Edge waves and surf beat

Abstract

Surf beat, wave motion at relatively low frequency (periods of 30–200 s), is often observed on beaches. However, even with modern instrumentation it is difficult to describe the spatial variation of the low‐frequency motion; consequently, the relative importance of a number of suggestions which, at least in theory, provide mechanisms for the generation of low‐frequency energy has never been established. Recent observations (e.g., Huntley, 1976) have reinforced the idea that edge waves, the free wave modes trapped at the shoreline, are a major component of low‐frequency energy. One of the most interesting explanations of surf beat suggests that the beating between particular pairs of incoming waves leads to resonant growth of edge wave modes, which may then dominate the low‐frequency spectrum (Gallagher, 1971). Empirical evidence is essential, as any theoretical development breaks down when the incoming waves break, a fundamental problem with Gallagher's (1971) model. To investigate the importance of this resonant interaction, the general interaction conditions were therefore used to design laboratory experiments in which both resonant and nonresonant conditions were expected. The experimental results show that the response at the beat frequency is stronger when the resonance conditions for edge wave growth are satisfied and that the response is in the form of the theoretically predicted edge wave mode, even when the incident waves are breaking. These results strongly suggest that surf beat is predominantly an edge wave phenomenon.