This section presents the results of the future climate risk assessment, demonstrating the likelihood and impact of the eight climate-hazards on the Trunk Road Network (TRN), for future scenarios of the mid-century and end of the century.

Flooding

Extreme rainfall events cause flooding of transport infrastructure and hubs, resulting in travel and freight delays, accidents and impacts on emergency services. CCRA3 reports a greater exposure to surface water flooding compared to river flooding. The current and future magnitude of risk is considered high for Transport Scotland.

Pluvial Flooding

Surface water (pluvial) flooding occurs after periods of heavy, sometimes short-duration, rainfall when excess water cannot be drained, often due to blocked or exceeded drainage capacity. Surface water flooding presents a significant risk to the TRN at present, and UKCP18 projections indicate that the risk from surface water, in the absence of further adaptation action, could double by the 2080s under a 4°C warming scenario.

Table 1: shows annual probability of a surface water flood for current and future scenarios based on CCRA3 uplifts

Current likelihood category (SEPA)	Length of TRN in probability band (km)	SEPA Baseline Probability (TS likelihood)	2050, pathway to +2°C warming by 2100	2050, pathway to 4°C warming by 2100	2080, pathway to 2°C warming by 2100	2080, pathway to 4°C warming by 2100
High	163	10% (High)	16.3% (Very high)	18.8% (Very high)	19% (Very high)	23.3% (Very High)
Medium	336	0.5% (Medium)	0.8% (Medium)	0.9% (Medium)	0.9% (Medium)	1.2% (Medium)
Low	387	0.1% (Low)	0.2% (Low	0.2% (Low)	0.2% (Low)	0.2% (Low)

Table 1 shows an overall, very high likelihood of surface water flooding across the TRN in the 2050s is derived from the future annual expected probability.

River Flooding

Fluvial flooding, or river flooding, occurs after periods of heavy, sometimes short duration rainfall which results in peak river flow events. Projections for Scotland indicate increases in extreme rainfall events, river runoff and peak flows which will exacerbate the risk of river flooding and increase the length of roads exposed to flooding. River defences may also be impacted causing a reduced standard of protection in the absence of further adaptation.

Table 2: shows annual probability of a fluvial flood for current and future scenarios based on CCRA3 uplifts

Current likelihood category (SEPA)	Length of TRN in probability band (km)	Current Probability	2050, pathway to +2°C warming by 2100	2050, pathway to 4°C warming by 2100	2080, pathway to 2°C warming by 2100	2080, pathway to 4°C warming by 2100
High	98	10% (High)	16.9% (Very high)	18.8% (Very high)	18.5% (Very high)	18.8% (Very High)
Medium	161	0.5% (Medium)	0.8% (Medium)	0.9% (Medium)	0.9% (Medium)	0.9% (Medium)
Low	201	0.1% (Low)	0.2% (Low	0.2% (Low)	0.2% (Low)	0.2% (Low)

Table 2 shows an overall, very high likelihood of fluvial flooding across the TRN in the 2050s is derived from the future annual expected probability, taking into account road lengths within each risk probability band in future.

Coastal Flooding

Sea levels have been increasing at a rate of approximately 1.4 mm per year since the beginning of the 20^th century, and the rate of rise is accelerating. Coastal flooding and erosion risk to roads is, therefore, expected to increase. The CCRA3 notes that current adaptation responses are inadequate to fully manage the risk from sea level rise and there is a need for greater understanding of current and future risk, and for ‘what if’ scenarios of high rates of change.

Table 3: shows annual probability of coastal flood for current and future scenarios based on CCRA3 uplifts

Current likelihood category (SEPA)	Length of TRN in probability band (km)	Current Probability	2050, pathway to +2°C warming by 2100	2050, pathway to 4°C warming by 2100	2080, pathway to 2°C warming by 2100	2080, pathway to 4°C warming by 2100
High	14	10% (High)	24.3% (Very high)	31.5% (Very high)	29.9% (Very high)	38.8% (Very High)
Medium	30	0.5% (Medium)	0.8% (Medium)	1.6% (Medium)	1.5% (Medium)	1.9% (Medium)
Low	40	0.1% (Low)	0.2% (Low	0.3% (Low)	0.3% (Low)	0.4% (Low)

Table 3 shows an overall very high likelihood of coastal flooding across the TRN is derived for future scenarios from the changes in annual expected probability, taking into account road lengths within each risk probability band in future.

River Scour

Structures built in or near river channels are at risk of scour, whereby flowing water can remove sediment from the foundations and compromise their structural integrity. Scour due to increased rainfall can lead to travel disruption, significant repair costs and the potential isolation of remote communities

The majority of catchments in Scotland which the TRN lies within have a medium susceptibility to scour, with hotspots of medium-high susceptibility in the River Avon catchment near Edinburgh and Glasgow Coastal catchment and to the North West. A high susceptibility to scour is noted to the south-west for the River Bladnoch and Water of Luce catchments which the TRN intersects.

Figure 1 shows an overall, high likelihood of river scour across the TRN in the 2050s is derived from the susceptibility of the TRN to scour due to its location in catchments with characteristics which are susceptible to river scour. In tandem with the future annual expected probability increases in fluvial flooding, there is likely to be an increased likelihood of fluvial flood events and therefore, indirect impacts from scour on assets such as bridges, culverts over water and walls supporting the carriageway adjacent to water.

Landslides

Increases in high intensity and/or prolonged rainfall can increase the number of slope failure incidents that impact the transport network. Variable ground saturation and periods of dry weather can lead to cracking and exacerbate this situation further. Upland and mountainous areas are more susceptible to slope failure and landslides because of their topography. Future soil moisture fluctuations due to increases in rainfall intensity and dry periods are projected to increase the risk of failure, particularly in autumn and winter. CCRA3 reports a medium future impact for Scotland.

The future likelihood scoring for landslides is based on UKCP18 projections of days which meet a threshold of greater than 64mm of rainfall, combined with antecedent rainfall thresholds (see Appendix E.3). Two future scenarios are assessed, RCP6.0 a medium emissions scenario and RCP8.5 a high emissions scenario for the mid-century (2050s) and end of century (2080s).

Figure 2 shows an overall, high likelihood of landslides across the TRN in the 2050s is derived from the UKCP18 projections of rainfall and antecedent rainfall.

Extreme Heat

High ambient temperatures lead to deformation of road surfaces and increase the thermal loading on bridges, parapets and vehicle restraints which can lead to expansion, bleeding and rutting. They can also cause subsidence of structures with spread foundations. Projections indicate that Scotland will experience higher average temperatures and a significant increase in the number of warmer (>25°C) days. However the frequency in days above 30°C does not dramatically increase. CCRA3 reports a high future impact for the UK.

The future likelihood scoring for extreme heat is based on UKCP18 projections of days which meet a threshold of greater than 30°C (see Appendix E.2). Two future scenarios are assessed, RCP6.0, a medium emissions scenario and RCP8.5, a high emissions scenario for the mid-century (2050s) and end of century (2080s). An example of the output to inform the likelihood score is presented in Figure X.

Figure 3 shows an overall, rare annual likelihood of extreme heat across the TRN in the 2050s is derived from the UKCP18 projections of days above 30°C.

Cold Spells

Cold temperatures with associated or antecedent precipitation, including snowfall, lead to significant, widespread disruptions across the TRN. Despite an increase in average winter temperatures, the intensity of winter precipitation is likely to increase, therefore although snow volumes and ice frequency are likely to decrease, high magnitude snowfall events may increase. CCRA3 reports a high future impact for the UK.

The future likelihood scoring for cold spells is based on UKCP18 projections of days which meet a threshold of less than 0°C and >1.2mm of precipitation (see Appendix E.4). Two future scenarios are assessed, RCP6.0 a medium emissions scenario and RCP8.5 a high emissions scenario for the mid-century (2050s) and end of century (2080s). An example of the output to inform the likelihood score is presented in Figure X.

Figure 4 shows an overall, high annual likelihood of cold spells across the TRN in the 2050s is derived from the UKCP18 projections of days less than 0°C and greater than 1.2mm of precipitation.

High Winds

Climate modelling suggests a higher frequency of extreme wind events in the second half of the 21st century, although quantified projections are difficult to estimate. CCRA3 reports a high future impact for the UK.

The future likelihood scoring for high wind gusts is based on UKCP18 projections for days exceeding a threshold of more than 35 mph. Due to low confidence in modelled outputs, the Met Office have not published UKCP18 Probabilistic projections, therefore the UKCP18 local climate model (2.2km) is used.

Figure 5 shows an overall, medium likelihood can be viewed for all time periods and emissions scenarios. A scaled likelihood is used to reach a 2050s medium emissions scenario to compare risk scorings across different hazards. Therefore, a medium likelihood is expected for wind for a 2050s, medium emissions scenario.