Most of the landslide dams are composed of loosely and widely graded cohesionless soil, it is susceptible to internal erosion during the rapid rising of upstream water level, and threaten the downstream residents' property and life safety. CFD-DEM coupling approach is often used to model the internal erosion, but it often costs lots of time due to the coupling. Based on the encountered forces of cohesionless soils of landslide dams during the process of seepage failure, this paper analyzed the fluid force applied to the particles and suggested a simplified method to modeling the internal erosion process with the discrete element method. In this method, the encountered fluid force of a particle in the flow field was applied to the DEM module in the form of an acceleration field. In this way, the amount of calculation required for the dual-module interactive iteration in the traditional CFD-DEM coupling method could be reduced, and it could effectively improve the calculation efficiency based on retaining the calculation effect of the CFD-DEM coupling method. Finally, considering the wide-graded non-cohesive dam soil material, the simplified ideal gap-graded sized particle model is used to compare the CFD-DEM coupling calculation results to verify the rationality and effectiveness of the fast calculation method proposed in this paper. It shows that the relative error is within 3.4%, which shows the reliability of the simplified simulation method. When a simplified simulation method is adopted to simulate specific cases, the specific magnitude of the error may changes. In terms of calculation speed, the running speed of the fast simulation method in this simulation is about 3.5 times than that of the CFD-DEM coupled simulation method, and when the number of particles increases, the calculation speed of the simplified simulation method increases significantly.
