Effect of grain sorting on gravel bed river evolution subject to cycled hydrographs: Bed load sheets and breakdown of the hydrograph boundary layer

Abstract The evolution of gravel bed rivers under cycled hydrographs and constant sediment supply is studied in this paper. With the aid of numerical modeling and flume experiments, previous research has indicated that under such water and sediment supply conditions, a river can reach a mobile‐bed equilibrium characterized by a hydrograph boundary layer (HBL) located near the upstream end of the river. The HBL defines the spatial region within which riverbed topography and grain size respond to the hydrograph, and downstream of which these bed characteristics are invariant. However, neither the governing physical mechanisms nor the general applicability of the HBL is yet well understood. In this paper, we implement a 1‐D morphodynamic model for this problem. The model is first validated with data for both flume experiments and numerical simulations. It is then implemented at field scale with grain size distributions varying from uniform sediment to very poorly sorted sediment mixtures. Our results show that the idea of an HBL is applicable in the case of uniform sediment, but breaks down in the case of poorly sorted sediment mixtures. In such cases, persistent low‐amplitude bed load sheets emerge along the entire channel reach, in addition to HBL‐like upstream boundary effects. Bed load sheets can be described as surface texture or grain size sorting waves in consonance with low‐amplitude elevation variations, and are ascribed to nonlinear advection terms in the governing equations that vanish for uniform sediment, but become significant when sediment is very poorly sorted.