Interaction among alluvial cover, bed roughness, and incision rate in purely bedrock and alluvial‐bedrock channel

A major control on bedrock incision is the interaction between alluvial cover and erosive mobile grains. The extent of alluvial cover is typically predicted as a function of relative sediment flux (sediment supply rate over bed load transport capacity, q bs / q bc ), yet little is known about how the bed roughness affects the alluvial cover. We performed field experiments with various flow discharges, sediment supply rates, grain sizes, and bed surface topographies. We then developed physically based models for estimating the threshold of sediment movement and the extent of alluvial cover, so as to include the effect of roughness change. The results for the threshold of sediment movement and the extent of alluvial cover obtained from our models show reasonable agreement with the results of the field experiments. We explored the sensitivity of the models to variations in sediment supply and bedrock relative roughness (bedrock hydraulic roughness height over grain size, k sb / d ). The results suggest the following: (1) a larger relative roughness yields a greater dimensionless critical shear stress required for initial sediment motion; (2) at a given sediment supply rate, the extent of alluvial cover is larger when the relative roughness is larger; (3) when the sediment supply rate and the relative roughness are small, throughput bed load moves over (and can abrade) a purely bedrock channel with no alluvial cover; and (4) the critical value of sediment supply rate below which throughput bed load transport occurs increases with decreasing relative roughness. The experimental results and analysis provide a framework for treating the (a) incisional morphodynamics of purely bedrock rivers by throughput bed load with no alluvial cover, (b) incisional/alluvial morphodynamics of mixed bedrock‐alluvial rivers, and (c) purely alluvial morphodynamics, as well as the transition between these states.