In the 2011 off the Pacific coast of Tohoku earthquake with huge tsunami, some breakwaters had unexpectedly terrible damages resulting in falling down or sliding caused by the tsunami forces directly. Reflecting on this fact, requirements of breakwaters in design have been changed so as to keep their performance even against unprecedentedly huge tsunami, i.e. the largest tsunami in history (around this 1,000 years). Nevertheless, achieving complete protection from such an immense tsunami is actually unrealistic, thus, the design concept should aim to keep their tenacity even during their collapse as long and as effectively as possible to gain time for evacuation of people. Following this concept, the Japan design guideline of　breakwater against tsunami [1] has been revised in 2013. However, unfortunately, details of the suffer mechanisms and even the tsunami force are still not studied sufficiently due to their complexities and, consequently, such prudent considerations are not introduced into the guideline so far. In order to resolve this problem, this study presents a simple and effective manner to evaluate the wave forces with implementation of hydraulic experiments targeting a breakwater under overflows. The proposed formula shows its good agreements with experimental results in a wide range. Its applicability to the additional structures, e.g. mound under the caisson, widening work, parapet, is shown through the additional experiments.
Key Words: Overflow, Tsunami, breakwater, wave force