The Hollandse Ijssel storm barrier
Why a dam?
The Hollandse IJssel (Dutch IJssel) connects Rotterdam with the North Sea. In the event of a flood, the river water would be unable to flow away because the rising seawater would stop it. The river would therefore easily burst its banks. There were two major reasons for finding a solution for the danger of flooding: firstly, the Hollandse Issel flows through the lowest lying area of the Netherlands. Secondly, this is one of most populous areas of the Netherlands.
The storm surge barrier
Initially, building a half-closed dam in the Hollandse IIssel did not seem a good idea, mainly because it would make shipping traffic impossible. Besides that, it had to be guaranteed that the water from the Hollandse IJssel could flow to the sea without any problems. A storm surge barrier, as used in the Oosterschelde, would bring relief. This storm surge barrier would only close at high water, so shipping traffic would not be faced with inconveniences throughout the rest of the year. Two towers were placed on both sides of the river and a pair of enormous doors measuring 80 meters wide were hung between the towers. These doors are able to be moved up and down vertically, and when there is a risk of flooding, the doors are lowered down into the water. In addition to the surge barrier, a lock was built for those ships that are too high to pass under the doors.