Standing edge waves on a pocket beach

Field measurements of edge waves obtained on a narrow pocket beach are desc ribed. The beach (named Usgo) is located on the north coast of Spain immedi ately to the west of the city of Santander and is exposed to the Atlantic O cean. The edge wave field on this beach is analyzed utilizing data from a l ongshore array of current meters located in about 3 m of water depth. Frequ ency spectra of longshore velocities display several significant peaks, sug gesting that the edge wave field on this pocket beach consists of several d ominant 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-ban ded edge wave field. We isolate four frequency peaks and examine the longsh ore variation of the motions at these frequencies and find convincing evide nce of a nodal structure. Utilizing the measured bathymetry, we compute the cross-shore profiles and theoretical wavelengths of edge waves at the domi nant frequencies using a numerical solution of the shallow water equations by Howd et al. [1992]. We find that edge waves at these frequencies are the oretically confined between the headlands since their amplitudes decay to i nsignificant levels within a cross-shore distance that is less than the off shore extent of the headlands. In addition, assuming perfect sidewall refle ctions, the theoretical nodal locations closely correspond to those inferre d from the measurements. We conclude that discrete standing edge waves resu lting from resonances related to the longshore width of the beach are prese nt on Usgo Beach.