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Pre.Deve-o 6.1. B. - Page 18 Orifice Sizing Calculation: where Drainage Q(Existing Post - ural age cxisung exuung Runoff Runoff Composite Existing Drainage Area Pervious Impervious Coefficient Coefficient Runoff Discharge Areas Size (SF) Surface (SF) Surface (SF) (Pervious) {Impervious] Coefficient (EJC, cis) A-1 47g0 4675 65 0.1 0.9 0.11 0.02 A-2 27669 20891 $778 0.1 0,9 0.30 0.34 A-3 2978 _2328 - 650 0.1� 0.9 0.27 0,03 _ A-4� 27972 22272 5700 0.1 0.9 0.26 0.30 jQ_j=0.11 x1.8x0.11=0.02cfsj Q,,zz = 0.30 x 1.8 x 0.64 = 0,34 cfsj jQ,x3=0.27x1.8x0.07=0.03 cfsj j Q,4=0.26 x 1,8x 0.64-0.30 cfsj Post -Development Hydrology IQroi - 0.11 x 2.1 X0.42 = 0.1.0 cfsj j Q,poz = 0.46 x 2.1 x 0.64 = 0.63 cfsj f QPD3= 0.43 x 2.1 x 0.07=0.06 cfsj Q,r,,x=0.52x2.1x0.66 =0.70 cfsj Discharge Calculation Method: Rational Method Q=CiA, where i''=1.8in/hr (10year 10minute) and i"=2.1ih/hr (30 year 10 minute) Post-Deve fopment Stormwater Storage: Attach me nt O Volume Stored ={Q - C ) x 3/2Tc, Tc =10 minutes Detention Volume Summary Table Storage Volume Basin Surface Rock Total Required Surface Volume Section Detention (CF) Area (SF) (CF) Volume Volume (CF) (CF) 66 122 34 49 83 266 I 567 190 227 417 26 108 28 43 71 353 992 272 397 669 IV1-(0.10-0,02)x3/7x10x 60=66ft3I 1/2 - (0.63 - 0.34) x 3�? x 10 x 60 = 266 f t3 1113 = {0.06 - 0.03) >< x 10 X 60 = 26 ft31 I V4 = (0,66 - C.30) x 3/7 x 10 x 60 = 320 ft' Orifice Sizing Orifice Sizing Calculation: where Drainage Q(Existing Post - Drainage Drainage New New Runoff Runoff Composite Develapme Areas Discharge) Area Pervious Impervious Coefficient Coefficient Runoff 0.7 _.... _ ...._-_-� Areas Size (SF) Surface (SF) Surface (SF) (pervious) (impervious) Coefficient tit Discharge A-3 0.03 0.611 32.2 2.75 ---4 0,9 TI A4- 0.30 0.611 32.2 (Q -PD, cis} A-1 4740 2868 1872 0.1 0.9 0.42 0.10 A-2 27669 14691 12978 0.1 0.9 0.48 0.63 A-3 T 297$ 1731 1247 0.1 0.9 0.43 0.06 1-4 27972 13408 14564 0.1 0.9 0.52 0.70 IQroi - 0.11 x 2.1 X0.42 = 0.1.0 cfsj j Q,poz = 0.46 x 2.1 x 0.64 = 0.63 cfsj f QPD3= 0.43 x 2.1 x 0.07=0.06 cfsj Q,r,,x=0.52x2.1x0.66 =0.70 cfsj Discharge Calculation Method: Rational Method Q=CiA, where i''=1.8in/hr (10year 10minute) and i"=2.1ih/hr (30 year 10 minute) Post-Deve fopment Stormwater Storage: Attach me nt O Volume Stored ={Q - C ) x 3/2Tc, Tc =10 minutes Detention Volume Summary Table Storage Volume Basin Surface Rock Total Required Surface Volume Section Detention (CF) Area (SF) (CF) Volume Volume (CF) (CF) 66 122 34 49 83 266 I 567 190 227 417 26 108 28 43 71 353 992 272 397 669 IV1-(0.10-0,02)x3/7x10x 60=66ft3I 1/2 - (0.63 - 0.34) x 3�? x 10 x 60 = 266 f t3 1113 = {0.06 - 0.03) >< x 10 X 60 = 26 ft31 I V4 = (0,66 - C.30) x 3/7 x 10 x 60 = 320 ft' Orifice Sizing Orifice Sizing Calculation: where Drainage Q(Existing Orifice C Q=4 a D z2gh Q = flow rate Cd g h (ft) Diameter Ca = discharge coefficient (0.611), Areas Discharge) (inch) D = 4Q g = gravitationla acceleration (32.2 ftls z), A-1 0.02 0.611 32.2 2,75 __T 0.7 _.... _ ...._-_-� Can29h = h depth of orifice A-2 0.34 0.611 32.2 3.5 2.6 A-3 0.03 0.611 32.2 2.75 ---4 0,9 TI A4- 0.30 0.611 32.2 2.4 4 x 0,02 D1= x12-0.7°xrzJ2x32.2x2.75 10,611 4 x 0.34 D2= x12=2.6" NJ 4 x 0.03 D3=ti0.611xnd2x32.2x2.75x12=0.9" 4 x 0.30 D4= x12=2.4" ti 0.611 xrrV2x32.2 x4.0 I M W 9.... B K F YEARS ENGINEER 3N. FIRS� M& 1730 N. FlRST E � - {no - SAN JOSE, CA 95112 www.bkf.com EXHIBIT B4 BASIN DETENTION CALCULATIONS Drawn: KH Checked: JM Approved: JM Job No.: 20176020 Date: 3/23/2018