Detection and measurement of wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 78

( Wilson & Makris , 2006 , 2008 ). Once deployed, they can be operated on a long-term or
regular basis and collect ready-to-process time series.
Passive acoustic measurements have been employed in a number of field observations
and laboratory experiments. Their first applications to wave-breaking studies were by
Farmer & Vagle ( 1988 ) in the field and by Melville et al. ( 1988 ) in the laboratory. Both
experiments showed that acoustic signatures of breaking waves can be used to identify
breaking events. Farmer & Vagle ( 1988 ) used a single hydrophone and found that the
mean distance between the breakers and the acoustic strength of the breakers depends on
the wind speed.
Ding & Farmer ( 1994 ) further advanced the technique. They developed a directional
array of hydrophones and a method to track individual breaking events out in the ocean.
The directional array made possible measurements of the phase speed of breaking events,
and showed it was related to the spectral scale of breaking waves (the wave period) and
therefore to the spectral scale at which dissipation occurs. Ding & Farmer ( 1994 ) obtained
interesting statistics on the frequency and spacing of breaking occurrences, on breaking
duration, dimension and speed, and some temporal and directional spectral characteristics
of breaking probability. They showed a number of distributions of the breaking probability
as a function of event speeds and event directions (which are analogues of the wave spec-
trum frequency and direction), but did not attempt to relate the magnitude and shape of the
distributions to the wave spectrum and thus to obtain the spectrum of energy dissipation.
Lowen & Melville ( 1991a ) extended and summarised results of earlier laboratory stud-
ies. They used measurements of the acoustic pressure and concluded that the duration of
the hydrophone signal above a background noise threshold is proportional to the breaking-
wave period. Their estimates also showed that, albeit small (i.e.

10 − 8 of the dissipated
wave energy), the acoustic energy radiated by breaking waves is proportional to the mechan-
ical energy dissipated.
This important finding is illustrated in Figure 3.1 reproduced from Melville et al. ( 1992 ).
The onset, impact and duration of the acoustic noise brought about by a breaking event are
clearly seen. The acoustic energy emitted in the course of breaking can easily be quantified.
These results provided a possible method for measuring temporal spectral scales of
breaking events, and even the dissipation related to those scales, using a single hydrophone.
In a spectral environment, that would potentially provide the breaking probability
( Section 2.5 ) and severity ( Section 2.7 ) and ultimately the spectral distribution of the
dissipation (2.20) .
In Lowen & Melville ( 1991a ) and Melville et al. ( 1992 ), the waves were made to
break by means of dispersive focusing of wave packets, generated mechanically, with a
pre-selected central frequency. The method was effectively developed for a single-wave
environment, and determination of the scales and energy losses of breaking waves in a
complex spectral environment was beyond the scope of the studies.
While showing a certain promise in investigating such an elusive characteristic of the
breaking process as wave-energy spectral dissipation, the technique, however, proved inap-
plicable in the field. This is, mostly, due to the much higher levels of background ambient

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