Detection and measurement of wave breaking - Breaking and Dissipation of Ocean Surface Waves - page 88

Figure 3.9 Time series of water elevation. Upper panel: ◦ , zero crossings; + ,crests; × , troughs.
Lower panel: water elevation with superimposed count of detected bubbles within 0 . 25 s intervals
at the optimal breaking-detection discriminant. Figure is reproduced from Manasseh et al. ( 2006 )
© American Meteorological Society. Reprinted with permission

on the energy loss due to breaking at the measured spectral frequencies. A combination of
the breaking-probability distribution and the bubble size can lead to direct estimates of the
spectral distribution of wave dissipation.
In order to examine the hypothesis that the bubble size is related to breaking severity,
the passive acoustic analysis was applied to data from a laboratory wave-maker. Waves
with a frequency of 0 . 75 Hz and various amplitudes were generated in a flume of width

1 215m at the School of Civil and Environmental Engineering of the University of

Adelaide. The water depth was 225

.

5mm above a sandy bottom. A vertical board 45mm
wide and 150mm deep was placed about 10m downstream of the wavemaker; its top was
30

±

3mm above the mean water level so plunging breakers were forced to form over this
barrier. Two capacitance probes measured the instantaneous water elevations, 640

±

± 5mm

upstream of the board and 560 5mm downstream of the board. A hydrophone (Bruel

& Kjaer 8103) with a diameter of 9

±

. 5mm was mounted 60 ± 2mm downstream of the

board with its tip 55 2mm below the mean water level. The probes and hydrophone were

approximately in the centre of the flume width. A typical acoustic time series is shown in
Figure 3.11 .

±