Lagrangian measurements of accelerations in the crests of breaking and broken waves.

The present investigation reports synoptic measurements and analysis of particle accelerations and kinematics of plunging crests in deep water wave groups. The unexpected occurrence of unusully large waves has been documented on numerous occasions. While little is known about the statistics of these waves, even less is known of the dynamical conditions under which they occur. Nonlinear interactions among individual waves travelling within a group on an opposing current have been identified as an important mechanism in the formation of giant waves in deep waters in the ocean. In this study, the non-linear packet-focusing technique is used to generate steep, deep water plunging waves in two laboratory flumes. In one of these flumes the plunging breakers were generated on opposing currents. The kinematics of these waves are measured just up-wave of the onset of plunging, and these results are compared to those of a superposition model, a modified stretching model, and a model based on Stokes 3(rd) order theory combined with a linear wave spectrum for an irregular sea on an opposing current, developed for the present study. The present model represents the velocity beneath the plunging breakers significantly better than the two other models.