A reliability rating system for flood embankments in the United Kingdom
PublisherUniversity of Strathclyde
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The probability of structural failure of flood defences is an important component in the modelling of flood risk. Flood embankments are a very common type of defence whose reliability assessment shows some problematic aspects. In particular it is difficulty to model the process of piping through the earthfill and, when assessing the safety of an extended flood defence network, a diffuse lack of knowledge (epistemic uncertainty) about some of the characteristics of the structures has to be addressed. This thesis presents a new methodology, named the Reliability Rating System, which makes possible the rapid quantification of the expected performance of flood embankments in extreme hydraulic conditions. The proposed methodology ranks and compare flood embankments according to their proneness to breaching using a performance indicator, called the Probability of Breaching in Extreme hydraulic conditions PBE. This performance indicator is related to probability of structural failure for a limited range of water levels above and immediately below the crest. The Reliability Rating System takes into account three failure modes: breaching initiated by grass cover failure in overtopping conditions, breaching induced by piping through the earthfill (through-piping) and breaching due to piping in the founding soil (under-piping). Credible mathematical models are available for the grass cover failure and under-piping and the probability of breaching induced by these failure modes can be calculated with the methods of reliability analysis. By contrast no credible process based mathematical model is currently available for through-piping. In this work the probabilities of breaching by through-piping have been estimated with a rigorous process of subjective judgement elicitation. In the case of uncertainty on some of the relevant characteristics the final users can rapidly study different possible scenarios thanks to the tabulated solutions presented here. Upper and lower bounds on the performance indicator PBE can be readily determined in order to handle the epistemic uncertainty typical of flood defences safety assessments.