Detection and measurement of wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 49

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Detection and measurement of wave breaking
Measurements and even detection of wave breaking are challenging tasks, particularly if
carried out by unattended devices in the open ocean. As a result, there are vast amounts
of wave records accumulated, and most of them contain breaking waves, but informa-
tion about the breaking cannot be extracted. There is an obvious need for methods and
instrumentation to directly detect breaking events and measure their properties, and for
the analytical means and criteria to identify the breaking waves in existing time series of
surface elevations.
Until recently, visual observations were arguably the only reliable means of breaking
detection. These are based on viewing and quantifying information on whitecaps produced
by breaking waves and are obviously biased towards large breakers (see Section 2.8 ). Over
the past two decades, more technological methods have become available, both contact
type and by remote sensing of the ocean surface or subsurface. These utilise acoustic (pas-
sive and active), optical (both visible and infrared range), reflective and other properties
of breakers which distinguish them from the more homogeneous background wave field.
Without giving a comprehensive review, we will mention here passive acoustic techniques
based on air bubbles ringing while being created during air entrainment (e.g. Lowen &
Melville , 1991a ; Ding & Farmer , 1994 ; Babanin et al. , 2001 ; Manasseh et al. , 2006 ), sonar
observations of bubble clouds produced by breaking wind-waves (e.g. Thorpe , 1992 ),
aerial imaging (e.g. Melville & Matusov , 2002 ; Kleiss & Melville , 2011 , 2010 ), infrared
remote sensing of breaking waves (e.g. Jessup et al. , 1997a ), radar observations of
microwave backscatter from breakers (e.g. Jessup et al. , 1990 ; Lowen & Melville , 1991a ;
Smith et al. , 1996 ; Phillips et al. , 2001 ), and conductivity measurements of the void frac-
tion produced by breakers (e.g. Lammarre & Melville , 1992 ; Gemmrich & Farmer , 1999 ),
amongst others. New analytical methods of detecting the breaking in wave records have
also become available, for example, by means of Hilbert transform (e.g. Huang et al. ,
1998 ) or wavelet analysis (e.g. Liu & Babanin , 2004 ) applied to the wave series.
This section gives a brief review of wave-breaking detection and estimation techniques.
We start from visual observations of whitecapping which have been a traditional way of
investigating the phenomenon and remain a valid and highly efficient method today. Con-
tact measurements of the breaking properties provide a means for direct estimates, which,
unlike visual observations, are not subject to human error. These are subdivided into two
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