Definitions for wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 16

Before this broken patch starts developing, the wavelet method will already be detecting the
breaking because the downward acceleration will exceed the threshold value while closing
the breaking onset. The visual technique implemented at the Black Sea and the acous-
tic technique implemented at Lake George, however, will not detect a breaking. The two
experimental techniques effectively make use of the occurrence of whitecapping as the
basic measurement point and, since there is no whitecapping, i.e. no 'broken patch', they
will fail to detect the wave crest as a breaking crest.
The developing stage is characterised by the lateral spread of breaking with a white-
capping appearance for the crest to pass over the measurement point, so a developing
breaker should be readily detected using whitecap-oriented measurements. But the devel-
oping breaker also exhibits an increase in wave front steepness before it subsides. Rapp &
Melville ( 1990 ) in their subsection 3.4 defined front steepness as the ratio of crest-to-front-
zero-crossing height to crest-to-front-zero-crossing length and showed that this is larger,
compared to the incipient breaking front steepness, for both spilling and plunging breakers.
As shown by Liu & Babanin ( 2004 ), even though the front steepness is not unambiguously
linked to the maximal instantaneous downward acceleration, this is an indication that the
over-limiting acceleration values may persist through the developing stage, and thus the
developing breaker will be detected by the wavelet method as well.
The relaxing or subsiding stage of breaking has not received as much attention in the
literature as developing breaking. Therefore it is not quite clear, for example, what will hap-
pen to the breaking crest once it has reached its maximal length according to Phillips et al.
( 2001 ) or when the front steepness of breakers, described by Rapp & Melville ( 1990 ), will
start to decrease. But at some stage it will start to decrease. For example, in his observations
of plunging breakers (see Section 2.8 about breaking types), Bonmarin ( 1989 ) described
the so-called 'splash phenomenon' due to interaction of the breaking-water jet with the
surface in front:
“The elevation of the splash of water can rise as high as the original plunging crest. When several
successive splash-up cycles occur, gradual decrease of the potential energy from one cycle to the next
is observed”.
In Figure 2.2 , a segment of a Black Sea record is plotted which shows surface elevations
with wave breaking marked by visual observations of whitecaps (dots) and by means of the
wavelet analysis (vertical bars). For the Black Sea waves shown, the mean front steepness
was 0.045. The second and the third breakers picked up by the visual method have front
steepness values of 0.052 and 0.075 which is greater than the mean steepness as one can
intuitively expect for a breaking wave. The first breaker, however, which was detected visu-
ally because a whitecapping crest propagated past the measuring wave probe, has a front
steepness of 0.011, well below mean wave steepness. Clearly, this broken wave, which still
carries a whitecapping patch, is not expected to be detected by the wavelet method based
on the acceleration criterion.
Figure 2.3 further illustrates the necessity for subdivision of the breaking process into
the three phases. It shows the properties of individual waves in the range of frequencies