or equal to the average land cover condition(Iwatershed)and the proposed improvements will create a total percent impervious cover(Ipost)which is less than or equal to the average land cover condition(Iwatershed). True (post 2% <_1„„„„,,d 16% No Situation 2: This consists of land development where the existing percent impervious cover(Iex;,ting)is less than or equal to the average land cover condition(Iwatershed)and the proposed improvements will create a total percent impervious cover(Ipost)which is greater than the average land cover condition(Iwatershed). True 'existing 0% <_Iwatershed 16% and False Ipas, 2% >Iwatershed 16% No Situation 3: This consists of land development where the existing percent impervious cover(Ier;,,;,,R)is greater than the average land cover condition(Iwatershed). False lexhtrwg 0% >Iwatershed 16% No Situation 4: This consists of land development where the existing percent impervious cover(lex;,t;,,a)is served by an existing stormwater management BMP(s)that addresses water quality. If the proposed development meets the criteria for development Situation I, than the low density development is considered to be the BMP and no pollutant removal is required. The calculation procedure for Situation I stops here. If the proposed development meets the criteria for development Situations 2,3,or 4,then proceed to STEP 4 on the appropriate worksheet. Prepared by Riki VanNiekerk 11/30/2018 Page 2 Stormwater Pollution Prevention Plan Water Quality Calculations VA DCR SWPP Performance Based Water Quality Calculation Procedure Project: OTV-Block Entry Feature Locality: Albemarle Co. Date: 30-Nov-18 Situation Worksheet No.2 SITUATION 2 True 'misting 0° 'watershed 16% and False 'post 2% 'watershed I 16% SITUATION 3 False 'existing 0% >'watershed 16% SITl CIION.4 False I.t; ; CURRENTL Y IN 3 PLACE? NO Summary of Situation 2 criteria:from calculation procedure S"I I:P I thru STEP 3,Worksheet 1: Area= 0.20 ac. I post= 2% I watershed= 16% I existing= 0% Step 4 Determine the relative pre-development pollutant load(Lore). Based on Existing Impervious Cover 1 Lpre(existing)=[0.05+(0.009 x Ie15ung)]x A x 2.28 (Equation 5-16) where: Lproexisting) =relative pre-development total phosphorous load(pounds per year) 'existing = Existing site impervious cover(percent expressed in whole numbers) A = applicable area(acres) Lpre(exigingl=[0.05+(0.009 x 0% )] x 0.204385 x 2.28 = 0.02 pounds per year 2 Lprelwaterahed)=[0.05+(0.009 x 1H.atershed)]x A x 2.28 (Equation 5-16) where: Lpwatershed) relative pre-development total phosphorous load(pounds per year) Lao-shed = average land cover condition for specific watershed or locality or the Chesapeake Bay default value of 16%(percent expressed in whole numbers) A = applicable area(acres) Lprelwatershed)=[0.05+(0.009 x 16% )] x 0.204385 x 2.28 = 0.09 pounds per year iStep 5 Determine the relative post-development pollutant load(Limo). Lpost = [0.05+(0.009 x'ppst)]x A x 2.28 (Equation 5-21) where: Lpost = relative post-development total phosphorous load(pounds per year) 1post = post-development impervious cover(percent expressed in whole numbers) A = applicable area(acres) Lpost=[0.05+(0.009 x 2% )] x 0.204385 x 2.28 0.03 pounds per year Step 6 Determine the relative pollutant removal requirement(RR). Prepared by Riki VanNiekerk 11/30/2018 Page 1 Stormwater Pollution Prevention Plan Water Quality Calculations RR = Lpwt-(0.9 X Lpre(esis(ng)) RR = 0.0 - 0.0 RR= 0.(31 pounds per year or RR = Lposl-Lprc(eatershed) RR = 0.0 - 0.1 RR= -0.06 pounds per year Use RR= -0.06 (lesser value) Prepared by Riki VanNiekerk 11/30/2018 Page 2 Stormwater Pollution Prevention Plan Water Quality Calculations Step 7 Identify best management practice(BMP)for the site. 1. Determine the required pollutant removal efficiency for the site: EFF = (RR+L )x 100 (Equation 5-22) where: EFF = required pollutant removal efficiency RR = pollutant removal requirement(pounds per year) Lpos1 = relative post-development total phosphorous load,Equation 5-21(pounds per year) EFF = -0.06 / 0.03 x 100= -189% 2. Select BMP(s)from Table 5-14 and locate on the site: Drainage Impervious Type Efficiency Area(s.f.) Area(s f) Description: BMP 1: BMP A BMP 2: BMP B BMP 3: BMP C BMP 4: BMP D BMP 5: BMP E 3. Determine the pollutant load entering the proposed BMP(s): Lamp = [0.05+(0.009 x IBMp)]x AproPBMP x 2.28 (Equation 5-23) where: Lamp = relative post-development total phosphorous load entering proposed BMP(pounds per year) IBMP = post-development percent impervious cover to proposed BMP (percent expressed in whole numbers) ApropBMP = drainage area to proposed BMP(acres) LBMP1=[0.05+(0.009 X 0% )] X 0.00 X 2.28 = 0.00 pounds per year LBMP2=[0.05+(0.009 x 0% )] x 0.00 x 2.28 0.00 pounds per year LBMP3=[0.05+(0.009 x 0% )] x 0.00 x 2.28 0.00 pounds per year LBMP4=[0.05+(0.009 x 0% )] x 0.00 x 2.28 0.00 pounds per year Lamps=[0.05+(0.009 X 0% )] x 0.00 X 2.28 0.00 pounds per year 4. Calculate the pollutant load removed by the proposed BMP(s): Lremoved = EffBMP x LBMP (Equation 5-24) where:Lrerr,o,„, = post-development total phosphorous load removed by proposed BMP(pounds per year) EffBMP = pollutant removal efficiency of BMP(expressed in decimal form) Lamp = relative post-development total phosphorous load entering proposed BMP,Equation 5-23(pounds per year) Lremoved/BMP 0.00 Pounds per year removed Lremoved/BM P. 0.00 Pounds per year removed Lremoved/Bi P. 0.00 Pounds per year removed Lremoved/BNIP 0.00 Pounds per year removed Prepared by Riki VanNiekerk 11/30/2018 Page 3 Stormwater Pollution Prevention Plan Water Quality Calculations 'removed/BMP 0.00 Pounds per year removed 5. Calculate the total pollutant load removed by the BMP(s): Lremoved/total = Lremoved/BMPI+L,,rtuved/BMP2+Lremoved/BMP3+•••(Equation 5-25) where: L ,,, =total pollutant load removed by proposed BMPs(pounds per year) Lremo,edrBMPI= pollutant load removed by proposed BMP No. 1,Equation 5-24 Lremoved/BMP2= pollutant load removed by proposed BMP No.2,Equation 5-24 Lremoved/BMP3= pollutant load removed by proposed BMP No.3,Equation 5-24 Lremoved/total 0.00 Total Pounds per year removed 6. Verify compliance: Lremoved/total>RR Lrcmoved/total= 0.00 Total Pounds per year removed Req'd Removal= -0.06 Total Pounds per year removed OK,Removal Rate Exceeded Prepared by Riki VanNiekerk 11/30/2018 Page 4 Stormwater Pollution Prevention Plan Water Quality Calculations Technology-based Approach Range of Impervious Cover Appropriate BMP Technology u • Vegetated Fitter Strip 16-21% • Grassed Swale • Constructed Wetlands 22-37% no> • Extended Detention • Retention Basin I • Bioretention Practices 38-66% ppc > • ED-Enhanced • Retention Basir II • Infiltration. • Sand F her 67-104% SDE • Infiltration(2WOV) • Retention Basin III WATER QUANTITY&QUALITY S.A.Req'd Project WQV(1"per imp.area)= 14.2 C.F. 5.00% 0.0 S.F. Project WQV(1/2"per imp.area). 7.1 C.F. 2.50% 0.0 S.F. BMP Efficiencies Vegetated Filter Strip 10% Grassed Swale 15% Constructed Wetlands 30% Extended Detention(2xWQV) 35% Retention Basin(3xWQV) 40% Bioretention Basin 50% Bioretention Filter 50% Extend Det.Enhanced 50% Retention Basin II(4xWQV) 50% Infiltration(1xWQV) 50% Sand Filter 65% Infiltration(2xWQV) 65% Retention Basin III(4xWQV+Bench) 65% Prepared by Riki VanNiekerk 11/30/2018 Page 5