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386 Chapter 11 Hydrology: Rainfall and Runoff

Area 5
Area 4
Area 7
Area 3

Area 6
Area 2

Area 1

Figure 11.9 Map Showing Areas of Approximately Similar Rainfall Characteristics

EXAMPLE 11.7 INTENSITY-DURATION-FREQUENCY RELATIONSHIPS FOR THE UNITED STATES
IN ENGLISH UNITS
Determine:
1. The intensity-duration-frequency equation for area 1 of the United States using the
English system and assuming the frequency of precipitation is 2 years.
2. Calculate the intensity (in./h and mm/h) of a rainstorm for a duration of 20 min.

Solution 1 (U.S. Customary System):
1. The intensity-duration-frequency from Eq. 11.19 and for n 1 is
A
i = (11.21)
t + d
From Table 11.10 for a frequency of 2 years,
5,230
i(mm/h) = (SI Units) (11.22)
t + 30
5,230
25.4 mm/hr
i(in./h) = (U.S. Customary Units)
t + 30
206
= (U.S. Customary Units) (11.23)
t + 30
2. The intensity (in./h) of a rainstorm for a duration of 20 min.
206
i(in./h) =
t + 30
206
=
20 + 30
= 4.12 in./h