CLAIM: “Rambling Fiona Hyslop gives 'no answers' on Queensferry Crossing closure” - Daily Express headline, 24 November 2024
DOORSTEP ANSWER: Traffic flow across the Forth was disrupted for all of 20 minutes, because safety concerns about ice accumulating high on the suspension cables of the Queensferry Crossing (due to Storm Bert) necessitated switching vehicles onto the Road Bridge. Ice accumulation is a problem on other UK suspension bridges, including across the Severn in England. The bridge has only been closed to traffic four times since it opened in 2017, according to the Scottish Government.
STORM BERT
On Sunday November 24, when Storm Bert was at its peak, the vital Queensferry Crossing across the Forth was closed to traffic for around 15 hours because of a risk of falling ice. One vehicle had its windscreen cracked. Such incidents are rare but not unknown in the UK. In 2009 ice fell from the two Severn crossings, resulting in smashed windscreens and scores of compensation claims from motorists.
Critics have suggested that exaggerated claims were made when the Crossing was first opened that it would not suffer the regular closures associated with the older Forth Road Bridge. Are these criticisms valid?
IS THE BRIDGE RESILIENT?
The 1.7 mile-long (2.7km) Queensferry Crossing was opened in 2017, as an insurance policy against the early obsolescence of the existing Forth Road Bridge. It was the biggest infrastructure project in Scotland for a generation and cost £1.35 billion to build. Delivered on time and on budget, the Crossing was considered a political triumph for the SNP administration under Alex Salmond. The bridge is 207m above high tide (683ft) and the steel bridge deck weighs 35,000 tonnes. The south caisson is the height of the Statue of Liberty. The Crossing carries about 24 million vehicle journeys per year.
The Queensferry Crossing was explicitly designed with resilience and ease of maintenance in mind. For instance, the suspension cables can be replaced as part of normal maintenance works without closing the bridge.
The major problem with the previous Road Bridge was the regular necessity to close access to high-sided vehicles due to strong winds in the Forth. According to the Met Office, Eastern Scotland is one of the windiest parts of the UK due to “being relatively close to the track of Atlantic depressions".
To resolve this problem, one of the key features of the Queensferry Crossing are the 3.5m-high baffle barriers designed to break up and deflect gusts of wind. Other bridges in the UK, such as the Second Severn Crossing, have some wind protection, but the Queensferry Crossing has shields across its full length. These baffles were modelled in a wind tunnel by Italian engineers. In February 2020, then transport minister Michael Matheson revealed that there had been 30 occasions when the new Queensferry Crossing remained operational where the old Forth Road Bridge would have been closed due to the high winds.
On any reasonable assessment, the new Crossing has resolved the major problem of closures due to high wind associated with the older Road Bridge.
DEALING WITH ICE
While that wind issue has been dealt with, new issues have emerged due to icing. In February 2020, two and a half years after the Crossing was opened, it was closed for the first time when accumulations of ice fell from cables on to the carriageway. Eight vehicles were damaged. The issue arose due to heavy snow being super-chilled into ice at the elevated heights at the top of the cables.
This problem has recurred, and again on November 24 2024. However, since the problem was first identified in 2020, sensors have been fitted on the cables to detect ice accumulations. These are linked to a new automated barrier system which diverts traffic onto the Forth Road Bridge, avoiding the need for traffic to divert via Kincardine. During this latest incident, the bridge authorities took the decision to close the Crossing at 10.20am. But traffic was redirected and moving across the neighbouring Forth Road Bridge by 10.50am – hardly a major impediment to communications.
Again, criticisms that the Crossing was made inoperable fail to take into account that transport links across the Forth were not severed due to an alternative route (across the old Road Bridge) being immediately available.
COULD THE ICE PROBLEM HAVE BEEN FORSEEN AND REDUCED?
The Queensferry Crossing is located at the same latitude as Labrador in Canada. Icing on cable suspension bridges is not uncommon in northern latitudes and has occurred in Denmark, Canada and Russia. The issue is not cold conditions per se but unpredictable and peculiar combinations of snow, wind velocity and wind direction. Added to the increase in extreme weather events due to climate change, this makes it virtually impossible to rule out icing on suspension cables. Nor are there yet any proven technical solutions to such periodic icing. What can be said is the Queensferry Crossing designers and authorities have been attentive to installing ice warning equipment, and to provide alternative routes in the case of emergency.
One question that has arisen is why ice accumulation is not such a problem on the older Forth Road Bridge? For one thing, the Crossing is 207 metres high (the tallest in the UK) compared to 150 metres for the Road Bridge, and temperatures drop dramatically with height. For another, the multiple, close-set cables of the Crossing – compared with the Road Bridge – induce a particular airflow pattern that affects icing.
Will a technical solution to icing be possible? One idea is to fit "collars" on each suspension stay which can be released to slide down the length of the cables to clear ice accumulations. These have been fitted on the Port Mann Bridge in Vancouver. Other approaches include vibrating the suspension cables, heating them, or using chemical de-icers. However, this technology is still at an experimental stage and research suggests that there will always be some unique combination of weather and wind that overpowers anti-icing defences. This is inherent in the climate change emergency.
FACT CHECK RATING: A freezing zero points for Unionist media. Expect more hot air from that source if anyone suggests the bridge should be named the Alex Salmond Crossing.
