Virginia; Non-Point Source Pollution Load Guidance Calculation Procedures - City of Suffolk

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Virginia; Non-Point Source Pollution Load Guidance Calculation Procedures - City of Suffolk
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  City of Suffolk, Virginia Department of Planning Non-Point Source Pollution LoadGuidance Calculation Procedures Step 1:Compile Site Specific Data (Note: unless otherwise noted, round to nearest 3 rd decimalplace.) (Note: One (1) acre equals 43,560 square feet.)1.  Area of Lot (excluding wetlands or water) = A CBPA Determination: Area of Lot (in square feet) ÷ 43,560 = A [ ] RPA [ ] RMA ____________ ÷ 43,560 = A [ ] IDA [ ] None A = ________________ acres2.  Current Impervious Area = I a ***(Skip this Equation if the Property Is Currently Undeveloped)*** Impervious area includes all surfaces covered by roofs, concrete, or gravel (including sidewalks anddriveways). This includes all structures regardless of whether they are set on a permanent foundation(including sheds, swimming pools, and gazebos). Do not include wooden open deck structures in thetotal. structures ________________ square feetparking ________________ square feetsidewalks ________________ square feetroadway ________________ square feetother ________________ square feetadditions ________________ square feetaccessory ________________ square feetTotal: ________________ (square feet) ÷ 43,560 = I a I a = ________________ acres3. Proposed Impervious Area (after proposed construction) = I b structures ________________ square feetparking ________________ square feetsidewalks ________________ square feetroadway ________________ square feetother ________________ square feetadditions ________________ square feetaccessory ________________ square feetTotal: ________________ (square feet) ÷ 43,560 = I b I b = ________________ acres Planning Department: 441 Market Street · P.O. Box 1858, Suffolk, Virginia 23439-1858 · Phone (757) 514-4060 · Fax (757) 5 14-4099  www.suffolk.va.usRevised 1 0/20/09    Enter these equations from the previous page: A = ________________ acres I a = ________________ acres I b = ________________ acres3.  Percent of site covered by impervious surfaces before development (Current) = I site a ***(Skip this Equation if the Property Is Currently Undeveloped)*** ( Total I a ÷   A ) x 100 = I site a ( __________ ÷ __________ ) x 100 = I site a ____________ = I site aI site a = __________ % (Note: Round to nearest whole number)4.  Percent of site covered by impervious surfaces after development (Proposed) = I site b (Total I b /  A ) x 100 = I site b ( __________ ÷ __________ ) x 100 = I site b ____________ = I site bI site b = __________ % (Note: Round to nearest whole number) Step 2:Determine the need to continue.16% = I watershed (Chesapeake Bay Watershed Default Impervious Area).If  I site b is less than or equal to 16%, STOP and submit analysis to this point.If  I site b is greater than 16%, CONTINUE ANALYSIS and go to Step 3. Step 3:Use Part A for new development on an unimproved (vacant) parcel oruse Part B for development proposed on a improved parcel (additions to anexisting dwelling or accessory structure) to calculate L pre. Part A Calculate Pre-Development Pollution Load = L pre *** Use only for new development on an unimproved (vacant) parcel *** 40.5 x [ 0.05 + (0.009 x I watershed ) ] x 0.26 x A x 2.72 ÷ 12 = L pre 2.043 x ________ x 2.72 ÷ 12 = L pre = ________ pounds per year of pollutants Planning Department: 441 Market Street · P.O. Box 1858, Suffolk, Virginia 23439-1858 · Phone (757) 514-4060 · Fax (757) 5 14-4099  www.suffolk.va.usRevised 1 0/20/09    Part B Calculate Pre-Development Pollution Load = L pre *** Use only for development proposed on a improved parcel (additions to anexisting dwelling or accessory structure) *** 40.5 x [ 0.05 + (0.009 x I site a ) ] x 0.26 x A x 2.72 ÷ 12 = L pre 40.5 x [ 0.05 + (0.009 x ________ ) ] x 0.26 x ________x 2.72 ÷ 12 =40.5 x [ 0.05 + ________ ] x 0.26 x ________ x 2.72 ÷ 12 =40.5 x [ ________ ] x 0.26 x ________ x 2.72 ÷ 12 =  L pre = ________ pounds per year of pollutants  Step 4:Calculate Post-Development Pollutant Load (L post ) = L post 40.5 x [ 0.05 + (0.009 x I site b ) ] x 0.26 x A x 2.72 ÷ 12 =40.5 x [ 0.05 + (0.009 x ________ ) ] x 0.26 x ________x 2.72 ÷ 12 =40.5 x [ 0.05 + ________ ] x 0.26 x ________ x 2.72 ÷ 12 =40.5 x [ ________ ] x 0.26 x ________ x 2.72 ÷ 12 = L post = ________ pounds per year of pollutants. Step 5:Calculate Pollutant Removal Requirement = RRL post - L pre [from Steps 3 (Part A or Part B) and 4 above] =  ( _________ - __________ ) =  RR = _________ pounds per year  To determine the overall BMP efficiency required (%RR) when selecting BMP options: RR ÷   L post x 100 (from steps 4 and 5 above) = (________ ÷ ________) x 100 =____________ x 100 = ____________ = %RR%RR = __________ % (Note: Round to nearest whole number)Planning Department: 441 Market Street · P.O. Box 1858, Suffolk, Virginia 23439-1858 · Phone (757) 514-4060 · Fax (757) 5 14-4099  www.suffolk.va.usRevised 1 0/20/09    Step 6:Worksheet C: Compliance Select BMP option from the list provided and list them below. Then calculate the pollutant loadremoved from each option. DO NOT LIST BMPs IN SERIES HERE .*** Note: If the proposed site impervious area (I site b ) is 20% or under vegetated BMPs may beused. If the proposed site impervious area (I site b ) over 20% engineered BMPs should be used.*** Removal Fraction of CBPA PollutantEfficiency x Drainage Area x LoadSelected (% ÷ 100) Served (*) Lpost = RemovedOption (expressed in decimal form) (expressed in decimal form) (lbs/yr) (lbs/yr) __________ _____ x__________ _____ x__________ _____ x_____ x _____ = __________ x _____ = __________ x _____ = _____ * The Fraction of CBPA Drainage Area Served is found by dividing the area draining to the proposed  BMP by the total lot area. Both numbers used in the equation must be in the same format (either square feet or acres). If the Load Removed (lbs/yr) is greater than or equal to RR (Pollutant removal requirement fromstep 5), then the selected BMP option is adequate to serve the proposed new development. Step 7:Worksheet D: BMP Design Calculate the proper size of BMP to determine storage volume based on type and required efficiency(typically BMPs are designed to accommodate the first half (1/2) inch of rainfall for the proposedimpervious area. If using engineered BMPs be sure to account for void area created by the use of aggregate material (gravel, etc.).Provide plot plan identifying the location of existing and proposed structures and the proposedBMPs. Additionally, provide all calculations and a cross section identifying the dimensions andmaterials of the proposed BMPs. Planning Department: 441 Market Street · P.O. Box 1858, Suffolk, Virginia 23439-1858 · Phone (757) 514-4060 · Fax (757) 514-4099  www.suffolk.va.usRevised 10/20/09  
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