Comparison of Coupled and Semicoupled Numerical Models for Alluvial Channels

Abstract

Contrary to general statements in the literature, the investigations reported in this paper show that semicoupled models are not necessarily unstable and give results comparable to those of coupled models. For this purpose, a two-dimensional model is developed to calculate bed variations in alluvial channels. Vertically averaged Navier-Stokes equations in transformed coordinates are numerically solved in conjunction with the sediment transport equation for the bed load. These equations are solved simultaneously in a fully coupled model and separately in a semicoupled model using the Beam and Warming alternating-direction implicit scheme. Both models are applied to predict bed-level changes due to sediment overloading and sediment shut-off. The computed results compare satisfactorily with the experimental results obtained in a laboratory flume. The results obtained by the semicoupled model are relatively close to that of the fully coupled model. Because the former algorithm permits any sediment formula and armoring effect to be easily incorporated, the semicoupled model appears to be more attractive for general applications to real-life systems than the coupled model.