LATERAL EROSION OF BEDROCK CHANNEL BANKS BY BEDLOAD AND SUSPENDED LOAD

Those who have seen the turbid and turbulent flow of a flooding mountain river know that bedrock rivers transport vast quantities of fine sediment in suspension. Could these tiny but energetic particles be responsible for eroding bedrock river banks and thus govern the width of actively incising channels? Surprisingly, this question has not yet been investigated mechanistically. Here 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 water surface. The model also predicts greater erosion within the suspended load layer than the bedload layer for the majority of 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.