Coupled DEM-CFD investigation on the formation of landslide dams in narrow rivers

Large-scale landslide dams can induce significant hazards to human lives by blocking the river flows and causing inundation upstream. They may trigger severe outburst flooding that may devastate downstream areas once failed. Thus, the advancement in understanding the formation of landslide dams is highly necessary. This paper presents 3D numerical investigations of the formation of landslide dams in open fluid channels via the discrete element method (DEM) coupled with computational fluid dynamics (CFD). By employing this model, the influence of flow velocity on granular depositional morphology has been clarified. As the grains settle downwards in the fluid channel, positive excess water pressures are generated at the bottom region, reducing the total forces acting on the granular mass. In the meantime, the particle sedimentations into the fluid channel with high impacting velocities can generate fluid streams to flow backwards and forwards. The coupled hydraulic effects of excess water pressure and fluid flow would entrain the solid grains to move long distances along the channel. For simulations using different flow velocities, the larger the flow velocity is, the further distance the grains can be transported to. In this process, the solid grains move as a series of surges, with decreasing deposit lengths for the successive surges. The granular flux into the fluid channel has very little influence on the depositional pattern of particles, while it affects the particle-fluid interactions significantly. The results obtained from the DEM-CFD coupled simulations can reasonably explain the mechanisms of granular transportation and deposition in the formation of landslide dams in narrow rivers.