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.Key words: Authors should not enter keywords on the manuscript, as these must be chosen by the author during the online submission process and will then be added during the typesetting process (see http://journals.cambridge.org/data/relatedlink/jfm-keywords.pdf for the full list)

1Observations show that maximally-steep breaking water wave crest speeds are much slower than 2 expected. We report a wave-crest slowdown mechanism generic to unsteady propagating deep water 3 wave groups. Our fully nonlinear computations show that just prior to reaching its maximum height, each 4 wave crest slows down significantly and either breaks at this reduced speed, or accelerates forward 5 unbroken. This finding is validated in our extensive laboratory and field observations. This behavior 6 appears to be generic to unsteady dispersive wave groups in other natural systems. 7 8 Introduction 9

The criterion for the initiation of breaking demonstrated numerically by Barthelemy et al. (2015) has been investigated in the laboratory for unidirectional wave groups in deep-water and extended to include conditions of moderate wind forcing. Thermal Image Velocimetry was used to compare measurements of the crest surface water particle velocity (U s ) with the wave crest velocity (C), as determined by an array of closely-spaced wave gauges. The energy flux ratio B x = U s /C that distinguishes maximum recurrence from marginal breaking was found to be 0.840±0.016 in good agreement with the numerically determined value of 0.855. Further, the threshold was found to be robust for different classes of wave groups of distinct characteristic steepness at the breaking threshold. Increasing wind forcing from zero to U λ/4 /C 0 =1.42 increased this threshold by 2%. Increasing the spectral bandwidth (decreasing the Benjamin-Feir index from 0.39 to 0.31) systematically reduced the threshold by 1.5%.