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Information from the National Weather Service
Flash flooding occurs within 6 hours of the rain event.
Flooding is a longer term event and may last a week or more.
Flash floods are the #1 weather-related killer in the United States!
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.
Even 6 inches of fast-moving flood water can knock you off your feet, and a depth of 2 feet will float your car! NEVER try to walk, swim, or drive through such swift water. If you come upon flood waters, STOP! TURN AROUND AND GO ANOTHER WAY.
Flooding Takes Many Forms...
RIVER FLOOD
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.
COASTAL FLOOD
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 sometimes referred to as tidal waves. These waves are produced by earthquakes or volcanic activity.
URBAN FLOOD
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.
FLASH FLOODING IN ARROYOS/WASHES
An arroyo is a water-carved gully or normally dry creek bed. Arroyos can fill with fast-moving water very quickly. Flash flooding at this arroyo in Arizona took only 58 seconds to develop.
ICE JAM
Floating ice can accumulate at a natural or man-made obstruction and stop the flow of water.
How can a foot or two of water cost you your life?
• Water weighs 62.4 lbs. per cubic foot and typically flows downstream at 6 to 12 miles an hour.
• When a vehicle stalls in the water, the water's momentum is transferred to the car. For each foot the water rises, 500 lbs. of lateral force are applied to the car.
• But the biggest factor is buoyancy. For each foot the water rises up the side of the car, the car displaces 1,500 lbs. of water. In effect, the car weighs 1,500 lbs. less for each foot the water rises.
• Two feet of water will carry away most automobiles.
NOAA WEATHER RADIO IS THE BEST MEANS TO RECEIVE WARNINGS FROM THE NATIONAL WEATHER SERVICE. The National Weather Service continuously broadcasts updated weather warnings and forecasts that can be received by NOAA Weather Radios sold in many stores. Average range is 40 miles, depending on topography. Your National Weather Service recommends purchasing a radio that has both a battery backup and a tone-alert feature which automatically alerts you when a watch or warning is issued.
What to Listen For...
• FLASH FLOOD OR FLOOD WATCH: Flash flooding or flooding is possible within the designated WATCH area be alert.
• FLASH FLOOD OR FLOOD WARNING: Flash flooding or flooding has been reported or is imminent take necessary precautions at once.
• URBAN AND SMALL STREAM ADVISORY: Flooding of small streams, streets, and low-lying areas, such as railroad underpasses and urban storm drains, is occurring.
• FLASH FLOOD OR FLOOD STATEMENT: Follow-up information regarding a flash flood/flood event.
The rule for being safe in a flooding situation is simple: HEAD FOR HIGHER GROUND AND STAY AWAY FROM FLOOD WATERS!
TAKE ACTION!
When a flash flood WATCH is issued - Be alert to signs of flash flooding and be ready to evacuate on a moment's notice.
When a flash flood WARNING is issued for your area, or the moment you realize that a flash flood is imminent, act quickly to save yourself. You may have only SECONDS!
Go to higher ground Climb to safety!
• Get out of areas subject to flooding. This includes dips, low spots, canyons, washes, etc.
• Avoid already flooded and high velocity flow areas. Do not attempt to cross flowing streams.
• If driving, be aware that the road bed may not be intact under flood waters. Turn around and go another way. NEVER drive through flooded roadways!
• If the vehicle stalls, leave it immediately and seek higher ground. Rapidly rising water may engulf the vehicle and its occupants and sweep them away. Remember, it's better to be wet than dead!
• Be especially cautious at night when it is harder to recognize flood dangers.
• Do not camp or park your vehicle along streams and washes, particularly during threatening conditions.
When you receive a FLOOD WARNING:
• If advised to evacuate, do so immediately.
• Move to a safe area before access is cut off by flood water.
• Continue monitoring NOAA Weather Radio, television, or emergency broadcast station for information.
During the flood:
• Avoid areas subject to sudden flooding.
• If you come upon a flowing stream where water is above your ankles, STOP! Turn around and go another way.
• Do not attempt to drive over a flooded road. The depth of water is not always obvious. The road bed may be washed out under the water, and you could be stranded or trapped.
• Children should NEVER play around high water, storm drains, viaducts, or arroyos.
After the flood:
• If fresh food has come in contact with flood waters, throw it out.
• Boil drinking water before using. Wells should be pumped out and the water tested for purity before drinking. If in doubt, call your local public health authority.
• Seek necessary medical care at the nearest hospital. Food, clothing, shelter, and first aid are available from the Red Cross.
• Do not visit disaster areas. Your presence might hamper rescue and other emergency operations.
• Electrical equipment should be checked and dried before being returned to service.
• Use flashlights, not lanterns, torches or matches, to examine buildings. Flammables may be inside.
• Report broken utility lines to appropriate authorities.
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