The role of dilatancy and permeability of erodible wet bed sediments in affecting erosion and runout of a granular flow: Two-phase MPM–CFD simulations

Liquefaction of saturated erodible bed sediments is often indicated as one of the main drivers for the bulking of geophysical mass flows and their extensive mobility, but this is especially difficult to capture in numerical models. In this work, we employ a two-phase depth-resolved MPM–CFD model to simulate a granular column collapsing over a water-saturated erodible bed. The solid phase is modeled using MPM, coupled with CFD to capture the behavior of the fluid phase, with a drag term accounting for the interactions between the two phases, regulated by the permeability of the medium. The model incorporates an elasto-plastic framework, utilizing a dilatant Mohr–Coulomb constitutive model based on Terzaghi's effective stress principle. The two-phase approach allows the emergence of pore pressures, liquefaction, erosion, and flow runout within the simulations. The model can naturally simulate the transition of the initially solid sediments into a liquid-like state when overridden by debris material. Through our numerical experiments, we quantify the effects of the initial dilatancy and permeability of the water-saturated bed on the runout dynamics and erosion. Contractive beds with lower permeability exhibit larger and persistent pore pressure buildup and erosion, resulting in longer runout compared to dilative beds, consistent with field and experimental observations. These findings emphasize the need to account for the hydro-mechanical properties of flow and erodible bed materials in predictive modeling of landslide hazards, moving beyond empirical formulations often used in depth-averaged models.