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 mudslides. 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
Rapid City, S.D.
15 inches of rain in five hours
$164M in damage
Source: National Weather
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. Flooding can occur
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 floodwaters,
stop, turn around and go another way!
* * *
historical NWS data
- May 31, 1889, Johnstown, Pa.
- December 1991, January 1992,
south central Texas
Flash flood events
* * *
takes many forms
Flash flooding occurs within four hours
of the rain event.
Flooding is a longer-term event and may
last a week or more.
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-NAH-meez), 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 two to six 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.
flooding in arroyos and 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 an arroyo in Arizona took only 58 seconds
Floating ice can accumulate at a
natural or man-made obstruction and stop the flow of water.
* * *
Listen for distant thunder. Runoff from
a faraway thunderstorm could be headed your way.
Look out for water rising rapidly.
In your automobile, look out for
flooding at highway dips, bridges and low areas. Nearly half of
all flash flood fatalities are auto-related!
Many flash floods occur at night. Be
prepared to take quick action.
* * *
How can a
foot or two of water cost you your life?
Water weighs 62.4 pounds 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 pounds of lateral force
is 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 pounds of water. In effect, the car weighs 1,500 pounds less
for each foot the water rises.
Two feet of water will carry away most
* * *
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
- 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
- Keep a stock of food that
requires little cooking and no refrigeration; electric power may
- 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 floodwater 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
your community can do:
- Assist hospitals and other
operations which are critically affected by power failure by
arranging for auxiliary power supplies.
- River and rainfall readings are
valuable to local emergency management agencies and the National
Weather Service 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 National Weather Service office or emergency management
agency for further information on local flood warning systems.
[to top of second column in
informed about the storm
Listen to NOAA Weather Radio,
commercial radio and television for the latest flash flood and 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 that
automatically alerts you when a watch or warning is issued.
What to listen for:
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:
These statements provide follow-up information regarding a flash
flood or flooding event.
- Flash flood or flood watch
The rule for being safe in a flooding
situation is simple: Head for
higher ground and stay away from floodwaters!
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
- Go to higher ground. Climb to
- Get out of areas subject to
flooding. This includes dips, low spots, canyons and washes.
- 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 floodwaters. 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
- When you receive a flood
to evacuate, do so immediately.
Move to a
safe area before access is cut off by floodwater.
Continue monitoring NOAA Weather
Radio, television or emergency broadcast station for information.
* * *
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:
1. 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
2. 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.
3. Implement your plan.
a. Post emergency telephone numbers by
b. Install safety features in your
house, such as smoke detectors and fire extinguishers;
c. Inspect your home for potential
hazards -- such as items that can move, fall, break or catch fire --
and correct them.
d. 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
e. Teach children how and when to call
911 or your local emergency medical services number;
f. 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.
A disaster supplies kit should
three-day supply of water -- one gallon per person per day -- and
food that won't spoil.
of clothing and footwear per person.
blanket or sleeping bag per person.
first-aid kit, including prescription medicines
tools, including a battery-powered NOAA Weather Radio and a
portable radio, flashlight, and plenty of extra batteries.
set of car keys and a credit card or cash.
Special items for infant, elderly or
disabled family members.
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 extinguishers
according to manufacturer's instructions. Replace stored water and
food every six months.
[National Oceanic and
Atmospheric Administration, National Weather Service, Federal
Emergency Management Agency and the American Red Cross]