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  • Writer's pictureSafer Highways

How abseilers and drones helped to turn a restored Victorian viaduct into a home fit for bats

Go for a stroll in the British countryside and there is a fair chance you will stumble across a remnant of the golden age of the railways. Disused tunnels, viaducts, bridges and embankments – they have all become much-loved parts of the landscape, bringing history alive and providing vital habitats for wildlife.

However, many of these structures still require maintenance and, historically, this hasn’t always been carried out with nature in mind.

A project in Scotland sought to explore a different way forward – one that married

technology with abseiling-based surveys to ensure the protection of local wildlife.

On the border between the West Lothian and Falkirk council areas, National Highways’ Historical Railways Estate (HRE) engineers began, years ago, to plot the restoration of the disused Westfield Viaduct, which is one of 3,100 former railway structures maintained by the government agency.

Completed in 1855 and then disused by the 1960s, the listed structure features 12 arches with spans of around 14 metres. In total, it stretches for 201 metres end to end, and stands 18 metres from the top of the arch to the bed of the river Avon below.

Colin McNicol, a civil engineer at HRE, explained that survey work had found some of the

wedge-shaped stones around the arches – known as voussoir stones – needed to be replaced, and that there was “water ingress from rain and weather” on the arches and piers that needed to be rectified.

“My predecessor decided that before the condition of [the Westfield Viaduct] deteriorated to such a point that it was in a poor state, the time was right to waterproof it and carry out repairs,” McNicol told ENDS.

Initial survey finds bats and otters

Following the development of a business case for the works and further surveys to develop a design for the repairs, a preliminary ecological appraisal (PEA) was undertaken in March 2018. The assessment involved a mapping of all habitats inside the survey area within 50 metres of the structure, and 200 metres up and downstream of it.

“The viaduct was assessed as having high potential for both summer roosting and hibernating bats,” Fiona Smith, a senior civil engineer at HRE, explained to ENDS.

“Surrounding habitats such as the river Avon, which runs under the structure, and woodland within the local area both provide excellent commuting and foraging opportunities for multiple species, as well as connectivity to the wider landscape,” Smith added.

And it wasn’t just bats. The PEA assessment also identified otter activity within 200 metres up and downstream of the viaduct. Three potential couches – above-ground daytime resting places for otters – were located during the survey, followed by camera trap monitoring in 2018, 2019 and a further otter survey in May 2021.

From this first assessment, it was determined that it was unlikely the otters nearby would be impacted by the proposed works. However, as Smith explained, camera traps were again deployed when the restoration process began to ensure monitoring remained in place.

Soprano pipistrelle bats

With an average head-body length of 3.5cm to 4.5cm – a size not much bigger than that of a 50p coin – Soprano pipistrelle bats are among the UK’s smallest. Credit: Getty images/Zdenek Macat

Once the PEA surveys had concluded, HRE followed it up with a ground-based, preliminary assessment of where bats were roosting within the viaduct.

While other projects have mapped this out by surveying bats when they emerge (at dusk) and return to roosts (at dawn), a choice was made for three dawn-only surveys in September 2018, June 2019 and July 2019, which is a “variation” – in Smith’s own words – to the Bat Conservation Trust (BCT) best practice guidelines.

“[This dawn-only approach] was considered a more suitable and robust approach for this large structure, allowing for a greater opportunity for bats to be tracked back to specific roosting locations across the site compared to a dusk survey,” she explained.

From these surveys, a picture of bat presence emerged. Four summer roosts were located across three spans of the structure. Three of the four belonged to soprano pipistrelle bats (the Pipistrellus pygmaeus), which are among the UK’s smallest bats, with the fourth occupied by Daubenton’s bats (Myotis daubentonii).

Drones and abseilers to recreate bat roosts

According to Smith, the surveys produced no evidence of high conservation value roosts on the viaduct, such as maternity roosts. The bats were found to be in low numbers, with a maximum of three bats in each roost.

However, as the repair of the structure was going to result in the loss of known roosting features for bats, a European Protected Species Licence was required from the government adviser Scottish Natural Heritage, now known as NatureScot.

This licence was subsequently granted. It set out compensatory measures, such as the incorporation of permanent roost features into the viaduct, to make up for the bat roosts that were about to be lost during the restoration works.

A search then got underway of the viaduct’s every potential roost feature to ensure that no bat would be blocked inside when the restoration began.

Over September and October 2019 – just ahead of the hibernation season – ecologists from Jacobs assessed the structure. Where locations were hard to reach, specialist abseilers were brought in to painstakingly check dozens of crevices in the masonry with camera-equipped endoscopes for signs of bats inside.

Abseilers and renovation workes over Westfield Viaduct

Armed with camera-equipped endoscopes, abseilers and restoration workers pored over the viaduct pre-renovation to ensure no bat would be blocked inside when restoration began. Credit: Historical Railways Estate

If a potential roost feature could not be fully inspected, a one way excluder was installed, allowing bats to leave a feature, but not to re-enter. These were left in place for seven consecutive days to make sure the bats had left the roost before being fully excluded.

The exclusion of bats from the viaduct spans, which is often done by packing roosts with a rag or other material, was undertaken in two phases to minimise impacts on the animals, Smith explained. Temporary bat boxes and bricks were then installed during both exclusions, in areas that are safe from the restoration works, to create alternative roosting provision.

Technology also played a role. As McNicol explained, the Westfield Viaduct was the first project where HRE deployed drones and high-definition cameras to confirm that existing bat exclusions were still working. This approach helped, he highlighted, to save operatives from having to carry out that work, minimising people’s proximity to bat features in the process.

Covid-19 forces new timings and licence extensions

As with so much else, the Covid-19 pandemic cast a shadow on the scheme’s schedule.

Bat exclusion devices were first put in place in 2019 but, due to Covid-19 restrictions, the main works could not begin until 2021 – a 16-month delay.

The impacts were felt by Balfour Beatty, the construction group that had won the contract to carry out the renovation. According to McNicol, the firm had initially decided to deploy suspended cradles underneath and either side of the viaduct, but social distancing forced a rethink.

“Because of the proximity of people working on those cradles, they had to go through a process to establish whether that was safe in terms of Covid-19,” McNicol said, adding that this delay had a knock-on effect on the overall project.

“We then had to go in and re-survey a lot of the bat features and the exclusions that had been put in previously. Some of those rags would have blown out or been pulled out by crows ... so we had to physically check that they were either intact or go through the process again,” he explained.

The team got to it, with bat-licensed ecologists and abseilers joining forces to reassess the integrity of prior exclusions and re-block those that needed re-blocking.

Bat brick by Westfield Viaduct

Installed during the renovation, the 27 bat compensation measures, such as this bat brick here, will help to maintain the viaduct as a “valuable, year-round roost” for local bat populations, HRE said. Credit: Historical Railways Estate

In parallel, the Covid-19 disruption forced the project to apply for an extension of the bat licence – and it was only the first. Further licence extensions were necessary after workers examined the structure and concluded that a “significant” volume of additional repairs was required, Smith explained.

Once it got going, the renovation of Westfield Viaduct stretched from June 2021 to December 2022. By the time it had concluded, 27 bat compensation features had been installed across the Victorian-era structure: 19 bat bricks, six bat tubes and two bat boxes.

The works, Smith said, resulted in “no loss” of habitat in this area, even while not necessarily delivering a biodiversity net gain. The bat compensation measures will help to ensure that going forward, the viaduct remains a “valuable, year-round roost” for local populations, she added.

Bats aside, no other permits for protected species were required, but Smith noted that “good working methods” and “ecological supervision” were observed elsewhere in the project, with potentially sensitive tasks such as vegetation clearance.

“This ensured the welfare of any other species that may have been inhabiting the structure or its immediate environs,” she added.

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