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Usually the main objectives of dredging is to recover material of value, or to create a greater depth of water.<ref name="About">{{cite web |title=EuDA - About dredging - Dredging |url=https://www.european-dredging.eu/Dredging |website=www.european-dredging.eu |access-date=3 July 2018 |language=en}}</ref> Dredging systems can either be shore-based, brought to a location based on [[Barge|barges]], or built into purpose-built vessels.
Dredging hascan significanthave environmental impacts: it can disturb [[Marine sediment|marine sediments]], leading to both short- and long-term [[water pollution]], destroydamage importantor destroy [[Marine ecosystem|seabed ecosystems]], and can release [[Legacy pollution|legacy human-sourced toxins]] captured in the sediment. These environmental impacts can significantly hurtreduce marine wildlife populations in some cases, contaminate sources of drinking water and interrupt economic activities such as fishing.
==Description==
===Suction dredgers===
[[Image:Pogłębiarka „Sawa”, głowica, Wisła, Saska Kępa, Warszawa 3.jpg|thumb|The [[dredge drag head]] of a suction dredge [[barge]] on the [[Vistula River]] in [[Warsaw, Poland]]]]
[[File:Sudging dredger Tuk87-47-10.jpg|thumb|sudging dredger in the [[Beaufort Sea]] used to build artificial islands for oil prospection]]
[[File:2022-10-06 01 Dredger PROVINCIA DE EL ORO NO I, III & IV.jpg|thumb|Cutter-suction dredger, ''Provincia de el Oro No 1'', in Puerto Bolivar, Ecuador]]
[[Image:Geopotes 14.jpg|thumb|The ''Geopotes 14'' lifting its boom on a canal in The Netherlands. (''gēopotēs'' is Greek for "that which drinks earth")]]
====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====
{{main|:Category:Dredgers}}
[[File:Dredger near the Dumbarton Bridge in California.JPG|thumb|[[Mallard II]] near the [[Dumbarton Bridge (California)|Dumbarton Bridge]] in 2021]]
As of June 2018, the largest dredger in Asia is "''[[{{MV |Tian Kun Hao]]''"}}, a {{convert|140 metres|m|adj=on}} long dredger constructed in China, with a capacity of 6,000 cubic meters per {{convert|6000|m3/hour}}.<ref name="xin">{{cite news |title=Asia's largest dredging vessel completes first sea trial |url=http://www.xinhuanet.com/english/2018-06/12/c_137249426_2.htm |archive-url=https://web.archive.org/web/20180613111953/http://www.xinhuanet.com/english/2018-06/12/c_137249426_2.htm |url-status=dead |archive-date=13 June 2018 |access-date=13 June 2018 |publisher=Xinhua |date=12 June 2018}}</ref> An even larger dredger, retired in 1980, was the U.S. Army Corps of Engineers ''[[{{ship|USAV |Essayons (|1949 ship)|Essayons]]''2}}, which was {{Convert|525.17|ft|m}} long.<ref>{{Cite book|url=http://hdl.handle.net/2027/uiug.30112008445915|title=The hopper dredge; its history, development and operation|series=Its Engineer manual, EM 1110-2-1410|publisher=Government Printing Office|year=1954|editor-last=Scheffauer|editor-first=Frederick C.|location=Washington, D.C.|hdl=2027/uiug.30112008445915}}</ref> The [[{{ship||Mallard II]]}}, a clamshell dredger that maintains [[levee]]s in [[San Francisco Bay]], has operated continuously since being built in 1936.<ref name="bcdc">{{cite web|url=https://bcdc.ca.gov/planning/reports/SaltPonds_Jun2005.pdf|title=Staff Report: Salt Ponds|publisher=San Francisco Bay Conservation and Development Commission|date=October 2005|access-date=2021-07-06|archive-date=2021-07-09|archive-url=https://web.archive.org/web/20210709183519/https://bcdc.ca.gov/planning/reports/SaltPonds_Jun2005.pdf|url-status=live}}</ref><ref name="twenty">{{cite web|url=https://www.southbayrestoration.org/sites/default/files/documents/2003.007.03edenlanding2020.12.18_bcdcoriginal_1.pdf|title=BCDC Permit NO. 2003.007.03|author=San Francisco Bay Conservation and Development Commission|publisher=California Department of Fish and Wildlife|date=2020-12-18}}</ref>{{rp|61}}
===Dredge monitoring software===
==Environmental impacts==
{{Globalize section|date=January 2023}}
Dredging can create disturbance todisturb [[aquatic ecosystem]]s, often with adverse impacts.<ref name="Brodie">{{Cite web|url=http://theconversation.com/dredging-set-to-swamp-decades-of-great-barrier-reef-protection-20442|title=Dredging set to swamp decades of Great Barrier Reef protection|last=Brodie|first=Jon|website=The Conversation|date=2 December 2013 |language=en|access-date=2019-09-21}}</ref><ref>{{cite journal | last1 = Newell | first1 = R.C. | last2 = Seiderer | first2 = L.J. | last3 = Hitchcock | first3 = D.R. | year = 1998 | title = The impact of dredging works in coastal waters: A review of the sensitivity to disturbance and subsequent recovery of biological resources on the sea bed | url =http://www.sussex.ac.uk/geography/researchprojects/coastview/dredging/Impact_of_Dredging_Oc_Bio.Ann_Rev.pdf | journal = Oceanography and Marine Biology: An Annual Review | volume = 36 | pages = 127–178 }}</ref> 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]].{{Citation needed|date=September 2017}}
Dredging can have numerous significant impacts on the environment, including the following:
The activity of dredging can create the following principal impacts to the environment:
* Release of toxic chemicals (including [[heavy metal (chemistry)|heavy metals]] and [[Polychlorinated biphenyls|PCB]]) from bottom [[sediment]]s into the [[water column]].<ref name="Bridges, T S. 2010">Bridges, T S., Gustavson, K. E., Schroeder, P., Ells, S. J., & Hayes, D. (2010). Dredging processes and remedy effectiveness: Relationship to the 4 Rs of environmental dredging. Integrated Environmental Assessment and Management, 6 (4), 619-630.</ref>
* Collection of heavy metal lead left by fishing, bullets, 98% mercury reclaimed [natural occurring and left over from gold rush era].<ref>{{Cite web|url=https://www.greatlakesnow.org/2019/06/restoring-howards-bay-legacy-lead-and-other-contaminants-are-targets-of-dredging-and-capping/|title=Restoring Howards Bay: Legacy lead and other contaminants are targets of dredging and capping.|last=Walsh|first=Melissa|website=Great Lakes Now|date=28 June 2019 |language=en-US|access-date=2019-09-22}}</ref>
* Short term increases in [[turbidity]], which can affect aquatic [[species]] [[metabolism]] and interfere with [[spawn (biology)|spawning]].<ref name="Brodie"/> Suction dredging activity is allowed only during non-spawning time frames set by fish and game (in-water work periods).<ref>{{Cite web|url=http://www.ukmarinesac.org.uk/activities/ports/ph5_3_2.htm|title=Timing of dredging and disposal as a means of minimising maintenance dredging impacts|website=www.ukmarinesac.org.uk|access-date=2019-09-22}}{{Dead link|date=August 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
* Secondary impacts to [[marsh]] productivity from [[sedimentation]]<ref name="fondriest.com">{{Cite web|url=https://www.fondriest.com/environmental-measurements/environmental-monitoring-applications/monitoring-turbidity-at-dredging-sites/|title=Monitoring Turbidity at Dredging Sites|website=Environmental Measurement Systems|language=en-US|access-date=2019-09-22}}</ref> and general [[Chemistry of wetland dredging|changes in wetland chemistry after dredging]].
* Secondary impacts to aquatic and benthic organisms' [[metabolism]] and mortality<ref>{{Cite news|url=https://www.theguardian.com/environment/2013/dec/11/the-facts-about-dredging|title=The facts about dredging|last=Milman|first=Oliver|date=11 December 2013|work=The Guardian|access-date=2019-09-21|language=en-GB|issn=0261-3077}}</ref>
* Possible contamination of dredge spoils sites<ref name="Bridges, T S. 2010"/><ref>{{Cite web|url=https://www.brisbanetimes.com.au/national/queensland/inquiry-to-probe-gladstone-harbours-leaking-bund-wall-20140107-30fnp.html|title=Inquiry to probe Gladstone Harbour's leaking bund wall|last=Moore|first=Tony|date=7 January 2014|website=Brisbane Times|language=en|access-date=2019-09-22}}</ref>
* Changes to the topography by the creation ofcreating "spoil islands" from the accumulated spoil.<ref>{{Cite web|url=https://spoilislandproject.org/spoil-island-project/|title=About the Spoil Island Project|website=Spoil Island Project|language=en-US|access-date=2019-09-22|archive-date=20 December 2019|archive-url=https://web.archive.org/web/20191220175716/https://spoilislandproject.org/spoil-island-project/|url-status=dead}}</ref>
* Releases toxic compound [[Tributyltin]], a popular [[biocide]] often used in [[anti-fouling paint]] banned in 2008, back into the water.<ref>{{Cite web|url=https://www.epa.gov/enforcement/case-summary-cleanup-settlement-addresses-contaminated-sediment-removal-blair-waterway|title=Case Summary: Cleanup Settlement Addresses Contaminated Sediment Removal in Blair Waterway, Washington State|last=US EPA|first=OECA|date=15 December 2015|website=US EPA|language=en|access-date=2019-09-22}}</ref>
The nature of dredging operations and possible environmental impacts causerequires that the industryactivity tooften be closely regulated and a requirement forrequires comprehensive regional environmental impact assessments withalongside continuous monitoring.<ref name="fondriest.com"/> TheFor example, in the U.S., the [[Clean 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 the [[United States Army Corps of Engineers|Army Corps of Engineers]].<ref>[[Clean Water Act]], {{USC|33|1311}}, {{USC|33|1362}}, {{USC|33|1344}}{{Citation needed|date=September 2017}}</ref> AsDue ato resultpotential of the potentialenvironmental impacts to the environment, dredging is often restricted to licensed areas only, with vessel activity monitored closely using automatic GPS systems.<ref name="fondriest.com"/>
==Major dredging companies==
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