The distribution function of extreme wave heights is characterized with a new parameterγ50 defined as the ratio of 50-year wave height to 10-year wave height. The threshold criterion for permissible sliding distance is set such that the probability of total sliding distance over 0.3 m during the lifetime is less than 0.10.
The Monte Carlo simulations are made for search of optimal design of caisson breakwaters. An extreme distribution with a large γ50 value demands a large caisson width in the location where the water depth is greater than about 2.5 times the equivalent deepwater wave height. In shallow waters, however, the effect of the distribution disappears because of the dominance of random breaking process. The optimal return period of design wave is 4 to 5 times the service lifetime in the offshore zone, but it becomes shorter than the lifetime in shallow waters.
