Figure 2-2. Open-water Placement Operations

Framework for Dredged Material Management

May 2004

Figure 2-2. Open-water Placement Operations

Pipeline dredges are commonly used for open-water disposal adjacent to

channels. Material from this dredging operation consists of a slurry with solids

concentration ranging from a few grams per liter to several hundred grams per liter.

Depending on material characteristics, the slurry may contain clay balls, gravel, or coarse

sand material. This coarse material quickly settles to the bottom. The mixture of dredging

site water and finer particles has a higher density than the disposal site water and

therefore can descend to the bottom forming a fluid mud mound. Continuing the

discharge may cause the mound to spread. Some fine material is "stripped" during

descent and is evident as a turbidity plume. Characteristics of the plume are determined

by: discharge rate, characteristics of the slurry (both water and solids), water depth,

currents, meteorological conditions, salinity of receiving water, and discharge

configuration.

The characteristics and operation of hopper dredges result in a mixture of water

and solids stored in the hopper for transport to the disposal site. At the disposal site,

hopper doors in the bottom of the ship's hull are opened, and the entire hopper contents

are emptied in a matter of minutes; the dredge then returns to the dredging site to reload.

This procedure produces a series of discrete discharges at intervals of perhaps one to

several hours. Upon release from the hopper dredge at the disposal site, the dredged

material falls through the water column as a well-defined jet of high-density fluid, which

may contain blocks of solid material. Ambient water is entrained during descent. After it

hits bottom, most of the dredged material comes to rest. Some material enters the

horizontally spreading bottom surge formed by the impact and is carried away from the

impact point until the turbulence of the surge is sufficiently reduced to permit its

deposition.