Dredging is a process to remove sediment . Dredging sediment to construct new ports and navigational waterways or maintain existing ones is essential for vessels to be able to enter shallow areas. Maintenance dredging is required because sediment suspended in the water eventually settles out, gradually accumulating on the bottom. If dredging were not done, harbors would eventually fill in and marine transportation would be severely limited. Dredging is also used to collect sediment (usually sand and gravel) for construction and other commercial uses. Hundreds of millions of cubic feet of sediment are dredged from marine bottoms annually in the United States and throughout the world.
One of the oldest types of dredging is agitation dredging, which uses a combination of mechanical and hydraulic processes and dates back over 2,000 years. An object is dragged along the bottom with the prevailing current; this suspends the sediment and the current carries the suspended material away from the area. Technology currently used to dredge sediment from a harbor, bay, or other marine bottom consists of hydraulic or mechanical devices. Hydraulic dredging involves suspending the sediment, which mixes with water to form a slurry , and pumping it to a discharge site. Mechanical dredging is typically used to dredge small amounts of material. It lifts sediment from the bottom by metal clamshells or buckets without adding significant amounts of water, and the dredged material is usually transferred to a barge for disposal at a particular site.
Most of the dredging that occurs in the United States is hydraulic dredging. Hopper dredges are vessels that employ hydraulic dredging, and they are often used in the open ocean or in areas where there is vessel traffic. The ship's hull is filled with dredged material and the ship moves the material to a designated disposal site where it is dumped through doors in the hull. Pipeline dredges use hydraulic dredging to remove sediment in nearshore areas, and the dredged material is discharged through a pipeline leading to a beach or diked area. Approximately 550 million wet metric tons of sediment are dredged from the waters of the United States each year, and an estimated one-third is disposed in the marine environment , accounting for the greatest amount of waste material dumped in the ocean. Of the dredged material dumped in the marine environment, 66% is disposed in estuaries. Two dozen marine disposal sites in the United States receive approximately 95% of all of the dredged material disposed at sea.
Dredged material is typically composed of silt , clay, and sand, and can sometimes include gravel, boulders, organic matter, as well as chemical compounds such as sulfides, hydrous oxides, and metal and organic contaminants. The grain size of the dredged sediment will determine the conditions under which the sediment will be deposited or resuspended if disposed in the marine environment.
The choice of where the dredged material should be placed depends on whether it is uncontaminated or contaminated by pollutants. If contaminated, the level of pollutants in the dredged material can also play a role in the decision of the type and location of disposal. Because many navigational channels and ports are located in industrialized areas, and because sediments are a sink for many pollutants, dredged material may be contaminated with toxic metals, organohalogens, petrochemical by-products, or other pollutants. Dredged material can also contain contaminants from agricultural and urban sources.
Dredged material with very little contaminants can be placed in a variety of locations and beneficially reused for beach restoration, construction aggregate, fill material, cover for sanitary landfills, and soil supplementation on agricultural land. The primary concerns over the disposal of uncontaminated dredged material in the marine environment are the physical impacts it can have, such as high turbidity in the water column, changes in grain size, and the smothering of bottom dwelling organisms. The ensuing alterations to the bottom habitat can lead to changes in the benthic community. Deposited dredged sediment is usually recolonized by different organisms than were present prior to the disposal of the dredged material. For example, disposal of sediment from a dredging project in Narragansett Bay, Rhode Island changed the bottom topography and sediment type, and this change in benthic habitat led to a subsequent decline in the clam and finfish fishery at the site and an increase in the lobster fishery. If the dredged material is similar to the sediment on which it is dumped, the area may be recolonized by the same species that were present prior to any dumping.
If dredged material is dumped in an area that has less than 197 ft (60 m) of water, most of the material will rapidly descend to the bottom as a high-density mass. A radial gradation of large-to-fine grained sediment usually occurs from the impact area of the deposition outward. Fine-grained material spreads outward from the disposal site, in some cases up to 328 ft (100 m), in the form of a fluid mud. It can range in thickness up to 3.9 in (1 dm). From one to five% of the sediment remains suspended in the water as a plume ; this sediment plume is transient in nature and eventually dissipates by dispersion and gravitational settling. The long-term fate of dredged material dumped in the marine environment depends on the location of the dumping site, its physical characteristics such as bottom topography and currents, and the nature of the sediment. Deep-ocean dumping of dredged material results in wider dispersal of the sediment in the water column. The deposition of the dredged material becomes more widely distributed over the ocean bottom than in nearshore areas.
Dredging contaminated sediment poses a much more severe problem for disposal. Disposing contaminated dredged material in the marine environment can result in long-term degradation to the ecosystem . Sublethal effects, biomagnification of pollutants, and genetic disorders of organisms are some examples of possible long-term effects from toxic pollutants in contaminated dredged material entering the food chain. However, attributing effects from placement of contaminated dredged material at a marine site to a specific cause can be very difficult if other sources of contaminants are present.
Dredged material must be tested to determine contamination levels and the best method of disposal. These tests include bulk chemical analysis, the elutriate test, selective chemical leaching , and bioassays. Bulk chemical analysis involves measurements of volatile solids, chemical oxygen demand , oil and grease, nitrogen , mercury , lead, and zinc. But this chemical analysis does not necessarily provide an adequate assessment of the potential environmental impact on bottom dwelling organisms from disposal of the dredged material. The elutriate test is designed to measure the potential release of chemical contaminants from suspended sediment caused by dredging and disposal activities. However, the test does not take into account some chemical factors governing sediment-water interactions such as complexation, sorption , redox, and acid-base reactions.
Selective chemical leaching divides the total concentration of an element in a sediment into identified phases. This test is better than the bulk chemical analysis for providing information that will predict the impact of contaminants on the environment after the disposal of dredged material. Bioassay tests commonly use sensitive aquatic organisms to measure directly the effects of contaminants in dredged material as well as other waste materials. Different concentrations of wastes are measured by determining the waste dilution that results in 50% mortality of the test organisms. Permissible concentrations of contaminants can be identified using bioassay tests.
If dredged material is considered contaminated, special management and long-term maintenance are required to isolate it from the rest of the environment. Special management techniques can include capping dredged material disposed in water with an uncontaminated layer of sediment, a technique which is recommended in relatively quiescent, shallow water environments. Other management strategies to dispose contaminated dredged material include the use of upland containment areas and containment islands. The use of submarine burrow pits has also been examined as a possible means to contain contaminated dredged material.
There is more than one law in the United States governing dredging and disposal operations. The General Survey Act of 1824 delegates responsibility to the Army Corps of Engineers (ACOE) for the improvement and maintenance of harbors and navigation. The ACOE is required to issue permits for any work in navigable waters, according to the Rivers and Harbors Act of 1899. The Marine Protection, Research, and Sanctuaries Act (MPRSA) of 1972 requires the ACOE to evaluate the transportation and ocean dumping of dredged material based on criteria developed by the Environmental Protection Agency (EPA), and to issue permits for approved non-federal dredging projects. Designating ocean disposal sites for dredged material is the responsibility of EPA. The discharge of dredged material through a pipeline is controlled by the Federal Water Pollution Control Act, as amended by the Clean Water Act (1977). This act requires the ACOE to regulate ocean discharges of dredged material and evaluate projects based on criteria developed by the EPA in consultation with the ACOE. Other Federal agencies such as the U. S. Fish and Wildlife Service and the National Marine Fisheries Service can provide comments and recommendations on any project, but the EPA has the power to veto the use of proposed disposal sites.
[Marci L. Bortman Ph.D. ]
Bokunwiewicz, H. J. "Submarine Borrow Pits as Containment Sites for Dredged Sediment." Wastes in the Ocean. Volume 2, Dredged Material Disposal in the Ocean, edited by D. R. Kester, et al. New York: Wiley, 1983.
Engler, R. M. "Managing Dredged Materials." Oceanus 33 (1990): 63-9.
Kamlet, K. S. "Dredge-Material Ocean Dumping: Perspectives on Legal and Environmental Impacts." Wastes in the Ocean. Volume 2, Dredged Material Disposal in the Ocean, edited by D. R. Kester, et al. New York: Wiley, 1983.
Kester, D. R., et al. "The Problem of Dredged-Material Disposal." Wastes in the Ocean. Volume 2, Dredged Material Disposal in the Ocean, edited by D. R. Kester, et al. New York: Wiley, 1983.
Kester, D. R., et al. "Have the Questions Concerning Dredged-Material Disposal Been Answered?" Wastes in the Ocean. Volume 2, Dredged Material Disposal in the Ocean, edited by D. R. Kester, et al. New York: Wiley, 1983.
Office of Technology Assessment. Wastes in Marine Environments. OTAO-334. Washington, DC: U.S. Government Printing Office, 1987.
Dredging refers to the removal of sediment from the bottom of freshwater bodies such as a river, lake, or delta (the region where a river empties into a larger water body such as an ocean), or from coastal saltwater areas.
The excavated sediment is discarded elsewhere. Dredging typically keeps rivers, bays, and coastal waters clear for the navigation of boats, and often needs to be accomplished at regular intervals because sediment will continue to accumulate as a river empties into a larger body of water, or due to deposition of eroded soil in the water.
If the dredged sediment is not contaminated, it can be recycled for other purposes including creating new land and providing foundations in the construction of buildings. Contaminated sediment must be treated to remove or degrade its pollutants, or contained in storage in order to avoid further harm to the environment.
Dredging is performed using specialized vessels. The sediment is typically scraped or suctioned from the bottom. If not done in a controlled manner, dredging can be damaging to life in the area and can disperse excess sediment in the overlying water.
Historical Background and Scientific Foundations
Dredging has been done for millennia. For example, archeological evidence indicates that dredging of the Tiber River was done during the reign of Emperor Nero in the first century AD. Then as now, the intent was to maintain shipping lanes. In Nero’s time, protective structures were also built along the coastal shoreline near Roman coastal ports in an effort to reduce erosion and so reduce the need for frequent dredging.
In ancient Rome, dredging involved sweeping a large bag attached to a shipboard boom along the bottom of the water body. The sediment-filled bag was then raised up and the contents dumped into an adjacent barge for transport elsewhere.
By the seventeenth century, dredging had become more sophisticated. Rotating wheels with attached buckets churned through the sediment, picking up the sediment and dumping it into a container as the buckets upended.
With the settlement of the East Coast and Gulf of Mexico regions of the United States and the development of sea-going trading, dredging had by the nineteenth century became a vital activity in U.S. seaports including Baltimore, Maryland, and New Orleans, Louisiana.
Dredging is accomplished now using several techniques. The most common is by sucking or vacuuming the bottom sediment into a tube that is positioned on a specialized vessel called a dredger. The sediment empties into the cargo hold of the dredger. When full, the sediment is off-loaded from the dredger either through doors that open and allow access to the cargo hold, or by reversing the action of the suction tube to pump the material out.
Typically, the sediment is vacuumed directly off the bottom of the water body. However, the suction hose can be equipped with a cutting device or a rotating screw-like device that loosens the sediment before it is suctioned up, similar to the operation of some designs of vacuum cleaner.
Other designs of dredgers include buckets attached to a rotating chain, a grabbing device similar to a front-end loader that is used in surface construction sites, a blade that is dragged across the bottom, and an injector device that gently applies a jet of water to the sediment. The blade and water jet systems are intended to move the sediment to another location on the bottom rather than to actually remove the material to the surface.
Impacts and Issues
Without dredging, the build-up of sediment would restrict boat traffic on rivers such as the Mississippi and Nile, and impede commercial shipping in coastal regions. As another example, the Fraser River that empties into the Pacific Ocean at Vancouver, British Columbia, Canada, carries over two million cubic meters of silt and sand downriver in the spring. This sediment must be dredged from the river’s mouth each year if commercial shipping into Vancouver is to continue.
Dredging is also an important means of minimizing flooding, since the build-up of sediment decreases the volume of water that can be contained in the area of the river. As well, the decreased capacity for water can cause the river to flow faster in that region. When more water is added such as runoff while the snow melts in the spring in northern climates, this decreased capacity combined with increased river flow can be too much to be contained by the river bank.
In areas where pollutants have accumulated in the sediment, dredging represents the only means of removing the toxic compounds.
These advantages of dredging come with the possibility of damage to the dredged region of the water course, which can adversely affect life in the immediate area and, if stirred up sediment is washed downriver, life farther removed from the dredge site. It is also conceiv-
WORDS TO KNOW
EROSION: As related to water, the wearing away of the surface material by running water.
RUNOFF: Water that falls as precipitation and then runs over the surface of the land rather than sinking into the ground.
SEDIMENT: Solid unconsolidated rock and mineral fragments that come from the weathering of rocks and are transported by water, air, or ice and form layers on Earth’s surface. Sediments can also result from chemical precipitation or secretion by organisms.
WETLANDS: Areas that are wet or covered with water for at least part of the year.
able that pollutants such as heavy metals that have accumulated deeper in the sediment and which persist for a long time can be stirred up and released into the water during dredging.
Thus, dredging must be done in a controlled manner and with knowledge of past activities in the particular region of the water course, and is often avoided in ecologically sensitive areas.
In September 2004, Hurricane Ivan deposited enough sediment to reduce the depth of the channel that serves as the port of New Orleans’s outlet to the Gulf of Mexico to 24 ft (7 m) in some areas, preventing deep-
draft shipping vessels from using the channel. Emergency dredging operations that followed topped $17 million. The following year, Hurricane Katrina obliterated more than 30 mi (48 km) of marshlands off the coast of Mississippi and Louisiana, transforming them into open water. Efforts to reclaim these valuable wetlands that serve as a buffer for storms include specialized dredging techniques that use an amphibious dredge, which can remove excess sediments from sensitive marshlands with minimal damage to the environment.
Bray, R. N. Environmental Aspects of Dredging. New York: Taylor & Francis, 2008.
O’Neill, Karen. Rivers by Design: State Power and the Origins of U.S. Flood Control. Raleigh, NC: Duke University Press, 2006.
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.
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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
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.
Hence dredge vb. 2 sprinkle with powder XVI; whence dredger2 box with perforated lid for sprinkling XVII.
Hence dredge vb. 1, dredger1 XVI.