at Portland in 1826, which was withdrawn in 1851. Coastal flooding has affected Portland's residents and transport for centuries—the only way off the island by land is along the causeway in the lee of Chesil Beach. At times of extreme floods (about every 10 years) this road link is cut by floods. The low-lying village of Chiswell used to flood on average every 5 years. Chesil Beach occasionally faces severe storms and massive waves, which have a fetch across the Atlantic Ocean. Following two severe flood events in the 1970s, Weymouth and Portland Borough Council and Wessex Water decided to investigate the structure of the beach, and coastal management schemes that could be built to protect Chiswell and the beach road. In the 1980s it was agreed that a scheme to provide storm protection with a 20% annual exceedance probability to reduce flood depth and duration in more severe storms. Hard engineering techniques were employed in the scheme, including a gabion running to the north of Chiswell, an extended sea wall in Chesil Cove, and a culvert running from inside the beach, underneath the beach road and into Portland Harbour, to divert flood water away from low-lying areas. At the start of the First World War, HMS Hood was sunk in the passage between the southern breakwaters to protect the harbour from torpedo and submarine attack. Portland Harbour was formed (1848–1905) by the construction of breakwaters, but before that the natural anchorage had hosted ships of the Royal Navy for more than 500 years. It was "the home of the Asdics," a centre for Admiralty research into asdic submarine detection and underwater weapons from 1917 to 1998; the shore base HMS Serepta was renamed HMS Osprey in 1927. During the Second World War Portland was the target of 48 air raids and a total of 532 bombs, although most warships had moved north as Portland was within enemy striking range across the Channel. Mulberry Harbour Phoenix Units can be seen at Black Barge beach, near Portland Castle. Portland was a major embarkation point for Allied forces on D-Day in 1944. Early helicopters were stationed at Portland in 1946–1948, and in 1959 a shallow tidal flat, The Mere, was infilled, and sports fields taken to form a heliport. The station was formally commissioned as HMS Osprey, which then became the largest and busiest military helicopter station in Europe. The base was