## Abstract

A probability distribution of the simultaneous maximum wave amplitude within a finite width along a virtual vertical plane placed in a 3-dim. irregular sea state is studied theoretically. The probability distribution differs considerably from the Rayleigh distribution even when the width of the structure is an order of 1/10 significant wave length. The difference increases with the interval width. Especially, appearant probability of large amplitude is considerably larger than that expected from the Rayleigh distribution. Coastal structures which were designed with sufficient safety margin have sometimes failes. There are possibilities that the structures were not attacked by "unexpectedly large waves" but large waves which could have been possibly expected if we had taken the width effect into account.
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