Journal of the Japan Society of Erosion Control Engineering, Vol.70, No.6, 2018

Study on fine sediment phase change factors and influence on debris flow behavior

Kana NAKATANI, Tomohiko FURUYA, Yuji HASEGAWA, Ken’ichiro KOSUGI and Yoshifumi SATOFUKA

In mountainous rivers, sediment transportation occur and flow with high sediment concentration such as debris flow cause huge damage in residential area. There are many studies about sediment transportation in steep mountainous area but most of them focused on large sediment. However, sediment distribution is wide from fine particles to large rocks and flow characteristic seems to be different. Recently, debris flow containing high concentration with fine particles occurred in volcanic regions, and they have been reported to show high flow‐ability comparing to stony debris flows. In debris flow simulations, methods considering fine sediment phase change as setting large fluid density have proposed and results have explained the actual event. However, the mechanism and behaviours of debris flows with fine sediment are not clear. In order to predict the run‐off process, it is important to reveal the behaviour and effect of fine particle in fluid phase. In this study, we conducted hydraulic experiment with fine sediment and coarse sediment. We presumed that part of the fine particles contribute to increase the fluid phase density when different size of particles exist or when mixture ratio of fine sediment and coarse sediment is different. We found that the density increase is affected by ratio of friction velocity and settling velocity, and fine particle diameter and concentration.

Key words:debris flow, fine sediment, phase change, hydraulic experiment, equilibrium concentration