Types of Flooding
There are several types of flooding.
Overbank flooding: What most people think of when they hear the word "flood." Filled to capacity because of heavy rain or melting snow, the water within a river overflows its banks and spreads across the land around it. Sometimes the area covered is wide and flat; water tends to spread out and be slow-moving, and may not appear to travel at all. Common in the Midwest, this kind of flooding can take days to dissipate. In mountainous areas, where water flows together through steep valleys, the flood water tends to move faster and linger for a shorter duration.
Flash floods: Water from floods can take time to build up, allowing the population in an area time to be warned in advance. But sometimes flooding occurs quickly. Flash floods gather steam within six hours of the events that spawned them. They are characterized by a rapid rise of fast-moving water. Fast-moving water is extremely dangerous — water moving at 10 miles an hour can exert the same pressures as wind gusts of 270 mph (434 kph), according to a 2005 article in USA Today. Water moving at 9 feet per second (2.7 meters per second), a common speed for flash floods, can move rocks weighing almost a hundred pounds. Flash floods carry debris that elevate their potential to damage structures and injure people.
Ice jam flooding: In cold temperatures, bodies of water are often frozen. Heavy precipitation can cause chunks of ice to push together and create a dam in what is known as ice jam flooding. Behind the dam, water begins to pile up, spilling over to the plains nearby. Eventually, the wall of ice breaks, and fast-moving water rushes downstream much like a conventional flash flood, destroying objects in its path. The water carries huge chunks of ice, which can increase damage to surrounding structures.
Coastal flooding occurs along the edges of oceans, and is driven predominantly by storm surges and wave damage. This kind of flooding is usually connected to hurricanes, tsunamis or tropical storms. When low pressures occur in a storm over the ocean, they suck the water toward the center. As long as the eye is over deep water, problems are minimized, but as the storm moves toward land it carries a dome of water that can exceed 25 feet (7.6 meters) in diameter. When the dome reaches the shoreline, it can cause significant damage. At the same time, waves breaking along the shoreline assault beaches and structures, with destructive potential. In a hurricane, 9 out of 10 deaths are caused not by wind but by fast-moving storm surge.
Engineering issues: Flooding may be caused by manmade issues, as well. A weakly constructed dam could receive a more substantive battering than it was designed for and give way, creating a flash flood in the regions downstream.