In this paper, we propose an estimation method associated with the effects of different rainstorm conditions on the timings of landslides occurrences. This method uses three-layer tank model calculating soil water index (SWI) which has been widely used in Japan in order to predict the timing of shallow landslide occurrences. The novelty of this method is due to use an each tank storage value. This method was applied to three landslides. One is shallow landslides occurred on 10 July 2010 in Shobara-city Hiroshima prefecture, and the others are deep seated landslides occurred on 6 September 2005 in Shiiba village and Misato town (former name : Saigo-village) Miyazaki prefecture. The results of this study were as follows : 1) Landslides occurred after the time when a tank storage value exceeded the past maximum storage value ; 2) In the comparison between a tank storage value and past maximum value, to use the return period (RP) of a tank storage value is valid ; 3) The difference of rainstorm conditions is able to be recognized as the difference of RP of each tank layer storage value ; 4) The timing of shallow landslides occurrence depended on the RP of the first-layer tank, and the timings of deep seated landslides depended on the RP of the second-and third-layer tanks. These results of this study suggest that it is possible to predict the timings of landslides occurrence despite differences in landslide scale.

Voluntary disaster prevention activities are important to prevent sediment-related disasters. However, there are not many areas where effective voluntary disaster prevention activities are being conducted. This study attempted to clarify residents' consciousness about voluntary disaster prevention activities and to understand the obstacles to participation in such activities in order to promote effective voluntary disaster prevention activities. A case study was conducted in the Moriyama District of Otsu City. A topographic analysis of the study site, a questionnaire survey among the residents, and interviews with officials of the voluntary disaster prevention association were carried out. The results of the questionnaire survey were analyzed using a chi-square test and residual analysis and revealed that residents' disaster prevention awareness is generally low. Their consciousness about disaster prevention varies according to the level of danger in the places where they live. Older residents had knowledge from past disasters, but their homes are often in relatively safe locations, and they were reluctant to be evacuated. Inhabitants who have emigrated in recent years often live along high-risk rivers and are more active in voluntary disaster prevention activities to some extent, but their knowledge about the possibility of disasters in the area is insufficient. Further elucidation and promotion of awareness is required. Knowledge about disaster risks and consciousness of disaster prevention varies depending on the place of residence and residence history. It is therefore necessary to inform residents about prevention activities after understanding the differences in their attitude.

In Mt. Unzen-Fugen-dake, debris flows occurred frequently after the Heisei eruption from 1990 to 1995 due to fine volcanic ash with a low infiltration rate. While the occurrence rate of debris flows decreased with the lapse of time after that, in recent years, debris flows have been occurring about once or twice a year at some streams. We researched the topographical and geological characteristics of recent major source area of debris flows in a previous work. As a result, a hypothesis was established positing that debris flows occur mainly because of the groundwater depending on the discontinuity of the infiltration rate and the differential erosion on the boundary of new pyroclastic-flow deposits and pre-eruptive ground surface below. The current situation in which debris flows are caused by these factors is considered to be the second stage, which follows the first stage in which low infiltration rate of volcanic ash caused debris flows. In this study, we verified that the hypothesis is generally consistent with the actual occurrence condition of debris flows at every stream covered with pyroclastic-flow deposits on eastern slope of Mt. Unzen-Fugen-dake, by researching the relationship among the location of pre-eruptive ground surface, the depth of pyroclastic-flow deposits, bed slope, catchment area, erosion depth and the actual occurrence conditions of debris flows, using the longitudinal and transverse cross sections from terrain models before and after the Heisei eruption. The topographical and geological factors that advance debris flows occurrence in the current situation were summarized as follows : (1) exposure of pre-eruptive ground surface with a low infiltration rate at a site on the stream bed, (2) a continuous steep stream with abundant pyroclastic-flow deposits below the site (1), and (3) exposure of pre-eruptive ground surface at stream bank of section (2). Furthermore, after suggesting the three factors above are likely to be completed when the lava landform is buried by pyroclastic flows, we proposed a simple guide for forecasting the source areas of debris flows at the second stage, namely “pyroclastic-flow deposits with a depth of less than around 50 m and a slope greater than 15 degrees that have buried the lava landform”.

It is thought that a large amount of sediment produced during a flood in the mountainous river channel will once stay in the river channel and will flow secondarily at the time of the subsequent flooding. But there are few examples of estimating sediment storage rate in mountainous river channel in Japan. On the other hand, use of LiDAR data is advanced in terrain measurement. In the Kinugawa watershed, remarkable sediment transport occurred due to collapse of hillside and debris flow during the September 2015 torrential rains in the Kanto and Tohoku region. Therefore, We tried to grasp the sediment storage rate on the mountainous river channel by differential analysis of the LiDAR data of before and after the September 2015 torrential rains in the Kanto and Tohoku region. The obtained sediment storage rate was about equivalent to the past report.