If a water level of 3.00 metres above NAP is anticipated for Rotterdam the Storm Surge Barrier in the New Waterway has to be closed. In these circumstances the Storm Surge Barrier computer - the Command and Support System (Dutch acronym BOS) instructs the Control System (BES) to shut the barrier. The BES implements the BOS's commands.

In the event of a storm tide, the docks are filled with water, so that the hollow gates start to float and can be turned into the New Waterway. Once the gates meet, the cavities are filled with water and the gates sink to the bottom, thus sealing off the 360 metre-wide opening. After the high water has passed the gates are pumped out and the structure begins to float again. Once it is certain that the next high water will not be another abnormally high one, the two gates are returned to their docks.

When the New Waterway is sealed off it is no longer possible for shipping to pass. The storm-surge barrier will only be closed in extremely bad weather – in probability once every ten years. A test closure will probably be conducted once a year in order to check the equipment. This will be done when there is little shipping. With the rise in sea levels the storm-surge barrier will need to close more frequently in 50 years time, namely once every five years.

Source: http://www.keringhuis.nl/engels/home_noflash.html