Weather sensors on the Queensferry Crossing allow us to monitor and respond to the specific weather conditions that we know can cause ice to form on the cables.
This video and the questions and answers below explain the issue in more detail.
Ice accretion
What exactly is the ice accretion problem? On four occasions since the Queensferry Crossing opened to traffic, ice has formed on the bridge’s towers and cables then fallen onto the carriageway below. This has occurred during a particular combination of weather conditions when wet snow has blown in and stuck to the cables, frozen and then fallen off.
Why does it affect the Queensferry Crossing in particular? Ice accretion is not unique to the Queensferry Crossing, however each bridge operates in a different climate and has different design details, so the mechanism by which ice forms and falls varies, as does the scope and effectiveness of potential solutions.
How does ice accretion form on the Queensferry Crossing? This phenomenon has occurred during a particular combination of weather conditions, when wind speed and direction, temperature, dew point and relative humidity all converge within specific parameters. In these conditions, wet snow can stick to the cables, freeze and then fall off within the time it takes for a squall to pass through – sometimes in as little as 10 to 15 minutes.
Is it an issue for the Forth Road Bridge? The main suspension cables on the Forth Road Bridge are very different to those on the Queensferry Crossing, with different dimensions and materials. There has been one occasion since the Forth Road Bridge opened in 1964 when it had to close for a few hours due to ice on its suspension cables.
How often will this happen? It’s too early to say how often this might happen in future. We know that it has happened four times since the cables were first installed during construction of the bridge in 2014.
Will the bridge close every time it snows/freezes? No it will not – the Queensferry Crossing has remained open throughout various instances of snow and severe weather that would have closed other bridges, not least the infamous “Beast from the East” in 2018. It is only in a specific combination of conditions that ice accretion is known to occur on the Queensferry Crossing.
What does it look like? What size are the ice pieces that could fall from the bridge? In the conditions outlined, wet snow sticks to the cables then freezes. Strips of frozen snow up to a metre across were observed falling from the cables in February 2020.
Can you predict when ice accretions will form? Our site-specific weather forecast alerts us when conditions conducive to ice accretion are possible, and a system of weather sensors on the towers and deck of the Queensferry Crossing allows us to monitor those conditions in real time so that we can take pre-emptive action as necessary to keep bridge users safe. Our understanding of the phenomenon should continue to improve, as the weather sensors gather data from future incidents.
Is there any possibility that ice accretions can form without the presence of known forecast indicators? We are confident that we understand the parameters within which this specific phenomenon can occur, and we will continue to refine this understanding with the help of the weather sensors on the bridge. But the Queensferry Crossing has only been open for four years, so it is possible that there could be other weather conditions, not yet experienced, which could cause ice to accrete in a different way.
If ice accretion occurs, how long will it last? Under what conditions will you reopen the bridge? Ice is known to form and fall from the Queensferry Crossing cables in a cycle lasting as little as 10 to 15 minutes, however in the event that there was a risk of ice falling from the cables, the bridge would remain closed for as long as the forecast and our weather sensors indicated ice accretion could still occur.
How dangerous could ice accretion be to drivers? The safety of bridge users and workers comes first, so we will close the bridge and divert the traffic in conditions that cause ice accretions to form and fall from the cables.
Can it damage the QC cables? Ice accretions have not caused damage to any part of the Queensferry Crossing.
Will QC cables bend or sag with the weight of the ice? Will that affect the bridge structure? Ice on the QC cables forms then falls within a relatively short timescale, sometimes as little as 10 to 15 minutes. No build-up of heavy ice has been observed.
Dealing with ice accretion
What’s been done to address the issue since it was first identified? Our ability to forecast ice accretion has been enhanced following observations and measurements of the conditions that caused the issue in previous incidents. A bespoke system of weather sensors, thermal sensors, and thermal and optical cameras has been installed to monitor these conditions in real time. Detailed operational procedures have been developed and tested to optimise the way we manage any similar incidents in future. Trials are being carried out of measures to speed up the diversion process if the Queensferry Crossing needs to be closed again. Research and development is ongoing to assess and test potential measures to mitigate against ice accreting on the bridge.
How will you respond if ice is forecast? If conditions conducive to ice accretion are forecast, BEAR Scotland staff will be mobilised to respond quickly. Ice monitoring teams will patrol and inspect the bridge and staff in the control room will closely monitor live data from the weather sensors, in consultation with expert meteorologists.
How do you deal with ice accretions once they form? The safety of bridge users and workers comes first, so we will close the bridge and divert the traffic if ice formations are observed on the tower or cables, or witnessed falling onto the carriageway.
Are operatives put at risk to deal with ice accretion? The safety of bridge users and workers comes first, so we will close the bridge and divert the traffic in conditions that cause ice accretions to form and fall from the cables.
Can you prevent ice accretion forming on the Queensferry Crossing? Our long term goal is to develop a solution that will prevent ice from forming on the cables, and the better we understand the conditions that cause this issue, the greater our chances of success. Every bridge has different design details and operates in a different climate, so there is no existing proven solution that is suitable for the Queensferry Crossing – any such solution will be bespoke.
Can we learn from similar issues on other bridges around the world? A review of measures to address ice accretion on other bridges has been undertaken. This indicates that, whilst many methods of prevention and removal have been studied, tested and deployed, no single method or technology has been found to be completely successful in mitigating the problem or removing ice build-up. Because of this, most operators simply close the bridge and wait for the ice to fall.
What preventative measures are you considering? The team has been carefully reviewing and examining a variety of possible options to determine which are worthy of further research and development. A number of options have been ruled out, however some are being taken forward for further investigations, testing and development. A peer review panel of international experts has been assembled to assist with this process.
Dirt has accumulated on the cables since the bridge opened to traffic in 2017 and this may play a role in helping ice to accrete. The cables on the north tower of the Queensferry Crossing were cleaned in autumn 2021 to allow us to measure the difference this makes in any future ice accretion events.
Laboratory tests are also being carried out at the Jules Verne Climatic Wind Tunnel in Nantes, a specialised research facility that allows us to recreate the conditions that cause ice accretion. These tests will measure the impact of cleaning, de-icing compounds and specialised hydrophobic and omniphobic coatings on full-size sections of Queensferry Crossing cable and a mock-up of the towers.
What are the criteria by which potential mitigation measures will be judged? It is a key requirement that installation and maintenance requirements for the retrofit of any mitigation measures should not be more disruptive than the problem being solved. Active measures involving ice expulsion can be equally as disruptive as the ice problem, due to carriageway closures being required while they are in operation. Reliability of any tower and cable components is paramount, as they may only be called into use occasionally and at short notice. Any solution should not risk altering the performance of the bridge and obviating the designers’ liability. Reaction time is imperative, given the short duration of ice events at the Queensferry Crossing – favouring options that prevent ice accretion rather than remove it. Any chemical products must be environmentally acceptable for use in the atmosphere above traffic and in a marine environment. Other factors such as the associated cost, success of the FRB as a diversion route, frequency of ice events and safety of road users will be considered as part of the decision-making process prior to any installation of a viable option for ice accretion mitigation.
Was this not predicted as an issue in the design of the bridge? Why was it decided to address this operationally rather than at design stage? Ice on the cables was considered as a potential issue during the design of the bridge, but was expected to be a relatively rare event in the climate experienced at the Forth. Rather than investing in an expensive preventative system that may only rarely be required, it was therefore decided to manage any ice accretion incidents through operational measures – i.e. by closing the carriageway on the rare occasions that it occurred.
Ice sensors
How many sensors are there? Where are they located? There are five clusters of weather sensors on the bridge – one at the top of each tower and two at deck level. Each cluster measures four different weather parameters. Nine optical and thermal cameras have also been installed on the towers for the purpose of monitoring ice – three on each tower – and an additional nine thermal sensors have been installed on the shortest stay cables to detect subtle changes in surface temperature. These are in addition to the thermal sensors installed during construction as part of the bridge’s structural health monitoring system.
What will the sensors do? The weather sensors allow us to monitor the four conditions that we know can cause ice accretion when they converge within specific parameters: wind speed and direction, temperature, dew point and relative humidity. Updated measurements are sent to the bridge control room every ten minutes.. This helps provide early warning of such conditions and allows us to measure and understand more accurately the conditions under which any future incidents occur. It is important to note that ice accretion will still be managed by closing the bridge should it happen again in the way that it has previously.
Will sensors stop ice forming? Sensors and cameras will not prevent ice accretion; however they will improve our understanding of the issue and give us some early warning when the conditions that cause ice accretion occur. They will also help us to confirm forecasts and determine when it is safe to reopen the bridge.
Diversion options
Can the Forth Road Bridge be used as a diversion route if the QC needs to close? The Forth Road Bridge is currently a dedicated public transport corridor and any other use would be to the detriment of public transport – however in emergency situations it is available to provide additional resilience as an emergency diversion route for motorway traffic. It should be noted that such a diversion would only be implemented overnight, outside peak periods. This is because it takes several hours to remove roadside barriers and lay large numbers of cones and signs, and this process would cause significant additional disruption to traffic during busy periods. Traffic will initially be diverted via the Kincardine Bridge, then switched to FRB overnight if the Queensferry Crossing is expected to remain closed for an extended period. Trials of the diversion implementation process have been carried out to ensure it is as efficient and effective as possible.
Why does traffic need to divert via Kincardine Bridge? Kincardine Bridge is the closest available diversion route if the Queensferry Crossing and Forth Road Bridge are not available for general traffic.
Why does it take so long to put the FRB diversion in place? The Forth Road Bridge is a public transport corridor and its approach roads are not designed for general traffic. To allow it to be used as a diversion route for Queensferry Crossing traffic, emergency crossovers need to be opened to link the Forth Road Bridge to the motorways north and south of the Forth. This involves stopping traffic on the M90 and A90, moving roadside barriers and laying a large number of cones and signs.
Can the Forth Road Bridge be used partially – even if not all lanes available? Trials are being carried out to test how long it takes to implement a diversion across the Forth Road Bridge using the slip roads on either side. This would not provide the same capacity as opening the emergency links, but could reduce the need for all traffic to divert via Kincardine during shorter term closures if it can be done more quickly. For longer term closures we would still open the emergency links north and south of the bridge to maximise capacity.
Is the Forth Road Bridge not a designated public transport corridor? Yes it is, however using it as an emergency diversion route may be appropriate in certain circumstances and help minimise overall disruption in the event that the Queensferry Crossing has to close.
How long will it take to re-open QC after weather conditions improve? If traffic is being diverted via Kincardine Bridge, then reopening the Queensferry Crossing will only take a few minutes. If traffic has been diverted to the Forth Road Bridge, it will take several hours and will be done outside peak periods to minimise disruption.
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