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ocean surface. In a way, however, this is a disadvantage too, when it comes to accurate
descriptions of the process and its properties. Phases of the breaking progress and its char-
acteristics are not necessarily commonly defined because they are left to obvious intuitive
understanding.
Thus, for example, wave-breaking onset is often investigated by measuring properties
of the breakers which have already produced whitecapping and related acoustic, optical or
other distinct signatures. It is argued in the topic that, if whitecapping is present, then the
wave is already collapsing, that is, the breaking is in progress, which is characterised by
physics different to that which led to the breaking onset. And it is hard, if not impossible to
understand the causes of the collapse by investigating its progress and consequences rather
than pre-history. In the case of wave breaking, this is even more complicated due to the
fact that the dynamics of the breaking progress differ essentially depending on the physics
which led to the breaking onset as mentioned above.
Therefore,
Chapter 2
suggested a definition of the breaking onset and classification of
different phases of the wave breaking. It defined the breaking probability and breaking
severity which is broadly employed throughout the topic, and a combination of which
determines the wave energy dissipation. Types of breakers were outlined too, and breaking
criteria described in some detail. Finally, some unrelated to breaking general knowledge
of wave features and terminology, frequently used in the topic, such as dispersion relation
and radiative transfer equation, were introduced for completeness.
Chapter 3
described methods of detecting and measuring wave breaking. There is a great
variety of these. We started from traditional means of observing the whitecaps, which have
regained importance in recent years in the light of advances of remote-sensing environ-
mental applications. Other methods include those visual and instrumental, experimental
and theoretical. Experimental techniques were divided into contact and remote-sensing
measurements, laboratory experiments and field observations. These were further subdi-
vided, laboratory methodologies into those dealing with deterministic and random wave
trains and fields, remote-sensing methods into acoustic, optical, infrared and radar appli-
cations, the acoustic techniques into active probing and passive listening. Theoretical
approaches were separated into two sections, dedicated to analytical and statistical
methods.
Contact measurements are direct and obviously most accurate, as they serve as cali-
bration means for remote-sensing techniques and theoretical approaches to begin with. It
should be emphasised that measuring the surface elevations (wave height) in very steep
and nonlinear waves, such as those at the breaking onset or in the process of breaking,
is not always sufficient to estimate the respective wave energies. Relationships between
the potential and kinetic energies in strongly nonlinear waves are not known, and therefore
measurements of the wave-motion velocity field are necessary to obtain the kinetic and thus
the total energy. The good news, however, is that the surface-wave nonlinearities appear to
decay rapidly away from the surface. Therefore, at least in deep water, we would expect the
deviations from linear wave orbits to be essential only very close to the interface, perhaps
even within the crest-trough volume.
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