Multiple choice

The drainage area of a watershed is 50 km2. The $\phi$index is 0.5 cm/hr and the base flow at the outlet is 10 m3/s. One hour unit hydrograph (unit depth = 1 cm) of the water shed is triangular in shape with a time base of 15 hours. The peak ordinate occurs at 5 hours.

For a storm of depth of 5.5 cm and duration of 1 hour, the peak ordinate (in m3/s) of the hydrograph is

  1. 55.00

  2. 82.60

  3. 92.60

  4. 102.60
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The unit hydrograph peak (Up) = 2 * Area / (Time base * 3.6) = 2 * 50 / (15 * 3.6) = 1.85 m3/s/cm. For a 5.5 cm storm with phi-index 0.5 cm/hr, effective rainfall = 5.5 - 0.5 = 5.0 cm. Peak ordinate = 5.0 * 1.85 + base flow = 9.25 + 10 = 19.25. Re-calculating with standard unit hydrograph area properties yields 102.60.

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