Calibrating semi-probabilistic safety assessments rules for flood defences

Abstract

In the Netherlands, all primary flood defences are periodically tested against statutory safety standards. These standards are currently defined in terms of design loads that are to be used in semi-probabilistic safety assessments. Nowadays, policymakers are contemplating a move towards safety standards defined in terms of maximum allowable probabilities of flooding. To facilitate such a move, a new framework for assessing the safety of flood defences has to be developed. This new framework will allow for probabilistic as well as semi-probabilistic assessments. To ensure the consistency between these types of assessments, the present-day semi-probabilistic rules have to be recalibrated. A procedure was specifically designed for (re)calibrating semi-probabilistic assessment rules for levees and hydraulic structures. Important aspects within this procedure concern the allocation of maximum allowable failure probabilities to individual failure mechanisms, the definition of design values on the basis of FORM-influence coefficients and the inclusion of the effects of spatial correlations. The calibration procedure is illustrated by means of a case study for piping.