MORPHODYNAMIC IMPACT OF SEA-BREEZE ACTIVITY ON A BEACH WITH BEACH CUSP MORPHOLOGY

Beach morphology and nearshore hydrodynamics were monitored over a num ber of sea breeze cycles on a beach with pronounced beach cusp morphol ogy in southwestern Australia. The action of the sea breeze resulted i n consistent changes to the incident wave field and beach cusp morphol ogy, and induced a diurnal cycle of beach change. The morphological ch anges were accomplished without an apparent sediment gain or loss, but involved a redistribution of sediment within the cusp morphological s ystem. During the sea breeze, the addition of locally-generated, short -period mind waves to the background swell resulted in an increase in wave height, a decrease in wave period and an intensification of the n earshore currents. The cusp morphology became increasingly subdued due to accretion in the embayment and, to a lesser extent, erosion of the horns. After the cessation of the sea breeze, the wind-wave energy le vel gradually decreased and the associated wind-wave period increased. Accretion on the cusp horns was accompanied by minor erosion of the e mbayment and resulted in an accentuation of the cusp morphology. The b uild-up of the cusp morphology was a consequence of the morpho dynamic feedback between the antecedent cusp morphology and the wave runup ch aracteristics. The wave runup was diverted from the horn into the emba yment, resulting in decreased backwash volumes and hence build-up of t he horn. In the cusp embayment, increased backwash volumes resulted in scouring and the suppression of potential swash events at the base of the beachface. Consequently, the proportion of infragravity-wave ener gy in the runup record was larger in the embayment than on the horn. T en hours after the sea breeze had stopped blowing, significant amounts of wind-wave energy were still present. It is suggested that these wi nd waves were generated a distance of almost 200 km south of the study area, implying that the presence of the local sea breeze may have reg ional implications for coastal processes.