A long concrete flume (n = 0.014) is laid on a slope of 0.001 and has a rectangular section 3 m wide. As a means of dropping water from this flume to another one, an open grille 3 m long consisting of longitudinal bars 75 mm wide with 25 mm gaps in between is laid in the bed of the flume. The discharge coefficient for the flow through the grille can be assumed equal to 0.5. (Note: the grille is a type of orifice)
Find the flow through the grille when a discharge of 8.5 m3/s is admitted at the upstream end of the channel. For what value of upstream discharge will the whole of the discharge pass through the grille?