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Home My WebLink AboutWPO200400100 Calculations 2004-08-19 Short Version BMP Computations For Worksheets 2 -6 Albemarle County Water Protection Ordinance: Modified Simple Method Plan: Longacre Farm Water Resources Area: Rural Area Preparer: NP Date: 19-Aug-04 Project Drainage Area Designation Site 0 f't'J v ti jt....X..., C4-P‘0'2 L storm pollutant export in pounds, L=[P(Pj)Rv/12] [C(A)2.72] (,) 1 Rv mean runoff coefficient, Rv=0.05+0.009(1) "u Pj small storm correction factor, 0.9 "i "' �� percent imperviousness I �Y Ntat i P annual precipitation,43" in Albemarle p A project area in acres in subject drainage area, A= 29.96 1-410 �`'�) C pollutant concentration, mg/I or ppm target phosphorus f factor applied to RR /'1 (_. ` tv�Q44 V required treatment volume in cy, 0.5"over imperv.area= A(I)43560(0.5/12)/27 LJ RR required removal , L(post)-f x L(pre) ! , %RR removal efficiency, RR100/L(post) �iD_ i Impervious Cover Computation (values in feet&square feet) O > Item 1q.3/7' pre-development Area post-development Area Roads Length Width subtotal Length Width subtotal 0 ,_, 0 0 0 0 0 L. 0 0 0 L___ 0 0 01 0 0 0 0' } 0 0 0j 16285 Driveways Length Width no. subtotal Length Width no. subtotal II and walks 0 0 0 0 200 8 6 9600 I 0 0 0 0 Ui 0 0. 0 i_ i j 0 0 0 9600' Parking Lots 1 2 3 4 1 2 3 4 01 0 0,1 0 0 Gravel areas Area Area 500 x 0.70= I 350' x 0.70= i 0 Structures Area no. subtotal Area no. subtotal 0 0 0 2000 6 12000 0 0 0 0 0 0 0 0 0 0 0 12000 Actively-grazed pasture& Area Area yards and cultivated turf 0 x 0.08= 0 240000 x 0.08= I 19200 Active crop land Area Area x0.25= 0 x0.25= 0 Other Impervious Areas 0 0 Impervious Cover 0% 4% I(pre) l(post) Rv(post)` V 0.09 88.1 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 42.31 16.44 -25.87 -157% Development Area 0.35 1.00 0% 0.05 4.62 8.22 3.60 44% Drinking Water Watersheds 0.40 1.00 1% 0.06 6.20 9.39 3.19 34% Other Rural Land *min. values Redevelopment(For Development Areas,existing impervious cover>20%) C f I(pre)* Rv(pre) L(pre) L(post) RR %RR Area Type 0.70 0.90 20% 0.23 42.31 16.44 -21.64 -132% Development Area 0.35 0.85 0% 0.05 4.62 8.22 4.29 52% Drinking Water Watersheds 0.40 0.85 1% 0.06 6.20 9.39 4.12 44% Other Rural Land Interim Manual, Page 70 rev. 16 Feb 1998 GEB `f rz, . gtD-Fti..76.z- (� Z•S &ie ee 1 A.n1% Wax AP-EA (6//2-SO s, LONGACRE FARM SUBDIVISION BIO-FILTER DESIGN CALCULATIONS DESIGN INTENT: Design the stone weir to handle the 10 year storm. HYDRAULICS: Area Number C Area(ac) CA 1 0.900 0.180 0.162 2 0.300 3.510 1.053 Weighted Totals: 0.329 3.690 1.215 Time of concentration= 5 min Period Intensity Peak Flow (years) (in/hr) (cfs) 2 5.40 6.56 5 6.35 7.72 10 7.10 8.63 25 8.00 9.72 Stone weir to handle a minimum of 8.63 cfs. DESIGN ASSUMPTIONS: Weir is trapezoidal with the following characteristics: Top Width = 5' Bottom Width = 2' Side Slope = 1.5 Depth = 1' Cross Sectional Area = 3.5 sq. ft. Hydraulic Radius = 0.624 Manning's Coefficient (based on d50) = 0.0352 Slope (assume 1%) = .01 Based on Mannings Equation and Continuity Equation, the capacity of this weir is 10.82 cfs which is greater than the 10 year storm requirement of 8.63 cfs. FINAL DESIGN: Weir is trapezoidal with the following characteristics: Top Width = 5' Bottom Width = 2' Side Slope = 1.5 Depth = 1' • Short Version BMP Computations For Worksheets 2 -6 Albemarle County Water Protection Ordinance: Modified Simple Method Plan: Longacre Farm Water Resources Area: Rural Area Preparer: NP Date: 19-Aug-04 Project Drainage Area Designation Site 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 I percent imperviousness P annual precipitation,43"in Albemarle A project area in acres in subject drainage area, A= 29.96 C pollutant concentration, mg/I or ppm target phosphorus f factor applied to RR V required treatment volume in cy, 0.5"over imperv.area= A(1)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 Length Width subtotal Length Width subtotal 0 0 0 0 0 0 0 0 0 F---- 0 0 0 ___. 1 (.. 0 0 0 0 0 0 0 16285 Driveways Length Width no. subtotal Len•th Width no. subtotal and walks 0 0 0 0 200- 8 6 9600 0 0 0 0 0 0 0 0 0 0111111111111111 0 9600 Parking Lots 1 2 3 4 1 2 3 4 0 0 0 0 0 Gravel areas Area Area I 500 x 0.70= 350 x 0.70= _ I 0 Structures Area no. subtotal Area no. subtotal�� 0 0 0 2000 6 12000 0 0 0 1 1 0 0 0 1 0 0 0 I 1 0 12000 Actively-grazed pasture& Area Area IAyards and cultivated turf 0 x 0.08= 0 240000 x 0.08= 19200 tive crop land Area Area x0.25= 0, x0.25= 0 Other Impervious Areas j n.mm 0 0 Impervious Cover 0% 496, l(pre) I(post) Rv(post) V 0.09 88.1 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 42.31 16.44 -25.87 -157% Development Area 0.35 1.00 0% 0.05 4.62 8.22 3.60 44%- Drinking Water Waters 0.40 1.00 1% 0.06 6.20 9.39 3.19 34% Other Rural Land *min.values Redevelopment(For Development Areas,existing impervious cover>20%) C f I(pre)* Rv(pre) L(pre) L(post) RR %RR Area Type 0.70 0.90 20% 0.23 42.31 16.44 -21.64 -132% Development Area 0.35 0.85 0% 0.05 4.62 8.22 4.29 52% Drinking Water Watersheds 0.40 0.85 1% 0.06 6.20 9.39 4.12 44% Other Rural Land Interim Manual, Page 70 rev. 16 Feb 1998 GEB - /z. 3/v-p'1�7 _ 67 2..C 7o pr i Pt vra,.4s AREA Zy/4-2--c"o s�J LONGACRE FARM SUBDIVISION BIO-FILTER DESIGN CALCULATIONS DESIGN INTENT: Design the stone weir to handle the 10 year storm. HYDRAULICS: Area Number C Area(ac) CA 1 0.900 0.180 0.162 2 0.300 3.510 1.053 Weighted Totals: 0.329 3.690 1.215 Time of concentration= 5 min Period Intensity Peak Flow (years) (in/hr) (cfs) 2 5.40 6.56 5 6.35 7.72 10 7.10 8.63 25 8.00 9.72 Stone weir to handle a minimum of 8.63 cfs. DESIGN ASSUMPTIONS: Weir is trapezoidal with the following characteristics: Top Width = 5' Bottom Width = 2' Side Slope = 1.5 Depth = 1' Cross Sectional Area = 3.5 sq. ft. Hydraulic Radius = 0.624 Manning's Coefficient (based on d50) = 0.0352 Slope (assume PA) = .01 Based on Mannings Equation and Continuity Equation, the capacity of this weir is 10.82 cfs which is greater than the 10 year storm requirement of 8.63 cfs. FINAL DESIGN: Weir is trapezoidal with the following characteristics: Top Width = 5' Bottom Width = 2' Side Slope = 1.5 Depth = 1'