Wave-breaking severity - Breaking and Dissipation of Ocean Surface Waves - page 222

0.03

0.02

0.01

0

−0.01

−0.02

0 2 4 6 8 10 12 14

time, sec

Figure 6.2 The influence of breaking on the time series. A segment of the surface elevation time
series with IMF = 1 . 8 Hz, IMS = 0 . 30 , U / c = 0. Solid line - surface elevations at the second
probe prior to breaking. Dashed line - same waves 1 . 2 s later at the third probe (shifted back and
superposed). The waves propagate from right to left. Figure is reproduced from Babanin et al. ( 2010a )
by permission

When the wind forcing is superimposed over the mechanically generated waves, its over-
all effect on the wave breaking is very significant. The wind marginally influences the
breaking probability, but its influence on the breaking strength is most essential.
With the ASIST tank length of about 15m and wavelengths of the order of

7m,
the fetch is short and the wind capacity to change the steepness of mechanical waves
is limited, even at strong forcing. Therefore, one would expect only a weak dependence
of the distance to breaking on the wind speed, which is what is seen in Figure 5.17 .
The wind, however, does alter this distance. In the experiment, gradual reduction of the
initial monochromatic steepness caused the waves to break further and further from the
wavemaker, all the way to the beach at the opposite end of the tank. If then the wind
was switched on and its speed gradually increased, the breaking-onset point was brought
somewhat back.
The breaking point, however, would not come too far from the beach, certainly not even
to the middle of the tank; this is not due to the lack of wind power. The ASIST facil-
ity is capable of producing hurricane-force winds. The reason was the breaking severity.
As the wind forcing was increasing and the distance to the breaking decreasing, so was
the breaking strength diminishing. In the end, already between probes 2 and 3 which
were 10

λ ≈ 0 . 5-0 .

59m from the wavemaker, the breaking became a mere toppling
of the very crest and effectively disappeared when the wind speed was increased
further.
This effect is demonstrated in Figure 6.3 which is analogous to Figure 6.2 except the
wind forcing is now superimposed and IMF

. 53m and 11 .

5 is different. The initial monochromatic
frequency had to be reduced in order to make the waves break between the same probes 2
and 3. As just mentioned, in the presence of the wind forcing it would take fewer wave-
lengths to the breaking point and therefore longer waves had to be used.

= 1 .