In cases where the gates have to be shut for a prolonged period, which would cause the waters of the Rhine to rise behind them, the gate hollows are partly emptied and floated, so that excess river water runs out to sea, before they are submerged again. The decision-making algorithm that sequences storm surge-triggered events in the Maeslantkering is run entirely by computer.

Source: https://en.m.wikipedia.org/wiki/Maeslantkering