BBC News Magazine

Page last updated at 14:26 GMT, Monday, 23 November 2009

Can you stop bridges collapsing in floods?

Northside bridge, Cumbria
Older bridges are often not on piles driven into the riverbed

The Magazine answers...

A number of bridges have collapsed or are on the verge of giving way in the Cumbrian floods. But is there any way to guarantee this doesn't happen?

The collapse of the Northside bridge in Workington is one of the more notable results of the catastrophic flooding of the past few days in Cumbria.

Another crossing, the Calva Bridge, is also on the verge of collapse.

The main issue for many bridges that are in danger is the effect of "scour". When a bridge is built on a river bed of gravel, the racing floodwater scours away the bed just downstream of the piers on which the bridge rests.

Modern bridges are made to withstand the type of flooding that might only occur once every 200 years
But no bridge is built to withstand unheard of levels of flooding

At the same time there is immense force on the bridge from the fast-flowing river. This is typically worse for a bridge with a number of arches than for a single-span bridge.

The force can intensify if trees and other debris pile against the bridge creating a dam effect.

And the pressure on the bridge is greatly increased if the flood waters reach the "deck" of the bridge.

Victorian bridges are often not "piled" - their piers are not built on piles driven deep into the river bed. Instead they are dug into the gravel bed at a more shallow level, says Alan Simpson, vice-chairman of the Institution of Civil Engineers, Scotland, and an expert on bridges and water.

"It is on the gravel bed of the river... people thought in those days that it won't get scoured away.

"In the past 20-30 years they have realised how much worse scour is than people used to think."

Incidents such as the collapse of railway bridges in Inverness in 1989, and at Glanrhyd in 1987, with the loss of four lives as part of a train dropped into the River Tywi, have enhanced understanding of how bridges get swept away in floods.

Infographic showing how bridges can collapse
1. Fast-flowing floodwater scours away the riverbed downstream of the piers on which a bridge rests
2. The torrent of water also puts immense force on the bridge, made worse if debris piles up creating a dam effect
3. The pressure is greatly increased if the floodwater reaches the deck, or top, of the bridge

Engineers have always known that scouring from flood-engorged rivers caused holes downstream of bridges. But it is only more recently they have realised the holes are actually bigger during the flooding than when it has abated.

And in bridge design, the height of the deck is well clear of the highest floodwaters expected.

"When the water reaches the deck then you suddenly get a lot more force being applied," says Mr Simpson. "Then you get a tree or two which jams against the piers and the deck and creates a kind of dam. You have got another area that the water is hitting."

In the case of older bridges that deck is not "glued" onto the piers of the bridge, but simply sits on it. Even newer bridges must have decks that move to allow for expansion of materials in the heat of summer.

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It is not practical to move an old bridge onto piles, and attempts to secure the riverbed around the piles are not easy. There are systems developed by engineering companies that effectively put a protective blanket on the riverbed, like matting or artificial grass, to try to keep the bed in place. One common solution is for massive boulders to be placed around the piers to take the force of the flood water.

The key thing, Mr Simpson emphasises, is that no bridge would have been built with unprecedented levels of flooding in mind.

These days, bridges, as is all infrastructure, are built to withstand the kind of extreme flooding that might happen once every 200 years - a 0.5% chance of it happening in any given year - with a margin on top of that.

But the level of flooding in Cumbria is completely unprecedented. Bridges could be built to withstand that incredible level of flooding but that would cost much more money, for something that was vanishingly unlikely.

It is also very difficult to assess the effects of scouring on a bridge while the flooding is still going on. The Calva bridge in Workington is closed because cracks were visible in the structure. But it takes divers to check the nature of the scouring.

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