The Thames Barrier, the 520 metre wide barrier on the river at Woolwich, has been raised more than 20 times in the last fortnight.

The barrier is used or raised to block the flow of water.  It can be used to prevent a high tide flowing into London and flooding 125 square kilometres of Central London, or it can be used to create an artificially low water level upstream for flood waters to drain into.

The barriers have helped prevent flooding twice in December when fierce storms led to storm surges and the highest tides in 60 years.  And they have been raised more than 20 times in the past fortnight because of severe rain to regulate river flooding which would have been worse that it already is.

The barriers were only raised 4 times in the 1980s, 35 times in the 1990s and more than 100 times since 2000, with 40 raises in 2014 alone.

Source: Financial Times