On This Day – 8th May

Officially opened on 8th May 1984 by Queen Elizabeth II, The Thames Barrier, Built approximately 3 km (1.9 mi) due east of the Isle of Dogs, prevents the floodplain of most of Greater London from being flooded by exceptionally high tides and storm surges moving up from the North Sea. When needed, it is closed (raised) during high tide; at low tide it can be opened to restore the river's flow towards the sea.

Built across a 520-metre (570 yd) wide stretch of the river, the barrier divides the river into four 61-metre (200 ft) and two, approximately 30 metre (100 ft) navigable spans. There are also four smaller non-navigable channels between nine concrete piers and two abutments.

The flood gates across the openings are circular segments in cross section, and they operate by rotating, raised to allow "underspill" to allow operators to control upstream levels and a complete 180 degree rotation for maintenance. All the gates are hollow and made of steel up to 40 millimetres (1.6 in) thick. The gates are filled with water when submerged and empty as they emerge from the river.

The four large central gates are 20.1 metres (66 ft) high and weigh 3,700 tonnes each. Four radial gates by the riverbanks, also about 30 metres (100 ft) wide, can be lowered. These gate openings, unlike the main six, are non-navigable.

Today's question – how many times has the barrier been closed since it has been in operation?

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