Understanding the fundamental dynamics of interaction between multi-phase geophysical flows and engineering structures is crucial for mitigating geophysical hazards. Specifically, liquid phase between particles induces matric suction which could play a significant part in regulating flow dynamics and warrants further consideration. In this study, flume model tests were conducted to investigate the effects of water content (0–30%) on the impact behavior of granular flows. The particle image velocimetry technique

was adopted to visualize the impact kinematics and the impact force was measured through a model barrier system. Results revealed that, besides geometric effects (kinetic sieving), mechanical effects (shearing and collision) are also vital for the mechanism of reverse segregation. At higher water contents, 20 and 30% in this study, discrete-surge impact, rather than a progressive impact process, was observed. The discrete surges induce impulses on the barrier. The discrete surges result from selforganization of unsaturated granular flows to overcome the enhanced shear strength induced by matric suction. Finally, a dimensionless index, namely the suction number, is used to quantify

the effect of suction on the dynamic behavior of granular flows. Even for large-scale geophysical flows, if the content of fine particles is high, effect of suction should not be neglected.