Linking Bedrock Topography to Grain Entrainment in Bedrock-Alluvial Channels

The dynamics of sediment grains on a bedrock surface are a key component of bedrock erosion within bedrock-alluvial channels. Grains in transport can erode the bed, whereas static cover provides protection. These dynamics are thought to be a function of the relationship between the roughness length of the bedrock (k) and the size of the sediment (D), with smaller D/k values inhibiting sediment mobility. However, we do not know a) the form of this relationship, nor b) the spatial scale over which k should be calculated. We present a new set of laboratory data to quantify the relationship between bedrock roughness and grain mobility, where mobility is measured as the pivot angle of a grain on the bedrock surface. Topographic data from rivers with different degrees of roughness and structural characteristics were 3D printed at a reduced scale, and fixed to a tilt table. Grains of different sizes were systematically dropped at different locations over each the printed surface, and the table was tilted until the grain moved. One surface was comprised of bedrock ribs, and so experiments were repeated with the ribs parallel and perpendicular to the downslope direction. Additional experiments were performed after incrementally covering 25% through to 100% of the surface with fixed sediment cover. We find that overall measured pivot angles decrease with increasing D/k, showing a similar relationship to those that have been identified in alluvial settings. However, for an individual surface, mean pivot angle does not necessarily decrease with increasing roughness because of the interaction between different sized grains and the different scales of roughness present on the surface. Rib direction also has a significant influence on mean pivot angle. The impact of sediment cover depends on the relative roughness of the cover and the bedrock surface. These data represent the first attempt to quantify these relationships, and have implications for predicting the critical shear stress of sediment grains in bedrock-alluvial channels.