Abstract
This deliverable contains the methodology and results from Task 3.2 of SafeWAVE project (Acoustic Modelling), which essentially is about the modelling of underwater sound propagation with the results obtained in Task 2.3 Acoustic Monitoring. For the project, 4 WEC systems in 4 different test sites were studied. The 4 locations were: Aguçadoura (Portugal), BiMEP (Spain), Mutriku (Spain) and SEM-REV (France).
For Aguçadoura, the highest acoustic disturbance distance obtained was 2.5 km for the 125 Hz component for the [2, 2.5) wave height bin, and the highest SL obtained was 174 dB, surpassing both 120 dB and 160 dB impact thresholds specified by the U.S. National Marine Fisheries Service for continuous and impulsive sounds respectively (NMFS, 2024). For the acoustic propagation maps, the SPL was calculated for the commissioning phase and it was seen that for all frequencies (62.5 Hz, 125Hz and 1000 Hz) it dropped below 120 dB within the first 10 meters. No operational data was obtained, so it does not fully represent the WEC's real state.
For BiMEP, the PENGUIN II WEC had a maximum Source Level (SL) of 146 dB re 1 m for the 62,5 Hz component for the [4, 8) wave height bin, which also surpass the 120 dB threshold for continuous sound. Nevertheless, when studying the propagation maps, the SPL dropped below 120 dB threshold (NMFS, 2024) within the first 10 meters for any of the frequencies (62.5 Hz, 125Hz and 1000 Hz) analyzed. When considering 2 devices (the ones needed for 1200kw), the affected area was estimated as follows: 3 km for 63 Hz and 125 Hz and 6 km for 1000 Hz.
For Mutriku, in Task 2.3 there was a hydrophone failure, and only two hydrophones gathered data. This case is the most different one out of the four WECs, as Mutriku is an onshore power plant, and can’t be categorized as a source that has “On” and “Off” states. In this case, the highest SL obtained was around 190 dB in the 62,5 Hz and 125 Hz frequencies for the [2, 2.5) wave height bin, surpassing also the 120 dB dB threshold (NMFS, 2024). However, it is suspected that the models introduced some artifacts in the calculation of the TL, as the SPL obtained in Task 2.3 showed coherent values, and the losses were unusually high. Bearing this in mind, for low frequencies (62.5 Hz, 125 Hz) it was found that 120db threshold dropped at 50 meters from the source, while for high frequencies (1000Hz) it did at 70 meters from the source.
Finally, for SEM-REV, also one hydrophone failed when obtaining the acoustic data in Task 2.3. This, combined with the lack of operational data, did not allow for a good characterization of the WEC signature depending of the WEC states. For the propagation maps, all frequencies (62.5 Hz, 125Hz and 1000 Hz), the SPL dropped below 120 dB threshold (NMFS, 2024) within the first 10 meters. On the other hand, when it is considered 8 devices (to reach 1200 kW), the affected area was estimated as follows: 30 meters for 63 Hz, 6 km for 125 Hz (with an average of 2 km), and 7.4 km for 1000 Hz.
Considering the impact threshold of 120 dB at SL (NMFS, 2024), it seems that all the studied WECs systems could negatively impact the cetaceans in all the studied areas at SL. Most of them significantly reduce these values within the first 10 meters, resulting in a relatively short affected area when compared to the entire acoustic map. However, to fully understand the potential ecological impact of the SLP obtained in this study and fully implement the impact assessment methodologies recommended by experts, such as TG-NOISE methodology, additional efforts are needed. It is essential to define the affected areas, identify the species present, and establish the LOBE for these species regarding the specificities of the production project (number of devices, operational regimes, etc). As offshore energy is expected to increase in the coming years, long-term studies are needed to fully understand the environmental impacts these new energy systems are having on the marine ecosystem.