Time-dependent reliability analysis of flood defences using gamma processes

Inclusion of time-dependent effects in flood risk analysis is useful to inform inspection and maintenance strategies. Several deterioration processes are incorporated in a reliability analysis of an earth embankment along the Thames Estuary based on multiple failure modes. Also, a biavariate gamma process model is developed to take correlated deterioration processes into account. That model is applied to motor crossing events that damage both the crest level and the vegetation on the rearslope. As neither process-based models nor statistical data on deterioration processes due to motor crossing are available, an expert judgmentbased model is developed. The model is valid under restrictive assumptions about stationarity, independency and linear proportionalities. The results highlight that the sensitivity of the reliability to a change in different random variables can significantly vary among the variables. That sensitivity has repercussions for the influence of the correlation model in this application. It is concluded that assumptions regarding the gamma process model can have a large impact on the time-dependent reliability, and should be made with care. When developing a time-dependent reliability model, the variables in the ‘snap shot’ reliability model for which the reliability is highly sensitive require most attention.