Method for Computing the Channel Protection Storage Volume (Cpv) Appendix D.11 Appendix D.11................... Method for Computing the Channel Protection Storage Volume (Cpv) The following procedure shall be used to design the channel protection storage volume (Cpv). The method is based on the Design Procedures for Stormwater Management Extended Detention Structures (MDE, 1987) and utilizes the NRCS, TR-55 Graphical Peak Discharge Method (USDA, 1986). "Compute the time of concentration (tc) and the one-year post-development runoff depth (Qa) in inches. "Compute the initial abstraction (Ia) [ I a = 200 CN − 2 ] and the ratio Ia P where P is the one-year rainfall depth (see Table 2-2). "With tc and Ia/P, find the unit peak factor (qu) from Figure D.11.1 and compute the one year post-development peak discharge qi = quAQa where A is the drainage in square miles. "If qi ≤ 2.0 cfs, Cpv is not required. Provide for water quality (WQv) and groundwater recharge (Rev) as necessary. "With qu, find the ratio of outflow to inflow (qo/qi) for T = 24 hours from Figure D.11.2 (use T=12 hours in USE III/IV waters). "Compute the peak outflow discharge qo = qo qi × qi "With qo/qi, compute the ratio of storage to runoff volume (Vs/Vr). Vs Vr = 0.683 − 1.43( qo ) + 1.64( qo ) 2 − 0.804( qo ) 3 qi qi qi "Compute the extended detention storage volume Vs = (Vs ) × Vr (note: Vr = Qa); Vr Vs Convert Vs to acre-feet by × A , where Vs is in inches and A is in acres. 12 "Compute the required orifice area (Ao) for extended detention design: qo qo = C 2 gho 4.81 ho where ho is the maximum storage depth associated with Vs. Ao = "Determine the required maximum orifice diameter (do) do = 4 Ao π . A do of less than 3.0” is subject to local jurisdictional approval, and is not recommended unless an internal control for orifice protection is used (App. D.8). D.11.1 Appendix D-11….Method for Computing the Channel Protection Storage Volume (Cpv) SCS Graphical Method of Determining Peak Discharge (qu) in csm/in for 24-Hour Type II Storm Distribution Figure D.11.1 1000 900 D.11.2 Unit Peak Discharge (qu), csm/in 800 700 Ia/P = 0.10 600 0.30 500 0.40 0.45 0.35 400 0.50 300 200 100 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Time of Concentration (tc) in Hours 0.8 0.9 1 Detention Time Versus Discharge Ratios ( qo qi Appendix D-11….Method for Computing the Channel Protection Storage Volume (Cpv) Figure D.11.2 ) 0.500 0.450 D.11.3 Ratio of Outflow to Inflow (qo/qI) 0.400 0.350 0.300 0.250 0.200 0.150 T =12 hr 0.100 0.050 T =24 hr 0.000 50 100 150 200 250 300 350 400 450 500 550 600 650 Unit Peak Discharge (qu), csm/in 700 750 800 850 900 950 1000