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ERDC TN-DOER-E14
August 2001
Characterization of Underwater Sounds
Produced by Bucket Dredging Operations
PURPOSE: This technical note presents a characterization of underwater sounds produced by
bucket dredging operations in Cook Inlet, Alaska. Both continuous sounds, such as those produced
by propellers, pumps, and generators, and repetitive sounds produced by particular dredging events
(e.g., dredge bucket striking the channel bottom, jaws of bucket clamping shut) are potential sources
of underwater noise. This note examines the intensity, periodicity, and attenuation of emitted sounds
from bucket dredging operations and how they are influenced by factors including substrate type,
hydrodynamic conditions, equipment maintenance, and skill of the dredge plant operator.
BACKGROUND/INTRODUCTION: In recent years concerns have been raised regarding under-
water sounds of anthropogenic origin and their potential impacts on aquatic organisms. Although
not the most frequently cited concern, noise-related disturbance has occasionally been cited as
justification for environmental windows for Federal navigation dredging projects in both marine
and freshwater systems (Reine, Dickerson, and Clarke 1998). Hypothetically, underwater sounds
may interrupt or impair communication, foraging, migratory, and other behaviors of marine
mammals. It has also been hypothesized that dredging-induced noise could block or delay the
migration of fishes through navigable waterways. Few data adequately characterizing sounds
emitted by dredge plants exist that would support objective decisions balancing the need to dredge
against relative risk to marine or aquatic resources. To obtain data to address this concern, field
investigations were undertaken to characterize underwater sounds of a typical bucket dredging
operation. Mechanical dredges produce a repetitive sequence of sounds generated by winches and
derrick movement, bucket impacts with the substrate, digging into substrate (especially gravel or
other coarse material), bucket closing, and the emptying of material into a hopper barge or scow.
Although underwater noise issues have arisen previously, they have been associated primarily with
petroleum exploration and industrial construction activities (Richardson et al. 1995). Their consid-
eration in conjunction with navigation dredging has little precedence. The authors are unaware of
any published studies that characterize sounds produced by dredges engaged in either navigation
channel maintenance or deepening operations. Studies by Greene (1985, 1987), Miles, Malme, and
Richardson (1987), and Miles et al. (1986) are among the very few relevant references that exist.
Given the general lack of knowledge on the topic, this investigation was undertaken to provide a
better technical foundation upon which to assess potential impacts of bucket dredging operations.
These findings will be interpreted in a biological response context in an ensuing publication.
METHODS
Field Recording: Underwater acoustic monitoring of bucket dredging (also known as mechanical
or clamshell dredging) operations in Cook Inlet, Alaska, was conducted during September 15-17,
1999, and August 16-17, 2000. Sound source audio of the dredging activities was recorded with a
RESON TC-4032 low-noise hydrophone with a built-in 10-dB preamplifier, which was connected

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