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event first. These can therefore be fine measurements which either require precise position-
ing of measuring devices or employ delicate high-precision instrumentation, impossible to
transport and deploy in the field, for example, particle image velocimetry (
PIV
).
In the content of this section, methodological and phenomenological differences between
directional and shortcrested waves will also be discussed. Another topic in this section,
which has been a discussion issue lately, is estimates of the wave-breaking dissipation
based on the dimensional argument which involves speed of breaker propagation.
Rather than just concentrating on passive observations and measurements, in the lab-
oratory an experimental effort can create conditions to simulate breaking by means of
a designated physical mechanism, for example by means of superposition of linear waves
achieved through use of frequency dispersion (e.g.
Cummins
,
1962
;
Davis & Zarnik
,
1964
;
Longuet-Higgins
,
1974
;
Rapp & Melville
,
1990
;
Griffin
et al.
,
1996
;
Meza
et al.
,
2000
),
or a superposition of nonlinear waves through amplitude dispersion (e.g.
Donelan
,
1978
;
Pierson
et al.
,
1992
), evolution of nonlinear wave groups (e.g.
Melville
,
1982
;
Babanin
et al.
,
2007a
,
2009a
,
2010a
), or simply by artificial means such as concentration of wave
energy because of converging channel walls (
Van Dorn & Pazan
,
1975
;
Ramberg
et al.
,
1985
;
Ramberg & Griffin
,
1987
) or over an obstacle or a submerged shoal (e.g.
Ramberg &
Bartholomew
,
1982
;
Manasseh
et al.
,
2006
;
Calabresea
et al.
,
2008
).
Conditions can also be created to exaggerate some wave-breaking features and extend
the breaking time in order to study the phenomenon in greater detail. Care must be taken
to properly scale down the observed and extended features, to clearly realise limits of
applicability of the modelled conditions, but the potential rewards of such laboratory efforts
can be very significant.
In this regard, an experiment by
Duncan
(
1981
) should be highlighted whose results were
broadly implemented and whose ideas stimulated very many related experiments, appli-
cations and studies (e.g.
Phillips
,
1985
;
Melville
,
1994
;
Phillips
et al.
,
2001
;
Melville &
Matusov
,
2002
;
Gemmrich
et al.
,
2008
, among many others). In the experiment, a steady
breaker was produced by means of towing a submerged hydrofoil with phase speed of the
breaking wave. The profile of the breaking waves, velocity distributions, turbulent wake
and other properties of the steady breaker were measured and investigated.
Of particular importance for subsequent studies and applications was
Duncan
's (
1981
)
conclusion that the energy dissipation rate in such a breaker can be described by a single
independent variable, the wave's phase speed
c
. Thus, the rate of energy loss per unit length
of the breaking front is proportional to:
)
∼
ρ
g
c
5
S
ds
(
.
c
(3.27)
Since in the linear or quasi-linear sense the phase speed translates into frequency/wave-
number, such a conclusion signifies the existence of a universal spectral function for wave
energy dissipation, and attempts have been undertaken to formulate and quantify such a dis-
sipation term (
Melville
,
1994
;
Phillips
et al.
,
2001
;
Melville & Matusov
,
2002
;
Gemmrich
et al.
,
2008
, among others).
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