MEASUREMENT OF OVERBANK FLOW IN A COMPOUND RIVER CHANNEL

The results of an experimental study of a compound river channel have been presented, including velocity distributions and stage discharge relationships. These have illustrated the effects of momentum transfer between deep and shallow flows, which include reduction in main channel velocity and discharge capacity, leading to a reduction in compound section capacity at depths above bankfull. The corresponding parameters on the flood plains are increased. Flow resistance relationships have been presented in terms of both Manning's and Darcy-Weisbach equations, showing the complex nature of flow resistance in compound channels and the dangers inherent in extrapolating inbank data for the analysis of overbank flows. Data from laboratory studies of smooth boundary channel have been shown for comparison. The errors incurred by applying conventional methods to compound channels have been quantified and found to lie in the range +/- 25%, depending on the particular method used.