Thames Barrier

 The Thames Barrier is a 1,710-foot wide movable flood control barrier across the River Thames just downstream from central London.  After a ten-year construction period, it was officially opened by Her Majesty Queen Elizabeth II on May 8, 1984.  Its mission is to prevent London from being flooded during periods of exceptionally high tides and/or storm surges.  The North Sea Flood of 1953, during which 307 people in the UK died, provided the impetus for construction of the Barrier, which cost approximately £530 million.  The Barrier consists of four major navigable spans (each 200 feet wide – the same width as the center span of Tower Bridge) and two smaller navigable spans (each 100 feet wide).  There are also four small non-navigable spans.  The flood gates are circular segments (arcs) that operate by rotating through 180 degrees.  When down, they allow the river to flow freely and ships to pass as usual.  They are raised to a fully vertical position for maintenance.  When raised approximately half-way, the gates block the flow of water.  When water levels for central London are forecast to exceed about 16 feet within the next nine hours, the process of raising the flood gates to block the flow begins.  River traffic is alerted.  The gates remain in position until the water level downstream becomes lower than that upstream of the Barrier.  There were four operational closures of the Barrier during the 1980’s, 35 operational closures during the 1990’s, and 75 operational closures between 2000 and 2009.  Geologists report that Britain is slowly tilting, with the north and west rising and the south and east subsiding, due to what is called the post-glacial rebound.  In addition, sea level worldwide is slowly rising.  Therefore, trends indicate that operational use of the Thames Barrier will continue to increase.