Intermittent subharmonic edge wave excitation with random incoming waves.

Edge waves are trapped near the shoreline by reflection and refraction, and propagate in the alongshore direction. Previous theoretical and experimental research described subharmonic edge waves excited by nonlinear interaction with incoming monochromatic waves that are strongly reflected at the shoreline. Random waves have largely been neglected, and our understanding of subharmonic edge wave excitation with realistic coastal waves is still extremely limited. We describe large scale laboratory experiments of subharmonic edge wave excitation. Results with monochromatic incoming waves confirm previous research. A new phenomenon, intermittent edge wave excitation, was observed with random incoming waves. Subhamonic edge wave amplitudes sometimes varied temporally, over many wave periods, between essentially zero (no edge waves) and the relatively large amplitudes seen with monochromatic incident waves. The observed details of intermittency (e.g. duration and energy of intermittently excited edge waves) depend on the directional and frequency spectrum of the random incoming waves. Numerical simulations, based on extension of monochromatic theory to random waves, also exhibit intermittency.