Modeling Steady Grain Sorting in River Bifurcations

Abstract: A striking feature of rivers is their ability to sort the sediments composing them. One of the chief causes for grain sorting consists in the selective transport of different sizes moving downstream. This process can be substantially influenced by the presence of lateral topographic variations, as those produced by channel bifurcations, in which water and sediment are diverted between two smaller anabranches. In particular, field and laboratory observations have shown that the asymmetric flux distribution commonly observed in actual bifurcations is associated with a coarsening of the most-carrying branch. Here, equilibrium sorting in river bifurcations is addressed through a quasi-2D model. Differently from previous works, a fully physically based description of the processes tied to a heterogeneous sediment mixture is introduced. The main mechanisms consist in the lateral pull of sediment due to a sloping bed upstream the bifurcation, and the variation of transport capacity in downstream branches. The model indicates that grain sorting tends to reduce the degree of flux asymmetry between branches for increasing heterogeneity of the mixture. Moreover, the uneven discharge distribution is associated with a different bed surface composition, with bed coarsening of the dominating channel and fining of the other branch. The reduced sediment transport asymmetry and bed surface fining enhance the transport of fine material in the less-carrying branch, thus contributing to keep it morphologically active. Finally, the model predicts an overall fining of bed surface material with respect to the feeder channel.