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"Dredge" redirects here. For other uses, seeDredge (disambiguation).
Contents
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1 Uses1.1 Relevance
2 Types of dredging vessels2.1 Suction2.1.1 Trailing suction2.1.2 Cutter suction2.1.3 Auger suction2.1.4 Jet-lift2.1.5 Air-lift
2.2 Bucket2.3 Grab2.4 Backhoe/dipper2.5 Water injection2.6 Pneumatic2.7 Bed leveler2.8 Krabbelaar2.9 Snagboat2.10 Amphibious2.11 Submersible2.12 Fishing2.13 Police drag
3 Disposal of materials4 Environmental impacts5 Major dredging companies6 Dredging in nature7 Images8 See also9 References10 External links
Dredging is anexcavation activity or operation usually carried out at least partly underwater, in shallow seas orfresh water areas with the purpose of gathering up bottomsediments and disposing of them at a different location.
This technique is often used to keep waterways navigable. It is also used as a way to replenish sand on some public beaches, where too much sand has been lost because ofcoastal erosion. Dredging is also used as a technique for fishing for certain species of edibleclams andcrabs, seefishing dredge.
A dredge is a device for scraping or sucking the seabed, used for dredging.
A dredger is a ship or boat equipped with a dredge.
Sometimes in American usage a dredger is called a dredge.
The process of dredging creates spoils战利品 (excess material), which are carried away from the dredged area. Dredging can produce materials for land reclamation开垦荒地 or other purposes (usually construction-related与建筑相关), and has also historically played a significant role ingold mining 采金业. Dredging can create disturbance in aquatic ecosystems水生生态系统, often with adverse impacts
Capital: dredging carried out to create a newharbour,berth orwaterway, or to deepen existing facilities场所 in order to allow larger ships access. Because capital主要 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.
Preparatory: work and excavation for futurebridges,piers ordocks/wharves, often connected withfoundation work. Maintenance: dredging to deepen or maintain navigable waterways orchannels which are threatened to become silted with the passage of time, due tosedimented 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 ofreservoirs or lakes.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 forflood orerosion control.Beach nourishment: mining sand offshore and placing on a beach to replace sand eroded by storms or wave action. This is done to enhance the recreational and protective function of the beaches, which can be eroded by human activity or by storms. This is typically performed by a cutter-suction dredge or trailing suction hopper dredge. Harvesting materials: dredgingsediment for elements likegold or other valuable trace substances. Seabed mining: a possible future use, recovering natural metalore nodules from the sea'sabyssal plains. Construction materials: dredging sand and gravels from offshore licensed areas for use in construction industry, principally for use in concrete. Very specialist industry focused in NW Europe using specialized trailing suction hopper dredgers self discharging dry cargo ashore.Anti-eutrophication: Dredging is an expensive option for the remediation of eutrophied (or de-oxygenated) water bodies. 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.Contaminantremediation: to reclaim areas affected by chemical spills, storm water surges (with urban runoff), and other soil contaminations. Disposal becomes a proportionally large factor in these operations. Removing trash and debris: often done in combination with maintenance dredging, this process removes non-natural matter from the bottoms of rivers and canals and harbors.Flood prevention: this can help to increase channel depth and therefore increase a channel's capacity for carrying water.
Relevance
Without the many and almost non-stop dredging operations world wide, much of the world's commerce would be impaired, often within a few months, since much of world's goods travel byship, and need to accessharbours or seas via channels. Recreationalboating also would be constrained to the smallestvessels. The majority ofmarine dredging operations (and the disposal of the dredged material) will require that appropriate licences are obtained from the relevantregulatory authorities, and dredging is usually carried out by (or for) harbour companies or corresponding government agencies.
Types of dredging vessels
Suction

The Geopotes 14 lifting its boom on a canal in The Netherlands. (gēopotēs is Greek for "that which drinks earth")
For suction-type excavation out of water, seeSuction excavator.
These operate by sucking through a long tube, like somevacuum cleaners but on a big scale.
A plain suction dredger has no tool at the end of the suction pipe to disturb the material. This is often the most commonly used form of dredging.[citation needed]
Trailing suction
A trailing suction hopper dredger (TSHD) trails its suction pipe when working, and 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 dredger in the world is currentlyJan De Nul's Cristobal Colon (launched July 4, 2008[1]); its sister ship Leiv Eriksson is under construction as of the end of 2008 (keel laid August 27, 2008, expected launch July 2009[2]). Main design specs for the Cristobal Colon and the Leiv Eriksson are: 46,000 cubic meter hopper and a design dredging depth of 155 m.[3] Next largest is HAM 318 (Van Oord) with its 37,293 cubic meter hopper and a maximum dredging depth of 101 m.
Cutter suction
A cutter-suction dredger's (CSD) suction tube has a cutter head at the suction inlet, to loosen the earth and transport it to the suction mouth. The cutter can also be used for hard surface materials like gravel or rock. The dredged soil is usually sucked up by a wear-resistant centrifugal pump and discharged through a pipe line or to a barge. In recent years, dredgers with more powerful cutters have been built in order to excavate harder rock without blasting.
The two largest cutter suction dredgers in the world are currently (as at August 2009) DEME's D'Artagnan (28,200 kW total installed power)[4] and Jan De Nul's J.F.J. DeNul (27,240 kW).[5]. Jan de Nul has by far the most heavy cutters in the market.
Auger suction
This process functions like a cutter suction dredger, but the cutting tool is a rotatingArchimedean screw set at right angles to the suction pipe. The first widely used auger dredges were designed by Mud Cat Dredges in the 1980s.
Jet-lift
These use theVenturi effect of a concentrated high-speed stream of water to pull the nearby water, together with bed material, into a pipe.
Air-lift
Anairlift is a type of small suction dredge. It is sometimes used like other dredges. At other times, an airlift is used, handheld underwater by adiver. It works by blowing air into the pipe, and that air, being lighter than water, rises inside the pipe, dragging water with it.
Bucket

Bucket dredging
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 or chain. Some bucket dredgers and grab dredgers are powerful enough to rip out coral to make a shipping channel throughcoral reefs.
Grab

Grab dredging in process inPort Canaveral,Florida
A grab dredger picks up seabed material with a clam shell grab, which hangs from an onboard crane or acrane ship, 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 with spuds.
Backhoe/dipper
A backhoe/dipper dredge has abackhoe like on someexcavators. A crude but usable backhoe dredger can be made by mounting a land-type backhoe excavator on apontoon. The three largest backhoe dredgers in the world are Vitruvius and Mimar Sinan (Jan De Nul) and 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 dredges material is loaded in barges. This machine is mainly used in harbors and other shallow water.
Water injection
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.
Pneumatic
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.
Bed leveler
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.
Krabbelaar
This is an early type of dredger which was formerly used in shallow water in theNetherlands. 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 isDutch for "scratcher".
Snagboat
Main article:Snagboat
Asnagboat is designed to remove big debris such as dead trees and parts of trees from rivers and canals.
Amphibious
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 inventedan amphibious dredger which was America's first steam-powered road vehicle.
Submersible
These are usually used to recover useful materials from the seabed. Many of them travel oncaterpillar tracks. A unique variant[6] is intended to walk on legs on the seabed.[7]
Fishing

Dredge haul including live clams and empty shells
Fishing dredges are used to collect various species ofclamsscallops,oysters orcrabs from the seabed. These dredges have the form of a scoop made of chain mesh, and are towed by afishing boat. Careless dredging can be destructive to the seabed. Nowadays some scallop dredging is replaced by collecting viascuba diving.
Police drag
In somepolice departments a small dredge (sometimes called a drag) is used to find and recover objects and bodies from underwater. The bodies may bemurder victims, or people who committedsuicide bydrowning, or victims of accidents. It is sometimes pulled by men walking on the bank.
Disposal of materials
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 giant 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.
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. Other times, it is pumped intobarges (also calledscows), which deposit it elsewhere while the dredge continues its work.
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. Current dewatering techniques employ either centrifuges, large textile based filters or polymerflocculant/congealant based apparatus.
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.
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.
Environmental impacts
Dredging can create disturbance toaquatic ecosystems, often with adverse impacts. 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.
The activity of dredging can create the following principal impacts to the environment:
Release of toxic chemicals (includingheavy metals andPCB) from bottomsediments into thewater column. Short term increases inturbidity, which can affect aquaticspeciesmetabolism and interfere withspawning. Secondary effects from water column contamination of uptake of heavy metals,DDT and other persistent organic toxins, viafood chain uptake and subsequent concentrations of thesetoxins in higherorganisms including humans. Secondary impacts tomarsh productivity fromsedimentation Tertiary impacts toavifauna which mayprey upon contaminated aquatic organisms Secondary impacts to aquatic and benthic organisms'metabolism and mortality Possible contamination of dredge spoils sites
Major dredging companies
Royal Boskalis Westminster (Netherlands)Jan De Nul (Belgium)Van Oord Dredging and Marine Contractors (Netherlands)DEME (Belgium)
Dredging in nature
Some animals have evolved adaptations to find their food by dredging:
DucksGrey whales: they filter seabed sand with theirbaleen
Images

The 'business end' (excavator) of aYukon dredge.

Profile view of this Yukon dredge tied up to aquay, note the size. The dredge conveys the spoils to the rear (left side) into a receiving vessel such as abarge.

Example of a trailing suction dredger: the Orisant in the port ofIJmuiden,Netherlands

Grab dredging inVictoria Harbor,Hong Kong

Stuyvesant

Essayons

Alexander von Humboldt of theJan de Nul fleet

Sand mining

April Hamer atLakes Entrance,Victoria
See also
UK Waterways portal
Nautical portal
Queen of the Netherlands (ship), a big dredgerWT Preston, asnagboatWeeks Marine, a dredging company
References
^http://www.sandandgravel.com/news/article.asp?v1=11114^http://www.sandandgravel.com/news/article.asp?v1=11221^http://www.jandenul.com^http://www.deme.be/^http://www.jandenul.com/jdn.html^http://www.nio.org/past_events/inchoe/dredging_eia.jsp#dredging_1^ "Concept of a mathematical model for prediction of major design parameters of a submersible dredger/miner" by Sritama Sarkar, Neil Bose, Mridul Sarkar, and Dan Walker, in "3rd Indian National Conference on Harbour and Ocean Engineering,National Institute of Oceanography", Dona Paula,Goa 403 004India, 7 - 9 December 2004
External links
Wikimedia Commons has media related to:Dredge ships
Directory of Dredgers (private photography series of dredgers)News and Equipment Exchange (Latest global news and equipment)Dredging NewsDredging and Spoil Disposal Policy (pdf)(from theAustralian Government)World of Boats at Eyemouth ~ Bertha Iron, Steam powered dredger or drag boat. Designed by Isambard Kingdom Brunel in 1844
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