Risk/opportunity:(from the Climate Change Risk Assessment for Scotland 2012):
FL8a Roads at significant risk of flooding TR1 Risk of traffic disruption
Narratives: Flooding and infrastructure
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
The proportion of road length directly at risk of flooding (for a 1:200 year event):
- All roads: 0.81% (fluvial), 1.23% (pluvial) and 0.34% (coastal)
- Trunk roads: 5.24% (fluvial), 2.13% (pluvial) and 0.73% (coastal)
The proportion of the road network that would be affected by this flooding (i.e. each section of road for which there is flood risk in at least one place along its length):
- All roads: 7.87% (fluvial) and 13.41% (pluvial)
- Trunk roads: 24.89% (fluvial) and 51.89% (pluvial)
Trunk roads in very remote areas: 42.3% forecast to be affected by pluvial flooding
 The assessment of this indicator has been undertaken for 0.5% probability (1:200 year) flood events only
 The ‘road network affected by flooding’ covers each section of road that is at risk of flooding at one or more sites. A section of road is defined as the length of road between two adjacent and connected junctions.
- Climate change projections suggest that flooding of road infrastructure in Scotland will become more extensive and more frequent.
- The trunk road network is more exposed to flood risk than the road network as a whole.
- The greatest risk to the road network is posed by pluvial source flooding
- Pluvial flooding poses the greatest flood risk to the road network in very remote rural areas
Flooding of the road network can result in physical damage to the road infrastructure and can present an accident risk to road users. Flooding can also disrupt the operation of the road network with knock-on consequences for many social and economic functions – such as delaying deliveries, preventing or delaying people from reaching work or disrupting healthcare services. Communities in remote areas are particularly vulnerable to road network disruption as they may rely more heavily on road transport than those living and working in other parts of Scotland and alternative routes (where these exist) often require long diversions.
There is 55,961 km of public road in Scotland, of which 6% is classified as the trunk road network (Transport Scotland, 2014). In terms of vehicular traffic, there were 43.84 billion vehicle kilometres driven in Scotland in 2013, the majority of which was by car (77.1%). Goods vehicles and buses constituted 20.1% and 1.4% of this total respectively (ibid).
64% of all personal journeys in Scotland in 2013 were made by car, whereas only 9% of journeys were made by bus. Car was the main mode of travelling to work for the majority of the population; 82% of households in rural areas travelled to work by car. In comparison, 57% of urban households travelled to work by car (ibid).
Freight transport in Scotland relies heavily on the road network. In 2010 (latest figure), 26,170 million tonne kilometres were moved by road. In comparison, 2,572 million tonne kilometres were moved by rail in 2010/11 (ibid).
This indicator uses modelled data to assess the risk to road networks as a result of flooding. Metrics include those that assess the length of road directly exposed to flooding (reflecting infrastructure damage and accident risk) and the length of road network that would be affected by such flooding (reflecting consequential operational costs, such as congestion and additional journey time).
Three separate factors were considered in the development of these metrics:
- The source of flooding – fluvial, pluvial and coastal
- The social and economic value of the road network (with trunk roads used as a proxy for value)
- The vulnerability of communities relying on the network (with rurality as a proxy for vulnerability)
Separate figures are given for a) the road length directly at risk of flooding and b) the road network that would be affected by such flooding. The latter helps to account for the networked nature of road infrastructure in a more meaningful way. A 0.15m flood depth threshold is used throughout as a measure of the depth at which driving may become unsafe.
Current figures are given in Table 1, and Figure 1 shows the road network in areas at risk of flooding, both for the road network as a whole (dark blue) and for the trunk road network only (light blue).
Table 1: Proportion of road network at risk of flooding (1:200 year event)
BT2a Proportion of road length directly at risk of flooding (all roads)
BT2b Proportion of road length directly at risk of flooding (trunk)
BT2c Proportion of network that would be affected by this flooding (all roads)
BT2d Proportion of network that would be affected by this flooding (trunk roads)
BT2e Proportion of road length directly at risk of flooding (all roads)
BT2f Proportion of road length directly at risk of flooding (trunk roads)
BT2g Proportion of network that would be affected by this flooding (all roads)
BT2h Proportion of network that would be affected by this flooding (trunk roads)
BT2i Proportion of road length directly at risk of flooding (all roads)
BT2j Proportion of road length directly at risk of flooding (trunk)
Figure 1 Proportion of road network located in areas at risk of flooding
Notes: Flood risk assessed for a 0.5% probability (1:200 year) flood event. Dark blue columns represent the road network as a whole; light blue columns represent the trunk road network only. The figure shows the both the directly at risk sections, and the sections that would be affected / impacted by this flooding
Figures 2 and 3 show the road network at risk of flooding for two example areas: the Central Belt and Lochaber and Mull. The figures show how flood risks to road infrastructure are spread across the areas depicted, reflecting the spatial distribution of watercourses and their catchments (fluvial flood risk), roads and other less permeable features (pluvial flood risk) and low lying coastal areas (coastal flood risk).
Figure 2 Distribution of fluvial, pluvial and coastal flood risk to the whole road network: Central Belt
Note: Trunk roads are shown as red lines. The map shows sections of the network at flood risk where flood depth is modelled to be 0.15m or greater.
Figure 3 Distribution of fluvial, pluvial and coastal flood risk to the road network: Lochaber and Mull
Note: Trunk roads are shown as red lines. The map shows sections of the network at risk of flooding where flood depth is modelled to be 0.15m or greater.
Risk to the road network in very remote rural areas
Road closures due to flooding in very remote rural areas warrant separate consideration. This is because communities in rural locations may be more vulnerable to road closures as rural roads are generally less networked – i.e. there are fewer alternative routes. In line with SEPA (undated), the use of very remote rural areas provides a proxy assessment of vulnerability, whereby vulnerability is construed as the rurality of the road’s location. (Note that the definition of ‘very remote rural areas’ is taken from the Scottish Government 8-fold urban rural classification (Scottish Government, 2014)). The road network in very remote rural areas is shown in Figure 4.
Figure 4 The road network in very remote rural parts of Scotland
Figure 5 shows the extent of flood risk to the road network in very remote rural parts of Scotland, while Figure 6 shows its spatial distribution. The network in these parts of Scotland has similar levels of exposure to fluvial and pluvial flood risk. It is notable that the greatest risk is posed by pluvial source flooding which has the potential to impact 42.4% of the trunk road network.
Figure 5 Road network within very remote rural Scotland located in areas at risk of flooding
Note: Dark blue columns represent the road network as a whole; light blue columns represent the trunk road network only. The figure shows the both the proportion of directly affected sections, and the proportion of the network that would be directly affected (impacted) by this flooding
Figure 6 Road network at risk of flooding in very remote rural parts of Scotland – spatial distribution of flood risk to the road network as a whole
Note: Trunk roads are shown as red lines. The map shows the road network at risk of flooding where flood depth is modelled to be 0.15m or greater.
 The coastal source assessment in SEPA’s Baseline Appraisal only provides data on the road network at direct risk from flooding (where flood depth would be greater than 0.15m). Thus there is no ‘affected / impacted section’ data available for coastal source flooding: Lauren Addis (SEPA Hydrologist), pers. Comm., Jan 26, ‘15.
 The use of very remote rural roads as a proxy for vulnerability is discussed in the Transport Overview report
Historic flood hazard and road network data is not available to assess past trends. However, historic climate data can help show how key aspects of climate (rainfall) have changed and affected biophysical systems (the hydrological response of catchments and watercourses) and, ultimately, the scale and magnitude of relevant climate risks (i.e. risk of road closure as a result of flooding).
Figure 7 shows the total precipitation for Scotland and Scottish regions between 1914 and 2011, with smoothed curves to show running averages. In each of the Scottish regions and for Scotland as a whole, the data shows a clear upward trend in winter precipitation from 1961 (Sniffer, 2014). Higher winter rainfall levels will have various impacts on biophysical systems (e.g. greater runoff volumes), which is likely to contribute to increased fluvial and pluvial source flooding.
Figure 8 shows days of heavy rain for Scotland and Scottish regions between 1961 and 2011 with smoothed curves to show running averages. There is a trend of increasing heavy rainfall in winter, particularly in north and west Scotland (Sniffer, 2014). Heavy rainfall may be a particular issue for pluvial flooding as roads are only designed to accommodate relatively short return period (5 year) events with no surface water flooding of the carriageway (Highways Agency et al, 2004). An increased frequency of heavy rainfall days may therefore be contributing to increased frequency and magnitude of pluvial flooding events affecting road infrastructure (i.e. as heavy rainfall becomes more frequent with climate change and overruns the limited design capacity of roads drainage).
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) have been used in conjunction with general aspects from the Scotland CCRA (HR Wallingford et al, 2012b) to understand how the risk of road closures from flooding might change in the future.
The Scotland CCRA (ibid) considered climate projections and impacts on biophysical systems to undertake a climate change risk assessment for Scotland. Climate change in Scotland is expected to result in increased winter rainfall and potentially increased incidence of intense summer rainfall events. In terms of impacts on biophysical systems, these projected climate changes are expected to lead to greater volumes of runoff, higher river flows and increased catchment wetness / waterlogging problems, particularly during winter months. These factors are expected to cause increased flooding from both rivers and surface water (note that that this could also be caused by an increase in intense summer rainfall events). Clearly the scale and magnitude of current and potential future flood risk depends on the configuration of the road network relative to floodplains and low lying coastal areas as well as the flood alleviation / road design measures in place, all of which will vary between the devolved countries, meaning that the data for England and Wales from the UK CCRA below can only be used as a broad indication of possible changes.
The CCRA for England and Wales indicated that the projected length of road at significant likelihood of flooding (where significant is defined as a 1.3% annual probability) would be between 13,000km and 16,000km by 2020 compared with a baseline of about 12,000km (Thornes et al, 2012). This equates to increases of between 8.3% and 33.3% from baseline with the range reflecting the different climate change scenarios considered in the assessment. The CCRA also highlighted how in addition to an increase in the overall length of infrastructure that could be affected, the frequency of flooding of infrastructure that is already located in the floodplain is expected to increase also (ibid). These projections do not account for any actions that could be taken to alleviate flood risk to roads (e.g. flood defence infrastructure, enhanced maintenance regimes) which may help to reduce overall flood risk (noting that these types of actions are picked-up in related indicator BT6). In summary the following projected changes are anticipated to take place in Scotland 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 (surface water) flooding
There are several key limitations to the assessment 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. This is not reflected in the modelling due to the granularity of SEPA’s flood hazard modelling process.
- Future consideration of road closures are based on English and Welsh data from the UK CCRA, which only provides a broad indication of possible future risks, and so should be interpreted with caution when applied to Scotland.
- This assessment is based on an early version of data that was generated by SEPA’s Baseline Appraisal flood hazard modelling Appraisal (SEPA, 2015b). The approach to and assumptions underpinning this model may be subject to refinement and changes in the future.
- The SEPA Baseline Appraisal (ibid) data that underpins the assessment includes some inaccuracies when classifying roads. As part of the proxy assessment of the value element of risk, a search term is used in all trunk road specific metrics 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.
- SEPA’s flood hazard modelling and the associated Baseline Appraisal data (SEPA, 2015) does not capture flood duration – i.e. in effect it presents a ‘snapshot’ of flood events based on flood extent, depth and velocity criteria. Whilst road infrastructure is susceptible to pluvial (surface water) flooding in that it is only designed to accommodate low return period / high probability flood events (5 year events) without flooding of the carriageway, drainage design for roads is such that surface water flooding should dissipate quickly, meaning therefore that flood duration should be relatively small (Highways Agency et al, 2004). This characteristic of flooding is not captured in SEPA’s flood hazard modelling and data and, as such, the assessment of pluvial metrics may paint a ‘worse picture’ than is actually the case.
- 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 (see Point 4 above), the number vehicles affected by a flood event, the journey purpose of those affected, the availability and the length of any diversion route amongst other factors.
- This assessment was only undertaken for 0.5% probability (1:200 year) modelled flood events. These are medium probability events. 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 will almost certainly 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)
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ClimateXChange (2016) Adaptation to Climate Change: Context and Overview for Transport Infrastructure Indicators. Available online at our Indicators and trends pages
Analysis and development of this indicator was undertaken by Dr Neil Ferguson (University of Strathclyde) and Peter Philips (Collingwood Environmental Planning Limited), utilising spatial data supplied by SEPA.
Katherine Beckmann, Heriot-Watt University / CXC contributed to this indicator.