Flume Experiments on the Erosive Energy of Bed Load Impacts on Rough and Planar Beds

Abstract

AbstractUnderstanding bed load impact dynamics on exposed bedrock in rivers is crucial to quantifying the erosive energy of a stream. Observations of the bed load saltation trajectories and impact energies are lacking, particularly in channels with nonplanar bed topography. In this study, we performed four flume experiments of saltating gravel to offer new insights on the dependence of particle impact dynamics on transport stage and bed topography. Our experiments used two different boundary shear stresses (τb = 36.5 and 25.4 Pa) and two different bed configurations, planar and nonplanar (bedrock disks uniformly oriented at 10° from the bed surface). For each experiment, we indirectly estimated the impact energies from the trajectories of particles with high‐speed video imaging and measured the erosion rates of rock samples embedded in the flume floor. The planar and nonplanar beds had negative and constant relationships, respectively, between energy delivered to the bed and transport stage. The nonplanar bed had a heavier tailed distribution of impact energy than the planar bed, which implies a greater number of rare highly erosive impacts. Probabilistic formulations of particle trajectories better predict the increase or decrease in erosion rate across experiments than deterministic regression equations. Our findings suggest that bedrock erosion models should consider a distribution of possible bed load impact energies, particularly for natural river channels that have rough surfaces.