Abstract
We investigate how the extreme wave statistics of a combined windsea and swell appears to be different from the extreme wave statistics of the corresponding windsea and swell partitions. We consider the situation of following long-crested windsea and swell in laboratory experiments and in simulations using a high-order spectral method (HOSM). We also consider the situation of short-crested windsea and swell crossing at nearly right angle, corresponding to the sea state when the Prestige accident happened, in hindcast simulations combined with HOSM. For cases when the two wave systems do not interact much, the combined wave system appears to be more Gaussian than the corresponding partitioned wave systems, consistent with the central limit theorem. This result is found for kurtosis and exceedance probability of envelope and crest height.
Highlights
Multiple wave systems constitute 15–25% of sea states observed in different locations around the world [3,20,21]
The exceedance probability is characterized by local variation, similar to what we previously found for the kurtosis
We have shown that extreme wave statistics of a long-crested windsea can be nearly unaffected by the presence of a following swell; at the same time as the extreme wave statistics of the combined following long-crested windsea and swell appears to be more Gaussian and less extreme
Summary
Multiple wave systems constitute 15–25% of sea states observed in different locations around the world [3,20,21]. Gramstad and Trulsen [16] computed the modification of the probability of freak waves in a windsea perturbed by a weak swell crossing at various angles. The increase was found to be minimal in the case of right angle between the swell and the windsea This regime corresponds to large separation between the two wave systems such that a partitioning is both feasible and desirable. 2 we describe a laboratory experiment in which we generated a unidirectional (long-crested) bimodal sea state of following swell and windsea, and in Sect. 8 we consider the multi-directional (short-crested) bi-modal sea state during the Prestige accident, with swell and windsea crossing at right angle [45]. Our main conclusion is that the extreme wave statistics of well-separated multimodal sea states can be better comprehended if analysis is done on the partitioned wave systems as well as the combined sea state
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