ERDC TN-DOER-E36 "Characterization of Underwater Sounds Produced by a Backhoe Dredge Excavating Rock and Gravel"

PURPOSE: This technical note characterizes underwater sound produced by a backhoe dredge during rock removal as part of the widening and deepening of New York/New Jersey Harbor. Both continuous sounds (e.g., engine and generator sounds transmitted through the hull) and repetitive, punctuated sounds (e.g., associated with bucket bottom contact and the repositioning of spuds) comprise a broad spectrum of dredging-emitted underwater sound sources. The various sound sources can be characterized in terms of intensity, periodicity, and attenuation with distance from the source. Likewise, the sounds must be placed into context with ambient levels of sound in the surrounding body of water. Such characterizations are required components of environmental assessments that address newly emerging concerns for detrimental impacts of underwater noise on many aquatic organisms. In order to adequately assess the risks associated with backhoe dredging operations, sounds were characterized with respect to sound pressure levels (SPLs) generated by this dredge type across the broad 20-Hz to 20-kHz spectrum. In addition, SPLs were measured in the 50to 1,000-Hz range generally detectable by fishes and the 100to 400-Hz range in which certain fish species show a greater sensitivity. Given the scarcity of existing accurate information quantifying underwater sounds generated by different dredge types and sizes, differences in geotechnical properties of material being excavated, and site specificity of working environments (i.e. bathymetry, hydrodynamic conditions, prevalence of non-dredging ambient sounds), this study fills important knowledge gaps that contribute to better-informed dredging project management practices.