Definitions for wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 40

Note that in the formally introduced definition (2.44) the primary breaker, which origi-
nally caused the energy loss, does not necessarily have to belong to the frequency range
f 1 <

f 2 .
Definitions (2.32) and (2.42) describe the breaking severity measured in physical space.
The first accounts for all energy losses that take place due to a breaking within a wave
group. As discussed above, these are breakings of dominant waves, although the spectral
impact may be distributed across all wave scales. The second definition relates specifically
to energy losses of waves short compared to the spectral peak f p according to (2.43) .The
most important significance of these definitions is that, together with measurements of the
breaking probability (2.36) and (2.4) , they allow us to estimate experimentally the spectral
dissipation function (2.21) . As already mentioned, this function has so far been the most
elusive and speculative property of wave modelling and forecasting (see e.g. Young &
Babanin , 2006a ; Babanin et al. , 2007c ; Babanin & van der Westhuysen , 2008 ; Babanin
et al. , 2010c ). It will be discussed in detail in Chapter 7 .

f <

2.8 Types of breaking waves: plunging, spilling and micro-breaking

Breaking waves are usually subdivided into three types: plunging, spilling and micro-
breaking (e.g. Weissman et al. , 1984 ). More detailed and complicated classifications are
also available. For example, Griffin ( 1984 ) introduces a collapsing type as a limiting case
of the plunging breaker and a surging-breaker type over a sloping beach.
The plunging breaker is the most commonly perceived picture of a breaking wave: at
breaking onset, the crest curves forward and forms a plunging jet that impacts and pen-
etrates the water surface in front of the wave, entrains air and turbulence deep under the
surface, and potentially can trap an air pocket between the jet and the former front face of
the wave, which will then disintegrate into large bubbles with corresponding consequences
in terms of gas exchange across the interface and particularly in terms of acoustic noise
produced by the event.
The spilling type is a less dramatic, but somewhat more frequent (e.g. Katsaros &
Atakturk , 1992 ) kind of breaking when the crest destabilises and collapses, spilling the
water over the front slope of the wave. Price ( 1970 , 1971 ) suggested a perturbation theory
leading to the spilling breaking. Qiao & Duncan ( 2001 ) used particle image velocimetry
(PIV) to investigate development of spilling breakers in the laboratory. They found that,
first, a bulge is formed which, however, does not result in formation of a jet as in the
plunging breaker described above. Rather, the bulge starts to slide down the front face of
the wave.
In many regards, the dynamics of the spilling breaker is similar to having multiple
smaller jets impacting the water surface; these jets also lead to formation of bubbles and
generation of turbulence. Essential also, as far as turbulence and dissipation are concerned,
are the shear stresses between the sliding-down bulge and the wave orbital motion. Plun-
ging breaking is more energetic in terms of energy/momentum loss from the waves and
correspondingly the energy/momentum transferred to the ocean, as well as in terms of