Debris flow is generally composed of a wide range of solid particles and viscous pore fluid. Interactions between solid particles and the resulted segregation in debris flows are affected by multiple parameters,and usually govern the rheological properties of debris flows and the flow mobility.Through the theoretical analysis,the effect of solids segregation on the flow mobility was demonstrated.Furthermore,granular debris flows with grains of mixed sizes were numerically modelled and the contact behaviour of solid particles was fundamentally studied using the Discrete Element Method (DEM).Compared to granular flows with uniform solid particles,the mechanisms of solids segregation and the effect on granular flow mobility were investigated.The results showed that that segregation can significantly increase the flow velocity of the front head and the length of the granular body on slopes.Due to the solids segregation,the shear rate developed in the granular body is greatly enhanced and no shearing-enhanced boundary layers near the sloping bed can be observed.
