EXTREME MULTI-DIRECTIONAL WAVES

Abstract

The present paper concerns the nonlinear description of large waves in which the wave energy is spread in both the frequency and the directional domains. A new series of experimental observations are briefly described in which a large number of deep-water focused wave groups were generated in a large wave basin. For each combination of frequency bandwidth and directional spread the intensity of the wave, measured in terms of a linear amplitude sum, was varied from a near-linear condition to the limit of incipient wave breaking. Comparisons between this laboratory data and a new fully nonlinear, multi-directional, numerical model are used to validate the modelling procedure and highlight the importance of the directionality. In particular, the results show that for a given linear amplitude sum an increase in the directional spread leads to a reduction in the magnitude of the nonlinear wave-wave interactions. Conversely, the maximum nonlinear crest elevation, observed just prior to the onset of wave breaking, increases with the directional spread assuming the frequency bandwidth remains constant. From a practical perspective, the paper demonstrates that an accurate representation of an extreme ocean wave requires a model that incorporates nonlinearity, unsteadiness and directionality. The present model satisfies these requirements.