Author(s): Tomihiro Iiboshi; Shiro Maeno; Keisuke Yoshida; Yusuke Tai; Akihito Takeuchi; Ryosuke Akoh
Linked Author(s): Keisuke Yoshida
Keywords: Landward bed scouring; Impulsive force; Tsunami overflow
Abstract: Coastal dikes in Iwate, Miyagi, and Fukushima prefectures in Japan were damaged severely by tsunami overflow consequent to the 2011 off-the-Pacific coast of Tohoku Earthquake. A main factor of the failure was landward bed scouring following the tsunami overflow. Coastal dikes were destroyed completely or partially because of embankment material outflows caused by scouring of landward beds. Because of this severe damage, the tsunami committee in Japan recommended several methods to produce resilient structures for coastal dikes. One method, landward toe protection works, reduces landward toe bed scouring. For earlier studies, we conducted small-scale experiments under steady overflow conditions to examine scouring characteristics. Results show that surface protuberances of landward toe protection works are more effective to reduce the scouring depth and scouring area. However, when an actual tsunami strikes, the considerably impulsive first arriving breaking wave is followed by a steady overflow state. Therefore, for this study, we conducted dam-break experiments to elucidate actual tsunami conditions. We examined different shape effects in landward slope works and toe protection works on landward bed scouring with two tsunami height levels. The results demonstrate that the scouring hole was formed closer to the coastal dike than under a steady flow condition. A large scouring hole was formed around the landward toe area without bed protection works. A surface protuberance of landward toe protection works is more effective to reduce the scouring depth and area. Furthermore, irrespective of the shape of the bed protection works, stepwise slope protection works are more effective at reducing the scouring depth and area.
Year: 2016