Editing Dredging (section)

===Mechanical dredgers ===
[[Image:Dredging technique schematic.png|thumb|Bucket dredging]]
Some bucket dredgers and grab dredgers are powerful enough to rip out coral to make a shipping channel through [[coral reef]]s.<ref name=mechanical>{{cite web |title=Mechanical Dredger |url=https://www.european-dredging.eu/Mechanical_dredger |website=www.european-dredging.eu |access-date=4 July 2018}}</ref>
[[File:ErfgoedLeiden LEI001016474 Baggervaartuig Hollandsch Diep 4..jpg|thumb|Old Dutch bucket dredging vessel ''Hollandsch Diep 4'']]

====Bucket dredgers====
A [[bucket (machine part)|bucket]] dredger is 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 [[Bucket chain excavator|chain]].<ref name=mechanical/>

====Grab dredgers====
[[File:Dredging the Gowanus Canal (05880).webm|thumb|A [[grab (tool)|grab]] dredger working in the [[Gowanus Canal]], a [[Superfund]] site in [[Brooklyn, New York City]]]]
A [[Grab (tool)|grab]] dredger picks up seabed material with a [[Clamshell bucket|clam shell bucket]], which hangs from an onboard crane or a [[crane barge]], 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''', steel piles that can be lowered and raised to position the dredge.<ref name=mechanical/>

====Backhoe/dipper dredgers====
A backhoe/dipper dredger has a [[backhoe]] like on some [[excavator]]s. A crude but usable backhoe dredger can be made by mounting a land-type backhoe excavator on a [[Pontoon (boat)|pontoon]]. 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|date=May 2010}} They featured [[barge]]-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.<ref name=mechanical/>

'''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.

====Bed leveler====
[[File:Bertha being demonstrated on the Exeter Canal Basin - geograph.org.uk - 1055804.jpg|thumb|Steam dredger ''[[Bertha (drag boat)|Bertha]]'', built 1844, on a demonstration run in 1982]] 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. The chain-operated steam dredger ''[[Bertha (drag boat)|Bertha]]'', built in 1844 to a design by [[Isambard Kingdom Brunel|Brunel]] and {{asof|2009|lc=y}} was the oldest operational steam vessel in Britain, was of this type.<ref>{{cite web|url=http://www.worldofboats.org/boats/view/bertha/21/bertha|title=Bertha|work=World of Boats|publisher=Eyemouth Marine Centre|access-date=27 December 2009|location=[[Eyemouth]], Scotland|url-status=dead|archive-url=https://web.archive.org/web/20110604060413/http://www.worldofboats.org/boats/view/bertha/21/bertha|archive-date=4 June 2011}}</ref>

====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 the Dutch word for "scratcher".<ref>https://www.researchgate.net/publication/360967741_Sediment_deficit_and_morphological_change_of_the_Rhine-Meuse_river_mouth_attributed_to_multi-millennial_anthropogenic_impacts#pf7</ref>

====Water injection==== 
<!--How to classify?-->
A water injection dredger (WID) 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 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. Water injection results in a lot of sediment in the water which makes measurement with most hydrographic equipment (for instance: singlebeam echosounders) difficult.
{{Further|Dredge valve}}

====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.{{citation needed|date=July 2018}}

====Snagboat====
{{Main|Snagboat}}

A [[snagboat]] is designed to remove big debris such as dead trees and parts of trees from North America waterways.{{citation needed|date=July 2018}}

====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 invented the Oruktor Amphibolos, an amphibious dredger which was America's first steam-powered road vehicle.<ref>{{cite book| last1 = Evans| first1 = Harold | last2 = Buckland | first2 = Gail | last3 = Lefer | first3 = David | year = 2006 | title = They Made America: From the Steam Engine to the Search Engine: Two Centuries of Innovators | publisher = Back Bay Books | location = Boston, MA | isbn = 0-316-01385-4 | ref = {{sfnRef|Evans, Buckland & Lefer}} | url = https://archive.org/details/theymadeamericaf00haro }}</ref>

====Submersible====
These are usually used to recover useful materials from the seabed. Many of them travel on [[continuous track]]. A unique variant<ref>{{cite web|url=http://www.nio.org/past_events/inchoe/dredging_eia.jsp#dredging_1 |title=National Institute of Oceanography, India |access-date=14 June 2013 |url-status=dead |archive-url=https://web.archive.org/web/20090112162856/http://www.nio.org/past_events/inchoe/dredging_eia.jsp#dredging_1 |archive-date=12 January 2009 }}</ref> is intended to walk on legs on the seabed.<ref>"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]] 403 004 India, 7–9 December 2004</ref>

====Fishing====
{{main|Fishing dredge}}[[File:Dedge haul including live clams and empty shells.jpg|thumb|Dredge haul including live clams and empty shells]]

Fishing dredges are used to collect various species of [[clam]]s, [[scallop]]s, [[oyster]]s or [[mussel]]s 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 a [[fishing boat]]. Clam-specific dredges can utilize hydraulic injection to target deeper into the sand.<ref>{{Cite web|url=http://safinacenter.org/2015/05/fishing-gear-101-dredges-the-bottom-scrapers/|archive-url=https://web.archive.org/web/20150526080221/http://safinacenter.org/2015/05/fishing-gear-101-dredges-the-bottom-scrapers/|url-status=dead|archive-date=26 May 2015|title=Fishing Gear 101: Dredges – The Bottom Scrapers|author=Kristjan F. Olgeirsson|date=19 May 2015|website=The Safina Center|language=en-US|access-date=2019-09-21}}{{clarify|reason=archived copies of  this url have a different author|date=October 2020}}</ref> Dredging can be destructive to the seabed and some scallop dredging has been replaced by collecting via [[scuba diving]].<ref name=SPUMS1991>{{cite journal |author=Walker, Margaret |title=What price Tasmanian scallops? A report of morbidity and mortality associated with the scallop diving season in Tasmania 1990. |journal=South Pacific Underwater Medicine Society Journal |year=1991 |volume=21 |issue=1 |url=http://archive.rubicon-foundation.org/9414 |access-date=16 July 2013 |archive-date=20 October 2013 |archive-url=https://web.archive.org/web/20131020041442/http://archive.rubicon-foundation.org/xmlui/handle/123456789/9414 |url-status=usurped }}</ref>