Reliability-based partial factors for flood defenses

Abstract

The Netherlands is protected against major floods by a system of primary flood defenses. These primary flood defenses have to comply with flood protection standards. Since 2017, these are defined in terms of maximum allowable probabilities of flooding. This is why a new set of tools and guidelines had to be developed, allowing for probabilistic as well as semi-probabilistic assessments. Semi-probabilistic assessments rest on a Load and Resistance Factor Design (LRFD) approach. Since major levee systems are essentially series systems with little to no redundancy, the difference between component and system reliability is essential for reliability analyses of flood defenses. This paper discusses the code calibration procedure that was developed to ensure consistency between probabilistic and semi-probabilistic assessments of flood protection systems and their components. Example applications are provided for two failure modes: slope instability and dune erosion. The newly calibrated semi-probabilistic rules allow practitioners to assess the reliability of flood protection systems on the basis of component-level semi-probabilistic LRFD analyses for individual failure modes.