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Using the same measurement techniques as those of Saket et al. (2017), we have investigated the breaking threshold proposed by Barthelemy et al. (arXiv:1508.06002v1, 2015b) but for different classes of unforced unidirectional wave groups in intermediate water depths in the laboratory. The threshold parameter $B_x = U_s/C$ (where $U_s$ is the horizontal surface water particle velocity at the wave crest and $C$ is the wave crest point speed) which distinguishes breaking from non-breaking waves was found to be 0.835 $\pm$ 0.005 with the experimental uncertainty of each data point of $\pm$ 0.020. This threshold is applicable to water depth to wave length ratios as low as 0.2 including the deep water conditions investigated by Saket et al. (2017). No dependence on peak spectral wavenumber was found at the scales achievable in a large-scale laboratory. The present study provides more robust and universal characterisation of breaking in transitional water than the empirical non-dimensionalisation of Nelson (1994). The limiting wave height to water depth ratios of marginally breaking deep and intermediate water waves remain within 10 % of Nelson's values. However, it is shown that the effect of wave grouping can produce waves in shallower water that are at least 30 % greater in height than the limit proposed by Nelson (1994). The present study supports use of limits based on McCowan (1894) and Miche (1944) for coastal engineering design for marginal breaking waves. Three-dimensional and more strongly breaking waves in shallower water may yield wave heights higher than those measured during this study.

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 (Us) with the wave crest velocity (C), as determined by an array of closely-spaced wave gauges. The energy flux ratio Bx = Us/C that distinguishes maximum recurrence from marginal breaking was found to be 0.840 $\pm$ 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{\lambda}/4/C0=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%.

Observations show that maximally-steep breaking water wave crest speeds are much slower than expected. We report a wave-crest slowdown mechanism generic to unsteady propagating deep water wave groups. Our fully nonlinear computations show that just prior to reaching its maximum height, each wave crest slows down significantly and either breaks at this reduced speed, or accelerates forward unbroken. This finding is validated in our extensive laboratory and field observations. This behavior appears to be generic to unsteady dispersive wave groups in other natural systems., Comment: 12 double spaced pages including 3 figures