Dredging is the removal of sediments and debris from the bottom of lakes, rivers, harbors, and other water bodies. It is a routine necessity in waterways around the world because sedimentation—the natural process of sand and silt washing downstream—gradually fills channels and harbors. Dredging often is focused on maintaining or increasing the depth of navigation channels, anchorages, or berthing areas to ensure the safe passage of boats and ships. Vessels require a certain amount of water in order to float and not touch bottom.
This water depth continues to increase over time as larger and larger ships are deployed. Since massive ships carry the bulk of the goods imported into the country , dredging plays a vital role in the nation's economy. Grab is mounted either on a dragline or hydraulic excavator. These can be of different types such as top open grab, top closed grabs and watertight grabs.
Grab dredgers can be of different capacities ranging between 1. Their capacity largely depends upon the crane power. Grabs are efficient in removing material in corners of dock and basins and also these make excavation closer to quay walls easier.
A revolving crane, fitted with a grab, placed on a hopper vessel or pontoon is known as a grab dredger. As the name suggests, it picks up the sediments at the seabed with a clam grabbing motion and discharges the contents.
Often used for excavating bay mud it also is useful to pick up clays and loose sand. These are also referred to as Dipper dredger and are somewhat similar to onshore excavators. These are used for harbour maintenance and shallow dredging. These are hydraulically driven excavators and consist of a half-open shell or a digging bucket which is capable of digging across a wide range of materials.
The shell or bucket is moved toward the machine and when filled, emptied in barges. Bucket capacity ranges between 0. Care should be taken while dropping this heavy and rigid bucket as it can cause damage to canal lining or quay walls.
Although they have few limitations where deep dredging is concerned but with some recent modern dredgers, deeper excavation is made quite easy. The key feature of hydraulic dredgers is that the material dredged by these type of dredgers is in suspension form and raised through the pumping system and fed to outlet pipes. These are most suitable for dislodging fine materials because it is easier to hold fine materials in the suspension than heavy gravels. Gravels and other powerful material can also be removed through hydraulic dredger by using greater power pumps.
Some common hydraulic dredgers are as under:. These are generally employed to remove sand or silt deposits from the seabed. They have a vertical suction pipe, which is pushed vertically inside the sand deposit and dredged material is sucked with or without water jet.
The dredged material is laden into barges or can directly to the reclamation area. These suction dredgers can be of two types viz. Rest of the working and design of cutter suction dredger is similar to the plain suction dredger but has the only difference that cutter suction dredger is equipped with a cutting tool which may be swinging arc.
CSD, as they are normally called, have a cutter head at the suction inlet which helps to loosen the earth and take it to the suction mouth. Used for hard surfaces like rock, CSDs suck up the dredged soil with the help of a wear-resistant pump and then discharge it through a pipeline or a barge.
A Trailing Suction Hopper Dredger is a self-driven dredging vessel. It consists of hoppers or trailers with bottom gates or valves. The material is loaded in hopper hydraulically when this hopper or trailer is filled, the bottom valves or gates are closed and the hopper is raised up with cranes or winches.
This dredger is most commonly used in open water: rivers, canals, estuaries and the open sea dredging. Suitable mostly for harbour maintenance and pipe trenching, a hopper dredger is a self-propelling vessel that holds its load in a large onboard hold knows as the hopper.
They can carry the load over large distances and can empty it by opening the bottom doors or by pumping the load offshore. Hopper dredges mostly dredge the soft non-rock soils and because of their high production rates can carry out land reclamation projects easily. It is a self-driven dredger that excavates sediments with strong water jets.
Strong water jet converts the sediments into suspension, as this suspension is heavier than water, it is carried away by water currents and gravity and disposed-off at a specific site. This type of dredger is generally used to dredge mud or fine sand bottoms and are more commonly used for harbour maintenance. There are a few different types of dredgers, some overlapping as hopper and cutter suction are both considered hydraulic dredges.
Mechanical dredge: these are the simplest type of dredgers. Mechanical dredging is manually removing material with a scooping bucket such as a backhoe, dipper, or clamshell.
Mechanical dredges are robust and work in tight areas, with minimal moving parts. Usually, a disposal barge, or scow, is required to hold the material.
Clamshells are used for loose material while backhoe and dippers are used for denser material. Bucket-chain: this is another type of mechanical dredger that uses a continuous chain of buckets that scoops material at the bottom and dumps it as the buckets tip over at the top. This is an old design that is no longer commonly used. Plain suction: these function like a big vacuum cleaner and just rely on suction alone to remove loose sediment. Cutter-suction dredges use a cutting tool that loosens material before removing it the same as a plain suction dredge.
This works for most materials because it breaks up the material and feeds it to the suction system. They can also deposit material directly to where it is needed, or store it on the vessel. Auger-suction dredges are similar, except that they burrow a hole into the surface. It works well for heavy-duty removal applications and deep removal. Jet-lift uses a high-volume stream of water to pull in material.
Hopper dredge is a vessel that can work in all types of sediments, is very nimble, and can transport the material to another location. They work well in rough seas, deep water, and in high-traffic areas. Dredger with a suction arm. Dredging vessel. Check out this video of Boskalis trailing suction hopper dredgers.
Hopper dredgers like the one above use dynamic positioning to stay on course for precise dredging. They lower a suction arm that uses water jets to loosen sediment, which is then sucked into the hopper to be stored. There are three options for the vessel to discharge the material: through rainbowing, or shooting the material out from a nozzle at the bow of the vessel, which works well when the intended target is land or just below water.
The sand can also be pumped to shore via pipelines for land reclamation or coastal projects. There are even doors in the bottom of the ship that can be opened to release the material to a specific location underwater. Where does all the dredged material go?
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