Optimising dredging operations and minimising environmental impact with advanced hydro-sediment 3D modelling

 

Lorient, a port city in the Brittany region of northwest France, is known for its maritime history and large fishing industry. Situated along the southern coast of the Bretagne Peninsula, it lies near the Groix Island, a protected Natura 2000 habitat. Lorient Agglomération and associated stakeholders (later called Lorient Consortium) launched the Dour-Glaz project to improve knowledge of fine sediment dynamics in the estuarine and coastal area and to support shared decision-making for environmental and sediment management. DHI developed a 3D hydro-sediment model to simulate natural turbidity and dumping operations over one year. The model, validated with field data, showed that direct turbidity plumes generated at each dumping event are quickly dispersed offshore, with minimal impact on sensitive habitats. It also revealed that the dumping site is semi-dispersive with slow evolution of deposit material, but with regular resuspensions by waves. An optimised scenario has suggested possible cost reduction while reducing ecological risks and supporting more sustainable, collaborative coastal management. 

 

Challenge

Lorient Consortium faced increasing expectations in managing sediment from harbour maintenance activities. While dredging operations are managed by each stakeholder, the cumulative effects of dumping operations have raised concerns, particularly given the proximity of the disposal site to Groix Island’s ecologically sensitive marine environment. The area’s designation under Natura 2000 adds regulatory pressure to minimise ecological disruption. 

However, the Consortium lacked a clear data-driven picture of how fine sediments behaved naturally in the inner and coastal region: how they move, settle or disperse. The challenge was to provide a strong scientific shared-knowledge to guide discussions among stakeholders — including port authorities, scientists and environmental bodies — in developing a new sediment management plan.

 

Solution

To tackle the challenges, DHI worked closely with Lorient Agglomération to develop a detailed understanding of fine sediment dynamics in the region. The first step was to create a 3D hydrodynamics model of the area and to couple a fine sediment model, simulating natural turbidity dynamics over an entire year. Calibration was performed using in-situ turbidity measurements, taken at key locations in the Lorient bay and offshore. This allowed the team to analyse how natural turbidity plumes behaved in the complex study area under different conditions, such as waves, tides and river flows.

 

Next, DHI carried out simulations for all dumping operations over 2022–2023 using near-field/far-field integration with MIKE 3 Mud Transport's 'Disposal' sub-module. This allows for accurate consideration of vessel types, effective dredged sediment volumes and operation schedules. The model precisely calculates each dumping event, the resulting turbidity plumes released along the water column and the disposed mass at the sea bottom. 

 

The model tracks suspended sediment concentrations dispersed into the water column, the evolution of deposited materials and the potential impacts on sensitive areas around Groix Island. Monthly analyses were performed to compare dumping induced turbidity plumes and natural turbidity variability in the entire coastal area.

 

Finally, the high-performance 3D model was used to simulate an optimised approach for dumping operations over the season, reducing both operation time and environmental impact.

 

Results

DHI’s solution provided significant value to Lorient Consortium by enhancing their understanding of sediment dynamics in the region. The modelling revealed the strong influence of wave and tidal conditions on turbidity plumes, helping the client better understand and assess how dumping activities could affect water quality. The results showed that dumping operations had a minimal environmental impact, with sediment plumes dispersing quickly and remaining low compared to natural levels. Only a small amount of sediment, about 0.13% of the total offshore dumped mass, returned to the inner bay. The insights also demonstrated that the dumping site is semi-dispersive, with minimal long-term effects on sensitive habitats around Groix Island. 

By simulating an optimised dredging plan, DHI demonstrated how to reduce operational time and minimise environmental risks. This will contribute to Lorient Consortium’s new management plan, lowering costs and ensuring more sustainable practices for the future.