Standing edge waves on a pocket beach

Abstract

Field measurements of edge waves obtained on a narrow pocket beach are described. The beach (named Usgo) is located on the north coast of Spain immediately to the west of the city of Santander and is exposed to the Atlantic Ocean. The edge wave field on this beach is analyzed utilizing data from a longshore array of current meters located in about 3 m of water depth. Frequency spectra of longshore velocities display several significant peaks, suggesting that the edge wave field on this pocket beach consists of several dominant modes. This is in contrast to observations on open coastal beaches [e.g., Oltman‐Shay and Guza, 1987], which show the existence of a broad‐banded edge wave field. We isolate four frequency peaks and examine the longshore variation of the motions at these frequencies and find convincing evidence of a nodal structure. Utilizing the measured bathymetry, we compute the cross‐shore profiles and theoretical wavelengths of edge waves at the dominant frequencies using a numerical solution of the shallow water equations by Howd et al. [1992]. We find that edge waves at these frequencies are theoretically confined between the headlands since their amplitudes decay to insignificant levels within a cross‐shore distance that is less than the offshore extent of the headlands. In addition, assuming perfect sidewall reflections, the theoretical nodal locations closely correspond to those inferred from the measurements. We conclude that discrete standing edge waves resulting from resonances related to the longshore width of the beach are present on Usgo Beach.