Risk/opportunity:(from the Climate Change Risk Assessment for Scotland 2012):
FL8a Roads at significant risk of flooding TR1 Disruption of road traffic due to flooding
SCCAP theme: Buildings and infrastructure
B1: Understand the effects of climate change and their impacts on building and infrastructure networks
B2: Provide the knowledge, skills and tools to manage climate change impacts on buildings and infrastructure
B3: Increase resilience of buildings and infrastructure networks to sustain and enhance the benefits and services provided
Current headline figures for the trunk road network are listed below :
- 0.21% of the trunk road network is at risk of high levels of traffic disruption as a result of fluvial flooding
- 0.9% of the trunk road network is at risk of high levels of traffic disruption as a result of pluvial flooding
- 0.06% of the trunk road network is at risk of high levels of traffic disruption as a result of coastal flooding
This risk-based indicator is designed to enable the potential for traffic disruption as a result of flooding to be tracked over time.
- Climate change projections suggest that flooding of road infrastructure in Scotland will become more extensive and more frequent resulting in disruption to traffic, particularly on roads carrying large traffic flows.
- Only a very small proportion of the trunk road network is at risk of high levels of traffic disruption as a result of flooding (under 2%). These areas are mostly in the central belt.
Flooding can disrupt the operation of the road network with knock-on consequences for many social and economic functions – e.g. delaying deliveries, preventing or delaying people from accessing employment, disrupting vital healthcare services etc.
Climate change will increase flood risk and without adaptation, traffic disruption will also increase. This indicator uses modelled data to assess the risk of traffic disruption as a result of flooding. It provides a range of metrics highlighting the different degrees of flood risk across the trunk road network as a function of traffic volume.
This indicator provides a proxy assessment of the risk of traffic disruption as a result of flooding on the trunk road network. The assessment uses average daily traffic volume thresholds to define the criticality of the road section so that road sections serving higher traffic volumes are considered to be more critical and therefore at risk of higher levels of traffic disruption , given the same probability of flood event occurrence (i.e. where risk is a function of event likelihood and consequence). This assessment defines three levels of traffic volume criticality (high, medium and low) and then determines the proportion of the trunk road network at risk of flooding which falls within each criticality level.
Table 1 below shows the proportion of the trunk road network at risk of flooding (fluvial, pluvial and coastal) within each criticality level threshold.
Overall, Table 1 indicates that for the three sources of flooding assessed, the proportion of the trunk road network falling within the high criticality level is lower than the medium and low levels. More specifically, for fluvial source flooding only 0.21% of the trunk road network serves high traffic volumes compared to 0.45% and 1.47% for medium and low traffic volumes respectively. In essence, whilst 2.13% of the trunk road network is exposed to a fluvial flood hazard overall, only 0.21% of the network can be considered at risk of high levels of traffic disruption.
The proportion of the trunk road network that can be considered high risk in terms of the traffic disruption related consequences of pluvial and coastal flooding is 0.9% and 0.06% respectively
Table 1. Proportion of trunk road network at risk of flooding for different levels of traffic disruption (1 in 200 year event)
Trunk road traffic disruption
Disruption (all levels)
Disruption (all sources)
Note: Metrics are presented in terms of ‘trunk road traffic disruption’ based on the level of traffic carried by sections of the trunk road network, where severe disruption corresponds to a high level of traffic, moderate disruption to a medium level of traffic and minor disruption to a low level of traffic. The figures represent the proportion of the trunk road network that would experience a given level of disruption (high, medium and low) should flooding take place, for a 1:200 year flood event. Further, the level of disruption provides a proxy for flood event consequence, therefore, taking flood likelihood as constant (1:200 year), the portion of the trunk road network where traffic disruption is severe can be considered high risk overall, where flood risk is a function of the likelihood of an event occurring combined with its consequences (see Table 4 for further information).
Geographically, sections of the trunk road network that are at ‘high risk’ of traffic disruption due to flooding (all sources) are focussed around the central belt of Scotland (M8, M73, M74, M77, M80, M9 and M90), reflecting the densely populated nature of the region and the associated high demand for transport. See Figure 1 below. Areas of pluvial and fluvial flood risk are across the region reflecting the spatial distribution of watercourses and their catchments (fluvial flood risk) and roads and other less permeable features (pluvial flood risk). There are two key areas at risk of high levels of traffic disruption due to coastal flooding where trunk road routes coincide with the Firth of Clyde and the Firth of Forth and areas of high traffic demand (M8 east of Langbank and the M9 south of Bridge of Allan).
Figure 1 Sections of the trunk road network that are at ‘high risk’ of traffic disruption due to flooding
Note: Sections of the trunk road network at risk of fluvial and pluvial source flooding are coincidental in some instances. Where this is the case the fluvial source flood risk (blue lines) may not show up on the map above.
 This assessment mirrors the assessment in BT2 Risk of road closures from flooding – in that the combined flood risk by source across all criticality thresholds in this is equal to relevant trunk road outputs from BT2.
Historic flood hazard and traffic volume data is not available to assess BT17 metrics in earlier years. However, past climate data shows how key aspects of climate (such as rainfall) have changed. Overall there is a clear upward trend in winter precipitation as well as increasing heavy rainfall in winter (Sniffer, 2014). It is likely that these climatic changes will have led to increased frequency and extent of pluvial and fluvial source flooding. In the context of BT17 however it is not possible to understand how the risk of traffic disruption as a result of flooding has changed without access to historic traffic volume (i.e. road criticality) data. Trends in traffic volume are influenced by a range of transport supply and demand factors such as extent and condition of the trunk road network, demographics, population distribution and economic factors.
The UK Climate Change Risk Assessment (HR Wallingford et al, 2012a; Thornes et al, 2012) assessed changes in flood risk to road infrastructure as a result of anticipated climate changes. Whilst this assessment was only undertaken for England and Wales (due to data availability) it provides a broad indication of what might happen in Scotland in the future given anticipated climate changes. Given this, transport specific aspects from the UK CCRA (ibid) can be used in conjunction with general aspects from the Scotland CCRA (HR Wallingford et al, 2012b) to understand how flood risk to road infrastructure might change in the future. BT2 provides a detailed description of the assessment undertaken. In summary however the following projected changes are anticipated to take place in the future:
- The proportion of the road network located in areas at risk of flooding is projected to increase
- Road infrastructure that is already located in the floodplain is expected to be affected by flooding more frequently
- Increased incidence of intense rainfall events may result in more frequent pluvial flooding
In relation to this indicator, the first projected change is of particular relevance. If the proportion of the road network located in areas exposed to flood hazards increases, additional road sections with potentially different traffic volume patterns would be exposed also. This would increase the overall exposure of the trunk road network. It may also be the case that the proportion of high, medium and low risk changes also.
Furthermore, changes in transport system supply and demand could also influence future disruption. Structural changes in population (e.g. a higher proportion of elderly people), population size and distribution, and wider economic factors will all affect transport demand. Changes in the transport system (e.g. new roads, road enhancements) will affect supply. All of these factors have the potential to influence traffic volumes and the spatial distribution of high, medium and low traffic volumes (criticality) and thus risk. It has not been possible to assess the potential influence of transport system supply and demand on future risks though the transport infrastructure overview document provides a summary of trends for key contextual indicators.
Given the nature of the data used to compute these metrics it is not possible to compare either the likelihood or the potential consequences of flood events to other types of disruptive event (such as road traffic collisions or other extreme weather events). Indicator BT4 Flood events affecting the road network supports the comparison of the frequency of flood events on the trunk road network to other types of incident.
There are several key limitations to the assessment of this indicator as summarised below:
- Roads in the floodplain are often raised above the ground surface on embankments. The difference in elevation afforded by these embankments is not always identified in flood modelling and mapping (Thornes et al, 2012). As such it may be the case that flood risk to road infrastructure is over estimated – i.e. where the embankment would raise the road out of the inundated area and this is not reflected in the modelling due to the granularity of SEPA’s flood hazard modelling process.
- The projections of what might happen in the future in terms of risk of road closures due to flooding are based on English and Welsh data only from the UK CCRA (see BT2 assessment also). Whilst this provides a useful broad indication of possible future risks, quantitative assessment is very limited, and the ‘future risk assessment’ / trends assessment is based on climate projections data only, as per the Scotland CCRA. Furthermore, it has not been possible within the scope of this project to consider how future changes in transport system supply and demand may influence future risk of traffic disruption due to flooding – i.e. given that the spatial distribution of traffic volumes (road infrastructure criticality) may change as a result of changing patterns of supply and demand which, in turn, would influence the spatial distribution of risk.
- The social and economic effects of flooding are not fully reflected in this assessment which would require a more in-depth understanding of the duration of the event, the number vehicles affected by a flood event, the journey purpose of those affected and the availability and length of any diversion route amongst other factors.
- The SEPA Flood Risk Management Strategy Characterisation Data (SEPA, 2015) that underpins the assessment of this indicator includes some inaccuracies when classifying roads. As part of this assessment process a search term is used to extract Baseline Appraisal outputs that relate to trunk roads only (see Table 4). The Baseline Appraisal’s classification of trunk roads is not entirely in agreement with Transport Scotland’s official list of trunk roads (Transport Scotland, 2015) meaning that the assessment for trunk roads only is a slight over estimate.
- These metrics have been derived at the national level only. It has not been possible to identify any regional differences in road network exposure to flood risk beyond the simple visual assessment of areas at risk of ‘high level disruption’ from flooding. As such, future iterations of the indicators could be expanded to include regional assessments (e.g. Local Plan Districts, Potentially Vulnerable Areas, catchments) to explore the degree to which climatic differences across Scotland are reflected in SEPA’s flood hazard modelling.
Due to resource constraints, assessment was only undertaken for 0.5% probability (1:200 year) modelled flood events. These are low probability events and are located at the more severe end of the flood event spectrum. Higher probability events (e.g. 1:10, 1:50) can be expected to affect a smaller extent of the network than 1:200 year events but on a more frequent basis. It should also be noted that the flood hazard modelling is based on historic data. As a result of climate change, the magnitude of a 1:200 year event may be greater than represented in this analysis.
 Trunk roads are identified in the Baseline Appraisal CLASSIFA field as ‘Primary Road’ or ‘Motorway’ (Lauren Addis – SEPA Hydrologist, personal communication, February 17, 2015)
HR Wallingford, AMEC Environment and Infrastructure, The Met Office, Collingwood Environmental Planning, Alexander Ballard Ltd, Paul Watkiss Associates, and Metroeconomica (2012a). UK Climate Change Risk Assessment [online]. Available at: https://www.gov.uk/government/publications/uk-climate-change-risk-assessment-government-report [accessed 22/05/15]
HR Wallingford, AMEC Environment and Infrastructure, The Met Office, Collingwood Environmental Planning, Alexander Ballard Ltd, Paul Watkiss Associates, and Metroeconomica (2012b). A Climate Change Risk Assessment for Scotland [online]. Available at: https://www.gov.uk/government/publications/uk-climate-change-risk-assessment-government-report[accessed 22/05/15]
HR Wallingford (2014). Indicators to assess the exposure of critical infrastructure in England to current and projected climate hazards [online]. Available at: http://www.theccc.org.uk/wp-content/uploads/2014/07/5-MCR5195-RT003-R05-00.pdf [accessed 22/05/15]
SEPA (undated). Flood Risk Management (Scotland) Act 2009 – Appraisal Method for Flood Risk Management Strategies. [not available online]
SEPA (2015) Flood Risk Management Strategy Characterisation Data
Sniffer (2014). Scotland’s Climate Trends Handbook [online]. Available at: http://www.environment.scotland.gov.uk/climate_trends_handbook/index.html [accessed 21/05/15]
Thornes, J., Rennie, M., Marsden, H., and Chapman L (2012). Climate Change Risk Assessment for the Transport Sector [online]. Available at: https://www.gov.uk/government/publications/uk-climate-change-risk-assessment-government-report [accessed 22/05/15]
Transport Scotland (2015). List of roads maintained under Section 2 of the Roads (Scotland) Act 1984 [online]. Available at: http://www.transportscotland.gov.uk/system/files/documents/tsc-basic-pages/Official%20List%20of%20Roads%201%20April%202015.pdf [accessed 21/05/15]
Analysis and development of this indicator was undertaken by Dr Neil Ferguson (University of Strathclyde University) and Dr Peter Philips (Collingwood Environmental Planning Limited).
Katherine Beckmann, Heriot-Watt University / CXC contributed to this indicator.
SEPA provided the bulk of the data that underpinned the assessment of this indicator.