The barrier protects central London against a storm surge, caused when a deep depression forms to the north of Scotland and progresses across the North Sea and south-easterly towards southern Scandinavia. When such a surge coincides with a high spring tide, the high winds associated with the depression can funnel the water up the Thames Estuary and cause surges of up to 3.5 metres (11.6 feet). The planners assessed that in the absence of a barrier, such a surge could inundate 45 square miles (117 km2) of land, put hospitals, power stations and the London Underground out of action and cause damage estimated in 1966 at £2.0 billion (about £50 billion at 2020 prices). The barrier was designed to provide a flood defence capable of resisting a once in 1000 year surge tide at a base date of 2030.