Geoscience Reference
In-Depth Information
Q
i
Inflow
Storage
Volume
q
o
Outflow
(Straight line
opproximation)
t
c
T
T + 1.5 t
c
Time
FIGURE 25.16
Trapezoidal hydrograph storage volume estimate.
Qt
QT
3
Qt
ic
od
oc
VQT
=
+
−
−
60
(25.5)
id
4
2
4
where
V
= Required storage volume (ft
3
).
Q
i
= Inflow peak discharge (cfs) for the critical storm duration (T
d
).
T
c
= Post-developed time of concentration (min).
Q
o
= Allowable peak outflow (cfs).
T
d
= Critical storm duration (min).
Regulatory ordinance or downstream conditions establish the allowable peak outflow. The criti-
cal storm duration (
T
d
) is an unknown and must be determined to solve for intensity
I
and to ulti-
mately calculate the peak inflow (
Q
i
). Therefore, a relationship between rainfall intensity
I
and
critical storm duration
T
d
must be established.
25.7.7.2 Rainfall Intensity
The rainfall intensity as taken from the IDF curves is dependent on the time of concentration (
t
c
)
of a given watershed. Setting the storm duration (T
d
) equal to
t
c
will provide the maximum peak
discharge. As stated previously, however, it does not necessarily generate the maximum volume of
discharge. Since this maximum volume of runoff is of interest, and the storm duration is unknown,
rainfall intensity
I
must be represented as a function of time, frequency, and location. The relation-
ship is expressed by the modified rational method intensity equation as follows:
a
bT
d
I
=
(25.6)
+
where
I
= Rainfall intensity (in./hr).
T
d
= Rainfall duration or rainfall intensity averaging period (min).
a
,
b
= Rainfall constants developed for storms of various recurrence intervals and various
geographic locations (Table 25.10).
The rainfall constants,
a
and
b
, were developed from linear regression analyses of the IDF curves
and can be generated for any area where such curves are available. The limitations associated with
the IDF curves, such as duration, or return frequency, will also limit development of the constants.
Table 25.10 provides rainfall constants for various regions in Virginia. Substituting Equation 25.6
into the rational equation results in the rearranged rational equation as follows:
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