Dredging

Dredging is the operation of excavating material from a water environment (sometimes temporarily created) for the purpose of improving existing water features; reshaping land and water features to alter drainage, navigability, and commercial use; construct dams, dikes, and other controls for streams and shorelines; and to recover valuable mineral deposits or marine life having commercial value. In all but a few situations the excavation is undertaken by a specialist floating plant, known as a dredger. Dredging is carried out in many different locations and for many different purposes, but the main objectives are usually to recover material of value or use, or to create a greater depth of water.[1]All dredges can generally be classified as suction or mechanical.[1]

Dredge (PSF)
A grab dredge

Description

Dredging is the form of excavation carried out underwater or partially underwater, in shallow waters or ocean waters. It keeps waterways and ports navigable, and assists coastal protection, land reclamation and coastal redevelopment, by gathering up bottom sediments 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 kilometre long pipelines. Disposal can be to infill sites, or the material can be used constructively to replenish eroded sand that has been lost to coastal erosion, or constructively create sea-walls, building land [1] or whole new landforms such as viable islands in coral atolls.[3]

History

Ancient authors refer to harbour dredging. The seven arms of the Nile 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. At Marseille, 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]

During the renaissance da Vinci drew a design for a drag dredger.

Dredging machines have been used during the construction of the Suez Canal from the late 1800's to present day expansions and maintenance.[5] The completion of the Panama Canal in 1914, the most expensive U.S. engineering project at the time, relied extensively on dredging.[6]

Draga cavafango Leonardo - Museo scienza e tecnologia Milano
Draga cavafango Leonardo 2 - Museo scienza e tecnologia Milano
Reconstruction of the mud-drag by Leonardo da Vinci (Manuscript E, folio 75 v.)

Purposes of dredging

  • Capital dredging: dredging carried out to create a new harbour, berth or waterway, or to deepen existing facilities in order to allow larger ships access. Because capital 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.
  • 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.
  • Maintenance: dredging to deepen or maintain navigable waterways or channels which are threatened to become silted with the passage of time, due to sedimented 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 reservoirs or lakes.
  • Harvesting materials: dredging sediment for elements like gold, diamonds or other valuable trace substances. Hobbyists examine their dredged matter to pick out items of potential value, similar to the hobby of metal detecting.
  • Fishing dredging is a technique for catching certain species of edible clams and crabs. In Louisiana and other American states, with salt water estuaries that can sustain bottom oyster beds, oysters are raised and harvested. A heavy rectangular metal scoop is towed astern of a moving boat with a chain bridle attached to a cable. This drags along the bottom scooping up oysters. It is periodically winched aboard and the catch is sorted and bagged for shipment.[7]
  • Preparatory: dredging work and excavation for future bridges, piers or docks or wharves, This is often to build the foundations.
  • Winning construction materials: dredging sand and gravels from offshore licensed areas for use in construction industry, principally for use in concrete. This very specialist industry is focused in NW Europe, it uses specialized trailing suction hopper dredgers self discharging the dry cargo ashore.
  • Contaminant remediation: to reclaim areas affected by chemical spills, storm water surges (with urban runoff), and other soil contaminations, including silt from sewage sludge and from decayed matter, like wilted plants. Disposal becomes a proportionally large factor in these operations.
  • Flood prevention: dredging increases the channel depth and therefore increase a channel's capacity for carrying water.

Other

  • Beach nourishment: this is mining sand offshore and placing on a beach to replace sand eroded by storms or wave action. This enhances the recreational and protective function of the beach, which are also eroded by human activity. This is typically performed by a cutter-suction dredge or trailing suction hopper dredge.
  • Peat extraction: dredging poles or dredge hauls were used on the back of small boats to manually dredge the beds of peat-moor waterways. The extracted peat was used as a fuel. This tradition is now more or less obsolete. The tools are now significantly changed.
  • Removing rubbish and debris: often done in combination with maintenance dredging, this process removes non-natural matter from the bottoms of rivers and canals and harbours. Law enforcement agencies sometimes need to use a 'drag' to recover evidence or corpses from beneath the water.
  • Anti-eutrophication: A kind of contaminant remediation, dredging is an expensive option for the remediation of eutrophied (or de-oxygenated) water bodies; one of the causes is like mentioned above, sewage sludge. 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.
  • Seabed mining: is a possible future use, recovering natural metal ore nodules from the sea's deepest troughs.[7]

Types of dredgers

Suction dredgers

Pogłębiarka „Sawa”, głowica, Wisła, Saska Kępa, Warszawa 3
The dredge drag head of a suction dredge barge on the Vistula River, Warsaw, Poland
Geopotes 14
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, see Suction excavator.

These operate by sucking through a long tube like some vacuum cleaners but on a larger 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.

Trailing suction

A trailing suction hopper dredger (TSHD) trails its suction pipe when working. The pipe, which is fitted with a dredge 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.

The largest trailing suction hopper dredgers in the world are currently Jan De Nul's Cristobal Colon (launched 4 July 2008[8]) and her sister ship Leiv Eriksson (launched 4 September 2009[9]). Main design specs for the Cristobal Colon and the Leiv Eriksson are: 46,000 cubic metre hopper and a design dredging depth of 155 m.[10] Next largest is HAM 318 (Van Oord) with its 37,293 cubic metre hopper and a maximum dredging depth of 101 m.

Cutter-suction

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.[11]

As of 2018, the most powerful cutter-suction dredger in the world is DEME's Spartacus, which is scheduled to enter service in 2019.[12]

Auger suction

The auger dredge system 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 in the 1980s. In 1996, IMS Dredges introduced a self-propelled version operates without anchors or cables. They were primarily used for sludge removal applications from waste water treatment plants; since then they have been used river maintenance and sand mining.

The turbidity shroud on auger dredge systems creates a strong suction vacuum, causing much less turbidity than conical (basket) type cutterheads and so they are preferred for environmental applications. This and the ability to convey material to the pump faster makes auger dredge systems more productive than similar sized conical (basket) type cutterhead dredges.

Jet-lift

These use 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 is a type of small suction dredge. It is sometimes used like other dredges. At other times, an airlift is handheld underwater by a diver.[13] It works by blowing air into the pipe, and that air, being lighter than water, rises inside the pipe, dragging water with it.

Mechanical dredgers

Dredging technique schematic
Bucket dredging

Some bucket dredgers and grab dredgers are powerful enough to rip out coral to make a shipping channel through coral reefs. [14]

ErfgoedLeiden LEI001016474 Baggervaartuig Hollandsch Diep 4.
Old Dutch bucket dredging vessel "Hollandsch Diep 4"

Bucket dredgers

A 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 chain.[14]

Grab dredgers

Grab Dredging Process
Grab (clamshell) dredging in process in Port Canaveral, Florida

A grab dredger picks up seabed material with a 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.[14]

Backhoe/dipper dredgers

A backhoe/dipper dredger has a backhoe like on some excavators. A crude but usable backhoe dredger can be made by mounting a land-type backhoe excavator on a 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). 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.[14]

Bed leveler

Bertha being demonstrated on the Exeter Canal Basin - geograph.org.uk - 1055804
Steam dredger 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, built in 1844 to a design by Brunel and now the oldest operational steam vessel in Britain, was of this type.[15]

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".

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 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.

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.

Snagboat

A snagboat is designed to remove big debris such as dead trees and parts of trees from North America waterways.

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.

Submersible

These are usually used to recover useful materials from the seabed. Many of them travel on continuous track. A unique variant[16] is intended to walk on legs on the seabed.[17]

Fishing

Dedge haul including live clams and empty shells
Dredge haul including live clams and empty shells

Fishing dredges are used to collect various species of clams, scallops, oysters or mussels 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.[18] Dredging can be destructive to the seabed and some scallop dredging has been replaced by collecting via scuba diving.[19]

Notable individual dredgers

A large Asian dredger

As of June 2018, the largest dredger in Asia is "MV Tian Kun Hao", a 140 metres long dredger constructed in China, with a capacity of 6,000 cubic meters per hour.[20]

Dredge monitoring software

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 uses Real Time Kinematic satellite navigation to accurately record where the machine has been operating and to what depth the machine has dredged to.

Transportation and 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 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 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. These pipes are also commonly known as dredge 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.[21] Some even had the pipes or hoses customised to exact dredging needs etc. Other times, it is pumped into barges (also called scows), which deposit it elsewhere while the dredge continues its work.

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.

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, Geotube containers, 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.

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.[22]

Environmental impacts

Dredging can create disturbance to aquatic ecosystems, often with adverse impacts.[23][24] 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 PCB) from bottom sediments into the water column.[25]
  • Collection of heavy metals lead left by fishing, bullets, 98% mercury reclaimed [natural occurring and left over from gold rush era].[26]
  • Short term increases in turbidity, which can affect aquatic species metabolism and interfere with spawning.[27] Suction dredging activity is allowed only during non-spawing time frames set by fish and game (in-water work periods).[28]
  • Secondary impacts to marsh productivity from sedimentation.[29]
  • Tertiary impacts to avifauna which may prey upon contaminated aquatic organisms.[30]
  • Secondary impacts to aquatic and benthic organisms' metabolism and mortality[31]
  • Possible contamination of dredge spoils sites[25][32]
  • Changes to the topography by the creation of "spoil islands" from the accumulated spoil.[33]
  • Releases toxic compound Tributyltin, a popular biocide used in anti-fouling paint banned in 2008, back into the water.[34]

The nature of dredging operations and possible environmental impacts cause the industry to be closely regulated and a requirement for comprehensive regional environmental impact assessments with continuous monitoring.[35] The U.S. 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 Army Corps of Engineers.[36] As a result of the potential impacts to the environment, dredging is restricted to licensed areas only with vessel activity monitored closely using automatic GPS systems.[37]

Major dredging companies

According to a Rabobank outlook report in 2013, the largest dredging companies in the world are in order of size [2]

Notable dredging companies in North America:

Images

Dredge in the Port of Oakland with San Francisco in the background

Grab (clamshell) dredge Njord of the Manson Construction Co. fleet in the Port of Oakland, California

Dredge No-4 (1)

The excavator of a Yukon dredge.

Lakes Entrance Dredger April Hamer

April Hamer at Lakes Entrance, Victoria Australia

OrisantDredger

The Orisant a trailing suction dredger in the port of IJmuiden, Netherlands

Hong Kong dredger123

Grab dredging in Victoria Harbour, Hong Kong

Alexander von Humboldt (empty)

Alexander von Humboldt of the Jan De Nul fleet

BurigangaDredge

Sand mining

HR-Morris-Pipeline-Dredge

HR Morris of the Manson Construction Co. fleet, a Cutter Suction Pipeline Dredge, working on Mission Bay, San Diego, California

Barge on Neva river

Dredge ship with barges on Neva bay in Saint Petersburg, Russia

Dredge ship top view - 01

Top view of a suction dredger on the Nandu River, Hainan, China

Cutterhead of dredge Bill Holman Louisville Kentucky USA Ohio River mile 607 July 2002 file a2g092

Cutterhead of dredge Bill Holman, Louisville, Kentucky, USA, Ohio River mile 607, July 2002

See also

References

  1. ^ a b c d "EuDA - About dredging - Dredging". www.european-dredging.eu. Retrieved 3 July 2018.
  2. ^ a b "Dredging: Profit margins expected to remain fairly healthy until 2018" (PDF). 2013. Retrieved 26 March 2018.
  3. ^ "What is China's 'magic island-making' ship?". BBC News. 6 November 2017. Retrieved 4 July 2018.
  4. ^ Morhange, Christophe; Marriner, Nick; Carayon, Nicolas. "The eco-history of ancient Mediterranean harbours" (PDF). Retrieved 1 July 2018.
  5. ^ "New Suez Canal: A Dredging Triumph". The Maritime Executive. Retrieved 21 September 2019.
  6. ^ "Dredging Up the Past: The Panama Canal". U.S. Aqua Services. Retrieved 21 September 2019.
  7. ^ a b "EuDA - About dredging - Dredging - Reasons for Dredging". www.european-dredging.eu. Retrieved 3 July 2018.
  8. ^ "Jan de Nul's mega trailer Cristóbal Colón launched - Dredging News Online". Sandandgravel.com. 7 July 2008. Retrieved 14 June 2013.
  9. ^ "Keel-laying ceremony for Jan de Nul's Leiv Eiriksson held - Dredging News Online". Sandandgravel.com. 1 September 2008. Retrieved 14 June 2013.
  10. ^ "Jan De Nul Group". Jandenul.com. 7 June 2013. Retrieved 14 June 2013.
  11. ^ "DEME orders world's most powerful cutter suction dredge". Marine Log. 1 March 2017. Retrieved 13 November 2018.
  12. ^ "Keel laying signals start of world's most powerful CSD". Baird Maritime. 21 December 2017. Retrieved 13 November 2018.
  13. ^ Robbins, R (2006). "USAP Surface-Supplied Diving". In: Lang, MA and Smith, NE (eds.). Proceedings of Advanced Scientific Diving Workshop: February 23–24, 2006, Smithsonian Institution, Washington, DC. Retrieved 5 April 2013.
  14. ^ a b c d "Mechanical Dredger". www.european-dredging.eu. Retrieved 4 July 2018.
  15. ^ "Bertha". World of Boats. Eyemouth, Scotland: Eyemouth Marine Centre. Archived from the original on 4 June 2011. Retrieved 27 December 2009.
  16. ^ "National Institute of Oceanography, India". Web.archive.org. Archived from the original on 12 January 2009. Retrieved 14 June 2013.
  17. ^ "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
  18. ^ November 30, Capt Kristjan F. Olgeirsson; 2018 (19 May 2015). "Fishing Gear 101: Dredges – The Bottom Scrapers". The Safina Center. Retrieved 21 September 2019.
  19. ^ Walker, Margaret (1991). "What price Tasmanian scallops? A report of morbidity and mortality associated with the scallop diving season in Tasmania 1990". South Pacific Underwater Medicine Society Journal. 21 (1). Retrieved 16 July 2013.
  20. ^ "Asia's largest dredging vessel completes first sea trial". Xinhua. 12 June 2018. Retrieved 13 June 2018.
  21. ^ "Dredging & Discharge: Dredge Hoses". Max Groups Marine. Retrieved 8 January 2019.
  22. ^ Fonti, V. "To remediate or to not remediate?". Atlas of Science.
  23. ^ Brodie, Jon. "Dredging set to swamp decades of Great Barrier Reef protection". The Conversation. Retrieved 21 September 2019.
  24. ^ Newell, R.C.; Seiderer, L.J.; Hitchcock, D.R. (1998). "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" (PDF). Oceanography and Marine Biology: An Annual Review. 36: 127–178.
  25. ^ a b 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.
  26. ^ Walsh, Melissa. "Restoring Howards Bay: Legacy lead and other contaminants are targets of dredging and capping". Great Lakes Now. Retrieved 22 September 2019.
  27. ^ Brodie, Jon. "Dredging set to swamp decades of Great Barrier Reef protection". The Conversation. Retrieved 21 September 2019.
  28. ^ "Timing of dredging and disposal as a means of minimising maintenance dredging impacts". www.ukmarinesac.org.uk. Retrieved 22 September 2019.
  29. ^ "Monitoring Turbidity at Dredging Sites". Environmental Measurement Systems. Retrieved 22 September 2019.
  30. ^ "Monitoring Turbidity at Dredging Sites". Environmental Measurement Systems. Retrieved 22 September 2019.
  31. ^ Milman, Oliver (11 December 2013). "The facts about dredging". The Guardian. ISSN 0261-3077. Retrieved 21 September 2019.
  32. ^ Moore, Tony (7 January 2014). "Inquiry to probe Gladstone Harbour's leaking bund wall". Brisbane Times. Retrieved 22 September 2019.
  33. ^ "About the Spoil Island Project". Spoil Island Project. Retrieved 22 September 2019.
  34. ^ US EPA, OECA (15 December 2015). "Case Summary: Cleanup Settlement Addresses Contaminated Sediment Removal in Blair Waterway, Washington State". US EPA. Retrieved 22 September 2019.
  35. ^ "Monitoring Turbidity at Dredging Sites". Environmental Measurement Systems. Retrieved 22 September 2019.
  36. ^ Clean Water Act, 33 U.S.C. § 1311, 33 U.S.C. § 1362, 33 U.S.C. § 1344
  37. ^ "Monitoring Turbidity at Dredging Sites". Environmental Measurement Systems. Retrieved 22 September 2019.

External links

Airlift (dredging device)

An airlift is device based on a pipe, used in nautical archaeology to suck small objects, sand and mud from the sea bed and to transport the resulting debris upwards and away from its source. It is sometimes called a suction dredge. A water dredge or water eductor may be used for the same purpose.Typically, the airlift is constructed from a 3-metre to 10 metre long, 10 cm diameter pipe. A controllable compressed air supply vents into the inside, lower end of the pipe (The input end always being the lower end). Compressed air is injected into the pipe in one to three second bursts with an interval long enough to let the resulting bubble to rise to the higher, output end of the pipe. The bubble moves water through the pipe sucking debris from the lower end and depositing it from the upper end of the pipe. Ejected debris can be either cast off (as in simply removing overburden) or collected in a mesh cage for inspection (as more often is the case in nautical archaeology). It is often designed to be hand-operated by a diver.Airlift pumps are used by water utilities, farmers and others to extract water from deep wells. In such cases the pipes can be 30, 60 or more meters deep underground. Airlift pumps are governed by the physics of two-phase flow.

Boskalis

Royal Boskalis Westminster N.V. is a Dutch dredging and heavylift company that provides services relating to the construction and maintenance of maritime infrastructure internationally. The company has one of the world's largest dredging fleets, a large stake in Smit International and owns Dockwise, a large heavylift shipping company.As of 2018, Boskalis employs around 10,700 employees and 900 ships. They operate in over 75 countries in six continents.

Bugeye

The bugeye is a type of sailboat developed in the Chesapeake Bay for oyster dredging. The predecessor of the skipjack, it was superseded by the latter as oyster harvests dropped.

China Harbour Engineering

China Harbour Engineering Company Ltd (CHEC) is an engineering contractor and a subsidiary of China Communications Construction Company (CCCC), providing infrastructure construction, such as marine engineering, dredging and reclamation, road and bridge, railways, airports and plant construction. It is the second largest dredging company in the world, carrying out projects in Asia, Africa, and Europe.Being a major operation arm of CCCC on international level, CHEC’s more than 70 branches and representative offices have a global foot print in over 80 countries with a human resource strength of more than 15,000 undertaking hundreds of international projects in the total turnover of US$16 billion dollars.

Data dredging

Data dredging (also data fishing, data snooping, data butchery, and p-hacking) is the misuse of data analysis to find patterns in data that can be presented as statistically significant when in fact there is no real underlying effect. This is done by performing many statistical tests on the data and only paying attention to those that come back with significant results, instead of stating a single hypothesis about an underlying effect before the analysis and then conducting a single test for it.The process of data dredging involves automatically testing huge numbers of hypotheses about a single data set by exhaustively searching—perhaps for combinations of variables that might show a correlation, and perhaps for groups of cases or observations that show differences in their mean or in their breakdown by some other variable.

Conventional tests of statistical significance are based on the probability that a particular result would arise if chance alone were at work, and necessarily accept some risk of mistaken conclusions of a certain type (mistaken rejections of the null hypothesis). This level of risk is called the significance. When large numbers of tests are performed, some produce false results of this type; hence 5% of randomly chosen hypotheses might be (erroneously) reported to be statistically significant at the 5% significance level, 1% might be (erroneously) reported to be statistically significant at the 1% significance level, and so on, by chance alone. When enough hypotheses are tested, it is virtually certain that some will be reported to be statistically significant (even though this is misleading), since almost every data set with any degree of randomness is likely to contain (for example) some spurious correlations. If they are not cautious, researchers using data mining techniques can be easily misled by these results.

The multiple comparisons hazard is common in data dredging. Moreover, subgroups are sometimes explored without alerting the reader to the number of questions at issue, which can lead to misinformed conclusions.

Dredge-up

A dredge-up is a period in the evolution of a star where a surface convection zone extends down to the layers where material has undergone nuclear fusion. As a result, the fusion products are mixed into the outer layers of the stellar atmosphere where they can appear in the spectrum of the star.

The first dredge-up occurs when a main-sequence star enters the red-giant branch. As a result of the convective mixing, the outer atmosphere will display the spectral signature of hydrogen fusion: the 12C/13C and C/N ratios are lowered, and the surface abundances of lithium and beryllium may be reduced.

The second dredge-up occurs in stars with 4–8 solar masses. When helium fusion comes to an end at the core, convection mixes the products of the CNO cycle. This second dredge-up results in an increase in the surface abundance of 4He and 14N, whereas the amount of 12C and 16O decreases.The third dredge-up occurs after a star enters the asymptotic giant branch and a flash occurs along a helium-burning shell. This dredge-up causes helium, carbon and the s-process products to be brought to the surface. The result is an increase in the abundance of carbon relative to oxygen, which can create a carbon star.The names of the dredge-ups are set by the evolutionary and structural state of the star in which each occurs, not by the sequence experienced by the star. As a result, lower-mass stars experience the first and third dredge-ups in their evolution but not the second.

Dredging Corporation of India

Dredging Corporation of India Limited, or DCI, is unit engaged in the business of dredging. DCI does dredging for Indian seaports exclusively. It occasionally dredges at foreign seaports in countries such as Sri Lanka, Taiwan and Dubai. It is mainly involved in maintenance dredging. Almost all the maintenance dredging in Indian seaports is carried out by DCI due to government regulations. DCI is also involved in capital dredging, beach nourishment, and land reclamation. The main seaports in which DCI does business are Visakhapatnam Port, Haldia, Kandla, Cochin Port and Ennore Port.

DCI is headquartered at Visakhapatnam and has project offices at many seaports in India. It reports to the Ministry of Shipping.

It is ISO 14001:2004 and ISO 9001:2008 certified.

Fishing dredge

A fishing dredge, also known as a scallop dredge or oyster dredge, is a kind of dredge which is towed along the bottom of the sea by a fishing boat in order to collect a targeted edible bottom-dwelling species. The gear is used to fish for scallops, oysters and other species of clams, crabs, and sea cucumber. The dredge is then winched up into the boat and emptied. Dredges are also used in connection with the work of the naturalist in marine biology, notably on the Challenger Expedition.

Gold mining

Gold mining is the resource extraction of gold by mining.

Hammonton, California

Hammonton (formerly, Dredgertown and Dredgerville) is an unincorporated community in Yuba County, California. It is located 3.5 miles (5.6 km) south of Browns Valley, at an elevation of 131 feet (40 m).

A post office operated at Hammonton from 1906 to 1957. The original name was due to gold dredging which took place in the nearby Yuba Goldfields. The current name is in honor of W.P. Hammon, gold-dredging company official.

Harbor

A harbor (American English) or harbour (British English; see spelling differences) (synonyms: wharves, haven) is a sheltered body of water where ships, boats, and barges can be docked. The term harbor is often used interchangeably with port, which is a man-made facility built for loading and unloading vessels and dropping off and picking up passengers. Ports usually include one or more harbors. Alexandria Port in Egypt is an example of a port with two harbors.

Harbors may be natural or artificial. An artificial harbor can have deliberately constructed breakwaters, sea walls, or jettys or they can be constructed by dredging, which requires maintenance by further periodic dredging. An example of an artificial harbor is Long Beach Harbor, California, United States, which was an array of salt marshes and tidal flats too shallow for modern merchant ships before it was first dredged in the early 20th century. In contrast, a natural harbor is surrounded on several sides by prominences of land. Examples of natural harbors include Sydney Harbour, Australia and Trincomalee Harbour in Sri Lanka.

Kajima

Kajima Corporation (鹿島建設株式会社, Kajima Kensetsu Kabushiki-gaisha) is one of the oldest and largest construction companies in Japan. Founded in 1840, the company has its headquarters in Motoakasaka, Minato, Tokyo. The company is known for its DIB-200 proposal. The company stock is traded on four leading Japanese stock exchanges and is a constituent of the Nikkei 225 stock index.Kajima's services include design, engineering, construction, and real estate development. Kajima builds high-rise structures, railways, power plants, dams, and bridges. Its subsidiaries are located throughout Asia, Oceania, Europe, and North America. A downturn in the construction industry during the latter half of the 1990s prompted Kajima to expand its operations to the environmental sector, specifically waste treatment, water treatment, soil rehabilitation, and environmental consulting.

Lake Needwood

Lake Needwood is a 75-acre (300,000 m2) reservoir in Derwood, Maryland, USA. Located east of Rockville, in the eastern part of Montgomery County, it is situated on Rock Creek. The lake was created by damming Rock Creek in 1965 with the goal of providing flood control and reducing soil erosion. Lake Needwood also protects the water quality of the creek by functioning as a retention basin to trap sediment from storm-water runoff.

The lake is part of Rock Creek Regional Park. Visitors can rent pedal boats, rowboats, and canoes, and a flat-bottom pontoon boat, the Needwood Queen, is available for rides. Also, the picnic areas surrounding the lake are popular locations for various events. Other park features include a visitors center and snack bar, hiking and biking trails, playgrounds, an archery range and Needwood Golf Course. About one mile (1.6 km) southeast is Lake Needwood's sister lake, Lake Frank.

The Rock Creek Trail begins at Lake Needwood and can be followed along the course of Rock Creek, ending at the Potomac River in Washington, D.C.

Mersin Harbor

Mersin Harbor is a seaport facility in Mersin, southern Turkey. As one of the largest harbors in the country, it is Turkey's main gateway to the Mediterranean Sea. It was constructed during the 1950s as a major government project.

San Elijo Lagoon State Marine Conservation Area

San Elijo Lagoon SMCA (SMCA) is a marine protected area that protects the lagoon near Encinitas in San Diego County on California’s south coast. The SMCA covers .44 square miles. The SMCA protects marine by limiting the removal of marine wildlife from within its borders. San Elijo SMCA prohibits take of all living marine resources except operation and maintenance, maintenance dredging, habitat restoration including sediment deposition, research and education, and maintenance of artificial structures inside the conservation area per any required federal, state and local permits, or activities pursuant to Section 632, or as otherwise authorized by the department.Boating, swimming, wading, and diving are prohibited within the conservation area.

Sliedrecht

Sliedrecht listen (population: 24,993 in 2017) is a town and municipality in the western Netherlands, in the province of South Holland. The municipality covers an area of 14.01 km2 (5.41 sq mi) of which 1.18 km2 (0.46 sq mi) is covered by water.

The first IKEA store in the Netherlands opened in Sliedrecht in 1978, but it closed in 2006. Sliedrecht is also the site of the Nationaal Baggermuseum (National Dredging Museum) - featuring dredging and salvage.

St. Sebastian River

The Saint Sebastian River is a river in the U.S. state of Florida, a tributary of the Indian River Lagoon west and north of the city of Sebastian.

Stanegarth

Stanegarth was built in 1910 as a steam-powered tugboat by Lytham Ship Builders Company for service with the British Waterways Board. She was converted to diesel power in 1957. The tug used to tow three dredging hoppers, each crewed by two men, on the trip to and from Gloucester to Purton.On 6 June 2000 she was scuttled at Stoney Cove to produce an artificial reef suitable for wreck diving. The wreck now sits in 20 metres (66 ft) of fresh water and measures more than 18 metres (59 ft) long with a beam of 5 metres (16 ft). A plaque attached to it reads "Stanegarth project by Stoney Cove and Diver Magazine June 2008".

Surface mining

Surface mining, including strip mining, open-pit mining and mountaintop removal mining, is a broad category of mining in which soil and rock overlying the mineral deposit (the overburden) are removed, in contrast to underground mining, in which the overlying rock is left in place, and the mineral is removed through shafts or tunnels.

Surface mining began in the mid-sixteenth century and is practiced throughout the world, although the majority of surface coal mining occurs in North America. It gained popularity throughout the 20th century, and surface mines now produce most of the coal mined in the United States.In most forms of surface mining, heavy equipment, such as earthmovers, first remove the overburden. Next, large machines, such as dragline excavators or Bucket wheel excavators, extract the mineral.

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