Wave Height Distributions of Laboratory Generated Bimodal Seas with Abnormal Waves

The study analyses bimodal data recorded in an offshore basin. The measured sea states represent two wave systems propagating at an angle of 60°, 90° and 120°, respectively, from each other. The wave spectra are separated into swell and wind sea components and the energy ratio between the areas under the associated curves (SSER) is estimated. The sea states are grouped into three classes with respect to SSER. Abnormal waves are identified in the three classes of SSER, although the majority of the cases pertain to wind-sea dominated wave fields. The probabilities of exceedance of the individual wave heights for the sea states with abnormal waves are estimated and approximated with probabilistic models, which take into account either the spectral bandwidth or the wave nonlinearities. Strong correlation is found between SSER and the fourth order normalized cumulant of the surface elevation, Λ40. The distributions for the maximum wave heights are constructed and compared with theoretical predictions depending only on the number of waves in the sample and Λ40. Furthermore, the probability of a wave to exceed a given threshold level conditional on Λ40 is compared with the theoretical estimates