A new coupled depth-integrated model incorporating 3D DDA on debris flow with large boulders

Debris flow frequently occurs in mountainous regions and can cause serious damages to residents and structures in susceptible areas. Conventional debris flow simulation (DFS) with a depth-integrated model has been widely used to study debris flow behavior. In DFS, two-phase fluid of water and solid is considered. However, large boulders are often entrained in debris flows in practice. Obviously, the behaviors of debris flow with large boulders cannot be correctly described by DFS with two-phase fluid model. Therefore, a new coupling method for simulating debris flow with large boulders is developed by combining three-dimensional discontinuous deformation analysis (3D DDA) with conventional DFS. The dynamic behavior of those entrained boulders in debris flow can be solved by 3D DDA while the kinematic and mechanic behaviors of two-phase fluid are predicted by conventional DFS. An interactive module is introduced to address the interaction between solid motion in 3D DDA module and fluid field in DFS module. The imposed interaction forces from fluid on solid block include buoyancy, drag force and static pressure. Drag effect between adjacent blocks in debris flow is evaluated empirically to make the calculated drag force more realistic. For the block resistance to the debris flow, an equivalent solution is proposed by time-varying elevation of terrain node. Subsequently, the developed 3D DDADFS method is validated by several simple examples. Further, a case study is carried out to discuss the kinematic and mechanic behaviors of debris flow with large boulders based on a real catchment. The dam safety under different dam material properties is investigated when debris flow with large boulders happens. The simulation results suggest debris flow with large boulders can be more destructive than a pure debris flow. The developed 3D DDA-DFS method is a promising approach to study debris flow with large boulders.