DREDGING AND PLACEMENT ALTERNATIVES

Technical Note DOER-N3

March 1998

overhead and in cross section, respectively, demonstrating the likely movement of sand into the

littoral system or onto the beach and fines moving offshore.

DREDGING AND PLACEMENT ALTERNATIVES: The selected purpose for nearshore place-

ment will to a large extent suggest the dredging and placement method used. In general, dredging

can be conducted either mechanically (clamshell, bucket, or dipper dredge) or hydraulically (plain

suction, cutterhead, hopper dredge). Usually, mechanically dredged material is placed in a

bottom-release scow from which it is discharged into the water column to settle to the seafloor.

Material (particularly fine-grained) dredged by this method tends to maintain bulk densities at or

near those of in situ conditions, which contributes to a more cohesive mound or feature at the

placement site. Mechanically dredged coarse-grained material (sands) also maintains high bulk

densities, but is more likely to lose coherence during descent through the water column. Generally,

however, mechanically dredged material, whether fine- or coarse-grained, is more conducive to

forming a lasting berm.

Hydraulic dredging involves fluidizing sediments for pumping. Hopper dredges fluidize bottom

sediments for pumping into their hoppers, which significantly reduces bulk density from in situ.

However, once in the hopper, sediment bulk density increases, though usually not to the predredging

levels. Hopper dredges discharge material through doors or a split hull from which the dredged

material settles to the bottom in much the same way as from a bottom-release barge or scow. When

released, the fluidized material is more susceptible to dispersion during descent through the water

column. Depending on the type of dredged material, however, densities that are nearly in situ can

be realized to create more stable features. Hydraulic pipeline dredges (cutterhead and plain suction)

fluidize material throughout the dredging and transport phases. Usually, pipeline-dredged material

is discharged to a confined disposal facility for drying and consolidation. In some cases, pipeline

dredges discharge directly into the water column, and evidence has shown that a feature can be

created with relatively fine-grained material using a pipeline dredge (Williams and Mathies 1996).

PLACEMENT SITE FACTORS: Many factors relating to mound or feature geometry should be

considered for nearshore placement of dredged material. Placement site geometry depends on a

wide range of factors including the placement objective, type and volume of material to be placed,

dredging and placement methods, environmental restrictions (areal and material composition),

existing profile, and restrictions on feature relief (from boating and navigation interests).

In situ sediments at the dredging site must be characterized to determine type of material and if it

is contaminated. Bathymetry of the proposed placement site defines the placement site location

and boundaries, and knowledge of the hydrodynamic climate (wind, waves, and currents) is

necessary to assess the potential physical behavior of dredged sediment and to properly site a mound

or berm to optimize stability, wave breaking, or nearshore/beach feeding benefits. In addition,

assessment of benefits and risks to submerged coastal habitats for shellfish, fisheries resources, and

endangered species is necessary to determine short- and long-term environmental impacts.

ECONOMIC CONSIDERATIONS: Economic considerations for nearshore placement of

dredged material are primarily related to the type of dredging equipment used and placement/dis-

charge method and location. Dredging costs are typically divided into two primary elements:

mobilization/demobilization (mob/demob) and unit cost. Mechanical and pipeline dredges have