Landslide-induced river damming is governed by transient momentum exchange between a moving landslide and receiving open-channel flow, yet this short formation stage is difficult to observe and predict. This study presents a fully Lagrangian multiphase smoothed particle hydrodynamics framework for resolving landslide intrusion, granular deposition, channel blockage, and backwater development under sustained inflow and outflow. The method couples an interface-aware multiphase SPH formulation ^[1] with buffer-layer open boundaries ^[2], allowing river hydraulics and topographic confinement to evolve within a finite reach.
Verification against multiphase and open-channel benchmarks and validation against flume tests demonstrate that the model captures interfacial deformation, partial-to-complete blockage, and rapid upstream impoundment. The framework is then applied to the 2008 Yangjiagou landslide dam in the Mianyuan River basin ^[3], where the simulated runout, channel-spanning deposit, and impounded-water distribution are consistent with field evidence (Fig. 1).
Results indicate that dam formation is controlled not only by bulk discharge but also by depth-dependent momentum redistribution, which regulates deposition pathway and blockage efficiency. This framework provides a process-based tool for assessing river-blocking landslide hazards in confined mountain valleys.

Landslide Dam,Smooth particle hydrodynamic method,multiphase flows,open channel flow,boundary condition