Critical depth prediction in straight compound channels

Critical depth is the depth at which the flow in an open conduit undergoes a transition from subcritical to supercritical regimes. The prediction of critical depth in a prismatic channel can be achieved to a high degree of accuracy. This is not the case for two-stage channels (main channel with flanking floodplain), also commonly known as compound channels. Traditionally, critical depth is predicted based on minimum specific energy, and this paper investigates the validity of this and several other methods of critical depth prediction for compound channels. It has been found that flow in compound channels does not undergo a sharp critical flow transition as occurs in prismatic channels. Instead, the change between fully subcritical and fully supercritical occurs over a depth range, whereby different regions in the cross-section maybe either sub- or supercritical. 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 mixed flow transition, rendering such predictions ineffectual. A new method for determining critical depth in a compound channel is presented and compared with other prediction techniques. Methods of incorporating the effects of the momentum interaction have been used to improve the prediction of the velocity profile within the channel and locate the mixed flow transition zone.