title: Strategies and outcomes for biophysical monitoring activities at tidal energy candidate sites in Australia
Creators: name: Scherelis, CC
Publisher: name: University of Tasmania
Year: 2021

description: Tidal energy is an emerging form of ocean renewable energy that utilises large, freestanding turbines in underwater or floating systems to generate electricity from tidally driven ocean currents. As the tidal energy industry moves towards commercial-scale array installations with multiple devices, interaction uncertainties between tidal energy devices and the marine environment have the potential to encumber tidal energy developments. Environmental impact assessment (EIA) studies are needed that inform about interaction concerns between tidal energy devices and the marine environment, meet regulatory monitoring requirements, and are feasible for tidal energy developers to implement and maintain. Identifying best practices and strategies for environmental monitoring procedures at tidal energy candidate sites, however, is contingent on studies that explore different environmental monitoring approaches and describe the benefits and limitations of the methodologies applied. This thesis presents findings from biophysical monitoring studies conducted in combination with tidal resource assessment campaigns performed by the Tidal Energy in Australia (AUSTEn) project. Two tidal energy candidate sites were investigated by the AUSTEn project: the Banks Strait, located in north-east Tasmania, and the Clarence Strait, located near Darwin, Northern Territory. Four discrete research studies were conducted involving mobile hydroacoustic surveys and long-term mooring deployments that housed a four-frequency biological echosounder, a five-beam Acoustic Doppler Current Profiler (ADCP), or both. The sampling procedure provided concurrent measurements of fish, hydrodynamic properties, and environmental conditions at the tidal energy candidate sites. Sampling locations were chosen based on depth, current speed, proximity to shore, and substrate suitable for commercial-scale tidal turbine installations. Tidal resources were characterised using the Nortek Signature500 ADCP. Given the prevalent usage of the Signature500 for tidal resource assessment campaigns and the advertised potential for its centre beam to also act as a biological echosounder, the instrument's biological monitoring capabilities for fish abundance and distribution were examined by conducting measurements concurrent to a multi-frequency biological echosounder over one month at the Banks Strait tidal energy candidate site. Post-processing techniques for hydroacoustics data collected in dynamic ocean environments were newly developed and optimised in combination with environmental parameters to obtain information about fish abundance and distribution metrics at the study sites. This involved a dynamic noise classification and removal approach that addressed varying noise signatures due to changing physical conditions (i.e. wind and current speed), a target-tracking and classification algorithm that evaluated and compared a target's actual and expected time-in-beam at different current speeds, as well as other established hydroacoustic processing methods (e.g. surface noise removal, dB-differencing, thresholding, etc.). Physical variables (i.e. current speed, turbulence, sheer stress) and environmental conditions (i.e. diel stage, temperature, local tidal conditions) were investigated for their relation to fish abundance, distribution, and behaviour in these highly dynamic ocean environments. Diel stage had the greatest influence on fish with a higher abundance, greater dispersion, and an elevated centre of mass occurring at night. At higher current speeds, fish increased in abundance, showed greater dispersion, and were located closer to the seafloor at the Banks Strait study site, while a preference for the upper water column was observed at the Clarence Strait in these conditions. A response to higher temperatures was also evident at the Banks Strait study site and showed increased fish densities and abundances that were sampled closer to the seafloor. Regions with increased turbulence parameters were generally avoided by fish at the Clarence Strait study site, and no significant changes were observed based on ow direction. Mobile surveys were only conducted at the Banks Strait study site and revealed a preference for fish to occupy areas with depths between 20 - 40 m. A preference for bottom-type was not observed for the bottom-types sampled (i.e. rock, gravel, sand). The Signature500 was found to generate improved fish abundance and distribution estimates over traditional ADCPs. With fish abundance estimates being correlated to those of a multi-frequency biological echosounder at r = 0:71, the Signature500 was shown to have value as a complementary biological monitoring tool during tidal resource assessments. However, given the remaining discrepancies and limitations regarding the sampling frequency, calibration process, and transducer beam width, it is recommended that more targeted EIA studies be conducted with dedicated biological echosounders for conclusive insights about biophysical interactions at tidal energy sites. Research outcomes are projected to provide insights about fish-turbine interaction potentials at the Banks Strait and Clarence Strait tidal energy candidate sites, help formulate and optimise monitoring practices for fish-current interactions in dynamic ocean environments, and aid in the development of a standardised EIA framework for tidal energy candidate sites. Regulatory bodies and industry developers can then adopt this framework to gain a more complete understanding about biophysical interactions at tidal energy sites, which allows for potential environmental impacts to be identified and mitigated early in the development process. In addition, it creates a baseline for environmental conditions at tidal energy candidate sites that provides context for EIA studies performed post-turbine installation.