Mixing, hypersalinity and gradients in Hervey Bay, Australia

Hervey Bay, a large coastal embayment situated off the central eastern coast of Australia, is a shallow tidal area (average depth = 15 m), close to the continental shelf. It shows features of an inverse estuary, due to the high evaporation rate (approx. 2 m/year), low precipitation (less than 1 m/year) and on average almost no freshwater input from rivers that drain into the bay. The hydro- and thermodynamical structure of Hervey Bay and their variability are presented here for the first time, using a combination of four-dimensional modelling and observations from field studies. The numerical studies are performed with the Coupled Hydrodynamical Ecological model for RegioNal Shelf seas (COHERENS). Due to the high tidal range (> 3.5 m) the bay is considered as a vertically well-mixed system and therefore only horizontal fronts a likely. Recent field measurements, but also the numerical simulations indicate characteristic features of an inverse/hypersaline estuary with low salinities (35.5 psu) in the open ocean and peak values (> 39.0 psu) in the head water of the bay. The model further predicts a nearly persistent mean salinity gradient of 0.5 psu across the bay (with higher salinities close to the shore)

Capturing the residence time boundary layer - Application to the Scheldt Estuary.

At high Peclet number, the residence time exhibits a boundary layer adjacent to incoming open boundaries. In a Eulerian model, not resolving this boundary layer can generate spurious oscillations that can propagate into the area of interest. However, resolving this boundary layer would require an unacceptably high spatial resolution. Therefore, alternative methods are needed in which no grid refinement is required to capture the key aspects of the physics of the residence time boundary layer. An extended finite element method representation and a boundary layer parameterisation are presented and tested herein. It is also explained how to preserve local consistency in reversed time simulations so as to avoid the generation of spurious residence time extrema. Finally, the boundary layer parameterisation is applied to the computation of the residence time in the Scheldt Estuary (Belgium/The Netherlands). This timescale is simulated by means of a depth-integrated, finite element, unstructured mesh model, with a high space-time resolution. It is seen that the residence time temporal variations are mainly affected by the semi-diurnal tides. However, the spring-neap variability also impacts the residence time, particularly in the sandbank and shallow areas. Seasonal variability is also observed, which is induced by the fluctuations over the year of the upstream flows. In general, the residence time is an increasing function of the distance to the mouth of the estuary. However, smaller-scale fluctuations are also present: they are caused by local bathymetric features and their impact on the hydrodynamics