Boulder concentration effects on sediment transport and deposition

Boulders in riverbeds significantly influence sediment transport and deposition by altering local flow patterns. This study employs coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) simulations to examine how boulder concentration affects sediment dynamics. Three boulder spacing scenarios, representing isolated, wake-interference, and skimming flow regimes, are evaluated for their impacts on flow structure, bed shear stress distribution, and sediment transport and deposition. The fluid phase is modelled using a large eddy simulation in a finite volume framework, while individual sediment grains are tracked with the discrete element method. The results reveal that increasing boulder concentration transforms the flow regime from isolated wakes behind individual boulders to a more coherent recirculation zone among neighboring boulders. This transition substantially reduces near-bed shear stresses between boulders and leads to a decline in total sediment transport rates. At high boulder concentration, sediment particles preferentially accumulate in sheltered inter-boulder corridors, forming stable depositional belts. In contrast, widely spaced boulders lead to isolated, localized deposition around individual boulders. To our knowledge, this is the first attempt to apply a coupled finite volume-DEM approach to simulate boulder-sediment interactions in open-channel flow, enabling analysis of sediment transport and deposition under varying boulder concentrations.