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.
June 9, 1972 Black Hills
Rapid City, SD
15 inches of rain in 5 hours
238 fatalities
$164M in damages
Source: National Weather Service
Identify where to go if told to evacuate. Choose several places...a friend's home or a motel in another town, or a shelter.
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, STOP! TURN AROUND AND GO ANOTHER WAY.
· 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 $61M.
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. Flash flooding at this arroyo in Arizona took only 58 seconds to develop.
Floating ice can accumulate at a natural or man-made obstruction and stop the flow of water.
Environmental Clues:
Distant thunder runoff from a faraway thunderstorm could be headed your way.
Water rising rapidly.
Look out for flooding at highway dips, bridges, and low areas.
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 (EMA) 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.
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. 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.
The rule for being safe in a flooding situation is simple: HEAD FOR HIGHER GROUND AND STAY AWAY FROM FLOOD WATERS!
Families should be prepared for all hazards that affect their area. NOAA's National Weather Service, the Federal Emergency Management Agency, and the American Red Cross urge each family to develop a family disaster plan.
Where will your family be when disaster strikes? They could be anywhere at work, at school, or in the car. How will you find each other? Will you know if your children are safe? Disasters may force you to evacuate your neighborhood or confine you to your home. What would you do if basic services water, gas, electricity or telephones were cut off?
Follow these basic steps to develop a
family disaster plan:
I. Gather information about hazards.
Contact your local National Weather Service
office, emergency management or civil defense office, and American Red Cross
chapter. Find out what type of disasters could occur and how you should
respond. Learn your community's warning signals and evacuation plans.
II. Meet with your family to create a plan.
Discuss the information you have gathered. Pick two places to meet: a spot
outside your home for an emergency, such as fire, and a place away from your
neighborhood in case you can't return home. Choose an out-of-state friend as
your "family check-in contact" for everyone to call if the family
gets separated. Discuss what you would do if advised to evacuate.
III. Implement your plan.
(1) Post emergency telephone numbers by phones;
(2) Install safety features in your house, such as smoke detectors and fire
extinguishers;
(3) Inspect your home for potential hazards (such as items that can move, fall,
break, or catch fire) and correct them;
(4) Have your family learn basic safety measures, such as CPR and first aid;
how to use a fire extinguisher; and how and when to turn off water, gas, and
electricity in your home;
(5) Teach children how and when to call 911 or your local Emergency Medical
Services number;
(6) Keep enough supplies in your home to meet your needs for at least three
days. Assemble a disaster supplies kit with items you may need in case of an
evacuation. Store these supplies in sturdy, easy-to-carry containers, such as
backpacks or duffle bags. Keep important family documents in a waterproof
container. Keep a smaller disaster supplies kit in the trunk of your car.
IV. Practice and maintain your plan.
Ask questions to make sure your family remembers meeting places, phone numbers, and safety rules. Conduct drills. Test your smoke detectors monthly and change the batteries at least once a year. Test and recharge your fire extinguisher(s) according to manufacturer's instructions. Replace stored water and food every six months.
Important terms to know: