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the turbine tower into the foundation and radiate into the surrounding water column and seabed (Tougaard et al., 2009). The resulting sound is described as continuous and nonimpulsive and is characterized by one or more tonal components that are typically at frequencies below 1 kHz (see The frequency content of the tonal signals is determined by the mechanical properties of the wind turbine and does not change with wind speed (Madsen et al., 2006).
Underwater measurements taken during the operation of one of the turbines at the BIWF contained sound that is hypothesized to be caused by aerodynamic noise from the turbine blade tips that was propagated through the air, into the water, and received on a hydrophone on the seabed at a range of 50 meters (Figure 3, bottom; J. Miller, Personal observation). This sound was measured to be around 71 Hz and was lower in level than fin whale vocal- izations recorded at the same time. This sound was only detectable during times when the weather was calm and there were no ships traveling in the area.
Sounds from Decommissioning
Since the first offshore wind farm decommissioning in 2015, a small number of offshore farms have been decom- missioned, but the decommissioning process is generally unexplored. As more wind farms reach the end of their design life, the decision will have to be made relating to extending operations, repowering, or decommissioning. Decommissioning is typically thought of as a complete removal of all components above and below the water surface, but there is research supporting a partial removal where some of the substructure would remain in place as an artificial reef for marine life (Topham et al., 2019). In general, sound would be generated as a by-product of the process used to remove the substructures, which could include cutting the foundation piles via explosives or water jet cutting (Nedwell and Howell, 2004).
Assessing Impact to Marine Life
Impulsive sounds, like those generated during impact pile driving, exhibit physical characteristics at the source that make them potentially more injurious to marine life compared with nonimpulsive sounds, like those generated during vibratory pile driving and wind tur- bine operation (Popper et al., 2014; Southall et al., 2019). Sound exposure is currently assessed based on the sound pressure received in the water column, but the resulting
particle motion in the water and sediment is also impor- tant when considering the potential impact to marine life sensitive to this stimulus. Additionally, the context under which an animal is exposed to a sound, in addition to the received sound level, will affect the probability of a behavioral response (Ellison et al., 2012).
Protective Measures to Mitigate
Sound Levels
Various mitigation methods can be employed during each phase of wind farm development to reduce the overall propagated sound levels and potential effect on marine life. Time-of-year limitations on construction are implemented to provide safeguards for specific pro- tected or susceptible species. Antinoise legislation in the Netherlands prohibits pile driving from July 1 through December 31 to avoid disturbance of the breeding season of the harbor porpoise (Tsouvalas and Metrikine, 2016). Off the US East Coast, an agreement was made between environmental groups and a wind farm developer to pro- vide protections for the North Atlantic right whale by not allowing pile driving between January 1 and April 30 when right whales are most likely to be present in the project area (Conservation Law Foundation, 2019).
The use of noise mitigation systems such as bubble cur- tains (see or physical barriers around the pile are commonly used to reduce the levels of sound generated during impact pile driving (Bellmann et al., 2017). These methods are a type of barrier system that work to attenuate the radiated sound levels by exploiting an impedance mismatch between the generated sound wave and a gas-filled barrier. Factors such as the water depth, current, and foundation type will influence the effectiveness of each system.
Ramp-up operational mitigation measures, in which the hammer intensity is gradually increased to full power, are also employed. This method aims to allow time for ani- mals to leave the immediate area and avoid exposure to harmful sound levels, although there are no data to sup- port the contention that this works for fishes, invertebrates, or turtles. Another mitigation method involves visually monitoring an exclusion zone around the piling activity for the presence of marine mammals. This zone is predefined based on the expected sound levels in the area and requires pausing piling activities if an animal is observed to reduce near-field noise exposure (Bailey et al., 2014).
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