September/October 2021 - 123

found at www.fisheries.noaa.gov/new-england-midatlantic/consultations/section-7-consultationtechnical-guidance-greater-atlantic,
does not include data
on very large piles however, such as those used in offshore
wind projects; in these situations, alternative data sources or
modeling would be required.
A better understanding of the realities
related to the evaluation and mitigation
of underwater noise ... will benefit project
owners, designers and contractors.
Mitigation Responses
Measures to mitigate underwater noise are regularly
required as permit conditions. These can potentially include
seasonal restrictions on pile driving, the " maintenance of an
acoustic corridor " where sound levels are below acoustic
thresholds for a portion of the river width to allow migratory
fish a zone of passage, and the use of source mitigation
measures, such bubble rings to attenuate noise levels. Bubble
rings were used on the Cuomo Bridge project, for example,
and consisted of perforated rings suspended around each
driven pile. The rings were supplied with compressed air
from barge-mounted compressors. The bubble rings
introduced entrained air into the water column and
successfully mitigated the noise levels at the source, but
also added a substantial amount of planning, effort and
cost to the pile-driving operation.
This project also had a permit condition that required
maintenance of a 5,000 ft (1,524 m) wide acoustic corridor
across the 3 mi (4.8 km) wide Hudson River in which noise
levels had to be kept below regulatory thresholds. This
condition required careful planning and coordination of
concurrent pile driving operations to ensure that the
corridor was maintained.
The width of the water body is an important
consideration for all projects when evaluating acoustic
impacts; the area ensonified by pile driving will occupy a
greater percentage of the available habitat in confined
environments such as small rivers or harbors. In larger
water bodies, it is easier to maintain a zone of passage
through or around a pile driving area. These mitigation
measures can have major impacts to project schedules
and budgets if not anticipated or improperly applied.
Other options to mitigate underwater noise, if
incorporated early in project decision making, can help
facilitate the permitting process. They include:
* Reducing the size of piles, which can result in smaller
noise impacts; generally, larger piles will produce
greater underwater noise levels.
* Modifying the pile material to reduce noise levels;
steel pipe piles generally produce greater noise levels
than concrete piles of a similar size.
* Considering the pile installation method, which
matters because impact hammers will produce
greater noise levels than vibratory hammers. The use
of " soft start " procedures can also reduce the exposure
of marine life; in a soft start, pile driving commences
with an initial period of low energy strikes followed by
waiting periods, which allows marine life to move
away from a pile.
As the body of knowledge around underwater noise and
marine construction has grown in recent years, the
permitting process has begun to become more
streamlined with user friendly tools such as the GARFO
Acoustics Tool. Nonetheless an experienced and
professional environmental team is necessary to help
guide a project through these issues. The proper handling
and ear ly incorporat ion of underwater noise
considerations into project planning can only lead to
better project schedule and cost outcomes.
DEEP FOUNDATIONS * SEPT/OCT 2021 * 123

http://www.fisheries.noaa.gov/new-england-mid-atlantic/consultations/section-7-consultation-technical-guidance-greater-atlantic http://www.dbmcontractors.com/

September/October 2021

Table of Contents for the Digital Edition of September/October 2021