Flow resistance and momentum flux in compound open channels

New formulations are presented for flow resistance and momentum flux in compound open channels. As implemented in the St. Venant equations, these formulations facilitate a physically enhanced approach to evaluating conveyance, roughness, stage-discharge relationship, and unsteady flood routing in compound open channels. An analysis using steady flow data from the well-controlled experiments at the large-scale Flood Channel Facility, FIR Wallingford, demonstrates the ability of the present approach to properly resolve the discontinuity of overall roughness across the main-channel bankfull level. Also, the proposed formulations are shown to be conducive to obviating the long-standing computational difficulty in unsteady flood routing due to small flow depths over flat and wide floodplains. The present work should find general applications in one-dimensional computation of river flows.