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no analytical theory to model, we have to simulate basic equations for two-phase flows,
including water filled with bubbles and air filled with droplets. Computationally, these are
very demanding models, but they have advanced very significantly over the past decade
(see
Section 7.2
).
Thus, applications which involve breaking severity have to rely on experimental data.
These are quite sparse and difficult to obtain as well, and the difficulties are many. Most
importantly, in order to estimate the loss of energy, contact measurements are usually
needed. That is, at the very least, the wave energy loss has to be estimated by direct meas-
urement immediately before and immediately after a breaking event. This is a complicated
exercise even in the laboratory because it involves a controlled breaking at a particular
location, between the wave probes.
While possible in principle (e.g.
Rapp & Melville
,
1990
;
Pierson
et al.
,
1992
;
Meza
et al.
,
2000
;
Manasseh
et al.
,
2006
;
Babanin
et al.
,
2007a
,
2009a
,
2010a
), the controlled-
breaking approach has many limitations. First of all, controlled breaking is often achieved
by artificial means, for instance through focusing many linear waves of different scales
by using their different phase speeds. It is quite likely that this is not the way, or at least
not the only way that breaking is controlled in general, and breaking severity in particu-
lar in natural environments (
Babanin
et al.
,
2009a
,
2010a
). Secondly, keeping controlled
breaking between two particular probes significantly limits the variety of wave proper-
ties and environmental dependencies that can be investigated. And finally, some processes
leading to wave breaking, and in particular the most important mechanism of modula-
tional instability of nonlinear waves trains, may be impaired in three-dimensional natural
field circumstances, which fact further depletes the generality of conclusions achieved in
two-dimensional wave tanks. See
Chapter 6
for more details.
In the field, estimating the breaking strength by means of measurement of wave-breaking
events before and after breaking is a challenge. Breaking is random, sporadic and infre-
quent, and the probability of it starting and ending right between two deployed wave gauges
is very low. To achieve this, long measurement series would have to be taken using an
extensive spatial array of wave probes, concurrent with detailed monitoring and recording
of information on wave breaking, i.e. the start, the end and the direction of propagation
of the breaking waves through the array. Depending on the instrumentation employed, a
significant complication of contact
in situ
measurements is the destructive power of wave
breaking which can damage the equipment, particularly as the most interesting breaking
events are the most severe. Significantly promising in this regard, both in terms of mea-
suring the waves losing their energy as they are breaking and in terms of being detached
from the direct wave-breaking impact, is the stereo-imaging technique which allows us
to video-record three-dimensional surface elevations over an area, with high resolution in
time and space (
Gallego
et al.
,
2008
).
While many remote-sensing techniques have been developed to investigate wave break-
ing, these are mostly intent on detecting the breaking events and measuring breaking
probabilities rather than the energy losses (see
Section 3.6
). In this regard, the bubble-
detection method of
Manasseh
et al.
(
2006
) is also significantly promising. The size of the
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