No abstract

Coastal flood defense systems can consist of multiple lines of defense. In case of a system with a front and a rear defense (e.g. a storm surge barrier and levees), the front defense can improve the reliability of the rear defense by reducing the load on this rear defense. This paper develops a framework in order to assess whether including the influence of such a load reduction influences the economically optimal safety targets of both defenses. The economic optimization is carried out using two approaches: a simplified method developed to explore the behavior of the economic optimization with a front and rear defense, and a numerical framework geared towards practical applications. The numerical framework provides more flexibility in defining risk, cost and damage functions, and emphasizes on the applicability and tractability of the necessary steps from an engineering perspective. Both approaches are used in a hypothetical case study in order to quantify the effect of including a load reduction on the economically optimal safety targets. The results indicate that if a front defense can create a significant risk reduction in a cost efficient manner, more efficient economically optimal safety targets can be found by including the load reduction.

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Abstract. Flood defence systems can be seen as multiple interdependent flood defences. This paper advances an approach for finding an optimal configuration for flood defence systems based on an economic cost-benefit analysis with an arbitrary number of interdependent flood defences. The proposed approach is based on a graph algorithm and is, thanks to some beneficial properties of the application, able to represent large graphs with strongly reduced memory requirements. Furthermore, computational efficiency is achieved by delaying cost calculations until they are actually needed by the graph algorithm. This significantly reduces the required number of computationally expensive flood risk calculations. In this paper, we conduct a number of case studies to compare the optimal paths found by the proposed approach with the results of competing methods that generate identical results. The proposed approach is set up in a generic way and implements the shortest-path approach for optimising cost-benefit analyses of interdependent flood defences with computationally expensive flood risk calculations.

In risk analysis of riverine flood defence systems, sections of flood defences are often considered separately, herewith ignoring their interdependence, e.g. due to the hydraulic response following dike breaches in the system. In previous studies it has been found that such interdependence can have a significant influence on flood risk estimates and the spatial distribution. In this paper a method is proposed for the economic optimisation of riverine flood defence safety levels from a river system perspective. In order to deal with the computational challenge of integrating the hydraulic interactions in an economic optimisation, a surrogate model was developed. Despite the many simplifications, this model yields reasonably accurate results within acceptable time. The application of the model to a case study in the Netherlands has shown that taking into account interactions between flood defences has significant influence on optimal long term strategies for flood defences. The results suggest that accounting for interdependence in setting safety standards and reinforcement prioritisation yields a significant return on investment both in terms of lower investment cost and in terms of reduced risks.