Predicting critical depth in straight compound channels

Abstract

Critical depth is the depth at which the flow in an open conduit undergoes a transition between supercritical and subcritical regimes. While the prediction of critical depth in a prismatic channel can be achieved to a high accuracy, the same cannot be said for two stage channels (main channel with flanking floodplain), also commonly known as compound channels. Traditionally, critical depth in a compound channel is predicted based on minimum specific energy. This investigation has shown that flow in compound channels does not undergo a sharp critical flow transition as in prismatic channels. Instead, the change between fully subcritical and fully super-critical occurs over a depth range, whereby different regions in the cross-section maybe either sub or super-critical flow. This has been termed a "mixed-flow" region. It has been found that the point of minimum specific energy does not coincide with either the beginning or the end of this critical transition, rendering such predictions ineffectual. Methods of incorporating the effects of the momentum interaction between the main channel and the floodplain have been used to improve the prediction of the velocity profile within the channel, and to produce subsection Froude numbers that can locate the key points of the critical transition zone.