Dredging 947692 225221741 2008-07-12T15:18:00Z JBazuzi 3144056 /* Backhoe/dipper */ ital. vessel names, remove irrelevent bit about spud poles and stationary dredges (maybe a new section or article?), fix grammar {{Unreferenced|date=August 2007}} {{redirect|Dredge}} [[Image:Dredge No-4 (1).JPG|thumb|The 'business end' (excavator) of a [[Yukon]] dredge.]] [[Image:Dredge No-4 (2).JPG|thumb|Profile view of above dredge tied up to a [[quay]], note the size. The dredge conveys the spoils to the rear (left side) into a receiving vessel such as a [[barge]].]] '''Dredging''' is an [[excavation]] activity or operation usually carried out at least partly underwater, in shallow seas or [[fresh water]] areas with the purpose of gathering up bottom [[sediment]]s and disposing of them at a different location, mostly to keep waterways navigable. 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 (though in American usage, there is no added letter). The process of dredging creates spoils (excess material), which are conveyed to a location different from the dredged area. Dredging can produce materials for land reclamation or other purposes (usually construction-related), and has also historically played a significant role in [[gold mining]]. Dredging can create disturbance in aquatic ecosystems, often with adverse impacts. ==Uses== ===Usage=== *'''Capital''': dredging carried out to create a new [[harbour]], [[berth]] or [[waterway]], or to deepen existing facilities in order to allow larger ships access. This process is usually carried out with a cutter-suction dredge. *'''Preparatory''': work and excavation for future [[bridge]]s, [[pier]]s or [[dock]]s/[[Wharf|wharves]], often connected with [[Foundation (architecture)|foundation]] work. *'''Maintenance''': dredging to deepen or maintain navigable waterways or [[Channel (geography)|channel]]s which are threatened to become silted with the passage of time, due to [[sediment]]ed 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 [[reservoir]]s 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 for [[flood]] or [[erosion]] 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''': dredging [[sediment]] for elements like [[gold]] or other valuable trace substances. *'''Seabed mining''': a possible future use, recovering natural metal [[ore]] nodules from the sea's [[abyssal plain]]s. *'''[[eutrophication|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. *'''[[Soil contamination|Contaminant]] [[remediation]]''': 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. ===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 by [[ship]], and need to access [[harbour]]s or seas via channels. Recreational [[boating]] also would be constrained to the smallest [[ship|vessel]]s. The majority of [[Marine (ocean)|marine]] dredging operations (and the disposal of the dredged material) will require that appropriate licences are obtained from the relevant [[regulatory]] authorities, and dredging is usually carried out by (or for) harbour companies or corresponding government agencies. ==Types of Dredging Vessels== [[Image:OrisantDredger.jpg|thumb|Example of a trailing suction dredger: the ''Orisant'' in the port of [[Ijmuiden]], [[Netherlands]].]] ===Suction=== These operate by sucking through a long tube, like some [[vacuum cleaner]]s. 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.{{Fact|date=August 2007}} ===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 currently Vasco da Gama (Jan De Nul) with its 33,000 cu.m. hopper and a maximum dredging depth of 135m. The next mega trailing suction hopper dredgers Cristobal Colon and Leiv Eriksson are actually under construction in Spain and should be delivered in 2008. Main design specs are the 46,000 cu.m. hopper and a design dredging depth of 155m. [http://www.jandenul.com] <!-- Deleted image removed: [[Image:XLarge cutterhead.jpg|left|300px|thumb|Replaceable pin tooth cutter head used on the largest cutter suction dredges manufactured by Ellicott Dredges]] --> ===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 and harder rock without blasting. The two largest cutter suction dredgers in the world are [http://www.deme.be/set_main.html Deme's] D'Artagnan (28,200 kW total installed power), and [http://www.jandenul.com/jdn.html Jan De Nul's] J.F.J. DeNul (27,240 kW). ===Auger suction=== This process functions like a cutter suction dredger, but the cutting tool is a rotating [[Archimedean screw]] set at right angles to the suction pipe. The first widely used auger dredges were designed by [http://www.mudcat.com Mud Cat Dredges] in the 1980s. ===Jet-lift=== This uses the [[Venturi effect]] of a concentrated high-speed stream of water to pull the nearby water, together with bed material, into a pipe. ===Air-lift=== An [[Airlift (dredging device)|airlift]] is a type of small suction dredge. It is sometimes used like other dredges. At other times, often an airlift is used handheld underwater by a [[underwater diving|diver]]. It works by blowing air into the pipe, and dragging water with it. ===Bucket=== A bucket dredger is a dredger equipped with a bucket dredge, which is a device that picks up [[sediment]] by mechanical means, often with many circulating buckets attached to a [[wheel]] or chain. Some bucket dredgers and grab dredgers are powerful enough to work through [[coral reefs]] to make a shipping channel. ===Grab=== [[Image:Grab_Dredging_Process.JPG|thumb|Example of the grab dredging process in [[Port Canaveral]], [[Florida]].]] A grab dredger picks up seabed material with a clam shell grab, which hangs from either an onboard crane or a [[crane ship]], or is carried by a hydraulic arm, or is mounted like on a [[dragline]]. This technique is often used in excavation of [[bay mud]]. Most of these dredges are crane barges with spuds. ===Backhoe/dipper=== {{Template:Unreferencedsection|date=July 2008}} A backhoe/dipper dredge has a [[backhoe]] like on some [[excavator|excavators]]. A crude but usable backhoe dredger can be made by mounting a land-type [[backhoe]] [[excavator]] on a [[Pontoon (boat)|pontoon]]. The two largest backhoe dredgers in the world were ''Tauracavor'' (Great Lakes), ''New York'' (Great Lakes) and ''Il Principe'' (Jan De Nul).{{Fact|date=February 2007}} Both feature a [[barge]]-mounted excavator. Small backhoe dredgers can be track mounted and work from the bank of ditches. A backhoe dredger is equipped with 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 injects water in a small jet under low pressure (low pressure because the sediment should not explode into the surrounding waters, rather it is carefully moved to another location) into the seabed to bring the sediment in suspension, which then becomes a [[turbidity current]], which flows away down slope, is moved by a second burst of water from the WID or is carried away in natural currents. Opposition claims that Water Injection Dredging is not a natural way of dredging while the side of the WID claims otherwise. As a side note: Water injection results in a lot of sediment in the water which makes measurement with most hydrographic equipment (for instance: singlebeam echosounder) difficult and should make use of filtering to produce better results. ===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 a [[bulldozer]] on land. ===Krabbelaar=== 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 [[Dutch language|Dutch]] for "scratcher". ==Other types== ===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]] [[excavator]]s 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 [[Oliver Evans#The Oruktor Amphibolos|an 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 on [[caterpillar tracks]]. [http://www.nio.org/past_events/inchoe/dredging_eia.jsp#dredging_1 This link] describes a type intended to walk on legs on the seabed. It is a summary of the article "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, [http://www.nio.org National Institute of Oceanography]", Dona Paula, [[Goa]] 403 004 [[India]], 7 - 9 December 2004: see http://www.nio.org for more information about [[publisher]] etc. ===Fishing=== There are types of dredges used for collecting [[scallop]]s or [[oyster]]s from the seabed. They tend to have the form of a scoop made of chain mesh. They are towed by a [[fishing boat]]. Scallop dredging is very destructive to the seabed, and nowadays is often replaced by [[scuba diving]] to collect the scallops. ===Police drag===<!-- This section is linked from [[Police drag]] --> In some [[police]] departments a small dredge (sometimes called a ''drag'') is used to find and recover objects and bodies from underwater. The bodies may be [[murder]] victims, or people who committed [[suicide]] by [[drowning]], 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 with [[slurry]], 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 into [[barge]]s (also called [[scow]]s), 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 polymer [[flocculant]]/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 to aquatic ecosystems, often with adverse impacts. In addition, dredge spoils may contain [[toxic]] chemicals that may have an adverse effect on the disposal area; furthermore, the process of dredging often dislodges chemicals residing in [[benthic]] substrates and injects them into the [[water column]]. The activity of dredging can create the following principal impacts to the environment: *Release of toxic chemicals (including [[heavy metals]] and [[Polychlorinated biphenyls|PCB]]) from bottom [[sediment]]s into the [[water column]]. *Short term increases in [[turbidity]], which can affect aquatic [[species]] [[metabolism]] and interfere with [[spawning]]. *Secondary effects from water column contamination of uptake of heavy metals, [[DDT]] and other persistent organic toxins, via [[food chain]] uptake and subsequent concentrations of these [[toxin]]s in higher [[organism]]s including humans. *Secondary impacts to [[marsh]] productivity from [[sedimentation]] *Tertiary impacts to [[bird|avifauna]] which may [[prey]] upon contaminated aquatic organisms *Secondary impacts to aquatic and benthic organisms' [[metabolism]] and mortality *Possible contamination of [[dredge spoils]] sites ==Gallery== <gallery> Image:Dredging technique schematic.png|Bucket dredging Image:Stuyvesant_Bow.JPG|''Stuyvesant'' Image:Essayons.JPG|''Essayons'' Image:Alexander von Humboldt (empty).jpg|''Alexander von Humboldt'' of the [[Jan Pieter De Nul|Jan de Nul]] fleet Image:BurigangaDredge.JPG|Sand mining <!-- Deleted image removed: Image:Punta Cana 03.29.06 012.jpg|''Ellicott 370HP Dragon Cutter Suction Dredge'' --> <!-- Deleted image removed: Image: Mud Cat.jpg|''Mud Cat MC2000 Auger Dredge'' --> Image:Lakes Entrance Dredger April Hamer.JPG|''April Hamer'' at [[Lakes Entrance]], [[Victoria (Australia)|Victoria]] </gallery> ==See also== *[[Queen of the Netherlands (ship)|Queen of the Netherlands]] *[[WT Preston]] *[[Weeks Marine]] ==External links== {{nautical portal}} *[http://www.dredgers.nl/ Directory of Dredgers] (private photography series of dredgers) *[http://www.idredge.com/ News and Equipment Exchange] (Latest global news and equipment) *[http://www.sandandgravel.com/ Dredging News] * [http://www.gbrmpa.gov.au/__data/assets/pdf_file/0003/7869/DredgeandSpoil.pdf Dredging and Spoil Disposal Policy] ([[pdf]])(from the [[Government of Australia|Australian Government]]) * [http://www.worldofboats.org/collection/view/bertha/21 World of Boats at Eyemouth ~ Bertha] Iron, Steam powered dredger or drag boat. 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