CREST INSTABILITIES OF GRAVITY-WAVES .1. THE ALMOST-HIGHEST WAVE

It is shown theoretically that the crest of a steep, irrotational grav ity wave, considered in isolation, is unstable. There exists just one basic mode of instability, whose exponential rate of growth beta equal s 0.123(g/R)(1/2), where g denotes gravity and R is the radius of curv ature at the undisturbed crest. A volume of water near the crest is sh ifted towards the forward face of the wave; the 'toe' of the instabili ty is at a horizontal distance 0.45R ahead of the crest. The instabili ty may represent the initial stage of a spilling breaker. On small sca les, the 'toe' may be a source of parasitic capillary waves.