can cause billions of pounds- worth of damage and many deaths. Many
people still choose to live on flood plains because the land is
good for farming and for building on. But if these areas are hit
by unusually stormy weather, the overflowing rivers can bring disaster.
>> In Bangladesh's
captial, Dhaka, people standing waist- high in floodwater queue
to collect clean drinking water.
When more than 15mm of rain falls in 3 hours, meteorologists describe
the conditions as a "flash flood". Here are some major reasons for
flash floods to occur.
sometimes occur when motionless or slow- moving thunderstorms produce
heavy rain over a particular area. When this happens, over 50 mm
of rain can fall in an hour. The ground very quickly becomes full
of water and cannot absorb any more. Water then runs along the surface
and into rivers, causing them to overflow.
snow can also cause flash floods, sometimes many hundreds of kilometres
from the areas of heavy snowfalls.
floods are very destructive, not only because there is so little
warning of them but also because the water flows so powerfully and
contains a lot of mud, sediment and boulders.
floodwater needs to drain away quickly, many cities have huge storm
drains to carry away rushing water. In coastal towns, sea walls
and dykes are built to keep out waves. To prevent rivers bursting
their banks, walls or embankment are sometimes raised. Special flood
barriers have been built in some large cities to protect them from
the water that sometimes surges upriver during storms.
The Thames Barrier was opened in 1984. It protects the people of
London from floods. If exceptionally high tides surge up the river,
20 metre- high steel gates close to protect the city.
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