Effects of defense-structure system for bridge piers on two-phase debris flow wakes

Defense structures are commonly constructed on the upstream side of bridge piers to mitigate the impact of potential debris-flow events. In this study, field investigations were carried out to obtain the geological conditions and geometric layout of engineering structures on a typical debris-flow site. A two-phase numerical model was adopted to simulate the dynamic processes of the debris flow rushing through a group of defense structures, and the V-shape debris-flow wakes detached from the defense structures were numerically studied. The results show that an oval-shape two-phase flow structure was formed due to the dynamic interaction between two neighboring debris-flow wakes. The shape of this flow structure is related to the viscosity, impact velocity, base fiction and structural arrangement. The corresponding center position and the maximum thickness of this superposed wakes are found to be correlated (nonlinearly or linearly) to the Froude number, Reynolds number, Manning friction number and lateral spacing of the defense structures. The present study improves the understandings toward the effects of the protective structural system on the debris-flow dynamics and can be used to optimize the structural design in debris-flow risk area.