The serious damage of breakwaters occurred in the tsunami caused by the 2011 off the Pacific coast of Tohoku earthquake. It is important to reinforce breakwaters with the concrete blocks and to give the function of tenacious structures. Hydrodynamic experiments had been performed to study the damage of concrete blocks armoring on rubble mounds for the over flow in tsunami. In the case of breakwater, the critical values of the overflow depth above the crest of breakwater, in which the damage of concrete blocks occurs, are affected by the height of breakwater above the still water level and the water depth from still water level to the foundation of reakwater. The positions where the damage of armor concrete blocks occur on the foundation change with the overflow depth above the crest of breakwater.
