WAVE GROUP FORCED NEARSHORE CIRCULATION

Forcing of the nearshore circulation system by incident wave groups is examined. Attention is focused on the generation of migrating rip currents and development of low frequency motion. Analysis of experimental results categorises the migrating rip currents as very low frequency motion in the FIG energy band. It is suggested that migrating rip currents, in addition to shear waves, are a valid generation mechanism for very low frequency motion in the nearshore region. Introduction The primary currents in the nearshore area are longshore currents, crossshore currents (or undertow), and rip currents. Longshore currents, generated by incident waves with an oblique approach to the shoreline, flow parallel to the beach carrying sediments downwave. Cross-shore currents develop to maintain the mass flux balance in the surf zone. They can transport sediments offshore, although they can not carry sediments far beyond the breaker line. Rip currents are more powerful surface currents and are a major mechanism for water exchange across the breaker line. They can transport large amounts of sediment a considerable distance beyond the surf zone. Numerous theories exist for the presence of rip currents as discussed by Dalrymple (1978). All these features of the nearshore circulation system are generally considered stationary processes. Underlying the nearshore currents is a low frequency motion which is typically unsteady and periodic in nature. This motion includes surf beat, edge waves, and far-infragravity motions. Much attention has been given to the characterization of these wave motions, but as yet, their generation mechanisms are not totally understood. The fundamental aim of the research work described in this paper was to study the nearshore circulation system and evaluate its response to forcing by offshore wave groups. In particular, we examined the generation of slowly migrating rip currents by the offshore wave groups, and the corresponding development of low frequency motions in the surf zone. Graduate Research Assistant, Center for Applied Coastal Research, University of Delaware, Newark, Delaware 19716 USA Professor, Center for Applied Coastal Research, University of Delaware, Newark, Delaware 19716 USA 729 730 COASTAL ENGINEERING -1990 Wave Groups and Low Frequency Motion in the Nearshore Region Initial discoveries of low frequency motion (frequencies < incident wave frequency, periods 30-300s) in the nearshore region are attributed to Munk (1949) and Tucker (1950). They observed oscillations in the water surface elevation which had periods of several minutes. Munk named these long, low frequency waves 'surf beat'. Tucker found the best evidence of surf beat during periods of long, regular groups of incident waves, and noted that the long waves were out of phase with the wave groups. Longuet-Higgins and Stewart (1962, 1964) demonstrated theoretically, using radiation stresses, that a small second-order undulation accompanies a first-order wave group, leading to the development of a set-down wave, a long, forced wave in the nearshore region. It was shown that the forced wave has the same wave length and period as the incident wave group and is 180° out of phase with the group, as observed by Tucker. They suggested that the surf beats seen by Munk and Tucker were seaward propagating free waves formed by the release of the forced, set-down waves as they are reflected near the coastline. Following these initial surf beat studies, the majority of research on the interaction of wave groups and the nearshore circulation system has focused on edge waves. Gallagher (1971) showed that non-linear interactions between wind waves, leading to an inter-frequency energy transfer, can initiate forced edge waves in the surf zone which can grow resonantly. In a laboratory study, Bowen and Guza (1978) considered both resonant and non-resonant interactions between incoming wave trains and confirmed that, when the frequency and longshore wave number of incident wave groups satisfy the edge wave dispersion relationship, theoretically predicted edge wave motions can be generated and maintained. Huntley, Guza and Thornton (1981) summarised that there are three different generation mechanisms for nearshore low frequency motion: progressive edge waves, standing edge waves (caused by an obstruction or topographic form), and forced second-order waves under incident wave groups. From their analyses of data from the National Sediment Transport Study field experiment at Torrey Pines Beach, California, they found the most important of these generation mechanisms to be progressive edge waves. They observed long-wave motion in the surf zone with frequencies and wave lengths that corresponded to those predicted by the edge wave dispersion relationship. However, they also found that other generation mechanisms were important, primarily forced motion due to standing edge waves. Tang and Dalrymple (1989) also worked with the NSTS data set, but concentrated on nearshore motions at much lower frequencies than those in the edge wave band. Figure 1 shows the edge wave energy identified by Huntley, Guza and Thornton, but there is also a significant pocket of energy in the lowest frequency band. Tang and Dalrymple demonstrated, through various statistical analysis methods, that offshore wave groups were significantly correlated with the measured, very low frequency motion (< 0.0015 Hz, periods > 660 sec). During the field experiment, observations were made of slowly migrating rip current cells in the surf zone which had the same wave length as the very low frequency motion. Tang and Dalrymple hypothesised that the incident wave groups forced a response in the surf zone in the form of migrating rip currents, which, in turn, had a very low frequency motion signature in wave number-frequency space. FORCED NEARSHORE CIRCULATION 731 LONGSHORE CURRENTS NOV. 20 . CONTOUR LEVELSi 2, 3, 5, E3 >8, m >10