Dredging

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Dredging

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.