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Stolte ( 1992 ) high-passed wave records at 1 Hz cut-off frequency and used visual obser-
vations to deduce empirical criteria: e.g.
+
2
.
5 cm-jump over 0
.
31 s to mark the start of a
breaker and
016 s for the breaker's end. Very interesting statistics for the
breaker velocity, height, length, momentum, spectral distribution of the breaking momen-
tum and parameterisations of the total and relative numbers of breaking were obtained.
Stolte ( 1992 ) also provided an experimental dependence of breaking severity on wind
speed, which is quite a unique result.
The contact means for detecting wave breaking and quantifying the breaking physics
do not have to be restricted to only measuring surface elevations or other surface prop-
erties. Lammarre & Melville ( 1992 ), Su & Cartmill ( 1992 ) and Gemmrich & Farmer
( 1999 ), for example, developed a technique based on conductivity measurements below
the surface. The breaking causes air-entrainment into the water column which is accom-
panied by a reduction of electrical conductivity. Effectively, the void fraction in the water
is detected which signifies and quantifies breaking events, their probabilities, severity and
other properties. Gemmrich & Farmer ( 1999 ) defined a wave as the breaking event if the air
fraction exceeds 8% in accordance with the theory of Longuet-Higgins & Turner ( 1974 ).
The method is quite sensitive and accurate, and when used in field experiments allows us
to obtain the spectral distribution of breaking probability ( Banner et al. , 2002 ).
Thus, the trial-and-error criteria based on interpretation of one or another kind of dis-
continuity/jump caused by the breaking can bring very fruitful and useful outcomes and
should not be underestimated. Some of them, however, are very involved and leave room
for uncertainties if they are to be repeated. For example, Stolte ( 1992 ) admits that his
criteria had to be modified for more- and for less-intensively breaking wave fields. With
respect to the Weissman et al. ( 1984 ) technique, Katsaros & Atakturk ( 1992 )wrote:
1
.
25 cm over 0
.
“Since the threshold, i.e. the absolute value of the measured band energy varies with wind speed
(stress), gustiness, underlying long waves, currents etc., it had to be determined individually for
each run”.
Gemmrich & Farmer ( 1999 ), who used the void-fraction criterion, pointed out:
“While this criterion utilizes a well-defined property of all breaking waves except microbreaking,
the definition of a suitable threshold again seems arbitrary and depends on the precise depth of the
measurement as well as other factors that we cannot control, such as the vertical gradient of air
fraction”.
3.4 Laboratory measurements in deterministic wave fields
Laboratory measurements may involve both contact and remote-sensing methods and deal
with either random (for example, wind-generated as in Section 3.3 ) or deterministic wave
fields. The principal difference of the latter, with respect to field observations, is that in
such controlled repeatable experiments the location of breaking is known. If so, the neces-
sary measurements of wave breaking can be planned without having to detect the breaking
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