Morphodynamics of reflective and dissipative beach and inshore systems: Southeastern Australia

Abstract

Field experiments involving direct measurement of surf and inshore current spectra, inshore circulation patterns, and depositional morphology have been replicated under different energy conditions and in several environmentally contrasting beach localities on the high-energy coast of New South Wales, Australia. The region exhibits compartmentalized beach systems and is dominated by a highly variable wind-wave climate superimposed on persistent high-energy swell ( T = 10–14 sec). Two general types of beach system occur: (1) predominantly reflective systems in which much of the incidentwave energy is reflected from the beach face; and (2) dissipative systems with wide surf zones and high turbulent energy dissipation. Reflective systems are characterized by steep, linear beach faces, well-developed berms and beach cusps, and surging breakers with high runup and minimum setup; rip cells and associated three-dimensional inshore topography are absent. Wave height and current spectra from reflective beaches consistently have their dominant peaks at incident wave and subharmonic frequencies, and cross-spectra indicate the existence of low-mode edge waves at those frequencies. Infra-gravity peaks are negligible. Under low-energy conditions subharmonic peaks are low relative to incident-wave peaks; however, increasing breaker height tends to be accompanied by increasing subharmonic dominance. Analyses of shore-normal currents near the bed show that under all conditions the strongest shoreward motions are induced by the incident waves; however, seaward motion near the beach face is subharmonic-dominated. Dissipative systems characterize the exposed open coast and are fronted by concaveupward nearshore (seaward of break) profiles and wide flat surf zones. Waves break 75–300 m seaward of the beach and dissipate much of their energy before reaching the beach, creating significant radiation-stress gradients and setup. Topography is much more complex and varied than in the case of reflective beaches; one or more bars, three-dimensional inshore topography and different scales of rip cells are normally present. The commonly occurring time-and-environment-dependent morphologic states can be classified into six general types. The greatest total dissipation is associated with Type 1 which prevails in the regions of most abundant inshore sediments or during and immediately after severe storms. Setup is highest and runup is lowest (relative to incident-wave height) with this type and the dominant energy near the beach is in the surf-beat part (80–120 sec) of the spectrum. As the bar migrates shoreward (Types 2 and 3) and beach face steepens and a deep trough develops within which the partially dissipated waves reform. Although the outer surf zone remains dissipative, synchronous and subharmonic resonance occurs near the beach face and, as with reflective beaches, low-mode edge waves form beach cusps. A conspicuous feature of Types 2 and 3 is the occurrence of pronounced resonant spectral peaks at 4 T ( ⋍ 40–50 sec ) within the trough and on the bar. Edge waves at this frequency may be responsible for the development of the crescentic bar forms (Type 3). Lower frequency surf beat peaks also remain present but are secondary. The peak at 4 T attenuates as Type 4 develops and is not present with Type 5 topography but the first subharmonic (2 T) becomes more pronounced, particularly with high tide. Type 6 morphology represents the fully accreted beach state and occurs only after prolonged periods of low swell. This type is a reflective beach with a steep linear beach face and a very high berm which remains continuous for long distances alongshore; rips are always absent. Wave and current spectra are also similar to those described for reflective beaches.