Multidisciplinary expertise of historical information for the characterization of water levels during storm and coastal flooding events

International audienceCharacterization of coastal water level reached during extreme events is a strong societal concern for a better coastal risk management. Historical archives related to storms and floodings are still not often considered whereas they could be used to improve knowledge on extreme sea levels. In this context, the French national Working Group (WG) “Historical Storms and Floodings” has been created to perform a multidisciplinary expertise of historical information. This WG includes researchers, statisticians and historians from different entities (IRSN, Artelia, BRGM, Cerema, EDF, LIENSs, Shom and Université Populaire du Littoral Charentais).A big challenge in such a multidisciplinary approach is the management of different types of information coming from various scientific contexts and practices. The data varies from historical tide gauge observations, to historical local press, scientific essays, eye-witness testimonies, minutes from town hall meetings etc. Each document used needs to be analyzed regarding the context in which it was written down and which were the objectives, which will then allow the definition of its trustability. These issues, the way there are addressed and the tools created within this working group will be presented.As a starting point, the database called TEMPETES ET SUBMERSIONS HISTORIQUES (“Historical Storms and Floodings”) was previously created by IRSN and has been provided to the WG members. This database aims at inventorying qualitative and quantitative information on storms and/or floodings that occurred on the English Channel and Atlantic coastlines. Currently, over 1500 sources describing more than 750 events are outlined for the period from the 16th century up to now, for French coastlines and neighbouring countries. Thanks to ongoing research projects and studies carried out by WG members this database is continuously enriched by new documents and events.In addition to the events themselves, water and surge levels are important variables of interest for the WG members. Most of the time, these values are not directly obtained from the available sources in the database and therefore a multidisciplinary expertise of the historical information is performed to estimate water levels reached during storm and flooding events. For this purpose, the available information in the database regarding one event is expanded with complementary data (such as historical city maps, sketches, profiles of the flooded dikes, historical local press, etc.), in order to elaborate a storm sheet. All this data is then used to reconstruct historical water levels i.e. total water levels and surge levels