Time-dependent reliability in flood protection decision making in The Netherlands

Since 2017 Dutch flood protection standards are defined as target flood probabilities that all primary flood defences have to comply with by 2050. Explicitly accounting for uncertainties in probability distributions of load and resistance is an integral part of estimating the actual flood probability. Based on such estimates, many flood defences will be reinforced in the coming years, for design lifetimes that are generally 25-100 years. Therefore it is important that we correctly take into account time-dependence of both load and resistance during the lifetime. Loads are typically uncorrelated from year to year, whereas strength parameters exhibit significant correlation over time. This correlation over time of strength parameters can significantly reduce the failure rate and increase the lifetime reliability of a flood protection structure. In this paper we show the implications of time-dependent reliability for a set of illustrative cases. We consider the effect of different degrees of temporal dependence on reliability, lifetime and relative cost savings. The cases show that for common configurations, the inclusion of time-dependent effects, especially the correlation in time of strength variables, can increase the lifetime of a flood protection structure by up to 50%.