Laboratory experiments on the flow of water over a downward step in an open channel

Abstract A turbulent jump forms below a step if the downstream level is low and above it if the level is high. Two intermediate stages are found to be possible. In the first, the rising surface just below the step becomes unstable in a cyclic manner, a breaking wave on it advancing towards the step and subsiding as it does so. In the second, with a slightly higher level downstream, a large steady wave arises below the step. Beneath it a bound vortex exists with a horizontal axis, and in the trough of the wave a turbulent jump occurs with relatively little commotion downstream. This second stage was seen below a large measuring weir across a river. For jumps below and above the step Rouse's assumptions concerning the pressure on the face of the step were verified by direct measurement. His theoretical formulae, which ignore wall friction, were found to be in good agreement with experiment for deep streams at low Froude numbers, but discrepancies appeared when the flow was shallow and fast.