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Federal Register / Vol. 86, No. 33 / Monday, February 22, 2021 / Notices
Note: Peak sound pressure Lpk has a reference value of 1 Pa, and cumulative sound exposure level LE has a reference value of 1Pa2s.
In this Table, thresholds are abbreviated to reflect American National Standards Institute standards ANSI 2013. However, peak sound pressure is defined by ANSI as incorporating frequency weighting, which is not the intent for this Technical Guidance. Hence, the subscript flat is being included to indicate peak sound pressure should be flat weighted or unweighted within the generalized hearing range. The subscript associated with cumulative sound exposure level thresholds indicates the designated marine mammal auditory weighting function LF, MF, and HF
cetaceans, and PW and OW pinnipeds and that the recommended accumulation period is 24 hours. The cumulative sound exposure level thresholds could be exceeded in a multitude of ways i.e., varying exposure levels and durations, duty cycle. When possible, it is valuable for action proponents to indicate the conditions under which these acoustic thresholds will be exceeded.

Ensonified Area Here, we describe operational and environmental parameters of the activity that will feed into identifying the area ensonified above the acoustic thresholds, which include source levels and transmission loss coefficient.
The sound field in the project area is the existing background noise plus additional construction noise from the planned project. Marine mammals are
expected to be affected via sound generated by the primary components of the project i.e., vibratory pile driving and removal. The maximum underwater area ensonified above the thresholds for behavioral harassment referenced above is 67.7 km2 26.1 mi2, and the calculated distance to the farthest behavioral isopleth is approximately 4.6 km 2.9 mi.
The project includes vibratory pile installation and removal and impact
pile installation. Source levels for these activities are based on reviews of measurements of the same or similar types and dimensions of piles available in the literature. Source levels for each pile size and activity are presented in Table 4. Source levels for vibratory installation and removal of piles of the same diameter are assumed to be the same.

TABLE 4SOUND SOURCE LEVELS FOR PILE DRIVING
Pile size and type
Source level at 10m
Hammer type SPLrms
11.5-inch H-Pile
14-inch H-Pile
48-inch Pipe Pile
Sheet Piles 19.69-inch and 25-inch

Impact
Impact
Vibratory
Impact
Vibratory

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, 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 R1 = the distance of the modeled SPL from the driven pile, and R2 = the distance from the driven pile of the initial measurement
Absent site-specific acoustical monitoring with differing measured transmission loss, a practical spreading value of 15 is used as the transmission loss coefficient in the above formula.
Project and site-specific transmission
Literature source
Peak 183
187
150
195
160

SEL
200
208
160
210
175

170
177
150
185
160

loss data for the Prudhoe Bay portion of AGDCs AK LNG project are not available; therefore, the default coefficient of 15 is used to determine the distances to the Level A and Level B harassment thresholds. However, as discussed in the Monitoring and Reporting section, AGDC will conduct SSV for pile driving. NMFS may adjust the shutdown zones and revise the Level A and Level B harassment zones, as appropriate, pending review and approval of the results of acoustic monitoring.
When the NMFS Technical Guidance 2016 was published, in recognition of the fact that ensonified area/volume could be more technically challenging to predict because of the duration component in the new thresholds, we developed a User Spreadsheet that includes tools to help predict a simple isopleth that can be used in conjunction with marine mammal density or
Caltrans Caltrans Caltrans Caltrans Caltrans
2015
2015
2015
2015
2015

12-in H-Pile.
14-in H-Pile.
12-in H-Pile.
60-in CISS Pile.
AZ Sheet Pile.

occurrence to help predict takes. We note that because of some of the assumptions included in the methods used for these tools, we anticipate that isopleths produced are typically going to be overestimates of some degree, which may result in some degree of overestimate of Level A harassment take. However, these tools offer the best way to predict appropriate isopleths when more sophisticated 3D modeling methods are not available, and NMFS
continues to develop ways to quantitatively refine these tools, and will qualitatively address the output where appropriate. For stationary sources such as pile driving, NMFS User Spreadsheet predicts the distance at which, if a marine mammal remained at that distance the whole duration of the activity, it would incur PTS. Inputs used in the User Spreadsheet, and the resulting isopleths are reported below.

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TABLE 5USER SPREADSHEET INPUT PARAMETERS USED FOR CALCULATING LEVEL A HARASSMENT ISOPLETHS

Spreadsheet Tab Used
Weighting Factor Adjustment kHz
Source Level
Number of piles within 24-h period a
Duration to drive a single pile minutes.

VerDate Sep<11>2014

20:43 Feb 19, 2021

11.5-inch H-pile
14-inch H-pile
14-inch H-pile
48-inch pipe pile
19.69-inch sheet piles
25-inch sheet piles
E.1 Impact pile driving.
2
170 dB SEL
26.09 b

E.1 Impact pile driving.
2
177 dB SEL
4

A.1 Vibratory pile driving.
2.5
150 SPLrms
8
15

E.1 Impact pile driving.
2
185 dB SEL
1.25

A.1 Vibratory pile driving.
2.5
160 SPLrms
15.24 b
18.9

A.1 Vibratory pile driving.
2.5.
160 SPLrms.
12.
24.

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