Detection and measurement of wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 74

to estimate, particularly in the field, it was concluded that the kinematic criterion (2.49) is
the most robust property to describe the breaking onset.
Special attention was paid to the severity of breaking in the circumstances. The severity
was defined in terms of wave groups (2.32) . It was found that
“spatially focusing and diffracting wave packets lost 34% and 18% of their energy, respectively, as
a result of plunging breakers and lost 12% and 9%, respectively, as a result of spilling breakers.
Comparable two-dimensional breakers with the same spectral shape lost 16% for plunging and 12%
for spilling”.
The breaking strength and its spectral distribution in the case of breaking caused by lin-
ear focusing, considered by Nepf et al. ( 1998 ) and Wu & Nepf ( 2002 ), is very different
to the severity distribution in the breaking event caused by amplitude dispersion ( Pierson
et al. , 1992 ) or by nonlinear modulation ( Babanin et al. , 2009a , 2010a ). The wave super-
positions, whether these are due to frequency or amplitude dispersion, concentrate wave
energy and thus create a steep wave which then becomes unstable and breaks. A very steep
wave can be brought about by a completely different physical mechanism, modulational
instability of wave trains. Such instability leads to formation of nonlinear wave groups,
within which rapid instantaneous concentration of wave energy occurs at some fetch. The
location of such an event, which results in wave breaking if the concentration leads to for-
mation of a steep enough wave, is repeatable in deterministic experiments, but it is not as
precise as that due to the superpositions of waves discussed above. Breaking happens at a
particular phase of the nonlinear group, close to the top of the group whose envelope is not
symmetric, and since the group usually comprises a non-integer number of carrier waves,
the exact position of the breaking oscillates ( Melville , 1982 ; Babanin et al. , 2007a , 2009a ,
2010a ).
The oscillation, however, is quasi-periodic and measurements of the breaking onset or of
other phases of breaking events can be conducted by a simple wave probe through recoding
a number of breaking events at the location ( Babanin et al. , 2007a , 2009a , 2010a ). Since
it is believed that the modulational instability may be one of the primary mechanisms
responsible for wave breaking in ocean wave fields, later in this topic significant atten-
tion will be paid to laboratory experiments dealing with breaking due to such instability
( Chapters 5 , 6 ).
3.5 Acoustic methods
Underwater ambient acoustic noise is generated through a number of possible sources
such as precipitation, formation of bubbles in a saturated condition (e.g. Blanchard &
Woodcock , 1957 ), biological sources (e.g. Chitre et al. , 2006 ), or breaking waves. In
most circumstances, the latter is by far the dominant source (e.g. Kerman , 1988 , 1992 ;
Farmer & Vagle , 1988 ; Thorpe , 1992 ; Felizardo & Melville , 1995 ; Bass & Hey , 1997 ;
Tkalich & Chan , 2002 ; Manasseh et al. , 2006 , among many others). Here, we will describe
the relation of ambient sound to wave breaking following Manasseh et al. ( 2006 ) and