Vertical sorting and the morphodynamics of bed form‐dominated rivers: A sorting evolution model

Existing sediment continuity models for nonuniform sediment suffer from a number of shortcomings, as they fail to describe vertical sorting fluxes other than through net aggradation or degradation of the bed and are based on a discrete representation of the bed material interacting with the flow. We present a new type of sediment continuity model that is based on a stochastic description of the bed surface rather than discrete bed layers. The model is aimed at conditions dominated by bed load transport wherein the river bed is fully covered by river dunes. Application of the model should be limited to spatial scales covering a significant number of bed forms. The resulting model, i.e., the sorting evolution model, is suitable for unsteady conditions, as it takes into account the time evolution of, for instance, vertical sorting in modeling net aggradation or degradation of the river bed. The present paper lists the various submodels of a morphodynamic model system to which the sorting evolution model is applied. We compare the results of the morphodynamic model system to measured data from two flume experiments. The new model shows reasonable results for the predicted time evolution of the vertical sorting profile, as well as the time evolution of the grain size distribution of the bed load transport. Yet the model does not properly include sorting mechanisms associated with partial transport and the winnowing of fines from the trough surface and subsurface. The model serves as a basis for a future simplification of the model into a new stochastics‐based bed layer type sediment continuity model in which vertical sediment fluxes are included in a parameterized way.