3D Hydrodynamics and Vertical Mixing in a stratified estuary

• DOI: 10.36334/modsim.2015.f9.jovanovic
• Publication Date: 2019-11-26T17:42:30Z
• Authors: Jovanovic, D.; Barnes, M. P.; Teakle, I. A.L.; Bruce, L. C.; McCarthy, D. T.
• Keywords: Water quality; 550; Yarra River; TUFLOW FV; 551; Salt wedge intrusion

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 ...