Coastlines are subject to multiple developments related to land use planning and the effects of climate change. These developments generally lead to an increase in the risk of coastal flooding. This article focuses on systems to protect against this hazard, and presents a bibliographical analysis on methods and operational tools to strengthen their resilience. This analysis is carried out by considering that a system of protection is a component of the territory to which it provides protection and that it is therefore necessary to study in depth the relations between this system and the various components of the territory (the natural environment, the built environment and the social and institutional environment). Based on this hypothesis, the concepts of risk and resilience applied to floods are specified and the protection and adaptation strategies commonly used in recent decades are described. This retrospective on concepts and strategies leads us to structure the state-of-the-art analysis on methods and operational tools in relation to two issues: 1) understanding risks; 2) adapting and transforming protection systems. In each case, the use of the concept of resilience implies a clear distinction between systemic and analytical approaches. Finally, this bibliographical analysis reveals the need to revise the protection system concept in order to promote the integration of these systems in the territories. Accordingly, new methodological developments could be considered using ecosystem and socio-economic approaches.

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Storm Xynthia, which hit the French Atlantic coast on February 28th, 2010, flooded vast territories despite coastal defences. This disaster highlighted the need to further study the behaviour of the coastal flood protection systems at an adapted geographical scale by considering the kinematics of the events. This objective has been achieved through a combination of conceptual input on the definition of protection systems, significant breakthroughs in the knowledge of the mechanisms governing the flooding, and via the improvement of strategies and methods dedicated to flood analysis and representation. The developed methodology was successfully tested on four sites submerged during Xynthia (Loix, Les Boucholeurs, and Boyardville, located in Charente-Maritime, and Batz-sur-Mer, located in Loire-Atlantique). This work is intended to guide the diagnosis of sites prone to marine flooding from the first investigations until the delivery of study reports. Beyond the usual focus on hydraulic structures, it provides guidelines to better analyse the interactions with the natural environment (sea, soil, dune, wetlands, etc.) and with the built environment (roads and urban networks, ponds used for fish farming, buildings, etc.). This systemic approach, which is applied to a territory considered as a complex adaptive system, is fundamental to understanding the reaction of a territory during a marine submersion event and subsequently developing adaptation or transformation strategies.

A better definition of the long term protection strategies against littoral hazards requires evaluating investment, management, and maintenance costs of various interventions on levees and other civil engineering structures, as well as on beaches and dunes. The cost study of coastal protections (Cerema 2018) aims at providing the assessment of the project global cost. Based on the study of a large number of projects completed in France, estimations were produced to enhance the reliability of these calculations at the scale of a site in the littoral zone or of an individual natural or artificial structure. For each structure type, the obtained results are essentially linear, surface, volume or mass costs (initial costs or annual average costs), as well as lifetime and intervention frequency. To obtain this information, the analysis mainly consisted of: • the creation of a typology which crosses « structure type » and « intervention type » which allows establishing homogeneous groups of characteristic operations; • the identification, for every group, of physical parameters influencing the associated costs, which allows the user of the manual to evaluate better the difference between the observed high and low values. This paper presents the context of the study, its methodological process, the progress of the investigations and the analyses, the obtained results, and finally the generic lessons learned.

The resilience of levee systems is intuitively associated with physical and technical measures applied to aide in the recovery or adaptation after a destructive event. However, facing a hazard whose characteristics are never fully anticipated and to which a fully predetermined response cannot be proposed, the responsiveness of levee managers depends primarily on their ability to make decisions that must necessarily be based on a sufficient level of information and be supported by appropriate methodological frameworks. Building on previous research about the response to the Xynthia storm, this paper demonstrates that the resilience of protection systems involves expanding the approaches for hazard characterization, flood protection system definition, and intervention modes. With climate change, and the current ecological and digital transitions, methodological developments in the field of flood protection should also encourage the mobilization of more varied disciplines and strengthen solidarity between stakeholders. These links must be woven in the long term, under normal conditions, to be put in place rapidly in emergency situations. Ultimately, methodologies and operational tools must be developed for all (both normal and emergency) circumstances, within the framework of a global, integrated and cohesive approach.