Velocity and Discharge in Compound Channels

The problem of discharge assessment in compound channels is described, including the difficulty of accurately modeling main channel and floodplain flows. Theoretical considerations show that the ratios of main channel velocity and discharge to floodplain values in a compound channel are independent of bed slope and are influneced only by depth and geometry. This is also true for other significant velocity and discharge ratios. Experimental confirmation of these theoretical predictions is presented using data from three symmetrical compound channel shapes. The relationships are described mathematically where possible, and the exponents and coefficients are evaluated. Comparisons are presented of measured discharges with values obtained using conventional methods that neglect the interaction between main channel and floodplains, showing the errors incurred by the conventional approach. The implications of these findings for compound channel analysis and design are assessed.