Opinions diverge at present as to whether extreme waves are "normal" extremes in an homogeneous population of waves, or the results of totally different generation mechanisms, such as for instance non linear interaction and phase locking of wave trains. Even in the case where extreme waves would belong to the same population as non-extreme ones, it is important to be able to reproduce them, in numerical simulations or in wave basins, with the adequate generation mechanisms in order to carry out tests with representative kinematics and actions on structures. Trial and error is not an acceptable selection way for reproduction methods: one must try to find what actual mechanism is in effect in nature to generate the extreme waves. The present paper intends to provide some insight into this issue by examination of the characteristics of the sea states during which extreme waves could be observed, for two locations, in the North Sea and in the Northeastern Atlantic. The sea states where occurrence of extreme waves with respect to the significant wave height was observed are thus characterized in terms of spectral bandwidth and multiple peakedness, of steepness, of non linearity, of wind conditions, of directional spreading when the information is available, and of the characteristics of the storm that contains them. Conclusions are proposed as to the nature of the generation mechanisms for extreme waves.
On several occurrences, unexpectedly large waves, refered to as "freak waves", have reportedly damaged ships and offshore structures, see for instance Ersdal & Kvitrud (1999) [2]. Such damages are cruel evidence of the existence of these waves, but only few measurements and studies are available to support incorporation of the related risks into the design procedures. Before consistent recommendations can be made
