Observations of wave motion are described, particularly of the horizontal particle velocities, up to a maximum of 25 m from the shoreline on Slapton Beach, Start Bay. For the first time on a natural beach, the observed velocities reveal directly the presence of short period edge waves, modes of wave motion trapped to the shoreline by refraction. These edge waves occurred at the first subharmonic (σ/2) of the incident wave frequency σ, and their exponential decay in amplitude with distance from the shoreline was consistent with edge waves of mode number zero with a longshore wavelength of 34 ± 6m. The implications of these observations are discussed in the light of recent laboratory experiments and theories of edge wave generation, and a possible swash interaction mechanism for generating subharmonic wave motion on Slapton beach is described.
A steady longshore current prevented these subharmonic edge waves from forming cuspate or crescentic features in the nearshore beach material. Small shoreline cusps observed on Slapton beach on other occasions were consistent with edge waves at the frequency of the incident waves themselves.