Flash Floods and River Floods


The catastrophic 1993 floods across the Midwest were caused by 77 events over several months where rainfall of greater than one inch (2.4 centimeters) occurred over areas 100–200 miles (160–320 kilometers) wide and 400–600 miles (640–960 km) long. The powerful and deadly force of flooding—especially flash flooding—is an underrated risk. On average, floods kill more Americans than lightning, tornadoes, or hurricanes. While the U.S. population has nearly doubled since 1950, the death toll from most weather hazards has dropped by half or more, but the flooding toll has risen slightly.

Flash floods are short-term inundations of small areas such as a town or parts of a city, usually by creeks and other smaller tributaries that flow into larger rivers. Heavy rain can produce flash flooding in a few hours even in places where little rain has fallen for weeks or months. In arid or semiarid regions, flash flooding can send a wall of water that turns a completely dry valley or canyon into a raging torrent in mere seconds.

If heavy rainfall occurs repeatedly over a wide area, then river or mainstem flooding becomes more likely, in which the main rivers of a region swell and inundate large areas, sometimes well after rainfall has ended.

Both flash flooding and river flooding threaten life and property, although the former causes more deaths and the latter more property and crop damage.

Research

NCAR research has explored the usefulness of radar data in assessing flood risk. The dual-polarization S-Pol radar, built at NCAR, can distinguish rain from hail, thus making it a valuable tool for estimating rainfall rates in hail-producing storms. Based on research at NCAR and elsewhere, the National Weather Service plans to add dual polarization to its network of Doppler radars over the next few years.

During the spring and summer, thunderstorms often move over similar tracks for days on end, increasing the risk of floods and flash floods. The catastrophic 1993 floods across the Midwest were caused by 77 events over several months where rainfall of greater than one inch (2.4 centimeters) occurred over areas 100–200 miles (160–320 kilometers) wide and 400–600 miles (640–960 km) long. NCAR scientists have analyzed these preferred corridors of movement.

Flooding and climate

A 100-year flood is one that has a 1%, or one out of 100, chance of being equaled or exceeded in any given year. The term gives an estimate of the probability that a flood of a certain size will occur, not when it will occur. There is nothing to prevent a second 100-year flood from occurring a year or even a week after the first one.

Because flood risk is the result of both environmental and societal factors, it is difficult to single out the impact of climate variations. Any alteration of global climate can bring either an increase or decrease in precipitation or flood events at a given location. Current computer models of climate are unable to project local variations with certainty.

Safety

It's easy to underestimate the power of water when compared to a multi-ton vehicle, but more people die in their vehicles than anywhere else during a flooding event. It takes only 18 inches of water to float a typical car. Sport utility vehicles will float like any other vehicle of similar size and weight. Walking through water deeper than your ankles can also be dangerous. Most safety experts give the same advice: climb to safety.

For more about climate and floods, see the flooding fact sheet.

thunderstorms • hail • lightning • tornadoes • floods • hurricanes • winter storms