How Old Bridges Collapse in Floods

The Calva Bridge in Cumbra, England is on the verge of collapse. Catastrophic flooding in the area over the past few days has already collapsed another bridge of similar construction.

An article on the BBC News website explains why Victorian bridges have a dangerous tendency to “scour away” in a flood. Rather than being “piled” like modern bridges, Victorian bridges are dug into the gravel bed at a more shallow level, said Alan Simpson, vice-chairman of the Institution of Civil Engineers, Scotland.

Experts have learned to better understand “scour” after the collapse of two railway bridges in the 1980s. But only recently have they realized “the holes are actually bigger during the flooding than when it has abated,” says the article.

Fast-flowing rivers create immense force. The effect is worse on bridges with a number of arches than for those with a single span. The pressure is worsened when trees and other debris pile against the bridge, and if flood waters reach the “deck” of the bridge.

Decks should be higher than expected floodwaters. “But the level of flooding in Cumbria is completely unprecedented,” says the article.

To read the BBC News article, please click here:
http://news.bbc.co.uk/2/hi/uk_news/magazine/8374616.stm