No abstract

Design codes and standards rely on generalised target reliability indices. It is unclear, however, whether these indices are applicable to the specific risk-profile of marine structures. In this study, target reliability indices for quay walls were derived from various risk acceptance criteria, such as economic optimisation, individual risk (IR), societal risk (SR), the life quality index (LQI) and the social and environmental repercussion index (SERI). Important stochastic design variables in quay wall design, such as retaining height, soil strength and material properties, are largely time-independent, whereas other design variables are time-dependent. The extent to which a reliability problem is time variant affects the present value of future failure costs and the associated reliability optimum. A method was therefore developed to determine the influence of time-independent variables on the development of failure probability over time. This method can also be used to evaluate target reliability indices of other civil and geotechnical structures. The target reliability indices obtained for quay walls depend on failure consequences and marginal costs of safety investments. The results were used to elaborate the reliability framework of ISO 2394, and associated reliability levels are proposed for various consequence classes. The insights acquired were used to evaluate the acceptable probability of failure for different types of quay walls.

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.