Considerations for Contaminated Material Dredging and Placement

During hydraulic dredging, the bottom material is fluidized, lifted via

pipeline by a centrifugal pump, and transported as a slurry. Material

dredged by hopper dredges is also considered hydraulic dredging because

of the fluidization process required to lift the material to the hoppers. Hy-

draulically dredged material is typically transported via pipeline to the dis-

posal site and discharged with large amounts of entrained water. For

hopper dredges, the material is transported in the hopper similar to a barge

or scow as with the mechanical dredging, but excess water that is en-

trained during dredging remains with the material, thereby making the ma-

terial less dense than when in situ or mechanically dredged. For both

cases of hydraulic dredges (pipeline and hopper), the less dense material

is more susceptible to stripping and creates a flatter feature covering a

larger area on the bottom (Sanderson and McKnight 1986).

Alternatives are available to increase the mounding potential of mate-

rial dredged by hydraulic means. For pipeline dredges, diffusers can be

employed to reduce the material exit velocity from the pipe and reduce dis-

persion. Pump-down pipes can be added to transfer the material closer to

the bottom and reduce losses due to stripping as the material falls through

the water column. For hopper dredges, the spread of material on the bot-

tom can be reduced by having the dredge come to a stop during placement.

Dredged material characteristics also contribute to mounding potential.

Cohesive and noncohesive materials will tend to mound when dredged us-

ing mechanical means and point dumped (i.e., from a barge). Both cohe-

sive and noncohesive material will tend to flow if hydraulically dredged

and point dumped (i.e., discharged from a pipe). In cases where a pump-

down pipe is incorporated for hydraulically dredged material, noncohesive

material tends to mound, while cohesive material tends to flow.

Table 2 summarizes available information on the mounding or flowing

characteristics of cohesive versus noncohesive sediments for various

dredging and placement methods. This information can be used in evaluat-

ing various equipment and placement techniques for a given set of site

conditions.

Considerations for Contaminated Material

Dredging and Placement

Placement of contaminated material for a capping project should be ac-

complished so that the resulting deposit can be defined by monitoring and

effectively capped. Therefore, the equipment and techniques for dredging,

transport, and placement must be compatible with that of the capping mate-

rial. Since capping is a contaminant control measure for potential benthic

effects, the contaminated material should be placed such that the exposure

of the material prior to capping is minimized. In most cases, the water col-

umn dispersion and bottom spread occurring during placement should also

be reduced to the greatest possible extent. This minimizes the release of

contaminants during placement and provides for easier capping. If the

placement of the contaminated sediment has potentially unacceptable

Chapter 5 Equipment and Placement Techniques