Semi-diurnal variation in beachface aggradation and degradation

Abstract

Sequences of morphologic change on the beachface at South Beach, Wollongong, N.S.W., were monitored for several semi-diurnal spring-tide cycles. The observations were made to establish temporal and spatial variation in the beachface response to tide, swash and groundwater interactions. It was anticipated that the sequences would differ under high and low wave energy conditions and on different parts of the same beach, where the foreshore morphology varied markedly. To test these propositions, two-dimensional beachface responses to tide, swash and groundwater level variation have been compared for two beachface profiles, one transecting the horn of a giant cusp and the other the embayment of the same cusp. The beach survey results were specified in two ways: (1) by examining the changes in sand-surface elevation at each survey point along the profiles; and (2) by transforming the survey data with a moving-axis system whose origin followed the intersection of the sea-level with the beachface over the tide cycle. In the first instance, the sequences of beach change along each profile were similar to those described from elsewhere ∗ ∗ Duncan (1964) and Strahler (1966). , although the profiles were undergoing different secular displacements. During the course of the survey, the salient was eroded and the embayment infilled. The net changes were determined by the dynamic balance of erosion versus deposition occurring with the flood and ebb of each tide cycle. Application of the moving-axis technique identified switching between erosional and depositional states in the swash zone on the salient. The swash zone changes were controlled by the tide and groundwater fluctuations. Beachface degradation began with the tide rise and terminated shortly after mid-ebb tide. Maximum degradation occurred when the beachface was most saturated, approximately 1 to 2 h after high tide. In both high and low energy conditions, the sequence of cut and fill was similar to that described in earlier work ∗∗ ∗∗ Duncan (1964). . Tidal migration of the sea-level and water-table effluent-line across the beachface established the location of the zones of erosion and deposition. Above the effluent-line, an unsaturated beach condition enhanced deposition. Below the line, the beachface was saturated and erosional conditions prevailed. The dynamic balance, given as the amount of erosion versus deposition over each tide cycle, was associated with the different levels of wave energy and sediment supply.