Keeping Our Underwater Highways Open
Material that falls to the bottom of a liquid is called sediment. If enough sediment deposits to build a shallow spot on the river or ocean bottom, it forms shoals. When a shoal in a navigation channel causes the bottom to become shallower than shown on nautical charts, this is a safety hazard. A ship or boat striking a shoal may be damaged, its cargo damaged, and in serious situations, the environment can be damaged if the ship's cargo is spilled into the waterway.
Dredging and Dredges
Underwater excavation is called dredging. After the initial excavation needed to establish a channel, dredging must be done periodically to keep it clear and safe for navigation. This is called maintenance dredging. Once sediments are dredged from the waterway, they are called dredged material.
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.
Today's modern dredgers use satellite information and computers to help them dig channels. Up until the 1970's, dredge captains used celestial navigation and markers placed on the riverbanks to guide their dredges. Now dredge captains use global positioning systems (GPS), which use satellite information to calculate the location of the dredge in the channel. On the dredge, information about the channel, the location of the shoal, and even the position of the dredge in the channel is likely to be displayed on a computer screen while they are working. Using computers to process and display information about the job and the dredge while they are working allows the dredging to be done with great efficiency. It saves time and money, and results in safer navigation channels.
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.
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 dump scows, are used in conjunction with the mechanical dredge. While one barge is being filled, another is being towed to the dump site. 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.
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.
Walking the Dredge Spuds
On pipeline and mechanical dredges, the spuds are the two large metal posts located on the stern of the barge. The dredge is held in place during dredging by sinking the spuds into the bottom of the waterway. During dredging operations, the dredge "walks" through the work area by picking up one spud and pivoting up one spud and pivoting around the other.
Cutterhead pipeline dredge
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.
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 is drained from the material and is 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.
Split hull hopper at ocean dredged material disposal site
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.
Sidecaster Merritt, Wilmington District
There are special hydraulic dredges called sidecasters and dustpan dredges. Both of these dredges are used to remove loosely compacted, coarse-grained material and place it in areas close to the navigation channel. They are not widely used. The dustpan dredges were specifically developed for jobs on the Mississippi River. Side-casting of dredged material, done mainly on some smaller projects, is also limited to fairly unique situations and environments.
Dustpan dredge JADWIN, Vicksburg District
Airlift dredges are special-use dredges that raise material from the bottom of the waterway by hydrostatic pressure. They have cylinders that operate like pistons. Material is drawn through the bottom of the cylinder. When it is full, the intake valve closes, trapping the material. Then, compressed air forces the material out through a discharge line to a waiting dumpscow or directly to a disposal site. Airlift dredges bring dredged material to the surface with a relatively small amount of water, which is good when environmental contamination is an issue.
Airlift pumps have not been widely used in the United States. They do not typically achieve high production rates, but are well-suited for projects where either site conditions or sediment quality concerns make other dredges inappropriate. They can be used in tight quarters around docks and piers, in rough seas, and in deep water.
In addition to dredging channels, engineers construct special structures to improve safety in channels or to help maintain channel depths. Some common navigation structures are locks and dams, jetties at ocean entrances, and pile dikes and wingwalls along river channels.
Jetties and Pile Dikes
These structures, shown to the left, are designed to force the water passing by them into the channel. The energy of the flowing water helps to keep sediments from settling and building shoals in the channel. By redirecting the flow of the river, pile dikes protect the bank from erosion, too.
Locks and Dams
Dams are built on rivers for many reasons, like flood control, irrigation, power generation, and navigation. Dams improve navigation in rivers by raising the level of the water in the river behind them. By controlling the amount of water passing through the dam, a constant water depth in the pool, or water ponded behind it, can be maintained. Boats can continue to travel on the river even during dry summer months when the river would ordinarily be too shallow.
Monongahela River locks and dams
For boats and barges to get past the dams, they need to use locks. Locks are watertight chambers that act like elevators for boats and barges on the river. In the lock, the boat enters at one level and the water level is raised or lowered -- raised if the boat is going upriver, lowered if going downriver- to enable the vessel to pass by the dam. This process is called locking through. The figure below depicts the steps of locking through in the downstream direction.
Dresden Lock and Dam, Illinois River
Rock Island District, Photographer: Carol Arney
Webdate: April 23, 2002
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