Dredging Makes Sense
Billions of cubic yards of material are removed from sites around the globe annually in an effort to keep the big ships and their cargo moving. Thus, maintenance of navigation channels helps the world economy by promoting efficient trade. Our forefathers recognized this and passed the General Survey Act of 1824, which established the U.S. Army Corps of Engineers’ role as the Federal water resource agency with the primary mission for constructing and maintaining a safe, reliable, and economically efficient navigation system.
Channels are kept deep and wide enough through dredging for safe movement of ships from deep ocean waters to the more than 200 deep-water harbors where imports are unloaded and exports loaded. Dredging, performed primarily by the Corps of Engineers at navigation channels and by Port Authorities at harbors, takes place in five major areas, and the materials removed differ in consistency and placement options (numbers correspond to sample map at right):
Main approaches (approach channel in ocean); dredged material is composed primarily of sand.
Bar channels (sandbars at inlets); dredged material is composed primarily of coarse-grained sand.
Entrance channels (to harbors); dredged material is composed primarily of sand to fine-grained silt and clay.
Berthing areas (harbors/ports); dredged material is composed primarily of silt and some sand.
Inland waterways (intracoastal waterways and river channels); dredged material is composed primarily of silt and sand.
A dredge is a machine that scoops or suctions sediment from the bottom of waterways or is used to mine materials underwater. People have been dredging channels in one way or another since primitive people began to irrigate crops. Until the early 1900s, dredges were crude and barely effective in keeping channels and harbors clean. Keeping the dredge in position in the channel, knowing how deep a channel was being dug, and even making accurate surveys of the completed channel, were a mixture of art and science. Experienced dredge captains and hydrographic surveyors (surveyors of the underwater topography) were able to produce remarkably good results, given the difficulty of their job.
While the onboard instrumentation of modern dredges is computer-assisted, the basic excavation methods of dredges have remained the same since the late 1800s. The three main types of dredges are mechanical dredges, hydraulic dredges, and airlift dredges.
Hydraulic dredges work by sucking a mixture of dredged material and water from the channel bottom. The amount of water sucked up with the material is controlled to make the best mixture. Too little water and the dredge will bog down; too much and the dredge won’t be efficient in its work. There are two main types of hydraulic dredges — pipeline and hopper dredges.
Hopper dredges are ships with large hoppers, or containment areas, inside. Fitted with powerful pumps, the dredges suck dredged material from the channel bottom through long intake pipes, called drag arms, and store it in the hoppers. The water portion of the slurry can be drained from the material and discharged from the vessel during operations. When the hoppers are full, dredging stops and the ship travels to an in-water disposal site, where the dredged material is discharged through the bottom of the ship.
Hopper dredges are well-suited to dredging heavy sands. They can maintain operations in relatively rough seas and because they are mobile, they can be used in high-traffic areas. They are often used at ocean entrances, but cannot be used in confined or shallow areas. Hopper dredges can move quickly to disposal sites under their own power, but since the dredging stops during the transit to and from the disposal area, the operation loses efficiency if the haul distance is too far.
A pipeline dredge sucks dredged material through one end, the intake pipe, and then pushes it out the discharge pipeline directly into the disposal site. Because pipeline dredges pump directly to the disposal site, they operate continuously and can be very cost-efficient. Most pipeline dredges have a cutterhead on the suction end. A cutterhead is a mechanical device that 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 for removal. Pipeline dredges are mounted (fastened) to barges and are not usually self-powered, but are towed to the dredging site and secured in place by special anchor piling, called spuds (see sidebar).
Cutterhead pipeline dredges work best in large areas with deep shoals, where the cutterhead is buried in the bottom. Water pumped with the dredged material must be contained in the disposal site until the solids settle out. It is then discharged, usually back into the waterway. This method of dredging is not suitable in areas where sediments are contaminated with chemicals that would dissolve in the dredging water and be spread in the environment during discharge.
Because the discharge line for pipeline dredges is usually floated on top of the water, they are not suited to work in rough seas, where lines can be broken apart or in high-traffic areas, where the discharge pipeline can be an obstruction to navigation. If there is a lot of debris in the dredging site, the pumps can clog and impair efficiency.
Mechanical dredges remove material by scooping it from the bottom and then placing it onto a waiting barge or into a disposal area. Dipper dredges and clamshell dredges, named for the scooping buckets they employ, are the two most common types.
Mechanical dredges are rugged and can work in tightly confined areas. They are mounted on a large barge and are towed to the dredging site and secured in place by anchors or anchor piling, called spuds. They are often used in harbors, around docks and piers, and in relatively protected channels, but are not suited for areas of high traffic or rough seas.
Usually two or more disposal barges, called scows, are used in conjunction with the mechanical dredge. While one barge is being filled, another is being towed to the disposal area. Using numerous barges, work can proceed continuously, only interrupted by changing dump scows or moving the dredge. This makes mechanical dredges particularly well-suited for dredging projects where the disposal site is many miles away.
Mechanical dredges work best in consolidated, or hard-packed, materials and can be used to clear rocks and debris. Dredging buckets have difficulty retaining loose, fine materials, which can be washed from the bucket as it is raised. Special buckets have been designed for controlling the flow of water and material from buckets and are used when dredging contaminated sediments.