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Environmental Impact of Dredging: Balancing Progress and Conservation

Posted on October 23, 2023

Dredging is vital for maintaining navigable waterways, supporting maritime trade, and removing contaminated sediments. However, it can disrupt ecosystems through increased turbidity, contaminant release, and habitat destruction. As a result, responsible management is needed to strike a balance between enabling economic progress through dredging and preserving the environment.

What Is Dredging Used For?

Dredging is excavating and relocating sediments from the bottom of harbors, rivers, lakes, and other water bodies. It is a routine necessity worldwide because the natural downstream flow of sand, silt, and other particles gradually fills channels and berthing areas critical for navigation and maritime trade.

Dredging primarily focuses on maintaining or increasing the depth of waterways for safe navigation by commercial and passenger vessels. As vessels grow, deeper channels have become essential, particularly for massive container ships and oil tankers. These larger vessels are crucial for global trade, making dredging indispensable to ensure shipping lanes and harbors are deep enough to accommodate them.

In addition to facilitating navigation and maritime commerce, dredging is also carried out for environmental remediation. Contaminated sediments containing industrial pollutants and municipal sewage discharges often accumulate in harbors and other industrialized water bodies. Dredging removes and isolates these harmful sediments from open water disposal, reducing the risk of ecological and human exposure to persistent toxins concentrated at the bottom of polluted sites.

Environmental Impacts of Dredging

Dredging activities can have adverse environmental consequences, ranging from localized effects near the dredging site to more far-reaching disturbances. The extent of these impacts depends on various factors, such as currents, tides, and other hydrological conditions, which can cause the consequences to propagate beyond the initial dredging area.

The extraction and disposal of bottom sediments alters the local physical conditions and ecological dynamics in affected water bodies. This process eradicates benthic organisms, such as small invertebrates, shellfish, and bottom-dwelling fish, while also dismantling intricate biological habitats that have developed over time on rock formations and substrates targeted for dredging.

In addition to the direct mortality of aquatic organisms, dredging indirectly affects local ecology by increasing water turbidity. As sediments are stirred and particles spill into the water, turbidity is raised, diminishing sunlight penetration crucial for aquatic plant and phytoplankton photosynthesis. This, in turn, diminishes photosynthetic activity and primary productivity, constraining the foundation of the marine food web.

Studies have found the increased turbidity caused by dredging activities can negatively impact the hunting and foraging abilities of seals, sea lions, and other marine mammals that rely heavily on eyesight for locating prey and navigating. Lastly, dredging in contaminated areas re-suspends pollutants like industrial chemicals, heavy metals, and toxins, making them bioavailable to surrounding organisms and entering wider food chains.

Current Regulatory Framework

Realizing the potential for environmental damage, international and national agencies have instituted permitting restrictions, sampling protocols, disposal guidelines, and standards to regulate dredging activities in recent decades.

At the international level, the United Nations Convention on the Law of the Sea (UNCLOS) governs mining and dredging in international waters via the International Seabed Authority (ISA). The International Maritime Organization (IMO) establishes global standards for shipping’s environmental aspects, including dredged material disposal, focusing on protecting seabed ecology and minimizing environmental impacts.

At the national level, government agencies, such as the US Environmental Protection Agency (EPA), uphold international guidelines by issuing permits, imposing equipment restrictions, conducting monitoring programs, and restricting disposal to pre-approved offshore sites.

Industry associations like the International Association of Dredging Companies (IADC) and research institutions offer technical guidance, advocate for best practices, and contribute to sustainable dredging methods.

Compliance with these regulations is paramount to ensure responsible and safe dredging, whereas violations can result in legal consequences and project delays.

Technology Innovations for Sustainable Dredging

In recent decades, improved technologies and more selective dredging techniques have enabled the extraction of required sediments while limiting dispersal and reducing environmental side effects. For example, the EU-funded Toolbot project developed a highly accurate remote-operated vehicle (ROV) that enables extremely selective and precise dredging, minimum turbidity, and optimized sediment transportation. This technology is particularly valuable for areas where standard dredging is restricted and for challenging locations like dams.

Haven Dredging, owned by Harwich Haven Authority, addressed emissions from dredging with their eco-friendly solution, Tiamat. This technology injects water into sediment and extracts diluted silt for release into the water column. It enables responsible sediment relocation within estuarine systems or through natural tidal currents, embodying the “Dredging with Nature” approach. Tiamat has already achieved a 50% cost reduction and a 65% decrease in greenhouse gas emissions, with a goal of a 90% reduction in the next two years.

Plume dispersion can be modeled using software tools like DHI Group’s PlumeCast to predict and mitigate ecosystem exposure. PlumeCast uses advanced mathematical modeling to streamline environmental assessments, optimize dredging operations, and mitigate risks while being accessible to experts and non-experts.

Balancing Act: Strategies for Dredging and Environmental Conservation

Incorporating environmental considerations into dredging projects can be achieved through several key strategies.

  1. Broad-Based Management: Evaluating dredging activities in the context of other waterway-related human activities to develop sustainable waterways while safeguarding natural resources.
  2. Management Tools: Various tools, including environmental windows, should be considered to minimize environmental impacts and enhance the efficiency of dredging and disposal operations.
  3. Setting and Monitoring Environmental Windows: Testing and refining various processes for assessing the need for environmental windows, along with their management and monitoring. This process should be piloted in a few districts to ensure consistency and effectiveness.
  4. Scientific Data and Information: Undertaking regular technical studies to assess the effects of dredging on biological resources, environmental stressors, and specific life stages. These studies should focus on integrating local and regional data and identifying gaps in scientific information for future research.
  5. Cross-Training Opportunities: Bridging the gap between engineering dredging projects and protecting biological resources by creating cross-training opportunities for resource managers and operators. This would help both parties understand each other’s challenges and needs.
  6. Stakeholder Collaboration: Proactively engaging stakeholders and partners to align project outcomes with their values, diversify benefits, and mitigate conflicts, ultimately contributing to the overall sustainability of the project.

Implementing adaptive management strategies

Adaptive management is increasingly advocated as the most promising approach to reconcile economic priorities with environmental concerns in dredging operations. It involves iterative cycles of goal-setting, ecosystem impact monitoring, data assessment, and refining practices to adapt to changing conditions.

Effective implementation relies on close coordination between project managers, regulators, scientists, and stakeholders, fostering ongoing learning and open communication. This approach ensures ecological protection while allowing major dredging projects to proceed.

Embracing adaptive management may require more initial effort in developing monitoring plans and impact thresholds, but it results in policies and practices that balance economic and ecological needs.

Successful Case Studies in Balancing Dredging and Environmental Preservation

Wheatstone LNG project

The Chevron-operated Wheatstone LNG Project in Western Australia involved extensive dredging and trenching to create navigable waters for LNG shipments and install a trunkline. Due to Western Australia’s stringent environmental regulations, the project implemented adaptive management to address potential environmental impacts from dredging, particularly protecting corals, macro-algae, seagrass, and filter feeder communities.

A tiered ‘trigger level’ approach for water quality monitoring was implemented to mitigate potential environmental impacts. These trigger levels, particularly for turbidity, determined various management responses, such as altering dredging methods, disposal procedures, or relocating the dredge area. Daily monitoring and assessment were performed, and benthic communities were regularly studied to protect critical habitats.

Proactive management using real-time data and forecast modeling helped address potential issues promptly, preventing unacceptable environmental impacts and allowing dredging activities to proceed effectively.

Vale Malaysia’s iron ore terminal project

In the development of Vale Malaysia Minerals Sdn Bhd’s iron ore terminal in Teluk Rubiah, Malaysia, an initial environmental impact assessment recommended the installation of a 14 km silt curtain to mitigate potential environmental impacts during dredging. However, due to cost concerns and uncertainty about its effectiveness, the dredging contractor proposed using adaptive management instead.

The adaptive management strategy involved daily and intensive dredge plume monitoring to ensure water quality remained within the required levels. Continuous communication among stakeholders enabled real-time adjustments, resulting in project completion ahead of schedule, safely, and at a significantly reduced cost without any environmental breaches.

The Path Forward

Finding an optimal balance between conservation priorities and development needs poses multifaceted socio-economic challenges without simple solutions. However, stakeholders increasingly recognize that the long-term sustainability of human communities and commerce fundamentally relies on maintaining the health of surrounding environments.

Collaborative efforts, including technological advancements, responsible operating practices, and adaptive management, offer a path to harmonize these priorities, ensuring a sustainable future.


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