BT26 Road and rail bridges vulnerable to scour

Transport is a means to an end supporting many different social and economic functions. Scouring of road and rail bridges can cause disruption to road and rail transport (e.g. as a result of speed restrictions, closures etc.) with knock-on consequences for these functions – e.g. delaying deliveries, preventing or delaying people from accessing employment, disrupting vital healthcare services, delaying rail freight etc. Scour risk to road and rail bridges is driven by the force exerted on foundations by river flood flows and also the river bed and sediment properties specific to the site. Therefore although scour risk is inherently site specific, the projected increases in peak flood flows as a result of climate change are expected to increase scour risk to bridge assets.

Related indicators:

BT4 Flood events affecting the trunk road network

BT12 Flood events affecting the railway network

Tables 1 and 2 and Figures 1 and 2 present current data on scour susceptibility of rail and trunk road bridges[1].  This includes data for the two metrics assessed which focus on the proportion of road and rail bridges that are high priority or highly susceptible to scour.  See Appendix 1 Table 4 Indicator Methodology for further information on scour classification of road and rail bridges.

Table 1 Scour assessment of rail bridges  (April 2015)

Table 2 Scour assessment of trunk road bridges (April 2015)

Figure 1. Scour susceptibility of Scotland Route (rail) underline bridge assets (April 2015)

Note: The Network Rail assessment includes five culvert assets though the vast majority of the assessment is on underline bridge assets (975 assets).

In the respective scour assessments of their bridge assets, Transport Scotland assessed 1,279 assets (trunk road bridges) and Network Rail 980 assets. In both assessments, only a small proportion of all assets were assessed as high priority or highly susceptible to scour: 3.52% in the case of trunk road bridges and 1.33% for rail.

In the case of road bridges, a large majority of assets (89.21%) were assessed as low priority or low susceptibility to scour (scour priority score 5) requiring no action to be taken (see Table 4).

For rail bridges, the distribution of scour priority scores is more even – only 41.73% of assets were assessed as low priority or low susceptibility (scour priority score 6) and 56.93% of assets fell within the middle scour priority or susceptibility categories (scour priority score of 2, 3, 4 or 5).

Figure 2. Scour susceptibility of trunk road bridge assets (April 2015)

Historic bridge scour assessment data is not available.  However, historic climate data shows how key aspects of climate (rainfall) have 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 relevant climate risks (i.e. scouring of road and rail bridges caused by increased magnitude and frequency of flood flows). 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 magnitude and frequency of flood flows in Scotland’s rivers. An additional issue with scour risk is the nature of bridge design – advances in structural design and understanding of scour mean that major modern bridges are rarely susceptible to scour (Thornes et al, 2012), especially where susceptibility is a function of foundation design (see Table 4). Indeed it is suggested that pre-20^thCentury bridges are most susceptible to scour as they generally have shallower foundations and are of variable and often unknown construction (ibid). As a result, the scale and magnitude of scour risk across the portfolio of road and rail bridge assets in Scotland will have changed with time and most modern bridges will exhibit very low scour susceptibility.  

The UK Climate Change Risk Assessment (HR Wallingford et al., 2012; Thornes et al., 2012) assessed changes in scour risk to road and rail bridges as a result of anticipated climate change. The contributory natural hazard in scour risk is river flows. Natural site conditions (river bed / sediments) play a part as well but these are not influenced by climate change. The CCRA estimated percentage increases in scour from present day conditions as a result of increased flow (flow increases of 10%, 20% and 30%). For all types of river bed (sediment) conditions (see Table 4), increases in river flow are expected to increase scour. For sand beds this may be anywhere between 2% and 9% and for gravel beds, anywhere between 2% and 65% (ibid). Furthermore, the CCRA and other research have assessed flow increases as a result of climate change in Scotland. For a medium emissions scenario, flow increases of 5-10% are expected by the 2020s, 15-20% by the 2050s and 20-30% by the 2080s (Kay et al., 2011).  Given this, it is anticipated that climate change will contribute to increased scour risk in the future, particularly for older bridge assets that have not been designed to withstand scouring. The CCRA did not attempt to quantify the overall increase in scour risk to all bridge assets due to the site specific nature of the risk (susceptibility) and an absence of data on bridge assets and river bed conditions.  

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

1. Within the scope of the project (time and resource) it was not possible to gain full details from Transport Scotland and Network Rail on the methodology used in scour assessments. In particular, it is unclear if or how information on river flow has been incorporated into the assessment – e.g. does the assessment just consider the susceptibility of the bridge assets or does it consider flow conditions at the site as well? Does the assessment consider scour susceptibility under a variety of flow conditions (return periods) and has the impact of climate change on river flow regimes been accounted for?
2. The compatibility of the Transport Scotland and Network Rail bridge asset scour assessments is uncertain. For example the exact methodology adopted in the assessments is unknown and the assessments use a different scoring system (the road bridge assessment has a five point system whereas the rail assessment uses six points). Given this, comparing like for like as per the two BT26 metrics may not be appropriate. 

Highways Agency, Transport Scotland, Welsh Government, and The Department for Regional Development Northern Ireland (2012). Design Manual for Roads and Bridges: Volume 1 Highway Structures – Approval Procedures and General Design [online]. Available at: http://www.standardsforhighways.co.uk/dmrb/vol1/section1/bd212.pdf [accessed 16/07/15]

HR Wallingford, AMEC Environment and Infrastructure, The Met Office, Collingwood Environmental Planning, Alexander Ballard Ltd, Paul Watkiss Associates, & Metroeconomica (2012). 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]

Kay, A.L., Crooks, S.M., Davies, H.N., and Reynard, N.S. (2011). An assessment of the vulnerability of Scotland’s river catchments and coasts to the impacts of climate change. Work Package 1 Report (Draft). Report produced for SEPA, April 2011

McLuskey, K. (2015). Personal communication with Keira McLuskey, Network Rail Environment Manager, April 20, 2015.

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]

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

Network Rail provided data on Scotland Route bridge asset scour assessments.

Transport Scotland provided data on trunk road bridge asset scour assessments.

Katherine Beckmann, Heriot-Watt University / CXC contributed to this indicator.