Spencer Gulf versus Coffin Bay: A comparison between inverse estuaries of vastly different sizes

This presentation describes and compares the dynamics and flushing behaviors of two inverse estuaries in South Australia of vastly different sizes - Spencer Gulf and Coffin Bay. Being subject to similar climatic conditions,... [ view full abstract ]

This presentation describes and compares the dynamics and flushing behaviors of two inverse estuaries in South Australia of vastly different sizes - Spencer Gulf and Coffin Bay. Being subject to similar climatic conditions, Spencer Gulf is ~300 km long, up to 150 km wide and has depths >20 m, whereas Coffin Bay is much smaller (25 km long, up to 10 km wide, water depths ~2.5 m). Despite this vast difference in size, both estuaries develop similar hypersaline conditions in their upper reaches. A hydrodynamic model is applied to both systems to explore their individual dynamical characteristics.

Based on the findings, Spencer Gulf can be dynamically separated into two different flow regimes. Lower Spencer Gulf is flushed via a density-driven exchange flow during austral winter months on a seasonal basis, while the upper reaches of Spencer Gulf are flushed only via quasi-diffusive exchange with waters from the Lower Spencer Gulf. Tidal currents are irrelevant in this flushing. As a consequence, upper Spencer Gulf becomes hypersaline year-round with salinities exceeding 40 g/l and the relative water age of Upper Spencer Gulf exceeds 1 year.

In contrast, Coffin Bay is diffusively flushed under the action of tides. Given the specific geometry of Coffin Bay, which consists of a series of sub-bays, the inner bays become hypersaline with salinities >45 g/l in late summer. Winter rainfall tends to remove these salinity anomalies in winter months. Nevertheless, the relative water age of the inner bays of Coffin Bay exceeds 100 days year-round, which is symptomatic of slow flushing.

Both the Upper Spencer Gulf and the inner bays of Coffin Bay are slowly flushed regions, for different dynamic reasons, that are most vulnerable to pollution. Special management efforts are required to protect such regions.