Scottish Road Network Climate Change Study: UKCP09 update Autumn 2011

9 Discussion of 2005 Study Recommendations

The Scottish Road Network Climate Change Study, 2005 made a number of recommendations to address the likely impacts on the trunk road network of the predicted trends in climate change. The likely impacts and recommendations were based on the most up to date information available at the time, which were the Climate Change Scenarios published in 2002 under the UK Climate Impacts Programme (UKCIP 02). In 2009, the UKCP09 climate change scenarios were released which superseded the 2002 projections. These scenarios are designed to provide improved and more detailed descriptions of the likely climate the UK will experience throughout the 21^st century. The previous sections of this report seek to present a broad understanding of the UKCP09 data and to provide an updated understanding of each of the relevant climatic variables, together with an understanding of how the UKCP09 projections compare to the equivalent projections of the earlier study.

This section of the report presents a review of the 28 adaptive recommendations made in the previous study and gives a broad assessment of the continuing validity of the recommendations in light of the updated understanding of likely future changes in the climate. Each of the recommendations made in the previous study is set out below, in numerical order, with a brief discussion of the on going validity of the recommendation in light of the UKCP09 scenarios.

9.1 Review of Recommendations

Recommendation 1 - revise the parameters for the design storm for surface water drainage performance. This could be achieved by continuing the use of historical information but for an increased return period, or by basing the approach on further research on rainfall changes arising from climate change.

Recommendation 1 is related to rainfall and was identified to address the concern that the increase in rainfall, suggested by climate change projections, could result in design elements which do not cater adequately for specific rainfall events where use is made of historical rainfall data.

This recommendation has been addressed by revision of the relevant design standard, HD33/06 of the Design Manual for Roads and Bridges (DMRB), where Paragraph 6.2 requires a sensitivity test of drainage systems for a 20% increase in rainfall intensity of the design storm. This recommendation was based on UKCIP02 projections which indicated that by the 2080s, extreme storm event rainfall depths were predicted to have increased between 10% and 30%. The increase is equivalent to a 1-year storm, 15 to 60 minutes duration, becoming on average a 2+/- 0.6-year storm when assessed on the present day rainfall depth - duration - frequency relationships.

Rain is one of the most important factors affecting the design and operation of the road network. Section 6 of this report presents an understanding of precipitation related variables based on UKCP09 data. With reference to the 2-year daily rainfall event, analysis of the UKCP09 data does not change the earlier finding based on the UKCIP02 data. It has been assumed for both studies that the 2-year daily rainfall event can be used as surrogates for adjusting short duration low return period events (ie 1 in 1-year and 1 in 5-year events typically used in surface water drainage situations). It is therefore concluded that the 20% increase in rainfall intensity of the design storm, currently implemented in the DMRB, in the main, continues to present a reasonable response to likely predicted changes in rainfall.

However, further consideration is required of the appreciable uncertainty which may surround the assumption that the changes to the 2-year daily rainfall event can be used as surrogates for adjusting short duration low return period event. There is also a range of uncertainty attached to the projected changes for this variable itself. For the design of particularly sensitive or critical parts of the trunk road network, it is recommended that consideration be given to a risk based approach whereby the drainage network is tested for a greater increase in rainfall intensity based on the projected 90 percentile change as an upper bound figure. This could be an increased of up to 40%.

Recommendation 2 - revise the parameters for the design storm for watercourse structures. This could be achieved by continuing the use of historical information but for an increased return period, or by basing the approach on further research on rainfall changes arising from climate change.

Recommendation 2 is related to rainfall and was identified to address the concern that the increase in rainfall, suggested by climate change predictions, could result in design elements which do not cater adequately for specific rainfall events where use is made of historical rainfall data.

Section 6 of this report presents an updated understanding of precipitation related variables based on UKCP09 data. Consideration of the central projected values (50 percentile values) for Glasgow, Dundee and Aviemore suggests that the change in extreme 10-year storm depths for the 2080s is predicted to increase by 15% to 40% compared to the current situation. This suggests that the current 100-year river flood would become the 40-year event by the end of the century. This compares to a 10% to 30% increase predicted in the previous study.

The principle underlying this recommendation is therefore unaltered and there remains a need to revise the parameters for the design storm for watercourse structures, to account for the projected changes. The assessment of the UKCP09 scenarios suggest that the magnitude of change for the 1 in 100-year storm depths may be slightly greater than previously assessed and this should be taken into consideration for current adaptive strategies. It is also noted that the current design guidance for watercourse structures in relation to climate change adaptation is less well defined than for surface water drainage systems. It is recommended that a renewed priority is assigned to this action.

It should be noted that the apparent greater increase for rarer events should be treated with some caution as, although it may be a true reflection of the likely change, it may also be a manifestation of the difficulty of projecting extreme events. An approach based on the 50 percentile value would therefore seem to be a prudent approach, with perhaps sensitivity testing for critical elements based on the 90 percentile value.

Recommendation 3 - identify locations where flooding of the road network has occurred and develop potential solutions for evaluation on a cost/benefit basis, prioritising those areas where repeated flooding has occurred.

Recommendation 4 - pre-emptively clear detritus from channels/watercourse structures in known areas of flooding risk in response to predicted heavy rainfall.

Recommendations 3 and 4 are related to rainfall and were identified to address the concern that the increase in rainfall suggested by climate change predictions could result in exacerbating flooding on the existing road network, particularly in areas already known to flood periodically.

Section 6 of this report presents an understanding of precipitation related variables based on UKCP09 data. The discussion following recommendations 1 and 2 show that in principle the analysis of the UKCP09 data does not change the earlier finding based on the UKCIP02 data that rainfall depths and frequency of events are predicted to increase. There is a reasonable correlation between the two studies for the projected changes to the 2-year daily rainfall event. The predicted increases for the 10-year 1-day rainfall depths are slightly higher based on the UKCP09 data compared to the UKCIP02 data.

Locations where flooding of the road network has occurred will remain at risk with a projected increase in the frequency of flooding. It is estimated that the 10-year 1-day design rainfall magnitude is likely to be as frequent as the 5-year event in the 2080s. The baseline 2-year one day rainfall event is also likely to be more frequent, becoming approximately a 1.3-year event.

The validity of recommendations 3 and 4 remains unaltered.

Recommendation 5 - undertake further research in respect of catchment runoff estimation parameters and provide guidance on possible risk-based design approaches, including evaluation of alternative solutions on a cost/benefit basis.

Recommendation 5 is related to rainfall and was identified to address the concern that the increase in rainfall combined with the changes in seasonal soil moisture condition suggested by climate change predictions, could result in catchments responding in a different manner to that described by current models, potentially increasing runoff volumes.

Section 6 of this report presents an updated understanding of precipitation related variables, including soil moisture deficit and groundwater recharge, based on UKCP09 data. The trends identified in this assessment are similar to those previously predicted based on UKCIP02 data. The UKCP09 data has provided further details to the previous assessment and has enabled a more informed view of soil moisture deficit and groundwater recharge to be taken.

The UKCP09 assessment supports the conclusion of the earlier study that changes in the climate could result in catchments responding in a different manner to that described by current models, potentially increasing run off volumes. Current adaptation strategies should therefore be maintained and reviewed in light of this updated assessment.

Given that the current adaptation strategy is linked to output from long term ongoing research, consideration should be given to reviewing the current strategy and adopting interim solutions to address this issue in the short term.

Recommendation 6 - consider using the Variable Message Sign (VMS) network to provide a greater level of locally relevant information to road users on predicted severe weather events, expressed in terms of probability of occurrence.

Recommendation 6 is not related to any one specific climatic variable in particular and was identified to address the concern that the general increase in severe weather events suggested by climate change predictions could result in a reduction in levels of service and safety for road users.

In general, the UKCP09 scenarios align with the previous assessment based on UKCIP02 scenarios and identify that there is likely to be an increased trend towards more severe weather events.

The basis of this recommendation therefore remains reasonable.

Recommendation 7 - review local experience on the durability of surface dressings and consider whether these or another intervention measure is appropriate for the location concerned.

Recommendation 7 is related to temperature and was identified to address the concern that the increase in temperature combined with the increase in rainfall suggested by climate change predictions, could result in changes to the basic design assumptions underlying specific design elements.

It was noted in the previous study that "while higher temperatures may affect the long-term durability of pavements, the current predictions (UKCIP02) do not suggest that this is likely to be a significant concern". As for the UKCIP02 scenarios, increases in temperature are predicted to occur across the whole of Scotland, based on UKCP09 data and various predicted scenarios are shown in section 5 of the report. As assessed previously, the magnitude of these increases are unlikely to be a significant concern with regard to the durability of surface dressings. The basis of this recommendation therefore remains reasonable.

It is also noted in section 8.2 that there are parts of England that currently experience high temperatures that are analogous to predicted future changes in the Scottish Climate. Adaptation techniques and actions can therefore build on the experience gained from managing high temperatures in other areas.

It has also been possible using the UKCP09 scenarios to gain an understanding of the potential change in the number of freeze-thaw days, which was not studied in the earlier report. This assessment shows a likely reduction in the number of freeze-thaw days per year, compared to the current baseline. The freeze-thaw cycle is an issue for pavement design and maintenance. However, a potential decrease in the number of freeze-thaw days is not a trend which gives rise to increased concern with regard to the durability of pavement materials.

Recommendation 8 - consider implications of an extended growing season when developing landscape designs, and specify slow -growing species where appropriate.

Recommendation 8 is related to temperature and was identified to address the concern that the increase in temperature combined with the increase in rainfall suggested by climate change predictions, could result in changes to the basic design assumptions underlying landscape design elements.

The analysis of the growing season produced unexpected results using the UKCP09 data. The issue has been raised with the UKCP09 technical team but is as yet unresolved.

UKCIP02 data indicated that the thermal growing season in the highlands will have increased from the baseline of 150 days to about 200 +/- 20 with increases in southern Scotland likely to be slightly higher. In the absence of an updated UKCP09 assessment, this remains the most current assessment of the anticipated changes to the growing season. The basis of Recommendation 8 therefore remains valid.

Recommendation 9 - design drainage systems with additional storage capacity, taking account of sustainable drainage techniques, where appropriate.

Recommendation 10 - introduce surface/sub-surface drainage during maintenance works where these do not exist at present, while recognising that this may not always be possible due to land-take constraints.

Recommendations 9 and 10 are related to rainfall and were identified to address the concern that the increase in rainfall suggested by climate change predictions could result in increased runoff which could influence the design decision on specifying particular drainage systems and/or introducing drainage systems into the road network where these have not previously been in place.

Section 6 of this report presents an updated understanding of precipitation related variables based on UKCP09 data. The discussion following recommendations 1 and 2 show that in principle the analysis of the UKCP09 data does not change the earlier finding based on the UKCIP02 data that rainfall depths and frequency of events are predicted to increase. There is a reasonable correlation between the two studies for the projected changes to the 2-year daily rainfall event. The predicted increases for the 10-year 1-day rainfall depths are slightly higher based on the UKCP09 data compared to the UKCIP02 data.

Recommendation 9 and 10 therefore remain a reasonable adaptation response to the predicted increases in rainfall. Introducing surface/sub-surface drainage during maintenance works will continue to improve the resilience of the trunk road network on an ongoing basis.

Recommendation 11 - clarify the investigation requirements for watercourse structures potentially susceptible to scour, in terms of periodic and post-event inspection requirements.

Recommendation 11 is related to rainfall and was identified to address the concern that the increase in rainfall suggested by climate change predictions could result in increased runoff events with the potential to cause scour at watercourse structures.

The basis of Recommendation 11 therefore remains valid.

Specific requirements for scour inspections are addressed by the current term management and maintenance contracts for the trunk road network. However, given the range of possible outcomes predicted for rainfall events and in particular the potential increased frequency of longer and rarer events, it would be prudent to review and where necessary strengthen investigation requirements for watercourse structures as the term maintenance contracts are reviewed.

Recommendation 12 - consider whether new scheme designs would be susceptible to full or partial closure in high winds, and to prepare a cost/benefit analysis to assess the opportunity for including wind barriers at these locations.

Recommendation 13 - continue the process of developing a High Winds Strategy for the trunk road network and consider the need for a similar approach on exposed sections of the local road network.

Recommendation 14 - undertake further research on likely changes in wind patterns, including seeking to reduce the significant uncertainty that exists in regard to this issue. This would enable further guidance to be given on future consideration of wind in the design and operation of the road network.

Recommendations 12, 13 and 14 are related to wind and were identified to address the concern that the increase in wind suggested by climate change predictions could result in increased disruption to road users at exposed locations that might not be adequately assessed where use is made of historical wind data.

No information is given in UKCP09 output with regard to wind and these recommendations have therefore not been reviewed further.

The assessment of this climatic variable, and appropriate adaptation strategies, remains as presented in the Scottish Road Network Climate Change Study, based on UKCIP02 data.

Recommendation 15 - consider when developing new designs whether these will be susceptible to coastal flooding, taking account of predicted sea-level changes and the possible effects of storm surges and consider how the design should take account of any such susceptibility.

Recommendation 15 is related to sea level rise and coastal flooding and was identified to address the concern that coastal flooding could be exacerbated by a combination of climate change predictions for various weather types that might not be adequately assessed where use is made of historical weather data.

UKCIP02 and UKCP09 scenarios both show, in principle, the same rising trend of increases in mean sea level, with differing values and ranges in certainty. The UKCP09 findings suggest that the mean sea level rise will be slightly greater than previously assessed. However, it is also noted that changes to wave regimes and surges remain poorly understood.

The basis of this recommendation remains valid. Current adaptive strategies should be maintained with consideration given to the updated UKCP09 scenarios when considering individual projects.

Recommendation 16 - update the Scottish Road Network Climate Change Study Report as necessary following publication of the UKCIP06 report.

The publication of this report addresses this recommendation.

Recommendation 17 - specify materials with appropriate stiffness to provide greater confidence that pavement deformation at high temperatures should not occur.

Recommendation 18 - consider periodic reviews of required pavement interventions, to determine whether high temperatures have been identified as a significant contributing factor.

Recommendations 17 and 18 are related to temperature and were identified to address the concern that the increase in temperature suggested by climate change predictions could result in changes to the basic design assumptions underlying particular pavement design elements.

As with the UKCIP02 scenarios, the UKCP09 scenarios continue to show predicted increases in the number of hot days per year. Overall the range of outcomes shown are not considered to be of any further significance in relation to material properties than was previously understood. The current recommendations and adaptive strategies therefore continue to represent a reasonable response to the predicted increases in temperature. It is also noted in Section 8.2 that there are parts of England that currently experience high temperatures that are analogous to predicted future changes in the Scottish Climate. Adaptation techniques and actions can therefore build on the experience gained from managing high temperatures in other areas.

Recommendation 19 - consider carrying out earthworks inspections under the principles of Technical Memorandum HD 41/03 'Maintenance of Highway Geotechnical Assets' pending further guidance on landslide risk management arising from the 'Landslide Study', prepared as a companion to this report.

Recommendation 19 is related to rainfall and was identified to address the concern that the increase in rainfall combined with the changes in seasonal soil moisture condition suggested by climate change predictions, could result in reductions in slope stability.

The basis of this recommendation remains valid and current adaptive strategies should be maintained.

Recommendation 20 - consider how enhanced communication could be developed with all parties involved in flood management.

Recommendation 20 is related to rainfall and was identified to address the concern that the increase in rainfall suggested by climate change predictions could result in increased runoff leading to more and larger flood events occurring.

This is a general recommendation which remains valid in response to the continuing predictions of increased extreme rainfall events. Consideration could be given to increasing the priority of this recommendation from "long-term" to "short-term".

It is noted that since the previous study the Flood Risk Management Act 2009 (Scotland) has been passed. This requires a much more integrated approach to the management and understanding of flooding across Scotland, and draws together SEPA, Local Authorities and Scottish Water as the three main agents in implementing the Act. It is recommended that Transport Scotland engage with that process to assess whether there are drivers for enhanced communication that may be of mutual benefit.

Recommendation 21 - was to consider how information on areas known to be at risk of flooding may be integrated with flood warning systems to provide early indication of potential hazards.

Recommendation 21 is related to rainfall and was identified to address the concern that the increase in rainfall suggested by climate change predictions, could result in increased runoff leading to more and larger flood events occurring.

Recommendation 22 - identify areas of the road network that require full or partial closure in high winds and consider the introduction of wind barriers at these locations. This consideration should include a cost/benefit analysis and, if necessary, an environmental assessment.

Recommendation 23 - consider whether enhanced parking/turning facilities at locations on the road network susceptible to full or partial closure in high winds would provide a cost effective operational benefit.

Recommendation 22 and 23 are related to wind and were identified to address the concern that the increase in the incidence of high wind events suggested by climate change predictions could result in increased disruption to road users at exposed locations on the existing road network.

No information is given in UKCP09 output with regard to wind and these recommendations have therefore not been reviewed further.

The assessment of this climatic variable and appropriate adaptive strategies, remains as presented in the Scottish Road Network Climate Change Study, based on UKCIP02 data.

Recommendation 24 - identify locations on the road network susceptible to coastal flooding. This should take account of predicted sea level changes and the possible effects of storm surges, to consider whether measures such as signage, sea-defences or re-routing are appropriate.

Recommendation 24 is related to sea level rise and coastal flooding and was identified to address the concern that coastal flooding could be exacerbated by a combination of climate change predictions for various weather types resulting in greater disruption on the existing road network.

The basis of this recommendation remains valid and current adaptive strategies should be maintained.

Recommendation 25 - carry out further research on specific aspects of winter conditions where data does not presently exist, to provide a greater understanding of the likely pattern of change in winter conditions that is expected.

Recommendation 26 - consider the predicted changes in winter conditions to determine how winter maintenance of the trunk and local road networks may be carried out on a cost-effective basis.

Recommendation 25 was identified to address the concern that although snowfall is expected to reduce and temperatures increase, there is insufficient information available at present to consider how the current pattern of freeze-thaw cycles may vary in the future.

Recommendation 26 was identified to address the concern that the predicted changes in winter conditions, once fully established, may be sufficiently significant to warrant a reconsideration of the strategic approach to winter maintenance.

Sections 5.2, 5.3 and 5.4 of this report present an updated assessment of frost days, freeze-thaw days and winter duration based on UKCP09 data. Freeze-thaw days and winter duration were not assessed in the previous study. The current data therefore provides a more informed picture with regard to general winter conditions. It is suggested that there may be a reduction in the winter period of 45 to 65 days by the 2080s medium emission scenario.

Further research and a greater understanding of the likely position of change in winter conditions remains a valid recommendation. However, given the impacts of natural and decadal fluctuations, these longer term trends are unlikely to affect current management and maintenance practices.

Recommendation 27 - consider developing a road user education programme on the appropriate response to different types of severe weather events.

Recommendation 28 - consider using the VMS network operated by Traffic Scotland (formerly NADICS) to support the above road user education programme.

Recommendations 27 and 28 were identified to address the concern that the general increase in severe weather events suggested by climate change predictions could result in a reduction in levels of service and safety for road users.

The updated UKCP09 assessment of predicted climate changes, remains reasonably well aligned with the UKCIP02 scenarios.

These are general recommendations which remain relevant.