An experimental investigation of gentle spilling breaking water waves

Gentle spilling breakers generated mechanically with a dispersive focusing technique are studied experimentally through measurement of the evolution of the surface profile and flow field in the crest. Froude-scaled generation conditions are used to produce waves with three average frequencies: {dollar}fsb0=1.42,{dollar} 1.26, and 1.15 Hz. The strength of the breaker at each frequency is varied by varying the overall amplitude of the wavemaker motion. It is found that in all cases the beginning of the breaking process is marked by the formation of a bulge in the profile at the crest on the forward face of the wave. The leading edge of this bulge is called the toe. As the breaking process continues, the bulge becomes more pronounced while the toe remains in nearly a fixed position relative to the crest. Capillary waves form ahead of the toe. At this stage of breaking, the maximum horizontal particle velocity is located at the forward face of the crest and is smaller than the phase speed of the crest. It is found that the height of the toe above the mean water level scales with the nominal wavelength, {dollar}lambdasb0(=g/(2pi fsbsp{lcub}0{rcub}{lcub}2{rcub})),{dollar} of the breaker, while the size and the shape of the bulge and the length of the capillary waves ahead of the toe are independent of {dollar}fsb0.{dollar} At a time of about {dollar}0.1/fsb0{dollar} after the bulge first becomes visible, the toe begins to move down the face of the wave and very quickly accelerates to a constant phase speed which scales with the wave crest speed. The fluid particle velocity in the vicinity of the toe is everywhere smaller than the phase speed of the toe; this indicates that toe motion is a wave like phenomenon. During this phase of the breaker evolution, the surface profile between the toe and the crest develops ripples which eventually are left behind the wave crest. Concurrent with the toe motion, a large amount of vorticity was generated and convected downstream and the turbulent flow quickly spreads over the crest.