compared to the storage capacity of the reservoir, it is assumed that the reservoir
would be full at the beginning of the design flood. However, if the capacity of the
reservoir is large compared to the volume of flood runoff, then the initial reservoir
elevation would be that expected at the beginning of the design flood or the elevation
obtained by a flood having an average frequency of once every 25 yr. The rate and
volume of inflow into the system may be determined utilizing the unit hydrograph
and projecting to the worst conceivable storm. The worst conceivable storm can be
estimated if sufficient long-term data are available for the watershed in question. The
other information needed concerns the nature of the discharge system. This may
consist of any combination of regulating outlets, penstocks, consumer demand, and
so on. In a small dam, the discharge may be considered through a culvert. To
determine the rate of flow through a culvert, the equation used is:
Q ¼ KA2gh
ð6:8Þ
in which
Q = the discharge in m
3 /s (cfs)
K = the coefficient of discharge
g = gravity, 9.81 m/s
2 (32.2 ft/s
2 )
h = the effective head on the center of the orifice in m (ft)
K may be calculated as:
1
1þ2gL= Cc 2 R h
À
Á
in which
C c = the Chezy coefficient.
L = the length of the conduit in m (ft).
R h = the hydraulic radius which = A/w p = the area (A) divided by the wetted
perimeter (w p ). Once the pipe is full, the value of R h = pipe diameter/4.
For discharge over a spillway, a sharp crested weir with no approach velocity may
be assumed. Here
Q ¼ cLH
3=2
ð6:9Þ
in which
Q = the rate of discharge in m
3 /s (cfs)
c ¼ the weir coefficient, which varies between 2.5 and 3.5
H ¼ the head above the spillway in m (ft)
L ¼ the length (width) of the spillway in m (ft)
258
D. B. Aulenbach et al.
would be full at the beginning of the design flood. However, if the capacity of the
reservoir is large compared to the volume of flood runoff, then the initial reservoir
elevation would be that expected at the beginning of the design flood or the elevation
obtained by a flood having an average frequency of once every 25 yr. The rate and
volume of inflow into the system may be determined utilizing the unit hydrograph
and projecting to the worst conceivable storm. The worst conceivable storm can be
estimated if sufficient long-term data are available for the watershed in question. The
other information needed concerns the nature of the discharge system. This may
consist of any combination of regulating outlets, penstocks, consumer demand, and
so on. In a small dam, the discharge may be considered through a culvert. To
determine the rate of flow through a culvert, the equation used is:
Q ¼ KA2gh
ð6:8Þ
in which
Q = the discharge in m
3 /s (cfs)
K = the coefficient of discharge
g = gravity, 9.81 m/s
2 (32.2 ft/s
2 )
h = the effective head on the center of the orifice in m (ft)
K may be calculated as:
1
1þ2gL= Cc 2 R h
À
Á
in which
C c = the Chezy coefficient.
L = the length of the conduit in m (ft).
R h = the hydraulic radius which = A/w p = the area (A) divided by the wetted
perimeter (w p ). Once the pipe is full, the value of R h = pipe diameter/4.
For discharge over a spillway, a sharp crested weir with no approach velocity may
be assumed. Here
Q ¼ cLH
3=2
ð6:9Þ
in which
Q = the rate of discharge in m
3 /s (cfs)
c ¼ the weir coefficient, which varies between 2.5 and 3.5
H ¼ the head above the spillway in m (ft)
L ¼ the length (width) of the spillway in m (ft)
258
D. B. Aulenbach et al.