EDGE WAVES AND THE LITTORAL ENVIRONMENT

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Several types of beach features seem to have a rather regular, longshore pattern. This pattern may indeed he sufficiently uniform to be described in terms of a recognisable longshore wavelength. A likely explanation for such features lies in the motion of edge waves, surface waves trapped by refraction to the shoreline. These waves, by themselves or by interaction with the normal, incoming surface waves breaking on the beach, can generate longshore features having a wavelength equal to or half the edge wave wavelength. If a broad spectrum of edge wave modes were present any longshore variation should appear rather irregular. The existence of regular features therefore suggests that a particular edge wave mode is often dominant, the characteristics of the dominant mode depending on the geometry of the nearshore area and the width of the surf zone. Any new, artificial structure stretching seawards provides new boundary conditions, almost certainly altering the characteristic of the edge wave spectra. This is particularly obvious in the case of a regularly spaced structure such as a s'et of groynes. A deeper understanding of the edge wave processes is needed so that the induced changes in the edge wave spectra are the least deleterious or, an intriguing possibility, advantageous.