Two-thirds of the planet's surface is covered with water in motion, evaporating, nourishing the land, providing drink, filling water tables and swelling streams and rivers. The extremes that cause flooding---storms and ocean events---are part of this "water cycle." In order to control the effects of one part of the cycle---flooding---we must manage our own land use and patterns of habitation.
Dams and levees made of earth and concrete were once the only flood-control devices used to hold back spring floods and storm waves, but the field of fluid dynamics has added more strategies to flood control over the past century. When water is forced out of one place by wind or more water, it moves to another, filling every space that is open to it until it is no longer under pressure. Any solid material that blocks its flow will be moved, and the debris that it carries along can do almost as much damage as the water itself. Water in its liquid form weighs about 3.63 Kilogram at sea level. If the volume of a sea wave is a million gallons, then a weight of 4,000 tons will be bearing down on anything in its path.
Managing Water's Movement
Every body of water is part of a system of pathways and collectors: streams, rivers, lakes and oceans. The water that flows through this system comes from the land that slopes downward into these pathways and collectors, areas we call "watersheds." When we build homes or alter landforms within these huge areas, the water has to go somewhere else, usually the least-elevated place available. The low places in a watershed around the bodies of water in it, the "floodplains," hold the key to flood management. Designing floodplains by excavating, building levees and adding engineered water retention ponds and waterways helps compensate for settlement in the attractive floodplain areas. Additional retention areas and levees or backup channels may result in islands in the flood and minimise damage. Today, governments that must manage watersheds try to discourage building in these sensitive areas, but many of these areas are filled with residential, business and recreational structures.
Building On the Edge
As long as the water's edge is beautiful, someone will want to build there, so emergency management recommends strategies to control the damage that is done by flash floods and storm surges. In addition to grading property to drain away from buildings, flood walls and swales (drainage ditches that fill only during flooding) serve to control floodwaters. Since water travels underground as well as above it, raising basements or eliminating sub-basements below the average flood level, a minimum elevation usually included in local building codes, can help minimise damage. In areas where elevating the entire structure is recommended---along seashores where hurricanes prowl, for example---stilts must be designed to stand against prescribed water pressures and anchored in concrete at a depth below where a surge would dig out the ground. Sewer backflow tanks, modifying some areas so water flows through instead of around or sealing specific areas of buildings to allow destruction of non-essential areas are mitigating strategies rather than ways to control flooding but may be a homeowner's only alternative to relocation.