Nonuniform flow in compound channel: A 1-D method for assessing water level and discharge distribution

This paper investigates 1-D modeling of nonuniform flows in compound channels. The issue is how to accurately predict both flow depth and mean velocity in the floodplain. A new model, called Independent Subsections Method (ISM), is presented here. Unlike classical 1-D models that solve a dynamic equation on the total cross section, the ISM estimates the water surface profile within each subsection. This enables the water level and the subsection mean velocities to be simultaneously calculated, without priority to any variable. In opposition to the Divided Channel Method (DCM), corrected DCM or the Exchange Discharge Model, the ISM assumes independent evolution of the discharge in each subsection of the compound channel. Indeed, this method does not assume equal head loss gradients in all subsections, and it does not impose the downstream discharge distribution. The ISM consists in a set of three coupled 1-D momentum equations (written within main channel, left-hand, and right-hand floodplains) and a mass conservation equation on the total cross section. Mass and momentum exchanges at the interfaces between subsections are explicitly accounted for. This method is validated against experimental data for developing flows in straight compound channel, flows in skewed compound channel, flows in a symmetric converging or diverging compound channel, and flows in an asymmetrical compound channel with an abrupt floodplain contraction. For the 46 runs, the ISM predicts flow depth and mean velocity in the floodplain with a maximum relative error of 8% and 19%, respectively. The ISM also appears to be a useful theoretical tool to improve our understanding of physical processes governing compound channel flows.