Temps de lecture :4 minutes
LAST winter’s floods on the rivers of central Europe were among the worst since the Middle Ages, and as winter storms return, the spectre of floods is returning too. (…) Perhaps it is time for a new response to flooding. Traditionally, river engineers have gone for Plan A: get rid of the water fast, draining it off the land and down to the sea in tall-sided rivers re-engineered as high-performance drains. But however big they dig city drains, however wide and straight they make the rivers, and however high they build the banks, the floods keep coming back to taunt them, from the Mississippi to the Danube. And when the floods come, they seem to be worse than ever.
No wonder engineers are turning to Plan B: sap the water’s destructive strength by dispersing it into fields, forgotten lakes, flood plains and aquifers. Progressive planners are tearing down banks, dykes and levees to return the rivers to their flood plains. They are putting back meanders and marshes to slow the flow, and even plugging city drains to encourage the flood waters to percolate underground. Rivers need room to flood, they say. And cities need to become more porous.
“The recent floods have provoked a completely new way of thinking,” says hydrologist Piet Nienhuis of the University of Nijmegen in the Netherlands. Back in the days when rivers took a more tortuous path to the sea, flood waters lost impetus and volume while meandering across flood plains and idling through wetlands and inland deltas. But today the water tends to have an unimpeded journey to the sea. And this means that when it rains in the uplands, the water comes down all at once.
Worse, whenever we close off more flood plain, the river’s flow farther downstream becomes more violent and uncontrollable. Dykes are only as good as their weakest link – and the water will unerringly find it. By trying to turn the complex hydrology of rivers into the simple mechanics of a water pipe, engineers have often created danger where they promised safety, and intensified the floods they meant to end.
Take the Rhine, Europe most engineered river. For two centuries German engineers have erased its backwaters and cut it off from its flood plain. The aim was partly to improve navigation, and partly to speed flood waters out of the Alps and down to the North Sea. Today, the river has lost 7 per cent of its original length and runs up to a third faster. When it rains hard in the Alps, the peak flows from several tributaries coincide in the main river, where once they arrived separately. And with four-fifths of the lower Rhine’s flood plain barricaded off, the waters rise ever higher. The result is more frequent flooding that does ever-greater damage to the homes, offices and roads that sit on the flood plain.(…)
“The focus is now on working with the forces of nature. Towering concrete walls are out, and new wetlands are in.“
To help keep London’s feet dry, the agency is breaking the Thames’s banks upstream and reflooding 10 square kilometres of ancient flood plain at Otmoor outside Oxford. Nearer to London it has spent £100 million creating new wetlands and a relief channel across 16 kilometres of flood plain to protect the town of Maidenhead, as well as the ancient playing fields of Eton College. And near the south coast the agency is digging out channels to reconnect old meanders on the river Cuckmere in East Sussex that were cut off by flood banks 150 years ago.
The same is taking place on a much grander scale in Austria, in one of Europe’s largest river restorations to date. Engineers are regenerating flood plains along 60 kilometres of the river Drava as it exits the Alps. They are also widening the river bed and channelling it back into abandoned meanders, oxbow lakes and backwaters overhung with willows. The engineers calculate that the restored flood plain can now store up to 10 million cubic metres of flood waters and slow storm surges coming out of the Alps by more than an hour, protecting towns as far downstream as Slovenia and Croatia. “Rivers have to be allowed to take more space. They have to be turned from flood-chutes into flood-foilers,” says Nienhuis.
And the Dutch, for whom preventing floods is a matter of survival, have gone furthest. A nation built largely on drained marshes and seabed had the fright of its life in 1993 when the Rhine almost overwhelmed it. The same happened again in 1995, when a quarter of a million people were evacuated from the Netherlands. Nienhuis says the subsequent inquest concluded that “the rivers had been straightened and the land had been built on so much that there was nowhere for the water to go“. Since then, the Dutch have resorted to Plan A by reinforcing dykes. But they have also broken one of the most enduring national stereotypes by giving engineers the go-ahead to punch holes in dykes, making plans to return up to a sixth of the country to soggy nature in order to better protect the rest.[…]
New Scientist magazine issue 2429 , page 26
January 10th 2004
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