Science of flash floods

U.S. flooding kills more than any other single weather hazard
Remy Melina and Karen Rowan MSNBC 11 Jun 10;

The flash floods that swept through Arkansas today, taking the lives of at least 16 people and leaving others missing, are a reminder of a little-known fact: Flash floods are the No. 1 cause of weather-related deaths in the United States, according to the National Weather Service.

Two key factors that lead to flash flooding are the intensity of the rainfall and its duration. For this reason, most flash flooding is caused by slow-moving thunderstorms, thunderstorms that move repeatedly over the same area, or heavy rains from hurricanes and tropical storms, according to the National Oceanic and Atmospheric Administration (NOAA).

Intense rainfall led to the Arkansas flash flooding, causing the Caddo and Little Missouri rivers to rise quickly overnight, sometimes faster than 8 feet (2.4 meters) per hour, according to news reports.

The water doesn’t always come from rain. A dam or levee failure, or a sudden release of water held by an ice jam can also unleash a flash flood. The topography of the region, the soil conditions, and ground cover also play significant roles.

The force of a flash flood can roll boulders, rip trees out of the ground, and destroy buildings and bridges.

True to their name, these floods occur suddenly – within a few minutes or hours. Rapidly rising water can reach heights of 30 feet or more, and to make matters worse, the same rains that produce flash floods can also trigger catastrophic mud slides.

Most flood-related deaths occur in automobiles, so NOAA advises that people do not attempt to cross water-covered bridges and avoid dips in the road or low-water crossings. Trying to cross even a small stream can be dangerous, because waters can rise rapidly.

On average, U.S. flooding kills about 150 people a year — more than any other single weather hazard, including tornadoes and hurricanes, according to the University Corporation for Atmospheric Research (UCAR). Most flood deaths are from flash floods, however, and about half of those are because people try to cross swollen streams or flooded roads.

Victims often underestimate the power of water when driving into flooded areas, UCAR scientists note, adding that it takes only 18 inches (46 centimeters) of water to float a typical vehicle. It only takes 2 feet (60 cm) of flowing water to sweep most vehicles downstream, and nearly half of all flood fatalities are auto-related, according to NOAA.

Flooding deaths have risen in recent decades, and the U.S. Congress's Office of Technology Assessment says that "despite recent efforts, vulnerability to flood damages is likely to continue to grow" as populations in flood-prone regions steadily increase.

Flash floods can occur along rivers, on coastlines, in urban areas and dry creek beds. River floods generally happen when river basins fill too quickly and water pours over the banks. Coastal flooding is common when tropical storms or hurricanes drive ocean water inland, or when tsunamis send water surging onto shore.

The pavement that covers urban areas prevents the natural soil from absorbing rainfall – in fact, urbanization increases runoff by two to six times over what would naturally occur, according to NOAA. Streets lined with tall buildings can be transformed into fast-moving rivers.

A flash flood moves quickly and can travel for miles beyond the original site of the storm, catching unwary hikers and motorists by surprise. Because flash floods can occur at any time of the year, it is important to always be aware of local weather reports, as the National Weather Service issues a flash flood warning whenever one is occurring or is imminent in specified areas.