On a unified breaking onset threshold for gravity waves in deep and intermediate depth water

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