Interaction Between Bedrock Roughness and Alluvial Cover: Field Experiments and Modelling

Abstract:

The surface area fraction of bedrock covered by alluvium is typically modeled as a function of sediment supply relative to the transport capacity, yet little is known about how the roughness of the underlying bedrock affects the alluvial cover. In this study, we performed field experiments under varied sediment supply rates, bedrock surface topographies and grain sizes. We then developed a physically-based model of the cover fraction as a function of the bedrock relative roughness (bedrock hydraulic-roughness over grain size), and tested the sensitivity of the model to changes in bedrock relative roughness and sediment supply.

The analytical results show that: 1) the cover fraction is smaller when the bedrock relative roughness is smaller; 2) when bedrock roughness is larger than alluvial roughness (clast-rough bedrock), the cover fraction gradually increases with sediment supply; 3) when bedrock roughness is smaller than alluvial roughness (clast-smooth bedrock), a fully exposed bedrock channel requires a relatively large sediment supply before any alluvial patch is formed, and as supply increases, rapidly transitions to a fully alluvial channel; 4) in the case of clast-smooth bedrock, the transition between a fully exposed bedrock channel and a fully alluvial channel has hysteresis.