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 Keywords: Two-phase debris flows ; Flow-obstacle-interactions ; High-resolution numerical simulations ; Shock waves ; Impact vacuum
