A wall of water hurtling down the North Sea, bursting over the Thames Barrier and swamping London makes a great story, but some awful science.

The Atlantic is the breeding ground for this fictional disaster. A storm with hurricane-force winds skirts around Scotland and turns south into the North Sea, bashing the East Coast with mountainous waves. So far, so good — this is the nightmare that flood forecasters fear most. Weather and geography make the North Sea a treacherous bottleneck, where a storm’s wind and low pressure pile up a huge bulge of seawater and ram it down the throat of the Straits of Dover.

This is the dreaded storm surge. But instead of a colossal battering ram seen in the film, all you would see is the sea gradually rising, like a tide coming in. And unlike the meteorologists left aghast at the unfolding disaster, there would be plenty of warning. A formidable network of coastal tidal gauges, monitoring buoys, weather satellites and computers would raise the alarm days ahead of a storm surge.

London has a long history of floods, though, as Samuel Pepys wrote on December 7, 1663: “There was the last night the greatest tide that ever was remembered in England to have been in this River: all Whitehall having been drowned.”

The storm surge of 1953 was the most powerful in recent times and ripped open flood defences along the East Coast and Thames Estuary, killing more than 300 people. London was spared only because the coastal floods diverted enough floodwaters away from the Thames.

That was the wake-up call to set up a flood warning system and build the Thames Barrier. These days it would take a mega-storm, something far bigger than the 1953 event, to breach the barrier, which is so unlikely at present that the film is really unbelievable.

If the unthinkable should happen, the number of people drowned is unlikely to reach 200,000 as in the film, because there would be enough warning to evacuate. However, the disaster of Hurricane Katrina in New Orleans shows the plight of trying to get the poor, elderly and sick to safety.

London’s greatest problem is complacency. The current defences are so good that they give a false sense of security — but climate change is whittling away the odds of another flood as sea levels rise, and eventually a storm surge will burst over the barrier, but probably not for 50 years or more. Plans are under way to make new defences to last another century. The costs could be huge, and the politicians will probably prevaricate, but Flood is a reminder of the catastrophe that could strike if nothing more is done to protect London.

Paul Simons writes the daily Weather Eye column in The Times

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