Modelling Extreme Water Level and its Components for Infrastructure Design: The case of Hook of Holland

Abstract

The projected increase in sea level is expected to increase the intensity of coastal flooding threatening communities living along the coast. This, in combination with population growth and urban expansion, calls for a better understanding of Extreme Water Levels (EWLs), the mechanisms generating them, and their components, i.e., astronomical tide and storm surge, since they drive the maintenance and design of flood protection systems. Netherlands' flood defense is crucial in facing the risk of flooding given its particular geographical configuration, its large number of inhabitants, and its high value of assets. For this, a better understanding of EWLs and their components is essential to assessing the quality of current structures and developing new adaptation strategies since they drive design and risk assessment procedures. Hence, in this paper, we investigate EWLs in Hook of Holland which represents a strategic location due to the inlet of the port of Rotterdam and the Maeslant storm surge barrier. Here, we present a stepwise procedure that starts by defining EWLs, assessing drivers of storm surges on observed sea levels via spectral analysis and coherence, and ends in estimating the statistics of EWLs based on multiple approaches, i.e., univariate extreme value analysis, copula functions, and Joint Probability Method (JPM). The results show that storms in the Southwest Delta have a duration of about 4 days and that EWLs components, i.e., surge and astronomical tide, present negative dependence (the Kendall's tau $\tau = -0.50$). From the comparison between statistical approaches to model EWLs and infer design values, results show that copulas and JPM lead to an overestimation of EWL. However, EWLs modeled via copulas fit better low quantiles.