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FLOODS: AMONG THE GREATEST NATURAL DISASTERS
March 15, 2001 Floods
are one of the greatest natural disasters known to mankind. Over
the past 50 years, flooding caused an average of almost $3.5
billion in damages and took more than 100 lives per year in the
United Statesmore than any weather-related event. Three-fourths
of all presidential disaster declarations are associated with
Flooding occurs whenever water
due to rain or snow melt accumulates faster than soils can absorb
it or rivers can carry it away.
The variety of floods is as diverse as the nation's natural resources
ranging from localized flash flood covering several city blocks
to massive flooding encompassing up to a quarter of the area
of the lower 48 states. Flooding can happen in the summer due
to torrential rains often associated with severe thunderstorms
and tornadoes, in the fall as a result of hurricanes, in winter
due to ice jams, and in the spring as a result of melting snows.
Flash floods are typically caused by short, intense rainfall
events over areas as small as a city to larger than a state.
Water pools in low spots such as underpasses and basements because
rain falls faster than the ground can absorb it. Widespread flash
flood events are characterized by pockets of heavy rain with
larger areas of lighter rain, making forecasting of exactly where
the worst flooding will occur difficult. Year in and year out,
flash floods take more lives than any other type of flooding.
The cumulative effect of widespread, prolonged flash flooding
can lead to flooding of major river systems.
capture the attention of the nation because of their destructive
power. While the effects of extreme winds and massive coastal
storm surges are widely recognized, the potential for severe
flooding is not always appreciated. Warm, moist tropical air
that serves to drive the winds in a hurricane can also lead to
widespread hazardous flooding. Rainfall is sometimes measured
in feet as opposed to inches. This deluge typically falls over
the period of a day or two, overwhelming the ability of streams
and rivers to carry it off, and resulting in extreme flooding.
Ice Jam Floods
When rivers clogged with ice rise rapidly due to rainfall and/or
snowmelt, the ice breaks up into chunks, some larger than an
automobile. These chunks of ice move downstream and can jam at
constrictions in rivers such as bends, bridge abutments or shallow
areas. The effect is much like a traffic jam that occurs if travel
lanes are closed due to an accident. The ice jam can act as a
dam, causing water to back up behind it. River levels behind
the ice jam can rise rapidly. On occasion, the ice jam can release
quickly, sending huge chunks of ice downstream in the torrent,
destroying everything in its wake.
Widespread excessive rain events produce flooding along waterways
throughout the United States. River flooding can range from minor
overbank events to massive, widespread inundation such as occurred
along the Missouri and Mississippi Rivers in the summer of 1993.
Such flooding may be caused by excessive rainfall alone, or a
combination of heavy rainfall and snowmelt.
Snow Melt Floods
Snow melts slowly enough that, by itself, it seldom causes flooding
in many parts of the country. However, warm, moist conditions
and heavy rain can combine with snow melt to cause dramatic winter
and spring flooding. In relatively flat areas in the Midwest,
river beds drop very slowly along the length of the river. As
a result, the water in the river glides slowly downstream. In
such areas, accumulation of melt water from over extensive snow-covered
areas can cause significant flooding. This situation is often
compounded by the effects of ice jams.
Flooding is sometimes also
caused by dam breaks or levee failures.
NOAA's National Weather Service monitors conditions that lead
to flooding 24 hours a day, seven days a week, and issues forecasts,
watches and warnings, because flooding can occur any place and
at any time. In the case of flash flooding, rapid dissemination
of these forecasts, watches and warnings is especially critical.
NOAA Weather Radio provides
up-to-the-minute flood warnings. Receivers can be set to provide
audible alarm even when they are turned off. This technology
is critical to saving lives, particularly during nighttime disasters.
In addition, NWS works closely with national, state and local
emergency managers to disseminate forecasts and warnings as well
as to support their flood response activities. Through the constant
infusion of technology, the National Weather Service continues
to improve flood forecasts with the goal of saving lives and
reducing damage. For example, Doppler radar provides a powerful
tool to provide pin-point warning information, particularly in
the case of flash floods. By making forecasts more accurate and
providing more lead time, public response to warnings appears
to have improved, resulting in savings of lives.
As the millennium begins, the
National Weather Service is implementing Advanced Hydrologic
Prediction Service (AHPS) which leverages new technology and
advances in hydrologic science to provide increasingly more accurate
forecasts. Not only will AHPS allow better forecasting and response
to flooding, but it will also transform water management activities,
leading to improved reservoir management and provide information
on how long drought conditions will affect river levels.
Unfortunately, most flood fatalities are not due to limitations
in the forecast system. All too often, people in vehicles literally
drive into harm's way. As little as two feet of water can float
an average car. Two to three feet of water can cause most sport
utility vehicles (SUVs) to be swept away. While it may appear
that water is not deep enough to cause problems, there is almost
no way of knowing if the roadbed itself has been eroded or undermined.
Avoid water, no matter how benign it may look. Don't gamble with
NOAA Flood Web Sites
Posted March 15, 2001