## Intensity DurationRecurrence Interval

Figure 16.8 shows typical intensity-duration-recurrence interval curves for a specific location. The lines in the chart represent the recurrence intervals of 2, 5, 10, 25, 50, and 100 years.

Note that both axes in the graph are logarithmic scales and must be carefully read. The time in the left half of the graph is in minutes. The time in the right half of the graph is in hours. To design water conservation structures, a graph appropriate for the area under study should be used.

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Time (minutes & hours)

Time (minutes & hours)

FIGURE 16.8. Example of rainfall intensity-duration-recurrence interval.

Problem: What is the expected rainfall intensity for a duration of 40 min and a 10 year recurrence interval.

Solution: Enter the table at 40 min along the horizontal axis. Move vertically until intersecting with the ten year curve. Move horizontally to the left until intersecting with the rainfall intensity axis. Read the rainfall intensity = 3.5 in/hr, Figure 16.9.

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