Rationale behind the revision of the NYCC salt spreading treatment matrix.
Prior to last season, the treatment matrix had been in use for many years and had not changed even though gritter spreading accuracy has greatly improved. Also, recent alterations to the guidance around salt spreading rates have convinced Officers that there was scope to reduce some of our spread rates.
The guidance followed comes from the National Winter Service Research Group (NWSRG), and is attached.
BACKGROUND TO NWSRG PRACTICAL GUIDE FOR WINTER SERVICE
The latest (2016) version of the UK Roads Liaison Group’s national code of good practice for highway maintenance matters, 'Well-managed Highway Infrastructure', no longer provides detailed guidance to practitioners regarding the delivery of the winter service.
Instead, and recognising the technical quality of its work and level of expertise residing within the NWSRG, the UK Roads Board, on behalf of the UKRLG, has requested the NWSRG to make its Practical Guide generally available to all practitioners and interested parties, as it is considered to constitute the best way of providing national best practice guidance on these issues.
The guidance goes into a lot of detail around the various external factors that influence how the salt that is spread works on, and is removed from, the road surface.
The key point of the guidance is that Authorities are to determine their own spread rate matrices that are appropriate for use on their own networks, as well as assisting them in determining which of those spread rates to utilise in response to a particular weather forecast, expected traffic and road conditions based upon this guidance.
The salient points to consider when determining our own matrix are:
1. Quality of the stored salt and accuracy of the gritter fleet.
2. Road temperatures expected during the period under consideration (usually the next 24 hours)
3. The amount of liquid water present at the time of spreading and the following period.
4. Traffic levels before, during and after spreading.
5. Wind speed and direction.
6. Residual salt present on the network.
7. Road surfacing type.
Last year the Corporate Director decision recognised the new guidance but restricted any reduction in spread rates to a minimum of 9g/m2, with further reductions only to be considered after a period of monitoring and experience. As no negative impacts were observed or detected, we now propose to reduce the spread rates further in line with the national guidance.
NYCC proposes to further amend its treatment matrix based on the following statements:
1. Quality of the stored salt and accuracy of the gritter fleet.
a) Our salt is stored under cover and assumed to be at the optimum moisture content of 2% to 4%, and this will be checked at points through the year.
b) The gritter fleet is modern, well maintained and calibrated at the start of every season. In season checks will be done and recorded. Therefore we will be using the rates suggested got “Good” spreader capacity.
2. Road temperatures expected during the period under consideration (usually the next 24 hours)
a) Our matrix will be divided into appropriate temperature bands.
3. The amount of liquid water present at the time of spreading and the following period.
a) Our matrix will be divided into dry, damp and wet for pre-treatments and ice, snow and freezing rain for post treatments.
4. Traffic levels before, during and after spreading.
Table 8.3 in the guidance:-
The guidance states:
It is anticipated that traffic levels on the great majority of local authority road networks will fall within the ‘Medium Traffic’ category during the times periods that most precautionary salting operations are undertaken. Therefore, the recommended spread rates provided in the matrices contained within sub-section 8.6 relate to the ‘Medium Traffic’ category.
It is not anticipated that many local authority roads will fall into the ‘High Traffic’ category and research has shown that salt losses do not increase significantly for traffic levels beyond 250 vehicles per lane per hour, as long as this traffic is moving normally.
However, it is likely that some precautionary salting operations undertaken by local authorities will include routes that fall into the ‘Light Traffic’ and ‘Congested Traffic’ categories. In these situations, it is important that spread rates are modified accordingly.
As all out pre-treatments are on the Priority One network, our spread rates for pre-treatments will be based upon Medium Traffic levels.
Rates for post treatments do not need to consider traffic levels.
5. Wind speed and direction.
The guidance states:
When treatments are carried out during high wind conditions, it is recommended that authorities monitor residual salt levels and carry out re-treatments if and where necessary. If this issue is considered to pose a significant risk, authorities may also wish to increase spread rates when carrying out precautionary salting operations during periods when forecast mean wind speeds are 20mph or higher.
Therefore our pre-treatment matrix for dry salting will show separate rates for when wind speeds are in excess of 20mph.
6. Residual salt present on the network.
The guidance states:
Residual salt from previous operations can reduce the spread rates required to prevent frost/ice formation. However if, when decision making, residual salt levels are relied upon to reduce instructed spread rates, it is important that such decisions are evidence based. As with all other pertinent information relating to winter service decision making, the supporting data should be recorded and retained.
For the purposes of the treatment matrix, the effect of any residual salt has been ignored. However, further guidance is being sought from the industry.
7. Road surfacing type.
The guidance asks that when spreading on porous asphalt, the spread rates provided in the guidance should be increased by 25% and the increased spread rate should be maintained for a distance of 1 kilometre ‘downstream’ of each porous section (in two-way traffic situations, the increased spread rate should be maintained for a distance of 1 kilometre at both ends of each porous section). For other negatively textured surfaces this increase should be between 10% and 25% for the first two years of the surfacing. For the purposes of our matrix this will be covered in a foot note.
The
recommended spread rates for dry salting in the guidance are shown
below:
