Question

The 4-h UH for a catchment is tabulated below :

$\begin{array}{cc}\text{Time (hr)}& \text{4-h UH ordinate}\\ & \left(m^3/sec\right)\\ 0& 0\\ 2& 8\\ 4& 20\\ 6& 35\\ 8& 38\\ 10& 32\\ 12& 16\\ 14& 14\\ 16& 10\\ 18& 8\\ 20& 6\\ 22& 3\\ 24& 0\end{array}$

The peak ordinate (in $m^3/sec$) of 8-h UH will be :

• A. 67
• B. 33.5
• C. 76
• D. 58

Answer 1

The correct option is B 33.5

$\begin{array}{ccccc}\text{Time}& \text{Ordinate of}& \text{Ordinate of}& \text{Combined}& \text{Ordinate of}\\ & \text{4-h UH}& \text{4-h UH}& \text{Hydrograph due to}& \text{8-h UH}\\ & \text{(col.1)}& \text{lagged by 4h}& \text{2 cm eff. rainfall}& \text{(col.3/2)}\\ & & \text{(col.2)}& \text{(col.3)}& \\ 0& 0& -& 0& 0\\ 2& 8& -& 8& 4\\ 4& 20& 0& 20& 10\\ 6& 35& 8& 43& 21.5\\ 8& 38& 20& 58& 29\\ 10& 32& 35& 67& \text{33.5}\\ 12& 16& 38& 54& 27\\ 14& 14& 32& 46& 23\\ 16& 10& 16& 26& 13\\ 18& 8& 14& 22& 11\\ 20& 6& 10& 16& 8\\ 22& 3& 8& 11& 5.5\\ 24& 0& 6& 6& 3\\ 26& -& 3& 3& 1.5\\ 28& -& 0& 0& 0\end{array}$

Peak ordinate of 8-h UH
$=33.5m^3/sec$