Emergence of short crestedness in originally unidirectional nonlinear waves

Ocean waves forced by the wind are multidirectional, even if the wind direction is steady. Directionality can occur due to superposition of long-crested waves propagating at angle to each other. Lateral modulation of the wave crests, however, also takes place for strictly unidirectional waves, due to nonlinear effects. Here short crestedness of unidirectional waves is investigated. A series of experiments on the lateral instability of monochromatic, deep water, unidirectional, initially long-crested steep waves was performed in a large wave basin. The cross modulation of wave crests is clearly visible; its magnitude depends on the wave steepness in the wave propagation direction. Its spatial scale is comparable with the wavelength. Thus, short crestedness is an inherent feature of nonlinear waves and should be taken into account when estimating directional properties of ocean waves