1 FAILURE MECHANISM OF KAMAISHI BREAKWATERS DUE TO THE GREAT EAST JAPAN EARTHQUAKE TSUNAMI Taro Arikawa 1 , Masaharu Sato 2 , Kenichiro Shimosako 3 , Iwao Hasegawa 4 Gyeong-Seon Yeom , 5 and Takashi Tomita 6 Many breakwaters were damaged by the Great East Japan Earthquake of March 11, 2011. The majority of the breakwaters were destroyed or deformed under tsunami overflow; however, the failure mechanism under tsunami overflow is not clear. Therefore, with the main objective of this report being to clarify the stability of breakwaters under tsunami overflow, hydraulic model experiments and numerical simulations were conducted with Kamaishi Bay breakwaters as the subject, and failure mechanisms of the trunk of the breakwaters were examined. Keywords: Kamaishi breakwater; tsunami overflow; stability of breakwaters INTRODUCTION The tsunami caused by the Great East Japan Earthquake collapsed many breakwaters and storm surge barriers. Views of this were recorded by video cameras at many places, and in many cases, it is presumed that the tsunami exceeded the heights of the breakwater etc., causing their failure. Among past studies focused on clarifying the mechanism of the failure of breakwaters caused by tsunami, Horiguchi and Yokota (1968) investigated the causes of the failure of the Kawaragi Breakwater in the Port of Hachinohe when it was struck by the Tokaichi Earthquake tsunami, reaching the hypothesis that it slid under the impacts of the water level difference inside and outside the port and the dynamic water pressure on its front surface. Tanimoto et. al. (1983) suggested that the failure of the landfill revetment then under construction in the outer harbor of Noshiro during the Japan Sea Chubu earthquake tsunami was a failure caused by the impact of a bore tsunami. But there has been no research on the stability of breakwaters when a certain period of time has elapsed after overflow, as occurred during this recent tsunami. So this research took Kamaishi as an example to perform hydraulic model experiments and numerical simulations of trunks of breakwater, in order to clarify the breakwater failure mechanism under tsunami overflow. Photo 1. The Kamaishi Bay mouth breakwaters (taken by Tohoku Regional Bureau) 1 Coastal and Ocean Engineering Field, Port and Airport Research Institute, 3-1-1 Nagase, Yokosuka, Kanagawa, 239-0826, Japan 2 Coastal and Ocean Engineering Field, Port and Airport Research Institute 3 Coastal and Ocean Engineering Field, Port and Airport Research Institute 4 ECOH CORPORATION, 2-6-4 Kitaueno, Taito-ku, Tokyo, 110-0014, Japan 5 Asia-Pacific Center For Coastal Disaster Research, Port and Airport Research Institute 6 Asia-Pacific Center For Coastal Disaster Research, Port and Airport Research Institute