A flood wall (or floodwall) is a man-made primarily vertical barrier designed to temporarily contain the waters of a river or other waterway which may rise to unusual levels during seasonal or extreme weather events. Flood walls are mainly used on locations where space is scarce, such as cities or where building levees or dikes would interfere with other interests, such as existing buildings, historical architecture or commercial exploitation of embankments.
Flood walls are nowadays mainly constructed from pre-fab concrete elements. Flood walls often have "flood gates" which are large openings to provide passage except during periods of flooding, when they are closed. As flood walls mostly consist of relatively short elements compared to dikes, the connections between the elements are critical to the failure of flood walls (QED in New Orleans).
The substantial costs of flood walls can be justified by the value of commercial property thus protected from damage caused by flooding. Flood walls are almost solely used in cities, notably:
In September 2005, following Hurricane Katrina, New Orleans, much of which is located below sea level, was substantially flooded after its system of levees and flood walls failed due to soil conditions and poor design.