If a bridge is built, the bridge pillars can change local marine currents and affect the mix between surface and deep water. A similar situation could occur if artificial islands are built for the ventilation of a tunnel.

Streamlined pillars and ventilation islands
The impact from bridge pillars and ventilation islands can be minimised by careful design. This requires close cooperation between bridge and tunnel designers and the experts who carry out the environmental impact studies. They must find technical solutions that take into account the environment and the construction itself.

Femern A/S is studying how bridge pillars and islands affect water flow using physical experiments, field measurements and mathematical simulation models. We are recreating conditions in the Fehmarnbelt on a small scale in indoor water tanks, and carry out tests of various designs of bridge pillars and islands to see the effect. Such experiments have previously shown that it may be worthwhile to limit the numbers of bridge pillars and ventilation islands and to streamline the underwater parts in order to minimise water resistance.

Using simulation models, we will go on to study the significance of the observations on conditions in the

Fehmarnbelt and nearby shipping lanes and for water exchange into the central area of the Baltic Sea.

Oxygen to the Baltic
The water in the Baltic Sea comprises a surface layer of brackish water and a bottom layer of saltier water. The division is very stable, and the mixing of the two water masses is therefore limited.

In the central Baltic in particular, where the water is very deep and moves slowly, the strata has consequences for deep water. It means that insufficient oxygen is brought to the deep water to compensate for the oxygen consumed when organic material sinks to the sea bed and decomposes. The result of this is not just oxygen depletion in the deep water, but also the upward spread of the oxygen-depleted zone.

Fortunately, the deep water is renewed at regular intervals. Particular weather conditions bring in a powerful deep water stream of salty, oxygen-rich water from the North Sea, which enters through the Belt Sea, passing through via the Oresund and Fehmarnbelt into the Baltic proper. This inflow is vital to the ecosystem, and is for example a precondition for the survival of the cod population.