SURF ZONE RESONANCE AND COUPLED MORPHOLOGY

Abstract

The edge wave hypothesis for periodic inshore morphology and circulation is tested for five beaches and is supported by resulting wave-current spectral and cross-spectral data. Beach types range from a reflective, narrow surf zone, case through various dissipative medium to high energy beaches including some with inshore bar-trough morphology and one broad surf zone troughless one. In all cases beachface reflectivity is moderately high (E < 2.5) and inshore resonance occurs, indicated by strong spectral peaks at lower than incident frequency with wave-current co-peaks being 90°out of phase. Several different edgewave frequency and mode combinations are indicated. The reflective beach shows an n = o subharmonic edgewave (i.e. at half incident wave frequency) which Guza and Davis (1974) predict as the most likely case, viz. the (o,o) triad. The troughless dissipative case shows a (1,0) edgewave triad; the same occurs in some bar-trough dissipative cases but in other cases is supplanted by the (o,o) sub-harmonic wave and/or by a lower subharmonic wave at h, incident frequency. The likelihood of a given edge .waveset appears to be regulated by surf friction, and a change of edge wave set appears likely to explain observed changes of inshore circulation.