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25 August 2026

Underwater drone monitors sediment in the Fehmarnbelt

A torpedo-shaped underwater drone plays an important role in monitoring water quality around the construction of the Fehmarnbelt tunnel. The survey continuously provide data on the particles stirred up from the seabed during the works. See how the work is carried out the video at the bottom of the page.

On the surface, it may look like an ordinary day at sea. But underwater, a torpedo-shaped drone moves through the Fehmarnbelt, collecting data on sediment in the water.

Sediement consists of loose material such as fine sand, clay, mud and small stones. When work is carried out on the seabed, some of this material can be stirred up and carried away by the currents. Sediment can have a negative impact on the surrounding marine enviroment, and Sund & Baelt therefore continuously investigate how much sediment the work disperes and how it moves through the water.

The survey are carried out by enviroment experts from the international consultancy and research organisation DHI. From the monitoring vessel Dhiva, the autonomous underwater drone is deployed along a pre-planned route. It takes measurements close to the area where the work is taking place, without being connected to the vessel by a cable. This method makes it possible to collect precise data close to the source while limiting disruption to the construction work and reducing health and safety risks.

The monitoring programme has followed the construction work since excavation of the 18-kilometre tunnel trench began. At that time, large quantities of material were dredged from the seabed. The work has now entered a new phase, in which the tunnel elements are immersed and secured in the trench using, among oher things, stone, gravel and a aggregate. Small amounts of sediment can also stirred up during these operations.

The purpose of the surveys is to document the project's impact on the marine enviroment and to ensure that the contractor's work remains within the established limits for sediment spil. The measurement data are therefore used by Sund & Baelt, the contractors and the authorities alike.

DHI and Sund & Baelt have collaborated on surveys, modeling and environmental consultancy since the first preliminary investigations in the mid-1990s. The partnership therefore spans almost 30 years and has followed the project through planning, regulatory approvals and the current construction phase.