Formation and erosion of sediment cover in an experimental bedrock-alluvial channel

Abstract

Sediment grains in a bedrock-alluvial river will be deposited within or adjacent to a sediment patch, or as isolated grains on the bedrock surface. Previous analysis of grain geometry has demonstrated that these arrangements produce significant differences in grain entrainment shear stress. However, this analysis neglected potential interactions between the sediment patches, local hydraulics and grain entrainment. We present a series of flume experiments that measure the influence of sediment patches on grain entrainment. The flume had a planar bed with roughness that was much smaller than the diameters of the mobile grains. In each experiment sediment was added either as individual grains or as a single sediment pulse. Flow was then increased until the sediment was entrained. Analysis of the experiments demonstrates that: 1) for individual grains, coarse grains are entrained at a higher discharge than fine grains; 2) once sediment patches are present, the different in entrainment discharge between coarse and fine grains is greatly reduced; 3) the sheltering effect of patches also increases the entrainment discharge of isolated grains; 4) entire sediment patches break-up and are eroded quickly, rather than through progressive grain-by-grain erosion, and 5) as discharge increases there is some tendency for patches to become more elongate and flow-aligned, and more randomly distributed across the bed. One implication of this research is that the critical shear stress in bedrock-alluvial channels will be a function of the extent of the sediment cover. Another is that the influence of sediment patches equalises critical shear stresses between different grain sizes and grain locations, meaning that these factors may not need to be accounted for. Further research is needed to quantify interactions between sediment patches, grain entrainment and local hydraulics on rougher bedrock surfaces, and under different types of sediment supply.