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.

Popular Abstract

The estuarine environment, the coastal zone where saline oceanic waters mix with freshwater rivers, has been heavily degraded due to anthropogenic stressors associated with historic industrial and urban development. Declining water quality, pollution, and over exploitation of natural resources have resulted in a drastically reduced health of these systems. The value of these environments extends from economic benefits of aquaculture and fisheries to social value for recreational users. An increasing understanding of the relationship between estuarine health and their value has shifted the focus to restoring these environments to their pre-anthropogenic state. One highlighted avenue to do this is through implementing oyster reefs. This is due to their ability to filter out pollutants and heavy metals from the water column, reducing sediment erosion from increased storm activity and rising sea levels associated with climate change, all while developing and maintaining habitat for various species on a system wide scale. While restoration projects have already been initiated worldwide, there is still a lack of understanding as to how such reefs will influence the natural transportation of sediments, relied on by various ecosystems in the estuarine environment. This is in part due to the difficulties of data collection in such a dynamic environment, creating difficulties for surveying and monitoring. The use of digital elevation models (DEMs) is key to understanding how and where sediment is being transported. The potential of new data collection techniques with aerial drones has the potential to overcome such difficulties while drastically improving data quality. Therefore it is the aims of this project to (1) study the influence of oyster reefs on the local sediment transportation around oyster reefs, and (2) incorporate and evaluate the application of drone technology to this study. To achieve both aims, an oyster reef located in the estuary of Port Hacking, Australia, was selected to collect topographic data using an aerial drone, and analyse results with wave, current, and sediment data to build an understanding of how different forcing’s influence how sediment moves and accumulates around an oyster reef. The results from this research identified a reduced amount of energy from waves and tidal currents in the leeward direction, creating a large region of sediment accumulation behind the reef that extends many times the size of the reef. Wave interaction with the reef illustrated similar characteristics 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 wave 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 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. The significant scale of influence of the reef, identified from the drone DEM, illustrates the possibility for reefs to interfere with the natural sediment transport within these dynamic estuarine systems. Therefore, future research should consider a broader scale perspective to understand potential impacts on the wider estuarine system. The high resolution and low resources required for drone data collection presents as a promising and feasible avenue to continue this, enabling larger scale and higher frequency surveys to monitor changes over time. Continuing such research is key to improving our understanding of the role oyster reefs 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.

• author: Johansson, Oskar Evert
• supervisor: Ana Vila-Concejo; Abdulghani Hasan
• organization: Dept of Physical Geography and Ecosystem Science
• alternative title: Understanding sediment transportation around oyster reefs, and the incorporation of aerial drone technology to better understand this
• course: GISM01 20211
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Geography, Geographical Information Systems, GIS, Physical Geography, Oyster Reef, Drone, UAV, Hydrodynamics
• publication/series: Master Thesis in Geographical Information Science
• report number: 131
• language: English
• additional info: External supervisor: Associate Professor Ana Vila Concejo, The University of Sydney
• id: 9043205
• date added to LUP: 2021-04-20 11:14:39
• date last changed: 2021-04-20 11:14:39