Encyclopedia
Dredging is miscellaneous excavation-type work or operation usually carried out underwater, in shallow sea or
fresh water area.
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.
In American usage any floating vessel equipped with dredging equipment is called a
dredge.
Types of dredging
- Capital dredging: carried out to create a new harbor or berth or waterway, or deepen an existing one in order to allow larger ships to use it. 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 trailing suction hopper dredge. Most dredging is for this purpose.
- Land reclamation: mining sand, clay or rock from the seabed and using it to construct new land. This is typically performed by a cutter-suction dredge or trailing suction hopper dredge.
- 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. This is typically performed by a cutter-suction dredge or trailing suction hopper dredge.
- Removing trash and debris from the bottoms of rivers and canals and harbors.
- Getting useful material from the seabed. One possible type in the future, is recovering natural metal ore nodules from the abyssal plains.
- Contaminant remediation. Dredging is often used to reclimate areas affected by chemical spills, storm water surges , and other soil contaminations. Disposal becomes an aproportionatly large factor in these operations.
- Anti-eutrophication. Dredging is an option for the remediation of eutrophied water bodies. As artificialy elevated phosphorus levels in the sediment aggrivate the eutrophication process, controlled sediment removal is occasionally the only option for the reclimation of still waters.
Suction dredgers
These operate by sucking through a long tube, like some
vacuum cleaners. A
plain suction dredger has no tool at the end of the suction pipe to disturb the material.
Trailing suction hopper dredger
A
trailing suction hopper dredger or
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. The largest hopper dredger in the world is Vasco Da Gama .and WD FAIRWAY 35000 m3
Cutter-suction dredger
In a
cutter-suction dredger or
CSD, the 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 D'Artagnan , and J.F.J. DeNul .
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 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 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. This technique is often used in excavation of
bay mud.
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 two largest backhoe dredgers in the world are Bean Excavations dredge Tauracavor and Great Lakes Dredge & Dock's dredge New York. Both feature a barge mounted Liebherr 996 excavator: see and .
Water injection dredger
A water injection dredger injects water in a small jet under low pressure into the seabed to bring the sediment in suspension, which then becomes a turbidity current, which flows away downslope, 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 witch makes measurement with most hydrografic equipment difficult and should make use of filtering to produce better results.
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:-
- .
- make a "pontoon dredger". It can float.
- 's Amphibious Dredger.
- 's Amphibious Dredger.
- 's Watermaster is said to be able to dredge and do various crane-type jobs in shallow water or on land.
- has 4 legs, each with a caterpillar track on its end.
- makes a backhoe dredger which has a pontoon hull and can float, and also has wheels on legs.
- Oliver Evans
...
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.
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, ", Dona Paula,
Goa 403 004
India, 7 - 9 December 2004: see http://www.nio.org for more information about
publisher etc.
Punaise is a submersible dredging pump station. See http://www.damendredging.com/html/en/specials.htm#PUNAISE .
made by 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 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 , which deposit it in the deep sea or on land.
When contaminated sediments are removed, or large volume inland disposal sites are unavailable, dredge sluries 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 .
In many projects, slurry dewatering is performed in large inland setteling pits, although this is becoming less and less proliferant as mechanical dewatering techniques continue to improve.
Similarly, many groups are performing research towards utilizing dewatered sediments for the production of concretes and construction block, although the high organic content of this material is a hindrance toward such ends.
History
WT Preston, a museum ship formerly a specialized river dredge, also called a "snagboat."
See also
- List of environment topics
