| Popis: |
Recent observations have shown that debris flows containing fine particles in volcanic regions exhibit greater mobility compared to stony debris flows. Recent researches have described that greater mobility occurred from fine sediment phase shift from solid phase to fluid phase in debris flow. In Japan, debris-flow research and sabo or erosion control planning widely apply the equilibrium concentration methods proposed by Takahashi. For considering fine sediment phase shift with the equilibrium method, it is proposed to set high fluid density. However, the mechanism of phase shift and behaviours of debris flows with fine sediment, are not fully understood. In this study, we conducted hydraulic experiments with sediment particles of two different diameters, defined as fine sediment and coarse sediment. We applied the equilibrium methods and took into account the increased fluid phase density due to the sediment phase shift to fluid. From the results, we found that part of the fine particles contribute to the increase in the fluid phase density. When conducting experiments, not only fine sediment, but some parts of coarse sediment behaved as a fluid. For considering the shift of sediment to fluid phase in debris flows, we presumed that the flow turbulence in debris flow affected. Regarding the sediment concentration, higher total sediment (coarse and fine) concentration increased the fluid phase density. A larger ratio of coarse sediment increased the fluid phase density more than when sediment contained only fine particles. It was speculated to occur from the flow turbulence owing to the mixture condition. Cases with smaller total sediment discharge showed higher fluid phase density though in same sediment concentration. We also found that the larger dimensionless tractive force showed a smaller ratio behaving as fluid phase, which was in contrast with the trends in recent studies. |
