Civil Engineering PE

Category - Hydrology

Which of the following assumptions is not made when applying the Rational method to calculate peak runoff in a small watershed?
  1. Rainfall is spatially and temporally constant for the duration of the event
  2. The rainfall duration is shorter than the time the time of concentration of the watershed
  3. The runoff coefficient incorporates all losses and diffusive properties of the watershed
  4. None of the above
Explanation
Answer: B - The assumption that the rainfall duration is shorter than the time of concentration of the watershed is not made when applying the Rational method to calculated peak runoff in a small watershed. The opposite assumption is made: the watershed is concentrated; therefore, the duration of the rainfall is equal to or longer than the time of concentration of the watershed. For this reason, the overall size of a watershed (also referred to as a “catchment”) should be kept small and the time of concentration assumed to be less than one hour.
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