Question

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 $m^3/s$

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

• A. $2.0m^3/s$
• B. $3.0m^3/s$
• C. $4.0m^3/s$
• D. $5.0m^3/s$

Answer 1

The correct option is C $4.0m^3/s$

$\begin{array}{cccccc}\left(1\right)& \left(2\right)& \left(3\right)& \left(4\right)& \left(5\right)& \left(6\right)\\ \text{Time}& \text{Ordinate}& \text{UH lagged}& \text{UH lagged}& \text{Ordinate of DRH}& \text{Ordinate of 3h}\\ & \text{of 1h UH}& \text{by 1h}& \text{by 2h}& \text{of 3cm in 3h}& \text{UH}\left(5\right)÷3\\ & & & & \left(2\right)+\left(3\right)+\left(4\right)& \\ 0& 0& -& -& 0& 0\\ 1& 2& 0& -& 2& 2/3\\ 2& 6& 2& 0& 8& 8/3\\ 3& 4& 6& 2& 12& 4\\ 4& 2& 4& 6& 12& 4\\ 5& 1& 2& 4& 7& 7/3\\ 6& 0& 1& 2& 3& 1\\ 7& & & 1& 1& 1/3\\ 8& & & 0& 0& 0\end{array}$

Thus at time t = 3h ordinate of 3h UH
$=4m^3/s$