Debris Effects on BRIdge resiliencE and Flooding

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Engineers, scientists and the lay public alike have long been troubled by the complex and sometimes catastrophic interplay between rivers and bridges. River bridges have their foundations built on river beds, which can undergo significant scour during flood events. The term scour denotes the erosion of the river bed soil that occurs as a result of fast and turbulent water flowing close to the bridge pier. When the descent of the bed caused by scour is too deep, the foundations may be undermined or simply incapable of sustaining the bridge, in which case the whole structure can be severely damaged or even collapse. Most bridge collapses that occur throughout the world are caused by this phenomenon. A further complication that occurs in rivers that transport large amounts of floating material (tree trunks, branches or human debris, often called 'debris') is the accumulation of this material at bridge piers forming a jam. These debris jams alter the flow by directing the velocity towards the bed, which exacerbates scour and increases the risk of bridge collapse. Debris jams also produce large forces on the pier, further contributing to the prospects of bridge failure. A wide body of research now acknowledges that debris ranks high among the causes of bridge failures around the world, being responsible for approximately 30% of bridge failures in the UK and the US. In addition to the threats posed to bridge safety, the blockage of bridges by debris leads to increased upstream water levels that can result in flooding of adjacent areas. As many bridges are located in urban areas-where costly assets are located and more people are exposed- debris jams can considerably increase the risk of flooding. The above problems are expected to become more frequent in the future as a result of i) increased river flows due to climate change, ii) increased number of bridges and population exposed to flooding that will follow from socio-economic growth. Recent events highlight the urgency for improved risk assessment and mitigation methods to build up the resilience of these key pieces of infrastructure. For example, during the 2009 and 2015 Cumbrian floods respectively 244 and 80 bridges were either damaged or lost.The project partners, Network Rail and the Environment Agency, are directly affected by the issues above. Network Rail owns more than 8,000 bridges near or close to waterways. The Environment Agency owns approximately 2,000 bridges over water courses. In addition, the Environment Agency is responsible for managing the risk of flooding from main rivers, estuaries and the sea. Until recently, the tools that were available to the project partners (as well as other stakeholders directly affected by these issues) to assess debris-induced risk were not based on strong science and were considerably limited. A recent assessment of these methods has demonstrated serious inconsistencies and inaccuracies, therefore rendering them inadequate for the management of risk of important infrastructure systems. Recent research conducted at University of Southampton has delivered a step-change in our understanding of how debris jams at bridge piers grow. These new results and predictive capabilities now pave the way for significant improvements in the way that the project partners assess risk, and therefore to a more effective use of resources to mitigate the risk of failure of critical infrastructures.The overarching aim of the project is to bridge the gap between this recent research and the practical needs of the project partners by translating the previous research into a robust, accurate and user-oriented methodology to assess the effects of debris jams on bridge resilience and flooding. The outcomes of the project will enable the project partners to take more informed decisions on how to prioritise investments to build up the resilience of critical infrastructure.