Effect of suspended load on sandbar instability

We analyze bars in sandy rivers when a significant portion of the sediment load is carried in suspension. First, we investigate the ability of a depth-averaged formulation of both the hydrodynamics and sediment transport recently developed for slowly varying flows to capture the main mechanisms of sandbar formation. The exercise turns out to be successful and opens the way to the development of analytical and numerical models of the nonlinear development of sandbars requiring a feasible computational effort. Second, we make use of the depth-averaged approach to analyze the nature of bar instability in sandy rivers applying Briggs criterion to distinguish between the convectively and absolutely unstable asymptotic response to an initial boundary value perturbation of bed topography. This analysis allows us to conclude that in the presence of suspended load, instability remains convective. The practical implications of such findings are discussed.