Effect of floating breakwaters on the flow in Os harbour, Hordaland, Norway

The municipality of Os is planning to deploy floating breakwaters in front of Os harbour to lower the waves in the port area. Floating breakwaters are a cost-effective alternative compared to fixed breakwaters, certainly in deep waters such as the Norwegian fjords.

Dam Engineering was involved in the environmental study to model the effects of the breakwaters on the currents and possible effects on wild salmon migration. The Os river, that discharges in the harbour, is one of the most valuable wild salmon rivers in the Bergen region and any negative effect on migration (~residence time in the harbour area) should be known beforehand.

At the location of the breakwater the water is highly stratified: a brakish toplayer and a saltier underlayer. The brakish toplayer is only a few meters thick and is fed by the fresh water discharge of the Os river and flows (generally) in the direction of the sea. The saltier underlayer flows in the opposite direction into the harbour.

The floating breakwaters are just inside the brakish part of the water column and that gives some (surprising!) effects on the currents and salinity in the area according to the 3D model.
Using the 3D finite-element model FINEL3d these flow circumstances are simulated. Both the present day situation and the floating breakwaters are simulated, so the difference in flow and salinity could be determined. The vertical resolution of the top layer of the model is 0.125m, while the horizontal resolution is 3m. Such a high resolution is required to accurately predict the currents in the stratified flow.