Gentle spilling breakers: crest profile evolution

The surface profile histories of gentle spilling breakers generated mechani cally with a dispersive focusing technique are studied experimentally. Frou de-scaled generation conditions are used to produce waves with three averag e frequencies: f(0) = 1.42, 1.26, and 1.15 Hz. At each frequency, the stren gth of the breaker is varied by varying the overall amplitude of the wavema ker motion. It is found that in all cases the beginning of the breaking pro cess is marked by the formation of a bulge in the profile at the crest on t he forward face of the wave. The leading edge of this bulge is called the t oe. As the breaking process continues, the bulge becomes more pronounced wh ile the toe remains in nearly a fixed position relative to the crest. Capil lary waves form ahead of the toe. At a time of about 0.1/f(0) after the bul ge first becomes visible, the toe begins to move down the face of the wave and very quickly accelerates to a constant velocity which scales with the w ave crest speed. During this phase of the breaker evolution, the surface pr ofile between the toe and the crest develops ripples which eventually are l eft behind the wave crest. It is found that the height of the toe above the mean water level scales with the nominal wavelength lambda(0) = g/(2 pi f( 0)(2)) of the breaker, while the size and shape of the bulge and the length of the capillary waves ahead of the toe are independent of f(0).