From September 23 to 24, 2022, heavy rainfall occurred in Shizuoka Prefecture due to Typhoon No. 15. A maximum 24-hour precipitation of 544 mm and a maximum one-hour precipitation of 125 mm were recorded. Precipitation was particularly heavy for short periods of time, with five of the 28 JMA stations in Shizuoka Prefecture recording one-hour precipitation amounts and eight recording three-hour precipitation amounts exceeding their maximum values since 1976. Three people were killed in the heavy rainfall disaster. One victim was killed by landslides, one by flooding, and one by falling into a river. Thousands of houses were damaged, especially in Shizuoka City, which was severely affected by the flooding. Shizuoka City experienced a major flooding disaster in 1974, but the scale of damage this time was likely smaller than that. In the eastern area of Shizuoka City, 76,300 households had their water cut off when water intakes were buried by the flood, and it took nearly 10 days to restore the water supply. All of the victims occurred in areas where flooding and sediment disaster were possible due to the topography of the area. Flooding in Shizuoka City is also likely to be within the range anticipated by the hazard map.

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Among the archaeological sites along the Saba River in Yamaguchi Prefecture, where collapse and debris flow frequently occurred due to the torrential rains on July 21, 2009, some of the archaeological sites are located in the high risk inundation area and sediment disaster area, and the former river bed adjoins or overlaps with some of them. In this paper, we analyzed the location, area and slope of each archaeological site in the Middle Ages (1185-1603). As a result, the 24 archaeological sites along the Saba River basin tend to form settlements where the local slope is gentle, and many archaeological sites are near the valley exit where debris flow is likely to occur.

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We collected and analyzed domestic and foreign video footage of woody debris masses carried by debris flow to clarify the qualitative characteristics of their movement. Woody debris masses were pushed by the front of the debris flows and moved down the flow section and the deposition section of the debris flow. In addition, it was estimated from the field survey that the woody debris masses contained about 90% of the volume of woody debris carried by the flows.

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This study aimed to analyze the social factors responsible for community disaster prevention activities in sediment disasters in Japan and Taiwan. It was found that in Japan, community disaster prevention activities are continuing, centered on these historical and institutional systems and plans. In Taiwan, ordinary citizens cooperate with the stakeholders as government and experts to establish the community disaster prevention system. Therefore, both countries could learn valuable lessons from each other. Japan could learn from Taiwan how to shrink the gap between governments, experts and local residents, while Taiwan could learn from Japan how to institutionalize community disaster prevention and planning activities for the unsustainable activities problem.

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