Infrastructure maintenance rethink needed as Storm Babet leaves sinkhole in historic bridge | New Civil Engineer

A sinkhole opened up on the 18th Century Bridge of Dun due to high water levels along the South Esk River when Storm Babet hit Scotland at the weekend. 

Built between 1785 and 1787 the Bridge of Dun is known as a masterpiece of masons art and crosses the low lying farmland around the South Esk river prior to it reaching the Montrose basin. 

Speaking to NCE, earth scientist and University of Hull vice chancellor David Petley, said that high levels of water flow through the smaller arch in the bridge allowed water to break through the stone facing of the bridge, and was a possible cause of the erosion of fill underneath the road surface.

“The rainfall events that we are now seeing are extreme, subjecting infrastructure to conditions that are beyond their design,” he said. “We should expect to see more of this type of situation. It is manageable, but will require substantial investment in our infrastructure; in cases of historic structures of this type, it will be particularly challenging.”

Damage to the Bridge of Dun was one of a number of incidents across a large area of south east Scotland caused by the storm, with engineering plans cancelled and emergency works carried out to take away fallen trees and other issues. 

The South Esk River was included in the 16 flood warnings issued by the Met Office for Scotland last Friday. According to Bill Harvey Associates masonry bridge specialist Hamish Harvey, past scouring can mean that bridges can become weaker over time. 

“I would assume that the water either scoured out the foundations or plucked stones from the masonry,” he said. “Once a start is made, it can take the rest of the arch out piece by piece allowing the fill to fall through. It is certainly the case that maintenance can make a difference to what happens in a flood. Deep mortar loss, or even missing stones, would leave the adjacent stones at risk. Past scour similarly makes catastrophic scour more likely.”

He added: “Scour of bridge piers can be exacerbated by changes to the hydraulic regime. If the bank upriver erodes, for example, you might end up with fast flood flow cutting across the pier instead of flowing straight through.

“At Bridge of Dun, the flood arch presumably rarely sees flow. The hydraulics high flow will always be novel in a sense.”

The Bridge of Dun wasn’t the only bridge affected by the recent storm, A wooden pedestrian crossing at Glen Clova in Angus was also seriously affected by the storm after intense rainfall caused the local river to rise above the walkway. 

It is unknown what condition the Bridge of Dun was in prior to the storm, but Petley believes issues such as land use and the creation of more areas for rainfall to collect upstream may need to be considered to ensure infrastructure downstream isn’t affected by ever-more-common extreme weather events. 

“The bridge will probably need to be protected against these high intensity floods, but the most effective approach will be to seek to retain more water upstream for longer to flatten the curve – i.e. lower the flood peak,” Petley said. “This will involve land use change and channel management in the catchment, a pattern that we should be seeking to replicate across all upland areas.”

Harvey warns that continual large storms can spell trouble for all kinds of bridges. “In the mess after this flood it would be unlikely that you could say whether the bridge [of Dun] was more exposed than it should have been, and there is no reason to imagine that a flood of this magnitude couldn’t do this to a well maintained bridge,” he said.

He added that the comment may not be applied to the Bridge of Dun itself without an investigation into the causes of the sinkhole.

Regarding bridge maintenance going forward, Harvey added: “There are certainly aspects of maintenance that will need much more attention [in the future], like clearing trees etc from bridge spans, especially flood spans which are often not as well maintained as they could be.”

Protections or upgrades to historic bridges could become more frequent. In 2021 the Grade-II listed Apperley Lane Bridge near Bradford was strengthened as part of a scheme to avoid the Boxing Day floods of 2015. One phase of the scheme saw the creation of a flood storage reservoir created to alleviate the flow of water downstream.

The Apperley Bridge itself was strengthened to withstand increased pressures exerted by the higher flow of water during storm or flood events. Transport Scotland may consider similar measures for historic bridges across the country.

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