2020 Volume 73 Issue 1 Pages 50-57
The Typhoon Hagibis brought record rainfall and induced many sediment disasters in the Toshin and Hokushin region of Nagano prefecture from October 12 to 13, 2019. In Toshin region, 48 hour accumulated precipitation from October 12 to October 13 analyzed by radar-AMeDAS, ranged from 200 mm to 500 mm. All the sediment disasters in this region occurred within the area with precipitation greater than 200 mm. In the Nukui River basin, 48 hour accumulated precipitation was greater than 400 mm. At the four sediment disaster sites in this basin, debris flows, or sediment gravity flows occurred within a few hours after the peak of precipitation. At each disaster site, only relatively small shallow landslides were found in the upstream area, while stream bed erosion was remarkable. The main cause of these disasters is thought to be a significant increase in the stream flow caused by the large amount of precipitation.
