Floods - Types, Predictions And Causes
Someone is experiencing a flood today, somewhere in the USA. It may only be a minor inconvenience, or it may be a major disaster, butfloods are an important and inevitable part of the way water ismade available to all parts of this planet.
So just how common are floods?
Well, every day in 2004, somewhere in the 48 states was experiencing flood conditions. Now 2004 was not a particularly wet year, yet the distribution of rain and snow throughout the year was enough to cause flooding somewhere.
(Yes, I realize that searching through a year's worth of flood reports is not one of life's highpoints, but the question arose, and I won't need to do it again.)
These statistics are restricted to floods which occur in stream and river systems and are divided into two major categories -
which I have
written about elsewhere, and the larger but more slowly
developing river floods. More restricted, but often just as
devastating, are the storm surges associated with hurricanes
and other severe storms, most frequent on the East and Gulf
And most devastating of all, although not weather related, are
the big tsunamis such as the one on Boxing Day in Southeast Asia.
But in this article I'll restrict myself to the large, extensive
and long lasting river floods.
Most of these develop gradually, but sometimes
devastating floods have resulted from the sudden
failure of a man made dam. These catastrophes,
which come with little warning, are rare and
hopefully getting rarer.
Although they usually were associated with heavy rain
which filled the dam before it broke, they are not
really part of the normal river floods, and are
not discussed here. However some of the more disastrous
dam failures are the subject of excellent books, DVDs and
videos, discussed in Part 3 of this review, More Flood Resources.
Part 2 reviews the Causes of River System Floods, and how
to record and understand floods using your home weather
station and other information.
Apart from the wealth of information on all aspects of
floods included in More Flood Resources (including
posters and photos), there is some great information at
Government websites such as NOAA, NWS and other national
weather agencies. You'll find links to some of the better
US sites, and they are well worth a visit.
Levee about to be overtopped.
Note how much lower the
floodplain to the right is.
They include the flood reports I mentioned above, which demonstrate that
floods are a natural part of the hydrologic cycle - the vertical
and horizontal transfer of water in all its states - vapor, liquid and solid - between
land, sea and the atmosphere. And most floods simply represent
a temporary loss of balance between the amount of rain or melted
snow and ice entering a river system and the river's
capacity to remove it.
Most river floods are minor spills of water over the banks of
the river on to the adjacent floodplain. They cause a little
inconvenience for a while, but provide natural irrigation and
soil replenishment through the coating of mud and silt they
Asheville NC flood, July 1916.
Bridge on left about to go.
Every so often the amount of water that must be moved vastly
overcomes the river's capacity to handle it, and huge amounts
of water spread out over the surrounding country. Houses and
businesses are flooded, transport is disrupted, lives are put
at risk and sometimes lost, and loss and damage to crops,
farmland and property reaches extreme levels. When disruption
is severe, a national disaster is declared and all efforts
are concentrated on rescue, recovery and restoration of
infrastructure, property and livelihoods.
Major floods on this scale usually take days, even weeks, from the time
the river first breaks its banks until the last water drains
away. Meanwhile the peak of the flood moves on down the river
to affect other communities.
When a major river system such as the Mississippi is in flood,
usually due to simultaneous major floods in several of its
tributaries, the flood peak can take months to clear the system.
That's why severe flooding can happen in areas where little
rain has actually fallen. Such a flood happened in 1993.
Forecasting a flood does nothing to reduce the height of the
floodwaters, but an accurate forecast goes a long way to
reducing the damage, cost and loss of property and life.
Evacuation plans, preparation of buildings and removal of
their contents before the arrival of the water, early
warnings to dam engineers and sewage plant operators, and
notice to barge and boat owners all help to reduce the
ultimate effects of a flood.
Forecasting has improved a great deal over the last 20
or so years and continues to do so. Some of the improvement
is the result of research projects, notably by the
National Weather Service's Hydrological Research
Laboratory, together with many other organizations.
Major improvements have also come with the development of
numerous river gauges, many of them automatic, and weather
stations, all linked by computer to central forecasting
Although computerization of data collection and analysis
has been a huge help, the task of flood prediction is
still not an easy one. Before an estimate of flood
height and arrival can be made, estimates have to be
made of three major components of a flood
- Base Flow - the amount of water already in the
stream, usually contributed by the natural release of
- Runoff, determined by the amount of rain or
melt water, ground and soil saturation, and the effect of
any dams or diversions, and
- Routed Flow, or the amount of water moving
down from upstream.
All this information needs to be compared to history
of previous floods to arrive at a best estimate for the height and duration of the flood.
Once compiled, the information is incorporated in flood
watches or warnings broadcast on
relevant websites. Information on river heights is also
rapidly made available through the River Heights section
on local NWS websites. Frequent updates are made to keep
everyone up to date with the flood's progress.
A good place to start looking for current Flood Information
Hydrologic Information Center's
By clicking on your location on the map on that page,
you'll be taken to the office coordinating flood
information for your area. The map there will allow you
to home in on the river you are concerned about. You'll
find a heap of information by drilling down through the
links, and you can bookmark the most relevant page for later.
Similar notification schemes are in place for
, although because the warning time is less,
and the floods move much faster, the detailed river
heights may not have as much immediate importance.
All the photos used in this article are reproduced by courtesy
NOOA Photo Library,
and were taken during floods in
the first few decades of last century. Historic and recent
photos in this collection are well worth a look
Like to Know More?
Apart from the links in the text above, and the excellent
books, DVDs, videos and other material mentioned in the
More Flood Resources pages, you can find out more about
the weather conditions that cause floods on the Severe
Severe Weather page.
More information on weather Stations can be found on the
Home Weather Stations
page, and there is also a
page to help you
set up your weather station.
Back to the Top, or return to the Home page.
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Last update 05/24/2011