Dredging

Dredging is miscellaneous excavator-type work underwater, usually in shallow sea or fresh water.
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.
American usage sometimes calls the ship or boat a dredge.

• These words also have other unrelated meanings: see #Other meanings of the word.

There are various types of dredgers.

• http://www.solentwaters.co.uk/Ships%20and%20Ports/Vessel%20Types%203/page4.html and http://www.solentwaters.co.uk/Ships%20and%20Ports/Vessel%20Types%203/page5.html describe several types of dredgers at length with images.
• http://www.clydesite.co.uk/clydebuilt/dredgers.asp is a list of dredgers built on the Clyde.

Types of dredging

• Capital dredging: carried out to create a new harbor or berth or waterway. It is usually carried out with a cutter-suction dredge.
• Maintenance dredging: deepening navigable seaports and waterways which have become silted with the passage of time, due to sand and mud deposited by water currents, until they may become too shallow for navigation. This is often carried out with a THSD = trailer hopper suction dredger. Most dredging is for this purpose.
• Removing sediment and trash and debris from the bottoms of rivers and canals and harbors.
• Land reclamation.
• Getting useful material from the seabed. One possible type in the future, is recovering natural metal ore nodules from the abyssal plains.
• Putting sand on beaches.

Suction dredgers

These operate by sucking through a long tube, like some vacuum cleaners.
A trailer suction dredger is so called because it trails its suction pipe when working.

Cutter-suction dredger

In a cutter-suction dredger or CSD, the suction tube has a cutter head at the end, to loosen the earth so it can be sucked up easier.

Auger suction dredger

This works like a cutter suction dredger, but the cutting tool is a rotating Archimedean screw set at right angles to the suction pipe.

Jet-lift dredger

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 dredger

An Airlift (dredging device) 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 diver. It works by blowing air into the pipe, and the air rises dragging water with it.

Bucket dredger

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 buckets attached to a wheel or chain.

Some bucket dredgers and grab dredgers are powerful enough to rip out coral reef to make a shipping channel.

Grab dredger

A grab dredger picks up seabed material with a clamshell grab, which hangs from an onboard crane, or is carried by a hydraulic arm, or is mounted like on a dragline.

Backhoe/dipper dredge

A backhoe/dipper dredge 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 largest backhoe dredger in the world, Bean Excavations dredge Tauracavor, features a barge mounted Liebherr 996 excavator: see Bean Excavation.

Water injection dredger

A water injection dredger injects water into the seabed to loosen the sediment, which then becomes a turbidity current, which flows away downslope or is carried away in natural currents.

Pneumatic dredger

Here, there is a chamber with inlets. The water is pumped out of it with the inlets closed. The inlets are then opened to let material in. The chamber is then pumped empty. The cycle is repeated. 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 the effect of a bulldozer.

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 for "scratcher".

Fishing dredges

There are types of dredges used for collecting scallops or oysters 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.

Amphibious dredger

Some of these are any of the above types of dredger, which can operate normally, or by extending legs 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. Some makes are:-

• Apollo Duck.
• Sandhurst Group Ltd (click on "Special Products" in the left margin) make a "pontoon dredger". It can float.
• Wright International's Amphibious Dredger.
• Richard Smalley's Amphibious Dredger.
• Inland Dredging Ltd's Watermaster is said to be able to dredge and do various crane-type jobs in shallow water or on land.
• This form by Gendredge Pty Ltd has 4 legs, each with a caterpillar track on its end.
• Susumo Engineering makes a backhoe dredger which has a pontoon hull and can float, and also has wheels on legs.
• Oliver Evans (1755-1819) in 1804 invented an amphibious dredger which was America's first steam-powered road vehicle.

Submersible dredger

These are usually used to recover useful materials from the seabed. Many of them travel on caterpillar tracks.

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, 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.

Punaise is a subersible dredging pump station. See http://www.dgn.nl/html/en/specials.htm#PUNAISE . (punaise is French for "bug".)

Spider made by Nexans crawls on the seabed on tracks and levels the seabed and lays cable. It is remote controlled from the surface.

I do not know if there are any neutrally-buoyant submarines which can dredge.

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 the dredgings

In a "hopper dredger", the dredgings end up in a big onboard hold called a "hopper", which has doors in its bottom. The excess water in the dredgings is spilled off by sedimentation: as the mud and sand settle to the bottom of the hopper, the water is siphoned from the top and returned to the sea to reduce weight and increase the amount of dredgings 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 opens the bottom hopper doors, dumping the slurry out. Or the hopper can be emptied from above. A suction hopper dredger is usually used for maintenance dredging.

Sometimes with a suction dredger the slurry of dredgings and water is pumped straight into pipes which deposit it on nearby land by pipes; or in barges (also called scows), which deposit it in the deep sea or on land.

External links to dredging companies

The companies marked # are claimed to be the largest dredging companies in the world.

• Bean Dredging
• Central Dredging Association
• Dredging Corporation of India Limited #
• Dredging International #
• Dredging Supply Company
• Ellicott Dredging
• Great Lakes Dredge and Dock Co. #
• IHC Holland
• International Association of Dredging Companies
• Jan de Nul #
• Royal Boskalis Westminster nv #
• Twinkle Co
• Van Oord Dredging #
• UDL Dredging
• Assemblers

See also

• list of environment topics

Other meanings of the word

• A dredge is also a piece of heavy equipment used to remove overburden in an open pit or strip mine, or, in the case of gold placer mining, to pick up ore and gravel and send it through sluices to remove gold and deposit the remaining rock, or tailings, behind the dredge.
• See Drag line excavator, otherwise called a dragline.
• In agriculture, dredge is the product of a harvest of barley and oats sown together.
• The verb dredge is sometimes used in cookery to mean sprinkling a powder (e.g. flour or icing sugar) over a food item.
• Dredger (comic character) was a tough Dirty Harry-type cop in the comic Action.