Dredging, as a process, is used to remove the deposits percolated underwater for the purpose of clearing the water pathway for ships to pass; to create adequate space to construct important bridges, dykes and dams and to weed out silt, intoxicants and pollutants from the bottom of the water.
What is Dredging Process?
Dredging process mainly involves carrying out excavation, of either naturally deposited sediments or man-made debris such as rocks, bottom sediments, construction debris, and refuse, and plant or animal matter on the bottom of either shallow seawater or freshwaters. In general terms, dredging implies digging up of the gathered sediments from the seabed and disposing them off at some other site.
The oldest known dredging activities are: dredging for peat excavation and maintenance dredging. Maintenance Dredging is a broader term which includes clearing of deposits and cleaning, widening or deepening of a water body using either a suction or scooping device (generally called a dredger).
Regular maintenance dredging is of huge importance in coastal regions which have large tidal activity and also in water bodies which are susceptible to become silted with sediments, sand and mud. The Lowlands of Netherlands and Flanders are the best example of such regions which require regular maintenance dredging.
For dredging, equipment called dredger is used to remove the deposited sediments from an inlet creek, waterway or ocean floor. Dredging process is a blended essence of following three independent elements: excavation, transportation of excavated material and then usage or proper disposal of dredged material.
History of dredging
With the beginning of civilization, there started the transportation of commodities by inland waterways and oceans. But this transportation depended on the ability of ships, which in turn largely depends on the water depth. Silting, the natural phenomenon of deposition of silt and sediments over the sea bed created a constant threat to the voyages of ships. People started fighting with the problem of siltation to ensure the safety of voyages but due to lack in equipment for removing siltation, they started manual digging up of the mud by hand which was not that efficient and limited to shallow waterways.
In 15th Century increase in trade at seas necessitated the development of some bed scratcher such as “Zeeuwse Krabbelaar”, which was a primitive bed leveller. These bed levellers cum scratchers were used to pick the sediments and dispose them. Development in these dredgers was carried out in certain phrases starting from ancient mills to modern suction dredgers.
Mills were developed around 1575, these were a sort of dredging equipment which was primarily employed for digging in ports. Mills had a rotating chain connected with wooden boards, these wooden boards dig up the mud.
At the primary stage of development of mills they were manually driven later they were powered by steam engines. Mills had gone obsolete in 1857 with the development of a suction dredger in the United States.
In 1867 there came a revolutionary development with the evolution of the design of suction dredger by a French engineer. He used successfully this suction dredger in dredging the Suez Canal. From then on, dredging by suction became more and more common.
Then came cutter suction dredger and trailing suction hopper dredgers in the 19th century. These are the modern dredgers and can avail efficient dredging. These were so efficient that they allowed shipping and dredging simultaneously without hindering the traffic.
Recent evolutions in dredging have optimisation of dredging process on its main focus rather than developing new dredgers. So standardizations of dredgers and equipment, and advancement in control and monitoring systems of dredgers improved dredging to a great extent.
Importance of Dredging
Dredging is an activity of huge importance in the maritime industry. It serves the following purposes:
- It helps in safer voyages by pacifying underwater traffic and by maintaining proper bottom clearance
- In some cases, dredging is indeed employed for the extraction of ocean gems such as shellfish and molluscs. So, dredging can help in exploring amazing varieties of these sea creatures.
- To channelize the construction of a network of bridges, dams or other civil engineering works in water, dredging is a primary requirement. These are only proper dredging tools which remove the requisite amount of underwater silt and other compositions and make the excellent constructions true that a civil engineer wonders
- Dredging is also important to make the marine eco-system pollution-free. It helps in eco-friendly disposal of deposited polluting toxicants and harmful materials, without causing any harm to the marine ecosystem.
- Dredging helps not only in preserving flora and fauna in water bodies but also used for recreational activities.
Type of Dredging Methods?
The dredging process is a combination of digging the soil in the water bed and removing or extracting that soil from the excavated surface. Hence, the types of dredging methods can be classified into three categories depending upon the how the debris is extracted from the site.
These methods are:
Mechanical dredging is the process in which the sediments are picked up using mechanical tools such as buckets, grabs etc.
Mechanical dredging is usually carried out near the shore line. Hence for removing sediment on land or shoreline, mechanical dredging is used.
The dredged sediment is picked up and placed in nearby land or in water or most of the times in another barge dedicated to carry the sediment. If the dredging is done near the shoreline, the sediment can be directly transferred to a truck or railway wagon.
The mechanical dredging operation can be done using a barge or operated from shore as well.
If the mechanical dredger is an onshore dredger it has a limitation of covering the area as it can only be used near the shore line. With barge type mechanical dredgers it can be operated in any water, however, it will be most effective near docks, piers etc.
The mechanical dredging can be a continuous process, however the quantity of the sediment will be limited to one scoop which is lifted every time to remove the sediment. The mechanical type is able to dredge hard compacted sediments and water carryover is way less than hydraulic dredging.
In the hydraulic dredging process, the sediment is removed from the dredged site by using pumps, usually centrifugal pumps, sucked into the pipe used to transport the sediment.
The sediment is mixed with water and made into a slurry mixture which makes it easier for the pump to transfer it. Depending on the pumping distance, a booster pump can be fitted in line to transfer the sediment to the nearest shore to maintain constant production rate. One of the major advantages of hydraulic dredging process is the elimination of additional transport medium or equipment as the sediments can be directly transported to shore facility, saving additional expenditure and time.
Advantages of hydraulic dredging are:
- Reduces capital costs
- Reduces use of equipment and transportation
- Reduces energy and emissions compared to mechanical dredging process
- Safe and efficient
- Hydrodynamic Dredging
The method of hydrodynamic dredging is normally used for maintenance of channel, port , harbour depths etc. If a new site needs to be freshly dredged, this method won’t prove useful and efficient. Once the area is dredged, it needs to be constantly maintained to avoid unsafe navigation by maintaining the required depths.
Related Read: Mastering Ship’s Navigation
This method utilises water injection technology, which injects large amounts of water using nozzles attached to a horizontal jet bar powered by pressurised pumps. As the water spray from the nozzle hits the water bed, it fluidizes the sediments, making them loose. These loose sediment which is close to the channel bed, flows down to the deeper areas due to natural current.
As the natural flow helps in sediment transport it makes this method very cost effective and much more environment friendly compared to the other two.
Different types of dredging ships or dredging vessels
As per the classification of the dredger, they can be further classified in to different types:
Bucket Dredger: The bucket dredger is usually a fixed type stationary dredger which rotates in arc by winches around the dredging site. The scrapping end is fitted with a bucket, which removes the sediment and when turned upside down, the sediments are unloaded on a brage.
Grab Type Dredger: This is a stationary dredger which has a grab as a dredging tool (Two equivalent scoops or shell operated hydraulically). Due to its design It is also known as clamshell dredger . There can be different designs of the grab and it can be used for deep water dressing.
Backhoe Dredger: Also known as fixed arm dredger, it is a stationary type dredger which is usually mounted on a barge or work near the banks. The dredging equipment is a half open shell with a fixed length hydraulic arm and is used in shallow waters and near harbour sites.
Suction Dredger: It is a stationary dredger normally used for mining sand. The suction pipe of this dredger is inserted into the sand deposit and water jets are used to bring the sand up from the excavation site. The sediment can be pumped by sucking the sediments into the pipeline and transferring it to the reclamation site or loaded into barges, depending upon the location and available transfer arrangement.
Cutter Suction Dredger: It is another stationary dredger with a cutter head as a dredging equipment to loosen the base which is to be dredged. Similar to the suction dredger, the sediments are sucked and pumped via a pipeline ashore or into barges. The cutter head can be of different designs and materials, depending upon the properties of the surface to be dredged.
Augur suction dredgers operate on the same principles as a cutter suction dredger, except that the mechanical cutting tool is a rotating Archimedean screw placed at right angles to the suction pipe
Trailing Suction Hopper Dredger: This is a self-propelled ship carrying the dredger equipment having a hold or hopper arrangement to fill it with excavated material while following a pre-set dredging operation. This type of dredger has an arrangement to open the bottom of the hold to unload the dredged material into the designated site. This kind of dredger is mainly used in open water such as canals, rivers, estuaries etc.
Reclamation Dredger: This is actually an assisting equipment in the dredging operation and not a dredger itself. It is used to empty the hopper barges sediments using suction pipe which can be lowered in the hopper barge hold. Additional water can be sprayed to make the sediment slurry for easier suction and transfer to the dedicated site or shore depot.
Hydrodynamic dredgers have only one type that is Water Injection Dredger, which has been explained earlier.
Few other types of dredger ships are:
Amphibious dredgers: These ships have the unique constructional feature of working out of the water surface using long legs as their base. They can be equipped with grabs, buckets or a shovel installation.
Air-lift dredgers: This dredger uses high pressure air jets instead of water jets for material flow at the mouth of the suction pipe.
Bed leveller: This type is used to level the bed surface which has recently been dredged. It consists of a long flat blade or heavy bar which is connected to a tugboat at the end and when it is pulled, it will level the surface on the dredged surface over short distances.
A hopper dredge is a ship that sucks up the sediment slurry and holds that slurry in the ship (hopper) until it gets to its destination A pipeline dredge sucks up the sediment slurry and pumps it through a pipeline directly to its destination
Effects of Dredging
But just like every coin has two sides, even dredging, in spite of all the positive attributes to the process, has negative points. These negative points are also important because they also tend to impact the existence and life of the marine habitat at large.
These negative effects of dredging are briefed as under:
- When dredging is carried out for marine engineering purpose or constructional work, a large amount of water is to be removed and stored at some other location. This creates a minor storage problem but extensively affects the life of flora and fauna prevalent in that water. This is due to change in mineral composition of water before and after both dredging and depositing it back in its source after construction.
- Dredging impacts the crops badly in the regions where it is carried out to de-silt the rivers which are primarily intended to quench the irrigation demand of the region. Because along with silt essential minerals also get dredged out leaving the resultant water and land lacking in nitrates and phosphates.
- Also if dredging is carried out to serve the cleaning purpose, the toxic waste to be removed alarms the safe disposal problem. If it is disposed on land to make the land barren and if in other less important water body harms the living creatures in that water body.
Related Read: Effects of Dredging on the Marine Environment
These ill-effects of dredging becomes insignificant in comparison to applications and importance of dredging but should be kept in mind and catered where necessary.
Dredging Industry has developed itself throughout the world in the last two decades.
It serves multi-purposes such as cleaning, maintenance, disposal, transportation, excavation, etc. at a time.
Dredging helps in making the water navigable and makes fishing easier even in shallow creeks. It helps in removing contaminants from the waterways and recreating damaged areas by reclamation works.
But an important aspect which we have to cater while dredging is safe dumping of material to be dredged. It should be disposed-off at a place where no significant landform and life-form get harmed. Also at some places dredging alters the mineral composition of the water; precautions should be taken, so as to oversee the alterations caused by the change in mineral composition.
By putting small things in place we can prevent the occurrence of a major catastrophe. Today with the advancement in dredging technology we have pushed dredging towards higher efficiency with lower environmental impact.
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