Definitions for wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 21

As seen in Figure 2.4 , there are small differences between wave-breaking detection tech-
niques, and therefore the breaking phases in deep water (Black Sea) and finite depth (Lake
George). Wave breaking in finite depths does exhibit some peculiarities which will be dis-
cussed in Chapter 5 . These peculiarities, however, are not of a principal nature and here,
quite likely, the minute differences observed are technical rather than physical.
Indeed, the wavelet analysis somewhat overestimates the total breaking percentage mea-
sured at Lake George and slightly underestimates that measured at the Black Sea compared
to the one-to-one line. At Lake George, the acoustic method used only allowed detec-
tion of the dominant breakers in

30% vicinity of the spectral peak ( Babanin et al. ,
2001 ), and therefore the wavelet method, whose performance is not limited by the spec-
tral peak band, may have an extra number of breakers contributing to the total statistics.
At the Black Sea, contrary to this, the breakers were detected visually and did not have an
upper-frequency bound except that of the physical capability of the observer to actually see
small whitecaps. Therefore, the wavelet detection method may have failed for small break-
ers whose profile was not sampled well enough but whose whitecapping was detected by
the observer, and this could lead to underestimation of the deep-sea breaking rates. Details
of the experimental techniques are described in Chapter 3 .
Another possible contributor to the observed deep-water-finite-depth variation can be
actual physical dissimilarity in relative durations of the breaking phases in the two envi-
ronments. Such dissimilarity, if it exists, should be further investigated and cannot be
addressed with certainty here. With some certainty, however, we can say that there appears
to be no difference in duration of the developing-breaking phase between the deep water
and finite depths. On the lower line, the Black Sea and Lake George points are scattered
evenly which means that the second-phase duration does not depend on whether breaking
occurs in deep or in finite-depth water.
The relative duration of the fourth, residual-breaking phase, with respect to the other
three phases, is much longer. While the total duration of the first three breaking stages
only accounts for a fraction of the wave period, the residual phase lasts for many wave
periods (see Section 2.3 ). It is essentially a different, post-breaking process of dissipation
of turbulence generated by the wave-breaking impact that occurs during the active phase
of a breaking event.
Alternative classifications of the wave-breaking phases, with different physics applicable
at the different phases, are also available in the literature. For plunging breaking, Bonmarin
( 1989 ) classified these phases as the overturning phenomenon, the splash-up phenomenon
and degenerated forward flow. Essentially these phases are similar to the second, third and
fourth (residual) phases introduced here. Even though Bonmarin ( 1989 ) also indicates his
overturning phase as a development towards the breaking point, in terms of this chapter it is
the breaking in progress, as the overturning phase is characterised by developing negative
asymmetry A s , which in our classification happens after breaking onset (see Section 5.1.1 ).
According to Bonmarin ( 1989 ), the characteristic timescale for the transition of the steep
progressive wave to the breaking point is of the order of T

±

/ 10 where T is the wave period.