from hub partner Geoscience Australia

The amount of energy delivered to the seabed by waves, tides and associated currents is one of the fundamental factors that shape the physical and biological character of the seabed. 

Exposure to this energy varies in intensity and duration both spatially and temporally and may represent an important control on the distribution of different types of seabed communities and of biodiversity. For example, biodiversity is likely to be high where disturbance occurs at a frequency that enables the establishment of transitional benthic communities.

The hub’s Surrogates Program is developing seabed exposure parameters for Jervis Bay, NSW; selected for study as an example of a relatively large sandy embayment. Parameters will be obtained from a hydrodynamic model (SWAN) of the bay, driven by a time-series of open ocean swell waves and calibrated with measurements of currents and waves within the bay (Figures 1 and 2). In this type of shallow setting, fine-resolution bathymetry data is essential for modelling wave refraction and diffraction (Figure 3), which strongly influence the spatial distribution of seabed shear stress.

To date, Geoscience Australia has collected over 3 months of data from the oceanographic moorings in Jervis Bay, which have been deployed at four different positions. The deployments will continue into the summer period to provide valuable calibration data that span the main seasonal wave regimes. Similar modelling is also being undertaken for the Hub’s study area on the Carnarvon Shelf, which is a much more exposed shelf setting.

Figure 1: Sontek PC-ADP acoustic Doppler current profiler and wave gauge being deployed in Jervis Bay. The instrument measures in detail (4 cm bin height) the velocity profile in the first meter above the seabed. This enables accurate estimation of the bed shear stress due to combined waves and currents. (Photo by T. Anderson).

Figure 2: Significant wave height and peak period measured in deep water on the outer shelf (red) and inside the bay (blue). As expected, the wave height inside the bay is considerably smaller, due to frictional attenuation of wave energy and the redistrbution of energy density by refraction and diffraction.

Figure 3: Nearshore modification of wave height and direction predicted by the SWAN model, for offshore waves with a significant height of 4 m, period of 12 s and direction of 160 degrees.