Simulating channel bifurcation flows with weirs for flood risk reduction

Abstract

Weir is widely used for controlling the flow discharges and water levels of a bifurcated river for flood risk reduction. The effects of weir on the bifurcation flow properties and hydrodynamic processes were not systematically studied by previous research, some of these have limitations and only showed that the discharge ratios were varied by the changes of weir height. Thus, there is a need to further explore on the effect of overall weir geometry and weir location on flow properties. In this study, an one-dimensional (1D) numerical model of Hydrological Engineering Centre-River Analysis System (HEC-RAS) has been applied to simulate an idealised channel with the applications of a variety of weir geometries at various locations. The model has been set to simulate a U-shaped main channel with two identical U-shaped bifurcated channels. Simulations have been undertaken for the weirs with cross-sectional shapes of rectangular, Cipolletti (trapezoidal), and V-notch (triangular). Comparisons of velocity profiles and water elevations with different Froude numbers have been undertaken. The results present the relationships of the outlet discharge ratio and velocity ratio to weir height, crest length, and crest angle ratio with different cross-sectional shapes and locations of weir. The findings show that flood risks could be potentially reduced by understanding the flow behaviours of channel bifurcation with presence of weirs as controlled structure.