Detection and measurement of wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 62

themselves, i.e. hydrodynamics rather than the wind, ultimately control the breaking process
of dominant waves.
The breaking of small waves, on the other hand, appears to correlate with the run-
ning variance of the acceleration. The spectrum of the acceleration is dominated by high-
frequency contributions into the variance (e.g. Babanin et al. , 1993 ). Thus, the correlation
signifies a connection of the small-scale breaking with variations of the level of equilib-
rium interval in the wave spectra and points to the two-phase behaviour of wave breaking
which will be an important discussion issue in breaking dissipation ( Section 7.3.4 ).
Thus, Babanin ( 1995 ) concluded that the short-term equilibrium of the wave spectrum
tail (i.e. level

α

in the spectrum shape formulation (2.7) ) was supported by changes in the
frequency of breaking occurrence of waves that belong to this tail (see also Section 8.2 ).
Breaking of the dominant waves controls the energy in the spectral peak region, that is the
enhancement

of the peak (2.7) . Overall breaking rates were also found to depend on the
spectral width which was formulated as

γ

m 0
f p F

ν = (3.20)

( f p )

following Belberov et al. ( 1983 ).
Observations similar to those by Holthuijsen & Herbers ( 1986 ) and Babanin ( 1995 )
have obvious limitations due to human error when marking rapid and suddenly happening
events, and due to the physical ability of the observer to only resolve large enough events.
Holthuijsen & Herbers ( 1986 )

“watched the buoy from either a nearby observation tower ( ∼ 100m from the buoy) or from a nearby
ship ( ∼ 50m from the buoy)”.

Their estimate is that they could see whitecaps of the minimum size of approximately
15 cm. Babanin ( 1995 ) watched the whitecaps occurring either inside an array of wave
staffs or on a drifting buoy. In the first case, the observer was located 16m directly above
the array and could resolve any breaking that produced whitecaps. In the case of the buoy,
the observer was watching this from a drifting research vessel, some 100m away, and had
a clear view of the whitecaps down to the size of the buoy (less than a metre ( Babanin
et al. , 1993 )).
Such limitations can be overcome to some extent by high-resolution video taping of the
wave surface. Having followed such an approach, Katsaros & Atakturk ( 1992 ) reported
quite unique statistics, including that of micro-breaking. Contrary to many expectations
(see Section 2.8 ), micro-breaking rates in their observations did not appear higher than
those for plunging and spilling breaking. This counter-intuitive conclusion can perhaps still
be explained by the difficulty of visual resolution of micro-breakers, even in the course
of repeated video observations. An important finding of Katsaros & Atakturk ( 1992 )is
that, while the overall amount of plunging/spilling breaking depends on average on the
background mean wind speed, the amount of micro-breaking does not. This can now be
explained by the cumulative effect of wave breaking, that is by the fact that small-scale