Numerical study of the hydrodynamics of a very shallow estuarine system - Coombabah Lake, Gold Coast, Australia

Coastal wetlands and estuaries are important environments providing significant habitats for flora and fauna species - often supporting commercial and recreational fisheries. These systems also act as filters for contaminants and sediments, and the absorption of wave energy. As a consequence of the ecological significance and the potential for anthropogenic disturbances and inputs into Coombabah Lake estuary (Australia), the lake and surrounding wetlands have been the focus of recent scientific study efforts. This estuarine lake (∼2 km² in size) is a very shallow (mean depth < 1 m) estuarine system that experiences a tidal range of 1.2 m, thus resulting in the continual exposure of large mud flats at low tide. Variations in water column physio-chemical and biological parameters and nutrient concentrations of the benthic sediments have previously been attributed to the hydrodynamic regime, hydrologic events, and sediment sources. In this study, a three-dimensional (3D) hydrodynamic model with unstructured mesh is setup to simulate the hydrodynamic regime and Bottom Boundary Layer (BBL) properties. In particular, the sensitivity of calibration parameters for a very shallow estuarine model is investigated. Model results are verified by recent intensive measurements. The hydrodynamic regime of the lake was found to be favorable for settlement of suspended sediments. The results reveal the necessity to correctly measure and use the appropriate bathymetry and bed roughness conditions in the numerical scheme for very shallow environments.