The need
Since 2011, the Departmental Directorates for Land and Sea (DDTM) have been responsible for providing technical support to the prefectures, acting as the Departmental Flood Coordinator (RDI - Référent Départemental Inondation), to help them foresee and manage flood crises as part of the ORSEC civil defence system. Although this assignment initially concerned river floods, it was extended in 2018 to include coastal flooding. In particular, the RDI must be able to interpret the forecasting data from Météo-France and deduce the expected impact on local assets. In the Arcachon basin, the DDTM33 asked BRGM to support the RDI mission by producing an atlas of submersion scenarios.
The results
The method applied is based on and adapts the principle of potential flood zones (ZIP -Zones d’Inondation Potentielles), already widely used for river flood risks. This is a cartographic atlas of coastal flooding scenarios for the 10 coastal municipalities around the basin for a set of fictitious but realistic weather and sea conditions. The scenarios are built using statistical analysis of forcing conditions (wind, waves, surges) and numerical modelling of land flooding, and represent a wide range of events, from the annual to the exceptional.
Thanks to a dedicated digital application, the atlas enables the on-call manager to interpret and translate the operational coastal forecasts of Météo-France into potentially submerged areas on land, working by analogy with the scenarios in the database.

BRGM is a long-standing partner and this new study is a logical follow-up to previous work. Thanks to the Bureau's scientific and technical skills, we are gaining a better understanding of our region. Today, the cartographic tool is being analysed by comparing it with recent meteorological events to improve its rendering. It already provides very useful information on the impact of storms on the Arcachon basin.
Using the results
The atlas is based on the selection of a tide level at the tide gauge and the search for a storm analogue in the database in relation to the weather and sea conditions forecast by Météo-France and the French naval hydrographic and oceanographic service (SHOM - Service hydrographique et océanographique de la Marine) at the Cap-Ferret buoy. Since the winter of 2023-2024, the on-call manager has had a dedicated digital application that can be installed on a PC and used offline. A first search tab allows users to enter tide level forecasts for Eyrac and storm conditions for the Cap-Ferret buoy. A second tab displays the three scenarios in the atlas that are closest to the scenario they are looking for, and allows them to compare the corresponding flood maps for the 10 municipalities. Extraction of the selected map(s) in pdf or GIS format enables the data to be used in other crisis management applications (SYNAPSE, etc.).