Hydrodynamics and Flushing of Coffin Bay, South Australia: A Small Tidal Inverse Estuary of Interconnected Bays

Using a three-dimensional hydrodynamic model and the concept of water age, this study explores the hydrodynamics and flushing characteristics of Coffin Bay, South Australia, which is a small inverse estuary of interconnected bays. Model findings suggest that the estuary is mainly tidally flushed throughout the year. Despite the existence of strong tidal flows >1 m/s in passages between bays of the estuary, the resultant tidal stirring zones are largely disconnected from each other. This disconnection induces a relatively slow flushing of the estuary's inner bays, where maximum water ages are 80-100 days. Given the tidal dominance in the flushing dynamics, a simple diffusion equation based on a total effective transverse diffusivity D (estimated from the model predictions) can be used to describe the flushing behaviour of the estuary. This equation is applied to derive the thermal response of Coffin Bay to either temperature variations in ambient shelf water or changes in surface heat fluxes. Findings indicate that, on timescales of 30 days, the inner bays of Coffin Bay respond predominantly to changes in surface heat fluxes, whereas the outer bay is more responsive to temperature variations in ambient shelf water. We quantify these individual responses in terms of spatially variable "thermal response factors," which are fundamental properties inherent in the estuary's flushing dynamics.