Nearshore construction projects often require installation of steel piles and
specific methodologies of installing piles may adversely affect nearby species of
concern. Water is an uncompressible medium which allows for easy
transmission of energy through the water column. Energy transmission through
water depends on the following factors for pile driving: 1) direct contact of the pile
and the water; 2) the depth of the water column; 3) the size of the pile; 4) type of
hammer; and 5) the energy of the hammer.
Seasonal restrictions are usually the best practicable alternative for reducing
habitat impacts when pile driving activities may adversely affect the species of
concern. When seasonal restrictions are not practical other options need to be
evaluated, including mitigation and noise attenuation devices. The broad array of
mitigation options will not be discussed as part of this presentation but instead
will focus on noise attenuation technologies. Dewatered cofferdams can be
effective noise attenuators as they eliminate direct contact of the pile with water
while driving. Also, introduction of a compressible medium, like air, near the
energy source can provide sound energy attenuation. An unconfined bubble
curtain is relatively inexpensive but is not effective in tidal marine projects or
rivers and may have unacceptable secondary effects. Secondary effects must
be evaluated as these can increase exponentially for larger projects. Secondary
effects to the environment include air quality degradation from air compressor
use, transportation costs for fuel and equipment, and the potential for injecting
oily compressed air into the water column. The confined bubble curtain works
well in nearly all environments. It usually is more expensive initially, relative to
an unconfined air curtain, but there is less potential for harmful secondary
effects.
Generally, seasonal restrictions will be adequate to protect species of concern.
However, there are conditions in which mitigation or one of the sound attenuation
options should be utilized. Site specific factors, including the amount of energy
being imparted into the environment, sensitivity of the species of concern,
current, accessibility, length of time needed to complete the project, and the cost
of the sound attenuation, should be factored into the permit evaluation.
Author: Alan Christopherson & Jennifer Lundberg, PND Incorporated
Publisher: DFI