Dredging is the excavation of material from a water environment. Possible reasons for dredging include improving existing water features; reshaping land and water features to alter drainage, navigability, and commercial use; constructing dams, dikes, and other controls for streams and shorelines; and recovering 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. Dredging has significant environmental impacts: it can disturb marine sediments, leading to both short and long term water pollution, destroy important seabed ecosystems, and can release human-sourced toxins captured in the sediment. Dredging is 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.
Dredging is a four-part process: loosening the material, bringing the material to the surface together extraction , transportation and disposal. The extract can be disposed of locally or transported by barge or in a liquid suspension in 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.
Ancient authors refer to harbour dredging. The seven arms of the Nile were channelled and wharfs built at the time of the pyramids BC , there was extensive harbour building in the eastern Mediterranean from 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. During the renaissance Leonardo da Vinci drew a design for a drag dredger. Dredging machines have been used during the construction of the Suez Canal from the late s to present day expansions and maintenance.
These operate by sucking through a long tube like some vacuum cleaners but on a larger scale. 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. As of [update] the largest trailing suction hopper dredgers in the world were Jan De Nul 's Cristobal Colon launched 4 July [8] and her sister ship Leiv Eriksson launched 4 September [9]. Main design specs for the Cristobal Colon and the Leiv Eriksson are: 46, cubic metre hopper and a design dredging depth of m.
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. As of , the most powerful cutter-suction dredger in the world is DEME 's Spartacus , which is scheduled to enter service in 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. Mud Cat invented the auger dredge in the 's.
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.
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. Some bucket dredgers and grab dredgers are powerful enough to rip out coral to make a shipping channel through coral reefs.
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. 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. A crude but usable backhoe dredger can be made by mounting a land-type backhoe excavator on a pontoon. 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. 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.
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 to a design by Brunel and as of [update] was the oldest operational steam vessel in Britain, was of this type. 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, 13 Aluminum Boat Trailer Wheels Weapon 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". 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. 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. A snagboat is designed to remove big debris such as dead trees and parts of trees from North America waterways.
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 Aluminum Boat Trailer Wheels And Tires Error 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 � in invented the Oruktor Amphibolos, an amphibious dredger which was America's first steam-powered road vehicle. These are usually used to recover useful materials from the seabed. Many of them travel on continuous track. A unique variant [18] is intended to walk on legs on the seabed.
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.
As of June , the largest dredger in Asia is " MV Tian Kun Hao ", a metres long dredger constructed in China, with a capacity of 6, cubic meters per hour. 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 Aluminum Boat Trailer Tires And Wheels Tracking to accurately record where the machine has been operating and to what depth the machine has dredged to. In a "hopper dredger", the dredged materials end up in a large onboard hold called a "hopper. 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.
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.
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. Dredging can create disturbance to aquatic ecosystems , often with adverse impacts.
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