Interaction of two-phase debris flow with obstacles

Landslides, debris avalanches and flows are common events in mountainous regions, causing tremendous damages to people and infrastructures. Their dynamics are substantially affected and altered by obstacles such as trees, big boulders and civil structures on their way. Appropriately designed and optimally installed obstacles, including braking mounds, catching or deflecting dams, in the flow path can drastically change the flow dynamics by deflecting, re-directing or stopping the debris mass. Such structures can substantially reduce the kinetic energy of the flow and associated risks. So, a proper understanding of the flow-obstacle-interaction is required to construct adequate defense structures. Here, we simulate a two-phase debris flow as a mixture of solid particles and viscous fluid down an inclined surface with tetrahedral obstacles of different dimensions, numbers and orientations. This is achieved by employing a physically-based general quasi-three dimensional two-phase mass flow model (Pudasaini, 2012) consisting of a set of non-linear and coupled partial differential equations representing mass and momentum conservations for both the solid- and fluid-phases. Simulations on mass flows are performed with a high-resolution and efficient numerical scheme that is capable of capturing rapid and detailed dynamics, including the strongly re-directed flow with multiple stream lines, mass arrest, strong shock waves and debris-vacuum generation and flow pattern formations, as the rapidly cascading mass suddenly encounters the obstacles. The estimated impact pressure is useful for designing the defense structures. The solid and fluid phases show fundamentally different interactions with obstacles, flow spreading and dispersions, and run-out dynamics and deposition. The observations are in line with natural debris flows and experiments. Our understanding of the complex interactions of real two-phase mass flows with multiple obstacles helps us to plan defense structures and constitute advanced and physics-based engineering solutions for the prevention and mitigation of risks caused by different gravitational mass flows.