Analysis of the risk associated to coastal flooding hazards: A new historical extreme storm surges dataset for Dunkirk, France

This paper aims to demonstrate the technical feasibility of a historical study devoted to French Nuclear Power Plants (NPPs) which can be prone to the extreme marine flooding events. It has been shown in the literature that the use of HI can significantly improve the probabilistic and statistical modeling of extreme events. There is a significant lack of historical data about marine flooding (storms and storm surges) compared to river flooding events. To address this data scarcity and to improve the estimation of the risk associated to the marine flooding hazards, a dataset of historical storms and storm surges that hit the Nord-Pas-de-Calais region during the five past centuries were recovered from archival sources, examined and used in a frequency analysis (FA) in order to assess its impact on the frequency estimations. This work on the Dunkirk site (representative of the Gravelines NPP) is a continuation of previous work performed on the La Rochelle site in France. Indeed, the frequency model (FM) used in the present paper had some success in the field of coastal hazards and it has been applied in previous studies to surge datasets to prevent marine flooding in the La Rochelle region in France. In a first step, only information collected from the literature (published reports, journal papers and PhD theses) is considered. A 1954 Coastal Engineering journal issue (Le Gorgeu and Guitonneau, 1954) on the reconstruction of the eastern dyke in Dunkirk has been more than a reference for this paper. It has indeed served as a main source of historical information (HI) in this study. Although this first historical dataset has extended the gauged record back in time to 1897, serious questions related to the exhaustiveness of the information and about the validity of the developed FM have remained unanswered. Additional qualitative and quantitative HI were extracted in a second step from many older archival sources. This work has led to the construction of storms and marine flooding sheets summarizing key data on each identified event. The quality control and the cross-validation of the collected information, which have been carried out systematically, indicate that it is valid and complete as regards extreme storms and storm surges. Most of the HI gathered displays a good agreement with other archival sources and documentary climate reconstructions. The probabilistic and statistical analysis of a dataset containing an exceptional observation considered as an outlier (i.e. the 1953 storm surge) has been significantly improved when the additional HI gathered in both literature and archives are used. As the historical data tend to be extreme, the right tail of the distribution has been reinforced and the 1953 exceptional event don't appear as an outlier any more. This new dataset provides a valuable source of information on storm surges for future characterization of coastal hazards.