FLOOD06389

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Urban Storm Runoff Inlet Hydrograph Study - Volume 4
• Date: 5/1/1975
• Prepared By: Cheng-lung Chen
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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depth-duration relations for the intermediate ratios of
15, 20, and 30 percen t are also plotted in Fig. 2,
Incorporating the values for the intermediate ratios
with those listed in Tables 1 and 2 results in the
standard intensity -duration relationships for various
ratios of I-hour to corresponding 24-hour rainfall
depth, as shown in Table 3 and Fig. 3. The storm
parameters, a, b, and c (Eq. 6) for the standard
intensity--duration curves can be evaluated by using
the method of least squares with the aid of an
optimization technique similar to the method of
steepest descent for optimizing an unconstrained
problem. The possible relationships between these
parameters and the ratio of I-hour to corresponding
24-11our rainfall depth were investigated. Such
relationships, if existent, can be used to estimate the
a, b, and c values at any location in the United States
without resorting to all of the 49 isopluvial maps in
the Technical Paper No. 40. For this method to be

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valid, at most two isopluvial maps for any frequency,
say 10-year I-hour and lO-year 24-hour, are needed
so that the ratio of l.hour to corresponding 24-hour
rainfall depth can be computed.

Since the ratio of I-hour to corresponding
24-hour rainfall depth in the Technical Paper No. 40
was shown to be nearly independent of frequency as
mentioned previously. accuracy-wise it does not
really matter which frequency of the two maps is
used as long as the present method is valid. However,
due mainly to convenience in application, there is
definitely an advantage of using 10-year I-hour and
lO-year 24-11our maps over the 2.year l.hour and
2-year 24-hour key maps, as will be compared later.

Applying the method of least squares and the
optimization technique to a set of the rainfall
intensities in Table 3 for the same ratio of I-hour to

2

3

6 12
DURATION (HOUri)

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Figure 2. Rainfall depth-duration diagram for the United States (after U.S. Weather Bureau Technical Paper
No, 40).

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