|
How do flash floods
occur?
How serious is
flood awareness?
How can a foot or two of water cost you your life?
How much
flood water can do harm?
What types
of flooding are there?
What types of
alerts are there?
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.
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.
Back to top
How serious is flood awareness?
Flooding is
the number one cause of weather-related deaths in the United States.
Nearly half of all
flash flood fatalities are auto related! In your automobile look out
for flooding at highway dips, bridges, and low areas. Much of flash
flooding occurs at night.
Back to top
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.
Back to top
How much flood water can do harm?
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.
Back to top
What types of flooding are there?
Flooding Takes
Many Forms 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 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.
NOTE: Coastal flooding caused by the storm surge associated with
hurricanes is described in publication NOAA/PA 78019, "Storm Surge and
Hurricane Safety."
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.
Back to top
What types of alerts are there?
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!
Back to top
|
