Modeling dependence and coincidence of storm surges and high tide: methodology, discussion and recommendations based on a simplified case study in Le Havre (France)

International audienceCoastal facilities such as nuclear power plants(NPPs) have to be designed to withstand extreme weatherconditions and must, in particular, be protected againstcoastal floods because it is the most important source ofcoastal lowland inundations. Indeed, considering the combination of tide and extreme storm surges (SSs) is a key issuein the evaluation of the risk associated with coastal floodinghazard. Most existing studies are generally based on the as sumption that high tides and extreme SSs are independent.While there are several approaches to analyze and character ize coastal flooding hazard with either extreme SSs or sealevels, only few studies propose and compare several approaches combining the tide density with the SS variable.Thus this study aims to develop a method for modeling dependence and coincidence of SSs and high tide. In this work,we have used existing methods for tide and SS combinationand tried to improve the results by proposing a new alternative approach while showing the limitations and advantages of each method. Indeed, in order to estimate extremesea levels, the classic joint probability method (JPM) is usedby making use of a convolution between tide and the skewstorm surge (SSS). Another statistical indirect analysis usingthe maximum instantaneous storm surge (MSS) is proposedin this paper as an alternative to the first method with the SSSvariable. A direct frequency analysis using the extreme totalsea level is also used as a reference method. The questionwe are trying to answer in this paper is then the coincidenceand dependency essential for a combined tide and SS hazardanalysis. The results brought to light a bias in the MSS-basedprocedure compared to the direct statistics on sea levels, andthis bias is more important for high return periods. It wasalso concluded that an appropriate coincidence probabilityconcept, considering the dependence structure between SSs,is needed for a better assessment of the risk using the MSS.The city of Le Havre in France was used as a case study.Overall, the example has shown that the return level (RL) estimates using the MSS variable are quite different from thoseobtained with the method using the SSSs, with acceptable uncertainty. Furthermore, the shape parameter is negative fromall the methods with a much heavier tail when the SSS andthe extreme sea levels (ESLs) are used as variables of intereest