Wave-breaking severity - Breaking and Dissipation of Ocean Surface Waves - page 219

6
Wave-breaking severity

On many occasions earlier in this topic, it has been mentioned and emphasised that
knowledge of breaking severity is as important as is understanding the physics driving
the breaking occurrence. While the latter, however, has received a lot of attention from
the wave-research community lately, our information on breaking strength, its variability,
environmental dependences and physics remains limited and fragmental.
If the breaking strength is defined as energy loss in a single breaking event ( Section 2.7 ),
then the breaking severity coefficient s can be identified in a number of ways, through
measurement of the individual breaking wave (2.24) , of the group where the breaking
occurred (2.32) , of spectra of the respective groups before and after the breaking (2.38)
and of short waves modulated by the longer wave only (2.42) . The magnitude of such a
coefficient varies greatly, from s 10% ( Rapp & Melville , 1990 , or even less as seen in

Figure 6.3 below) up to 99% (2.31) based on the Black Sea estimates (see also Bonmarin ,
1989 ; Babanin et al. , 2010a , 2011a ).
Such a range of change of course cannot be disregarded or substituted with some
mean value in applications that involve the breaking severity. A typical application is
the wave-energy dissipation function S ds employed in wave forecast models (2.21) ,
(2.61) and (5.40) . As defined in (2.21) , it can in principle be directly determined as a
product of the breaking probability and breaking severity, but since more or less definite
parameterisations of the latter are not available a set of inventive indirect methods have
been elaborated to estimate the dissipation function (see e.g. and The WISE Group ,
2007 ,forareview).
In other applications, such as, for example, engineering aspects of the wave-breaking
impact, the breaking-strength magnitude needs to be known explicitly rather than as an
element of the overall energy dissipation. More than that, statistical values such as those
involved in obtaining the averaged dissipation term may not be helpful in this regard, as
extreme or individual events may be sought. Therefore, quantifying and parameterising the
breaking severity is an important outstanding task of wave-breaking studies.
Chapter 6 is shorter than other chapters in this topic for obvious reasons. There are
not many experimental dependences and no theoretical approaches to discuss. In the two
sections, indicative laboratory ( Section 6.1 ) and field ( Section 6.2 ) results will be outlined
in addition to those already mentioned in the definition Section 2.7 .

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