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An isolated 3-h rainfall event on a small catchment produces a hydrograph peak and point of inflection on the falling limb of the hydrograph at 7 hours and 8.5 hours respectively, after the start of the rainfall. Assuming no losses and no base flow contribution, the time of concentration (in hours) for this catchment is approximately

1. 8.5
2. 7.0
3. 6.5
4. 5.5

Correct answer: 5.5

Solution

The time of concentration is defined as the time it takes for water to travel from the most distant point in the catchment to the outlet. Since the peak occurs at 7 hours and the point of inflection on the falling limb at 8.5 hours, the time of concentration is typically estimated as the time to peak minus the time it takes for the hydrograph to start falling, which in this case suggests a value around 5.5 hours.

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