Lateral Erosion of Bedrock Channel Banks by Bedload and Suspended Load.

Bedrock rivers carry large amounts of fine sediment in suspension. We developed a mechanistic model for erosion of bedrock channel banks by impacting bedload and suspended load particles that are advected laterally by turbulent eddies (advection‐abrasion model). The model predicts high lateral erosion rates near the bed, with rates decreasing up to the water surface. The model also predicts greater erosion within the suspended load layer than the bedload layer for many typical sediment supply and transport conditions explored. We compared the advection‐abrasion model with a previously derived model for lateral erosion of bedrock banks by bedload particles deflected by stationary bed alluvium (deflection‐abrasion model). Erosion rates predicted by the deflection‐abrasion model are lower, except within limited conditions where sediment is transported near the threshold of motion and the bed is near fully covered in sediment. Both processes occur in bedrock rivers at the same time, so we combined the advection‐abrasion and deflection‐abrasion models and found that the lateral erosion rate generally increases with increasing transport stage and relative sediment supply for a given grain size. Application of our combined‐abrasion model to a natural bedrock river with a wide distribution of discharge and supply events, and mixed grain sizes, indicates that finer sediment dominates the lateral erosion on channel banks in low sediment supply environments and can be as important as coarser sediment in high sediment supply environments. Plain Language Summary: The width of bedrock rivers is controlled by the competition between vertical and lateral erosion against a backdrop of vertical uplift. Both processes are governed by the quantity and size of sediment that is transported. Bedrock rivers transport small particles suspended in the water column and large particles along the bed. Compared with large particles, bedrock erosion by impacts of small particles transported in suspension is poorly understood. Here, we develop a model to explore whether the small particles can be responsible for eroding bedrock channel banks. The new model shows that particles transported in suspension cause higher erosion on channel banks than the particles transported along the bed, except where bedrock channels are especially steep. Compared with a model of large particles transported along the bed, the new model predicts higher bank erosion rates, except in bedrock rivers where sediment covers the whole bed and is transported near the threshold of motion. We find that small particles cause more lateral erosion than large particles when the river transports only small amounts of sediment. Small particles can be as important as large particles when the river transports large amounts of sediment. Key Points: A new model is developed for lateral bank erosion by bedload and suspended load particle impacts from advection by turbulence eddiesThe model predicts higher lateral erosion within the suspended load layer than the bedload layer for many sediment supply and transport conditionsCombining the model with a bedload‐deflection‐based abrasion model indicates that finer sediment is important for eroding bedrock banks [ABSTRACT FROM AUTHOR]