Natural and human-made diversions of flow and sediment toward side channels in lowland rivers

The division of sediment at river bifurcations is crucial for the morphodynamics of anastamosing rivers, distributary delta channel networks, and rivers controlled by infrastructure. Many river bifurcations are strongly asymmetric and have a planform where a small channel branches off to the side. Such a configuration is also typical for human-made diversions of water and sediment into canals. At asymmetric bifurcations, the division of sediment is influenced by the secondary current, which is caused by the turning of the flow toward the side. The secondary currents steer especially water from the lower parts of the water column into the side branch. As the sediment concentration close to the bottom is high, side branches can receive a disproportionately large fraction of the incoming sediment load, relative to the water discharge. Lateral diversions have been extensively studied with physical and numerical experiments, with the goal to either mitigate or exploit this effect. Here, we present a comprehensive analysis using an idealized model, revealing how the division of sediment is influenced by the width and depth of the branches. We show that the excess of sediment that is diverted into the side branch is lower when the inlet to the side branch is wider and shallower. This may have larger implications for the stability of delta channel networks, as inlets to side branches tend to be locally wider, which likely contributes to their morphological stability. We apply the idealized model to explain water and sediment diversion to a secondary channel created by a longitudinal training dam in the River Waal in The Netherlands.