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See the famous barrier in action during its annual test closure. Gleaming silver and 520 meters across, the Thames Barrier straddles the river at Woolwich Reach in East London. Built to protect London from tidal surges from the North Sea, it's one of the capital's most dramatic landmarks as well as a remarkable feat of engineering. We'd be in big trouble without it. As sea levels rise and the south eastern corner of the British Isles tilts slowly downward, high tide in central London is rising at a rate of approximately 75cm a century. Add the effect when an area of low pressure moves eastwards across the Atlantic (which can raise the level of the seawater beneath it by up to a third of a metre), combined with the impact of high winds from the north and you can get surge tides of up to four metres heading up the Thames Estuary towards the capital. When heavy rain causes flood water from upstream to meet a high surge tide things get even more serious. Each month individual gates are closed to allow for maintenance and training and once a year there's a full closure, which means that above the barrier it's low tide all day. This year's full test closure will take place on Sunday October 9.