Framework for Dredged Material Management
Bucket or clamshell dredges remove the sediment being dredged at nearly its in
situ density and place it on a barge or scow for transportation to the disposal area.
Although several barges may be used so that the dredging is essentially continuous,
disposal occurs as a series of discrete discharges. Barges are designed with bottom doors
or with a split-hull, and the contents may be emptied within seconds, essentially as an
instantaneous discharge. Often sediments dredged by clamshell remain in fairly large
consolidated clumps and reach the bottom in this form. Whatever its form, the dredged
material descends rapidly through the water column to the bottom, and only a small
amount of the material remains suspended. Clamshell dredge operations may also be used
for direct material placement adjacent to the area being dredged. In these instances, the
material also falls directly to the bottom as consolidated clumps.
Dredge hoppers and scows are commonly filled past the point of overflow to
increase the load. The gain in hopper or scow load and the characteristics of the
associated overflow are dependent on the characteristics of the material being dredged
and the equipment being used. There is little debate that the load can be increased by
overflow if the material dredged is coarse grained or forms clay balls, as commonly
occurs with new-work dredging. For fine-grained maintenance material, significant
disagreement exists as to whether a load gain can be achieved by overflow.
Environmental considerations of overflow may be related to aesthetics, potential effects
of water-column turbidity, potential effects of deposition of solids, or potential effects of
sediment-associated contaminants (Palermo and Randall 1990).
Open-water disposal sites can be either predominantly nondispersive or
predominantly dispersive. At predominantly nondispersive sites, most of the material is
intended to remain on the bottom following placement and may be placed to form
mounds. At predominantly dispersive sites, material may be dispersed either during
placement or eroded from the bottom over time and transported away from the disposal
site by currents and/or wave action. However, both predominantly dispersive and
predominantly nondispersive sites can be managed in a number of ways to achieve
environmental objectives or reduce potential operational conflicts. Additional discussion
of open-water disposal processes is found in Chapter 4.
2.4.2 Description of Confined Disposal
Confined disposal is placement of dredged material within diked nearshore or
used in this document in its broadest sense. CDFs may be constructed as upland sites,
nearshore sites with one or more sides in water (sometimes called intertidal sites), or as
island containment areas as shown in Figure 2-3.
The terms "confined disposal facility," "confined disposal area," "confined disposal site," "diked disposal
site," and "containment area" all appear in the literature and refer to an engineered structure for
containment of dredged material. The confinement dikes or structures in a CDF enclose the disposal area
above any adjacent water surface, isolating the dredged material from adjacent waters during placement. In
this document, confined disposal does not refer to subaqueous capping or contained aquatic disposal (see