Study on two-phase dynamic behaviours within non-homogeneous debris flow

Abstract

Non-homogeneous debris flows consisting of a wide range of grain size are commonly modelled as two-phase flows. In adopting such an approach, a critical grain diameter to separate the solid and liquid phases within such debris flows can be determined through the principles of minimum energy dissipation. In this paper, an improved analytical approach using the resistance formula of water flow and the mass conservation law is presented to determine the velocity of solid and liquid phases within a non-homogeneous debris flow. Some of the dynamic parameters are validated against experimental data measured from the Jiangjia gully. The results show that, for the majority of non-homogeneous debris flows tested, the liquid phase exhibits higher velocity than the solid phase. However, as the bulk density of the debris flow increases, the solid phase tends to have higher velocity than the liquid phase. These findings are shown to have important implications on the vertical grading patterns of bed deposits. The field observations indicate that non-homogeneous debris flows with bulk density being significantly lower, close to and significantly higher than the critical value seem to exhibit, respectively, normal, mixed and inverse grading patterns in the alluvial fan deposits.