This first stage of a laboratory study of the turbulent flow-field induced by wave-breaking involves the application of a flow-visualization technique from which external appearance and internal flow-structures of the surf zone have been examined. Plunging breakers were generated in a wave flume, whilst suitable light sources were deployed. To trace the water particle motion, fluorescent coatings in combination with suitable illuminating light sources, such as an Argon-Ion laser and ultraviolet lamps, were used. Whilst a particular light source was suitable for studying the water body in the surf zone, it might not be for studying particle motion in the air-bubble zone having a high concentration of air entrainment. It was found that the motion in the surf zone was highly three-dimensional with a dividing region formed beneath the impinging surface rollers, and that the breaking phenomenon was cyclic and deterministic in character.

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To investigate the evolution of wave modulation and wave breaking, a series of elaborate experiments were conducted in a super wave flume (300 m × 5.0 m × 5.2 m) at Tainan Hydraulics Laboratory. Two wave trains of initial regular waves and initial imposed sidebands were generated by a wave maker with active wave absorption using an electro-hydraulic server feedback control system. The wave profiles were recorded by 66 high-resolution capacitance-type wave gauges positioned along the wave flume, and connected to a specially designed data acquisition system to collect a large number of parallel inputs in the long flume. Experimental data were obtained on wave modulation and related wave breaking. The long time evolution of the nonlinear wave train and the corresponding wave spectra are demonstrated for both breaking and non-breaking cases. Specifically, the periodic modulation and demodulation of wave train is observed at post-breaking stage.

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