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Home My WebLink AboutWPO201300032 Other 2018-07-24PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 1 Pagel of 3 STEP 1 Determine the applicable area (A) and the post -developed impervious cover (Ipos)• Applicable area (A)* = 1.093 acres Post -development impervious cover: other: structures = acres parking lot = acres roadway = acres acres acres Total — 0.148 acres Ipost = (total post -development impervious cover _ A) x 100 = 13.52 * The area subject to the criteria may vary from locality to locality. Therefore, consult the locality for proper determination of this value. STEP 2 Determine the average land cover condition (lwatmhed) Dr the existing impervious cover (Iexistfng)• Average land cover condition (T,atershed) If the locality has determined land cover conditions for individual watersheds within its jurisdiction, use the watershed specific value determined by the locality as Iwatershed- _ o/O Iwatershed Otherwise, use the Chesapeake Bay default value: % 5D-5 PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 1 Page 2 of 3 Ex1n_g_irrlpervious cover (Texistind: Determine the existing impervious cover of the development site if present. Existing impervious cover: other: structures = acres parking lot = acres roadway = acres acres acres Total = 0.00 acres ,existing = (total existing impervious cover - A*) x 100 = 0.00 % * The area should be the same as used in STEP 1. STET' 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: ,/ Situation 1: This consists of land development where the existing percent impervious cover (Iexisting) is less than or equal to the average land cover condition (Iwatershed) and the proposed improvements will create a total percent impervious cover (Ip°s) which is less than or equal to the average land 13.52 0 Ipost /^ � Iwatershed 1 6 5D-6 PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 1 Page 3 of 3 Situation 2: This consists of land development where the existing percent impervious cover (Iexisting) is less than or equal to the average land cover condition (Iwatershed) and the proposed improvements will create a total percent impervious cover (Ipos) which is greater than the average land cover condition (Iwatershed)- Iexisting 0.00 % < Iwatershed 16 %; and Ipost 13.52 % i Iwatershed 1 6 % Situation 3: This consists of land development where the existing percent impervious cover (Iexisting) is ,greater than the average land cover condition (Iwatershed)• existing 0 �" > Iwatershed % Situation 4: This consists of land development where the existing percent impervious cover (Iexisting) is served by an existing stormwater management BMP(s) that addresses water quaff. If the proposed development meets the criteria for development Situation 1, than the low density development is considered to be the BMP and no pollutant removal is required. The calculation procedure for Situation 1 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. 5D-7 PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5D W_.orksheet2_ :_Situation-2 Page 1 of 4 Summary of Situation 2 criteria: from calculation procedure STEP 1 thru STEP 3, Worksheet 1: Applicable area (A)* = 1.093 acres Ipost = (total post -development impervious cover - A) x 100 = 13.52 % 0 Iwatershed _ o �n Or Iwatershed — 16 �° Iexisting = (total existing impervious cover _ A*) x 100 = 0.00 % Iexisting 0.00 % 5 Iwatershed 16 %; and 0 Ipost 3.52 /n > Iwatershed 16 °/^ STEP 4 Determine the relative pre -development pollutant load (Lprd. Lpre(watershed) = [0.05 + (0.009 x Iwatershed)] x A x 2.28 (Equation 5-16) where: Lpre(watershed) = relative pre -development total phosphorous load (pounds per year) Iwatershed = average land cover condition for specific watershed or locality Dr the Chesapeake Bay default value of 16% (percent expressed in whole numbers) A = applicable area (acres) Lpre(watershed) — [0.05 + (0.009 x 16 )] x 1.093 x 2.28 0.48 pounds per year 5D-9 PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 2 • Situation 2 Page 2 of 4 STEP 5 Determine the relative post -development pollutant load (LP.j. Lpost = [0.05 + (0.009 x 4,,j] x A x 2.28 (Equation 5-21) where: Lpost = relative post -development total phosphorous load (pounds per year) 1post = post -development percent impervious cover (percent expressed in whole numbers) A = applicable area (acres) Lpost = [0.05 + (0.009 x 13.52 )] x 1.093 x 2.28 0.43 pounds per year STEP 6 Determine the relative pollutant removal requirement (RR). RR = Lpost - Lpre(watershed) -0.05 pounds per year STEP 7 Identify best management practice (BMP) for the site. 1. Determine the required pollutant removal efficiency for the site: EFF = ( RR = Lpost ) x 100 (Equation 5-22) where: EFF = required pollutant removal efficiency (percent expressed in whole numbers) RR = pollutant removal requirement (pounds per year) Lpost = relative post -development total phosphorous load (pounds per year) EFF = ( — ) x 100 5D-10 PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 2 _;.situation -2 - Page 3 of 4 2. Select BMP(s) from Table 5-15 and locate on the site: BMP 1: Biofilter #2 BMP 2: BMP 3: 3. Determine the pollutant load entering the proposed BMP(s): LBMP = [0.05 + (0.009 x IBMP)] x A x 2.28 (Equation 5-23) where: LBMP = relative post -development total phosphorous load entering proposed BMP (pounds per year) IBMp = post -development percent impervious cover of BMP drainage area (percent expressed in whole numbers) A = drainage area of proposed BMP (acres) LBMP1 = [0.05 + (0.009 x 13.52 A x 1.093 x 2.28 0.43 pounds per year LBMP2 = [0.05 + (0.009 x )] x x 2.28 = pounds per year LBMP3 = [0.05 + (0.009 x )] x x 2.28 pounds per year 5D-11 PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 2 , Sit ion 2. Page 4 of 4 4. Calculate the pollutant load removed by the proposed BMP(s): Lremoved = EffBw x LBMP (Equation 5-24) where: Lremoved = Post -development pollutant load removed by proposed BMP (pounds per year) EffBw = pollutant removal efficiency of BMP (expressed in decimal form) LBW = relative post -development total phosphorous load entering proposed BMP (pounds per year) LremovedBMPI = 0.65 x 0.43 = 0.28 pounds per year LremovedBMP2 = x = pounds per year Lremoved/IMP3 = x = pounds per year 5. Calculate the total pollutant load removed by the BMP(s): Lremoved/total = Lremoved/BMPl + Lremoved/ MP2 + Lremoved/BMP3 + - .. (Equation 5-25) where: Lremoved/total = total pollutant load removed by proposed BMPs Lremoved/BM21 = pollutant load removed by proposed BMP No. 1 Lremoved/BMP2 = pollutant load removed by proposed BMP No. 2 Lremoved/BM93 = pollutant load removed by proposed BMP No. 3 Lremoved/total = 0.28 + + +. . 0.28 pounds per year 6. Verify compliance: Lremoved/total 2 RR Z Biofilter #2 removes 0.28 pounds per year of phosphorous. The drainage area of Biofilter #2 is less than (I watershed) 16% impervious but we need to go above and beyond to show its removal. There is some science to show that providing whips on the 2:1 slope open space will reduce the amount of pounds per year phosphorous more than the Biofilter #2. The trading nutrient reductions nonpoint source BMP in the Chesapeake Bay Watershed Appendix A gives 0.50 pounds per year of phosphorous for each #rg�pf land conversion going from Pasture to Forest in the James River Basin west of 1-95. While we are not applying to become our own nutrient credit bank, adding 0.56 acres of whips essentially converts the grassed area to forest and we would get 0.28 pounds per year of phosphorous removed.