Large transient waves generated through modulational instability in deep water

The long-term evolution of nonlinear wave train in deep water with varied initial wave steepness is investigated experimentally in a super wave flume (300 m long, 5 m wide, 5.2 m deep). The initial wave train is the combination of one carrier wave and a pair of imposed sideband components. Increasing modulation of wave train is observed due to sideband instability until a critical value which either initiates wave breaking or reaches the maximum modulation. The observed maximum local wave steepness increases rapidly with the increase of the initial wave steepness, and levels off at initial wave steepness roughly equal to 0.15 despites that the data exhibits a little scattering. The normalized crest elevation at peak modulation increases rapidly with initial wave steepness and approached a maximum value almost equal to 3.5 which corresponds to initial wave steepness around k_ca_c = 0.15. The results reveal that the large transient wave such as freak wave could be generated during the propagation of nonlinear wave trains in deep water through sideband instability.