The ordinates of the 2-hour unit hydrograph as 60 minutes time interval starting from time t = 0 are $0, 2, 5, 4, 2, 1 and 0 m^{3} / s$ . A storm of 60 minutes duration resulted in an effective rainfall of 30 mm. The average storm loss and base flow were found to be 0.3 cm/hour and $5 m^{3} / s$ respectively. The peak of the resulting direct runoff hydrograph would be _____ $m^{3} / s$ .
18

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Solution

The correct option is A 18
$\begin{matrix} Time & 2 - h r UH & Lag 2hr & S_{A} (S - curve) & Lag~1hr & S_{A} - S_{B} & ord of 1hr UH S-curve add & S_{B} (S - curve) & = \frac{S_{A} - S_{B}}{\frac{T}{D} = \frac{1}{2}} \times 1 cm 0 & 0 & - & 0 & - & 0 & 0 1 & 2 & - & 2 & 0 & 2 & 4 2 & 5 & 0 & 5 & 2 & 3 & 6 3 & 4 & 2 & 6 & 5 & 1 & 2 4 & 2 & 5 & 7 & 6 & 1 & 2 5 & 1 & 6 & 7 & 7 & 0 & 0 6 & 0 & 7 & 7 & 7 & 0 & 0 \end{matrix}$

$Peak of DRH = Peak of 1 hour UH \times \frac{R c m}{1 c m}$

$= 6 \times \frac{3}{1}$

$= 18 m^{3} / s$

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