CONCLUSION

ERDC TN-DOER-E14

August 2001

CONCLUSION: Navigation dredging is common in coastal waters. Measured sound levels from

bucket dredging activities can exceed ambient levels out to considerable distances as was seen in

this study. Sounds produced from bucket dredging operations were quite variable, depending on

the phase of the operation. In this study the strongest sound was associated with the bucket striking

the channel bottom, although absolute sound levels were highly dependent on substrate type.

Dredging in coarse sediments produced the most intense sounds, whereas bottom contact in

unconsolidated mud emitted considerably less intense sound. The second strongest sound source

was the winch motor that pulled the loaded clamshell bucket back to the surface. This noise

exceeded the bucket striking the channel bottom event when dredging in soft sediment. The bucket

digging event produced noises at peaks similar to those generated by the winch motor, but only

when dredging in coarse sediment. In comparison with other dredging events, the bucket closing

event generally produced the least intense sounds. Additionally, sounds from this event were

sporadic and were not always audible. Of all events evaluated, noise produced from dumping

material into the hopper barge attenuated most rapidly with increasing distance from the sound

source. No audible sounds from this event were detected beyond 1,200 m.

When estimating the level of noise that may be introduced into a shallow coastal environment from

bucket dredging, consideration of sediment type may be very important. In general, considerably

less noise is generated when dredging in soft sediment. Maintenance dredging produced sounds

that extended outward only to 3,000 m in this study, and these were most audible for the bucket

striking the channel bottom. Many of the other events such as digging into the soft sediment were

audible at comparatively short distances from the sound source. Other factors that may be important

in bucket dredge sound production include size of the bucket (not directly evaluated herein) and

state of repair of the various types of equipment involved. Additional factors influencing received

noise levels include hydrodynamic conditions, notably prevailing suspended sediment loads and

sea state conditions, as well as skill of the dredge plant operator. With respect to the latter,

considerable sound variation was observed among repetitive dredge cycles due to variation in speed

of the winching operations, the position and orientation of the bucket relative to the water surface

when the downward plunge began, and the consequent angle at which the bucket impacted the

bottom.

These data should provide a better foundation for consideration of environmental concerns stem-

ming from underwater noise associated with bucket dredging operations. Sounds produced by

hydraulic hopper draghead dredges and hydraulic pipeline cutterhead dredges will be characterized

in ensuing notes.

ACKNOWLEDGMENTS: This underwater sound characterization study is a joint effort by the

U.S. Army Engineer District, Alaska, Anchorage, AK, and the U.S. Army Engineer Research and

Development Center, Vicksburg, MS, under the Dredging Operations and Environmental Research

(DOER) Program. The authors wish to express thanks to Mr. Guy McConnell, Planning Division,

Alaska District, for providing support for field efforts in 1999 and 2000. Manson Dredging

graciously provided vessel support and cooperated fully by adjusting their dredging operations to

satisfy field data collection requirements. Dr. Peter Scheifele of the National Undersea Research

Center at Avery Point, CT, provided helpful assistance with sound data analyses.