INFRAGRAVITY WAVES AND NEARSHORE BARS IN PROTECTED, STORM-DOMINATED COASTAL ENVIRONMENTS

Field measurements of run-up oscillations were conducted on fetch-restricted, storm-wave dominated beaches in Denmark. Experiments took place under conditions with breaker heights of 1.5-3 m and wave periods of 5-7 s. Statistically significant spectral peaks were identified in the infragravity regime. The frequencies of these peaks were compared to frequencies predicted by the edge wave cut-off model, originally proposed by Huntley (J. Geophys. Res., 81, 1976), and it was found that the waves were probably cut-off mode edge waves with the mode number corresponding to the number of nearshore bars at the locality in question. The waves were usually standing alongshore, but during an experiment in which a very high energy level prevailed, the edge waves were probably progressive. The selection of only one or two cut-off modes may be the result of an interrelationship between the topographic cut-off and offshore wave groups. The nearshore region was surveyed following storms. It was found that there was generally a good correlation between bar positions and rhythmic dimensions and the theoretical cross-shore and longshore structure of the recorded edge waves. It is concluded that infragravity waves may be important in the formation, migration and morphology of nearshore bars, with the number of bars depending on the edge-wave mode number. © 1990.