WBI2017 Code calibration : reliability-based code calibration and semi-probabilistic assessment rules for the WBI2017

The Netherlands is protected against major floods by a system of primary flood defenses. The primary flood defenses have to comply with the flood protection standards from the Water Act. These were updated in January 2017. They are now defined in terms of maximum allowable probabilities of flooding. In the past, the standards were defined in terms of exceedance probabilities of loads that primary flood defenses should be able to safely withstand. Periodic safety assessments are carried out to establish whether the Dutch primary flood defenses comply with the flood protection standards from the Water Act. Because of the change in the type of standard, a new set of tools and guidelines had to be developed for assessing the safety of primary flood defenses: the WBI2017. The WBI2017 consists of simple screening methods as well as probabilistic and semi-probabilistic methods for detailed assessments. Semi-probabilistic methods rest on a partial safety factor approach. This approach allows practitioners to evaluate the reliability of flood defenses without having to resort to probability calculus. To ensure consistency between probabilistic and semi-probabilistic assessments, the WBI2017’s semi-probabilistic assessment rules have been code calibrated. This means that appropriate design values (partial safety factors and representative values) have been defined for use in semi-probabilistic assessments. For reasons of consistency, efficiency and transparency, a standardized code calibration procedure was developed. This report provides an overview of this procedure and discusses its application to the following failure mechanisms: 1. internal erosion (uplift, heave and piping), 2. slope instability (macro instability), 3. dune erosion, 4. block revetment failure caused by wave impacts, 5. asphalt revetment failure caused by wave impacts, 6. grass revetment failure caused by wave impacts, 7. grass revetment failure caused by wave run-up.