NOAA Miami Library

(Any link in red will guide you outside to AOML server!)


National Oceanic and Atmospheric Administration
National Weather Service
July 1992
American Red Cross

FLASH FLOODS. #1 weather-related killern 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 thun derstorms, thunderstorms repeatedly moving over the same area, or heavy rains from hurricanes and tropical storms.

June 9, 1972 - Black Hills
Rapid City, SD
15 inches of rain in 5 hours 238 fatalities
$1 64M in damages Source: National Weather Service

PLAN AHEAD: Identify where to go if told to evacuate. Choose several places...a friend's home or a motel in another town, or a shelter.

GO TO HIGHER GROUND! Know your area's flood risk. For information, call your local National! Weather Service office, Red Cross chapter, or local emergency management agency.

Check your homeowner's or renter's insurance. Homeowners' policies do not cover flooding. Contact your insurance agent to find out how to get flood insurance.


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,


* Dam Break

- May 31, 1889 Johnstown, Pennsylvania ... the worst flood in United States history ... 36-40 ft. wall of water... 2,200 dead.

* River Flood

- December 1991/January 1992 South-central Texas ... wide spread river flooding on the Guadalupe, Brazos, Trinity and Colorado River Basins ... up to 17 inches of rain ... 15 dead ... damages $ 100M .

* Flash Flood Events

- June 14, 1990 -- Shadyside, Ohio ... 4 inches of rain in less than 2 hours produced a 30 foot high wall of water ... 26 dead ... damages $6-8M.
- August 1, 1985 -- Cheyenne, Wyoming ... 6 inches of rain in 3 hours ... 12 dead ... damages $61 M.


Flash flooding occurs within 6 hours of the rain event. Flooding is a longer term event and may last a week or more.

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 floodlng.

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.

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 (tsoo-na -mez), 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. "
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.


- Listen for distant thunder-runoff from a faraway thunderstorm could be headed your way.
- Look out for... Water Rising Rapidly

Nearly half of all flash flood fatalities are auto related!
- In your automobile, look out for... flooding at highway dips, bridges, and low areas.


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.

Before the flood What YOU can do:

- Know your flood risk and elevation above flood stage. - Do your local streams or rivers flood easily? If so, be prepared to move to a place of safety. Know your evacuation routes.
- Keep your automobile fueled; if electric power is cut off, gas stations may not be able to operate pumps for several days. Store drinking water in clean bathtubs and in various containers. Water service may be interrupted.
- Keep a stock of food that requires little cooking and no refrigeration; electric power may be interrupted.
- Keep first aid supplies on hand.
- Keep a NOAA Weather Radio, a battery-powered portable radio, emergency cooking equipment, and flashlights in working order.
- Install check valves in building sewer traps to prevent flood water from backing up into the drains of your home.
- Assemble a disaster supplies kit containing: first aid kit, canned food and can opener, bottled water, rubber boots, rubber gloves, NOAA Weather Radio, battery-powered radio, flashlight, and extra batteries.

What YOUR community can do?

- Assist hospitals and other operations which are critically affected by power failure by arranging for auxiliary power supplies.
- River/rainfall readings are valuable to local emergency management agencies (FEMA) and the National Weather Service (NWS) in assessing flood conditions and taking appropriate actions. Advanced warning provided by early detection is critical to saving lives. Automatic flood detection systems are available commercially for flood-prone communities. Contact your local NWS office or emergency management agency for further information on LOCAL FLOOD WARNING SYSTEMS.

STAY INFORMED ABOUT THE STORM by listening to NOAA Weather Radio, commercial radio, and television for the latest flash flood/flood WATCHES, WARNINGS, and ADVISORIES.

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. The 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


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: - 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. - 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. Flammable may be inside.
- Report broken utility lines to appropriate authorities.