Flood Safety

Turn Around Don't Drown
Photo from just outside of Tremont, IL

Flash Floods

The #1 Weather Related Killer in the United States!

The NWS says flooding is the number one weather-related killer in the U.S., with about 100 people losing their lives in flood-related incidents each year. Since 1995, 19 people have died from floods in Illinois, most of those involving people in vehicles trying to cross a flooded roadway.

How Do Flash Floods Occur?

Several factors contribute to flash flooding. The two key elements are rainfall intensity and duration. Intensity is the rate of rainfall, and duration is how long the rain lasts. Topography, soil conditions, and ground cover also play an important role.

Flash floods occur within a few minutes or hours of excessive rainfall, a dam or levee failure, or a sudden release of water held by an ice jam. Flash floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Rapidly rising water can reach heights of 30 feet or more. Furthermore, flash flood-producing rains can also trigger catastrophic mud slides. You will not always have a warning that these deadly, sudden floods are coming. Most flood deaths are due to FLASH FLOODS.

Most flash flooding is caused by slow-moving thunderstorms, thunderstorms repeatedly moving over the same area, or heavy rains from hurricanes and tropical storms.

Occasionally, floating debris or ice can accumulate at a natural or man-made obstruction and restrict the flow of water. Water held back by the ice jam or debris dam can cause flooding upstream. Subsequent flash flooding can occur downstream if the obstruction should suddenly release.

Flooding takes Many Forms

Flooding along rivers is a natural and inevitable part of life. Some floods occur seasonally when winter or spring rains, coupled with melting snows, fill river basins with too much water, too quickly. Torrential rains from decaying hurricanes or tropical systems can also produce river flooding.

Winds generated from tropical storms and hurricanes or intense offshore low pressure systems can drive ocean water inland and cause significant flooding. Escape routes can be cut off and blocked by high water. Coastal flooding can also be produced by sea waves called tsunamis (tsoo-n„ -m z), sometimes referred to as tidal waves. These waves are produced by earthquakes or volcanic activity.

NOTE: Coastal flooding caused by the storm surge associated with hurricanes is described in publication NOAA/PA 78019, "Storm Surge and Hurricane Safety."

As land is converted from fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall. Urbanization increases runoff 2 to 6 times over what would occur on natural terrain. During periods of urban flooding, streets can become swift moving rivers, while basements can become death traps as they fill with water.

An arroyo is a water-carved gully or normally dry creek bed. Arroyos can fill with fast-moving water very quickly.