Debris-flow is a kind of two-phase fluid, which contains liquid-phase with slurry and solid-phase with coarse grains. The resistance characteristics, which can be divided into internal resistance and external resistance, is a critical factor for the study of kinematic and dynamic of debris-flow.The external resistance caused by interaction between debris-flow and gully-bed and gully-wall depends on the roughness and geometric shape of gully and properties of debris-flow.The debris-flow can be classified into viscosity debris-flow and two-phase debris-flow for understanding their internal resistance characteristics. For the former, the solid-phase and liquid-phase during the motion have no obvious slip velocity, therefore viscosity debris-flow could be identified as a kind of homogeneous mixing fluid and modeled by pseudo-homogenous fluid. However, for the two-phases debris-flow, the solid-phase and liquid-phase have obvious slipvelocity;the stress forces due to solid-liquid interaction, such as viscosity drag force and virtual mass force etc., plays critical role, and thus the stress due to interactions of solid grains, which comes from granular flow, is a key factor. The Fine contents in liquid-phase slurry has a huge effect on the resistance character of a debris flow and can change the resistance character of liquid-phase slurry, including yield stress and viscosity coefficient. Furthermore, the viscosity coefficient of slurry can have effects on stress due to interaction of solid grains. In this study, a comprehensive resistance model of debris-flow, including internal resistance and external resistance, has been proposed.
