September/October 2021 - 122

GPS track of HDR vessel monitoring for presence of stunned or injured fish during active pile driving
spawning behaviors that involve
migration and are critical to species
that are completing their life cycle.
The noise from pile installation can
be impulsive or continuous. Impact pile
driving results in impulsive noise,
which is characterized by a higher peak
and a rapid rise in sound pressure level
that has the greatest potential for
impact. Vibratory pile installation or
drilled shafts result in lower intensity,
but continuous, sound pressure levels.
Impact Assessments
For both types of noise sources,
exposure is assessed in terms of both
peak sound pressure level and
cumulative; in other words, the highest
instantaneous noise level an organism
was exposed to is evaluated, as well as
the cumulative amount of noise
exposure an organism experienced
over the entire pile installation.
There are establ ished sound
pressure level criteria used by the
National Marine Fisheries Service
(NMFS) to evaluate the potential for
physiological and behavioral impacts
122 * DEEP FOUNDATIONS * SEPT/OCT 2021
to marine life. Evaluating the distance
from each pile at which each of these
thresholds is reached is an important
early step of the environmental review
process. The distance to each threshold
will vary based on pile size, material,
installation method and the acoustic
environment of the project area. The
distance to the acoustic thresholds can
be estimated using sophisticated
acoustic models or from databases.
The models are often proprietary
and require careful vetting and
explanation to regulators and entail a
substantial expense. The results of
acoustic models are used to compare
project alternatives and can be of
critical importance when evaluating
the potential for impact or incidental
take of ESA-listed species.
The validation of model results with
in-field measurements is a critically
important step. As an example, during
the permitting process for the Mario M.
Cuomo Bridge project, the modeling
results were found to overestimate the
distance to regulatory thresholds when
compared to in-field measurements of
a test pile. It was hypothesized that the
presence of construction barges in the
area immediately around the driven
pile provided a measure of noise
mitigation that was not included in the
model. This was of critical importance,
as the distance to the acoust ic
thresholds was used in part to
determine the potential for incidental
take of endangered Atlantic and
shortnose sturgeon.
Database Option
As more and more projects nationally
have collected underwater noise
measurements, a robust dataset has
been compiled and incorporated into
the NMFS Greater Atlantic Region
Fisheries Office (GARFO) Acoustics Tool;
this spreadsheet-based tool uses
measurements from a variety of
surrogate piles as inputs for formulas
that estimate the distance to acoustic
thresholds, greatly facilitating noise
impact evaluations. For most common
pile installation projects, this tool
would be appropriate to use in lieu of
intensive modeling efforts. The tool,

September/October 2021

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