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Dredging is theexcavation of material from a water environment. Possible reasons for dredging include improving existingwater features; reshaping land and water features to alterdrainage,navigability, and commercial use; constructingdams,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 floatingplant, known as a dredger.
Usually the main objectives of dredging is to recover material of value, or to create a greater depth of water.[1] Dredging systems can either be shore-based, brought to a location based onbarges, or built into purpose-built vessels.
Dredging can have environmental impacts: it can disturbmarine sediments, creatingdredge plumes which can lead to both short- and long-termwater pollution, damage or destroyseabed ecosystems, and releaselegacy 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.
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Dredging isexcavation carried out underwater or partially underwater, in shallow waters orocean waters. It keeps waterways and ports navigable, and assists coastal protection, land reclamation and coastal redevelopment, by gathering up bottomsediments 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.[2]
Dredging is a four-part process: loosening the material, bringing the material to the surface (together extraction), transportation and disposal.[1]
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 tocoastal erosion, or constructively create sea-walls, building land[1] orwhole new landforms such as viable islands incoral atolls.[3]
Ancient authors refer to harbour dredging. The seven arms of theNile were channelled and wharfs built at the time of the pyramids (4000 BC), there was extensive harbour building in the eastern Mediterranean from 1000 BC and the disturbed sediment layers gives evidence of dredging. AtMarseille, 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.[4]
TheBanu Musa brothers during the Muslim Golden Age in while working at the Bayt-Al-Hikmah (house of wisdom) in Baghdad, designed an original invention in their book namedBook 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.[5]
During the renaissanceLeonardo da Vinci drew a design for a drag dredger.
Dredging machines have been used during the construction of theSuez Canal from the late 1800s to present day expansions and maintenance.[6] The completion of thePanama Canal in 1914, the most expensive U.S. engineering project at the time, relied extensively on dredging.[7]
These operate by sucking through a long tube like somevacuum cleaners but on a larger scale.
A plain suction dredger has no tool at the end of the suction pipe to disturb the material.
A trailing suction hopper dredger (TSHD) trails its suction pipe when working. The pipe, which is fitted with adredge 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.[citation needed]
As of 2008[update] the largest trailing suction hopper dredgers in the world wereJan De Nul'sCristobal Colon (launched 4 July 2008[11]) and her sister shipLeiv Eriksson (launched 4 September 2009[12]). Main design specifications for theCristobal Colon and the Leiv Eriksson are: 46,000 cubic metre hopper and a design dredging depth of 155 m.[13] Next largest isHAM 318 (Van Oord) with its 37,293 cubic metre hopper and a maximum dredging depth of 101 m.
A cutter-suction dredger's (CSD) 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 (for example gravel deposits or surface bedrock) where a standard suction dredger would be ineffective. They can, if sufficiently powerful, be used instead of underwater blasting.[14]
As of 2024[update], the most powerful cutter-suction dredger in the world isDEME'sSpartacus, which entered service in 2021.[15]
Theauger dredge system functions like a cutter suction dredger, but the cutting tool is a rotatingArchimedean screw set at right angles to the suction pipe. Mud Cat invented the auger dredge in the 1970s.[16]
These use theVenturi effect of a concentrated high-speed stream of water to pull the nearby water, together with bed material, into a pipe.
Anairlift is a type of small suction dredge. It is sometimes used like other dredges. At other times, an airlift is handheld underwater by adiver.[17] It works by blowing air into the pipe, and that air, being lighter than water, rises inside the pipe, dragging water with it.
Some bucket dredgers and grab dredgers are powerful enough to rip out coral to make a shipping channel throughcoral reefs.[18]
Abucket dredger is equipped with a bucket dredge, which is a device that picks upsediment by mechanical means, often with many circulating buckets attached to a wheel orchain.[18]
Agrab dredger picks up seabed material with aclam shell bucket, which hangs from an onboard crane or acrane barge, or is carried by a hydraulic arm, or is mounted like on adragline. This technique is often used in excavation ofbay mud. Most of these dredges are crane barges withspuds, steel piles that can be lowered and raised to position the dredge.[18]
A backhoe/dipper dredger has abackhoe like on someexcavators. A crude but usable backhoe dredger can be made by mounting a land-type backhoe excavator on apontoon. The six largest backhoe dredgers in the world are currently the Vitruvius, the Mimar Sinan, Postnik Yakovlev (Jan De Nul), the Samson (DEME), the Simson and the Goliath (Van Oord).[citation needed] They featuredbarge-mounted excavators. Small backhoe dredgers can be track-mounted and work from the bank of ditches. A backhoe dredger is equipped with a half-open shell. The shell is filled moving towards the machine. Usually dredged material is loaded in barges. This machine is mainly used in harbours and other shallow water.[18]
Excavator dredge attachments
The excavator dredge attachment uses the characteristics of cutter-suction dredgers, consisting of cutter heads and a suction pump for transferring material. These hydraulic attachments mount onto the boom arm of an excavator allowing an operator to maneuver the attachment along the shoreline and in shallow water for dredging.
This is a bar or blade which is pulled over the seabed behind any suitable ship or boat. It has an effect similar to that of abulldozer on land. The chain-operated steam dredgerBertha, built in 1844 to a design byBrunel and as of 2009[update] was the oldest operational steam vessel in Britain, was of this type.[19]
This is an early type of dredger which was formerly used in shallow water in the Netherlands. It was a flat-bottomed boat with spikes sticking out of its bottom. As tide current pulled the boat, the spikes scraped seabed material loose, and the tide current washed the material away, hopefully to deeper water.Krabbelaar is the Dutch word for "scratcher".[20]
A water injection dredger uses a small jet to inject water under low pressure (to prevent the sediment from exploding into the surrounding waters) into the seabed to bring the sediment in suspension, which then becomes aturbidity current, which flows away down slope, is moved by a second burst of water from the WID or is carried away in natural currents. Water injection results in a lot of sediment in the water which makes measurement with most hydrographic equipment (for instance: singlebeam echosounders) difficult.
These dredgers use a chamber with inlets, out of which the water is pumped with the inlets closed. It is usually suspended from a crane on land or from a small pontoon or barge. Its effectiveness depends on depth pressure.[citation needed]
Asnagboat is designed to remove big debris such as dead trees and parts of trees from North America waterways.[citation needed]
Some of these are any of the above types of dredger, which can operate normally, or by extending legs, also known as spuds, so it stands on the seabed with its hull out of the water. Some forms can go on land.
Some of these are land-type backhoe excavators whose wheels are on long hinged legs so it can drive into shallow water and keep its cab out of water. Some of these may not have a floatable hull and, if so, cannot work in deep water.Oliver Evans (1755–1819) in 1804 invented the Oruktor Amphibolos, an amphibious dredger which was America's first steam-powered road vehicle.[21]
These are usually used to recover useful materials from the seabed. Many of them travel oncontinuous track. A unique variant[22] is intended to walk on legs on the seabed.[23]
Fishing dredges are used to collect various species ofclams,scallops,oysters ormussels from the seabed. Some dredges are also designed to catch crabs, sea urchins, sea cucumbers, and conch. These dredges have the form of a scoop made of chain mesh, and are towed by afishing boat. Clam-specific dredges can utilize hydraulic injection to target deeper into the sand.[24] Dredging can be destructive to the seabed and some scallop dredging has been replaced by collecting viascuba diving.[25]
As of June 2018, the largest dredger in Asia isMV Tian Kun Hao, a 140-metre (460 ft) long dredger constructed in China, with a capacity of 6,000 cubic metres per hour (59,000 cu ft/ks).[26] An even larger dredger, retired in 1980, was the U.S. Army Corps of EngineersEssayons, which was 525.17 feet (160.07 m) long.[27] TheMallard II, a clamshell dredger that maintainslevees inSan Francisco Bay, has operated continuously since being built in 1936.[28][29]: 61
Dredgers are often equipped with dredge monitoring software to help the dredge operator position the dredger and monitor the current dredge level. The monitoring software often usesReal Time Kinematic satellite navigation to accurately record where the machine has been operating and to what depth the machine has dredged to.[citation needed]
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In a "hopper dredger", the dredged materials end up in a large onboard hold called a "hopper." A suction hopper dredger is usually used for maintenance dredging. A hopper dredge usually has doors in its bottom to empty the dredged materials, but some dredges empty their hoppers by splitting the two-halves of their hulls on large hydraulic hinges. Either way, as the vessel dredges, excess water in the dredged materials is spilled off as the heavier solids settle to the bottom of the hopper. This excess water is returned to the sea to reduce weight and increase the amount of solid material (or slurry) that can be carried in one load. When the hopper is filled withslurry, the dredger stops dredging and goes to a dump site and empties its hopper.[citation needed]
Some hopper dredges are designed so they can also be emptied from above using pumps if dump sites are unavailable or if the dredge material is contaminated. Sometimes the slurry of dredgings and water is pumped straight into pipes which deposit it on nearby land. These pipes are also commonly known asdredge hoses, too. There are a few different types of dredge hoses that differ in terms of working pressure, float-ability, armored or not etc. Suction hoses, discharge armored hoses and self-floating hoses are some of the popular types engineered for transporting and discharging dredge materials.[30] Some even had the pipes or hoses customised to exact dredging needs etc. Other times, it is pumped intobarges (also calledscows), which deposit it elsewhere while the dredge continues its work.[citation needed]
A number of vessels, notably in the UK and NW Europe de-water the hopper to dry the cargo to enable it to be discharged onto a quayside 'dry'. This is achieved principally using self discharge bucket wheel, drag scraper or excavator via conveyor systems.[citation needed]
When contaminated (toxic) sediments are to be removed, or large volume inland disposal sites are unavailable, dredge slurries are reduced to dry solids via a process known as dewatering, to minimize thedredge plume. Current dewatering techniques employ either centrifuges,geotube containers, large textile based filters or polymerflocculant/congealant based apparatus.[citation needed]
In many projects, slurry dewatering is performed in large inland settling pits, although this is becoming less and less common as mechanical dewatering techniques continue to improve.[citation needed]
Similarly, many groups (most notable in east Asia) are performing research towards utilizing dewatered sediments for the production of concretes and construction block, although the high organic content (in many cases) of this material is a hindrance toward such ends.[citation needed]
The proper management of contaminated sediments is a modern-day issue of significant concern. Because of a variety of maintenance activities, thousands of tonnes of contaminated sediment are dredged worldwide from commercial ports and other aquatic areas at high level of industrialization. Dredged material can be reused after appropriate decontamination. A variety of processes has been proposed and tested at different scales of application (technologies for environmental remediation). Once decontaminated, the material could well suit the building industry, or could be used for beach nourishment.[31]
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Dredging can disturbaquatic ecosystems, often with adverse impacts.[32][33] In addition, dredge spoils may containtoxic chemicals that may have an adverse effect on the disposal area; furthermore, the process of dredging often dislodges chemicals residing inbenthic substrates and injects them into thewater column, where they become toxicdredge plumes.[citation needed]
Dredging can have numerous significant impacts on the environment, including the following:
The nature of dredging operations and possible environmental impacts requires that the activity often be closely regulated and requires comprehensive regional environmental impact assessments alongside continuous monitoring.[36] For example, in the U.S., theClean Water Act requires that any discharge of dredged or fill materials into "waters of the United States," including wetlands, is forbidden unless authorized by a permit issued by theArmy Corps of Engineers.[41] Due to potential environmental impacts, dredging is often restricted to licensed areas, with vessel activity monitored closely using automatic GPS systems.[36]Town Of Oyster Bay v. Commander Oil Corporation, Court of Appeals of New York (2001) , showed a case in which the dredger had received a permit from the Army Corps of Engineers preventing the nearby city (Town Of Oyster Bay) to stop the dredging.
According to aRabobank outlook report in 2013, the largest dredging companies in the world are in order of size, based on dredging sales in 2012[2]
Notable dredging companies in North America
Notable dredging companies in South Asia
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