A river diversion canal discharges into a concrete flume of rectangular section running down a steep slope. the walss and the floor of the flume are to be 300 mm thick, and the allowance fro freeboard is to be 300mm. For Q=32m^3/s, find the width of the flume that will minimise the cross-sectional area of the concrete at the head of the steep slope, where the flow is critical. Calculate also the cross-sectional area of the concrete for this canal width.