Investigation of sediment transport processes near tidal stream devices in Australia

Interest for renewable energy has increased recently with the motivation to create a sustainable energy future for the next generation. Of the potential technologies, tidal stream turbines are one of the most advanced marine renewable energy technologies, but still face challenges due to the difficulty to acquire data and uncertainties about their influence. The influence of tidal turbine installation on the seabed is one key object of concern for tidal energy developers as part of the environmental impact assessment of these systems. In most promising areas such as shallow straits with sandy bed nearby, tidal motion influences the bedform and sand waves are often present. As a result, studies of not only the hydrodynamics but also the local sediment dynamics are important elements for environment influence and site assessments. This study aims to provide a better understanding of sediment transport processes at high energy sites, including the influence of anthropogenic activity such as tidal energy extraction on these sites. This work also aimed to increase our understanding of sediment data collection requirements for the calibration of numerical models. Hydrodynamic and sediment transport models were developed using MIKE3 and MIKE21 software packages to investigate sediment processes and the influence of Tidal Energy Converters (TECs). The models were first applied to idealised tidal channels to understand their capabilities and the behaviour of different sediment transport formulae and evaluate the difference between two-dimensional and three-dimensional models. The models were then developed for a potential tidal energy site in Australia, Banks Strait (Tasmania) in an area that exhibits the formation of large sand waves at the seabed. For this promising site, two models were used: firstly, calibration and validation of the required high-resolution hydrodynamic model was performed followed by the investigation of the hydrodynamic changes induced by different arrays of TECs. In a second step, sediment transport was modelled under the action of tides and waves with and without tidal arrays. Special attention was given to quantifying model sensitivity from sediment parameters under various hydrodynamic conditions to enhance our knowledge of the mechanisms driving sediment dynamics in such highly energetic sites. Emphasis was also placed on the model calibration methods using a varied set of in-situ data collected during the 2018/2019 Australia Tidal Energy (AUSTEn) campaign. Assessment of 2D and 3D models in both a theoretical channel and in a real-world application at the Banks Strait site revealed that 2D models were sufficient to give an accurate general overview of the impact of tidal farm deployment on hydrodynamic conditions. Validation of the high-resolution 2D hydrodynamic model in Banks Strait showed good agreement (R=0.92 for currents in 2D) with the large dataset of in situ data. The large-scale influence of tidal farms was first assessed with a tidal forcing and it was found that only the farm with 300 turbines displayed non-negligible influence on the hydrodynamics with changes in current speed and bed shear stress reaching 7km from the tidal arrays. For the dynamics of sediment without energy extraction, the numerical analysis showed a constant shift of the sand waves profile in the east direction under the action of tides and waves, consistent with field observations. This migration is strongly linked with the tidal asymmetry, with a residual current towards the east. Results for the tidal energy extraction cases showed that tidal farms of more than 24.5MW (rated power) start altering the circulation of residual currents leading to changes in the sediment dynamics. Findings from this research provide novel insights: a 2D approach is sufficient for regional assessment, a better understanding of sediment dynamics at high energy sites and of the ocean response to large-scale tidal arrays. This will assist tidal project developers in selecting the best methods to perform environmental assessments related to tidal turbines.