Effect of grain-sorting waves on alternate bar dynamics: Implications of the breakdown of the hydrograph boundary layer

Abstract. Understanding the morphological responses of gravel-bed rivers to changes in external forces (e.g. water and sediment supply conditions) is a critical concern in river science and engineering. However, this remains a challenging issue because river responses are highly dependent on the distance from the source point where such environmental changes occur. Here, we focus on the short-term effects of flood-scale non-equilibrium sediment supply on the downstream alternate bar dynamics in poorly sorted gravel-bed rivers using a numerical morphodynamic model. Specifically, we perform a two-dimensional morphodynamic calculation using iRIC-Nays2DH in a straight channel under repeated cycles of an unsteady water hydrograph and a constant supply of poorly sorted sediment. Under the well-sorted sediment cases, the upstream non-equilibrium sediment supply can affect only a limited distance from the upstream end (i.e. the hydrograph boundary layer). However, the inclusion of a poorly sorted sediment disrupts this concept, leading to the migration of low-amplitude bedload sheets far downstream. In this context, the upstream water and sediment boundary conditions may affect the far-downstream river dynamics through the migration of bedload sheets. The numerical results show that the migration of bedload sheets and the associated fine sediment transport greatly affect the alternate bar dynamics and change their texture. However, this effect of bedload sheets on bars cannot propagate across the entire channel and disappears completely in the alternate bars located further downstream. These results suggest that the upstream non-equilibrium sediment supply condition in poorly sorted sediment has a non-negligible role in downstream alternate bar dynamics even far from the sediment feed point. However, this effect becomes negligible in the further downstream reaches as long as active and larger morphological changes, such as alternate bars, greatly disperse the bedload sheets.