Extreme wave statistics in co-propagating windsea and swell

We investigate how the extreme wave statistics of a windsea is modified by a following swell, by means of laboratory experiments and simulations using a High Order Spectral Method (HOSM) of long-crested sea. The windsea spectrum is kept equal for all cases, while the swell is altered. Analysis of the combined wave system gives the impression that the mixed sea has milder extreme wave statistics than the windsea alone, especially when the nonlinearities in the two systems clearly differ. Upon partitioning the mixed sea into windsea and swell, the windsea part is found to be nearly unaffected by the swell and to be governed by essentially the same statistics irrespective of a swell being present or not. This result is found for skewness, kurtosis and exceedance probability of envelope and crest height.