Landslide- and debris-flow-induced river blockage is a critical hazard-chain process in which a dense granular mass enters an open water body, deposits, and progressively obstructs conveyance. In contrast to conventional landslide-generated tsunami studies that emphasize impulsive wave generation, blockage prediction requires resolving multiphase deposition–hydrodynamic feedback under large density/viscosity contrasts, strongly non-Newtonian granular behaviour, and realistic inflow/outflow. We present an open-boundary extension of an interface-aware, conservative δR⁺-SPH framework tailored to landslide/debris-flow damming in a numerical “river/lake”. The method combines δ⁺-type stabilization with phase-aware particle shifting and divergence cleaning to maintain sharp interfaces, stable pressures, and near-constant phase volumes during violent multiphase transients [1]. Open waters are constructed with inlet/outlet boundary conditions using a buffer-zone particle strategy. The granular phase is represented using viscoplastic non-Newtonian or μ(I)/visco-inertial rheology consistent across dry–transitional–submerged regimes, while interfacial shear stresses across multi-viscosity phases are treated using a harmonic-mean interaction viscosity [2]. Verification is conducted with stratified multi-viscosity Poiseuille flow and Rayleigh–Taylor instability benchmarks. Validation against large-scale flume experiments of granular landslides interacting with water and three-dimensional deformable granular-slide wave tests demonstrates that the proposed model reproduces runout, deposit geometry, free-surface response, and evolving blockage metrics with robust agreement. These results indicate that δR⁺-SPH provides a physics-resolving, mesh-free tool for quantifying dam-formation dynamics and for supporting hazard assessment and engineering design for landslide/debris-flow-induced river blockage. REFERENCES [1] Y. Ma, M. Asai, and Z. Han et al, An interface-aware, conservative δR-Plus-SPH for granular slide–water interaction across subaerial–subaqueous regimes, Available at SSRN 5678842. [2] A. Shakibaeinia and Y.-C. Jin, MPS mesh-free particle method for multiphase flows. Comput. Methods Appl. Mech. Engrg., Vol. 229, pp. 13-26, 2012.