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Federal Register / Vol. 86, No. 23 / Friday, February 5, 2021 / Notices
account for an elastic seabed. MONM
RAM incorporates bathymetry, underwater sound speed as a function of depth, and a geoacoustic profile based on seafloor composition, and accounts for source horizontal directivity. The PE
method has been extensively benchmarked and is widely employed in the underwater acoustics community, and MONMRAMs predictions have been validated against experimental data in several underwater acoustic measurement programs conducted by JASCO. At frequencies greater than 2
kHz, MONM accounts for increased sound attenuation due to volume absorption at higher frequencies with the widely used BELLHOP Gaussian beam ray-trace propagation model. This component incorporates bathymetry and underwater sound speed as a function of depth with a simplified representation of the sea bottom, as subbottom layers have a negligible influence on the propagation of acoustic waves with frequencies above 1 kHz. MONM
BELLHOP accounts for horizontal directivity of the source and vertical variation of the source beam pattern.
Both propagation models account for full exposure from a direct acoustic wave, as well as exposure from acoustic wave reflections and refractions i.e., multi-path arrivals at the receiver.
The sound field radiating from the pile was simulated using a vertical array of point sources. Because sound itself is an oscillation vibration of water particles, acoustic modeling of sound in the water column is inherently an evaluation of vibration. For this study, synthetic pressure waveforms were computed using the full-wave rangedependent acoustic model FWRAM, which is JASCOs acoustic propagation model capable of producing timedomain waveforms.
Models are more efficient at estimating SEL than SPLrms. Therefore, conversions may be necessary to derive the corresponding SPLrms. Propagation was modeled for a subset of sites using the FWRAM, from which broadband SEL to SPL conversion factors were calculated. The FWRAM required intensive calculation for each site, thus a representative subset of modeling sites were used to develop azimuth-, range-, and depth-dependent conversion factors. These conversion factors were used to calculate the broadband SPLrms from the broadband SEL prediction.
Two locations within the SFWF were selected to provide representative propagation and sound fields for the project area see Figure 1 in SFWF COP, Appendix J1. The two locations were selected to span the region from shallow to deeper water and varying distances to
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dominant bathymetric features i.e., slope and shelf break. Water depth and environmental characteristics e.g., bottom-type are similar throughout the SFWF, and therefore minimal differences were found in sound propagation results for the two sites Denes et al., 2018. The model also incorporated two different sound velocity profiles related to in situ measurements of temperature, salinity, and pressure within the water column to account for variations in the acoustic propagation conditions between summer and winter. Estimated pile driving schedules Table 6 were used to calculate the SEL sound fields at different points in time during pile driving.
The sound propagation modeling incorporated site-specific environmental data that describes the bathymetry, sound speed in the water column, and seabed geoacoustics in the construction area. Sound level estimates are calculated from three-dimensional sound fields and then at each horizontal sampling range, the maximum received level that occurs within the water column is used as the received level at that range. These maximum-over-depth Rmax values are then compared to predetermined threshold levels to determine acoustic ranges to Level A
harassment and Level B harassment zone isopleths. However, the ranges to a threshold typically differ among radii from a source, and might not be continuous because sound levels may drop below threshold at some ranges and then exceed threshold at farther ranges. To minimize the influence of these inconsistencies, 5 percent of the farthest such footprints were excluded from the model data. The resulting range, R95percent, is used because, regardless of the shape of the maximumover-depth footprint, the predicted range encompasses at least 95 percent of the horizontal area that would be exposed to sound at or above the specified threshold. The difference between Rmax and R95percent depends on the source directivity and the heterogeneity of the acoustic environment. R95percent excludes ends of protruding areas or small isolated acoustic foci not representative of the nominal ensonified zone see Figure 12;
SFWF COP Appendix J1.
The modeled source spectrum is provided in Figure 7 of the SFWF COP
Appendix J1. The dominant energy for both pile driving scenarios maximum and standard is below 100 Hz. Please see Appendix J1 of the SFWF COP for further details on the modeling methodology Denes et al., 2020a.

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South Fork Wind will employ a noise mitigation system during all impact pile driving of monopiles. Noise mitigation systems, such as bubble curtains, are sometimes used to decrease the sound levels radiated from a source. Bubbles create a local impedance change that acts as a barrier to sound transmission.
The size of the bubbles determines their effective frequency band, with larger bubbles needed for lower frequencies.
There are a variety of bubble curtain systems, confined or unconfined bubbles, and some with encapsulated bubbles or panels. Attenuation levels also vary by type of system, frequency band, and location. Small bubble curtains have been measured to reduce sound levels but effective attenuation is highly dependent on depth of water, current, and configuration and operation of the curtain Austin, Denes, MacDonnell, & Warner, 2016;
Koschinski & Ludemann, 2013. Bubble curtains vary in terms of the sizes of the bubbles and those with larger bubbles tend to perform a bit better and more reliably, particularly when deployed with two separate rings Bellmann, 2014; Koschinski & Ludemann, 2013;
Nehls, Rose, Diederichs, Bellmann, &
Pehlke, 2016.
Encapsulated bubble systems e.g., Hydro Sound Dampers HSDs, can be effective within their targeted frequency ranges, e.g., 100800 Hz, and when used in conjunction with a bubble curtain appear to create the greatest attenuation.
The literature presents a wide array of observed attenuation results for bubble curtains. The variability in attenuation levels is the result of variation in design, as well as differences in site conditions and difficulty in properly installing and operating in-water attenuation devices.
A California Department of Transportation CalTrans study tested several systems and found that the best attenuation systems resulted in 1015
dB of attenuation Buehler et al., 2015.
Similarly, Dahne et al. 2017 found that single bubble curtains that reduced sound levels by 7 to 10 dB reduced the overall sound level by 12 dB when combined as a double bubble curtain for 6 m steel monopiles in the North Sea.
Bellmann et al. 2020 provide a review of the efficacy of using bubble curtains both single and double as noise abatement systems in the German EEZ
of the North and Baltic Seas. For 8 m diameter monopiles, single bubble curtains achieved an average of 11 dB
broadband noise reduction Bellmann et al., 2020. In modeling the sound fields for South Fork Winds proposed activities, hypothetical broadband attenuation levels of 0 dB, 6 dB, 10 dB,
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