  Horizontal Lagrangian velocities and accelerations at the surface of steep
water-waves are studied by Particle Tracking Velocimetry (PTV) for gradually
increasing crest heights up to the inception of a spilling breaker. Localized
steep waves are excited using wavemaker-generated Peregrine breather-type wave
trains. Actual crest and phase velocities are estimated from video recorded
sequences of the instantaneous wave shape as well as from surface elevation
measurements by wave gauges. Effects of nonlinearity and spectral width on
phase velocity, as well as relation between the phase velocity and crest
propagation speed are discussed. The inception of a spilling breaker is
associated with the horizontal velocity of water particles at the crest
attaining that of the crest, thus confirming the kinematic criterion for
inception of breaking.
