Abstract. The design of effective coastal protections requires an adequate estimation of the annual occurrence probability of rare events associated with a return period up to 10 3 years. Regional frequency analysis (RFA) has been proven to be an applicable way to estimate extreme events by sorting regional data into large and spatially distributed datasets. Nowadays, historical data are available to provide new insight on past event estimation. The utilisation of historical information would increase the precision and the reliability of regional extreme's quantile estimation. However, historical data are from significant extreme events that are not recorded by tide gauge. They usually look like isolated data and they are different from continuous data from systematic measurements of tide gauges. This makes the definition of the duration of our observations period complicated. However, the duration of the observation period is crucial for the frequency estimation of extreme occurrences. For this reason, we introduced here the concept of "credible duration". The proposed RFA method (hereinafter referenced as FAB, from the name of the authors) allows the use of historical data together with systematic data, which is a result of the use of the credible duration concept.
Au cours de la dernière décennie, la tempête Xynthia et d'autres ont marqué l'actualité de par leur intensité et les dégâts causés lors de leur passage sur le territoire français. Ces événements peuvent donner l'impression qu'ils n'ont été "jamais vu par le passé", mais le recensement des tempêtes plus anciennes (1953, 1987, 1999…) montre que de tels phénomènes surviennent en fait régulièrement sur le littoral métropolitain. Dès lors, la collecte et l'analyse d'informations historiques apparaissent incontournables pour améliorer la prévention des risques littoraux, en particulier liée à la submersion marine. Afin de mutualiser les informations disponibles en France, une base de données relationnelle et spatiale a été développée au sein de l'IRSN. À ce stade, plus de 650 événements de tempête et/ou de submersion sur le littoral français et des pays voisins ont pu être recensés, sur une période s'étendant de 1500 à nos jours. En parallèle à l'élaboration de cette base de données, un groupe de travail interdisciplinaire s'est constitué. Il intègre des ingénieurs, chercheurs, statisticiens et historiens appartenant à différents organismes. Un axe majeur de travail est l'intégration de nouvelles données historiques issues de diverses archives dans la base de données, ainsi que leur interprétation et la quantification des niveaux marins associés.
Abstract. To withstand coastal flooding, protection of coastal facilities and structures must be designed with the most accurate estimate of extreme storm surge return levels (SSRLs). However, because of the paucity of data, local statistical analyses often lead to poor frequency estimations. The regional frequency analysis (RFA) reduces the uncertainties associated with these estimations by extending the dataset from local (only available data at the target site) to regional (data at all the neighboring sites including the target site) and by assuming, at the scale of a region, a similar extremal behavior. In this work, the empirical spatial extremogram (ESE) approach is used. This is a graph representing all the coefficients of extremal dependence between a given target site and all the other sites in the whole region. It allows quantifying the pairwise closeness between sites based on the extremal dependence. The ESE approach, which should help with have more confidence in the physical homogeneity of the region of interest, is applied on a database of extreme skew storm surges (SSSs) and used to perform a RFA.
The use of historical sources and data to improve knowledge of past extreme events is no longer to be demonstrated. Old quantitative and qualitative data on water levels available in non-digitized tidal charts, tide gauge records, newspapers, city council registers, engineers' reports, etc. can be used to reconstruct water levels and skew surges that occurred during a storm. Data from primary and/or secondary historical sources require a historical critical approach. This paper presents an interdisciplinary study that required a combined work between historians, geographers, geologists and engineers. In this study, nine different variables that can lead to uncertainties on the quantification of historical skew surges have been identified , such as tidal predictions, chart datum references or the reliability of the historical data. Starting from the historical critical method developed by historians over decades, we propose a three-step method to assess the quality of historical documents and define their level of reliability. The method is applied to two case studies: the storm of 16 th November 1940 and the storm of 15-16 th February 1941. Interesting results are provided and the study shows that by paying attention to the potential causes of uncertainties, by ensuring the reliability of historical sources and data through the use of the historical documents quality method, by deepening historical researches and by combining different scientific fields and methods, it is possible to reduce uncertainties and errors when quantifying historical skew surges.
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