Non-dissipative effects of breaking on the wave field - Breaking and Dissipation of Ocean Surface Waves - page 337

Table 8.1 Environmental conditions of records
used. Here, U 10 is the wind speed at 10m height,
f p is peak frequency, H s is significant wave height.

Run U 10 , m / s f p , Hz H s , m

4 6.6 1.33 0.05

8 11.9 0.60 0.16

9 12.0 0.52 0.13

10 8.1 0.77 0.08

11 10.6 0.57 0.08

14 8.2 1.12 0.06

15 7.3 0.60 0.07

24 6.4 1.22 0.05

this issue is important for the alterations of the air-sea exchanges in these extreme situa-
tions (see Section 9.1.3 ). Indeed, Kudryavtsev & Makin ( 2001 ) and Makin & Kudryavtsev
( 2002 ) have parameterised breaking into their phase-resolvent model for ocean wind-
waves and they attribute up to 50% of the total wave-induced stress to the breaking. Clearly,
such model estimates need to be validated observationally.
The measurements were conducted at the Lake George field experimental site during
active wind-generating situations. The prevailing environmental conditions for the set of
records analysed here are summarised in Table 8.1 .
The breaking-detection methodology relied on detecting enhanced acoustic noise at
three bottom-mounted pressure sensors attached to the base of the wave-gauge array frame.
The setup is described in detail in Donelan et al. ( 2005 ). The breaking waves generate an
enhanced acoustic pressure at high frequencies which was sensed clearly by the collo-
cated hydrophone (e.g. Babanin et al. , 2001 ; Manasseh et al. , 2006 , where two different
methodologies for breaking detection were investigated, see also Section 3.5 ). The same
pressure was also detected by the pressure probes, and Babanin et al. ( 2007b )reliedona
third method that uses these probes.
Thus, the breaking-detection procedure was based on 'hearing' the breaking, but it was
also verified by visual means - i.e. by 'seeing' the breaking. This was done using video
records of waves at the measurement location. The video record was taken at the rate of
25 images per second, the same as the sampling frequency of most of the other measure-
ments. All these measurements were synchronised. Once a breaking event was registered,
a zero-crossing analysis was used to identify that whole wave as a breaker and to measure
its relevant properties (i.e. period, height and steepness).
Figure 8.3 illustrates the phenomenon, which is quantified in the subsequent figures in
this section. Figure 8.3 a displays the surface elevation , measured by the wave resistance

wire. In the segment shown, the waves around the 2nd, 7th, 12th and 14th seconds were
identified as breaking by repeated viewing of the video record. Figure 8.3 b demonstrates

η