FLOOD05148

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Urban Storm Drainage Criteria Manual
• Date: 5/1/1984
• Prepared By: UDFCD
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

DRAINAGE CRITERIA MANUAL

RAINFALL

5. INTENSITY - DURATION CURVES FOR RATION METHOD

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5.1 General Procedure
To develop Intensity-Duration curves for the Rational Formula Method of
runoff analysis take the I-hour depth(s) obtained from Figures 2-1 through
2-10 and multiply by the factors in Table 5-1.

Table 5-1

Factors For Durations of Less Than I-Hour
Duration (minutes) 5 10 15 30
Ratio to I-hour Depth 0.29 0.45 0.57 0.79
Source: NOAA Atlas 2, Vol. III, Colorado 1973

5.2 Example -- Development of Intensity-Duration Curves
Develop a series of intensity--duration curves for the 2, 10 and 100-year
frequencies to be used ir conjunction with the Rational Method formula for
Townshi p 3 South, Range 61; West. From the ra infa ll-dura tion-frequency maps,
we obtain the following (in inches): .

1 hour depth:

2 Year
0.95

10 Year
1.60

100 Year
2.60

Take the one hour depth for each frequency and multiply by the factors
given in Table 5-1 for 5, 10, 15 and 30 minutes and convert the results to
average intensities for each duration. Table 5-2 illustrates the
computational process which results are then plotted on Figure 5-1. Fiqure 5-1
may then be used with the Rational Formula Method to calculate near runoff
rates.

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