Quantification of risks to bridges from erosion and blockage: An elicitation of expert views

The aim of the project is to quantify uncertainties in the vulnerability of bridges and culverts to blockage or scour, in order to support better guidance and risk models for infrastructure managers and their partners, such as Network Rail and the Environment Agency. The approach will be a formal 'expert elicitation' to quantify the fragility of bridges and culverts at risk of scour or blockage.

Erosion and blockage are two significant hazards for major infrastructure networks where they cross rivers or smaller watercourses. In the UK there are estimated to have been 15 fatalities due to flood/scour failure of a structure since the 1840s (RSSB, 2004). In recent years notable incidents include the Glanrhyd railway bridge in Wales, which collapsed in 1989 due to scour of a pier, resulting in four fatalities when a train attempted to cross the collapsed bridge and fell into the river. The Lower Ashenbottom viaduct in Lancashire failed in June 2002 as its central pier collapsed, partially due to scour during a flood event but exacerbated by the presence of debris. In the 2009 Cumbria floods, seven road and foot bridges failed due to the combination of scour and hydrodynamic loading. The collapse of the Northside road bridge in Workington caused one fatality and massive disruption.

Whilst catastrophic bridge failures are rare, blockages of culverts and bridges, even over relatively small rivers, can cause flooding and erosion of safety-critical earthworks. Even minor incidents can lead to additional operational costs and risks for infrastructure operators, including those associated with debris clearance and emergency structural inspections. For the wider public, these incidents can cause disruption because of operational measures such as speed restrictions, delays, time-table changes or diversions. With nearly 10,000 bridges over watercourses on the rail network alone, the scale of the asset stock is significant. Despite industry efforts over the years, there remains much uncertainty about the individual resilience of these assets, and there is limited quantitative knowledge of this uncertainty.

The uncertain and disparate nature of information about scour and blockage probabilities indicates that a formal elicitation of expert judgements will be useful in seeking to draw out a synthesis of current knowledge. Inevitably, uncertainty has a major influence on a risk assessment and on any associated decisions in circumstances such as this; a structured procedure for eliciting expert judgements from a range of opinions is needed to obtain a rational consensus on the appropriate level of uncertainty quantification to use in the appraisal of contributory factors. Soliciting expert advice for decision support is not new. Generally, however, it has been pursued on an informal basis. However, an unstructured approach is rarely, if ever, entirely satisfactory to all parties. Neither is it likely to be immune to legitimate criticism or auditing from one side or another. To address these shortcomings, structured expert judgement makes it possible to tie the whole process to stated and transparent methodological rules, with the goal of treating expert judgements in the same way as 'normal' scientific data, in a formal decision process.
Various methods for assessing and combining expert uncertainty are described in the literature. Until recently, the most familiar approach has been one that advocates a group decision-conferencing framework for eliciting opinions, but other approaches now exist for carrying out this process more objectively. Prominent amongst these is the expert weighting procedure known as the Classical Model, formulated by Cooke (1991). Drawing on the knowledge and expertise of UK and international experts, this project will use Cooke's Classical Model to quantify uncertainties in the vulnerability of bridges and culverts to blockage or scour - to support better guidance and risk models for infrastructure managers.