Floods: When water attacks

Russell McLendon has a long, information-packed piece on flooding in Mother Nature Network: … The 2010 flood season may be unusually severe, but as many U.S. watersheds grow more crowded, paved and stormy, their residents will likely face similar dangers long into the future. And while floods are famously difficult to forecast — with influences ranging from rainfall and snowmelt to soil moisture, ground cover and local geology — people's precautions and reactions can still mean the difference between life and death. …A single flood may fit multiple categories, but floods are generally classified as one of the following:

• Flash floods: Most deaths and damage from floods are due to flash flooding — "a rapid and extreme flow of high water into a normally dry area, or a rapid rise in a stream or creek above a predetermined flood level," according to the National Weather Service. …

• Slow river floods: Rising waters may spur flash floods in steep, narrow river basins, but in flatter, wider ones, flooding tends to be slow, shallow and long-lasting. Flat floodplains can remain inundated for days or even weeks, but these floods are at least usually easier to predict than flash floods. ….

• Coastal floods: Storms and earthquakes are the two leading causes of ocean floods. Hurricanes push walls of sea water ashore when they hit land, creating a saline flash flood known as a "storm surge." Storm surges are often responsible for the majority of deaths from tropical cyclones, as was the case in New Orleans following Hurricane Katrina in 2005. ….

• Ground failures: Some floods attack from below, as the water table rises to the surface and washes away chunks of topsoil. This can cause a variety of ground failures, including "subsidence," or sinking soil, and "liquefaction," a process in which water-soaked sediment loses strength and acts like a liquid….

• Lake floods: Most lakes experience fluctuating water levels, but they usually don't "flood" the way rivers do because lakes typically have outlet streams or rivers to help them drain. But not all lakes have such outlets, and these "closed-basin lakes" are prone to potentially catastrophic floods if their water level rises too high. ….

….Scientists with the National Wildlife Foundation reported in January that the recent flurry of winter storms indicates U.S. weather is already showing symptoms of climate change, although many experts discourage such specific attributions. Climate change could affect flooding in several ways, though, depending on the region and type of flood in question…..

The 1979 Easter flood in Jackson, Mississippi, from the National Weather Service