Geoscience Reference
In-Depth Information
The subplots of
Figure 8.4
demonstrate different wave and wind properties as functions
of the magnitude of the chosen
b
t
value.
Figure 8.4
a shows such a dependence for
T
∗
=
T
br
/
T
tot
, the ratio of observed breaking duration to the total duration. If
b
t
is chosen very
low, all waves will be identified as breaking and the ratio will approach 100%. If
b
t
is
excessively high, no breaking waves will be detected in the record and
T
∗
will asymptote
to zero. For high values of
b
t
well above the threshold, but below the ultimate value, the
method will select particularly severe breakers, depending on the strength of their acoustic
impact.
The acoustic method demonstrates the expected behaviour for the breaking duration in
general, with noticeable differences between the three wind-forcing situations at low-to-
intermediate values of
b
t
. For example, for
b
t
=
1, the method would identify some 80%
of waves as breaking at the two lighter winds, but only 70% at the 11
s wind. The
latter signifies a relatively smaller contribution of particularly 'noisy' events to the total
noise, which is most likely caused by a higher level of ambient acoustic noise at strong
winds.
The second subplot,
Figure 8.4
b, shows the corresponding dependence of
E
∗
=
.
9m
/
E
tot
,
the relative contribution of breaking waves to the input energy flux to the waves. It imme-
diately exhibits the expected effect of the enhancement of the wind input. If there were no
enhancement, the ratio
E
br
/
E
br
/
E
tot
would follow the
T
∗
=
T
tot
dependence, tending to
asymptote to zero for severe but rare events. It is clearly not the case for strong breakers
with
b
t
greater than about 2.5, which means that the relative contribution of those events
to the total flux is greater than their relative duration.
The bottom panel in
Figure 8.4
is the most informative subplot in this figure, and under-
pins our choice of threshold level
b
t
=
T
br
/
2
.
5. In this panel, plotted as a function of the
T
∗
. This ratio should be 1 for the case when the
flux
E
br
had no enhancement compared to the flux that would occur during the period
T
br
if
the waves were not breaking. Since the breaking waves usually have larger amplitudes than
those not breaking, to avoid any influence of wave steepness on the instantaneous energy
fluxes,
E
tot
has been normalized by the significant wave height
H
s
of the non-breaking
waves
H
nb
, and
E
br
by the significant wave height of the breaking waves
H
b
.
The ratio
E
is seen to be close to or below 1, up to a threshold value of
b
t
around
2.5, and corresponds approximately to the stage where the threshold starts detecting the
breakers and not detecting the non-breakers. Furthermore, for
b
t
=
E
∗
/
threshold property
b
t
, is the ratio
E
=
5, all waves with a
bottom pressure exceeding this threshold certainly break. This was verified by viewing
the synchronised video imagery for a representative subset of the records. Therefore, this
bottom pressure threshold was adopted throughout the analysis to register the breaking
waves.
Finally, with respect to this subplot, it should be noted that waves with acoustic signa-
tures below the bottom-pressure threshold of
b
t
=
2
.
5 may or may not break - the bottom
pressure-sensing system did not detect breakers reliably near the threshold. That is why
the enhancement curves can depart from unity. These data were not taken into account in
order to deal with genuinely breaking waves only.
2
.
Search WWH ::
Custom Search