The hydrodynamic impacts of Estuarine Oyster reefs, and the application of drone technology to this study

Abstract: The impacts of climate change are being seen within the estuarine environment through erosion and shoreline retreat, associated with sea-level rise and changes in storm activity. Restoration oyster reefs have been identified as a working-with-nature strategy to protect and restore these shoreline environments, stabilising sediment, and reducing erosion. Despite restoration projects already being initiated worldwide, their local hydrodynamic impacts are still poorly understood. This is in part due to the dynamic environment that reefs exist in, creating difficulties for surveying and monitoring. The use of digital elevation models (DEMs) is crucial for observing and analysing the hydrodynamics and morphological responses to these large three-dimensional living structures, and the dynamic marine environment has proven problematic for past surveying techniques. Advances in drone technology has the potential to overcome the difficulties of modelling in dynamic marine environments, creating higher resolution and more accurate models. The use of such technologies, paired with hydrodynamic observations, may enable a better understanding of how oyster reefs are influencing the environments in which they exist. It is therefore the aims of this thesis to (1) study the influence of estuarine oyster reefs on the local hydrodynamics and sediment morphology, and (2) incorporate and evaluate the application of drone technology to this study. To achieve these goals, an oyster reef located in the microtidal estuary of Port Hacking, Australia, was selected to collect wave, current, sediment, and topographic data of the reef and surrounding sediment substrate. The results from this research identified the presence of hydrodynamic protection in the leeward direction from both tidal currents and local wind waves, developing a significant sediment accretion area within this protection zone, extending many times the size of the reef. Wave attenuation characteristics by the reef were found to be similar to man-made engineering structures, such as sea walls/breakwaters, where waves were able to pass over the reef freely when the reef is completely submerged, but blocked from passing when the reef is exposed. Evidence of higher energy wave dissipation was also present, likely from artificial sources such as local boat and ferry traffic. These results build on past research that has identified an increased amount of sediment on the leeward side of the reef, by illustrating the spatial patterns and extent of influence oyster reefs have on the local morphology and surrounding sediment substrate. Such findings emphasise the use of oyster reefs as a natural engineering structure, mimicking the functionality of man-made structures while providing additional ecosystem services by creating, developing, and maintaining habitat for other species on an estuary wide scale. However, the significant scale of hydrodynamic and morphological influence from reefs highlights their possibility to interfere with the natural sediment transport within these dynamic estuarine systems, a process which is relied upon by various ecosystems. Therefore, a broader scale perspective must be taken to understand restorative and hydrodynamic impacts on the wider estuarine system. The high resolution and low resources required for drone data collection presents as a highly promising and feasible avenue to continue these studies, enabling higher frequency surveys to monitor changes over time. Continuing research into oyster reef restoration is key to improving our understanding of the role oyster reefs can play in restoring the health of already degraded estuarine systems, with the potential to benefit the vast communities that both rely on and enjoy their natural resources.