[1] The results of laboratory measurements on limiting freak waves in the presence of currents are reported. Both dispersive spatial-temporal focusing and wave-current interaction are used to generate freak waves in a partial random wave field in the presence of currents. Wave group structure, for example, spectral slope and frequency bandwidth, is found to be critical to the formation and the geometric properties of freak waves. A nondimensional spectral bandwidth is shown to well represent wave group structure and proves to be a good indicator in determining limiting freak wave characteristics. The role of a co-existing current in the freak wave formation is recognized. Experimental results confirm that a random wave field does not prevent freak wave formation due to dispersive focusing. Strong opposing currents inducing partial wave blocking significantly elevate the limiting steepness and asymmetry of freak waves. At the location where a freak wave occurs, the Fourier spectrum exhibits local energy transfer to high-frequency waves. The Hilbert-Huang spectrum, a time-frequency-amplitude spectrum, depicts both the temporal and spectral evolution of freak waves. A strong correlation between the magnitude of interwave instantaneous frequency modulation and the freak wave nonlinearity (steepness) is observed. The experimental results provide an explanation to address the occurrence and characteristic of freak waves in consideration of the onset of wave breaking.

Incipient breaking of unsteady waves on sheared currents is experimentally investigated. A new wave-generation technique, based on the iterative frequency-focusing concept with the consideration of effects of Doppler shift and current shear, is developed. The surface displacement, the wavelength, and the phase speed of waves at the breaking onset on shear currents are measured. It is found that the steepness of unsteady, incipient breaking waves is altered by the sign and magnitude of current shear ͑or vorticity͒. A current with a positive shear, as would be the case in a wind-driven current, reduces the steepness of an unsteady incipient breaking wave. A negatively sheared current, such as the jet-like ebb current at a tide inlet, leads to steeper incipient breaking waves. The magnitude of reduction/increase in wave steepness is proportional to the strength of a current shear. In particular, a negative shear can alter the wave steepness more significantly in comparison to a positive shear of the same magnitude. Interestingly, the trend of crest-trough asymmetry with respect to the change of a current shear is in contrast to the limiting wave steepness. A positively sheared current can dramatically increase crest-trough asymmetry for unsteady waves. Dimensionless amplitudes of unsteady waves at incipient breaking are well correlated with surface current drifts. Positive/negative surface drifts lead to the reduced/increased dimensionless wave amplitudes. However, the change in dimensionless wave amplitude of unsteady waves is much smaller than that of steady waves.

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