How we defend against floods

An estimated 37 per cent of the world’s population
live within 100 kilometres (62 miles) of the coast
– many in major cities like London, New York and
Tokyo. They are at risk of fl ooding from unusually
high tides and storm surges.
Many cities protect themselves from this threat
with hi-tech barriers, dams or sluice gates. For
example, the Thames Barrier downstream of
London is a set of gates that lie fl at on the riverbed
when open. The gates rotate upright when a storm
surge heads inland from the North Sea, or heavy
rainfall raises the river level of the Thames enough
that normal tides will cause fl ooding. Each gate can
hold back 90,000 tons of water.
Other fl ood defence systems have permanent
dams with sliding sluice gates to control water fl ow.
The Oosterscheldedam in the Netherlands – the
largest tidal barrier on Earth – is nine kilometres (5.6
miles) long and has 62 colossal gates; these raise to
make the structure watertight.
New Orleans was flooded in Hurricane Katrina in
2005 – one of the deadliest storms in US history.
Much of the city lies below sea level and depends
on man-made embankments to keep out water.
During the hurricane, these levees broke. The US
Army is renovating the levees and building new
defences, including a pumping station able to pump
540 cubic metres (19,000 cubic feet) of rainwater
every second – enough to empty an Olympic-sized
swimming pool in five seconds.

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