New Evidence of Breakpoint Forced Long Waves: Laboratory, Numerical, and Field Observations

Numerical modeling and analysis of laboratory and field data are combined to show new evidence of breakpoint forced long waves. The laboratory data and numerical modeling show that the breakpoint can oscillate at frequencies not directly linked to the wave group envelope for specific bichromatic wave groups. The breakpoint therefore oscillates at frequencies where incident bound long waves are not present, enabling direct identification of breakpoint forced waves. Spectral analysis and cross-shore wave propagation patterns indicate that the breakpoint forced waves occur at subharmonic wave groups frequencies, the repeat frequency of the short waves within the group, and at frequencies associated with triad interactions. The results indicate that in this case the long waves are generated by breakpoint forcing rather than by energy transfer during shoaling of the incident waves. The slowly varying shape of the short waves in the groups and wave merging at the breakpoint controls the modulation of the breakpoint in these cases. The spectral analysis is applied to two field data sets where the breakpoint oscillation, incident waves and shoreline oscillation were measured, and infragravity wave generation is again identified at discrete frequencies that are not present in the incident wave group envelope, with merging of waves at the breakpoint again evident. The magnitudes of the generated infragravity waves outside the surf zone are also dependent on the normalized surf zone width. Both sets of observations provide further evidence for the generation of infragravity waves by the breakpoint forcing mechanism in the field.