Most of the goods we use every day, from fuel to food, clothes to construction materials, arrive in Ireland by ship. As an island nation, we rely heavily on maritime trade. According to the Central Statistics Office (CSO), in 2024, total throughput for all Irish ports reached 51 million tonnes, a 7% increase on 2023 (47.8 Mt). Dublin Port remains the busiest, handling approximately 49% of the country’s total cargo tonnage and 57% of vessel arrivals. The Port of Cork handled approximately 21% of the national total while Shannon Foynes, Irelands largest bulk port, carried approximately 22%. To keep our ports safe for large vessels to navigate, the seabed in busy harbours and navigational channels needs to be regularly deepened and widened. This is done through a process called dredging.
What is Dredging?
Dredging is the removal of sediment, silt, and debris from the bottom of harbours, rivers, or shipping channels. Over time, natural processes like sedimentation can cause these areas to become shallower, making it difficult for ships to navigate safely. With increased drainage upriver, sedimentation is a growing issue in most of our ports meaning more and more dredging is required to keep the ports open. Dredging helps maintain the necessary depth for vessels to enter and leave ports, a key part of keeping trade flowing.
There are different types of dredging:
- Capital dredging involves deepening or widening areas for the first time, often during port expansions or new developments.
- Maintenance dredging is ongoing work to keep existing channels/berths clear.
- Remedial dredging focuses on improving environmental conditions, such as removing polluted sediments from industrial areas.
The type of dredger used depends largely on the seabed composition. Mechanical dredgers operate on the principle of ‘hand-packing,’ using equipment such as buckets, grabs, and backhoes to physically scoop or excavate material from the seabed. Hydraulic dredgers remove material by mixing it into a water-based suspension and pumping it through pipelines, making them ideal for fine sediments but also capable of handling heavier materials with more powerful pumps, which include the Trailing Suction Hopper Dredger, Cutting Suction Dredger and Water Injection Dredger.
From Dublin Port, Shannon Foynes and the Port of Cork, dredging supports Ireland’s economy by enabling larger, more efficient ships to deliver goods quickly and safely.
What About Marine Mammals?
Ireland is home to a rich variety of marine mammals, including harbour porpoise, bottlenose dolphin, and grey and common seals. Marine mammals are protected under Irish and EU law, and some live year-round in or near active ports like Dublin Bay and in the Shannon Estuary.
One of the main concerns during dredging is the underwater noise produced by the machinery. While dredging is generally not as loud as operations like pile-driving or blasting, it can still disturb marine mammals if not properly managed. Marine mammals rely on sound to navigate, feed, and communicate, and may be affected by increased noise and activity. The impact depends on the frequency and intensity of noise produced.
The Role of Marine Mammal Observers
To reduce any potential risks, IWDG Consulting uses qualified Marine Mammal Observers (MMOs) to oversee dredging operations in Ireland. These professionals are trained to carry out visual monitoring before and during dredging operations, implement mitigation measures and ensure compliance with environmental guidelines issued by the National Parks and Wildlife Service (NPWS), specifically the Guidance to Manage the Risk to Marine Mammals from Man-made Sound Sources in Irish Waters (2014).
IWDG Consulting has been providing MMOs for dredging projects across Ireland, including in Dublin Port, Port of Cork, Shannon Foynes, Port of Waterford, and other regional ports. We have also worked in the major fishing ports of Killybegs and Castletownbere and at a smaller local scale such as Kilrush Marina and Belfast Lough. Our MMOs conduct pre-start watches for at least 30 minutes before dredging begins, monitoring a 500-metre mitigation zone. If a marine mammal is observed, the MMO will delay the start of dredging until the mitigation zone has been clear for at least 30 minutes. MMOs are deployed on dredging vessels or may observe from land.
In some cases, acoustic monitoring has been used not only to detect the presence of marine mammals during dredging campaigns, but also to help MMOs identify more suitable disposal sites at the time of dumping. Passive acoustic monitoring can be configured to send real-time alerts to MMOs when marine mammals are detected in the vicinity. By highlighting areas of higher or lower activity, this data allows MMOs to advise on relocating dumping operations to minimise potential disturbance. This real-time, proactive approach strengthens mitigation measures by enabling operations to avoid sensitive areas and reduce the risk of impacts on marine mammals. In addition to acoustic and visual monitoring, IWDG MMOs also collect behavioural data to support long-term conservation by helping assess how marine mammals respond to dredging activity in Irish waters.
What Does the Science Say?
Environmental monitoring has played a key role in major dredging projects at Dublin Port, including the Alexandra Basin Redevelopment (ABR) and MasterPlan2 (MP2), as well as ongoing maintenance dredging. IWDG Consulting has used static acoustic monitoring devices, initially C-PODs and now F-PODs, to detect harbour porpoise and dolphins at the offshore spoil grounds, where dredged material is deposited. The spoil grounds for Dublin Port operations lie within the Rockabill to Dalkey Island Special Area of Conservation (SAC), a protected site designated for harbour porpoises. Monitoring echolocation activity in this area helps assess whether dredging operations may be causing displacement or behavioural changes in marine mammals.
Dublin Port is also funding a PhD research project at ATU Galway to investigate whether dredging affects harbour porpoise habitat use including feeding behaviour, using the acoustic data collected during the ABR Project (2017–2021). Harbour porpoises are detected daily within the Rockabill to Dalkey Island SAC, and underwater acoustic devices can distinguish regular echolocation clicks from “feeding buzzes.” By analysing the ratio of buzzes to clicks, this research found that feeding activity increased during three out of four dredging campaigns compared to non-dredging periods. This suggests that dumping material at the spoil grounds may actually increase food availability. With appropriate mitigation measures in place, these findings indicate that dredging operations do not disrupt porpoise feeding as once feared, an important insight for future port developments.
A Balanced Approach
Dredging plays a vital role in maintaining Ireland’s ports and supporting economic growth but protecting our marine environment is just as important. Thanks to the work of IWDG Consulting MMOs, adherence to NPWS guidelines, and ongoing scientific research, it’s possible to find the right balance between these two priorities.
As our ports expand and prepare for future developments like offshore wind, it is essential that dredging continues to be carried out, but with care. With trained IWDG Consulting MMOs in place, we can help safeguard whales, dolphins, porpoises and seals while keeping Ireland’s maritime future moving forward.
Cloadagh Russell
IWDG Consulting Manager
consulting@iwdg.ie
References
Statistics of Port Traffic Quarter 4 and Year 2024 – Central Statistics Office
IADC | International Association of Dredging Companies
Different Types of Dredgers Used in the Maritime Industry
NPWS (2014) Guidance to Manage the Risk to Marine Mammals from Man-made Sound Sources in Irish Waters (2014)
Kootstra. C, Russell, C., Berrow, S., and O’ Brien, J. (submitted) The harbour porpoise (Phocoena phocoena) in Dublin Bay. How does dredge disposal affect foraging behaviour? Marine Mammal Science.