Section 1 Introduction - National Marine Fisheries Service - NOAA

NBK at Bangor EHW-1 Pile Replacement Project Final Marine Mammal Monitoring Report
average WRA boat measured level and the average propagation rates from measured vibratory
sound levels for all piles, the calculated distance to the 120-dB ranged from 821 to 13,000 m
(Table 4). Sound levels during soft starts were typically lower than those levels at the initiation
and completion of continuous vibratory driving. However, levels during continuous driving
varied considerably and were at times lower than those produced during the soft starts. It is
difficult to assign a level that describes how much lower the soft start sound levels were than
continuous levels.
The APE 200-6 vibratory hammer was used to install and remove all piles. Table 5 summarizes
the difference between the modeled and actual harassment sound thresholds for vibratory pile
driving. As stated previously, the maximum underwater sound levels recorded at 10 meters from
the pile was 174 dB RMS (re: 1µPa) and the average was 166 dB RMS (re: 1µPa). Using the
average SPL and assuming practical spreading loss (15LogR), the source levels produced from
vibratory driving were below the injury threshold for both cetaceans (180 dB) and pinnipeds
(190 dB), therefore there was no risk of injury to any marine mammal during pile driving and
removal events associated with the EHW-1 PRP. Even using the maximum SPL produced, the
source level of pile driving were always less than 190 dB RMS, and the maximum distance to the
180 dB RMS isopleth would have been less than 4 meters from the pile. Since no cetaceans ever
approached or entered within the WRA (over 500 meters from the pile) there was still no risk of
injury to cetaceans even if the sound pressure levels exceeded the 180 dB RMS isopleth on
occasion.
Table 5. Modeled and Measured Marine Mammal Behavioral Harassment RMS Sound
Level Thresholds for Vibratory (120 dB) Pile Driving
Threshold
Airborne Sound
Modeled Distance
(Average, m)
Measured Distance (Average, m)
120 dB 10,300 † 4,396
† Average of predicted thresholds to the North (13,840 m) and South (6,760 m) in Hood Canal
Airborne measurements were taken using two microphone systems placed along the shoreline on
the north and south (removed 7 October 2011) of the project site; also, one microphone was
placed on a vessel within the WRA. No injury thresholds exist for marine mammals exposed to
airborne sound, and no behavioral harassment threshold exists for cetaceans exposed to airborne
sound. The behavioral harassment thresholds for pinnipeds are 90 dB RMS re: 20 µPa (harbor
seals only) and 100 dB RMS re: 20µPa (all other pinnipeds).
Summaries of RMS sound levels at these locations, and associated threshold distances, are
shown in Table 6. Prior to the initiation of the EHW-1 PRP, the distance to the 100 dB
(unweighted) airborne harassment zone for vibratory pile driving was predicted to extend 9 m
from the pile. Based on the measurement of average RMS Leq levels and applying a 20 Log10
propagation rate, the harassment zone was 10 m from the pile for all pile sizes. Likewise, the
distance to the 90 dB (unweighted) harassment zone for harbor seals was predicted to extend
28 m from the pile. Based on the measurement of average levels and applying a 20Log10
propagation rate, the actual harassment zone was 33 m for all pile sizes (Table 7).
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