Planning a Dredging Project
Level 2

Dredged Material Disposal
Disposal site selection for dredged material is one of the most important and challenging parts of planning a dredging project. In this section, some of the most common dredged material disposal methods are described.

Ocean Placement of Dredged Material
Ocean Dredged Material Disposal Sites (ODMDS) are primarily used for material coming from inlets, coastal entrance bars, or main coastal navigation waterways. Typically, in ocean placement, a hopper dredge or towed barge sails to a designated area in the ocean, where the hull (bottom) of the vessel is opened. The dredged sediments drift to the bottom. Only clean dredged material may go to ocean sites.

There are over 100 Corps and EPA-designated ODMDS around the coast of the continental United States, Alaska, and Hawaii. On average, about 60 million cubic yards of dredged material is taken to these sites each year. The Corps of Engineers and EPA spend hundreds of thousands of dollars annually to monitor and manage the ODMDS. The Corps has developed computer simulation models to help predict the impacts of ocean disposal, which are used by project planners in the site selection and project planning processes.

Additional Resources: ADDAMS models for dredged material dispersion EPA/Corps Guidance on the Development of Site Management Plans for Ocean Dredged Material Disposal Sites

Beach Nourishment
Beach nourishment is the placement of dredged material on or near the beach, usually to replenish an eroding beach or protect an eroding wetland. The dredged material is generally sand coming from inlets, coastal entrance bars, or main offshore waterways. Both hopper dredges and pipeline dredges can be used for beach nourishment sites. When hopper dredges place sand offshore along the beach, natural processes carry it onto the beach over a long period of time.

Only clean dredged material can be used for beach nourishment. This method of dredged material disposal is considered a beneficial use of dredged sands. It is used in coastal areas all around the country. In the state of Florida, in recent years, between 1 and 3 million cubic yards of sand dredged from navigation channels has been placed on beaches annually.

Confined Disposal Facilities (CDFs)

Buffalo Harbor CDF Huron Harbor CDF Reclaimed CDF in Cleveland Harbor

In Confined Disposal Facilities (CDFs), dredged material is placed behind dikes, which contain and isolate it from the surrounding environment. A CDF consists of a mixture of dredged material and water that is pumped into an area that is divided into several smaller areas, called cells. As the water moves between the cells, it slows, the dredged material settles out, and finally, clean water is discharged from the site. There are three types of CDFs: Upland, Shoreline, and Island. The difference in the three types of CDFs is their location. Upland CDFs are on land, above the line of high water and out of wetland areas. Shoreline CDFs are constructed over the sea or lake bottom and are attached to the shoreline on at least one side. Island CDFs are constructed offshore, but in relatively shallow water.

CDFs can be used for any type of dredged material, coarse or fine-grained. Usually, pipeline dredges pump material directly from the dredging site into the CDF, which is the least expensive way to put the material in the site. In special cases, where the CDF is far removed from the dredging site or a pipeline dredge isn't used for the dredging, barges or hopper dredges may take dredged material to the site, where it is re-pumped into the CDF.

Preparing and caring for a CDF requires a substantial commitment of time and money by local and Federal government agencies. Sometimes a CDF can be designed so that there can be other uses for the land during and after the site is used for dredged material disposal. In the Great Lakes, shoreline CDFs have been used to contain and isolate contaminated sediments from the environment.

Flow-lane and Within-banks Placement
Some waterways are in high-energy river systems with rapidly flowing water and strong currents. The energy of the water causes shoals made of coarse sand to form and move downstream relatively quickly. In these systems, flow-lane or within-banks disposal may be used. The methods are similar. The figures below show schematics of flow-lane and in-banks disposal.

In both cases, dredged material is placed at a site in or along the river that is subject to river erosion. The site is filled temporarily, until the energy of the river moves the sand out again. Within-banks disposal refers to the temporary use of eroding banks, the river thalweg, sandbars, or man-made islands. It is common on the Mississippi River. Pipeline and dustpan dredges use this disposal option.

Flow-lane disposal of dredged material refers to the placement of materials in water within or adjacent to the navigation channel. It is similar to the "thalweg" disposal on the Mississippi River. (The thalweg of a river is the part of the channel where the water has its greatest velocity.) Flow-lane disposal is commonly used in the Columbia River by both hopper and pipeline dredges.

In some rivers, clean dredged material may be placed in or adjacent to
the navigation channel to be carried off again by the energy of the river

Capped Disposal
Sometimes, but not in most locations, dredged material is contaminated with metals, chemicals, or other things. If the contamination is bad enough, it might be harmful to the environment to put the material in a typical in-water site or a typical CDF. In these cases, special handling will be used to prevent the contaminants from re-entering the environment during dredging and disposal.

The objective of capped in-water disposal is to isolate contaminated material from the environment by capping, or covering, the contaminated material with clean material -- usually sand. The contaminated dredged material is placed on a level bottom or in deep pits or bottom depressions. Then clean material is dropped on top. Specific engineering principles are used in the design and placement of the cap to ensure that it successfully isolates the contaminants and stays in place. Caps are designed so that currents, waves, or the burrowing bottom creatures will not erode the protective layer over time. In addition, the caps are continually monitored to look for signs of failure.

Capped disposal for the isolation of contaminated sediment is practiced worldwide. In the United States, capped disposal sites have been used successfully in several places, Long Island Sound in New Bedford, Massachusetts, Puget Sound in Washington State, the New York Byte, and at other locations.

Selecting a New Disposal Site

Whether planning a new waterway or routine maintenance dredging, the toughest dredging decisions are seldom about which dredge to use, but rather, where to put the dredged material once it's dug. Dredged material must be placed in a legally designated disposal area.

Why is Site Selection Controversial? Economics & the Environment In selecting dredged material disposal sites, the Corps of Engineers must balance the wise expenditure of tax dollars with the requirement to avoid unacceptable environmental impacts. The cost of a waterway and the benefits to the community are usually the easiest things to evaluate because they can be reflected by a dollar value. It is often harder to judge the worth of possible beneficial or harmful effects on the environment because the traditional measure of value - dollar value - can't always be used. The worth of beauty, archeological remains, or wildlife habitat is what the community at large thinks it is. Comparing dollars to non-economic losses is like comparing apples to oranges. Placing the appropriate value on environmental effects, whether they are beneficial or harmful effects, is at the heart of most controversy in selecting new disposal sites. The community is not usually of a single opinion on such matters. Different stakeholders will hold different views. Sometimes it takes a national law, like the Endangered Species Act, to dictate priority, when "value" is left to judgement. It is in this public decision-making arena where sound science, social empathy, and good negotiating skills are most important.

Whenever possible, dredged material is put where it can be used for another beneficial purpose -- like fill for airport runway construction or building bird habitat. When that's not possible, the other disposal options already described must be used. In the best of circumstances, there is a designated site close by. But when sites become full or unavailable for some other reason, a new site must be found.

Selecting a disposal area is often controversial, pitting economic concerns against environmental values. In most cases, the cost of dredging the project increases dramatically the further the disposal site is from the dredging site. But a disposal site can change the land, shore, or ocean bottom and its habitat or commercial value far into the future. In highly industrialized areas, like the areas around city ports, much of the land may already be tied up in commercial development and unavailable.

There are laws that regulate both where and under what conditions the placement of dredged material can occur. All states and some Federal agencies have formal procedures and issue permits (a permit is the government's written permission) for the placement of dredged material to ensure that the environment is protected. For disposal in or near fresh water, the state must agree that the placement will not harm the water quality of the river or lake. For disposal in the ocean, the Corps of Engineers and the Environmental Protection Agency pick disposal areas in places that will minimize impacts to the environment.