Geoscience Reference
In-Depth Information
TABLE 25.10
Rainfall Constants for Virginia
(Duration, 5 min to 2 hr)
Station
Rainfall Frequency
Constants
a
1
2
117.7
19.1
5
168.6
23.8
10
197.8
25.2
2
2
118.8
17.2
5
158.9
20.6
10
189.8
22.6
3
2
130.3
18.5
5
166.9
20.9
10
189.2
22.1
4
2
126.3
17.2
5
173.8
22.7
10
201.0
23.9
5
2
143.2
21.0
5
173.9
22.7
10
203.9
24.8
a
Constants are based on linear regression analyses of
the IDF curves contained in the Virginia Department
of Transportation (VDOT)
Drainage Manual
.
a
bT
QC
=
A
(25.7)
+
d
where
Q
= Peak rate of discharge (cfs).
a
,
b
= Rainfall constants developed for storms of various recurrence intervals and various
geographic locations (Table 25.10).
T
d
= Critical storm duration (min).
C
= Runoff coefficient.
A
= Drainage area (acres).
Substituting this relationship for Q, Equation 25.5 then becomes:
t
a
bT
C
A
c
+
VC
a
bT
QT
3
Qt
d
od
oc
=
AT
+
−
−
60
(25.8)
d
+
4
2
4
d
2 5 . 7. 7. 3 Maximum Storage Volume
The first derivative of this storage volume equation, Equation 25.8, with respect to time is an equa-
tion that represents the slope of the storage volume curve plotted vs. time. When this equation is set
to equal zero and solved for
T
d
, the slope of the storage volume curve is zero, or at a maximum, as
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