Multi-phase-flow modeling of underwater landslides on an inclined plane and consequently generated waves

•Underwater landslides and the resultant waves are simulated using a three-phase flow model.•A smooth bed yields a faster sliding and larger waves than a rough bed.•The effects of particle size differ between the rough bed and the smooth bed. An underwater landslide is a complex multiphase flow phenomenon with potential to cause large waves and destruction. Particle size is an important parameter that affects flow behaviors of multiphase flows. This numerical study examines underwater granular landslides on an inclined plane and their resultant waves with the purpose of exploring how the particle size affects landslide kinematics and wave amplitudes. A rheology-based multi-phase flow model is employed, newly incorporating the Navier-slip condition. Smooth and rough beds that reflect ideal and practical conditions, respectively, are simulated. Five types of particle are used; they are very fine sand, medium sand, coarse sand, granules, and pebbles. The granular masses are initially in the loose-packing condition. Our numerical results demonstrate that a smooth bed results in the faster landslides and the larger impulsive waves than a rough bed. The effects of particle size differ between the two bed conditions. For the rough bed, the relationship between the landslide speed and the particle size is negative, and so is the relationship between the wave amplitude and the particle size. For the smooth bed, the two relationships become concave and non-monotonic.