平成24年7月九州北部豪雨災害にみる洪水時の河川横断構造物と流木の危険性－玉来川下流域を対象として－

抄録

Heavy rainfall which we had never experienced occurred in the northern part of Kyushu Island on July 2012. A record-breaking rainfall was observed at several stations. This heavy rainfall caused sediment disaster, river flooding and inundation that claimed many casualties and missing in Ooita, Kumamoto and Fukuoka Prefecture. In Taketa City, Ooita prefecture, two people died and more than 250 houses were inundated due to the flooding of the Tamarai River which flows through the Taketa City. Especially, the water level rose rapidly and reached about 2.5m from a top of a levee in the Haitabaru area, downstream part of the Tamarai River. A field observation and a numerical simulation were carried out in the Tamarai River Basin in order to find out the reason why the water level had risen like that. Results from calculations showed that the increase in the water level due to a decrease of just 10 percent in the cross-sectional area of the river was estimated to be 3m. Because of heavy rainfall, a large amount of driftwoods were produced in the upstream area and accumulated at the river bridge, New Azou Bridge. As a result the cross- sectional area of the river decreased and the rapid increase in the water level occurred.