The Maeslantkering storm surge barrier
Moveable storm surge barrier
The original plans for the Delta Works did not include this particular barrier because the authorities wanted to keep the ports of Rotterdam and Antwerp accessible. However, when it was found that the dykes that were originally planned were insufficient to protect the population, it was decided to build a moveable storm surge barrier.
The huge barrier doors are in fact floating pontoons that can be filled with water. The additional weight makes them sink and turns them into a massive barrier. After the situation returns to normal the water is pumped out of the pontoons, which are then stored in a dry location. Everything is computer-controlled and represents another example of Dutch hydraulic engineering that has allowed the Dutch to survive the water.
Find more information here.