Two-phase depth-resolved numerical model captures debris flows entraining water-saturated sediments

The importance of erosion processes in influencing the long-distance travel of geophysical mass movements (such as debris flows, rock and snow avalanches, and landslides) is well recognized. However, numerical modeling of these processes remains difficult and is frequently overlooked. Typically, researchers have neglected entrainment or employed empirical models, where the entrainment parameters must be back-calculated to achieve the observed erosion volume and runout. Instead, in this work, we use a two-phase depth-resolved model, within an elasto-plastic framework, utilizing a dilatant Mohr-Coulomb constitutive model based on Terzaghi's effective stress principle. This model effectively captures large deformations and the interactions between solid and liquid phases in water-saturated soils subjected to overriding granular flows. Consequently, it naturally simulates bed liquefaction—the transition of initially solid soil into a liquid-like state—when overridden by debris material. The simulations reveal that the initial characteristics of the bed material, such as its permeability and consolidation degree, are crucial in influencing pore pressure generation and dissipation, degree of bed material mobilization and flow travel distance, consistent with observations from natural events. The study highlights the need to consider ground hydrological and geotechnical properties when predicting landslide hazards while also offering a detailed quantitative analysis of how bed mechanical properties influence the potential for liquefaction. Since bed material properties can potentially be measured through laboratory and field tests, the two-phase depth-resolved model has the capabilities to predictively simulate real events.