Bedload sediment transport in the turbulent flow over a rough bed with an array of boulders*

Bedload transport is a complex phenomenon significantly influenced by near-wall turbulence, where coherent structures and velocity fluctuations can cause the onset of motion and mobilize sediments downstream. Namely, particles are subject to the competition of instantaneous local stresses with resistive forces and particle collisions. Transport is also affected when large immobile boulders, induce flow separation and large-scale unsteady vortices that can either hide or expose smaller mobile grains to the local flow. With the objective of improving our understanding of the effects that an array of boulders placed on a rough bed produces on bedload transport, we perform Large Eddy Simulations (LES) coupled with an immersed boundary method for the boulders and the rough bed. Based on the double-averaged (DA) methodology we explain the effects caused by the turbulence generated at the scale of the roughness elements through the new terms on the DA momentum and energy budgets. We also evaluate the effective shear stress to estimate bedload fluxes, considering form-induced stresses and excluding drag. The turbulent flow is then coupled to the Lagrangian tracking of sediment particles based on the discrete element method (DEM) to evaluate how spatial flow variations influence transport. Throughout this work we characterize the spatial variability of bedload fluxes and other sediment quantities such as particle velocity and activity in regard to local flow properties for two different mobility conditions.