A discrete element method and computational fluid dynamics coupled investigation on the hydrodynamic erosion of soil–rock mixtures

Hydrodynamic erosion is widespread in colluvial and reservoir-bank deposits, where gap-graded soil–rock mixtures (SRMs) are highly susceptible to erosion under hydraulic conditions. However, the mechanisms governing hydrodynamic erosion in gap-graded SRMs remain unclear. In this study, a coupled discrete element method and computational fluid dynamics framework is developed to investigate the hydrodynamic erosion of gap-graded SRMs, in which the discrete element method tracks particle motion and contact forces, while the computational fluid dynamics module computes pore-scale fluid flow based on Darcy's law. Two-way coupling is achieved by exchanging porosity and fluid drag forces at each time step, allowing the fluid field and particle dynamics to be updated. The numerical approach is validated through two laboratory tests for simulating hydrodynamic erosion. A series of numerical simulations are conducted to investigate the influence of rock content (RC), rock particle shape, and hydraulic gradient on hydrodynamic erosion behavior. The results indicate that higher RC generally leads to greater cumulative fine particle loss, accompanied by highly heterogeneous flow fields with preferential erosion near boundaries. Irregular rock particles further restrict pore connectivity, suppressing continuous seepage channels while enhancing localized clogging.