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Heroic vision, state-of-the-art engineering and sheer physical courage together span the mouth of the Seine and take the breath away. JONATHAN GLANCEY reports. Photographs by JEAN GAUMY

Imagine the whole of the Champs Elyses from the Place de la Concorde to the Arc de Triomphe raised 50 metres into the air, the Seine flowing beneath it. Imagine this and you will have a good idea of the scale and audacity of the Pont de Normandie, the world's latest, longest and most stunning cable-stay bridge, which opened in January.

Cable-stay bridges are successors to the great suspension bridges that, since the Brooklyn Bridge of 1883, have challenged engineers to span rivers, bays and ravines with increasing refinement and calculated bravura. A titanic spider's web of concrete and steel cable, the Pont de Normandie stands sentinel over the mud flats of the Seine estuary, linking the ports of Honfleur and Le Havre. It has changed this famous waterscape for good, and certainly not for the worse. Had Monet painted Impressions: soleil levant today, rather than at the same spot 123 years ago, he would have plashed oil on canvas in the complex shadow of the bridge. It is to these estuarine mud flats that the Impressionists flocked, revolutionising 20th-century art, much as engineers revolutionised architecture and our perception of the landscape.

The Pont de Normandie, however, is less revolutionary than evolutionary. It has been under discussion since 1972, when the engineering principles on which it was built were being applied for the first time, but the work wasn't started until 1988.

The press blurb for the opening of the bridge claims, rather bizarrely, that it is a "technical showcase on a par with EuroDisneyland". The Pont de Normandie is far more impressive than that, as Jean Gaumy's photographs show. Gaumy, a Magnum photographer, has spent the past five years recording the building of the world's most advanced bridge. He has watched as hundreds of thousands of gallons of concrete have been poured into foundations as deep as 20-storey office blocks; as 2,000kilometres of cable have taken up the weight of 33 sections of concrete and steel carriageway. He has been witness to the expenditure of ten million hours of human skill and 1,450 million French francs (£200million). He has watched engineers pore over 5,000 pages of computer-generated calculations and recorded the daring of death- defying young mountaineers who have hung like spiders from this steel web in winds of up to 80mph to weave the metal fabric together.

The scale and ambition of the bridge are daunting. From each of the twin 214-metre concrete towers embedded in the 150 million-year-old rock bed, 50 metres below a shifting tide of Normandy sand, peat, clay and gravel, 184 stays stretch out to carry the weight of the box girder dual carriageway spanning the estuary. The bridge was built section by section, stay by stay until the final centre section - designed like an inverted aircraft wing - was raised up from below to slot accurately into place.

Of course, the whole structure will sway in the wind, but dampers throughout its length have been designed to control it. However, to cross a river bridge for the first time is never less than an act of faith. Have engineers Demare, Lavour and Deroubaix got it right? Could there be just one tiny error in those 5,000 pages of computer printouts? Unlikely, yet bridges are forever in delicate balance and even battle with the brute force of nature; this and the weight of 6,000 vehicles a day paying between 30 and 70francs each for the privilege of saving 20 minutes on the journey from Le Havre to Honfleur.

To date, no one is quite sure how far the cable-stay bridge can be stretched; cautious souls, the world's engineering experts say that a 1,000-metre span is the limit; the span of the Pont de Normandie is 856 metres. This excludes the approach ramps. The total length of the bridge is 2,200metres, well over a mile. And for all the engineers' caution, a bridge such as this remains a leap of the imagination carried aloft by miles of cables and reams of checked and rechecked calculation. Such a bridge requires immense skill and daring in its construction, and unless wholly jaded by life in the motorway age, most of us remain thrilled by the sight of it, as we are by the Alps seen for the first time. The Pont de Normandie, says Gaumy, is "like Concorde in comparison to the jumbo jets of the suspension bridge age".

Back on earth, the bridge is the essential link in the new Route des Estuaires stretching across France from the Belgian to the Spanish border. It will save, says the Le Havre Chamber of Commerce and Industry (the organisation that has funded and built the bridge), 300,000 traffic hours in its first year, which means shorter journeys, less pollution and more trade for Normandy and the Loire. And beneath its technological wizardry, conservationists are nurturing and extending the mud flats, bringing back birds scared away by the works and creating an impression of harmony between motorway man and nature that Monet would have been hard-pressed to capture in oil. Jean Gaumy has done it very beautifully on film instead.