Dredging
Dredging is the excavation of material from a water environment. Possible reasons for dredging include improving existing water features; reshaping land and water features to alter drainage, navigability, and commercial use; constructing dams, dikes, and other controls for streams and shorelines; and recovering valuable mineral deposits or marine life having commercial value. In all but a few situations the excavation is undertaken by a specialist floating plant, known as a dredger.
Usually the main objectives of dredging is to recover material of value, or to create a greater depth of water. Dredging systems can either be shore-based, brought to a location based on barges, or built into purpose-built vessels.
Dredging can have environmental impacts: it can disturb marine sediments, creating dredge plumes which can lead to both short- and long-term water pollution, damage or destroy seabed ecosystems, and release legacy human-sourced toxins captured in the sediment. These environmental impacts can reduce marine wildlife populations, contaminate sources of drinking water, and interrupt economic activities such as fishing.
Description
Dredging is excavation carried out underwater or partially underwater, in shallow waters or ocean waters. It keeps waterways and ports navigable, and assists coastal protection, land reclamation and coastal redevelopment, by gathering up bottom sediments and transporting it elsewhere. Dredging can be done to recover materials of commercial value; these may be high value minerals or sediments such as sand and gravel that are used by the construction industry.Dredging is a four-part process: loosening the material, bringing the material to the surface, transportation and disposal.
The extract can be disposed of locally or transported by barge or in a liquid suspension in pipelines. Disposal can be to infill sites, or the material can be used constructively to replenish eroded sand that has been lost to coastal erosion, or constructively create sea-walls, building land or whole new landforms such as viable islands in coral atolls.
History
Ancient authors refer to harbour dredging. The seven arms of the Nile were channelled and wharfs built at the time of the pyramids, there was extensive harbour building in the eastern Mediterranean from 1000 BC and the disturbed sediment layers gives evidence of dredging. At Marseille, dredging phases are recorded from the third century BC onwards, the most extensive during the first century AD. The remains of three dredging boats have been unearthed; they were abandoned at the bottom of the harbour during the first and second centuries AD.The Banu Musa brothers during the Muslim Golden Age, while working at the Bayt-Al-Hikmah in Baghdad, designed an original invention in their book named Book of Ingenious Devices, a grab machine that does not appear in any earlier Greek works. The grab they described was used to extract objects from underwater, and recover objects from the beds of streams.
During the renaissance Leonardo da Vinci drew a design for a drag dredger.
Dredging machines have been used during the construction of the Suez Canal from the late 1800s to present day expansions and maintenance. The completion of the Panama Canal in 1914, the most expensive U.S. engineering project at the time, relied extensively on dredging.
Purposes
- Capital dredging: dredging carried out to create a new harbour, berth or waterway, or to deepen existing facilities in order to allow larger ships access. Because capital works usually involve hard material or high-volume works, the work is usually done using a cutter suction dredge or large trailing suction hopper dredge; but for rock works, drilling and blasting along with mechanical excavation may be used.
- Land reclamation: dredging to mine sand, clay or rock from the seabed and using it to construct new land elsewhere. This is typically performed by a cutter-suction dredge or trailing suction hopper dredge. The material may also be used for flood or erosion control.
- Maintenance: dredging to deepen or maintain navigable waterways or channels which are threatened to become silted with the passage of time, due to sedimented sand and mud, possibly making them too shallow for navigation. This is often carried out with a trailing suction hopper dredge. Most dredging is for this purpose, and it may also be done to maintain the holding capacity of reservoirs or lakes.
- Harvesting materials: dredging sediment for elements like gold, diamonds or other valuable trace substances. Hobbyists examine their dredged matter to pick out items of potential value, similar to the hobby of metal detecting.
- Fishing dredging is a technique for catching certain species of edible clams and crabs. In Louisiana and other American states, with salt water estuaries that can sustain bottom oyster beds, oysters are raised and harvested. A heavy rectangular metal scoop is towed astern of a moving boat with a chain bridle attached to a cable. This drags along the bottom scooping up oysters. It is periodically winched aboard and the catch is sorted and bagged for shipment.
- Preparatory: dredging work and excavation for future bridges, piers or docks or wharves, This is often to build the foundations.
- Winning construction materials: dredging sand and gravels from offshore licensed areas for use in construction industry, principally for use in concrete. This very specialist industry is focused in NW Europe, it uses specialized trailing suction hopper dredgers self discharging the dry cargo ashore. Land based old river beddings can be processed in this manner too.
- Contaminant remediation: to reclaim areas affected by chemical spills, storm water surges, and other soil contaminations, including silt from sewage sludge and from decayed matter, like wilted plants. Disposal becomes a proportionally large factor in these operations.
- Flood prevention: dredging increases the channel depth and therefore increase a channel's capacity for carrying water.
Other
- Beach nourishment: this is mining sand offshore and placing on a beach to replace sand eroded by storms or wave action. This enhances the recreational and protective function of the beach, which are also eroded by human activity. This is typically performed by a cutter-suction dredge or trailing suction hopper dredge.
- Peat extraction: dredging poles or dredge hauls were used on the back of small boats to manually dredge the beds of peat-moor waterways. The extracted peat was used as a fuel. This tradition is now more or less obsolete. The tools are now significantly changed.
- Removing rubbish and debris: often done in combination with maintenance dredging, this process removes non-natural matter from the bottoms of rivers and canals and harbours. Law enforcement agencies sometimes need to use a 'drag' to recover evidence or corpses from beneath the water.
- Anti-eutrophication: A kind of contaminant remediation, dredging is an expensive option for the remediation of eutrophied water bodies; one of the causes is like mentioned above, sewage sludge. However, as artificially elevated phosphorus levels in the sediment aggravate the eutrophication process, controlled sediment removal is occasionally the only option for the reclamation of still waters.
- Seabed mining: is a possible future use, recovering natural metal ore nodules from the sea's deepest troughs.
Types
Suction dredgers
These operate by sucking through a long tube like some vacuum cleaners but on a larger scale.A plain suction dredger has no tool at the end of the suction pipe to disturb the material.
Trailing suction
A trailing suction hopper dredger trails its suction pipe when working. The pipe, which is fitted with a dredge drag head, loads the dredge spoil into one or more hoppers in the vessel.When the hoppers are full, the TSHD sails to a disposal area and either dumps the material through doors in the hull or pumps the material out of the hoppers. Some dredges also self-offload using drag buckets and conveyors.
the largest trailing suction hopper dredgers in the world were Jan De Nul's Cristobal Colon and her sister ship Leiv Eriksson. Main design specifications for the Cristobal Colon and the Leiv Eriksson are: 46,000 cubic metre hopper and a design dredging depth of 155 m. Next largest is HAM 318 with its 37,293 cubic metre hopper and a maximum dredging depth of 101 m.
Cutter-suction
A cutter-suction dredger's suction tube has a cutting mechanism at the suction inlet. The cutting mechanism loosens the bed material and transports it to the suction mouth. The dredged material is usually sucked up by a wear-resistant centrifugal pump and discharged either through a pipe line or to a barge. Cutter-suction dredgers are most often used in geological areas consisting of hard surface materials where a standard suction dredger would be ineffective. They can, if sufficiently powerful, be used instead of underwater blasting., the most powerful cutter-suction dredger in the world is DEME's Spartacus, which entered service in 2021.