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Sand extraction identified as major factor in Portuguese bridge collapse

EXTRACTION OF sand from the River Douro has emerged as a major contributing factor in last month's collapse of the Ponte de Ferro road bridge in Entre-osRios, northern Portugal.

Up to 70 people died when a coach and two cars plunged into the swollen waters on the night of Sunday 4 March. Scour undermining the piled foundations of one of the bridge's six masonry piers during flood conditions is seen as the most likely cause of the tragedy, said to be Europe's worst ever road disaster.

Early on, Portuguese government engineers had already blamed the local sand extraction industry for dramatically altering the flow regime under the bridge, and NCEI has since learnt that cargo ships transporting the sand between sites are frequently seen striking the bridge piers.

A geotechnical engineering expert from the Laboratorio Nacional de Engenharia Civil (National Civil Engineering Laboratory), principal research officer Rui Correia, has been appointed to head the official investigation into the disaster, but the circumstances that led to the collapse are as much political as they are technical, and lead right to the Portuguese government.

The River Douro, or river of gold, is so named because of the high concentration of sand that is suspended in its waters. Sand extraction is big business along the entire length of the river.

However, Entre-os-Rios's position at the confluence of the rivers Douro and Tamega - both hundreds of metres wide - makes it a particularly good location for extraction of what is a valuable material.

Where the rivers converge the water slows near to the south bank and the sand settles out, making extraction easy. The Ponte de Ferro is less than 100m downstream of this point.

The state-run Institute of Navigation recognised the potential risk this posed to bridges along the river, and acted to solve the problem in 1998 by imposing a '10km' rule for sand extraction.

This banned sand extraction from the river at any point up to10km upstream of a bridge.

However, the sand extraction industry is a major part of the local economy around Entre-osRios and, rather than abandon the site at Ponte de Ferro, the operators merely changed its function.

Officially, the actual extraction has shifted to sites downstream of the bridge, with the site at Ponte de Ferro used as a distribution centre for transferring the sand from ship to truck.

This move may have only served to worsen the situation by encouraging 50m long cargo ships to manoeuvre in the fast flowing waters next to the 115 year old bridge.

The local population is not happy. Antonio Grilo, art and design teacher at the local school, says: 'They have already broken our roads with lorries carrying loads 10% to 20% heavier than legally permitted. Now it's the bridge.

'With the speed of the river, and its currents, the ships are always hitting the piers. The locals are saying that maybe the pier that collapsed got hit one time too many.'

Oporto National University civil engineering design professor Antonio Adao da Fonseca, who is president of the European Council of Civil Engineers, is even concerned about the impact on the river's flow regime: 'With the speed of the water in floods reaching 5m/s, any modification to the upstream banks may well have an impact.'

And as the extraction equipment at the site near the bridge appears to show few signs of two years of inactivity, da Fonseca also questions whether the extraction firm is adhering to the rules: 'Temptation comes to us all the time. If you have such a nice piece of sand, it may be hard not to extract it.

'The Institute of Navigation's resources are good, but they are only so good. It is hard to enforce a rule if the public do not understand the implications of breaching it.'

While Grilo bemoans the sand extraction industry and the modern developments plaguing his country, another modern development is also likely to have contributed to the collapse - electricity generation.

The construction of three hydroelectric dams by national power company Electricidade de Portugal within 15km of Entre-os-Rios raised the water level at the bridge to 18m to 20m.

The Carrapatelo and Crestuma dams were built in the late 1960s and 1970s spanning the Douro upstream and downstream of the Ponte de Ferro.

The Torrao dam across the Temega was constructed 15 years ago.

Da Fonseca is reluctant to blame the electricity company.

'EDP has huge resources and would have studied everything thoroughly, and would have considered the implications on the river bed. But they would only have been theoretical studies.' However, he concedes that the rapid flow in the River Douro makes it a special case.

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