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.
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
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.
Hurricanes consistently 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
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
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 your life.
NOAA Flood Web Sites