Author(s): Akihiro Hashimoto; Akira Tai; Toshimitsu Komatsu
Linked Author(s): Akira Tai, Akihiro Hashimoto
Keywords: Heavy rainfall; Flooding; Driftwood accumulation; Structures built over a river
Abstract: Driftwoods during flood events may lead to accumulations and blockages at river bridges or weirs, and can result in an increase in backwater that may lead to flooding of nearby areas. Recently, torrential rains which are triggers of landslides have happened frequently and this trend would increase because of climate change. This may lead to increase driftwood production. The flood event in July 2012 in Japan highlighted the need to investigate driftwood-related problems. Therefore, this paper presents flood risk due to driftwoods accumulation and blockage at structures built over a river, especially bridges. Field observation and numerical simulation were conducted to estimate the effect of driftwoods accumulation upon backwater rise. A 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 landslides, river flooding and inundation that claimed many victims in Oita, Kumamoto and Fukuoka Prefecture. In Taketa City, Oita 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 2.5 m from a top of a levee in the Haitabaru area, downstream part of the Tamarai River. Results from calculations showed that a decrease of just 10 percent in the cross section area of the river would result in the water level rise of about 3 m.
Year: 2014