Modeling Steady Grain Sorting in River Bifurcations

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.River bifurcations occur when the main channel course splits into two secondary branches. The partition of sediments between these branches is essential to define whether bifurcations are morphologically stable in time, preventing sedimentation, and abandonment of the smaller channel. In this paper, we formulate a mathematical model to study how sediments of different sizes divide at bifurcations in morphodynamic equilibrium. Specifically, we consider two classes of sediment particles (e.g., fine and coarse gravel) and we investigate the role of the key mechanisms driving the sediment transport: the gravitational effect, which tends to move downhill coarse particles; and the selective transport effect, which promotes the mobility of coarse particles within the most-carrying branch. These two mechanisms are found to bring contrasting effects, but the net results are clear. First, flow asymmetry is found to reduce with respect to the case where sediment particles of uniform size are considered. Second, the bed composition of the smaller branch becomes finer, contributing to maintain a significant sediment transport. These effects can explain patterns observed in gravel-bed river bifurcations, and suggest how adjustments of bed composition can contribute to maintaining all branches active in the long term.Grain sorting reduces water and sediment flux asymmetry between downstream bifurcatesThe less-carrying branch undergoes significant fining, which enhances its sediment transport capacityThe overall bed composition of downstream branches is finer than that of the feeder channel