Risk Assessment of Masonry Bridges Under Flood Conditions: Hydrodynamic Effects of Debris Blockage and Scour

Recent flooding events such as those of winter 2013/14 in the South West of UK have highlighted the importance of having greater resilience in our transport infrastructure. The failure of bridges or even a reduction in service during and in the aftermath of floods can lead to significant direct and indirect costs to the economy and society, and hamper rescue and recovery efforts. For example, 29 bridges collapsed or were severely damaged during the 2009 floods in Cumbria leading to nearly £34m in repair and replacement costs, and significantly larger economic and societal costs.

This research aims to enhance the resilience of our transport infrastructure by enabling practitioners to assess the risks to bridges from debris accumulation in the watercourse, a leading cause of bridge failure or damage during floods both in the UK and world-wide. It will address an important industry need as there is currently no guidance available for practitioners to evaluate the hydrodynamic effects of debris blockage at bridges and in particular, at masonry bridges, which are most susceptible to debris blockage. Floating debris underneath or upstream of a bridge can significantly increase downstream flow velocities, which can worsen scour around piers and abutments. It can also increase water levels on the bridge and thereby cause large lateral and uplift pressures, which are especially problematic for masonry bridges since they rely on self-weight of masonry and fill to transfer load.

This project will aim to understand and characterize the hydrodynamic effects of debris blockage through a combination of laboratory experiments in flumes and computational fluid dynamics (CFD) modelling. It will then develop a risk-based approach for assessing the scour, and uplift and lateral forces at individual bridges due to debris blockage during flood conditions, and incorporate this approach within existing guidance for the assessment of bridges under hydraulic action. The project will be carried out by a multi-disciplinary research team with a strong track record of generating impact, and assisted by an industry consortium composed of major stakeholders involved in UK bridge management.

Scour and hydrodynamic forces arising from floating debris are noted as major causes for bridge damage and failure during floods. Research commissioned by the Rail Safety and Standards Board highlighted debris as the main factor in 20 out of 69 water-related failures of railway bridges. In the USA, drift or floating debris is cited as the primary cause for over one-third of bridge failures. The costs of bridge failure or a reduction in service can be significant, particularly when including the indirect economic and societal costs, which are often an order of magnitude greater than the direct costs of structural repair and replacement. For example, in the aftermath of the Cumbrian floods of 2009, disruptions in the transport network led to a one-mile trip becoming a 21-mile round trip in some cases. The increased travel time, in addition to causing significant distress to the general public, was estimated to cost businesses as much as £2m per week. There is therefore an urgent need to develop methods for assessing and protecting structures against the risk of debris-induced hydrodynamic effects, especially with more frequent and severe floods predicted due to climate change. This research will fulfil this practical need by addressing existing knowledge gaps that prevent characterizing the hydrodynamic effects of debris blockage at bridges, and subsequently, implementing the knowledge into a supplementary technical note for CIRIA's manual on scour at hydraulic structures (C551). C551 is the leading industry guidance in the UK for assessing and protecting structures under hydraulic action, and is currently undergoing a revision that will be completed in the last quarter of 2014.

Findings from this research will contribute immensely towards creating a climate resilient transport infrastructure in the UK. Implementation in C551 will improve management of bridges within the nation's transport networks by enabling engineers to identify structures vulnerable to the effects of debris blockage and hence to efficiently target scour mitigation and protection measures. This will drastically reduce downtime of bridges during and in the aftermath of floods, and therefore bring major savings to the UK economy and significantly enhance the quality of life. A better managed transport infrastructure will also help greatly with rescue and rehabilitation efforts and assist in faster post-flood recovery.

The research team will be guided by an industry steering committee composed of members representing the major stakeholders in the management of UK bridge infrastructure. The steering committee will meet every 6 months at Exeter starting with an initial kick-off meeting at the commencement of the project. The committee includes major bridge owners and operators, consultants, stakeholder bodies and authorities involved in publishing policy and guidance. This proposal recognizes the ongoing CIRIA project on revision of C551 and the importance of incorporating the developed methodology into the revised C551. The industry steering committee therefore includes the CIRIA project manager responsible for C551 revision as well as representatives from companies leading the revision project and those sitting on its project steering group. Towards the end of the project, the research team will also organize a workshop to present the developed methodology and a number of illustrated case studies to major UK industry stakeholders and researchers with expertise and interests at the interface of flooding and bridge management.

Minimizing flood-related damage to infrastructure is a major challenge not only in the UK but also around the world. The team will utilize its strong links with leading research groups across the world in flood risk modelling, CFD and bridge management through the Centre for Water Systems (CWS) and the Structures and Dynamics Group (SDG) at UoE, and HWU's Institute for Infrastructure and Environment (IIE) to disseminate findings.