Tuesday, August 21, 2007

After the flood

[This is the pre-edited version of my Lab Report column for the September issue of Prospect.]

Can there be a pub in the country that has not witnessed some sage shaking his head over his pint and opining “Well, if you will build on a flood plain…”? These bar-room prophets are, as usual, merely parroting the phrases they have heard from Westminster, where flood plains have become the talk of the House. “Gordon Brown has to accept the inconvenient truth that if you build houses on flood plains it increases the likelihood that people will be flooded”, says shadow local government secretary Eric Pickles. But the chief executive of the National Housing Federation counters that “there’s simply no way we can’t build any more new homes because of concerns about flood plains… much of the country is a flood plain.”

But what exactly is a flood plain? Perhaps the most pertinent answer is that it is a reminder that rivers are not, like canals, compelled to respect fixed boundaries. They are, in fact, not things at all, but processes. Surface water flow from rain or snow melt, erosion, and sediment transport combine to produce a river channel that constantly shifts, redefining its own landscape. The meanders gradually push back the surrounding hill slopes and smooth out a broad, flat valley floor, thick with fertile sediment: the perfect setting for agrarian settlements, or so it seems. The catch is that when the river waters rise above the banks, there is nothing to hold them back from washing across this wide plain. Levees may try, but struggle against the fact that a river’s curves are always moving slowly: the Mississippi shifts its tracks by up to 20 m a year. One of the fundamental problems for building near rivers is that buildings stay put, but rivers don’t.

What’s the solution? To judge from recent events, it hasn’t changed much in a hundred years: you pile up sandbags. But some precautions are still little heeded: replacing soil with concrete exacerbates the dangers by increasing runoff, and the inadequacies of Britain’s Victorian drainage system are no secret. There’s nothing particularly sophisticated about flood defence: it’s largely question of installing physical barriers and gates. But permanent walls can create conflicts with access and amenity – no one would tolerate a three-foot wall all along the Thames. And some areas are simply impossible to protect this way. So there’s no real call for new science or technology: it’s more a matter of recognizing that flood threats now have to be considered routine, not once-in-a-lifetime risks.

The UK floods were the worst for 60 years, and claimed at least nine lives. But the tribulations of a soggy summer in Gloucester are put in perspective by the situation in Asia. In China, heavy rainfall in the north brought flooding to the Yangtze, and the combined effects of storms has affected one tenth of the population. In a reversal of the usual situation where the parched north envies the moist south, a heatwave in the southern provinces has left more than a million short of drinking water. Meanwhile, an unusually intense monsoon has devastated parts of India and Bangladesh, killing more than 2000, displacing hundreds of thousands from their homes and affecting millions more. A map of the flooded areas of Bangladesh is almost surreal, showing more than half the country ‘under water’.

There’s little new in this, however. Low-lying Bangladesh floods to some degree most years. The Yellow River is commonly known as China’s Sorrow, which has brought recurrent catastrophe to the country’s Great Plain well over a thousand times in history despite herculean efforts to contain its flow with dikes. A flood in 1887-8 created a lake the size of Lake Ontario and, one way or another, killed an estimated six million.

But perhaps surprisingly, some in China have been more ready than in the West to blame the recent events on global warming. Dong Wenjie, director-general of the Beijing Climate Centre, claims that the frequency and intensity of extreme weather events are increasing, and that this “is closely associated with global warming.” Well, maybe. No single event can itself be interpreted one way or the other. The most one can really say is that it is in line with what global warming predicts, as the hydrological cycle that moves water between the seas and skies intensifies – although that by no means implies more rain everywhere. That regional variation, in fact, was a central component of the recent claim by scientists to have detected evidence of global warming on 20th-century rainfall: computer models predict that this influence has a particular geographical fingerprint that has now been identified in the data. It’s a clear sign that the future predictions of more extreme weather – droughts as well as floods – need to be taken seriously.

One question so far given rather little consideration is what this implies for the major hydraulic engineering projects underway in Asia. Ten years ago, specialists in water-resource management were predicting that the problems evident with existing big projects, such as the Aswan Dam on the Nile, might curtail the era of mega-dams and suchlike. Now that looks unlikely: China’s Three Gorges dam is basically complete, and both China and India seem set on ambitious and controversial schemes to transfer waters between their major rivers. The South-North Water Diversion Project in China is scheduled to deliver water to Beijing in time for the Olympics from over 1,000 km away, while the massive Interlinking Rivers project in India would convert the entire country into a grid of waterways controlled by dams, with the aim of alleviating both flooding and drought.

Both of these schemes are already fraught with economic, environmental, social and scientific questions. The prospect of greater variability and more extremes of rainfall can only make the issues more uncertain, and prompts us to shed the illusion that we understand what rivers can and will do.


JimmyGiro said...

If the great volcano eruptions of the past have caused global cooling due to the SO2 they launch into the stratosphere, then maybe global warming can be mollified by issuing sulphur hoppers to all high altitude aircraft engines. To deploy albedo enhancing cloud formation whenever the aircraft sees sunlight whilst flying in the thermosphere.

If the idea takes off, I expect 10% :))

Philip Ball said...

You're quite right Jim - seeding the atmosphere with sulphur to create more aerosol scattering is one of the geo-engineering solutions htat has been proposed as an emergency measure, in this case by Nobel laureate Paul Crutzen. It would be a very drastic move - let's hope it doesn't come to that.

I'm glad my suspicions were correct - nice to hear from you after so many years. By the way, I think I usually used to win at table tennis.

JimmyGiro said...

Damn those Nobel Laureates, coming over here and pre-empting our ideas!

By the way, the cleaner ladies told me to let you win, so you wouldn't break so much furniture.

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