At last the lesson King Canute tried to teach his subjects, that no man can turn back the tide, has finally sunk in. After 1,000 years of building sea walls and grabbing more and more land from the sea, carefully built sea defences are, in many places, beginning to be deliberately destroyed.

Although, at £400 million a year, more is still being spent on 20,000 miles of flood defences than at any time in history, the Environment Agency and the Government have accepted that all of Britain cannot be protected from the sea. As the East and South of England sink below sea level at increasing speed, the plan is that more and more low lying land will be abandoned. Experiments have shown that a sea wall costs £5,000 a metre to build and maintain. However, retreating 80 metres inland and allowing a salt marsh to form breaks up the tide and waves, so the new sea wall sheltering behind it costs only £400 a metre to do the same job. The problem for the Environment Agency and the Government is that the sea is rising all the time, and the natural salt marshes are being washed away, leaving more and more sea wall exposed.

The problem is made worse by two factors. The South-East continues to tilt into the sea as a result of the "rebound" of the Scottish mountains from the last Ice Age, and the sea level is rising every year because global warming is both melting glaciers and causing thermal expansion of the oceans. Sea levels may rise by a metre (about 39 inches) in some places in fifty years. Add to that the effect of higher tides and large areas vulnerable to a storm surge. There is no suggestion that urban areas will be abandoned, but like Canvey Island in the Thames Estuary, "drowned" in the 1953 flood, they may become islands surrounded by sea defences while agricultural land is used as a buffer against floods.

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