Correlation Effect in Probabilistic Design against Piping in Multi-Functional Flood Defences

Abstract

The multi-functional flood defences are one of the many solutions proposed for climate adaptation in deltaic areas. Similar concepts flood management structures such as Delta-dikes, un-breachable dikes or super-levees can also be found in literature. Yet all of them have two key design requirements in common. The first one is to improve their reliability given the uncertainty in future projections of water level change. The second one is the optimization of their dimensions in order to make them safe enough. In the case of the Netherlands, the failure mechanism of piping is one of the main concerns of the designers and flood risk managers. The large uncertainties that come from the heterogeneity of the soil properties are coped by moving towards a more complete probabilistic design. However correlation between input parameters is one of the sources of uncertainty that must be solved. In the present study, the effect of possible correlation between grain size and hydraulic conductivity is investigated. A straight forward approach is implemented by the inclusion of joint bivariate distributions generated from copulas. This random sampling method allows to induce not only the degree of correlation of two variables but their different tail dependence behavior as well. The generated random samples are propagated with a Monte Carlo algorithm using the piping Sellmeijer limit state function in order to estimate the reliability of the analyzed flood defence. The present study showed that correlation has an important effect in the design of this kind of structures which is even more severe when implemented according to the actual Dutch legislation. Therefore a robust soil investigation can be more cost effective than the actual over dimensioning of the structures.