  Nonlinear wave interactions affect the evolution of steep wave groups, their
breaking and the associated kinematic field. Laboratory experiments are
performed to investigate the effect of the underlying focussing mechanism on
the shape of the breaking wave and its velocity field. In this regard, it is
found that the shape of the wave spectrum plays a substantial role. Broader
underlying wave spectra leads to energetic plungers at a relatively low
amplitude. For narrower spectra waves break at a higher amplitudes but with a
less energetic spiller. Comparison with standard engineering methods commonly
used to predict the velocity underneath extreme waves shows that, under certain
conditions, the measured velocity profile strongly deviates from engineering
predictions.
