Author(s)

K. Roscoe, S. Caires, F. Diermanse & J. Groeneweg

Abstract

As part of the flood risk assessment in The Netherlands, extreme hydraulic boundary conditions (HBC) at coastal water defenses are calculated to assess the safety and sufficiency of the defenses. For this purpose, extreme offshore values of wave height and wave period with a frequency of occurrence of 1/10,000 years need to be estimated. The estimation is carried out using measurements at nine offshore locations in the North Sea, with a record length of 30 years. To estimate extreme wave heights and wave periods, the Generalized Pareto Distribution (GPD), which follows from extreme value theory, was fitted to peaks-over-threshold (POT) at each of the nine measurement stations. The fitted GPD parameters and consequently the extrapolated extreme values are particularly sensitive to the choice of threshold. A reproducible method was employed for threshold selection based on the anticipated behavior of the GPD shape parameter as the threshold approaches the domain of attraction. A modified regional frequency analysis (MRFA) was subsequently carried out to spatially smooth the GPD shape parameters. Final GPD fits were made using the shape-parameter output of the MRFA to produce final extrapolated extreme value estimates. The extreme values were compared with those resulting from an operationally-used distribution function in The Netherlands known as the conditional Weibull distribution. Keywords: extreme value theory, flood risk, threshold selection, generalized Pareto distribution, extreme wave parameters.

Keywords

extreme value theory, flood risk, threshold selection, generalizedPareto distribution, extreme wave parameters