The friction coefficients of debris flows over a rigid bed from several previous experiments were compiled in a preliminary investigation on the classification of phase transitions in debris flows. The collected friction coefficients were compared to the theoretical values of the friction coefficients in the relationship with the relative flow depth on the basis of sediment particle size (h/d) under various conditions. The friction coefficients of debris flows with h/d values less than 20 agreed closely with the theoretical value for boulder debris flows derived from the constitutive equations, while the friction coefficients with h/d values in the range 1000-10,000 agreed roughly with the theoretical value for turbulent water flows. The friction coefficients with h/d values of 30-300 exceeded the theoretical value for both debris and turbulent water flows. These intermediate debris flows were observed in experiments involving turbulent mud flows. However, a review of these experiments revealed that they may have included debris flows in which the turbulent structure was not well developed, and could be considered as debris flows in transition from laminar to turbulent flows. In some of the transitional debris flows, an interface dividing the flow structure into an upper turbulent-flow layer and a lower debris-flow layer was observed as reported for sediment-laden flows. The friction coefficient for transitional debris flows was modeled considering the shift of this interface. The model was able to explain the value for transitional debris flows, inferring that phase transition in debris flows from laminar to turbulent flows is induced by the shift of the interface.

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This study analyzes the mechanism of the landslide event at Hsiaolin Village during Typhoon Morakot in 2009. This landslide event resulted in 400 deaths. The extremely high intensity and accumulative rainfall events may cause large-scale and complex landslide disasters. To study and understand a landslide event, a combination of field investigations and numerical models is used. The landslide area is determined by comparing topographic information from before and after the event. Physiographic parameters are determined from field investigations. These parameters are applied to a numerical model to simulate the landslide process. Due to the high intensity of the rainfall event, 1,675mm during the 80h before the landslide event, the water content of soil was rapidly increased causing a landslide to occur. According to the survivors, the total duration of the landslide run out was less than 3min. Simulation results indicated that the total duration was about 150s. After the landslide occurrence, the landslide mass separated into two parts by a spur at EL 590 in about 30 to 50s. One part passed the spur in about 30 to 60s. One part inundated the Hsiaolin Village and the other deposited at a local river channel and formed a landslide dam. The landslide dam had height between 50 and 60m and length between 800 and 900m. The simulation result shows that the proposed model can be used to evaluate the potential areas of landslides induced by extremely high intensity rainfall events.

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Landslides may cause huge sediment disasters. To mitigate such sediment-induced disasters, the behavior of the landslide must be predicted, in addition to the time, the location of occurrence, and the scale of the landslide. This paper proposes a two-dimensional numerical simulation for landslides. The Mayuyama landslide of 1792, which was triggered by volcanic activity, caused one of the largest disasters in the world. To reproduce sediment movement resulting from this landslide, 2-D numerical simulation and topographical analysis are discussed. The topography of Mt. Mayuyama before the failure, the topography of the slip surface, and the characteristics of the landslide material are estimated for conducting numerical simulation. Results suggest that landslide volume is about 150 million m3 or more, the landslide probably reached the sea in only a minute, and the event may have been almost finished in a couple of minutes. Landslide velocity upon reaching the sea is estimated at 100 m/sec and the thickness of landslide front is estimated at 30 m, which are enough to generate a tsunami causing a huge disaster along the seashore of Ariake bay.