Energy dissipation across the wave spectrum - Breaking and Dissipation of Ocean Surface Waves - page 232

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Energy dissipation across the wave spectrum
In previous chapters, we have considered in some detail dissipation of wave energy in the
course of individual wave-breaking events. This makes clear physical sense as breaking
is an intermittent rather than continuous process, and the breaking rates in realistic cir-
cumstances are of the order of a few percent. That is, only a few waves out of a hundred
break at any given spot or any given time, and that is sufficient to keep the energy bal-
ance in a wave system that experiences a consistent and uninterrupted energy input from
the wind.
As discussed in Chapter 6 and throughout the topic, it is often the case that more than
one wave is affected in a single breaking occurrence. The dynamics of these waves is
locked and coupled in many ways, and as far as the wave-energy dissipation is concerned,
these energy losses are impossible to separate. Therefore, it usually makes sense to look at
the properties of the wave group where the breaking took place rather than investigate the
dynamics of some individual wave.
One way or another, there are no theoretical or even experimental approaches that would
allow us to describe the breaking-dissipation process as such, in terms of some decay rate,
as, for example, gradual energy decline due to the action of viscosity in fluid flows. First
of all, unlike most if not all other dissipation causes, breaking dissipation has a start and an
end. The former, as discussed throughout this topic, is most likely due to the water surface
becoming too steep and collapsing, and the reason for the end of breaking at this stage is
perfectly unclear.
Then, gradual decline, i.e. constant dissipation rate, is perhaps not an option as a breaking-
dissipation characteristic either. Plunging breaking has been portrayed above as a series of
jets and splash-ups, and similar, although perhaps less intermittent behaviour should be
applicable to spilling breakers with their multiple-jet structure.
Apart from the collapse of the water system, whose dynamics can apparently proceed
in a number of different ways since it leads to very different outcomes in terms of energy
loss as discussed above, breaking is also accompanied by aeration of the water, ejection
of spray, generation of turbulence and other features. These would require complex two-
phase-fluid modelling approaches, capable of reproducing rotational motions and multiple
exchanges across the interface at various scales. The modelling should be conducted in
three dimensions for two principal reasons: the breaking starts at the centre of the wave
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