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  Figure 5. Seasonal variability of underwater sound propagation in the BIWF area showing transmission loss (TL) predictions in decibels for a 200-Hz sound source in September 2015 (summer; a) and December 2015 (winter; b). The source depth (Zs) in the model was 15 meters and the receiver depth (Zr) was 20 meters. The corresponding sound speed profiles (SSP) are shown. The TL was higher in the summer compared with the winter conditions. Reproduced from Lin et al., 2019, with permission.
Exploiting seasonal differences in the water tempera- ture and salinity and its effect on underwater sound propagation could also be used to mitigate the impact of pile-driving noise by scheduling wind farm construction during seasons of high expected acoustic transmission loss. For example, the pile driving for the BIWF occurred during the summer season but had the construction occurred during the winter season, the received SELs at ranges greater than 6 kilometers could have been up to 8 dB higher (Figure 5) due to lower water tempera- tures causing larger acoustic impedance contrast at the seafloor (water-bottom interface) and a more isovelocity, or constant, sound speed profile (Lin et al., 2019). This difference in received sound levels is significant and high- lights the effect the environmental conditions have on the overall sound propagation.
Conclusion
Ancillary sounds of varying levels and characteristics are generated during each phase in the development of an off- shore wind farm. The highest amplitude sound is expected during the impact pile-driving part of the construction phase and potentially during the decommissioning phase depending on the methods employed to remove the wind turbine foundations. The installation methods used for each turbine foundation type will result in different levels and types of sounds radiated into the marine environ- ment. The sound levels can be reduced using physical barriers, and the sound exposure of marine life can be mitigated through monitoring methods and time-of-year
restrictions on sound-generating activities. The potential for acute sound exposure of marine mammals and fishes is currently assessed based on the generated sound pres- sure levels in the water column, but other factors such as the particle motion in the water and sediment and the behavioral response of marine life are important factors to evaluate. Although the construction and decommissioning phases take on the order of months to complete, offshore wind farms are designed to operate for minimum of 20–25 years. With the continued development of offshore wind farms worldwide there will be additional opportunities to measure the underwater sound generated during all phases and assess any potential long-term effect of this sound on the marine environment.
References
Amaral, J. L., Miller, J. H., Potty, G. R., Vigness-Raposa, K. J., Fran- kel, A. S., et al. (2020). Characterization of impact pile driving signals during installation of offshore wind turbine foundations. The Journal of the Acoustical Society of America, 147(4), 2323-2333. https://doi.org/10.1121/10.0001035.
Bailey, H., Brookes, K. L., and Thompson, P. M. (2014). Assessing environmental impacts of offshore wind farms: Lessons learned and recommendations for the future. Aquatic Biosystems 10, 1-13. https://doi.org/10.1186/2046-9063-10-8.
Bellmann, M. A., Schuckenbrock, J., Gündert, S., Michael, M., Holst, H., and Remmers, P. (2017). Is there a state-of-the-art to reduce pile-driving noise? In J. Köppel (Ed.), Wind Energy and Wildlife Interactions, Springer, Cham, Switzerland, pp. 161-172. https://doi.org/10.1007/978-3-319-51272-3_9.
Conservation Law Foundation. (2019). Protective Measures for North Atlantic Right Whales. Available at https://tinyurl.com/tj8awyb. Accessed February 27, 2020.
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