Turbulence dynamics and grain-size interaction effects on bedload transport flux

Near-bed coherent structures and turbulent stresses play an important role in the dynamic of bedload transport. Sediment motion is the result of the competition between instantaneous local hydrodynamic stresses with resistive forces and particle collisions. Large immobile particles induce an additional spatial and temporal variability in bedload fluxes caused by flow separation, and large-scale unsteady vortices that can either hide or expose smaller mobile grains to variable shear stresses and pressure fields. However, such variations are ignored with the commonly used reach-averaged estimations of bedload transport capacity. To improve our understanding of the dynamics of bedload transport processes and their interplay with larger immobile grains, we perform LES of the turbulent flow through an array of boulders placed in a rough bed, coupled with a Lagrangian model for mobile sediment particles. We analyze bedload fluxes across the domain and provide quantitative insights into how sediment motion is influenced by the dynamics of the flow induced by the larger particles. We also offer new perspectives on the fundamental mechanisms of particle entrainment and transport, and their implications for river morphology and bedload transport estimations at larger scales.