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Question

A triangular unit hydrograph for 5 hr duration is having peak discharge of 45 cumecs occurring at 5 hours, and having a base width of 20 hours. If the value of ϕ - index is 0.5 cm/hr then the resulting peak discharge of direct runoff due to an average rainfall of 20 cm occurring for 10 hours duration is ______ m3/s.
  1. 562.5

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Solution

The correct option is A 562.5
Time Ordinate of 5 hr UH
0 0
5 45
10 30
15 15
20 0

Rainfall excess = 200.5×10=15cm
A 10 hour hydrograph can be obtained by superimposing two 5 hours U.H lagged by 5 hours


Time Ordinate of 5 hr UH 5 hr UH lagged by 5 hr 10 hr DRH 10 hr UH
0 0 - 0 0
5 45 0 45 22.5
10 30 45 75 37.5
15 15 30 45 22.5
20 0 15 15 7.5
25 0 0 0


Peak discharge of resulting direct runoff = QP=37.5×15=562.5m3/s

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