Particle Collisions Control Stable Bed Configuration Under Weak Bedload Transport Conditions

Abstract:

Sedimentary bed configurations that are stable under weak bedload transport conditions can be divided into two groups: (1) meso-scale features that influence flow and sediment transport through roughness and drag partitioning effects (e.g. ripples and dunes), and (2) grain-scale features that can effectively be ignored at the macroscopic scale (e.g. lower-stage plane bed and microforms). These groups produce distinct sedimentary structures and are thought to be separated by a transition in process regime characterized by the onset of flow separation and scour. However, the physical mechanisms responsible for this transition are poorly understood. Previous studies suggest that interactions between moving particles lead to stabilized bed disturbances that initiate nonlinear coarsening. This study presents a quantitative interpretation of this hypothesis that is tested using experimental observations of particle motion over stable and unstable planar topography. We find that the microform/mesoform transition corresponds to the transition from rarefied to congested transport quantified by a dimensionless Knudsen number. Theory presented herein enables prediction of fluvial bed configuration from shear stress and grain size under weak transport conditions.