FAQ: Queensferry Crossing ice accretion

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.

The video above and the questions and answers below explain the issue in more detail.

Ice accretion

1. What exactly is the ice accretion problem?
   On three occasions since the Queensferry Crossing was constructed, the bridge has been temporarily closed to traffic as a safety precaution after ice formed on the bridge’s towers and cables then fell onto the carriageway below.
2. How does ice accretion form on the Queensferry Crossing?
   The 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.
3. 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.
4. 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.
5. How often will this happen?
   It’s still too early to be sure how often this might happen in future. The Queensferry Crossing has been closed to traffic due to falling ice four times since the cables were first installed during construction of the bridge in 2014.
6. 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.
7. 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.
8. 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 will continue to improve as the bridge’s weather sensors gather data from future incidents.
9. 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. But the Queensferry Crossing has only been open since 2017, so it is possible that there could be other weather conditions, not yet experienced, which could cause ice to accrete in a different way.
10. If ice accretion occurs, how long will it last and under what conditions will you reopen the bridge?
    Ice is known to form and fall from the Queensferry Crossing in a cycle as short in duration as 10 to 15 minutes, however in the event that there is a risk of falling ice which could present a hazard, the bridge will remain closed for as long as our observations, the forecast and our weather sensors indicate there is a risk to bridge users.
11. 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 present a risk to bridge users. Traffic speed may also be reduced prior to any decision to close the bridge.
12. Can it damage the cables?
    Ice accretions have not caused damage to any part of the Queensferry Crossing.
13. Will the Queensferry Crossing’s cables bend or sag with the weight of the ice? Will that affect the bridge structure?
    Ice on the 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

14. 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.
15. 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 for as long as falling ice presents a risk to bridge users.
16. Can you prevent ice accretion forming on the Queensferry Crossing?
    Every bridge has different design details and operates in a different climate, so there is no existing proven means of preventing ice accretion that is suitable for the Queensferry Crossing – any such solution will be bespoke. Our long-term aspiration is to develop a solution that prevents ice from forming on the cables, however the most effective solution may still prove to be diverting traffic via the Forth Road Bridge.
17. 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. A system of automated barriers has been installed to speed up the process of diverting traffic via the Forth Road Bridge if the Queensferry Crossing needs to be closed. Research and development has been carried out to assess and test potential measures to mitigate against ice accreting, and all of the bridge’s cables were cleaned in advance of winter 2022/23 after laboratory tests indicated this has a beneficial effect.
18. What preventative measures have you considered?
    The team carefully reviewed and examined a variety of possible options to determine which were worthy of further research and development. A number of options were ruled out, however some were taken forward for further investigations, testing and development. A peer review panel of international experts was assembled to assist with this process.
    Laboratory tests were carried out at the Jules Verne Climatic Wind Tunnel in Nantes, a specialised research facility that allowed us to recreate the conditions that cause ice accretion. These tests measured 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. The results indicated that cleaning the cables had a beneficial effect by increasing the time taken for wet snow to adhere to the high density polyethylene stay pipes encasing them. Dirt had accumulated on the cables since the bridge opened to traffic in 2017 and this may have played a role in helping ice to accrete. All of the Queensferry Crossing’s cables were therefore cleaned in 2022, using specially-developed machines that can be winched up the cables to jet-wash them remotely. However, we cannot guarantee that cleaning will completely eliminate the risk.
19. 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 Forth Road Bridge 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.
20. Can we learn from similar issues on other bridges around the world?
    A review of measures to address ice accretion on other bridges worldwide has been undertaken. This indicated 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.
21. 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.

Weather sensors

22. 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.
23. What do 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.
24. Do sensors stop ice forming?
    Sensors and cameras do not prevent ice accretion; however they improve our understanding of the issue and give us some early warning when the conditions that cause ice accretion occur. They also help us to confirm forecasts and determine when it is safe to reopen the bridge.

Diversion options

25. Can the Forth Road Bridge be used as a diversion route if the QC needs to close?
    Traffic on the M90 can be be diverted via the Forth Road Bridge if the Queensferry Crossing has to close. A system of automated barriers has been installed that significantly reduces the time it takes to open this diversion – find out more here.
26. Is the Forth Road Bridge not a designated public transport corridor?
    Yes it is, however using it as an emergency diversion route is appropriate in certain circumstances and helps minimise overall disruption in the event that the Queensferry Crossing has to close.