Destruction patterns and mechanisms of coastal levees on the Sendai Bay Coast hit by the 2011 tsunami

A mega tsunami hit the Sendai Bay Coast on March 11, 2011, overtopped coastal levees and intruded into far inland while sweeping houses, people and others away. Eighty percent of the levees, which rimmed the coast to protect the land from storm surges together with the wind waves were broken in various degrees of damage by the tsunami. The national and local governments decided to rebuild the levees to be durable even for mega tsunamis. This requirement motivates us to find the destruction mechanism of the coastal levees. We conducted field investigations and collected the tsunami records, aerial photos and tsunami videos. Especially, the video taken from the helicopter “Michinokugo” which flew along the Sendai Coast to the south during the attack of the tsunami’s leading wave enables us to see the breaking process. Integrated analysis leads to two step mechanisms of the destruction: the first step of breaking the upper structure of the levees by the surging bore of the leading wave and the second step of expanding erosion by the return flow concentration.