Question

The ordinates of the 2-hour unit hydrograph as 60 minutes time interval starting from time t = 0 are $0,2,5,4,2,1\text{and}0m^3/s$. 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 $5m^3/s$ respectively. The peak of the resulting direct runoff hydrograph would be _____ $m^3/s$.

• A. 18

Answer 1

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

$\text{Peak of DRH}=\text{Peak of 1 hour UH}\times \frac{Rcm}{1cm}$

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

$=18m^3/s$