Characteristics of Slope Failures Seen outside the Designated Landslide-prone Areas

As global temperature rises and weather pattern changes, sediment disaster risks have been increasing in Japan. When Typhoon Hagibis hit East Japan in October 2019, about 30% of landslides occurred outside the designated landslide-prone areas. Such sediment disasters outside the designated areas draw public attention for fear of their increases with advances in climate change. This research aims to clarify characteristics of slope failures that have occurred outside the landslide-prone areas in the Kanto region (Greater Tokyo region) in recent three years. Intensities of heavy rainfall as well as basic topographic and geological features that may have caused failures are examined. The analysis indicates that such slope failures are more likely to occur as a result of long-term rainfalls rather than short-term rainfalls. Moreover, long-term rainfalls with a higher Soil Water Index (SWI) exceeding 10-year return period may trigger larger-scale landslides and are accompanied with a longer runout distance. It may suggest that large-scale slope failures of “slide” and “flow” types have a relatively high probability to occur on gentle slopes with a gradient of less than 30 degrees as a result of abnormal long-term rainfall.