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11.15 Intensity-Duration-Frequency Relationships 383
The results are as shown in Table 11.9 and are used in constructing Fig. 11.7. Similar calcula-
tions for the 1-year, 2-year, and 11-year storms underlie the remaining members of the family of
curves in Fig. 11.7.

Table 11.9 Duration-Intensity Relationship for Example 11.5

Duration, min 5 10 15 20 30 40 50 60 80 100
Intensity,
in./h 6.50 4.75 4.14 3.50 2.46 2.17 1.88 1.66 1.36 1.11

Intensity,
in./h 1.0 1.25 1.5 1.75 2.0 2.5 3.0 4.0 5.0 6.0
Duration, min 116.0 89.9 70.0 52.5 46.7 29.0 25.7 16.0 9.3 7.5
Conversion factor: 1 in./h 25.4 mm/h

7
Intensity of precipitation in inches per hour 5 10-year storm
6

4

3

2

1
5-year storm
0 1-year storm 2-year storm
0 10 20 30 40 50 60 70 80 90 100
Time or duration in minutes
Figure 11.7 Intensity-Duration-Frequency of Intense Rainfalls.
Conversion factor: 1 inch/hour = 1 in./h = 25.4 mm/h

11.15 INTENSITY-DURATION-FREQUENCY RELATIONSHIPS

Time-intensity curves such as those in Fig. 11.7 are immediately useful in the design of
storm drainage systems and in flood-flow analyses. For purposes of comparison as well as
further generalization, the curves can be formulated individually for specific frequencies
or collectively for the range of frequencies studied.
Good fits are usually obtained by a collective equation of the following form:
cT m
i = n (11.18)
(t + d)
where i and t stand for intensity and duration as before; T is the frequency of occurrence, in
years; c and d represent a pair of regional coefficients; and m and n represent a pair of