Characterization of Underwater Sounds Produced by Bucket Dredging Operations

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