Effect of Sediment Transport Boundary Conditions on the Numerical Modeling of Bed Morphodynamics

Experimental sediment transport and river morphologic studies in laboratory flumes can use two sediment-supply methods: an imposed feed at the upstream end, or a recirculation of sediment from the downstream end to the upstream end. These methods generally produce similar equilibrium bed morphology, but temporal evolution can differ. The adjustment of natural rivers may be reproduced by both modes. Nevertheless, computer models of river morphodynamics typically use a sediment-feed boundary condition, which can impact the simulated evolution of transient features such as bedforms. The effect of sediment transport boundary conditions on bedform dynamics was analyzed through numerical experiments using a two-dimensional, depth-averaged sediment transport model. Two different boundary conditions were imposed at the inlet (constant sediment feed and sediment recirculated from the outlet) for two bedform scales (dunes and bars). The type of sediment transport boundary condition greatly influenced dune development. The sediment-recirculating condition produced a more dynamic bed morphology with dunes of higher amplitude. The associated zones of higher shear stress had a direct impact on the hydrodynamics and patterns of sediment transport. In the case of the bar bed morphology, the simulated bars had similar mean length and height for both sediment boundary conditions. However, the sediment-recirculating case produced a more dynamic bed, in which the dominant bar length varied over time. Finally, the simulated bed morphology with bars, agreed much better with that observed when using a calibrated sediment transport equation to match sediment discharges instead of the standard empirical sediment transport equations available in literature.