Detection and measurement of wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 60

part of the measurement is concerned, still, for many years visual observations remained
the most reliable, or sometimes the only means of detecting breaking events. These included
manual tagging of breaking waves in the surface elevation records ( Weissman et al. , 1984 ;
Holthuijsen & Herbers , 1986 ; Stolte , 1992 , 1994 ; Babanin , 1995 ) or video-taping of the
surface, accompanied by collocated measurements of the waves ( Katsaros & Atakturk ,
1992 ; Babanin et al. , 2001 ). In the laboratory experiments or field observations, tagging
can be performed in real time while observing recorded breaking waves and marking them
electronically. In the case of video-taping, such tagging can be done by means of repeated
watching of the recorded videos, which reduces the possibility of human error.
In any case, the observations deal with visible whitecapping, that is with the breaking-
in-progress stage of the process (see Section 2.2 ). Therefore, they mostly concentrate
on studies of breaking probability (see Section 2.5 ) and dependences of the breaking
occurrence on wave, wind and other environmental characteristics.
The first major observation of the kind was by Holthuijsen & Herbers ( 1986 ). In many
regards, it was a breakthrough in experimental studies of wave breaking, particularly in
field conditions. One of the authors was watching a wave buoy and
“triggered a radio signal each time an active whitecap was seen to pass under the buoy. The signal
was recorded synchronously with the buoy signal on one tape in an onshore station, thus identifying
breaking waves with an 'on-off' signal”.
Then wave-by-wave analysis of individual waves was conducted by means of the zero-
crossing method. Probability and statistics of wave height, crest height, wave period, steep-
ness and asymmetry of the breaking waves were studied for the first time and dependence
of the breaking probability on wind speed was considered.
In particular, it was concluded that
“the breaking occurs at wave steepness values much less than the theoretically expected steepness of
a limiting wave” (2.47) .
This issue has been discussed in Section 2.1 above, and we now know that measurements
of a wave that already exhibits whitecaps cannot provide estimates of the limiting steepness
at breaking onset.
Other outcomes of the papers, for example, conclusions on the connection of breaking
occurrence with the structure of wave groups, remain valuable up-to-date knowledge on
wave breaking. Holthuijsen & Herbers ( 1986 ) concluded that the probability of a wave
group having at least one breaker was higher for longer wave groups. In their observations,
almost 100% of long groups, i.e. those consisting of seven waves, produced a breaker.
Overall, 69% of all breaking waves happened within a wave group. Those were breaking
close to the centre of wave groups, slightly ahead of this centre. As will be discussed
later, such observations are consistent with the present understanding of the two-phase
behaviour of wave breaking (see Young & Babanin , 2006a , Chapters 5 and 6 ) and with the
asymmetric shape of nonlinear wave groups due to modulational instability ( Shemer et al. ,