Low-frequency changes of sediment volume on the beachface at Warilla Beach, New South Wales, 1975–1985

Abstract

Characteristic patterns of shore-parallel and shore-normal movements of sediment on a sandy beach are described in this study. The sediment movements are apparent as low-frequency variations in the volume of sediment stored above mean low water spring tide (MLWST) level on Warilla Beach, N.S.W. They were determined by an empirical orthogonal-function (EOF) analysis of beach profile records. The profiles were surveyed from eighteen stations along a 2 km long beach at least monthly, generally fortnightly, for over 10 years, from July 1975 until August 1985. Results from the EOF analysis confirm and substantially extend an earlier study of Warilla Beach when the first 5 years of the beach survey record was examined ∗ ∗ Eliot and Clarke (1982a) and Clarke and Eliot (1982, 1983a). . The importance of low-frequency beachface change identified in the earlier, shorter study was confirmed from the extended data set; zones of maximum variability, linked with rip-current activity, are consistent in both studies, and similar patterns of alongshore sediment movement in the swash zone were determined from both analyses. The principal differences between the two sets of results highlight beachface responses to processes operating in different time scales. Amplitude spectra from time series describing the characteristic modes of sediment movement along and across the beachface have periodicities of 2.0, 1.7 and 1.0 years. Beach changes with periods close to 3 years are apparent in the record, but cannot be resolved by standard spectral analysis techniques. Each period is associated with a characteristic pattern of beach change; the triennial and 1.7 year cycles with alongshore sediment transport, and the biennial cycle with onshore-offshore sediment exchange. Eigenvectors from the EOF analysis identify the characteristic patterns of beach change from survey data specified to highlight the shore-parallel sediment movement along 0.5 m deep, horizontal segments of beach, and shore-normal sediment movement along the beach profiles. In the analysis of shore-parallel variation in beach profile configuration, the fundamental eigenvectors explain between 70 and 80% of beachface variance in each instance, and different patterns of sediment movement were identified for each horizontal slice. On the upper part of the beachface, all eigenvectors identified a zone of low variability along the shoreline skirting a rockwall. Below berm crest, in the swash and intertidal zones of the beach, and away from the rockwall, the northern half of the beach was highly variable. Second mode eigenvectors (cellular-flux modes) explain a further 5 to 14% of the variance and describe sediment exchanges between discrete zones in each horizontal segment of the beachface. Zones of greatest variability occurred landward of places where rip currents frequently develop. Conversely, zones of low variability are coincident with areas of beachface in the lee of places where inshore bars frequently form. Analysis of sediment movement along each profile is consistent with the beach changes described. The fundamental eigenvectors account for 91 to 99% of the variance on each profile. They describe an onshore-offshore movement of sediment through the intertidal zone of the beach.