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Dredging: A Necessary and Growing Marine Industry

But What are the Environmental Impacts of Moving ‘Muck’?



Dredging as part of restoration efforts at Prime Hook after Hurricane Sandy.  ©US Fish and Wildlife Service/Flickr/Public Domain
Dredging as part of restoration efforts at Prime Hook after Hurricane Sandy. ©US Fish and Wildlife Service/Flickr/Public Domain

Dredging rivers, channels, and ports is a common, global practice today that serves many purposes. Its importance to commerce, navigation, waterways maintenance, and preserving and protecting marine life is leading to a booming industry. The international market size for dredging is estimated to reach $18 billion by the end of 2024 and reach $25 billion in 2034, according to Fact.MR, a global market research group.


But especially for those who reside near dredging sites, it can be an eye-opener to dig more deeply into why dredging is done, how it is regulated, and to find ways to mitigate its environmental impacts.


Dredging is the process of removing accumulated material (sediments, debris, etc.) from water bodies, often to accommodate the passing of boats and ships. It is needed to maintain or increase the depth of navigation channels and harbors to guarantee that ships will not damage themselves or the water bottom. Thus, dredging depth tends to increase with ship size (such as with cruise and cargo ships).


Dredging Process

The dredging process generally involves three steps: excavation, transport, and utilization or disposal.


Excavation involves dislodging material through the use of a dredger (such as mechanical or hydraulic, each with their own advantages and limitations) from the bottom of a water body. Solid particles (sand, silt, clay, and shells) mostly comprise the excavated material, but potential contaminants including heavy metals (mercury, cadmium, arsenic, etc.), toxic substances (benzene, pesticides, etc.), and organic matter can be found, depending on the site.

Solid particles (sand, silt, clay, and shells) mostly comprise the excavated material, but potential contaminants including heavy metals (mercury, cadmium, arsenic, etc.), toxic substances (benzene, pesticides, etc.), and organic matter can be found, depending on the site.
Mechanical dredging to clear the Mississippi River.  ©US Army Corps of Engineers/Flickr. Public Domain
Mechanical dredging to clear the Mississippi River. ©US Army Corps of Engineers/Flickr. Public Domain

The excavated material is then transported, depending on the dredging method, to a utilization, disposal, or intermediate treatment site. Disposal sites include confined aquatic disposal (CAD), open water disposal, nearshore or upland confined disposal facility (CDF), and solid waste landfills. Dredged materials can be dumped into landfills even if contaminated as long as they are not hazardous waste.


The dredged material transported to confined disposal facilities has various potential uses (“beneficial uses”), such as for Superfund site remediation, construction material for roads, and habitat restoration. Otherwise, dredged material is commonly dumped in the ocean but with certain criteria such those recognized by the Environmental Protection Agency, including site location, interference with marine operations, and monitoring. In the US, dredging is typically managed through the Dredging Quality Management Program by the US Army Corps of Engineers.


Ports and Land Restoration

Cargo ships and passenger liners depend on dredging—ports are made accessible for the sake of both local and international economies and the safe passage of large ships. Throughout the US, over 400 ports and 25,000 miles of navigation channels are dredged and maintained. Dredging keeps smaller harbors clear for recreational use and is also useful for flood prevention by reducing sea levels. Occasionally dredging is necessary to lay pipes or construct pilings.

Throughout the US, over 400 ports and 25,000 miles of navigation channels are dredged and maintained.

In situations such as the recent Francis Key Scott Bridge collapse in Baltimore, Maryland, dredging becomes critical. The port in Baltimore is the gateway for a vast amount of imported goods, and commerce was impacted. Dredging needed to commence almost immediately to free the cargo ship; two other channels were dredged to allow marine transport to continue. Already 2,900 tons of debris have been dredged from the channel.


In 2015, the Army Corps of Engineers restored Mordecai Island in New Jersey. Erosive wave action had undermined this wildlife habitat to the point that approximately 50% of its area was essentially eroded, separating the island into two parts. Using dredged materials from the New Jersey Coastal Waterway, the separation between the two parts was filled in, and intertidal vegetation was planted on the deposited sediment to help secure the two lobes of the island and provide habitat for shorebirds. However, maintenance is crucial—the island may ultimately split into two lobes again if there is no additional sediment and stabilization by planting.


Aerial image of Mordecai Island, New Jersey, after restoration efforts. The central mound with vegetation on top is the sediment placement area.  ©R. Giannelli, NOAA NCCOS
Aerial image of Mordecai Island, New Jersey, after restoration efforts. The central mound with vegetation on top is the sediment placement area. ©R. Giannelli, NOAA NCCOS

Environmental Dredging

Environmental dredging is conducted to protect fish, people, and wildlife by containing the spread of contaminants to a widespread body of water. Contaminated sediments typically comprise undisturbed residuals (due to incomplete removal from initial dredging) and generated residuals (from dredging operations). Particularly in areas near cities or industrial activities, there is likely to be run off discharging a variety of pollutants.


For any environmental dredging project, the four “Rs”—resuspension (of sediments), release (of contaminants), residuals, and risk (assessment)—should be carefully considered, according to the Army Corps of Engineers.


Dredging can manage cyanobacterial blooms and resultant eutrophication by removing the topmost organic and nutrient-rich sediment. Eutrophication can increase the amount of plant and algae growth in estuaries and coastal waters through elevated levels of nitrogen and phosphorus. The National Oceanic and Atmospheric Administration estimates that approximately 65% of estuaries and coastal areas of the United States are already affected.


Hydro-Raking and Bio-Dredging

In areas where the water depth is not too deep (such as ponds, lakes, or shallow rivers), a mechanical hydro-rake can be used to remove organic matter and sediment. A hydro-rake is either a floating pontoon or barge fitted with a backhoe and rake attachments. It uses high pressure water jets to remove aquatic vegetation and can be used in water up to a depth of about 10 feet. Hydro-rakes can remove vegetation (emergent species, floating leaf species, and submersed species) while creating less sedimentation and environmental impact. However, it must be done with care otherwise it can damage underwater structures or habitats.

Biological dredging, or bio-dredging, can be done to reduce algae blooms and pollutants through the use of beneficial bacteria and organisms. The beneficial organisms can reduce excess nutrients and potentially reduce heavy metals accumulated in the sediment. However, this method is costly, time-consuming, can disrupt existing aquatic ecosystems, and introduces additional contamination.


Potential Drawbacks 

Even with careful management and consideration, dredging faces various environmental challenges, such as sediment disruption, habitat destruction (such as seagrass), water quality degradation, noise pollution (disrupting marine life), invasive species spread, chemical contamination, erosion and shoreline impact, and destruction of underwater cultural and heritage sites.

[D]redging faces various environmental challenges, such as sediment disruption, habitat destruction, water quality degradation, noise pollution, invasive species spread, chemical contamination, erosion and shoreline impact, and destruction of underwater cultural and heritage sites.

In rivers, excessive dredging and desilting can increase flood risk to downstream communities. It can also undermine fish habitats, cause sediment resuspension, and can sometimes churn up old industrial pollutants like heavy metals that have settled to the water bottom, adding to contamination levels. 


Dredging is also used for sand mining. The UN Environment Programme (UNEP) estimates that 50 billion tons of sand is used globally per year with 4 billion and 8 billion tons of marine sand extracted every year. Sand is essential for making the concrete that goes into construction and roads.


A trailing suction hopper dredger used to load sand in Belgium.  ©Wikimedia/Gordon Leggett (CC BY-SA 4.0)
A trailing suction hopper dredger used to load sand in Belgium. ©Wikimedia/Gordon Leggett (CC BY-SA 4.0)

Ocean floor dredging can destroy sensitive marine ecosystems. Often this dredging is unregulated and occurs in areas intended to be marine sanctuaries. In September 2023, the Marine Sand Watch was created by UNEP to monitor ocean floor dredging by employing short-range radio signals to chart the movement of larger boats, accounting for 60% of all dredging vessels worldwide. Algorithms and AI are used to analyze the motions of these boats and determine if dredging activity has occurred.  


Concerns from Dredging Projects

Several environmental groups filed a lawsuit in August 2022 (ultimately rejected by a federal judge in 2023) to prevent a dredging project in Puerto Rico’s biggest port on the grounds that it would be damaging to marine life—including corals, manatees, and sea turtles—if the $62 million dollar project to dredge the Port of San Juan went forward. The removal of nearly 3 million cubic yards of the marine floor will deepen the channel of San Juan Bay for large tankers to serve a natural gas terminal on Puerto Rico’s northern coast. While many see the expansion of the harbor as an investment in Puerto Rico’s future, othersincluding environmentalistsare asking, “At what cost?” 


The Savannah Harbor Expansion Project, which operated from 2015 to 2020, included an environmental assessment, including impacts on fish, plankton, and migratory bird species. As a result, the harbor project included a fish passage for the adversely impacted shortnose sturgeon, an endangered species.


Meanwhile, there can be controversy over how dredging and dredging materials are handled, as in the case of a CAD project in the Patapsco River in Maryland. Becoming involved with local dredging projects can go a long way in ensuring that thorough assessments of dredging sites are conducted to determine possible negative environmental impacts.

 

*Kate Pugnoli is an Arizona-based freelance journalist and former educator who works with nonprofit organizations. Her area of interest is in addressing environmental issues impacting marine biodiversity and conservation.

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