Effects of landward slope protection and toe protection work shape at coastal dikes on landward bed scouring caused by tsunami overflow

摘要

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 the landward bed. Because of this severe damage, the tsunami committee in Japan recommended several methods to produce resilient structures for coastal dikes. One method is landward toe protection works to reduce the landward toe bed scouring. It can improve the coastal dike stability. However, the relation of protection work shapes to landward bed scouring characteristics remains unclear. Therefore, we particularly examined the effects of landward slope and toe protection work shapes on landward bed scouring. We conducted small-scale experiments to clarify the effects of works with different characteristics. The obtained results demonstrate that a large scouring hole was formed around the landward toe area in any shape of the slope protection works without bed protection works. Landward bed protection works can reduce size of the scouring hole. A surface protuberances of landward toe protection works is more effective to reduce the scouring depths and areas.