Physical Processes of a Shallow Subtropical Estuarine System : Coombabah Lake, Gold Coast, Australia

Abstract

Estuaries are of immense importance to many communities. It has been estimated that 60 to 80 % of commercial marine fishery resources depend on estuaries for part, or all of, their life cycle. The characteristics of estuarine flow, water quality and sediment conditions are important as they play a critical role in the functionality and health of these systems. This study utilised both field data and numerical modelling technique to help enhance our understanding of the physical processes of a very shallow subtropical estuarine system. This study first quantified various salt flux components within the shallow subtropical estuarine system Coombabah Lake in Gold Coast, Queensland, Australia to better understand the system’s physical processes for assisting future management decisions in this ecologically and economically significant region. Residual water transport was identified as the dominant factor influencing residual salt transport, which alternates direction frequently. This study then developed a new simple and robust traversing system to measure flow properties within estuarine bottom boundary layers to estimate two important parameters used in numerical modelling of aquatic systems: bed shear stress and bed roughness height. Four commonly-employed techniques: (1) Log-Profile; (2) Reynolds stress; (3) Turbulent Kinetic Energy; and (4) Inertial Dissipation used to estimate bed shear stresses from velocity measurements were also compared. Bed shear stresses estimated with these four methods agreed reasonably well; of these, the Log Profile method was found to be most useful and reliable for the unstratified conditions studied. A three-dimensional hydrodynamic model with unstructured meshes utilising the MIKE3 FM modelling system and simulated the hydrodynamic regime was set up for Coombabah Lake to assist with enhancing our understanding of the hydraulic properties within this shallow sub-tropical estuarine system. The sensitivity of calibration parameters of a very shallow estuarine model was also investigated. The model utilised the hydraulic data collected by the newly developed traversing system and that collected during the first part of the study used to examine salt flux dynamics. The hydrodynamic regime of the lake was found to be favourable for settlement of suspended sediments. The results also revealed that the correct bathymetry is the most important parameter for accurate modelling, followed by appropriate bed roughness in the numerical scheme for very shallow environments. This study finally provided an understanding of the sediment dynamics within Coombabah Lake and the surrounding waters. It utilised ten days of observed hydrodynamic and sediment data and employed the three-dimensional model with unstructured meshes utilising the MIKE3 FM modelling system. Sediment dynamics of the lake were found to be dominated by advection process driven by tides with wave and wind playing minor roles – even though the system was shallow. Simulation results agreed well with field data and supported the aforementioned findings. Correlation between TSS and turbidity was very poor; therefore, the employed automatic data logging system (turbidity meters) was determined inappropriate for the estimation of TSS concentration in the very shallow subtropical estuarine system.