- Constructed automatically around each Coastal Lowland tile belonging to the city; it protects them from flooding when sea level rises due to Climate change.
- Initial Production cost and per turn maintenance are variable based on the number of Coastal Lowland tiles in this city and the current sea level. The formula is (80 x coastal lowland tiles) + (80 x coastal lowland tiles x flood level).
- If constructed after some of the city's tiles have been flooded, those tiles can be repaired in full and used again, along with all that's on them. (Does not affect submerged tiles; these are lost forever and cannot be recovered.)
- Will not be pillaged if the city is captured.
- Must be built in a city with one or more Coastal Lowland tiles.
- Cannot be Purchased with Gold.
This building is a must-have for any city on the coast whose seashore doesn't consist of predominantly highland tiles (either Hills or such that aren't defined as Coastal Lowland). For such cities land is already scarcer than for inland cities, and having some of that land submerged in the later stages of the game is no fun at all, especially if the submerged tiles contain districts or wonders. Climate change is practically inescapable after the Atomic Era, and it can ruin your strategic goals for any coastal city. Granted, you could salvage some lost land with late game improvements such as the Seastead, but the usefulness of coastal tiles can never be matched by that of land tiles.
The catch when building Flood Barriers is that their price goes up along with climate change! Sometimes the price will practically double in the course of the construction (if the climate keeps worsening) and you will witness a never-ending project bogging down your production queue forever. And the only way to construct a Flood Barrier instantly is if you're the Suzerain of the Valletta city-state! This is why it's advisable to build Flood Barriers as soon as you're able, instead of waiting until the water rises to your chin. You can, however, use Military Engineer charges to complete 20% of the construction.
There is a long history of constructing earthworks to prevent flooding of low-lying habited areas. Levees to contain rivers have been built almost as long as urban centers have existed. Sea walls prevent the sea from inundating populated areas along the coast under most circumstances. These engineering feats are effective and can be long lasting if maintained—and they must be maintained or they can fail catastrophically.
The Pondicherry sea wall was built in 1735 and protected that city from a tsunami in 2004 that caused intense devastation in many other areas in the Indian Ocean.
- The Flood Barrier's model is at least partly based off the Oosterscheldekering.