Uncertainties and errors in predictions by morphodynamic models of rivers with nonuniform sediment are usually attributed to shortcomings pertaining to the submodel of sediment transport. This mistakenly neglects shortcomings in the submodel of sediment continuity, which describes the vertical sorting and the bed surface composition, whereas the latter is one of the main input parameters for calculating sediment transport and thus morphological changes. Hirano [1971 , 1972] was the first to develop a sediment continuity model for nonuniform sediment. Since this commonly used Hirano active layer model and its variants suffer from a number of shortcomings, the authors have developed a new type of sediment continuity model that describes the bed composition and vertical sorting fluxes without distinguishing discrete bed layers. This continuum sorting model is aimed at conditions dominated by bed forms and bed load transport. It is based on (1) the Parker-Paola-Leclair framework for sediment continuity, (2) the Einstein step length formulation, (3) a newly developed lee sorting function, and (4) a newly developed method to account for the variability in bed form trough elevations. The resulting model is deterministic in the computation of the vertical sorting profile and is probabilistic in terms of the riverbed surface due to the presence of dunes.
Blom, A., & Parker, G. (2004). Vertical sorting and the morphodynamics of bedform-dominated rivers: a modelling framework. Journal of geophysical research. Earth surface, 109(F2), [F02007]. https://doi.org/10.1029/2003JF000069