Oslo Fjord gets artificial respiration 

Having suffered from oxygen depletion for several years, the innermost part of the Oslo Fjord, Bunnefjorden, will now get artificial respiration. In spite of improved sewage purification techniques, oxygen levels in the fjord have not increased sufficiently. The poor water circulation at great water depths makes the marine environment in the fjord particularly sensitive, and the problem is now so serious that all flora and fauna have disappeared at depths below 80 m.

"This is the first time a marine restoration project has been aimed at a sea area of this size using such direct measures. We are pumping surface water down to a depth of 150m at seven cubic metres per second. This is equivalent to the water flow rate of a small river," says COWI Project Manager Carsten Jürgensen. He believes that the same method can be applied in other Danish and Norwegian fjords.

Together with its Norwegian subsidiary, Hjellnes COWI, and Professor Flemming Bo Pedersen of the Technical University of Denmark, COWI won the competition for ideas on how to restore oxygen levels in the fjord towards the end of 2001. The competition was launched by the technical committee working to restore the fjord's marine environment. COWI is about to commence the detailed design.

Innovative thinking

The concept behind the project is to transfer the light surface water of Bunnefjorden down to the bottom of the fjord using pumps. The pumps are to circulate the deep water while reducing the weight difference between the surface fresh water and the heavy, saline deep water. This will enhance the water renewal rate in Bunnefjorden, which in turn will revitalise the marine environment in the entire fjord.

The technical solution, which can be established and run at a relatively low cost, is however rather challenging. The more than 100m long PCB pipes must be anchored at the bottom of the fjord and fitted with the necessary electrical installations. Furthermore, the project team is aware that establishing pumps of that size in the fjord comes at a price.

"Experience from Danish lakes that have received artificial respiration through oxygenation or air injection shows that often improvements do not last once the respirator is turned off. We are aware that interfering with nature's ways can be risky. That is why this assignment has required innovative thinking and a great understanding of physics and hydraulics," says Carsten Jürgensen.

Published 28.01.2002