Rivers and flooding
Causes of flooding
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The main cause of flooding is when a lot of rain falls in a few days, especially
following a prolonged dry period.
The ground becomes so wet that
no more rain can penetrate the
earth. This rain runs into the rivers
and the water level quickly
becomes much higher. Rivers
cannot cope with this extra water
and the water breaks over the
river banks.
•
Countries like Bangladesh, where
the land is flat with high
mountains, are often flooded in
Spring when snow and ice melt as the weather becomes warmer
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Cyclones, hurricanes, and typhoons can cause floods. These are very strong
winds which form over seas and blow towards coastlines. The winds make the
waves in the sea much higher. The waves then crash onto the beach and
flood the flat land by the coast.
•
Earthquakes shake the earth, causing big waves called tsunamis. The
tsunamis travel towards the shore and flood the flat coastal land.
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Unusually high tides in Spring time can cause floods by an estuary. High
waves, caused by the high tide, rush up the river estuary. There is too much
water in the river so the surrounding land is flooded when the river banks
burst.
•
Dam failure can cause a flood. If engineers do not open enough of the dam
holes in times of heavy rainfall, the water builds up behind the dam in the
reservoir. The water can then spill over the top of the dam.
The impact on people and places
Even a small, slow-flowing stream or gentle river can
cause serious damage to people and the places in which
they live and work. People like to live near to rivers - in
the past mainly for food, water, transport and
protection. Even today people enjoy the peace and
tranquillity offered by flowing water.
Flooding can turn even the most harmless looking
watercourse into a raging torrent of large-scale
destruction - buildings may prove no obstacle to its
power; food crops may be ruined leading to food
shortages and even starvation; lives, both human and
animal, may be lost through drowning disease and
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homelessness.
Why do Rivers Flood?
Floods are natural occurrences. They mainly
happen when the river catchment area (land
that feeds water into the river and the streams
that flow into the main river) receives greater
than usual amounts of water (eg through
rainfall or melting snow). The river cannot
cope and this extra water causes the level of
the water in the river to rise and a flood to
take place. This flooding may take place at any
point along the river course and not necessarily at the place where the extra
water has entered.
Floods can be beneficial
Although man often endeavours to protect himself and his property from the
often-damaging effect of floods, man can use them to his advantage.
The people of Egypt used the regular event of the River Nile flood to their
advantage. Until 1970, the Nile had flooded every Spring as the mountain snows
melted. This flooding of the floodplains alongside the river enriched the soils
leaving behind rich silt in which crops could be grown.
Controlling flooding
Man has long tried to control and prevent the damaging effects of flooding
rivers. River engineers may build artificial flood banks, straighten the river
course, or dredge the riverbed to make it deeper. All of these methods of control
can work, although they often have a negative effect on the river as a habitat.
The Environment Agency http://www.environment-agency.gov.uk has a major
role in the control of river flooding and the protection of people and their
property. Their job is a difficult one has there is an increasing tendency of rivers
towards 'flash flooding' - this is where rainfall or melting ice and snow result in a
rivers level rising extremely quickly, often far more quickly than in past years.
Sandbags to protect property
People can put sand bags around their houses, and by cracks in doors and
conservatories. Windows can also be boarded up to stop water from coming into
the house. If flood water does get into the house, people can use a pump to
remove this water. This takes a lot of time, and damage can still be done.
People can evacuate their houses, if there is time. They may take precious
belongings, pets and essential items such as clothes, food and money. They will
have to stay somewhere else during the flood. Damage is still done to the
property and has to be dealt with when the owners return.
Raising river banks
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Governments can spend money to make river banks higher using soil or
concrete. This is called a levee. When there is more water in the river, it should
stay in the channel as the banks are higher.
Flood wall
Sometimes a flood wall can be built by the river bank to protect the town from
floods. If the river bursts its banks, the water should stay in front of the flood
wall. The water will not reach the town as it is trapped between the river bank
and the flood wall.
Dams
Dams are also used to control floods. In times of really heavy rainfall, more
holes can be opened to allow more water to flow into the river downstream. The
water can be released quickly before it has a chance to build up.
Lake Vyrnwy Dam, Wales
Floodways
Floodways are artificial river channels which can be built to divert the flow of
extra water in rivers. The water then has two channels to flow through in times
of heavy rainfall, meaning that the river banks should not burst.
Flood barriers
Flood barriers can be built across rivers. These are like walls and are similar to
dams. They are usually built near the mouth of a river. Flood barriers protect
against floods caused by high tides or strong winds. The flood gate is closed to
keep the water out of the river. In times of normal water level, the gates are
kept open to allow ships to pass through. The Thames Flood Barrier is an
example.
The Thames Barrier the world's largest movable flood barrier, spans 520 metres
(a third of a mile) across the Thames at Woolwich Reach, South East London,
where it protects the UK's capital city from flooding.
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Floods in Mozambique
The floods in Mozambique in 2000 were devastating in their extent and impact
on life. These floods claimed many lives and left
thousands of survivors homeless and with shortages in
food, clean water and employment. Amazing stories of
survival against the odds and bravery were reported
daily in newspapers worldwide - including the mother
who gave birth to a child in a treetop, where she was
escaping the rising floodwaters waiting to be rescued by
helicopter.
Activities
• Identify a river catchment area - Use a map or atlas to identify a major river
near to where you live, and use tracing paper to identify and draw its source
and its mouth. Then trace on the streams, brooks and smaller rivers that
feed into your main river - these are called the tributaries. The source of each
of these tributaries marks one point on the boundary of the river - joining
these points up shows the approximate limits of the catchment area of your
river.
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Use an atlas to locate and mark Boscastle and the course of the River
Valency and its tributaries on a blank outline map of Cornwall.
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The Boscastle flood happened 52 years to the day after the disaster that
devastated Lynmouth in North Devon. Find out more about this flood and
compare the two.
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Try searching for information on how the Ancient Egyptians used the regular
annual flooding of the River Nile to help their agricultural activities.
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Why do you think that so many cities developed close to rivers despite the
obvious risks?
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What flood prevention methods can you find in your local area?
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