In Part 1 of this chapter on Floods , we reviewed the main types of floods, and recognized river system floods as the most common form. Year by year, theyare the most dangerous, disruptive events affectingmany people's lives, and are the most expensive interms of property destruction and repair costs.
The other common form of flooding -
is discussed elsewhere. They are usually rapidly
developing but brief events, often unexpected, and
usually result from very heavy local rainfall
in the headwaters of a stream.
But back to the river system floods. Let's look at
what's behind them.
Causes of Floods
I don't think I'll surprise you by stating that the most important
cause of flooding is widespread, heavy rain. Heavy rain is
hard to ignore, particularly when it continues for days. Its effects
are magnified when the rain bringing system extends over most of
a river catchment area, so that all tributaries are soon at bank
full stage or are in flood. When they meet the main channel, its
capacity for removing the water is rapidly exceeded, and
extensive flooding is inevitable.
Heavy rain itself can be difficult to define, except after the
event. In some areas where rainfall is normally light and
spread out, falls of 3-4 inches may be enough to cause flooding.
In other areas, falls of this size may be common, but
the stream channels have adapted to handle the amount of water,
and no flooding occurs.
And while it is true that a major rainfall event can overcome
all natural conditions and man made structures that may minimise
flooding, even prolonged drought, there are many other factors
that can contribute to major flooding.
Saturated or Frozen Ground. In normal times, soil
has a great capacity to soak up a large amount of water,
preventing it from running off. But when the ground is already saturated
or frozen, all rain or melt water moves over the top and into
Full streams. Quite often, if the ground is saturated
water is already running off and streams and rivers are
close to bank full stage. Any extra rain will rapidly cause
Full dams. Partly full dams can be used as a flood
control measure by intercepting and holding back the early
run off. In some cases, if a flood is expected to occur, water
authorities will begin releasing water before the expected peak
to create some capacity in the dam to reduce flooding
downstream. Although this may cause early nuisance flooding, it
will help reduce the effects of the main flood.
But if the streams and the dams are already full, once again
almost all the rain will end up in the river.
Despite everyone's best efforts, these three conditions tend
to occur together and reinforce each other. This situation
contributed to the Mississippi Flood of 1993, when a series
of heavy thunderstorms over several weeks eventually
overwhelmed natural and artificial checks on flooding.
Snowcover. As a general rule, ten inches of freshly
fallen snow is equivalent to one inch of rain. But over
winter, it compacts, and a ten inch snowpack spread over
a large area can add a lot of runoff to a river system
if it melts quickly.
Ice Jams. Two sorts of ice jams can form and
Local freezing can dam up unfrozen water behind the ice,
generally only causing local flooding. The same thing
can happen when ice breaks up, is transported downstream
as blocks and sheets, and forms a dam at a natural or
man made obstruction, such as a bridge.
Apart from its potential to damage or destroy bridges and
other structures, and its contribution to upstream flooding,
the main threat of such an ice dam is if it suddenly breaks.
Dangerous downstream flash floods can result, and usually
every effort is made to remove the ice jam as quickly as
possible. Log jams pose similar threats.
Frozen ground, a heavy snowpack, and ice jams all contributed
to devastating flooding on the Red River at Grand Forks,
North Dakota during Spring, 1997. This combination was
probably enough for a major flood, but the situation was
made far worse by events months earlier. Record Fall rains
were followed by a quick and sustained freeze.
So not only did record amounts of melting snow have to get
away, but so too did the frozen Fall rainwater. Ice jams
forming during the thaw also contributed to the record
Out of control steamship breaks levee,
Junior, Louisiana, 1927
Floods and Home Weather Stations
River floods, by their nature, are best studied after theevent. Flash floods, covered elsewhere,are most commonly associated with individual thunderstorms.
Unless a flood is caused by a single episode of widespreadand persistent and heavy rain, it can be very difficult toput your finger on the initial cause of a flood while it is happening.
As we have seen, the start of the 1997 Red River ND floodbegan almost half a year before.
Similarly, the 1993 Mississippi Flood developed over sometime. Was the cause in the first of the train of thunderstorms that moved through the region, or was it in the first thunderstorm after the ground was saturated and the streams were running full, or was it at some other time. It can be hard to tell.
But if you have experienced a flood in your area, it isan interesting exercise to go back through records andthe archives of the NWS to try and pick the point atwhich the flood first became likely, as well as anysubsequent events which made the flood inevitable, such as further heavy rain.
Reports on river heights, estimates of flood peaks,and predicted extent of flooding can also be illuminating, and at worst will give you an idea of thevast amount of information available to the officialflood forecasters at the time when it's all happening.
All the photos used in this article are reproduced by courtesyof the NOOA Photo Library, and were taken during floods inthe 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 excellentbooks, DVDs, videos and other material mentioned in the More Flood Resources pages, you can find outmore about the weather conditions that cause floods onthe Severe Weather page.
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