Scottish Road Network Climate Change Study: UKCP09 update Autumn 2011

10 Conclusions and future considerations

In 2009 the UK Climate Projections (UKCP09) were released. These scenarios are designed to provide improved and more details descriptions of the likely climate that the UK will experience throughout the 21^st century.

The UK Climate Projections provide projections of climate change for the UK, giving greater spatial and temporal detail, and more information on uncertainty than previous UK climate scenarios. The purpose of this report is to update the findings of the Scottish Road Network - Climate Change Study (2005) including its recommendations of adaptive strategies to minimise future climate change impacts.

Overall, the assessment shows that the trends in the projected changes in climatic variables remain generally consistent with the findings of the earlier study, based on the UKCIP02 climate change scenarios. It is concluded that the understanding of the general picture of climate change and its likely effects on the design, operation and maintenance of the trunk road network has not significantly altered. The 28 recommendations made in the Scottish Road Network - Climate Change Study 2005 therefore continue to represent a reasonable response to the predicted changes in the climate.

The further understanding, provided by the assessment of the UKCP09 climate change scenarios, will assist in supporting the ongoing adaptive actions. Ongoing actions and strategies which are in place to progress each of the individual recommendations, which in principle remain relevant, should nevertheless be reviewed and strengthened if required, to ensure that they remain robust and are continuing to deliver the desired outcomes.

Of the climatic variable assessed as part of this study, rain and potential changes in rainfall can perhaps be identified as one of the most important factors affecting the design and operation of the road network. Whilst the affects of changes in other climatic variables are not to be down played or overlooked, the effects of flooding may arguably be felt more frequently and visibly affect larger parts of the network than changes in other climatic variables. It could be argued that an element of prioritisation is therefore required to address this potential threat to the road network.

The implementation of new road schemes, which have been completed recently or are ongoing, will have considered and made allowance for the effects of climate change in their design, to some extent. It is therefore the existing trunk road network that remains the most vulnerable to the effects of flooding and changes in storm events and durations. In the absence of robust adaption measures, it is likely that the historic incidence of flooding and disruption due to weather events across the network will increase in frequency and magnitude.

Assuming that the changes to the 10-year daily rainfall event can be used as surrogates for adjusting rarer events suitable for considering design circumstances of natural watercourses, the following observation is made:

• By the 2080s peak river flood flows of 10-year to 100-year are 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.

Assuming that the changes to the 2-year daily rainfall event can be used as surrogates for adjusting short duration low return period events (typically used in surface water drainage situations), the following observation is made:

• By the 2080s a 1-year storm of 15 to 60 minutes would become on average a 2 +/- 0.6-year storm when assessed on the present day rainfall depth-duration-frequency relationships. (Similarly by the 2020s the storm would become a 1.4 +/- 0.2-year storm).

The range of uncertainty associated with the above predictions should however be recognised. For example increases by the 2080s could conceivably be almost zero or as high as 75%.

Notwithstanding the overall conclusion presented above, there are a number of further issues worth highlighting with regard to future adaptation activities. These are discussed below.

The probabilistic nature of the UKCP09 projections provides better definition of the inherent uncertainties behind climate change projections and this report provides a fuller understanding of the envelope of uncertainty that can presently be attached to the projections. This is a more realistic way of presenting the data. However, it is necessary for those considering adaptation measures, to consider how to balance the probabilistic nature of the projections together with the associated risks and consequences to the issue that is being managed. An agreed approach should therefore be developed to guide decision makers on how to address the envelope of uncertainty that accompanies the projections. This approach should be developed across all sectors of the Scottish Government so that a consistent framework exists for decision making.

The consequences of not taking forward robust adaptation activities are likely to be increased disruption to the road network and wider economy. The quantification of the costs of increased disruptive weather events, in particular flooding, and the disruption to the wider sustainable economy may assist in prioritising future spending requirements.

In order to make best use of available resources, it is sensible to prioritise adaptation activities and focus on those elements of the network where the likelihood and consequences of disruption are the greatest. The next renewal of the trunk road term maintenance contracts will include the implementation of a risk assessment process to identify and assess areas that have been repeatedly disrupted due to events such as flooding or high winds. This will enable targeted action to be taken as appropriate. Consideration might also be given to a wider vulnerability study in terms of the existing infrastructure's ability to cope with the projected change, consideration of past events and relative importance of individual routes with regard to access to services and people, and the significance of the particular issue in terms of cost and safety.