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Abstract
Water wave breaking is a dominant dynamical process of the upper ocean,
inducing strong flow-turbulence-wave interactions and air-sea exchanges. A
fundamental and long-standing gap in the understanding of wave breaking is
how to characterise and predict the onset of breaking.
The threshold for the onset of breaking proposed by Barthelemy et al.
(arXiv:1508.06002v1, 2015) has been investigated intensively in the
laboratory for different classes of two- and three-dimensional wave groups in
deep and transitional water in the absence and presence of wind. Thermal
Image Velocimetry was used to compare measurements of the wave crest
surface water particle velocity with the wave crest speed determined by an
array of closely-spaced wave gauges.
For the first time, a threshold crest point surface energy flux ratio (Bx) that
distinguishes maximum group recurrence from marginal group breaking has
been established for gravity waves. The critical value of Bx was found to be
0.835 ± 0.005 with an experimental uncertainty of each data point of ±0.020.
The breaking threshold is robust for different types of unidirectional and
directional wave groups. Very weak dependence on wind forcing and
group bandwidth is demonstrated. No dependence on relative water depth
was observed. If there is a dependence on peak spectral wavenumber, it is
weak and negligible for the scales achievable in a large-scale laboratory.
This study provides more robust and universal characterisation of breaking
in transitional water than the empirical non-dimensionalisation of Nelson
(1994). The effect of wave grouping can generate marginally breaking waves
in shallower water that are at least 30 % greater than the limit proposed by
Nelson. The study supports use of a limit at least that recommended by
McCowan (1894)/Miche (1944) for coastal engineering design in transitional
and shallow water until it is demonstrated that there is negligible risk
of strongly breaking group waves achieving higher breaker indices.