Dredge Bucket Comparison Demonstration at Boston Harbor

PURPOSE: Sediment resuspension and loading characteristics of a Conventional (open-faced) clamshell bucket, an Enclosed clamshell bucket, and a CableArm@ clamshell bucket were studied under similar operating and environmental conditions in Boston Harbor during August 1999. Monitoring was conducted to characterize each bucket’s near and far field sediment resuspension characteristics. Bucket-loading characteristics were investigated with regard to water-to-solids ratios dredged by the different buckets. Documentation of sediment resuspension in the water column and loading characteristics with conventional and enclosed clamshell buckets (a bucket type that includes both the contractor-built Enclosed and the CableArm buckets) will assist U.S. Army Corps of Engineers (USACE) Districts in making bucket selection decisions and provide data for the fate of dredged material numerical model verification. BACKGROUND: The Boston Harbor Navigation Improvement Project (BHNIP) deepened tributaries to the inner harbor and associated berthing areas. Extensive coordination with resource agencies and other interests resulted in the decision to use in-channel confined aquatic disposal (CAD) cells for placement of contaminated sediments dredged with an Enclosed clamshell bucket. To determine the effectiveness of this new contaminated sediment management option, the U.S. Army Engineer District, New England, the Massachusetts Port Authority and dredging contractor conducted extensive monitoring. The USACE Monitoring Completed Navigation Projects (MCNP) program contributed to the monitoring to evaluate the effectiveness of in-channel CAD cells at Boston Harbor. One study area of the MCNP investigation is the investigation of relative amounts of sediment resuspension associated with different bucket types. Because a significant fraction of the sediments dredged on the BHNIP had elevated levels of some contaminants, the Commonwealth of Massachusetts, Department of Environmental Protection required that an enclosed bucket be used to reduce sediment resuspension and the potential for water quality impacts. However, the contractor performed dredging in a normal fashion, attempting to dredge as efficiently as possible to keep production high and costs low. Tests performed by the water-quality measurement contractor showed no exceedances of the water criteria with either of the approved buckets, the Great Lakes Dredge and Dock (GLDD) Enclosed bucket or the CableArm navigation bucket. However, the New England District expressed concern that the Enclosed buckets were adding additional water to the already soft and weak sediments, possibly causing a further reduction of the bearing capacity of the sediments. This reduction of bearing capacity would, in turn, make the capping operation even more difficult.