Discharge Assessment in Compound Channel Flow

A series of laboratory tests were performed to measure the boundary shear stresses in a compound channel consisting of a main channel and associated shallow floodplains. Four different floodplain roughnesses were used. From these results apparent shear stresses across three different assumed main channel/floodplain interface planes (vertical, horizontal and diagonal) were calculated. The data was used with data from previous studies to derive a statistical relationship between the apparent shear stress on the vertical interface and several easily calculated parameters of the channel geometry and hydraulic characteristics. The accuracies of different methods of discharge calculation were shown to be dependent upon the assumptions they make regarding the magnitude of the apparent shear stress on the particular interface chosen. An apparent shear stress ratio was proposed which was found to be a useful yardstick in selecting the best method of discharge calculation for particular channel and flow conditions. Generally it was found that at low floodplain depths using the horizontal or diagonal interfaces gave better discharge results than the vertical.