2019 Volume 71 Issue 5 Pages 11-18
In the mountain stream channels around Nagiso-dake of the Yomikaki area, a debris flow occurred in Nashizawa river on July 9, 2013. Similar disasters were observed in the adjacent streams of Nashizawa river in 1965 and 1966. The characteristics of these disasters indicated that there were no large scale collapsed areas. It was assumed that the debris flow occurred due to the secondary movement of the sediments bed. We considered this fact that the occurrence of debris flow and its cycle are greatly affected by the location and the amount of sediment accumulated in the river bed in the mountain stream. Therefore, we identified the location to monitor the amount and sediment accumulation in the mountain stream and a periodical monitoring to predict the occurrence of debris flow due to the secondary movement of the sediments bed. In the current study, by utilizing these measurement techniques, we analyzed the relationship between the situation of debris flow occurred in the Nashizawa river in 2013 and the distribution of sediments in the four river streams near Nagiso-dake and the Hontani river around Ena mountain. As a result, in order to predict the occurrence of debris flow, we identified the location where sediment deposition should be monitored and estimate the amount of sediment distributed there. In the future, by continuing monitoring, it is possible to predict the occurrence of debris flow due to the secondary movement of sediments bed.
