Posted on September 26, 2017
By Cory Levins, ASC
Dredging is the process of excavating sediment deposits, trash, and other debris from the harbor floor and disposing of them elsewhere. An essential aspect of harbor 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.
Do the benefits of dredging outweigh the costs?
Dredging Methods
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
•Clamshell – Clamshell devices use a bucket with two sides, which close together to pick up materials from the harbor 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, and much of the Panama Canal was dredged this way. However, this method is slower than others and has been phased out by more efficient technology over time.
•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 harbor dredging and creating shipping channels. However, many bucket dredges are powerful enough to rip through coral, sparking controversy among conservationists.
•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 harbors, as it is the best way to excavate sediment from tighter spaces.
Suction Type Dredges
•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 harbor 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.
•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
Dredging is an essential requirement for the maintenance of harbors, 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 harbors 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.
Tourism
If a harbor 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.
Habitat Creation
The material excavated during dredging can be used in many intuitive ways to create new habitats for fish, birds, and other wildlife. Rocks collected during the dredging process can be repurposed for use in state-approved artificial reef programs, which have been constructed in many areas around the U.S.
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.
Oyster Reef Restoration
Over-harvesting, pollution, and increased sedimentation can all lead to the eventual destruction of oyster beds. The New York/New Jersey Harbor has suffered a dramatic loss of its oyster beds over the last few generations, and this permanently altered the structure and function of the area’s ecosystem. Using dredged materials to reconstruct these oyster reefs could potentially restore this habitat and bring biodiversity back to the area.
“Renourishment” of Onshore Areas
The material removed from the ocean floor during dredging is usually a mixture of sand, mud, and rock. This material can be used to replenish coastal areas that have been worn away by erosion, replacing the land that has been lost over the years. If the material collected is sand, this can be used to extend the beach area of the region for recreational use or to attract visitors to the area.
Removal of Pollutants and Contaminants
Sediment deposits formed near cities and industrial areas 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.
Economic Impacts
The practice of dredging has played a vital role in the development of our global economy and the world’s economic trade systems. Without continued maintenance of our harbors and waterways, it would be impossible to import the necessary amount of goods to meet our ever-growing demand. Keeping our trading systems efficient is a crucial aspect of contributing to our global economy, which benefits from the effects of dredging in many ways.
Boosting Business
Dredging can allow a port or harbor to receive much larger ships with ease, contributing to the trade and economic development of that area. A greater number of large vessels frequently entering the harbor also creates hundreds of jobs in that area, benefitting the economy of the nearby community. A shipping port that can accommodate the passage of larger vessels also encourages visitation to the area, further boosting the economy of the surrounding towns and cities.
Social Development
Historically, the spread of information and ideas has been facilitated by large numbers of people routinely traveling between harbors, both within countries and internationally. This exchange of information has led to accelerated social development and improved the knowledge and awareness of communities throughout the world.
Globalization is what has made our world prosperous and well-educated, and, while in modern times we have other methods of communicating, this past social development led to revolutions, improved education, and economic progress across the continents.
Environmental Impacts
The impacts of dredging on the environment are varied and diverse, with both beneficial and harmful effects observed on wildlife and habitats surrounding dredging sites.
Beneficial Impacts
•Creation and Restoration of Animal Habitats – Materials excavated from the harbor floor during dredging can be repurposed and used to create artificial reefs, oyster reefs, terrestrial islands, or beaches. This material creates new habitats for all kinds of bird, fish, and mammalian species, boosting population numbers and increasing biodiversity in the local ecosystem.
•Reduction of Erosion and Flooding – Sediments collected from dredging can also be used in beach replenishment, to restore parts of the land that have worn away because of erosion. This replenishment extends the habitat of local wildlife, and can also prevent flooding, protecting people and animals living in coastal regions.
•Removal of Contaminants – Dredging can also be used as a form of remediation for sediments that are contaminated by pollutants, which often find their way into the water around larger cities and industrial sites. Physically removing the sediment, and, therefore, the contaminants, removes potentially harmful toxins from the environment. This removal promotes the health and well-being of the surrounding aquatic life, reduces the likelihood of exposure in the human population, and prevents the spread of pollutants to other areas. The removed material is either treated and repurposed following excavation or is disposed of in an environmentally friendly manner.
Negative Impacts
•Damage to Coral Reefs – Some methods of dredging are notoriously damaging to coral reefs and can cause extensive physical damage to corals. This destruction of a natural habitat can have a devastating impact on the local ecosystem, as the populations of fish and aquatic mammals that ordinarily live and feed on the reefs declines, significantly reducing the overall biodiversity of the area.
•Smothering of Seagrasses – Sediments that are not properly excavated and either repurposed or disposed of are often simply dumped elsewhere. Suddenly moving large amounts of sediment to a new location can negatively impact the local wildlife in many ways and can smother corals and seagrass. Seagrass is the main food source for many marine mammals, including manatees and sea turtles, and cutting off this food supply can seriously harm their population numbers.
•Release of Toxic Chemicals and Heavy Metals – Disturbing sediment on the ocean floor can release toxic chemicals and heavy metals into the water column. These can negatively impact the health of many marine species and can potentially contaminate food sources meant for human consumption.
•Increase in Turbidity – Dredging obviously causes a great deal of disruption to the ocean floor, which can disturb and release particles of sand and other minerals into the water. This release increases the turbidity (or cloudiness) of the water, which has been found to interfere with the normal breeding and feeding habits of aquatic life. The increase in turbidity can have such a profound effect on the spawning habits of fish that certain methods of dredging, such as suction dredging, are only permitted during non-spawning time frames.
The Future of Dredging
Harbor dredging is a fundamental aspect of harbor maintenance, allowing ports to function efficiently and adapt to increasing demands for goods from overseas. Larger trade ships create opportunities for an increase in business and prosperity and contribute significantly to the overall global economic climate.
Although dredging undoubtedly furthers the welfare of the people who benefit from improved trade, there is still a great deal of controversy surrounding the practice, due to its ill effects on the wildlife habitats and the ecosystem of any area to be dredged. Dredging is known to drastically disrupt coral reefs and increase turbidity, which disturbs the feeding and breeding habits of many aquatic creatures. However, dredging must continue for us to maintain effective trading practices.
Many dredging companies are actively seeking ways of continuing the essential practice of dredging in a way that allows for greater environmental sustainability. Future potential research aspects of dredging involve addressing the issue of environmental impact, to provide a method of continuing the practice without harming the surrounding ecosystems.
oMore Environmentally Friendly Dredging Methods – New dredging technologies are constantly in development, and methods with a reduced negative environmental impact are among the techniques most likely to be used in the future. These measures for developing new dredging technologies aim to reduce their carbon emissions and their impact on wildlife habitats.
•Developing Cumulative Effects Analysis – Designing a method of examining the cumulative effects of dredging on the environment is important for creating a more accurate picture of the environmental impact of the process. This accuracy will allow for more accurate critical analysis of the techniques used, and how they can be adapted to reduce their impact on the wildlife and ecosystems around dredging sites.
•Assessment of Contaminated Dredging Material – The development of techniques to determine the levels of contaminants in sediment is important, as it allows for a more accurate analysis of the potential threats posed to human and animal life.
Harbor dredging has paved the way for more efficient trade throughout the world and has formed a fundamental aspect of our global economic development. By creating opportunities for the enhanced movement of people and goods, it has undoubtedly furthered the development of humans across cultures and continents and must continue to ensure further advances in our economy.
The environmental impact of dredging is widely contested, with some claiming benefits to local habitats and wildlife and others fiercely opposing the practice. However, with new research and dredging techniques in constant development, there are now strict regulations surrounding where and what times of the year dredging can be performed.
Most companies aspire to adopt techniques of dredging that allow the process to be done efficiently and effectively, with minimal negative effects on the surrounding environment. This minimizing will allow for our continued success in global trade and tourism, without doing so at the expense of our aquatic wildlife.
Source: ASC
