Characterization of Underwater Sounds Produced by Dredges

Potential effects of underwater noise produced by dredging operations on a variety of organisms have emerged as a concern of environmental agencies. For example, it has been hypothesized that dredging-induced noise could block or delay the migrations of fishes through navigable waterways. Few data exist that adequately characterize sounds emitted by dredge plants that would support objective decisions balancing the need to dredge against relative risk to a fishery resource. To obtain data to address this concern, field investigations were undertaken to characterize underwater sounds typical of bucket, hydraulic cutterhead, and hopper dredging operations. Bucket dredges produce a repetitive sequence of sounds generated by winches, bucket impact with the substrate, bucket closing, and bucket emptying. In contrast, cutterhead dredges generate relatively continuous sounds as the cutterhead rotates while embedded in the substrate. Hopper dredge sounds consist of a combination of sounds emitted from two relatively continuous sources: engine and propeller noise similar to that of large commercial vessels, and sounds of dragheads moving in contact with the substrate. The intensity, periodicity, and spectra of emitted sounds differ greatly among dredge types. Components of underwater sounds produced by each type are influenced by a host of factors including substrate type, geomorphology of the waterway, site-specific hydrodynamic conditions, equipment maintenance status, and skill of the dredge plant operator. Dredge sound characterization data must be integrated with knowledge of auditory thresholds and behavioral responses of those aquatic organisms perceived to be at risk in order that potential impacts can be accurately assessed. Ultimately, should problematic effects be detected, sound characterization data can lay the groundwork for effective mitigation measures.