Protects Coastal Lowland tiles in this city from flooding. Initial build cost and per turn maintenance are variable based on the number of Coastal Lowlands in this city and the current sea level. If constructed after some of a city's tiles have been flooded, those tiles can be repaired in full. Submerged tiles however are never recoverable.
Must be built in a city with one or more Coastal Lowland tiles. Will not be pillaged if the city is captured.