Numerical Simulation of Dynamic Alluvial Patterns in Mixed Bedrock-Alluvial Channels

Mixed bedrock-alluvial channels are characterized by exposed bedrock surfaces alternating with discontinuous alluvial sections, which may in turn develop bedforms such as bars, dunes, and ripples. The bedrock, alluvial patches, and bedforms may exert different levels of resistance on the flow and thereby cause feedbacks in shear stress, sediment transport, and channel evolution. Quantitative numerical prediction of patterns of sediment cover in mixed bedrock-alluvial rivers is challenging due to this spatio-temporal variability in roughness. Here we use a two-dimensional morphodynamic model to explore how slope, sediment supply, and initial alluvial cover affect the development and evolution of sediment cover in mixed bedrock-alluvial channels. The model employs a particle saltation and saturation model for the treatment of sediment transport over an exposed bedrock bed, and HLLC flux to capture the possible transition and discontinuity of the flow condition caused by abrupt surface skin roughness changes at the interface between bedrock and alluvial cover. A roughness correction is applied to account for dynamically evolving topography such as incipient and migrating bedforms. Model results from simulations of dynamics in straight, rectangular channels are compared with published experimental observations to evaluate the model's ability to reproduce main features of mixed bedrock-alluvial channels such as the process of alluviation, bedform development over the bedrock surface, and the degree of bedrock exposure. This work quantifies the effects of channel slope, characteristics of sediment and bedrock, and sediment supply to capacity ratio on overall patterns of alluviation. In steeper slope channels, the bed undergoes a sudden transition from no initial sediment cover to full alluviation when the sediment supply exceeds a near-capacity threshold. In lower slope channels, form drag becomes a dominant factor controlling patterns of alluviation because additional resistance due to macroscale topography variation decreases flow velocity and sediment transport capacity, leading to a gradual transition in bedrock exposure in response to the changes in sediment supply.