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Le 2 octobre 2020, une violente tempête touche la France, et des pics de précipitations historiques sont retenus dans le sud-est de la France. Au poste Météo-France de Saint-Martin-Vésubie on enregistre plus de 500mm de précipitations en moins de 24h. Les différentes vallées de l’arrière pays niçois (vallée de la Vésubie, de la Tinée et de la Roya) sont dévastées. Très médiatisée, la catastrophe a donné lieu à de nombreux retours d’expériences qui seront le point de départ de l’étude. Le but étant de synthétiser les différents retours d’expériences pour savoir ce qui en est ressorti et ce qui n’a pas été dit pour analyser les conclusion. Certaines zones de flous sont à creuser, en particulier autour de trois piliers de la gestion du risque : l’aménagement du territoire, l’information/prévention et la gestion de l’alerte.

International audience; As part of ongoing projects to integrate the capacity to model the dispersion of radionuclides in rivers at the IRSN's Technical Crisis Centre (CTC) (Institute for Radiological Protection and Nuclear Safety) during accident or incident situations, the prioritization and selection of watercourses (WC) to be modelled appears necessary in order to identify those most likely to present a health, environmental and social challenge in the more or less long term, taking into account the diversity and variability of accident scenarios. From an initial dataset comprising more than a hundred thousand rivers, a pre-selection based on their length enabled the extraction of 1876 WC to be prioritized by considering the elements of vulnerability in their vicinity, whatever the accident scenario selected, according to a multi-expertise and multi-criteria methodology. The selection of WCs was carried out by considering the most important WC based on their length.In order to prioritize the watercourses according to their vulnerabilities, a deliberative and multidisciplinary assessment process was set up at IRSN. This process was based on a multi-criteria decision support approach to identify: (i) the relevant characterization grid for all WC according to the available data, (ii) attributes characterizing vulnerability around watercourses, (iii) the data and knowledge available, (iv) criteria for prioritizing watercourses, (v) the choice of a multi-criteria prioritization procedure for the WCs, (vi) the identification of the sets of weights for the criteria, and (vii) the prioritization of the 1876 WC.Several sets of weights for the criteria were identified by the experts individually and collectively. The robustness of the hierarchies projected on the territory was analyzed.This paper proposes to report on the contributions of multi-criteria and multi-actor approaches to assist expertise and decision-making to support crisis management or to consolidate risk prevention approaches. In this respect, the territorial issue of prioritizing the WCs by considering the elements of vulnerability in the vicinity, whatever the accident scenario selected, raises the question of transition and sustainability in two ways. The first is the question of the transition is between emergency and post-accident. The last is the question of the "knowledge transition" and the way in which collective modelling processes help to promote the transition from a body of individual knowledge to a common body of knowledge (e.g. knowledge mobilized during emergency and during post-accident).

International audience; The energy and territorial transition in which French and European environmental policy is embedded raises the issue of sustainable management of radioactive waste. In this context, IRSN has launched an innovative deliberative action-research approach based on the issue of the management of so-called "very low activity" waste (VLLW). A multi-actor, multi-expertise and multi-criteria approach has been set up for this purpose. This paper proposes an overview of the contributions and limitations of the multi-actor and multi-criteria approach for the sustainable management of VLLW waste. These aspects are discussed following a three phases approach. The core methodology focus on the management scenarios that are considered by experts and stakeholders based both on a "reasoned" voting protocol and a multi-criteria hierarchical process (multi-expertise and multi-stakeholder). This paper presents at an early stage a way to frame a methodology.