Hydrodynamic Modelling of Coral Reefs: Ningaloo Reef-Western Australia

As with all coral reef systems, the ecology of Ningaloo Reef is closely linked to water circulation which transport and disperse key material such as nutrients and larvae. Circulation on coral reefs may be driven by a number of forcing mechanisms including waves, tides, wind, and buoyancy effects. Surface waves interacting with reefs have long been known to dominate the currents on many coral reefs. This forcing is provided by wave breaking on the forereef that causes a local increase in the mean sea level (the “setup”) that is responsible for driving the cross-reef flow. For this project, we are developing a coupled wave-circulation numerical model of Ningaloo Marine Park-located in northeast of Australia, using an extensive field data set collected from April-May 2006 in an ~5 km region around Sandy Bay, to validate its performance. The analysis of field data collected on the forereef, reef flat and in the channel revealed a strong correlation between the incident wave height and currents inside the reef lagoon and channel. A nearshore numerical wave model (SWAN) which simulates wave transformation due to the effects of shoaling, refraction, diffraction, and dissipation by both bottom friction and wave breaking was chosen to simulate waves across the system. The model uses a finely-resolved computational grid (~20 m resolution on the reef) and incorporates high resolution bathymetric data provided from hyperspectral imagery. The model is forced with offshore wave conditions measured during the 2006 field campaign and model output is compared with an array of wave gauges deployed along cross-reef transects from the forereef slope to the lagoon. Following successful validation, results from SWAN, particularly the 2D radiation stress gradients, are used by the circulation model to include the wave-driven circulation.