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Surges result from a coincidence of meteorological conditions. One is a trough of low atmospheric pressure over the North Atlantic, causing the water to rise in a kind of hump. When this low moves southward into the North Sea powered by northerly gales, the hump is funneled into the Thames estuary. If the tide is rising as well, the result can be a huge mass of water growing ever higher as the river narrows near London.
To make matters worse, Britain is slowly tilting, apparently an aftereffect of the last great Ice Age, with the northwest of the country rising and the southeast sinking at about a foot a century. London, built on a foundation of clay, is settling even faster; and normal tides are becoming higher and higher.
Because continuing to raise the walls along the Thames would cut off London's view of the river and ruin the historic waterfront, government officials years ago began looking for another answer to the flood peril. In 1972, after weighing dozens of schemes, they settled on a novel system of floodgates proposed by Engineer Charles Draper, 47. The entire barrier, a 521-meter (1,710 ft.) structure, will span the Thames 14 km (8.5 miles) downstream from London Bridge. It has ten openings, six of which are large enough to accommodate the more than 5,000 seagoing ships that are expected to pass through each year.
The gates in these larger openings are actually sections of cylinders, rounded on one side, flat on the other, with a disc at either end. Under ordinary circumstances, they will lie flat-face up in rounded troughs planted in the riverbed. But if there is a threat of flooding, each of the gates will be rotated 90° by hydraulic rocker arms, housed at either end inside well-designed piers (which resemble the futuristic opera house in Sydney, Australia). The rotation raises the gates to a vertical position, turning them into five-story-high barricades against the rising waters. Though the gates weigh 3,200 tons each, they can be raised in only 30 minutes.
Now nearly two-thirds completed, the project is expected to cost more than $800 million. Additional flood defenses along the banks downstream (to contain the water blocked by the barricade) are expected to put the total tab of taming the Thames at more than $1.5 billion. Meanwhile, the Greater London Council is taking precautions against any flooding that might occur before the great Thames barrier is in place. With the onset of winter gales, the council is stepping up distribution of instructions about what to do in case the waters begin to rise. Says one poster: "If you live, work or travel through the Flood Risk Area, you should learn the Thames Flood Drill now. Cut it out and keep it handy. We hope you'll never need it. There's a 1 in 50 chance you will."