Gce Model Questions Vol1 2017 Geography Question 46

An ungauged catchment has an area of \(12\ \text{km}^2\). Land use is 40 % forest (runoff coefficient 0.15), 30 % agriculture (0.30), 20 % urban (0.70) and 10 % water bodies (0.05). The main channel length is \(3\ \text{km}\) and its average slope is 0.02. Rainfall intensities for storms of 10 min, 30 min and 60 min are \(80\ \text{mm\ h}^{-1}\), \(70\ \text{mm\ h}^{-1}\) and \(55\ \text{mm\ h}^{-1}\) respectively. Using the Kirpich formula \(T_c = 0.0195 L^{0.77} S^{-0.385}\) (with \(L\) in metres and \(S\) dimensionless) to estimate the time of concentration, select the appropriate intensity for the storm duration equal to the calculated \(T_c\) (choose the next longer duration if an exact match is unavailable) and then compute the peak discharge with the rational method \(Q = \frac{C i A}{360}\) (where \(C\) is the weighted runoff coefficient, \(i\) the selected intensity in \(\text{mm\ h}^{-1}\) and \(A\) the catchment area in \(\text{km}^2\)). Give \(Q\) in \(\text{m}^3\text{s}^{-1}\) to two decimal places.