An improved approach to stage-discharge and velocity distribution prediction in straight compound channels using CFD models

Abstract

An approach for applying 3-D CFD models to estimate uniform flow stage-discharge relationships and velocity distributions in straight compound channels is presented in this paper. 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 and floodplain interface is sometimes ignored or is accounted for by assuming a constant eddy viscosity. This potentially results in an over-estimation of conveyance capacity in compound channels. The proposed approach represents an advance on these methods and uses the 3-D CFD model SSIIM with k-ε 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 Phase A straight channel tests undertaken at the large-scale UK Flood Channel Facility (FCF) and is also compared with predictions from Divided Channel Methods (DCMs).