The `holy grail' for the winter maintenance industry is to determine the level of salt present on roads to better inform managers of the requirements to treat with re-application of salt or not. At present, decision makers, both in the UK and internationally, rely on weather station sensors embedded in the road which only provide a small window sample as to the overall route network's condition. The reliance on limited road sensor data invariably leads to risk averse over salting of roads.
In conjunction with leading academic experts from Heriot Watt University, the Dundee City Council and Tayside Contracts Road Maintenance Partnership has achieved the goal of determining a configurable value of radar reflectivity to measure salt levels. This system has been trialled, corroborated and the findings and data shared with the winter maintenance community so all responsible for winter road maintenance can benefit from this innovative development. The system developed has the potential to radically change how the winter maintenance industry operates and to achieve significant financial savings without a reduction in service standards.
The challenge
In Scotland salt use varies from year to year depending on the severity of the weather. In the last season (2018/19), there were 350,000 tonnes of salt used on Scotland's roads. In the year prior (2017/18), which featured the `beast from the east' weather event, the salt use for the season was 740,000 tonnes. During exceptionally severe winters such as 2010/11, the usage figure can be in excess of 1,000,000 tonnes. At an approximate average cost of £50 per tonne, the value outlaid annually in Scotland on salt use alone is significant. Coupled with the cost of salt is the cost of deploying the gritters which can double the expenditure. Dundee's salt use averages 8,500 tonnes per annum with 4,400 tonnes used last season (2018/19) and 11,500 tonnes the year before (2017/18).
In an average year, Dundee City Council incurs £800,000 in operational route treatment costs, a figure which is variable dependent on the weather and the number of treatments deployed in the season. Nationally in Scotland the variable cost of winter gritting averages approximately £50,000,000. The assessed benefit of the system developed is targeted to reduce salt use and pre-treatment requirements by 10%. This target has been established based on the initial trials findings of route length residual salt measures compared to the window samples attainable at weather station sites. The targeted savings equate to an annual reduction of 850 tonnes in salt use in Dundee, providing a £85,000 potential cost saving. If deployed nationally throughout Scotland, the potential annual savings to Scottish local government would be £5,000,000 per annum with 50,000 tonnes less salt deployed on Scotland's roads each year.
In addition to the cost associated with gritting roads, there are environmental implications. In urban areas such as Dundee the vast majority of runoff is discharged through drainage carrier pipes to water treatment plants prior to release to tidal estuary. However, in rural inland areas runoff is often received by ditches which discharge to watercourses. Low levels of salt run off have a minimal effect on ecology, however continuous salt deployment during prolonged low temperatures can heighten the concentration of sodium chloride and potassium chloride to the detriment of water quality and aquatic wildlife such as plankton and fish inhabiting inland lochs. Reduction of unnecessary salt deployment is a key to sustainably managing the environmental impact of winter maintenance operations. In addition to the effect on water purity, reducing salt usage has a consequential impact on the carbon footprint of winter operations by lessening haulage of material to stockpiles and secondary transportation during route deployment.
Actions/approach taken
During the 2018/19 winter season Dundee City Council, in partnership with Tayside Contracts and an academic team of microwave scientists from Heriot Watt University, embarked on a trial to develop a system of identifying the level of residual salt on road surfaces utilising vehicle mounted technology to scan gritting routes. The trial was structured with development tasks aimed to achieve the objective of using less salt in winter maintenance treatments and accordingly saving money and reducing the environmental impact of operations.
The trial involved fitting radar technology to a test gritter and linking to the gritter's onboard computer. The gritter was run on a set winter route to record residual salt volumes in various road states, with the data recorded, analysed and corroborated against secondary data feeds. On conclusion of the trials, a set of frequency signatures were established to provide a means of establishing salt volume from radar reflectivity magnitude.
The project team consisted of engineers from Dundee City Council, operational staff and mechanics from Tayside Contracts, and academic scientists from Heriot Watt University. Each element of the team provided specialist input into their area of expertise and all learned from each other to the benefit of the project's development.
The funding for the development and trials were secured from a £30,000 industry innovation fund provided by Peacock Salt Ltd aimed at achieving advancements in winter maintenance technology and practices. Three applicant teams were shortlisted by Peacock Salt Ltd and the project was successful in being awarded the fund due to the demonstrated scientific rationale for the technology, coupled with the clear plan for development, testing and evaluation of the system's accuracy in performance.
Impact and results
The system development and trials were completed during the 2018/19 winter season with results verified in April 2019 to establish the radar frequency signatures of the various road surface states. This information can now be used to inform winter treatment decision makers of the accurate route residual salt concentration to aid in their determining re-treatment requirements. It will also allow for salt conservation by lessening deployment in pre-treatment actions undertaken prior to freezing temperatures.
The findings of the 2018/19 season trials and system development have been shared via a technical report produced for the Society of Chief Officers of Transportation Scotland (SCOTS) and the Association for Public Service Excellence (APSE). To further publicise this development and share best practice within the Scottish Winter Maintenance community, our Roads Maintenance Partnership Manager delivered a presentation to the annual winter maintenance industry conference `Cold Comfort Scotland 2019' on 25 April 2019. To inform the international community of this development, the technical report was shared with the UK National Winter Service Research Group which feeds into the International Road Federation to provide reach to a further 118 countries.
How is the approach being sustained?
Subject to funding being secured to live run the next phase of the development it is targeted to deploy route monitoring of residual salt in Dundee during the 2019/20 winter season. During the next season it is planned to compare the actual treatment plans completed with full road residual salt monitoring against the theoretical action plans produced using the previous system of weather station data reliance to determine more accurately the salt use and deployment savings which will be calculated in monetary, resource and carbon saving values.
The findings of the live route trials and vehicle automation development will again be made public and shared with the winter maintenance community to make available the system data and allow broader uptake and further advancement of this innovative development.
Lessons learned
Accurately identifying and recording the quantity of salt on roads prior to re-application has always been a major goal of the winter maintenance community, both in the UK and internationally. At present, all roads authorities determine the volume of residual salt from weather station sensors embedded in the road at strategic locations. On average there is one sensor for every 100 miles of road, meaning the window sample is extremely limited on the overall road network condition. This is coupled with the sensor being very small in area - approximately the size of a match box. When determining whether top up reapplication of salt is required, the winter managers view the salt levels recorded at these sensors to identify whether the salt levels are sufficient to prevent refreezing of the road surface. The sensors also provide the record to demonstrate the roads authorities' diligence in applying salt in the event of road traffic accident.
In conjunction with leading microwave science academics from Heriot Watt University, the Dundee City Council and Tayside Contracts Roads Maintenance Partnership have developed a system which can scan the road surface and identify the level of residual salt present. The system utilises a vehicle mounted radar scanner and the project team have been able to identify the radar reflection magnitude frequency signature for varying levels of salt present on the road surface. These readings have been cross referenced and corroborated against actual salt deployment records and the weather station data to verify the readings.
For many years a system to identify residual salt has been seen by practitioners as the `holy grail' in winter maintenance technology, with a desire for a system capable of accurately identifying salt levels from vehicle mounted driven route coverage. The findings of the trials undertaken this year have been shared with the UK roads community and also the international community with details of the signature frequencies of alternate road states together with the method of analysis made public. This information can allow driven route scanning of the network as part of the treatment decision making process, reducing the reliance on small weather station window samples to determine salt concentration and allowing targeted supplementary treatments to be deployed.
Next steps
The findings achieved to date have already provided a significant advance in treatment analysis, and further trials are planned for the 2019/20 season (subject to funding) to link the scanning technology to the gritters' automatic deployment system to start and stop spreading and vary spread rates as determined by the residual salt encountered when passing over. Scotland's only gritter manufacturer, James Cuthbertson Ltd based in Biggar, has confirmed their commitment to partake in this further development trial in 2019/20 which will build upon the award-winning automation development previous completed in 2016/17 to create the UK's first retrofitted system of pre-planned spreading automation.
For further information on this case study, please contact
Ewan Macnaughton
ewan.macnaughton@tayside-contracts.co.uk
01382 834149).