The URL can be used to link to this page

Home My WebLink AboutWPO200400080 Calculations 2021-05-14PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 1 Page 1 of 3 STEP 1 Determine the applicable area (A) and the post -developed impervious cover (IPoat)- Applicable area (A)* = O•96 acres Post -development impervious cover: other: structures = 6 43l �a 6Fes parking lot -- 916°0 taeres roadway = acres acres Total — 0.37 acres (31,6"�i ); t" sne T 5\ ar POPC40 Soe 1004000w) \� {�paav(g 5oP d�`loont0 S1E� 51� 1post = (total post -development impervious cover A) x 100 = * 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 (Iwate.hed) orr the existing impervious cover Average land cover condition (Iwater�i ed): If the locality has determined land cover conditions for individual watersheds within its jurisdiction, use the watershed specific value determined by the locality as I wnh d. watershed _ o �U Otherwise, use the Chesapeake Bay default value: V lwete.d = 16% 5D-5 APPROVED by the Albemarle County Community Development Department Date 07/16/2021 File WP0200400080 Amendment 1 PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 1 Page 2 of 3 Existing impervious cover (I_: Determine the existing impervious cover of the development site if present. Existing impervious cover structures = acres parking lot = acres roadway = acres other: acres — acres Total — O acres PER P1 4"veo soPaooy000vo Ie _ (total existing impervious cover., A*) x 100 = Q % The area should be the same as used in STEP 1. 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: Situation 1: This consists of land development where the existing percent impervious cover (1e,d,,ng) is less than orequal to the average land cover condition (I,pau,,,ed) and the proposed improvements will create a total percent impervious cover (Ip.) which is less than or equal to the average land cover condition (kamtshed). .. post u �� Iwatershetl 5D-6 PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 1 Page 3 of 3 V Situation 2: This consists of land development where the existing percent impervious cover (Ie) is less than ore ual to the average land cover condition (I,.I,1ed) and the proposed improvements will create a total percent impervious cover (I,,� which is greater than the average land cover condition (I .1.1 ). Iex6 ing Q %° ^igaterahetl 6 %: and Iv-t -g %>Iwetenhed j6 Situation 3: This consists of land development where the existing percent impervious cover (Ie ) is greater than the average land cover condition (I .t.,h ). Ies'sting % > Iwatenhed % Situation 4: This consists of land development where the existing percent impervious cover (I,,sti g) is served by an existing stormwater management BMP(s) that addresses water quality. 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. 513-7 PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 2 : Situation 2 Page I of 4 Summary of Situation 2 criteria: from calculation procedure STEP 1 tiro STEP 3 Worksheet is Applicable area (A)* = O•" acres IPO3, _ (total post -development impervious cover = A) x 100 = + % (watershed — % or 1w.t=Md = 16% Ie,hti g = (total existing impervious cover— A*) x 100 = O % /° ° Ievsting 24atershed �r. °/ °> and Ip., 4'3, 9 % > katerah d 16 % STEP 4 Determine the relative pre -development pollutant load (Lpj. Lpre(watershed) _ [0.05 + (0.009 x Iware,shea)] x A x 2.28 (Equation 5-16) where: Lprecwa,,r�hea> = relative pre -development total phosphorous load (pounds per year) Iwa� nad = 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) Lpre(wahrshed) _ [O.05 + (0.009 x I_ 6 )] x q,$6 x 2.28 0, 3$ 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 (Lpos,). Lpog, _ [0.05 + (0.009 x Ipos,)] x A x 2.28 (Equation 5-21) where: Lpos, = relative post -development total phosphorous load (pounds per year) Ipos, = post -development percent impervious cover (percent expressed in whole numbers) A = applicable area (acres) Lpo , _ [0.05 + (0.009 x k a 1] x p. 8 5 x 2.28 0. pounds per year STEP 6 Determine the relative pollutant removal requirement (RR). post I'pre(warershad) RR = 0-. ?5 r p.3{3 0•47 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 - Lpps,) 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 =( 0,97 - O',. )xtoo 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: ')t BMP 5az Nat[y BMP 3. Determine the pollutant load entering the proposed BMP(s): Law = [0.05 + (0.009 x IB,,)] x A x 2.28 (Equation 5-23) where: - LBNV = relative post -development total phosphorous load entering proposed BMP (pounds per year) IBw = post -development percent impervious cover of BMP drainage area (percent expressed in whole numbers) A = drainage area of proposed BMP (acres) Lam _ [0.05 + (0.009 x a, $ ] x 0 26 x 2.28 O , SS pounds per year Ln�p2 _ [0.05 + (0,009 x )] x x 2.28 pounds per year LB m = [0.05 + (0.009 x )] x x 2.28 pounds per year pvAo`xo �nen`'urw F"iw ,� ?, 63a S a �Pta Agjpe 'ao soP aOo4oc�a�o sK6�+ `�3� P« Mate( USMH 1R/6r. 3,N^°lG v"Vz\`SaesNG lsuYa6LzwES` >cR B x�ZFRS: GtFaa PLANNING P�RPc+SFS� A5Sw1E -tHA1 Tt-k FtAOR AAZA or-- -t FQ 13XOCi iF'neTap� C�hS7,.N tn,-cu �E A Mews Mvrl 01' .•, # 1'i.07. or- -TKlti YMPERksCA R0.EA oN _NS Qj04NA6C SKfY) iF i1�C FaPSc 1.0 '%i0VS of RvAOF' ;5 'TO Gar-t0.6-ATEO." ASS�niNF Tt APfp� E� W� aoO40�3o t9aasz6�c-R vko4�kov_Q T+cE: ^AY:MVA PKcSV'A0A0v.5 ERR 5D-11 AvAx�+o�e rtY me TxhLGw<z�M zs Tt--W PA<mx TKr- f37n�rr.7�d5 FLoaa RfZFA vvavl)p Ctaw4L of 14tE vq, AREA, 4. y'/, ar 1Np. PAEA t O,oy (6G431 tj,C00) = 64t Pk-"' &-iv, OE-4au V-Laka AREA 'x '�Lrz J�ifpovkP) SOP a=40amop SK<-Vt 5S, A UTIL'11ER r-Lwk Or- wr,a QRo47aE� t CO NC Lu S= 0&): IkMOvcd ` RO 2a" 40090 :-N-Ceto o TA u5E A 7PAGZT RQ of Gb" PERFORMANCE -BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 2 : Situation 2 Page 4 of 4 4. Calculate the pollutant load removed by the proposed BMP(s): Lr.mored = EffBxe X L,,,, (Equation 5-24) where: L1 .0 .d = Post -development pollutant load removed by proposed BMP (pounds per year) EffBw = pollutant removal efficiency of BMP (expressed in decimal form) LB, = relative post -development total phosphorous load entering proposed BMP (pounds per year) Lremov.dBMPl = 0I65 X Q, '� = 0155 pounds per year Lr.mavedB"2 = X = pounds per year Lremay.d/BW3 = X = pounds per year 5. Calculate the total pollutant load removed by the BMP(s): Lremoved![otal — LremovedMTI + Lremove"I T2 + Lremoved/BM93 + • • • (Equation 5-25) where: Lmmoved/wN, = total pollutant load removed by proposed BMPs L,m,, /Bi 81 = pollutant load removed by proposed BMP No. 1 LremovedBIR2 = Pollutant load removed by proposed BMP No. 2 4emm,e"NT3 = pollutant load removed by proposed BMP No. 3 LremovedNotal — �• + +„ . = pounds per year 6. Verify compliance: Lremovedttotel ERR 0,55 �t 0-l1-I NOTE» rAlK MvM ?Hpdotl> Ov` (A rctk A?N0,vC0 ap014000Q0=- 0A1 RPQ""ift a- NCR- xn au'oo or, Kaatjl t<E t10F1L'teg wr�tl 65'/, M s p,55 Ib %yt, 5D-12