Floods may be caused by a variety of events. Hurricanes immediately come to mind, but flooding can also occur as a result of the spring thaw of snow and heavy rains. This article is the fourth in a series of charts providing information on tornadoes, hurricanes, wildfires, floods, earthquakes, blizzards, and volcanoes.

Flood Factors

Sometimes, floods occur when there is a problem with a dam or levee near a populated area. There are more than 80,000 dams in the U.S., and about one-third of them pose “high” or “significant” danger to life and property if they fail.

One memorable example is the failure of the levees on Lake Ponchartrain and Lake Borgne in New Orleans, Louisiana during Hurricane Katrina in 2005. The storm surge, or increased water level, for a Category 3 storm like Katrina is typically 9 to 12 feet (3 to 4 meters) above normal. The storm surge for Hurricane Katrina, however, reached as high as 30 feet (9 meters); for context, the threshold for the storm surge of a Category 5 storm is 18 feet (5.5 meters) or higher.

Another problem affecting floods, particularly in urban areas, is inadequate drainage. Large amounts of rain falling onto impervious surfaces such as roads can overwhelm the local drainage system, leaving the water with no place to go. Many areas flood with heavy spring rains and snow melt when the drainage systems are overwhelmed.

Flash Floods

Flash floods are a particularly dangerous sub-type of flood, defined by NOAA as flooding that begins within six hours, though often within three hours or even less, of heavy rainfall. Flash floods are particularly dangerous because it leaves people with less time to prepare themselves and their homes for evacuation. It is essential to get to higher ground as quickly as possible.