A multi-hazard cascading risk model for coastal rail infrastructure: numerical modelling & engineering failure analysis

The February 2014 extratropical cyclonic storm chain, that impacted the English Channel (UK) and Dawlish in particular, caused significant damage to the main railway connecting the southwest region to the rest of the UK. The incident caused the line to be closed for two months, £50 million of damage and an estimated £1.2bn of economic loss. This incident highlighted the urgent need to understand the cascading nature of multi-hazards involved in storm damage and their impacts on coastal railway infrastructure. This study focuses on the Dawlish railway where a seawall breach caused two months of railway closure in 2014. I used historical and contemporary data of severe weather damage and failure analysis to develop a multi-hazard risk model for the railway. Twenty-nine damage events caused significant line closure in the period 1846-2014. For each event, hazards were identified, the sequence of failures were deconstructed and a flowchart for each event was formulated showing the interrelationship of multiple hazards and their potential to cascade. The most frequent damage mechanisms were identified: (I) landslide; (II) direct ballast washout and (III) masonry damage. I developed a risk model for the railway which has five layers in the top-down order of: (a) Trigger (storm); (b) force generation; (c) common cause failure; (d) cascading failure and (e) network failure forcing service suspension. Armed with the multi-hazard cascading risk model, I go on to collate eyewitness accounts, analyse sea level data, and conduct numerical modelling in order to decipher the destructive forces of the storm. My analysis reveals that the disaster management of the event was successful and efficient with immediate actions taken to save lives and property before and during the storm. Wave buoy analysis showed that a complex triple peak sea state with periods at 4-8 s, 8-12 s, and 20-25 s was present, while tide gauge records indicated that significant surge of up to 0.8 m and wave components of up to 1.5 m amplitude combined as likely contributing factors in the event. Significant impulsive wave forces were most likely the initiating cause of the damage. Reflections off the vertical wall caused constructive interference of the wave amplitudes that led to increased wave height and significant overtopping, our numerical simulations suggesting up to 16.1 m3/s/m (per meter width of wall). With this information and using engineering judgment I conclude that the most probable sequence of multi-hazard cascading failure during this incident was: wave impact force leading to masonry failure, loss of infill, and failure of the structure following successive tides. The multi-hazard cascading risk model developed in this research is applicable for other infrastructure under a variety of natural hazards. Examples are presented in this research. Given the current global climate emergency and sea level rise, it is expected that the results of this work will provide an important contribution to infrastructure resilience to natural hazards.