Flash Flooding

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Flash Flooding
Flash Flood 1 weather-related killer in the
United States!
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What is a flood?
State Street River
What is a flood? A flood results from days of
heavy rain and/or melting snows, when rivers rise
and go over their banks.
The "State Street River" (floodwater from City
Creek) rushes through downtown Salt Lake City,
Utah, in May 1983. Rapid melting of a
record-water-content snowpack caused the most
widespread flooding in the State's history,
resulting in damages totaling 621 million.
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What is a flash flood?
What is a flash flood? A flash flood is a
life-threatening flood that begins within 6 hours
and often within 3 hoursof the rain
event. Flooding is a longer term event and may
last a week or more.
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What is a flash flood?
A flood is typically associated with a defined
body of water, but flash floods can occur in
normally dry areas with no visible stream channel.
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What is a flash flood?
Consequently, included in the flash flood
definition is the rapid inundation of normally
dry areas, roads, underpasses, and buildings.
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What is the cause of flash flooding?
Most flash flooding is caused by slow-moving
thunderstorms, thunderstorms repeatedly moving
over the same area, or heavy rains from
hurricanes and tropical storms.
Midwest flooding caused by rain storms, August
2007
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Hurricane induced flood
Floyd,1999 North Carolina
Hurricane Floyd,1999
Katrina, 2005 New Orleans, LA
Hurricane Katrina, 2005
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How dangerous could flash floods be?
Clear Fork Road, WV, July 14, 2001 -- This house
was torn apart by the flooding of the nearby
Clear Fork Creek.
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.
Panther, WV, May 8, 2002 -- Heavy silting
happened on many creeks and rivers as a result of
the violent flash flood that struck southern West
Virginia on May 2, 2002.
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How dangerous could flash floods be?
Flash flood-producing rains can also trigger
catastrophic mud slides. You will not always have
a warning that these deadly, sudden floods are
coming.
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Flooding Types
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.
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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.
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Rainfall and Runoff Relationship
Flash floods are rapid-onset hydrologic events
that can be very difficult to forecast. A
combination of high rainfall rate with rapid and
often very efficient runoff is common to most
flash flood events. Ice jam formation on rivers
can also play a role in rapid-onset flash
flooding, particularly upstream of the ice jam.
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Soil Influences
The three critical soil properties to consider
when assessing the risk of flash flooding are
soil moisture, soil texture, and soil profile.
Soil moisture is often considered the most
important soil factor for rapid runoff and flash
flooding. There is some validity to this
reasoning in that if the soil is saturated there
is no room for additional rainfall to infiltrate,
and all rainfall becomes runoff regardless of the
soil texture.
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When rain is very intense...
Flash floods occur with intense rainfall rates
that often exceed the soil's infiltration
capacity even when the soil is dry.
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Soil Influences Soil Texture
Certain soil textures such as clay, and to some
extent silt, can be associated with low
infiltration rates and result in rapid runoff
during intense rainfall. Therefore, runoff from
intense rainfall is likely to be more rapid and
efficient with clay soils than with sand.
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Soil Influences Soil Texture
Although sandy soils permit greater infiltration
of intense rainfall, rapid runoff can occur if
there is only a thin layer of soil. For example,
if an impermeable layer of rock underlies a thin
layer of soil, that soil layer can saturate
quickly and result in large amounts of runoff.
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Flash flooding facts
1. Flood can occur nationwide. 2. Flash floods
are the 1 weather-related killer in the U.S.
3. Nearly 80 of all flash flood fatalities are
auto related! 4. Many flash floods occur at
night....be prepared to take quick action. 5. A
mere 2 feet of water can float a large vehicle or
even a bus. This is why you should never drive
through flooded roads. Just 6 inches of rapidly
moving flood water can knock a person down.
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Watch and warning
FLOOD WATCH - means that an overflow of water
from a river is possible for your area. FLASH
FLOOD WATCH - means that flash flooding is
possible in or close to the watch area. Flash
Flood Watches can be put into effect for as long
as 12 hours, while heavy rains move into and
across the area. FLOOD WARNING - means flooding
conditions are actually occurring in the warning
area. FLASH FLOOD WARNING - means that flash
flooding is actually occurring in the warning
area. A warning can also be issued as a result of
torrential rains, a dam failure or snow thaw.
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What should we do in case of flooding?
The rule for being safe in a flooding situation
is simple HEAD FOR HIGHER GROUND AND STAY AWAY
FROM FLOOD WATERS!
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!