This report studies the ability of flood defences to reduce the risk of flooding. Storm surge barriers are a specific type of flood defence. Their function is to reduce the hydraulic loads, the water levels and the wave conditions on the dikes located behind the storm surge barrier. Safety standards for flood defences are established to ensure a minimum level of safety, accounting for the performance of flood defences, which is determined by their probability of failure.
The applicable requirements for storm surge barriers have developed over time. Various assumptions have been made in deriving these requirements, because the applicable standards for flood defences and the corresponding methods to assess their safety have developed. Specifically, assumptions have been made regarding the influence of storm surge barrier performance on the failure probability of dikes. The objective of this study is to build a framework that includes this relationship in a requirement for storm surge barriers.
The framework is built by analysing the relationships that define the flood protection system. It is considered that a reduction in storm surge barrier performance increases the water levels in front of dikes, which increases the dike failure probability and the risk of flooding. The framework built in this study calculates a requirement for structural failure of storm surge barriers based on an economic optimisation at flood protection system level. The dike failure probability is calculated by expressing storm surge barrier performance in terms of water levels in front of the dike and by representing the resistance of a dike to a certain water level using fragility curves. In the next step, investment functions are used to translate failure probabilities into costs to determine the minimum, and therefore optimum, costs at flood protection system level.
The framework is first applied in a generic context to outline the steps and potential applications of the framework. Second, the framework is applied to a schematised representation of the Eastern Scheldt, focusing on the case-specific aspects. The framework is applied to identify the criteria that should be considered in safety standards for storm surge barriers. A requirement for storm surge barriers considers the dike failure probability as a function of storm surge barrier performance. The maximum allowed dike failure probability relates to the maximum allowed consequences of flooding. Furthermore, the variation in dike investment costs compared to the variation in storm surge barrier investment costs is important to derive a requirement that corresponds to an economically optimum flood protection system.