An automated barrier system to ease diversions onto the Forth Road Bridge in the event of falling ice forcing a closure of the Queensferry Crossing could be in place by this winter season, BEAR Scotland has said.
Currently, it can take up to five hours to install a diversion to the Forth Road Bridge.
Ice forced closures to the Queensferry Crossing in February and December 2020 and January 2021 and vehicles were damaged on at least one occasion.
Bear Scotland’s unit bridges manager for south east Scotland, Chris Tracey, told Cold Comfort Scotland delegates that the automated barrier was already in the design and early manufacturing stages.
‘The automated barriers in normal running will sit in the central reserve and the verges and can be automatically opened up within five minutes to form a natural diversion route so traffic comes off the Forth Road Bridge and back onto the M90, and down on the M90 on the Forth Road Bridge. This should make it a lot quicker with no need for traffic management or advanced signage. The road will be marked to help this,’ he said.
He added that when the Queesferry Crossing was built it was ‘designed to not make it easy for traffic to get from Queensferry to the Forth Road Bridge,’ which now operates as a public transport corridor.
Mr Tracey added: ‘We still have the original crossovers from Queensferry onto the Forth Road Bridge north and south, but they are not designed in such a way that you can just open them and just let traffic run. The best we have done so far is five hours to get these open and running when we are using the crossovers.’
The problem is caused by the unique climate around the Queensferry Crossing, which causes a slushy snow to gather on the bridge's towers and cable stays, particularly during local squalls.
BEAR has been able to identify with increasing certainty the exact climatic conditions that cause the problem, including a number of factors that must be in place concurrently.
Mr Tracey explained: ‘Observations made to date indicate that the following climatic meteorological criteria existing concurrently may likely be associated with ice formations:
Relative humidity (RH) – a measure of moisture content of the air and influenced by dew point and air temp
- RH% above 90% = be aware
- RH above 95% = be very aware
- RH above 97% = expect issues
Squalls were observed as influencing RH by 5% within minutes if blowing through. Therefore, any RH above 85% should be monitored.
Dew point - this is the temp at which moisture comes out of the air but does not fall as rain.
- Dew point = below +2C trending towards 0C
- Squalls were observed as influencing dew point y 3C within minutes of blowing through.
Air temperature
- Air temp below 1C and above 0C produces ‘wet snow’
- Squalls were observed as reducing air temperature by 3.5C within minutes of blowing through. Therefore any air temps below 4.5C should be monitored.
BEAR has also been working with consultant Arup and specialist Norwegian consultant KVT to develop a bespoke weather forecast model. KVT looked at a model developed by NASA, Mr Tracey revealed, which provides specific predictions of snow particle liquid water content – key to the prediction of wet snow.
The model ‘appears to produce a substantially lower false-positive rate than the current system,’ Mr Tracey said, adding that both systems would run concurrently this winter.
‘KVT has produced a bespoke numerical weather prediction model using the open-source WRF modelling package. The WRF model provides explicit predictions of snow particle liquid water content, which is needed for accurate predictions of wet snow.’
BEAR has also carried out testing in laboratory conditions in France to analyse ways of tackling the ice accretion problem Queensferry suffers from. These will continue this year, Mr Tracey said adding that chemical options were ‘still on the table’ to prevent the ice from forming.
‘We are not looking to go down the reactive route we need to be preventative,’ he said.
In conjunction with VSL, BEAR has also developed a machine to clean the bridge stays, which has proven successful in recent trials, while the laboratory testing suggested that cleaning the grime from the bridge towers and stays could help alleviate ice accretion.
Mr Tracey told Highways: ‘Ideally, we will clean all the stays over summer and autumn in conjunction with designing these automated barriers. We are hopeful we will get the automated barriers in place. Pretty sure we can do it for the north side whether we can do it for the south side is more problematic because of a BP pipeline [on that site].
‘We will also carry out further wind tunnel laboratory testing and focus on some of the coatings and the clean stays and do longer trials and get more information.’
Conclusions from laboratory testing:
The surface condition impacts the amount and distribution of snow accreting to the high-density polyethene (HDPE) surface of the cable stays. The roughness of the stay pipe appears to control the accretion behaviour. Once the snow starts to accrete to the initial layer of snow ice, this effect becomes irrelevant.
Most of the tested coatings caused the snow ice to slide along the inclined stay pipe. It could not be established clearly for each coasting whether this was triggered or aided by repelling the liquid phase or by reducing the interface bond.
The cleaning of the stay pipes appears to have a positive impact on the behaviour. The soiled and abraded surfaces appear to accrete more snow/ice based on the increased specific surface and roughness. The cleaning reinstates the ability of the surface of the cable stays to repel water and snow to some extent and enables a homogenous distribution of snow accretes.
The temporary and short-term coatings (de-icing fluid and wax emulsion) performed slightly better in their rig level groups considering the accretion thickness and the accreted mass. The cleaned specimens achieved comparable results and showed leading performance in the overall average.
Cold Comfort 2022 (introducing Climate UK), the 29th Annual Winter Service Conference and Exhibition, takes place at the Harrogate Convention Centre on 25 and 26 May 2022.
