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Federal Register / Vol. 86, No. 167 / Wednesday, September 1, 2021 / Notices
modeling. We used the Navys realistic, site-specific averaged median ambient noise measurement of 129.6 dB RMS re 1 mPa for the Level B harassment threshold in San Diego Bay NAVFAC
SW, 2020. It should be noted that based on the bathymetry and geography of San Diego Bay, sound will not reach the full distance of the Level B harassment isopleths in all directions.
To determine the most appropriate and conservative Level B harassment isopleths, we compared two methods and selected the isopleth between each method that was largest, thus providing the greatest coverage for the Level B
harassment zone. Level B harassment isopleths were considered appropriate based on the distance where the source level reached the 129.6 dB ambient value. The two methods compared the empirical data provided in the Navys Compendium for work at Naval Base Point Loma NAVFAC SW, 2020 with the Practical Spreading Loss model using a transmission loss coefficient of 15, as described above. Results of each method are shown in Table 5 and described below.
For the Compendium method, the average and maximum sound levels in dB re 1 mPa measured at the source 10
m and then at various far-field distances typically showed a monotonic decline in average and maximum sound pressure levels distance increased. The Navy chose to use the average values for two main reasons: 1 Consistency with using the average median L50 ambient values; and 2 average source values were used for the same activities in the Pier 6 project nearby 86 FR 7993, February 3, 2021. However, some level of variability in the recorded sound pressure levels was present where noise levels will drop to ambient levels and then increase to higher levels at greater distances. An example of this will be measurements for the 84-inch caisson removal by a single wire saw. At source 10 m, the average and maximum source levels exceeded the ambient noise levels for both measurements at the source 136.1 and 141.4 dB re 1 mPa;
140.9 and 146.5 dB re 1 mPa, respectively. At far-field distances >20
m, the averages show variability with a gradual decline and then a subsequent increase, i.e., 140.8 dB re 1 mPa at 20 m and 134.8 at 40 m, then 137.1 dB re 1
mPa at 60 m. The distance where sound was measured ends at 283 m from the source with an average level of 130.3 dB
re 1 mPa and a maximum level of 137.0
dB re 1 mPa, both in exceedance of the ambient level. These instances could be attributed to the presence of vessel traffic at distance from the acoustic recorder, causing some interference or
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competing background noise to the pure sound measurements of the wire saw or to random variation from other acoustic effects related to the specific location of the hydrophone. In any event, the distance at which the sound declined below ambient was not always entirely clear and the Navy was unable to develop a consistent criterion to determine the likely distance at which sound decreased below ambient or to account for factors like the topography or hydrophone location. Therefore we describe the analysis of the Navy Compendiums field data for each pile removal method individually below.
For the 13-inch polycarbonate piles with pile clippers the Navy believes that at between 300 and 400 m 984 to 1,312
ft, a majority of the background noise measured is directly related to traffic transiting to/from the Everingham Brothers Bait Company EBBCO bait barges which are to the southwest of the project area. Boat traffic for that specific route ranges from small boats to large recreational/commercial fishing vessels and traffic is nearly constant throughout the day. Because of that, the Navy believes values between those distances will likely be artificially high relative to the transmission loss associated with the project-related activities.
Furthermore, in the turning basin the slope rises up from a max depth of 20.12
m 66 ft to 11.58 m 38 ft between 200
to 400 m 656.17 to 1,312.34 ft. As is evidenced by the Navys acoustical model for south-central San Diego Bay see the Naval Base Point Loma Pier 6
project at https
www.fisheries.noaa.gov/action/
incidental-take-authorization-navalbase-san-diego-pier-6-replacementproject-san-diego, changes in bathymetry i.e., channel walls act as noise attenuators. Therefore, the Navy estimated the Level B harassment isopleth for this source at 350 m, smaller than the Practical Spreading Loss model prediction of 423 m. Given the uncertainty discussed above, we used the 423 m distance for the Level B harassment isopleth.
For the one pile clipper on concrete pile source, the Navy again believes the Compendium data were influenced by boat activity and topography of the channel. In this particular case, Table 39
of the Compendium shows that the average dB level at 215 m was 129.0 dB
RMS. However, the two measurements at 309 m were split, one higher and one lower than the value at 215 m. The Navy decided that Understanding that acoustics is not an exact science, we evaluated the data and chose a distance 250 m that fit the data average noise levels dropped below 129.6 dB at
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between 215 and 309 m. As this 250
m distance exceeded the practical spreading loss model distance of 145 m, we chose the 250 m distance for the Level B harassment isopleth.
For the two pile clipper on concrete pile source the Navy decided that Because the project footprint is parallel to the shoreline, we created a monitoring zone that used a source level of 150 dB, but at two points at the extreme north and south of the project footprint see Fig 63 in the IHA
application because we felt that this would generate a more conservative zone that led to an estimate of the Level B harassment isopleth of 250 m. As this 250 m distance exceeded the practical spreading loss model distance of 229 m, we chose the 250 m distance for the Level B harassment isopleth.
For the underwater chainsaw the Navy noted the transmission loss 27logR was steep when compared to other equipment, but the source value was in line with the pile clippers.
Because of the very steep TL value, we looked at the perceived far-field data points for the clipper activities and chose a distance that was in-between the drop off to ambient for the chainsaw from 26 to 45 m and the clippers 250
m. The Navy estimated the Level B
harassment isopleth for this source at 45
m, smaller than the Practical Spreading Loss model prediction of 229 m. Given the uncertainty discussed above, we used the 229 m distance for the Level B harassment isopleth.
For the diamond wire saw the Navy again believes the Compendium data were influenced by boat activity and topography of the channel. The available data are from caissons which consist of 1.5 inch thick hardened steel shells filled with concrete, and with wooden piles in the center of the concrete. For lack of information on wire saws, the Navy evaluated the likely far-field values for the potential zones based on the 84-inch caissons Table 34
in the Compendium, which had more data at multiple distances. The Navy felt that this was a valid approach based on the similarity of the average noise data at 40 m 132.5 dB for 66-inch caisson, 134.8 for the 84-inch caisson.
Per Table 34, using the average dB
values at distance, the data shows a drop below 129.6 dB RMS at 200 m, but a rise again at 283 m. If you plot the regression curve based on the average 84-inch data, we cross the ambient threshold at approximately 350 m . . .
Because the data at far-field distances was variable, we chose a monitoring zone 350 m that was based on the available real-time data. . . . Our assumption is that, if a wire saw were
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