Analysing salinity exchange during operation of ship lock

The Mahlbusen lake at Dornumersiel on the North Sea coastline of Germany has a size of about 11 hectares and is just one m deep. The lake has a high recreational value both and a high ecological importance. It is currently being drained by a sluice and pumping station into the North Sea.

The authorities plan the extend the lake area, build a new marina for sailboats, and expand the hydraulic structures with a ship lock so boats can pass between Mahlbusen and the North Sea. However, the operation of the ship lock will impact the salinity in Mahlbusen. An environmental and hydraulic analysis was carried showing increased salinity levels in Mahlbusen during the summer months. This salinity in the summer months is caused mainly by the increased number of openings to the North Sea.

The modeling showed that each transfer operation may result in a maximum amount of 483 cubic meters of salt water being transferred into Mahlbusen. In reality much lower salinities are expected in the summer months, as tidal fluctuation may alleviate the salt accumulation.

This can be verified in a detailed analysis, which was not part of the present investigation. In such an analysis more appropriate boundary conditions could be introduced in MIKE 11, but as a preliminary analysis this study has given valuable information about worst case scenario and the potential impacts on the ecosystem in Mahlbusen.