Types of Dredging
Types of Navigational Dredges
1. Clearing and Snagging
Snagboats are used to break up logjams and to clear debris, sunken vessels, and dilapidated piers that are or might become hazardous to navigation.
2. Mechanical Dredges
Several types of mechanical dredges are used. Dipper dredges and clam shell dredges are the two most common. Mechanical dredges are rugged and capable of removing hard-packed materials or debris. They can be worked in tight areas and are efficient when large barges are used for long-haul disposal. Mechanical dredges have difficulty retaining loose, fine materials in buckets, do not dredge continuously like pipeline dredges, and may need added controls when handling contaminated sediments. Mechanical dredges place the material into barges for transport to the placement location.
3. Hydraulic Dredges
The two primary types of hydraulic dredges are the cutterhead pipeline dredge and the self-propelled hopper dredge. Advantages of cutterhead pipeline dredges include their ability to excavate most materials, to pump directly to a disposal site, to dredge almost continuously, and to dredge some types of rock without blasting. However, cutterhead pipeline dredges have limited capability in rough weather; have difficulty with coarse sand in swift currents; and, for the most part, are not self-propelled. In addition, the necessary pipeline can be an obstruction to navigation and, when handling debris in sediment, the removal efficiency is diminished.
Self-propelled hopper dredges can operate in rough water and move quickly to a jobsite under their own power. The dredging operation does not interfere with other traffic. Work progresses quickly and is economical for long haul distances. Hopper dredges are limited to work in deep waters, but they cannot dredge continuously. Excavation is less precise than with other dredges, and this dredge type has difficulty dredging steep banks and consolidated materials. Specialty dredges such as the dustpan dredge and sidecaster dredge are used to remove loosely compacted coarse-grained material at rapid shoaling sites or in areas where the sediment is needed adjacent to the navigation channel.
Information Courtesy of the U.S. Army Corps of Engineers
Note: Environmental dredging, conducted to remove contaminated sediments, may require modification of the technologies discussed above to insure that contamination is not resuspended. A mechanical dredge, for instance, can use a sealed bucket to bring contaminated sediments to the surface without releasing toxic material.