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bubbles formed in the course of breaking can be determined by passive acoustic means,
i.e. by recording the noise produced by a breaking wave and quantifying the characteristics
of this noise. Essentially, bubble size appears to be connected to the strength of breaking.
Thus, the new acoustic technique, once it is fully validated and calibrated, has the poten-
tial to allow measurements of breaking severity by means of underwater hydrophones. The
hydrophones are very small devices, easy to deploy, cheap and therefore cost-effective even
if lost in a field experiment. Their power consumption is low which would allow long-term
deployment and observations with a power supply based on batteries. Also, importantly,
because the sound attenuation in water is relatively low, the hydrophones can be deployed
well below the surface, even at the bottom in the case of finite-depth field sites, thus avoid-
ing the destructive impact of wave-breaking events (see Babanin et al. , 2001 ; Young et al. ,
2005 ; Manasseh et al. , 2006 ).
In the meantime, reliable and comprehensive understanding of breaking severity is the
principal issue for all applications involving dissipation of wave energy, which thus range
from engineering problems to general topics of air-sea interaction, extending as far as cli-
mate modelling. Indeed, in order to estimate the rate of energy loss from the wave field,
whether it is then converted into energy of impact on engineering structures or into mix-
ing of the upper ocean, it is necessary to know how frequently the waves break and how
much energy they lose in a breaking event. If we denote the breaking severity, i.e. the mean
energy loss due to breaking of waves of a particular scale f as E s (
f
)
, then the amount of
energy dissipated per wave crest at this scale D
(
f
)
is:
D
(
f
) =
b T (
f
)
E s (
f
).
(2.20)
In terms of the spectral dissipation function S ds employed in wave forecast models, the
dissipation rates per unit of time are:
n
(
f
)
E s (
f
)
n
(
f
)
N
(
f
)
D
T .
(
f
S ds (
f
) =
=
E s (
f
) =
(2.21)
t
N
(
f
)
t
This practical definition can be used to estimate the spectral distribution of dissipation
by experimental means ( Manasseh et al. , 2006 ; Babanin et al. , 2007c ). It has, however,
the same obvious issues with the spectral band (2.5) , as does the practical definition of
breaking probability in Section 2.5 , because, again, n
(
f
)
and E s (
f
)
in the right-hand side
are not spectral densities but are statistical quantities.
The wave energy per unit area is (e.g. Young , 1999 )
1
8 ρ w
2
E
(
f
) =
gH
(
f
)
(2.22)
where
is the density of water, and the normalised potential wave energy can be expressed
simply in terms of the wave height H
ρ w
(
f
)
2
E
(
f
) =
H
(
f
)
.
(2.23)
 
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