Overbank flow depth prediction in alluvial compound channels

Algorithms based on simple one-dimensional stage–discharge models for compound channels have been developed and tested for overbank flow data from the UK Flood Channel Facility and the University of Ulster channel with straight and meandering planforms. The proposed divided resistance approach takes into account the grain resistance, the bed form resistance and the roughness characteristics of the floodplain but it was found to give unsatisfactory prediction of the flow depth for compound channels with rough floodplains. A simple algorithm for stage–discharge prediction based on a lumped resistance approach was then proposed. It requires calibration of the overbank Manning n for a relative depth zero and a coefficient of proportionality, and this is achievable if measurements of the flow depth and velocity are taken for at least three overbank flow discharges. The application of the lumped approach to various flume and field data showed good agreement between the measured and predicted flow depths.