Climate Change: Storm warning

Will devastating floods hit Britain next month? And what if they do?

From his throne on the shore at Bosham, Sussex, Canute, the 11th-century Danish king of England, addressed the sea: "You are subject to me, as the land on which I am sitting is mine, and no one has resisted my overlordship with impunity. I command you, therefore, not to rise on my land, nor to presume to wet the clothing or the limbs of your master."

Inevitably, his feet got wet. And thus began the British battle with the sea, one that the waves have won time and again.

When the threat of another imminent victory for the sea emerged last week, angry eyes turned to the Government, which has been quietly cutting its spending on flood defences. Fears that they would soon face the worst floods in a generation swept through communities along Britain's coasts after scientists warned of unusually high tides in September and October.

A tidal storm surge of the sort that devastated the east coast 50 years ago could bring New Orleans-style flooding to Hull, Portsmouth and Cardiff, it was reported. And despite the billions of pounds poured into coastal defences over the past half century, we were said to be unprepared for the littoral apocalypse that was about to pound through low-lying villages.

Such a disaster is certainly possible, but is it probable? Britain's rulers, and their engineers, have become more ambitious - presumptuous even - since Canute's day. The Government spends £800m a year on flood defences, yet it is still getting its feet wet; Britain suffers an average of £1.4bn in flood damage annually. More than two million properties - worth £200bn and housing four million people - are at risk.

And the danger is set to swell massively over the course of the century. Damages would rise to £27bn a year by 2080 under the worst scenario considered in the Foresight Future Flooding report, a study commissioned by the Department of Trade and Industry. "In all the scenarios we looked at, flood risk is going to increase a lot," said Edward Evans, a consulting engineer and the lead technical adviser on the report.

Even the most optimistic scenario looked at by the Foresight think tank, with low economic growth and an emphasis on environmentally friendly policies, would see annual damages nearly double by 2080. And much less money would be available for building new defences.

While flooding is possible throughout Britain when rivers swell with rainwater, the biggest danger is along the coasts and tidal estuaries, particularly those on the east and south coasts. The Thames Barrier protects London for now, but already plans for its replacement in 2030 are being prepared. Other towns and cities are less well defended.

Rising sea levels due to global warming account for only a quarter of the £27bn forecast in the report. A tiny percentage comes from the gradual sinking of the south-east, a see-saw response to the rebound of the Scottish Highlands since the glaciers retreated at the end of the last ice age. But most of the wealth at risk is due to development on vulnerable flood plains, Mr Evans said. And that's unlikely to change. Cities, the densest concentrations of society's assets, tend to be sited on river crossings or coasts. One only has to look to New Orleans to see what can happen when the defences around a city fail during a severe storm.

So what is the Government doing about this threat to life and property? Not what you might want if you lived on a coastal floodplain or cliff top. In a daring U-turn four years ago, after several years of preparation, the Environment Agency adopted a new policy of "managed retreat" from the sea. Instead of relying solely on giant concrete defences, it is putting more of its faith in sand dunes and salt marshes, nature's way of absorbing both the water and the energy it carries. The policy is summed up in its new catchphrase, "making space for water". Whitehall, it seems, has at last learnt Canute's lesson.

But as the Department for Environment, Food and Rural Affairs cut its budget for sea defences by £15m this year, the Association of British Insurers warned that many properties might become uninsurable. The Government, some feared, had forgotten another lesson, from the most awful storm in living memory.

A gale had been blowing down the east coast of England for 36 hours by the evening of 31 January 1953. In Hunstanton, on the Norfolk coast, police had warned residents on South Beach Road - an area of caravans and holiday chalets that had been flooded in the past - about the weather, but hadn't insisted they evacuate. Just before 8pm, sirens called firefighters to their station; the tide was breaking on the wall of the promenade and their pumps were needed.

By dawn, the toll wrought in Hunstanton could be counted - 31 dead from drowning or exposure, including several children. In total, 312 were killed in Britain and 224 off shore, including 132 on the MV Princess Victoria, one of the first roll-on-roll-off ferries. A thousand miles of coast was inundated and 24,000 properties seriously damaged.

And Britain got off easy. In the Netherlands, the storm claimed more than 1,800 lives and a 10th of the country ended up under water. In one of many heroic incidents, the mayor of Nieuwerkerk commandeered the river ship de Twee Gebroeders and ordered its captain to sail it into a breach in the Groenendijk, saving two whole provinces and three million people.

The sea wall at Hunstanton, which had a gap at its southern end in 1953, is today complete. The top is as wide as two motorway lanes and a borough councillor, Bryan Bullivant, assures me as we stroll along it on a blustery, overcast afternoon that it cost just as much to build. Deep concrete steps lead down to the beaches, which are protected from erosion by zig-zagging wooden groynes. To the north, past the doomed lighthouse (expected collapse: 2075) are the spectacular two-tone cliffs - chalk over sandstone. Inland, over the curved concrete wave reflector, we can see the roofs of caravans. In a 1953-size stormsurge, they would be completely submerged.

Mr Bullivant is untroubled despite his ward's sad history. "There's no such thing as a surprise flood any more," he says. The Storm Tide Forecasting Service, a joint project of the Met Office and the Proudman Oceanographic Laboratory in Liverpool, uses computer models to warn of storm surges up to 48 hours in advance. The Environment Agency's flood control centre at the Thames Barrier then notifies its regional command posts, which send out automated voice warnings. And in Hunstanton, an additional force of volunteers is mobilised to knock on the doors of every property at risk.

Meteorologists describe the 1953 storm as a one-in-a-250-year event. (It arrived exactly a quarter of a millennium after its predecessor, the Great Storm of 1703 chronicled by Daniel Defoe). So while a repeat could happen at any time, the probability of such a combination of tides and weather coming this year is low.

Tides vary according to overlapping cycles, a phenomenon first explained by Sir Isaac Newton in 1686. The shortest are the fortnightly spring tides, when the Earth, Moon and Sun are in line. The combined gravity of the Moon (70 per cent) and the Sun (30 per cent), is enough to raise the seas, not straight up, but laterally, creating bulges, one of which is dragged anticlockwise around the North Atlantic as the Earth rotates. When this bulge reaches the tip of Scotland, it turns to begin an anticlockwise tour of the North Sea.

A proxigean spring tide happens every 18 months, when the Moon's closest approach to Earth, 8 per cent nearer than average, coincides with it being a new moon. If this happens at one of the equinoxes, around the 21st of March or September, they will be higher still, because then the Sun is above the equator where there is more water for its gravity to act upon. Together, these factors form an irregular cycle lasting four and a half years.

In the open ocean, tides are tiny, typically around 50cm, but above continental shelves, such as the large one surrounding Britain, they rise much higher, as there is less water to be raised. Furthermore, the North Sea is shaped like a funnel, so as the tidal bulge moves towards the Strait of Dover, it is squeezed between the approaching coasts.

If a severe storm coincides with a high tide, it can drag the sea's surface even higher. Every millibar fall in pressure adds a centimetre to the sea level, so a strong storm could raise it by half a metre. Winds can add another two metres. The result would be seas up to five metres higher than average, with waves on top - more than enough to slosh over some sea walls, and with the power to knock holes in weaker defences.

When the next big storm comes Britain's defences are likely to be quite different from today's. The Environment Agency aims to turn 500 acres a year into salt marshes, says Phil Rothwell of the Environment Agency. And sea defences at places such as Freiston, Lincolnshire, are being breached.

"We refer to a coastline, but it's never been a line," he says. "Now we have the option of letting some places become wetter, which is what nature wants us to do." Like Canute, we're realising that even in the 21st century, we can't command the seas.