We revisit the classical but as yet unresolved problem of predicting thebreaking onset of 2D and 3D irrotational gravity water waves. This studyfocuses on domains with flat bottom topography and conditions ranging from deepto intermediate depth (depth to wavelength ratio from 1 to 0.2). Ourcalculations based on a fully nonlinear boundary element model investigatedgeometric, kinematic and energetic differences between maximally recurrent andmarginally breaking waves in focusing wave groups. Maximally steep non-breaking(maximally recurrent) waves are clearly separated from marginally breakingwaves by their normalised energy fluxes localized near the crest region. On thesurface, this reduces to the local ratio of the energy flux velocity (here thefluid velocity) to the crest point velocity for the tallest wave in theevolving group. This provides a robust threshold parameter for breaking onsetfor 2D and 3D wave packets propagating in uniform water depths from deep tointermediate. Warning of imminent breaking onset was found to be detected up toa fifth of a carrier wave period prior to a breaking event.
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