Multiple choice

Ordinates of a 1-hour unit hydrograph at 1 hour intervals, starting from time t = 0, are 0, 2, 6, 4, 2, 1 and 0 m3/s.

Ordinate of a 3-hour unit hydrograph for the catchment at t = 3 hours is

  1. 2.0 m3/s

  2. 3.0 m3/s

  3. 4.0 m3/s

  4. 5.0 m3/s
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C Correct answer
Explanation

To convert a 1-hour unit hydrograph (UH) to a 3-hour UH, use the method of superposition. Sum the ordinates of the 1-hour UH shifted by 0, 1, and 2 hours, then divide by 3. At t=3: (Ordinate at t=3 + t=2 + t=1) / 3 = (4 + 6 + 2) / 3 = 12 / 3 = 4.0.

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