Dredging is the process of excavating sediment deposits, trash, and other debris from the harbour floor and disposing of them elsewhere. An essential aspect of harbour maintenance, dredging is routinely performed in ports around the world for a variety of reasons.
The main purpose of this process is to maintain the depth of existing ports or to create new Shipping Channels for Larger Vessels. Larger ships require a greater depth of water to access certain areas, and dredging is sometimes necessary to prevent them from running aground when they dock.
The global economy relies on shipping as a means of bringing trade and tourism to countries all over the world. Regular removal of excess sediment is therefore crucial for the maintenance of the shipping channels regularly used by large vessels.
Other benefits of dredging include flood prevention, restoration of onshore land, and the removal of contaminants from the sea floor.
Although dredging is undoubtedly necessary, there is a great deal of controversy surrounding the process, with several groups believing the process should be halted altogether for a range of environmental and economic reasons.
There are two main types of equipment used in dredging; scoop-type dredges, which lifts away sediment bit by bit, and suction-type dredges, which removes sediment at a consistent volume.
SCOOP TYPE DREDGES
1. CLAMSHELL – Clamshell devices use a bucket with two sides, which close together to pick up materials from the harbour floor. This device is usually suspended from an onboard crane and can remove large amounts of sediment with each scoop. Historically, this method has been widely used. However, this method is slower than others and has been phased out by more efficient technology over time.
2. BUCKET – This method uses many separate buckets, often attached to a wheel or chain, dragged along the ocean floor to collect and remove materials, which then travel back to the vessel to be emptied. Teeth are often placed at the opening of the buckets to allow them to dig more deeply into the sediment. This method is very effective for long distances, making this a popular choice for harbour dredging and creating shipping channels. However, many bucket dredges are powerful enough to rip through coral, sparking controversy among conservationists.
3. DIPPER – The dipper is a similar concept to the clamshell, but the bucket has only one side and is connected to a hydraulic arm, which lowers the bucket into the water to scoop materials from the ocean floor. This method is commonly used in harbours, as it is the best way to excavate sediment from tighter spaces.
SUCTION TYPE DREDGES
1. Cutter Suction Dredges – This type of dredging can be used to remove coral and soft rocks, making it an effective but highly disruptive method of increasing harbour depth. The dredger’s suction tube has a cutting mechanism at the opening, which breaks off and mashes up materials before they enter the tube. Cutter-suction dredgers are most commonly used in areas with hard surface materials, such as gravel or bedrock.
2. Auger Suction Dredges – These function in a similar way to the cutter suction dredgers, but the cutting tool used is an Archimedean Screw, which is set at right angles to the tube.
BENEFITS OF DREDGING
Benefits Dredging is an essential requirement for the maintenance of harbours, allowing them to remain functional and efficient, but there are many other economic and environmental benefits of dredging.
ACCOMMODATING LARGER VESSELS
Creating deeper channels in harbours allows for the passage of larger vessels, allowing for increased trade in and out of the area. The bulk of all goods imported into a country usually arrive on colossal ships, so ensuring their safe passage is vital for the health of the nation’s economy.
If a harbour can accommodate very large ships, this can make it a potential destination for cruise liners. These enormous vessels carry tens of thousands of people and can significantly boost the tourism industry of a city that is both appealing and accessible by cruise ship.
The material excavated during dredging can be used in many intuitive ways to create fish, birds, and other wildlife. Rocks collected during the dredging process can be used for resettling of reefs.
These offshore sites have been subsequently discovered to be productive fishing areas, suggesting they create suitable habitats for fish and other marine life. Dredged material can also be used to create islands and coastal uplands, providing valuable habitats for birds and other wildlife. These habitats increase biodiversity and promote the conservation of native species, preserving the ecosystem and promoting a more sustainable availability of natural resources.
REMOVAL OF POLLUTANTS AND CONTAMINANTS
Sediment deposits formed near cities and industrial are often contaminated with pollutants which usually find their way into the ocean from point sources such as sewer openings and industrial discharges. In this situation, dredging is often performed to limit to exposure of people and wildlife to potentially harmful contaminants and to prevent them from spreading to other areas of the Ocean.
WHY IS ROTOTANK™ INTERESTED IN THE DREDGING OF HARBOURS ?
Preservation of Our Oceans is part of RotoTank™ ethos, Protecting and preserving the oceans for the future generations to come. We highlight the problems with waste plastic on all our social media platforms to remind the General public to be mindful of pollution.
We as a company became more aware of the Plight and the problems associated pollution when we started our design and manufacture process of our RotoTank Pipe floats for the Dredging Community for the various Harbours Around the South African Coast Line.