Flooding and Flash Flooding
Image courtesy NOAA.
Flood: Any high flow, overflow, or inundation by water which causes or threatens damage.
Flash Flood: A rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level, beginning within six hours of the causative event (e.g., intense rainfall, dam failure, ice jam). However, the actual time threshold may vary in different parts of the country. Ongoing flooding can intensify to flash flooding in cases where intense rainfall results in a rapid surge of rising flood waters.
(Source: National Weather Service)
During the 20th Century, floods were the number one natural disaster to cause the loss of lives and property, according to the USGS. Flooding can occur in a number of situations, including heavy downpours in strong thunderstorms, or during the spring when snowpack is melting. Record flood events have occurred along the Mississippi and Missouri rivers in years such as 1927, 1973, 1993, and 2011. During the spring, snow melt from winter snows flows into rivers in the upper U.S., most of which converge into the Mississippi. After winters of extreme snowfall, rivers can swell far into their flood plains and wreak havoc on river towns. Flash flooding is a rapid and extreme flow of high water into a normally dry area, beginning within six hours of the causative event (e.g. a strong thunderstorm).
Flooding Caused By Tropical Cyclones
In tropical storms and hurricanes, wind speeds and surge are not the only danger—flooding and flash flooding have claimed the most lives in tropical cyclones from 1970 to 1999 (unlike the historic 2005 hurricane season, where storm surge claimed thousands of lives). Flash flooding will occur in creeks, streams, and urban areas within hours of torrential rain. These floods can reach heights of 30 feet or more. Streets can be turned into rivers, and underpasses become deadly.
Deaths caused by the effects of tropical cyclones in the U.S. 1970-1999
Image courtesy NOAA.
Turn Around, Don't Drown
The National Weather Service's "Turn Around, Don't Drown" program warns people of the danger of driving through flooded areas. The Center for Disease Control estimates that over half of all flood-related drownings occur when a vehicle is driven into flood water. The second highest percentage of drownings are from people who walk into or near flood waters.
People generally underestimate the force of moving flood water. Just
two feet of water can move or lift a car, even a truck or SUV. Only
six inches of water is necessary to sweep you off your feet. If flooding occurs, take the following precautions:
- Move to higher ground and stay away from low-lying flood-prone areas
- Do not allow children to play in flood waters, no matter how fun it might look
- Never drive on a flooded road
- Do not set up camps along streams or washes when there's a chance of rain or thunderstorms
- Be extra cautious during nighttime flooding situations
The National Weather Service issue the following flood-related advisories:
A hazardous flood event could develop.
The expectation of a flood event has increased. Usually this means that somewhere within the watch zone, a flood is expected. If you're in the watch area, you should pay attention to Weather Radio or local news in case a warning is issued.
Flash Flood Warning
Flash flood warnings, flood warnings, or flood advisories are issued when flooding is occurring or imminent.