BT9 Disruption risk to railway services as a result of flooding

Transport supports many social and economic functions. In 2013/14, 86.3 million passenger journeys were made by rail in Scotland which represents 16% of all public transport journeys.  Rail patronage has increased by 35% since 2004/05 and constitutes a growing share of public transport journeys.  In contrast, there has been a decline of 8.2% in the amount of freight (tonnes) lifted by rail between 2002/03 and 2012/13.  The modal share of freight (in tonne-km) carried by rail in Scotland in 2010 was 7% (Transport Scotland, 2014).

Flooding of rail infrastructure can cause disruption to rail transport with knock-on consequences for these functions – e.g. preventing or delaying people from accessing employment, delaying rail freight etc. Climate change predictions suggest that flooding of rail infrastructure will become more extensive and more frequent. This indicator will provide insight into any changes in such disruption.   

Note that BT9 metrics measure the length of rail at direct flood risk (for a 1:200 year flood), for each of high, medium and low rail traffic volumes.  It was not possible during the course of this project to obtain data that would show the length of rail routes that would be affected by one or more directly flooded sites.  It may be possible to address this in the future if additional resources are made available.  For example, if the Edinburgh – Glasgow line is flooded then a useful representation of the issue could include one or both of the following:

• total Length of the ‘Major Route’ affected – in this example approx. 40 miles, and
• the distance between the two stations separated by the flooding site (ie the length over which a replacement bus service would have to run).

Related indicators:

BT8 Railway network at risk of flooding

BT12 Flood events affecting the rail network

BT16 Rail network benefitting from flood protection measures

Table 1 shows current figures for expected levels of service disruption from flooding.  These are based on the amount of rail traffic carried through a section at flood risk (of a 1:200 year flood event).  Thus severe disruption corresponds to a rail section with a high level of rail traffic that is affected by one or more flooded sites, moderate disruption to a flood site on a line with medium traffic load, and so on. The figures represent the proportion (by length) of the rail network that would experience disruption should flooding take place.

Table 1  Proportion of rail network affected by flooding   (1:200 year event)

For all three sources of flooding assessed, the potential for disruption to railway services appears to be low as the majority of the rail network exposed to flood hazard serves low volumes of rail traffic. This also means that overall flood risk to the network is lower as well (where flood risk is assessed as a function of the likelihood and consequences of an event).

Figure 1 illustrates the rail network at risk of fluvial flooding.  It shows that for fluvial source flooding only 0.02% of the exposed rail network serves high rail traffic volumes compared to 0.38% and 2.22% for medium and low traffic volumes respectively. In essence, whilst 2.62% of the rail network is exposed to a fluvial flood hazard (see BT8 assessment), only 0.02% of the network can be considered at high risk of flooding overall when the rail traffic disruption related consequences of flooding are taken into account. Similarly, the proportion of the rail network at high risk of flooding overall from pluvial and coastal sources is 0.74% and 0.00% respectively.

The location of flood risks to the rail network for severe and moderate levels of flooding are shown on Figure 2.

Sections of the rail network exposed to flood hazard carrying high rail traffic volumes are termed ‘high criticality lines’. These represent the potential for severe levels of rail traffic disruption as well as higher flood risk overall. Medium criticality lines represent moderate disruption levels.

Figure 2 Sections of the rail network exposed to severe and moderate levels of rail traffic disruption due to fluvial (LH map) and pluvial (RH map) flood hazards (1:200 year event)

Sections of the rail network that carry high and medium rail traffic volumes are focussed around the central belt of Scotland (e.g. the Glasgow to Edinburgh via Falkirk line; the various Glasgow and Edinburgh commuter lines), reflecting the densely populated nature of the region and the associated high demand for transport. The East and West Coast Mainlines also carry medium levels of rail traffic and are therefore exposed to moderate levels of disruption from flooding (though this assessment doesn’t account for the strategic importance of these lines or the number of passengers using these services which may be higher due to the use of higher capacity trains). This is indicated in Figure 2. Areas of pluvial and fluvial flood risk are spread around the region reflecting the spatial distribution of watercourses and their catchments (fluvial flood risk) and impermeable features or sites with poor surface water drainage provision (pluvial flood risk).

Historic flood hazard and rail traffic volume data are not available. However, historic climate data shows how rainfall has changed leading to impacts on biophysical systems (e.g. hydrological response of Scotland’s catchments and watercourses) and ultimately changes to the scale and magnitude of the  risk of rail network flooding. 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[1].  It is not possible to show how the risk of rail traffic disruption due to flooding has changed without access to historic data on disruption of railway services. Rail service patterns will be driven in part by demand which is subject to change over time due to various drivers (e.g. population distribution, other demographic factors, distribution and type of economic activity, and so on).

The UK Climate Change Risk Assessment (HR Wallingford et al, 2012a; Thornes et al, 2012) assessed changes in flood risk to rail 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. As such, transport specific aspects from the UK CCRA can be used in conjunction with general aspects from the Scotland CCRA (HR Wallingford et al, 2012b) to understand how flooding related impacts to rail infrastructure might change in the future.[1]

In terms of potential future flooding related climate risks to rail infrastructure, the results of the CCRA for England and Wales indicated that the projected length of railway at significant likelihood of flooding (where significant is defined as a 1.3% annual probability) would be between 2,000km and 2,600km by 2020 compared with a baseline of about 2,000km (Thornes et al, 2012). This equates to a possible increase of between 0% and 30% with the range reflecting the different climate change (emissions) 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 (ibid). These projections do not account for any actions that could be taken to alleviate flood risk to railways (e.g. flood defence infrastructure, enhanced maintenance regimes, etc) which may help to reduce overall flood risk (see indicator BT16). Whilst these projections are focussed on England and Wales (and therefore the specific regional climate projections and rail network issues and configuration therein), they provide a useful proxy of what may happen in Scotland.

 In summary the following projected changes are anticipated in future:

• The proportion of the rail network located in areas at risk of flooding is projected to increase
• Rail 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

The assessment above does not account for possible future changes in rail service patterns (e.g. frequency, number, location) which would affect the degree to which the rail network is exposed to flooding related disruption. It has not been possible to collate data on current and future rail service plans so possible future flooding related climate risks to the rail network, as a function of changing patterns of service delivery, have not been assessed.

There are several key limitations to the assessment as summarised below:

1. Major railways located 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). Therefore,  flood risk to rail infrastructure may be over-estimated – i.e. where the embankment would raise the railway out of the inundated area and this is not reflected in the modelling due to the granularity of SEPA’s flood hazard modelling process.
2. The projections of what might happen in future in terms of flooding related climate risks to the rail network are based on English and Welsh data only from the UK CCRA. This provides a broad indication of possible future risks only.  Furthermore, the assessment of future climate risks has not considered any data on rail service plans / possible future changes to service delivery.  
3. Network Rail’s Network Links Layer which was used in this analysis includes double sections of track at many locations as well as railway siding etc.  Thus the total length of track used in this analysis does not correspond with the length of track published by Transport Scotland (2014) which counts one kilometre of single or double-track as one kilometre of route length.
4. Rail outputs from SEPA’s Flood Risk Management Strategy Characterisation Data are based on a GIS intersect operation of flood extent polygons and rail network polylines. Within this, flooding of rail infrastructure is considered to occur regardless of flood depth or velocity. 
5. 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.

HR Wallingford, AMEC Environment and Infrastructure, The Met Office, Collingwood Environmental Planning, Alexander Ballard Ltd, Paul Watkiss Associates, & 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, & 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., & 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 (2014) Scottish Transport Statistics, No. 33, 2014 Edition.  Available at http://www.transportscotland.gov.uk/statistics/j357783-00.htm [accessed 11/08/2015]

Context and Overview for Transport Infrastructure Indicators (ClimateXChange Report)

SEPA provided spatial data on rail network flood risk assessment from an early version of the Flood Risk Management Strategy Characterisation Data (SEPA, 2015)

Network Rail provided spatial data for the Scotland Route.

The analysis and development of this indicator was undertaken by Dr Neil Ferguson (University of Strathclyde) and Dr Peter Phillips (Collingwood Environmental Planning Limited)