Download files
Abstract
Embayed beaches differ from open coast beaches due to the control exerted by headlands, leading to alongshore gradients in wave height, surfzone width and beach state. Circulation and morphodynamic behaviour of embayed beaches are poorly understood. This study involves observations at three embayed beaches around Sydney, Australia. Methods involve Lagrangian observations of currents and bathymetric surveys.
At Whale Beach (600 m long), Lagrangian observations of an entire embayment, including headland rip currents, are presented. Headland geometry and wave direction were found to influence rip current trajectories, and degree of bathymetric non-uniformity correlated with current velocities. Surfzone exit rates increased along the beach from the protected headland to the exposed end.
At Bondi Beach (850 m long), observations of morphodynamic response to storm-groups over a three week period are presented. Offshore significant wave heights reached 5.6 m, with a 10-day average >3 m. A mega-rip developed at the exposed headland, while the protected end maintained a low-tide-terrace. Alongshore non-uniformity was found to decrease near the shoreline, and increase in the outer-surfzone and nearshore.
At Shelly Beach (2.3 km long), the rip current hazard was investigated, testing the efficacy of 'float' and 'swim parallel' strategies. Human subjects entered three rip channels with Lagrangian measurements of background flow. Swimming parallel was found to be a shorter duration strategy, with lower failure rate. However both strategies failed in some instances and neither can be recommended as a sole preference.
Finally, all data are analysed to investigate the degree of bathymetric control on very low frequency (~10 min) current variability due to surfzone eddies. A classification scheme is introduced ranging from: (i) strong bathymetric control where currents exhibit high mean velocities, with velocity pulsation or rip cell oscillations, to (ii) no bathymetric control where randomly directed surfzone eddies occur on planar bars.
This study broadens understanding of several aspects of embayed beach dynamics. In particular, it demonstrates that alongshore wave-height and beach state gradients on embayed beaches will force alongshore changes in: (i) current velocities, trajectories and variability; (ii) cross-shore exchange; (iii) morphodynamic response to storms; and (iv) the level of hazard to bathers.