Abstract
Chickerell BioAcoustics was contracted by the European Marine Energy Centre (EMEC) to carry out work to further characterise the ambient noise field at their Fall of Warness tidal energy test site in Orkney, UK. The project was funded by Scottish Natural Heritage (SNH) and follows on from previous acoustic characterisations of the test site carried out by the Scottish Association for Marine Sciences ( SAMS), Oban, UK (funded by Highlands & Islands Enterprise).
The work carried out by SAMS went some way towards characterising the baseline noise present at the site in the absence of any tidal energy converter (TEC) devices. This work quantified the ambient sound levels at the site in both ebb and flood conditions. Investigating the origins of the sounds was outwith the scope of the SAMS studies and this has been addressed in the current study.
The scope of the current SNH-funded project was to incorporate a review and identify any requirements for additional analysis of existing acoustic surveys at the site and to extend the acoustic dataset to provide a more comprehensive characterisation of the test site. This encompassed investigation of noise sources, including noise contributed by the presence of TECs. While the characterisation of noise from specific devices operating at the site was outwith the scope of this project, such characterisation could be an area of consideration for future work.
The current project called for three surveys to be carried out using drifting acoustic recorders. Initial surveys were carried out using the existing EMEC drifting hydrophone equipment but later surveys used an upgraded system, the Drifting Acoustic Recorder and Tracker (DART) system developed by Chickerell BioAcoustics and EMEC.
This report presents the results of analysing the data collected to date and shows that flow noise is a significant contributor to the ambient noise field within the test site. Other significant contributions to the noise field include shipping noise and noise from TEC devices operating in the test site. The report concludes that the noise from the TECs operating on site during the survey period is unlikely to significantly impact marine mammals using the area.
Main findings
Data collected and analysed during this project have enabled the ambient noise at the EMEC tidal test site to be characterised in the presence of some operating tidal energy converter devices. The following conclusions can be drawn from this study:
- The ambient noise levels measured are higher than those suggested by Urick (1975) for shallow water sites, but lower than those measured by Wilson & Carter (2008).
- The noise level of frequencies above 1 kHz can vary by at least 26 dB across the site; this is believed to be primarily due to flow-induced noise generation.
- Rain can raise ambient noise by up to 30 dB and this extends to lower frequencies than flow noise.
- It was not possible to locate the individual noise sources accurately as this was outwith the scope of this project, but future studies could investigate this further.
- Further work will be required to fully understand the effect of tidal flow on ambient noise based on experience gained from processing this dataset.
- Based on the data gathered during this study, it seems unlikely that the noise generated by tidal energy converters operating within the site would have a significant impact on marine mammals. However, further detailed studies will be required in order to gain more understanding of this.