Many breakwaters were damaged by the March 11, 2011 Tohoku Earthquake off the Pacific coast. The majority of the breakwaters were destroyed or deformed under tsunami overflow; however, the failure mechanism under tsunami overflow is not clear. It is supposed that caissons slid from the mound under tsunami overflow from video analysis of Kamaishi breakwaters, of which about half were damaged. Therefore, with the main objective of this report being to clarify the stability of breakwaters under tsunami overflow, hydraulic model experiments were conducted with Kamaishi Bay as the subject, and disaster mechanisms of the head and trunk of the breakwaters as well as the countermeasures were examined. The following are the main results.

- Failure mechanism of the mouth of the port
　At the mouth of the port, submerged breakwaters are washed away by the primary positive waves, exposing the counterweight. Then, it is thought that scouring, especially near the head of breakwaters is caused by outgoing flow, and finally, the head of breakwater is caused to tilt.

- Failure mechanism of the trunk of breakwaters
　The experimental results have shown that negative pressure on the backside of the caissons under tsunami overflow, rather than the difference of pressure due to the water level difference between the sea side and harbor side, caused the horizontal force on the caissons to increase. This experiment made clear that in this tsunami, the ratio of increase is about 10% of the pressure of water level difference. In addition, it indicates that the dispersion of sliding resistance force of the rubble mound and the negative pressure on the backside make the difference of damage level of breakwaters.

- Countermeasures
　This experiment showed that installing the counterweight behind the caisson is an effective method to increase the sliding stability, but protection from scouring due to overflow is still necessary.