Sediment movement induced by deep]seated rapid icatastrophicj landslides
and location of damage occurrence

Yuki NISHIGUCHI, Taro UCHIDA, Yasutaka TANAKA, Jun’ichi KAMBARA, Ryosuke OKUYAMA,
Junya HINA, Tomoo MATSUBARA and Wataru SAKURAI

In steep mountainous regions, not only soils but also weathered bedrocks were sometimes sliding simultaneously. These landslides often move rapidly and triggered debris flow and sometimes induced landslide dam. In this study, these landslides are referred to “deep]seated rapid (catastrophic) landslide". This study excludes slow failures of a more chronic nature, such as deep]seated chronic landslide, deep]seated gravitational creep or rock flow, from the deep]seated rapid landslide. To date, there is no adequate information about probability of damage occurrence due to deep]seated rapid landslide. Since there is a variety of sediment movement type, it is important to assess what type of sediment movement may occur. In this study, we compiled existing three databases about deep]seated rapid landslide inventories, which developed by Public Works Research Institute. So, we used about300landslide inventories to characterize volume of landslide, sediment movement types and location of damage occurrence. We roughly classified the sediment movement which caused damage into three types, landslide mass, debris flow and landslide dam breach and showed the ratio of occurrence of each sediment movement type. Consequently, we found that, when the landslide volume exceeded1million m3, the ratio of disasters due to landslide dam breach was high. In contrast, if the landslide volume was smaller than1million m3, the ratio of disaster due to debris flow became large, instead of landslide dam breach. Then, we tested the correlation between deep]seated rapid landslide scale and the extension of damaged area. We found positive correlation between volume of landslide dam lake and the distance from landslide to the lower end of damaged area due to landslide dam breach. Also, we showed positive correlation between landslide area and the distance from landslide to the lower end of damaged area due to debris flow. Finally, we proposed empirical relationship between probability of damage occurrence, landslide scale and distance from landslide.