Study on the effect of entrainment on the behavior of debris flows using a 3D Coupled Eulerian–Lagrangian finite element method

Debris flows are one of the most hazardous surface processes. The entrainment of bed sediment is an essential process in the debris flows. The volume and potentially destructive power of debris flows can be increased by the entrainment of bed sediment. Numerical analysis of debris flows with and without entrainment was conducted to confirm the effect of entrainment on debris flows mobility. To simulate entrainment of bed-sediments by debris flows, the numerical analysis was focused on the change in the properties of the soil layer. In order to take into account the solid-like elastoplastic behavior before failure and fluid-like viscous behavior after failure, the two constitutive models were combined using the deviatoric shear strain of the soil layer induced by the debris flow. The model for the simulation of debris flows over erodible beds was validated using published experimental data. To quantify the effects of entrainment on the behavior of debris flows, the flow length, angle of inclination, and erodible depth were controlled in numerical analysis. The depth of the entrained soil layer increased as the depth of the wetting band increased. The results of the analysis were represented in the velocity, thickness of debris flows reaching the bottom area, and impact pressure. It was confirmed that the velocity of debris flows decreases with increasing viscosity and the velocity of debris flows arriving later appears to be faster as the wetting depth increases. Entrainment of the bed-sediment by debris flowing could either increase or decrease its mobility depending on the flow length. The maximum velocity and thickness of debris flow increased with the increasing slope inclination angle.