Different approaches to handling vertical and streamwise sorting in modeling river morphodynamics

This paper presents an assessment of the strengths and weaknesses of four sediment continuity models for nonuniform sediment by applying these models to an aggradational flume experiment that is dominated by nonuniform sediment and dunes. The author makes simulations of the flume experiment using four numerical morphodynamic model systems to which the following sediment continuity models are applied: the commonly applied active layer model (A), a two-layer model (B), a sorting evolution model while assuming bed form size to be regular (C1), and a sorting evolution model while taking into account the variability in bed form geometry (C2). The model systems that incorporate the variability of bed form geometry, i.e., the two-layer model and the sorting evolution model with irregular dunes, show an improved prediction of the adaptation timescale of the composition of both the bed surface and the transported sediment, as well as the vertical sorting profile. This is because including the variability of bed form geometry enables the model system to account for sediment being stored (temporarily) at elevations that are exposed to the flow less frequently. Future application of both models to field cases is difficult, however, as the two-layer model is not sufficiently generic and may lead to ellipticity of its set of equations, whereas the sorting evolution model requires a very small numerical time step.