Stage–discharge prediction in straight compound channels using 3D numerical models

An improved approach for applying three-dimensional (3D) computational fluid dynamics (CFD) models to estimate uniform flow stage–discharge relationships and velocity distributions in straight compound channels is presented. Commonly used modelling approaches tend to be over-specified. For a given flow and water level, desired results are obtained through calibration of resistance coefficients that can be artificially high and vary with changing flow conditions. Furthermore, the momentum interaction at the main channel–floodplain interface is sometimes ignored or is accounted for using a constant eddy viscosity. This potentially results in an overestimation of conveyance capacity in compound channels. The proposed approach represents an advance on these methods and uses a 3D CFD model with k– turbulence closure in a predictive capacity where a flow together with physically realistic resistance coefficients are specified. Downstream water levels are then iteratively adjusted until uniform flow conditions are established in the channel. The approach is validated against benchmark experimental data obtained from the largescale UK Flood Channel Facility and is compared with predictions from divided channel methods.