Field observations of long-period, surf-zone standing waves in relation to contrasting beach morphologies

Abstract

Field observations of water-surface oscillation and horizontal flow velocity spectra were carried out in the surf zones of contrasting beach-inshore morphodynamic states. The observations were made under conditions of long-period (7.5-11 s), moderate-energy (height 1 .2-2.3 m) swell on different beaches encompassing the full range of states from the steep. reflective extreme to the flat, dissipative extreme and including intermediate states involving different scales of rhythmic surf-zone topography and rip circulations. Experiments on all types of topography reveal standing surf-zone oscillations at periods longer than incident wave period. At least some of these appear to be edge waves. The actual frequencies are dependent on the morphodynamic state of the surf zone and beach. The standing waves with shortest period are zero-mode subharmonic edge waves which consistently occur on highly reflective beaches where they cause cusps spaced at one-half the edge-wave length. The standing waves with lowest frequency were observed on the flattest and most dissipative beach and comprised a wide band of infragravity frequencies. These oscillations could cause the multiple parallel bars. Intermediate frequencies occur in the presence of the pronounced bar-trough and rhythmic topographies of intermediate morphodynamic states.