Breaking for 2D and 3D gravity wave groups in deep and transitional water

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.