Those long waves spread under the peak in the energy spectrum are of greatest concern for the safety of ships and marine structures, and coming onshore for the Integrity of the shore environment. Furthermore, at sea measurement of breaking waves show that the breaking waves are about twice as energetic as the wave of mean height, Holthuijsen & Herbers (1986). Upon observing breaking waves in deep water, it is natural to ask: Why do sea waves break? What is the hydromechanics? Are there appropriate criteria for the inception of breaking? How can we model the breaker as it evolves? Although the subject is 140 years old and great progress has been made in recent decades, these large basic questions still remain largely unanswered. That is why the fundamental study of breaking waves is so challenging. In this lecture we review our understanding of the breaking of these energetic waves at sea in deep water. We have the advantage that they are predominantly long crested, so that we can approximate them, at least prior to breaking as planar waves. Breaking is itself characterized by a forward motion of the crest, a steepening of the forward face of the wave, and an accompanying increase in the water velocity at the crest, q*. From everything we know, when q* becomes as large as the wave celerity, c, then the Irreversible overturning process becomes inevitable, Figure 1. Therefore, this condition, q*≥c, is a good criterion to separate breaking and non-breaking waves in practice; Melville and Rapp (1988), In a tank experiment using a laser velocimeter, have proven this criterion and have shown that wave height itself, or even rapid rises in wave height, cannot distinguish breaking waves without error. That is why the fundamental study of breaking waves is so challenging.

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