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Federal Register / Vol. 86, No. 21 / Wednesday, February 3, 2021 / Notices
7997

TABLE 3PROJECT SOUND SOURCE LEVELS
Pile driving activity
Estimated sound source level at 10 meters without attenuation
Method
Pile type
Vibratory Extraction

12-inch timber/plastic
20 and 24-inch concrete 16-inch steel
20-inch concrete
12 to 24-inch concrete ..
12-inch timber/plastic
20-inch concrete
20 and 24-inch concrete 16-inch fiberglass

Water Jetting
Underwater Chainsaw
Small Pile Clipper
Large Pile Clipper
Impact Hammer

dB RMS
152
160
160
158
150
154
161
176
153

dB SEL

dB peak



166
144

188
177

Data source and proxy
Greenbusch Group 2018.
Caltrans 2015, Table I.22, 24-inch steel sheet.
Caltrans 2015, Table I.22, 24-inch steel sheet.
NAVFAC SW 2020, 24 30-inch concrete.
NAVFAC SW 2020, 16-inch concrete.
NAVFAC SW 2020, 13-inch polycarbonate.
NAVFAC SW 2020, 24-inch concrete.
Caltrans 2015, Table I.21, 24-inch concrete.
Caltrans 2015, 13-inch plastic.

Note: SEL = single strike sound exposure level; dB peak = peak sound level; rms = root mean square.
Source level was 147 dB at 17m from source, back calculated to 150dB using transmission loss coefficient of 15.
Average of the peak values was 166 and that value was used in modelling in DellOsto and Dahl 2019 rather than the absolute peak we recommend for use in the user spreadsheet, SEL calculated from assumed strike rate in DellOsto and Dahl 2019.

During pile driving installation activities, there may be times when two pile extraction methods pile clippers, water jetting, underwater chainsaws or vibratory pile removal are used simultaneously. The likelihood of such an occurrence is anticipated to be infrequent, will depend on the specific methods chosen by the contractor, and would be for short durations on that day. In-water pile removal occurs intermittently, and it is common for removal to start and stop multiple times as each pile is adjusted and its progress is measured. Moreover, the Navy has
multiple options for pile removal depending on the pile type and condition, sediment, and how stuck the pile is, etc. When two continuous noise sources, such as pile clippers, have overlapping sound fields, there is potential for higher sound levels than for non-overlapping sources. When two or more pile removal methods pile clippers, water jetting, underwater chainsaws or vibratory pile removal are used simultaneously, and the sound field of one source encompasses the sound field of another source, the sources are considered additive and
combined using the following rules see Table 4: For addition of two simultaneous methods, the difference between the two sound source levels SSLs is calculated, and if that difference is between 0 and 1 dB, 3 dB
are added to the higher SSL; if difference is between 2 or 3 dB, 2 dB are added to the highest SSL; if the difference is between 4 to 9 dB, 1 dB is added to the highest SSL; and with differences of 10 or more dB, there is no addition NMFS 2018b; WSDOT 2018.

TABLE 4RULES FOR COMBINING SOUND LEVELS GENERATED DURING PILE REMOVAL
Difference in SSL

Level A zones
0 or 1 dB
2 or 3 dB
4 to 9 dB
10 dB or more ..

Add Add Add Add
3 2
1 0

dB
dB
dB
dB

to to to to
the the the the
higher higher higher higher
source source source source
level level level level
Level B zones

Add Add Add Add
3 2
1 0

dB
dB
dB
dB

to to to to
the the the the
higher higher higher higher
source source source source
level.
level.
level.
level.

Source: Modified from USDOT 1995, WSDOT 2018, and NMFS 2018b.
Note: dB = decibels; SSL = sound source level.

There is also the possibility that impact installation of piles could happen simultaneously with any of the non-impulsive removal methods over large portions of the project as described above. On days when this occurs the Level A harassment zones would be based on the zones calculated for impact pile driving while the Level B
harassment zone would be the largest of the zones for whatever construction methods are being used that day.
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Level B Harassment Zones Transmission loss TL is the decrease in acoustic intensity as an acoustic pressure wave propagates out from a source. TL parameters vary with frequency, temperature, sea conditions, current, source and receiver depth,
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water depth, water chemistry, and bottom composition and topography.
The general formula for underwater TL
is:
TL = B Log10 R1/R2, where TL = transmission loss in dB
B = transmission loss coefficient; for practical spreading equals 15
R1 = the distance of the modeled SPL from the driven pile, and R2 = the distance from the driven pile of the initial measurement
The recommended TL coefficient for most nearshore environments is the practical spreading value of 15. This value results in an expected propagation environment that would lie between spherical and cylindrical spreading loss conditions, which is the most appropriate assumption for the Navys
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proposed activity in the absence of specific modelling. For this project however, the Navy did model sound propagation for the impact and vibratory hammering methods DallOsto and Dahl 2019. For all other pile removal methods we used the practical spreading value.
The Navy determined underwater noise would fall below the behavioral effects threshold of 126 dB rms for marine mammals at distances of less than 10 to 7,140 m depending on the pile types and methods Table 5. 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. Because the Navys as yet unhired contractor has not
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