The Thames Barrier spans the river at Woolwich Reach, where the river is 520 metres wide. It is an amazing feat of engineering built to control flooding along the River Thames. Construction began in 1972 and took ten years to complete. The barrier is formed from 10 moveable underwater gates, which are supported by large steel half-domes placed across the river.

When the barrier is not in use, the gates are open, allowing the free movement of river traffic. The gates can close in minutes, forming a continuous steel wall that seals off the upper river from the sea.

Over 80 members of staff at the Environment Agency operate the barrier and its associated flood defences. The Barrier Controller makes the decision to close the barrier based on the predicted height of the incoming tides. Dangerous conditions can be forecast up to 36 hours in advance. Ideally, the barrier is closed three to four hours before the peak of the incoming tide reaches the site. This allows time for local shipping to be informed by the Port of London Authority.