Page 422 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch11_357-397.qxd 8/2/10 9:01 PM Page 382

382 Chapter 11 Hydrology: Rainfall and Runoff
By pooling all observations irrespective of their association with individual storm
records, a generalized intensity-duration-frequency relationship is obtained. The following
empirical equations are of assistance in weeding out storms of low intensity from the
analysis of North American station records:
i 0.6 12>t for the northern U.S (U.S. Customary Units) (11.16a)
i 1.2 18>t for the southern U.S (U.S. Customary Units) (11.17a)
Here, the rainfall intensity in in./h and t is the duration in min. Two equivalent intensity
equations using the SI units of i (mm/h) and t (min) are
i 15.24 304.8>t for the northern US (SI Units) (11.16b)
i 30.48 457.2>t for the southern US (SI Units) (11.17b)
For a duration of 10 min, for example, intensities below 3 in./h (76.2 mm/h) in the south-
ern states need not receive attention. The storm recorded in Example 11.4 exhibits double
this intensity.
Storm rainfall can be analyzed in many different ways. However, all procedures nor-
mally start from a summary of experience such as that shown in Example 11.5. The results
may be used directly, or after smoothing (generally graphical) operations that generalize
the experience. The developed intensity-duration-frequency relationships may be left in
tabular form, presented graphically, or fitted by equations.

EXAMPLE 11.5 DETERMINATION OF TIME-INTENSITY RELATIONSHIP FOR A GIVEN FREQUENCY
The number of storms of varying intensity and duration recorded by a rain gauge in 45 years
for New York City is listed in Table 11.8. Determine the time-intensity values for the 5-year storm.

Table 11.8 Record of Intense Rainfalls for Example 11.5
Duration, Number of Storms of Stated Intensity (in./h) or More
min
1.0 1.25 1.5 1.75 2.0 2.5 3.0 4.0 5.0 6.0 7.0 8.0 9.0
5 123 47 22 14 4 2 1
10 122 78 48 15 7 4 2 1
15 100 83 46 21 10 3 2 1 1
20 98 64 44 18 13 5 2 2
30 99 72 51 30 21 8 6 3 2
40 69 50 27 14 11 5 3 1
50 52 28 17 10 8 4 3
60 41 19 14 6 4 4 2
80 18 13 4 2 2 1
100 13 4 1 1
120 8 2

Conversion factor: 1 in./h 25.4 mm/h

Solution:
If it is assumed that the 5-year storm is equaled or exceeded in intensity 45>5 9 times in 45 years,
the generalized time-intensity values may be interpolated from Table 11.8 by finding (a) for each
specified duration the intensity equaled or exceeded by nine storms, and (b) for each specified
intensity the duration equaled or exceeded by nine storms.