Locally and Externally-Driven Dynamics of a Large Semi-Enclosed Bay in Southern Australia

Abstract

Port Phillip Bay in southern Australia is linked to a large ocean strait through a narrow entrance. The circulation within the Bay is dependent on the frequency and magnitude of oscillations coming from the ocean as well as the local forcing. Field investigations and numerical model investigations were established to analyse (1) current and sea level variance, (2) the relationship between the Bay hydrodynamics and those of the adjacent ocean and (3) the link between hydrodynamics and dispersal, particularly horizontal and vertical shear dispersion, in the semi-enclosed water body. A large field program was undertaken and two- and three-dimensional versions of nested numerical hydrodynamic models of the Bay and adjacent Strait were developed. Approximately 98% of the sea level variance in the nearby Strait and 96% within the Bay was found to be predictable using empirical and numerical techniques to describe the tides, coastal-trapped waves, barometric pressure and local wind-forcing. Over a complete year, coastal-trapped waves arriving from the west accounted for 3% of the total variance. Non-isotropic behaviour at low frequency was found to be related to non-linear interaction of currents through the entrance and over the flood-tidal delta. Vertical shear dispersion dominated the dispersal and effective horizontal eddy diffusivities of 325 m² sec^-1 were noted. The Port Phillip Bay numerical model simulated M2 sea level amplitudes with an average error of 1.2% and M2 phases with an error of 3.8%, both of which were well within the scatter in the field data. The predicted and measured tidal currents had a correlation coefficient of 0.98. There was a 13% average deviation between predictions and measurements of low frequency wind-driven currents within inner Port Phillip Bay but a wind factor of 1.07 applied to the wind speeds eliminated this deviation.