The Thames Barrier is a flood control structure on the river Thames, located downstream of central London. Designed by Rendel, Palmer and Tritton, it was finished in 1982 and officially opened in 1984. It's purpose is to protect London from being flooded by an exceptionally high tide moving up from the sea, exacerbated by a storm surge. At low tide it is opened to release water flowing down the Thames. The barrier divides the river into four 61m and two 31m navigable spans and four smaller non-navigable channels between nine concrete piers and two abutments. The flood gates across these openings are circular segments in cross section, raised by hydraulics and operated by rotating. It is the world's second largest movable flood barrier, the largest being the Maeslantkering in The Netherlands.