Flood control refers to all methods used to reduce or prevent the detrimental effects of flood waters.
London is protected from flooding by a huge mechanical barrier across the River Thames, which is raised when the water level reaches a certain point
Venice has a similar arrangement, although it is already unable to cope with very high tides.
The largest and most elaborate flood defenses can be found in the Netherlands, where they are referred to as Delta Works with the Oosterschelde dam as its crowning achievement.
Currently the Saint Petersburg Flood Prevention Facility Complex is to be finished by 2008, in Russia, to protect Saint Petersburg from storm surges. It also has a main traffic function, as it completes a ring road around Saint Petersburg. Eleven dams extend for 25.4 kilometres and stand eight metres above water.
Another elaborate system of floodway defenses can be found in the Canadian province of Manitoba. The Red River flows northward from the United States, passing through the city of Winnipeg. To protect the city from floods, the Manitoba government undertook the construction of a massive system of diversions, dikes, and floodways
The New Orleans Metropolitan Area, 35% of which sits below sea level, is protected by hundreds of miles of levees and flood gates. This system failed catastrophically, with numerous breaks, during Hurricane Katrina in the city proper and in eastern sections of the Metro Area, resulting in the inundation of approximately 50% of the Metropolitan area, ranging from a few inches to twenty feet in coastal communities.
Floodgates are adjustable gates used to control water flow in reservoir, river, stream, or levee systems.
Radial gates are rotary gates consisting of cylindrical sections. They may rotate vertically or horizontally.
Bulkhead gates are vertical walls with movable, or re-movable, sections. Movable sections can be lifted to allow water to pass underneath (as in a sluice gate) and over the top of the structure.
Hinged crest gates, are wall sections that rotate from vertical to horizontal, thereby varying the height of the dam. They are generally controlled with hydraulic power, although some are passive and are powered by the water being impounded.