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et al. , 1992 ) and wind-wave momentum/energy exchange (e.g. Babanin et al. , 2007b )are
of essential importance.
The pre- and post-breaking physics are not entirely disconnected, and the outcome of
breaking collapse appears to 'remember' the 'input' which made the wave break. This will
be discussed in more detail in Chapter 7 . Here, we would like to emphasise that among
wave-breaking definitions, breaking in progress needs to be considered separately and can
be further subdivided into distinctly different phases.
For classification of wave-breaking phases, we will follow the logic suggested by Liu &
Babanin ( 2004 ). They envisaged and quantified in terms of relative duration a single break-
ing event as passing through several distinct stages from both the external appearance of
breaking and the underlying physics involved. These stages are incipient breaking, devel-
oping breaking, subsiding breaking and residual breaking. The first stage leads to breaking
onset as described in Section 2.1 . Developing breaking and subsiding breaking are differ-
ent phases of breaking in progress. Residual breaking is a follow-up dynamic impact of the
breaking event, rather than wave breaking as such and it will be discussed in Section 2.3 .
Liu & Babanin ( 2004 ) aimed at testing the Liu ( 1993 ) breaking-detection approach
based on the wavelet technique, by means of field data (see Section 3.7 ). Wavelet analysis
indicated wave breaking if the downward acceleration, obtained from a surface-elevation
series, exceeded a predetermined threshold value. The data were obtained under a variety of
wind-wave conditions in deep water in the Black Sea and in a finite-depth environment in
Lake George, Australia. Both data sets included synchronised time series of surface eleva-
tions and wave-breaking marks. Both had been extensively used to study breaking statistics
for different wave spectra and in different environmental conditions, and therefore detailed
results of the analyses were available for comparison ( Babanin , 1995 ; Babanin & Soloviev ,
1998a , b ; Banner et al. , 2000 ; Babanin et al. , 2001 ).
In the Black Sea, wave breaking was detected and marked visually ( Babanin , 1995 ), and
in Lake George detection was conducted by acoustic means ( Babanin et al. , 2001 ). Overall
statistics of the number of breaking events and their frequency of occurrence matched the
outcomes of the wavelet technique very well. Detailed examination of the results, how-
ever, indicated essential differences. The wavelet approach, along with the measurements,
was generally successful in capturing breaking-wave events on many occasions, although
on some other occasions one of them failed to detect a breaker while the other indicated
that breaking had occurred. This is not unexpected as both measurements, i.e. detection
of breaking by visual or acoustic means and the theoretical wavelet approach, should be
anticipated as relevant to different phases of wave breaking.
At the incipient stage, the wave begins to find its continuous surface becoming difficult
to sustain, so it is about to break, but is not breaking yet. An incipient breaker does not
have whitecapping coverage as the breaking crest does not turn over. This is how Phillips
et al. ( 2001 ) described development of the crest breaking:
“A single breaking event is generally initiated at some point on the wave crest and spreads laterally
so that its average length is of order half its ultimate length, the width of the broken patch”.
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