Keeping Our Underwater Highways Open
Water carries sand and soil, as well as other floating material, like logs, along with it as it flows toward the ocean. Where the water slows down, the sand and soil drop to the bottom of the river channel. When the sand and soil drop to the bottom, they are called sediments.
Sediments can build up and become shoals, or high spots in the navigation channel. This is a danger for the boats and ships using the channel. If a vessel grounds, or strikes the bottom, the vessel and its contents may be damaged. In serious situations, the environment can be damaged if the ship's cargo is spilled into the waterway. Obviously, it is very important to keep the channels clean!
Dredging and Dredges
For the major waterways in the United States, the U.S. Army Corps of Engineers is the agency responsible for seeing that the channels are kept clear. Dredges are used to maintain the depth in navigation channels. A dredge is a machine that scoops or sucks sediments from under the water. Digging under water is called dredging. There are a few different types of dredges.
Mechanical Dredges - Mechanical (mi - can -ik-all) dredges remove material by scooping it from the channel bottom and then placing it onto a waiting barge or into a disposal area. Dipper dredges and clamshell dredges, named for the type of scooping buckets they use, are the two most common types.
Mechanical dredges are often used in areas protected from waves and sea swells. They work well around docks and shallow channels, but not usually in the ocean.
Hydraulic (Hi-draw-lik) Dredges - Hydraulic dredges work by sucking a mixture of dredged material and water from the channel bottom. There are two main types of hydraulic dredges -- the pipeline dredge and the hopper dredge.
Cutterhead Pipeline Dredge - The cutterhead pipeline sucks dredged material through one end, the intake pipe, and then pushes it out the discharge pipeline directly into the disposal site. The "cutterhead" is a mechanical tool on the end of the intake pipe. It has rotating blades or teeth to break up or loosen the bottom material so that it can be sucked through the dredge. Some cutterheads are rugged enough to break up rock.
Pipeline dredges are often used in river channels, bays, and harbors where there is plenty of space to work and not many waves. They are seldom used in the ocean.
Hopper Dredges - Hopper dredges are ships that have large containment bins, or hoppers, built into them. They have powerful pumps and long suction pipes called drag arms. At the end of the drag arm is a flat, slotted foot, called the draghead.
Split hull hopper at ocean dredged material disposal site
As they move slowly along, hopper dredges suck sediments from the channel bottom and store it in the hoppers, not unlike a vacuum cleaner. When the hoppers are full, dredging stops and the ship travels quickly to a deepwater disposal site, where the dredged material drops through doors on the bottom of the ship.
Hopper dredges are self-powered, so they can move to the dredging and disposal site by themselves. They can move out of the way of other ship traffic and are often used in channels where there is a lot of other ship traffic. They can be used at ocean entrances where there are waves and rough seas. But, because they have to be moving to dredge, they are not used in confined areas, like small harbors.
Airlift Dredge - Airlift dredges are special-use dredges that raise material from the bottom of the waterway by air pressure. They have cylinders that operate like pistons. Material is sucked through the bottom of the cylinder. When it is full, a valve closes, trapping the material in the cylinder. Then, compressed air forces it out through a discharge line. Because they do not suck very much water with the sediment, they can use a barge for disposal or pump directly to a disposal site on land.
Airlift dredges can be used in deep water and in very tight places. Because they don't mix much water into the material, they are good for some special dredging jobs when the sediments have been polluted with chemicals.
Today's modern dredges use satellite information and computers to help them dig channels. Not so long ago, dredge captains relied on the stars and markers placed on the riverbanks to guide their dredging. Now they use the global positioning system (GPS), which uses satellites and can give the location of a dredge in a channel to within inches of its true position. On the dredge, all the information about the channel, the location of the shoal, and even the position of the dredge is shown on a computer screen. The computers help the dredgers do a better job in less time. And that means that they'll be able to keep channels cleaner and safer for navigation.
Computer-assisted dredging controls on the dredge ESSAYONS
Webdate: April 23, 2002
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