令和元年台風第19号等の豪雨によって2019年10月に関東地域で発生した土砂災害について

抄録

The number of sediment disasters caused by Typhoon Hagibis in October 2019 reached 952 as of December 24, 2019, which is the largest number of typhoon-related sediment disasters since 1982. The sediment disasters caused by this typhoon are distributed over a wide area, mainly in East Japan. In this report, the authors describe three landslides that occurred in Sagamihara, Tomioka, and Chiba. The common feature of these three landslides is that all of them has high fluidity, which seemed to result in the serious damage. It is thought that all the three have a deep relationship with the Kanto loam area consisting of volcanic ash layers. An assessment of rainfall amount that triggered the disasters by return period showed that the largest was Sagamihara, then Chiba, and finally Tomioka. Looking at the volume of landslide, the largest was about 15,000 m³ in Sagamihara, followed by about 3200 m³ in Tomioka, and 250 m³ in Chiba. In addition, the time delay, which is defined as the time from the point when the Soil Water Index (SWI) exceeds 2-year return period value to the time when the slope failure occurs, is about 12 hours in Sagamihara, about 8 hours in Tomioka, and about 2 hours in Chiba, respectively. The 2-year return period of SWI is obtained by applying extreme value analysis to the values of fixed duration between 1981 and 2010. And the result that the larger landslides require the longer time until the landslide occurrences is in harmony with the previous study. These results suggest that it is possible to evaluate an evacuation time according to the scale of landslides in the same standard time scale.