A reevaluation of coastal embayment rotation: The dominance of cross-shore versus alongshore sediment transport processes, Collaroy-Narrabeen Beach, southeast Australia

Over 30 years of wave and beach survey data at Collaroy-Narrabeen Beach in southeast Australia are analyzed to investigate the extent to which shoreline variability within this coastal embayment is dominated by shoreline oscillations due to cross-shore sediment exchange or shoreline rotations due to alongshore exchange between the beach's extremities. Offshore wave data are derived from both buoy measurements and ERA-40 reanalysis data. EOF analysis of the monthly shoreline data suggests that the dominant mode of shoreline variability (60% of variability) is an onshore-offshore sediment exchange that is greater at the more exposed northern end of the beach than at the predominantly sheltered southern end. This mode is primarily associated with variability in wave energy/storms, which by the nature of the wave climate of this region chiefly occurs from the south-southeast. The secondary mode of shoreline variability (26% of variability) is a rotational signal between either end of the beach, which coincides with 24% of the offshore wave climate shifting between the south-southeast and east as well as changes in the wave period and wave energy/storms. A distinct annual cycle is identified in this rotation signal. It is concluded that alongshore sediment transport processes associated with embayment rotation is more subtle than previously thought and a refined conceptual model of coastal embayment variability in this region is presented highlighting the dominance of cross-shore sediment exchange processes.