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HomeMy WebLinkAboutWPO201300032 Calculations 2014-09-13 Short Version BMP Computations For Worksheets 2 -6 Albemarle County Water Protection Ordinance: Modified Simple Method Plan: Cascadia Subdivision Water Resources Area: Development Area Preparer: Dominion Engineering Date: 9/13/2014 Project Drainage Area Designation GRASS SWALE#1 L storm pollutant export in pounds, L=[P(Pj)Rv/12][C(A)2.72] Rv mean runoff coefficient, Rv=0.05+0.009(1) Pj small storm correction factor,0.9 1 percent imperviousness P annual precipitation,43"in Albemarle A project area in acres in subject drainage area, A= 0.54 C pollutant concentration, mg/I or ppm target phosphorus f factor applied to RR ✓ required treatment volume in cy,0.5"over imperv. area= A(I)43560(0.5/12)/27 RR required removal , L(post)-f x L(pre) %RR removal efficiency, RR100/L(post) Impervious Cover Computation(values in feet&square feet) Item pre-development Area post-development Area -I! Roads&Parking Length Width subtotal Length Width subtotal I 0 0 0 0! 0 0 ! 0 0 0 0 0 0 0 0 0{ 0 l - 0 0 0 0!. Driveways - Length Width_ no. subtotal Length Width no. subtotal and walks 0 0 0 0 0 0 0 0 1 _ . 0 0 0 0 0 0 0 0 [:1-11 0 0 0 0 0 0 0 1 2 3 4 1 2 3- 4 0 0 I I 0 ,Gravel areas Area Area I x0.70= 0 0 x0.70= 0 Structures Area no. subtotal Area no. subtotal 0 0 0< 0 0 2880 0 0 0 0 0 0 0 0 0 0 j 0 2880 Actively-grazed pasture& Area Area ards and cultivated turf 962676 x 0.08= 0; 120001x 0.08= 960 Active crop land Area Area x0.25= 0' I x0.25= 0 Other Impervious Areas(Future Development"Town"Phase) l Ol 0 Impervious Cover 04 l(pre) l(post) Rv(post) V 0.20 5.9 New Development(For Development Areas,existing impervious cover<=20%) L C f I(pre)* Rv(pre) L(pre) L(post) RR %RR Area Type 0.70 1.00 20% 0.23 0.76 0.65 -0.11 -17% Development Area 0.35 1.00 0% 0.05 0.08 0.33 0.24 75% Drinking Water Watersheds 0.40 1.00 1% 0.06 0.11 0.37 0.26 70% Other Rural Land *min.values Redevelopment(For Development Areas,existing impervious cover>20%) L---( C f I(pre)* Rv(pre) L(pre) L(post) RR %RR Area Type • 0.70 0.90 20% 0.237- 0.76 0.65 -0.03 ,.,. -5% Development Area 0.35 0.85 0% 0.05 0.08 0.33 0.26 8% Drinking Water Watersheds 0.40 0.85 1% 0.06 0.11 0.37 0.28 '75% Other Rural Land ro Interim Manual, Page 70 rev. 16 Feb 1998 GEB Short Version BMP Computations For Worksheets 2 -6 Albemarle County Water Protection Ordinance: Modified Simple Method Plan: Cascadia Subdivision Water Resources Area: Development Area Preparer: Dominion Engineering Date: 9/13/2014 Project Drainage Area Designation GRASS SWALE#2 L storm pollutant export in pounds, L=[P(Pj)Rv/12][C(A)2.72] Rv mean runoff coefficient, Rv=0.05+0.009(1) Pj small storm correction factor,0.9 percent imperviousness P annual precipitation,43"in Albemarle A project area in acres in subject drainage area, A= 1.34 C pollutant concentration,mg/I or ppm target phosphorus f factor applied to RR ✓ required treatment volume in cy,0.5"over imperv. area= A(I)43560(0.5/12)/27 RR required removal , L(post)-f x L(pre) %RR removal efficiency, RR100/L(post) Impervious Cover Computation(values in feet&square feet) Item pre-development Area post-development Area Roads&Parking Length Width subtotal Length Width subtotal 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ._ 0 Driveways Length Width no. subtotal Length Width no. subtotal and walks i 0 0 0 0 0 0 0 0: 0 0 0 0 0 0 0 0 0 0 0,_ 0, 0 0; 0 - 1 2 3 4 1 2� 3 4 0 0 I - 0 Gravel areas Area 1 Area • x0.70= 0 0 x0.70= 0 Structures Area no. subtotal Area no._ subtotal 0 0 _ 0 0 6350 0 0 0' 0 0 0 0 0 0, _ ��, 0' 6350 Actively-grazed pasture& Area Area _ yards and cultivated turf 962676,x 0.08= 0 33000,x 0.08= 2640 Active crop land Area Area lx 0.25= 0 i x0.25= 0 Other Impervious Areas(Future Development"Town"Phase) o 0 Impervious Cover 04 l(pre) ((post) Rv(post) V 0.19 13.9 New Development(For Development Areas,existing impervious cover<=20%) C f I(pre)* Rv(pre) L(pre) L(post) RR %RR Area Type 0.70 1.00 20% 0.23 1.89 1.55 -0.34 Development Area 0.35 1.00 0% 0.05 0.21 0.78 0.57 ' Drinking Water Watersheds 0.40 1.00 1% 0.06 0.28 0.89 0.61 ''' Other Rural Land ®___.___ *min.values Redevelopment(For Development Areas,existing impervious cover>20%) C f I(pre)* Rv(pre) L(pre) I L(post) RR %RR Area Type 0.70 0.90 20% 0.23 1.89 1.55 -0.15 .;,-10% Development Area 0.35 0.85 0% 0.05 0.21 0.78 0.60 % Drinking Water Watersheds 0.40 0.85 1% 0.06 0.28 0.89 0.65 Other Rural Land Interim Manual, Page 70 rev. 16 Feb 1998 GEB