Shoreline variability at a low-energy beach: Contributions of storms, megacusps and sea-breeze cycles

The temporal and spatial variations in shoreline position were analysed at a low-energy beach in southwestern Australia fronted by an offshore reef to examine the relative contributions of storm events, megacusps and diurnal sea-breeze cycles to the beach morphologic response. The shoreline position did not exhibit a seasonal sinusoidal erosion/accretion pattern as often observed at open coast sandy beaches, but instead a dynamic morphology dominated by storm and recovery cycles that generated fluctuations of ~10 m in the shoreline position. During low swell recovery periods, the beach rapidly developed megacusps (wavelength ~100 m, amplitude ~5 m), which despite generating local erosive changes within the megacusps embayments, the beach still tended to accrete during these low swell conditions. In most cases, the embayments coincided with locations of rip currents suggesting that rip currents might contribute to the formation of the megacusps. Additionally, high-frequency measurements collected over a series of diurnal sea-breeze cycles typical of the site during summer months demonstrated that the erosive impact due to sea breezes was much smaller than that observed during storms, despite wave heights often being similar during these events. While the beach eroded during intensification of the diurnal sea-breeze, the beach mostly exhibited net daily accretion due to the low-energy swell that dominated the wave spectrum during the overnight and morning hours. The formation and alongshore migration of megacusps during sea-breeze cycles also contributed to beach accretion, overwhelming the erosion due to the consecutive sea-breezes events after a few days.