Dredging: The Double-Edged Sword of Marine Development

Since ancient times, waterways such as rivers and lakes have served as vital transportation routes for people and businesses moving goods across long distances. Dredging helps maintain these waterways by removing built-up sediment and sand, ensuring smooth passage for vessels.

Let’s take a deeper look into dredging, why and how it’s done, and how it impacts the environment.

What is Dredging?

Dredging is the process of removing sediment, plant material, and/or rock from the beds of waterways such as rivers, lakes, and canals with a dredge.

Some commonly dredged materials include:

• Sand
• Silt
• Clay
• Gravel
• Coral
• Rock
• Boulders
• Peat

Coastal dredging is a crucial practice that goes hand-in-hand with many coastal and marine infrastructure developments.

There are three main types of dredging projects: 

• Maintenance dredging: Maintains previously authorized depths in channels, harbors, canals, and other established water passages. This ensures continued safe navigation for ships and boats.
• Capital dredging: Carves out new channels, deepens existing ones, or expands port facilities. This creates space for larger ships, increases cargo handling capacity, or establishes entirely new ports and harbors.
• Remedial dredging: Combats various forms of pollution (like heavy metals, toxic chemicals, or industrial waste) and focuses on cleaning up contaminated areas and improving environmental quality.

Why is Dredging Important?

Dredging is an extremely important process in the maritime industry and, in many cases, is seen as a necessary process.

Some of the reasons dredging is needed include:

• Maintains Navigation: Ensures the safe passage of watercraft by reducing underwater obstructions and maintaining good bottom clearance.
• Supports Coastal Development: Allows for the creation of new ports, canals, and even land reclamation projects. This fosters economic activity and trade.
• Protects Coastlines: Beach nourishment, where sand is dredged from offshore to replenish eroding beaches, is crucial to safeguarding coastal communities from storm surges and flooding.
• Improves Water Quality: Removes pollutants like industrial waste from waterways, improving the health of the ecosystem.

How Does Dredging Work?

In the process of dredging, a dredge (a large flat marine vessel) is used to either scoop, drag, or pump up sediments and debris from water bodies.

The persons operating the dredge will lower a rotating head which is used to loosen up debris on the water body floor. While this is happening, a pump is used to suck up the material and place it on the dredge.

The removed sand and sediment are then transported to a pre-approved location for their final processing.

What Happens To Dredged Material?

The final location of the removed sediment depends on the site, the operations of the surrounding area, and local laws and regulations.

In some coastal areas, sand dredged from the coastline is used to rebuild, widen, and restore beaches (beach nourishment) that have begun to wash away due to coastal erosion.

If the sediment is deemed non-toxic and there is no alternative use for it, some dredging companies will drop their loads far out at sea.

Some dredged material is used for construction purposes, especially when the dredging is done to create space for development. Although not all sediment can be used, some may be helpful for changing landscapes and building foundations.

If the dredged material can’t be reused and ocean disposal isn’t feasible due to distance or safety concerns, it’s taken to a special containment facility. These confined disposal facilities are essentially sealed enclosures designed to prevent the dredged material from escaping back into the surrounding environment and contaminating it.

That said, environmentalists are constantly looking for new ways to sustainably reuse this dredged material. Some applications include:

• Restoration of Superfund sites
• Restoration of Brownfield lands
• Restoration of Green Space habitats
• Construction material (such as building roads)

Environmental Impacts of Dredging

Although dredging is seen as a necessary process, both for safety and functionality in our society, it does not come without environmental risks. In fact, dredging has been seen to have a major negative impact on the surrounding environment.

Dredging often disrupts lightweight sediment, causing it to drift downstream. This can have unseen environmental consequences beyond the immediate dredging area, highlighting the need for stricter measures to assess and mitigate its ecological impact.

Dredging disrupts the underwater environment in several ways. The churning action can destroy fish breeding grounds, destabilize riverbanks, and smother aquatic plants with settling silt. This reduces crucial oxygen levels and blocks sunlight, hindering plant growth.

A study published in the ICES Journal of Marine Science suggests that dredging has an array of negative effects on marine animals, as well as those that live close to water bodies.

According to the study, some direct impacts include:

• Collisions: Animals such as otters, alligators, whales, and fish often collide with dredges, or their propellers.
• High Noise Levels: Mammals that rely on sound for hunting are greatly affected by the noisy process of dredging. This sound also disrupts communication and can chase away prey from otherwise abundant areas.
• Sediment Plumes: Increased turbidity in the water can create sediment plumes that can travel great distances downstream, thus expanding the area of impact by a significant amount.

Moreover, the destruction of habitats and the potential spread of toxic sediments pose a threat not just to the environment and marine life, but also to human health.

Environmental Benefits of Dredging

Although dredging is seen, for the most part, as a negative environmental practice, some possible positives could arise from the activity.

The disturbance of sediment has been seen to enhance the diversity and abundance of fauna near dredged areas. This could be due to nutrients being released from sediment plumes which temporarily increases the availability of food sources for marine mammals.

Changes in topography can aid some predators in prey detection and capture. For example, bottlenose dolphins in Florida, have been found to prefer hunting in previously dredged areas utilizing the altered seabed to become more efficient hunters.

Dredging can briefly stir up food sources for filter feeders like oysters. This may lead to temporary growth spurts. However, this boost is often short-lived as the initial abundance is replaced by a decline in food diversity and overall availability.

Eco-Friendly Alternatives to Dredging

Traditional dredging has massive impacts on the environment, not only in the immediate area but in many surrounding habitats. Not to mention it’s incredibly expensive and often leaves a lot of waste products.

Luckily, environmentalists have been working on ways to conserve the ocean and water bodies.

One environmentally conscious alternative is biological dredging.

Bio-scientists have discovered a means of introducing specialized bacteria into water bodies that accelerates the decomposition of plant material and removes excess nutrients from the water.

The bacteria break down the organic matter into harmless byproducts like carbon dioxide and water, essentially “digesting” organic matter. This reduces the buildup and improves water quality.

Compared to mechanical dredging, bio-dredging offers several advantages:

• Eco-Friendly: It leverages natural processes and minimizes disruption to the ecosystem
• Cost-Effective: It’s generally less expensive than mechanical dredging
• Sustainable: By breaking down organic matter, it reduces the release of harmful nutrients

Unfortunately, bio-dredging has its limitations. Whilst it is most effective for organic matter, it may not be suitable for removing inorganic materials like sand or clay. It can also be a slower process compared to mechanical dredging.

FAQs