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