Circulation pattern in an Australian estuary

This project aims to quantify from three-dimensional modelling the impact of climate variability and extreme weather events upon the coastal circulation of Hervey Bay.

The bay is a large embayment situated of the central eastern coast of Australia. It is home to several endangered species including Dugongs, Humpback Whales and Leatherback turtles and it is being utilised extensively for aquaculture and impacted upon by fisheries, tourism and pollution from agricultural runoff and coastal industries. Due to high evaporation and low river runoff many Australian estuaries and large coastal embayments are characterised by inverse flows with salinity increasing toward the coast and river mouths.

Compared to the traditional concept of an estuary, the circulation within an inverse estuary is reversed with outward flow of high-salinity below an incoming flow of low-salinity water. This circulation pattern is driven by high evaporation and low river runoff. The inverse estuarine circulation transports locally produced Hervey Bay Water away from the shallow coastal source regions and discharges this water into the open ocean. Since Australia’s climate is one of the worlds driest and locally characterised by high year-to-year rainfall variability, extreme weather events and climatic trends are likely to significantly impact upon the coastal circulation.

We seek to investigate this impact of climate variability and extreme weather events upon the coastal circulation of Hervey Bay through the further development and application of a general ocean circulation model. The development of the model will be supported through field observations. Lagrangian and Eulerian modelling techniques and diagnostics will be implemented into the model to estimate flushing times and regional residence times. Sensitivity studies including the effect of waves and projected anthropogenic sea-level rise will illuminate the basic physical mechanisms and responses of the circulation and sedimentation processes in the bay. Of particular interest are (a) the production and flow of saline water away from its coastal source region toward the open ocean as a function of the evaporation and precipitation balance, and (b) extreme weather events such severe storms and cyclones that lead to dramatic discharges of freshwater.

Current drying trends observed in coastal and southeast Queensland are expected to impact upon the balance between evaporation and river-runoff. This in turn impacts upon the production of saline Hervey Bay Water and leads to changes in the estuarine circulation. Changes due to climatic variability on a larger scale have already been documented for the circulation of the eastern Australian coast and are most likely linked to marine ecosystem changes. In addition to seeking answers to the above questions, it is proposed to investigate how current regional climatic trends will alter the physics of the Bay in the future.