Applications of hydrodynamic modelling in Western Port Bay

Western Port Bay (WPB) is a large, shallow and complex embayment in southern Victoria that opens into Bass Straight via narrow passages either side of Phillip Island. The hydrodynamic behaviour of WPB is a complex amalgam of... [ view full abstract ]

Western Port Bay (WPB) is a large, shallow and complex embayment in southern Victoria that opens into Bass Straight via narrow passages either side of Phillip Island. The hydrodynamic behaviour of WPB is a complex amalgam of meteorological, oceanographic and catchment processes operating at a range of scales from meters to kilometres. The objective has been to develop a hydrodynamic and water quality modelling platform that has the capacity to simulate the effects of these processes at sufficient resolution to allow a better understanding of the fate and transport of allochthonous and autochthonous agents throughout the bay. Initially the model was applied to assess two very different processes. The first assessed the fate and transport of dissolved chemical agents supplied to the bay via the northern creeks that are fed by fringing agricultural catchments. Model tracers were used to demonstrate a 3-log reduction in concentrations of chemicals (using conservative assumptions about their in-situ chemical reactivity) in the northern tidal flats where flushing is slowed by a 'squeeze-box' effect generated by the dual tidal passages around Phillip Island. In the well-flushed southern passages there was a far greater decrease in concentration. In a second assessment, Lagrangian particle dynamics were used to predict the fate and transport of mangrove seeds. The results were used to identify the littoral regions that received a significant supply of seeds and provided favourable conditions for seed establishment. Currently, a third and more comprehensive study is underway to simulate nutrient and sediment dynamics and the impact these have on the health of seagrass. Through a flexible java-based software platform the study will integrate model and data streams from system to seagrass-bed scales. At the system scale, the model design will accommodate ocean boundary conditions that are derived from regional oceanographic models and observations. At the small scale, algorithms that describe the physiology of seagrass growth will be developed and tested in the context of the dynamic bay environment provided by the system-scale model. The platform will enable a seamless process for land-use, climate change and management scenario modelling that links oceanographic, estuarine and ecosystems scales.