Geoscience Reference
In-Depth Information
Storm duration >
t
c
I
d
= Storm duration or rainfall
intensity averaging period
Q
P
=
Cl
d
A
Storm duration =
t
c
Q
P
=
Cl
t
c
A
Q
p
Q
p
t
c
Time
2.5
t
c
t
c
T
d
T
d
+ 1.5
t
c
Time
10-year recurrence interval
hydrographs for various rainfall
averaging periods.
25
10
20
20
Rainfall averaging
periods in minutes
15
30
40
50
10
60
70
Allowable Release
Rate
5
10
20
30
40
50
60
70
Time (minutes)
FIGURE 25.13
Modified rational method hydrographs.
longer than the
t
c
may not produce as large a peak rate of runoff, but it may generate a greater volume
of runoff. By using the modified rational method, the designer can evaluate several different storm
durations to verify which one requires the greatest volume of storage with respect to the allowable
release rate. It is this maximum storage volume that the basin must be designed to detain.
The first step in determining the critical storm duration is to use the post-developed time of con-
centration (
t
c
) to generate a post-developed runoff hydrograph. Rainfall intensity averaging periods
(
T
d
), representing time periods incrementally longer than the
t
c
, are then used to generate a “family”
of runoff hydrographs for the same drainage area. These hydrographs will be trapezoidal with the
peak discharges (
Q
i
) based upon the intensity (
I
) of the averaging period (
T
d
). Figure 25.13 shows the
construction of a typical triangular and trapezoidal hydrograph using the modified rational method,
and a family of trapezoidal hydrographs representing storms of different durations.
Note that the duration of the receding limb of the trapezoidal hydrograph (Figure 25.13) is set
to equal 1.5 times the time of concentration (
t
c
). Also, the total hydrograph duration is 2.5
t
c
vs. 2
t
c
.
This longer duration is considered more representative of actual storm and runoff dynamics. It is
also more analogous to the SCS unit hydrograph where the receding limb extends longer than the
rising limb.
The modified rational method assumes that the rainfall intensity averaging period is equal to
the actual storm duration. This means that the rainfall and runoff that occur before and after the
rainfall-averaging period are not accounted for. Therefore, the modified rational method may under-
estimate the required storage volume for any given storm event.
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