The one in Holland is a system of louvres that can be raised to keep out floodwater during the big storms they get every winter. Most of the country is below sea level, and the old medieval dykes used to fail from time to time with catastrophic results...so they built these things. I gather they have yet to fail despite some of the biggest North Sea storms ever recorded in the past 7 or 8 years.
what exactly "is" that?
