The Response of the Wave-driven Circulation at Ningaloo Reef, Western Australia, to a Rise in Mean Sea Level

Ningaloo Reef extends some 280 km along the western coast of Australia, and consists of a barrier reef ~1-6 km offshore with occasional gaps, backed by a shallow lagoon. The reef morphology thus has some similarities to submerged breakwaters, which largely attenuate wave energy by wave breaking over the reef flat. This wave breaking generates radiation stress gradients that produce setup and wave-driven mean currents across Ningaloo Reef. A section of reef at Sandy Bay was chosen as the focus of an intense 6week field experiment and numerical modelling and wave modelling of the region was conducted using SWAN coupled to the 3D circulation model ROMS. The circulation model was forced by radiation stresses provided by SWAN, and was configured with a 50 m grid resolution that incorporated high-resolution hyperspectral bathymetry. The response of the reef currents to potential forcing mechanisms (wind stress, wave height and tidal level) was investigated with the field data and the modelling, which revealed the dominant role that wave breaking plays in driving the mean circulation and flushing of this reef system. The physics of nearshore processes such as wave breaking, wave setup and mean flow across the reef were investigated in detail by examining the various momentum balances established in the system. The magnitude of the terms in this balance were sensitive to changes in mean sea level, e.g. the wave forces decreased as the water depth increased (and hence wave breaking dissipation was reduced). This led to changes in the intensity of the circulation of the reef-lagoon system, thus highlighting the sensitivity of waves and currents over reefs in response to a potential rise in mean sea level.