各種計測技術を活用した恵那山および南木曽岳周辺渓流における土砂流出特性の把握

摘要

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.%平成26年7月9日，長野県木曽郡南木曽町読書地区の南木曽岳西側斜面に位置する梨子沢において土石流が発生し，下流集落において死者1名，負傷者3名，家屋全·半壊10棟の甚大な被害が発生した（以下，H26南木曽災害という）。読書地区では，隣接する渓流においても昭和40年，昭和41年に同様の災害を経験しているなど，古くから土砂流出を繰り返している地区であることが記録に残っており，これら土砂流出の有無とそのサイクルは，渓流内の河道に徐々に蓄積された土砂の多寡に大きく影響を受けていることが報告されている。