Stationary placement

Placement Options, Restrictions,

and Tolerances

Several options are possible for placement of materiel using hopper

dredges, barges, or pipeline dredges, depending on the particular needs for

the project. These include stationary placement, placement at multiple

points or along multiple lanes, or options aimed at spreading materials

over large areas.

Stationary placement

Stationary placement is where the tug/barge or hopper dredge comes to

essentially a complete stop for disposal. This method is ideal for concen-

trating the material to minimize mound spread. Dredged material will

settle to the bottom without the imparted vessel velocity and associated

turbulence and thus reduce total mound coverage. On its capping projects,

the New England District has specified that the dredged material be

placed while the barge is stationary or moving at less than 2 knots. The

disadvantage of this method is the loss of vessel control by the operator

during placement. Most operators prefer some forward movement of the

vessel, particularly if waves, winds, and/or currents are strong enough to

affect positioning. Vessel speeds up to 2 to 3 knots are preferred in the

open ocean. However this scenario will increase the mound spread as the

material is released over a greater area. In some cases this greater spread

may be desirable to prevent creation of too much relief or to spread mate-

rial evenly over a larger disposal area.

The time required for material to exit a barge or hopper should also be

considered when specifying stationary or moving placement. Material

exit time depends on the barge opening width, time to open, and type of

material being placed. In general, barges open in 20 to 60 sec to a width

of approximately the bin width. Barge modifications (including installa-

tion of false sides) can be made to effectively increase the opening

width/bin width ratio thus facilitating material exit, though this is an ex-

treme (and costly) modification. Typically, sandy material will exit the

barge in 30 sec to 2 min, and fine-grained material will take 10 to 30 sec

to exit. For split-hull hopper dredges, exit time can take from 3 to 5 min

for sandy material, with fine-grained material exiting in roughly 30 sec,

with silty sand mixtures exiting in about 2 to 5 min. Hopper dredges with

doors and pocket barges require longer times for the material to exit. For

example, the STUYVESANT (industry hopper) has 20 hopper doors, and

sandy material takes approximately 5 min to exit (Sanderson and

McKnight 1986).

An often encountered problem during the disposal phase is that as the

hull is opened and material begins to exit the barge, some material will

form a bridge across the hull opening and thereby reduce the rate of dis-

charge. Additionally, the material may bridge to the extent that it will not

fall until the hull has opened beyond the angle of repose of the material.

When this occurs, this bridged material can discharge quickly and exit

the barge with a large initial velocity. The net effect can be an increased

Chapter 5 Equipment and Placement Techniques