Scour is a major risk for bridges with pad foundations in rivers prone to high velocity flows.
Older bridges are particularly at risk, as long term changes in the hydrological characteristics of a river basin can create flows never envisaged by the designers.
Two of Britain's rail bridges, the Glanryhd viaduct in Wales and the Inverness bridge, were brought down by scour in the late 1980s.
Four train passengers died in the Glanrhyd disaster in 1987, but an early morning inspection by Scotrail prevented a repeat of the tragedy two years later in Inverness.
Both structures, like the Ponte de Ferro bridge in Portugal, stood on pad foundations that were undermined by scour during flood conditions.
More recently, the Kaoping bridge in Taiwan collapsed, injuring more than 20 (NCE 7 September 2000).
Investigations in Taiwan focused on the extensive illegal sand and gravel extraction which had taken place close to the foundations of the collapsed pier. They also covered the possibility that earlier anti-scour measures had contributed to the problems.
Scour takes two forms. Clear water scour is triggered as river flows increase above a certain critical velocity - which depends on the physical characteristics of the river bed material. Riverbed is washed away from around the piers, with the worst effects often concentrated immediately upstream of the pier.
Live bed scour is caused when very high water velocities erode the riverbed and piers are battered by debris rushing downstream. Even bridges built on piled foundations are at risk.
Pier geometry and orientation are major factors influencing the rate of scour.
During high water flood conditions the direction of flow can be significantly different to that assumed by the designers, increasing the scour potential.
But perhaps the biggest risk comes from the extraction of sand and gravel close to the piers, as in Taiwan and at Ponte de Ferro Dave Parker