An investigation into three-dimensional waves has been carried out using Particle image velocimetry The wave field contains a single frequency with a uniform angular spreading within a given range {- a, a) The waves are designed such that the wave energy is focused at a given point Several a values between 30 and 60 degrees were tested Angular spreading was found to have great effects on the breaking characteristics and kinematics Two types of breakers were observed, with the first being plunging breaking and the second being spilling breaking The extreme wave kinematics were enhanced by the increased spreading The maximum velocity to deep water celerity ratio of unity seemed to be the upper limit for spilling breakers and the lower limit for the plungers The maximum velocity did not, in general, appear at the crest The limiting crest velocity was found to be the value of the deep water celerity, which is In line with the observation of Griffiths (1989)


Wave breaking has been a subject under vigorous investigations in recent years As more and more advanced measuring techniques such as LDA (Laser-Doppler Anemometry), PIV (Particle Image Velocimetry) and FOS (Fibre Optical Sensors) become available for scientific researches, much wider scope of wave-associated problems are being examined Using LDA, Griffiths' made a detailed parametric study of shoaling of two-dimensional mono-chromatic waves on a beach of various slopes The deep water celerity was found to be the limiting crest velocity In pursuing an understanding of the intricate nature of breaking waves, many researchers developed various numerical methods which allowed them to examine breaking wave properties in detail A good example of this is that of Cokelet (1979) These numerical methods can, however, only be used to simulate two-dimensional breaking waves, although some of them can in principle be expanded to deal with three-dimensional cases The existing experimental works are also largely limited to two-dimensional conditions Studies of three-dimensional wave breaking in laboratories and open seas are difficult and rare Su (1982) reports an interesting phenomenon where a two-dimensional Stokes wave develops into fully three-dimensional breakers as a result of three-dimensional instability Kjeldsen et al (1981) reported some field measurements of breaking waves where surface elevations were utilised to derive a description of surface properties

To take the large step from two-dimensional measurements to fully three-dimensional measurements would involve the introduction of a large number of parameters and would require a research programme on a very large scale Choice of spectrum (P-M, JONSWAP), spreading angle, water depth, currents, selection of measuring point and definition of breaking all influence the final form of the breaking wave For this reason it is first necessary to isolate the individual parameters and measure their effects She et al (1991) looked at the addition of two angled wave crests and concluded that the velocity field of two crossing wave trains was more severe than that of a sinusoidal two-dimensional wave The effects of angular spreading on wave breaking was further examined by She et al (1992) and the surface parameters, such as breaking wave height and steepness, were found to be strongly influenced by angular spreading, underlying the importance of three-dimensional effects

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