3D Hydrodynamics and vertical mixing in a stratified estuary

Estuaries are commonly classified by their flow characteristics and the extent of salt and fresh water mixing observed under normal conditions. Highly stratified, "salt-wedge" estuaries are characterised by a well-defined horizontal halocline, with a fresh surface water layer forming above the saline coastal water. Salt-wedge estuaries have large fluvial to tidal flow ratio and typically occur along microtidal coasts where the tidal range is less than 2 m. The mixing of fresh river water and saline coastal water in estuaries is primarily determined by turbulent mixing and to a much lesser extent molecular diffusion (e.g. Masselink and Hughes, 2003). Under low turbulent energy conditions the river and coastal water masses remain segregated. As turbulent mixing increases, such as during a flood event, the estuary may temporarily transition to a "partially" or "well-mixed" condition.

The hydrodynamics and vertical mixing in a stratified estuary has been explored using high-resolution datasets and numerical models. The hydrodynamics and vertical structure in the Yarra River estuary (Melbourne, Australia) was observed using a combination of ADCP (Acoustic Doppler Current Profiler) and EC/T (Electrical Conductivity and Temperature) instruments. The observed features of the estuary and position of the halocline were subsequently simulated using a 3D Non-Linear Shallow Water Equation (NLSWE) solver coupled with turbulent mixing and atmospheric exchange models. The key aspects of the numerical modelling approach required to accurately capture the vertical structure of the Yarra River estuary included:

The inclusion of approximately 200 urban stormwater discharge inputs,

A hybrid z-coordinate with surface sigma-layer model mesh vertical discretization, and

Coupling of the 3D hydrodynamic model with a two-equation vertical turbulence scheme.

The coupling of the hydrodynamics with the vertical turbulence scheme was an essential component of the modelling system. Following this approach, the 1D (vertical) transport equations of momentum, salt and heat are calculated and used by NLSWE solver in the 3D circulation calculations. Efficient integration of the 3D NLSWE was achieved through a mode splitting scheme, whereby different components of the governing equations were updated using an appropriate timestep selected by taking into account physical and numerical convergence and stability considerations.

This model will ultimately form the basis for a 3D hydrodynamic-microorganism model through the coupling with the Aquatic EcoDynamics (AED(2)) modelling library. It is anticipated that this tool will be used by industry partners (Melbourne Water) to make scientifically-informed management decisions for improvement of water quality in the Yarra River estuary.