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11.17 Estimates of Storm Runoff 391

EXAMPLE 11.9 ESTIMATION OF RUNOFF BY THE RATIONAL METHOD
3
Determine the mean runoff flow, in ft /s and L/s, for a 5-year recurrence rainfall assuming the
runoff coefficient for the semiresidential area in Massachusetts is 0.45, the concentration time is 10
2
min, and the drainage area a is 10,000 m (2.47 acre; 1 ha).
Solution 1 (U.S. Customary System):

Q = cia
= (0.45) (i) (2.47 acre)
3,330>25.4
i (in./h) =
t + 19
3,330>25.4
=
10 + 19
= 4.52 in./h
Q = (0.45) (4.52) (2.47)
3
= 5 ft /s

Solution 2 (SI System):
Q cia (11.24)
2
Q (0.45) (i) (10,000 m )
From Fig. 11.9, Massachusetts is in Area 3. From Table 11.10, the intensity-duration-frequency for
area 3 and a frequency of 5 years is

3,330
i =
t + 19
3,330
=
10 + 19
= 115 mm/h
= 0.115 m/h
Therefore,
2
Q = (0.45)(0.115 m/h)(10,000 m ) using Eq. 11.24b
3
= 518 m /h
3
3
= (518 m /h) * (1,000 L/m )>(60 * 60 s/h)
= 143 L/s runoff rate
Q = 2.76 (0.45) (115) (1) using Eq. 11.24d
= 143 L/s

11.17.2 The Unit-Hydrograph Method
In dry weather, or when precipitation is frozen, the residual hydrograph or base flow of a
river is determined by water released from storage in the ground or in ponds, lakes, reser-
voirs, and backwaters of the stream. Immediately after a rainstorm, the rate of discharge
rises above base flow by the amount of surface runoff entering the drainage system. That
portion of the hydrograph lying above base flow can be isolated from it and is a measure of