Dredging is the maritime transportation of natural materials from one part of the water environment to another by specialised dredging vessels. It involves collecting and bringing up, fishing up or clearing away or out material and / or any object from the bed of a river, sea, etc.; transporting it to the relocation site and unloading the material or object. The purpose for dredging can be maintenance of the depth or the deepening of navigation accesses or channels, it can also be land reclamation, coastal protection, seabed stabilisation for the offshore energy installations or the removal of contaminated sediments. In fact, in a usual dredging cycle, self-propelled trailing suction hopper dredgers, barges and other ships spend the majority of their time sailing back and forth between extraction sites and placement sites, transporting materials between the port and the borrow site; sailing between borrow sites; and sailing from sites where material has been extracted to unloading or placement sites.
Maintenance Dredging is the activity of keeping existing watercourses, harbour basins, etc., at the required nautical and / or hydrological depth by removing siltation. The environmental effects of such an operation are in general of minor importance and limited to the effects of the extraction operation itself and the relocation operation when no beneficial use of the dredged material is available. Environmental issues increase in importance when the material to be dredged is polluted.
Capital Dredging is the activity of creating new civil engineering works by means of dredging, such as harbour basins, canals, etc., and the deepening of existing waterways, approach channels. Capital dredging is carried out in virgin soil, which in general is unpolluted. The effects on the environment are limited to the actual working site(s), where the existing habitat or ecosystem is (temporarily) removed.
The activity of extracting minerals with an economic value from underwater deposits. Mineral dredging takes place to mine for instance gold, diamonds, tin, so-called mineral sands (ilmenite, rutile, zircon), phosphates; but also for sand, clay and gravel. The non-valuable fractions dredged along with the mined mineral(s) are in general dumped back in the mined area. The environmental effects of mineral dredging are comparable with capital dredging, the resulting "landscape" however might cause other environmental problems (e.g. with deep gravel pits).
Environmental Remedial Dredging:
Environmental Remedial Dredging is the activity of removing polluted sediments from rivers, harbour basins, etc. Environmental dredging therefore often will be a special type of maintenance dredging. The removal of polluted sediments just because their presence might cause a hazard to public health has created a new type of project and ecosystems. Environmental aspects must be taken into account during all phases of the execution of environmental dredging works.