Dredging

Dredging is the process of excavating or removing sediments from the bottom of lakes, rivers, estuaries , or marine (ocean) locations. Sediment excavation or dredging is conducted for multiple purposes. These purposes include navigation, mineral extraction (mining), construction activities (e.g., laying underwater pipeline), and the environmental cleanup of polluted sediments.

Dredging is generally conducted by floating construction equipment and is accomplished by mechanical, hydraulic, or hydrodynamic (agitation) processes. Mechanical dredges generally employ drag lines, open or closed clam shell buckets, or an endless chain of buckets to excavate the sediment and place it in a container such as a barge or scow. The dredged sediment is then transported in the barge or scow for beneficial use at a location on land or in the water (e.g., construction material, fill or habitat enhancement), to a nearby disposal site, or in some cases, to an aquatic disposal site at a lake, river, estuary, or ocean.

Hydraulic pipeline dredges use a suction pipe connected to an excavation device (like a huge vacuum cleaner hose with a digger at its end) for removing the dredged sediment from the bottom. In the process, the removed sediment mixes with the overlying water to form the resultant dredged material. The sediment is then pumped hydraulically by a pipeline to a location intended for beneficial use (e.g., beach nourishment or construction fill), to an adjacent aquatic placement location, or to an upland placement facility for storage for later beneficial or commercial uses. Contaminated sediments may be transported to off-site treatment or disposal facilities or to a contained aquatic disposal site. The nonaquatic disposal alternative for contaminated sediments is much more environmentally complex when plant, animal, air (volatile), and surface and groundwater (leachate) pathways for contaminants must be controlled.

Hydraulic dredging may also be accomplished by a self-propelled ocean-going dredging vessel (e.g., hopper dredges) that will store the sediment and entrained water in a large hopper for transport to an ocean disposal site, for beneficial-use placement in the nearshore zone for beach nourishment, or for transport to a land-based containment facility. A special-purpose self-propelled hydraulic dredge known as a side caster excavates the sediment (e.g., entrance channel sand) and immediately pumps the material to a location adjacent to the channel, but down drift of nearshore natural prevailing currents. The currents rapidly disperse the sediments down coast, beneficially adding to the normal coastal sand movement.

Hydrodynamic dredging (agitation dredging) is a process whereby the bottom sediment is physically disturbed by mechanical (e.g., a boat's propeller) or hydraulic means (e.g., water jets). The sediment is not excavated and removed from the water body. The suspended material simply moves away from the dredging site as a result of the natural prevailing currents. The sediment never leaves the water body and is not moved or transported in a vessel or container. There is no resulting disposal or discharge from hydrodynamic (agitation) dredging.

The vast majority of dredging in the United States occurs for navigation purposes as deep channels and berths are needed for ports in lakes, rivers, estuaries and the nearshore ocean to accommodate large commercial or military vessels. These ships are an integral part of U.S. trade and also necessary for defense purposes. About 350 million tons of dredged sediments are excavated annually in U.S. waters to maintain navigation. A large percent of dredged material is clean, approximately 90 percent, and suitable for a wide variety of useful purposes, including placement back into the water at an approved aquatic disposal site. In industrial and highly urbanized areas that account for about 10 percent of the total U.S. dredging, sediments are polluted with industrial and sewage contaminants along with runoff from nearby land areas. As such, these sediments must be thoroughly tested by chemical and toxicological means and disposed of in an environmentally acceptable manner. Some aquatic areas are so heavily polluted that the sediments must be removed for cleanup from the water body and disposed of in a secure disposal facility.

Dredging for an environmental cleanup can be very controversial because of the significant expense, and the need for an environmentally suitable disposal alternative and proof that the cleanup is necessary, then effective. Environmental dredging has been used in more than thirty U.S. locations with mixed success. These sites are currently under review regarding the long-term usefulness of dredging. As a result, significant controversy (technical and political) exists as to the overall effectiveness of clean up dredging and the transfer of environmental and human health risk when huge quantities of sediment are removed from a water body and placed in an upland location. Comparative risk assessment of all practical alternatives is necessary to resolve these controversies.

see also Abatement; Bioaccumulation; Cleanup; Ocean Dumping; PCBs (Polychlorinated Biphenyls); Risk; Superfund; Water Pollution.

Bibliography

boyd, m.b., et al. (1972). "disposal of dredge spoil; problem identification and assessment and research program development." technical report h-72-8. vicksburg, ms: u.s. army engineer waterways experiment station.

palermo, m.r.; engler, r.m.; and francingues, n.r. (1993). "the united states army corps of engineers perspective on environmental dredging." buffalo environmental law journal 2:243–254.

internet resource

ceda, iadc, pianc. (1997). guidance documents on dredging. guide 4: machines, methods and mitigation. the netherlands: iadc secretariat. also available from www.iadc-dredging.com.

pianc. (2000). dredging: the facts. brussels, belgium: international navigation association. also available from www.pianc-aipcn.org.

pianc. (2001). dredging: the environmental facts. where to find what you need to know. brussels, belgium: international navigation association. also available from www.pianc-aipcn.org.

u.s. environmental protection agency web site. available from http://www.epa.gov/hudson.

Robert M. Engler

dredging process of excavating materials underwater. It is used to deepen waterways, harbors, and docks and for mining alluvial mineral deposits, including tin, gold, and diamonds.

The Dutch at an early period cleared their canals of silt with a pole to which was attached a bag held open by a steel ring. The apparatus, operated from the side of a stationary barge, was dragged along the bottom and then emptied into the barge.

Modern dredging equipment may be divided into four main classes. The grab dredge is used where the amount of excavation is relatively small. It consists of one or more grab buckets, operated by cranes mounted on a vessel or barge or sometimes on the shore. Each bucket has jaws that are hinged together. The bucket is lowered to the bottom with its jaws open and pointing down. When it sinks into the material to be dredged, its jaws close. The material can then be lifted to the surface and discharged into a hopper for removal to a disposal area. The dipper dredge, also known as the boom-and-dipper assembly, is similar in appearance to a land power shovel. It is used extensively in canal construction and was employed in the cutting of the Panama Canal.

The ladder-bucket dredge, a more elaborate type, is generally mounted on a self-propelling vessel built with a longitudinal well in the center, open to the water beneath for a considerable length. Mounted and hinged over the well is a long steel frame, which may be raised or lowered at will; it is equipped with a long string of buckets passing over sprockets at each end. The buckets, operating through the well, scoop up material from the bottom and discharge it into a chute that projects over the vessel's side to a hopper barge moored alongside or into a receiving hopper in the dredge itself.

The suction dredge, or hydraulic dredge, an entirely different type, is used principally where material such as sand or mud is to be removed. It consists of a flexible pipe connected at one end to a powerful centrifugal pump. At the other, open end there is usually a device designed to break up the material to be dredged. The open end of the pipe is lowered to the bottom, where the material to be dredged is mixed with water, pumped up, and then discharged into hopper barges. There the heavy material settles, and the surplus water is allowed to overflow.

Material from these dredges is sometimes pumped through pipes for long distances and used to build up low-lying ground. Hopper barges made to carry away and sink the material brought up by dredges are of a special type. In the space where the material is carried are hinged doors, or flaps, held closed by chains and opening downward. Around the space are watertight compartments to give the barge buoyancy. When the dredge is above the disposal area, the bottom doors are released and the material discharged; the doors are then closed again by winches.

dredge¹ / drej/ • v. [tr.] clean out the bed of (a harbor, river, or other area of water) by scooping out mud, weeds, and rubbish with a dredge. ∎  bring up or clear (something) from a river, harbor, or other area of water with a dredge: mud was dredged out of the harbor | [intr.] they start to dredge for oysters in November. ∎  (dredge something up) fig. bring to people's attention an unpleasant or embarrassing fact or incident that had been forgotten: I don't understand why you had to dredge up this story. • n. an apparatus for bringing up objects or mud from a river or seabed by scooping or dragging. ∎  a dredger. dredge² • v. [tr.] sprinkle (food) with a powdered substance, typically flour or sugar: dredge the bananas with sugar and cinnamon.

dredge 2 †sweetmeat containing spice. XVI. alt. of ME. drag(g)e, dragie (XIV) — OF. dragie, (also mod.) dragée, with medL. drageia, dragētum, dragāta obscurely rel. to L. tragēmata, Gr. tragḗmata species, condiments.
Hence dredge vb. 2 sprinkle with powder XVI; whence dredger² box with perforated lid for sprinkling XVII.

dredge, an iron, wedge-shaped contrivance with a small net attached, by which shellfish are brought up from their beds. Towed along the bottom by a smack, the iron wedge loosens the hold of the shellfish and guides them into the net.

See also fisheries.

dredge 1 instrument for dragging the bed of a river, etc. XVI. rel. in some way to early Sc. dreg (XV), which may be — MDu. dregghe; but the final cons. of the Eng. word suggests a native orig.
Hence dredge vb. 1, dredger¹ XVI.

dredging Sprinkling food with flour, sugar, etc. Fish and meat are often dredged with flour before frying, while cakes and biscuits are dredged with sugar as a decoration.

dredge •allege, dredge, edge, fledge, hedge, kedge, ledge, pledge, reg, sedge, sledge, veg, wedge •straight edge