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Poster presentation

The energy of water masses is a first-order factor that controls the essential physicochemical dynamics of a water body. Its study allows one to understand the roots of the processes that occur at the water-mass, water-atmosphere and water-sediment interfaces. The analysis of the Thomas Pond in the Brenne region gives a valuable overview of energy stock evolution on a yearly scale. It highlights the direct impact of this evolution on thermal stratification and the potential for evaporation and exchange with the atmosphere. The study of evaporation remains challenging due to the complexity of the energy processes and factors involved. Its estimation using formulas, which are mostly empirical, is one of the most used means for studying the process. The studied pond shows a natural stratification during the summer season, however often fragile and disturbed by other climatic factors such as wind and precipitation. This disruption leads to increased exchanges between the pond and the atmosphere. The methods used to estimate pond-atmosphere exchanges, namely evaporation, vary in values ranging between 1 mm/d to > 15 mm/d. Among these methods, three stand out and seem to give reasonable values. This observation is based on the noticeable drop of the pond’s water level during the period of non-communication with the outside, which corresponds to 65 mm. The energy required for this evaporation varies between 600 W/m2 and 1500 W/m2, except for the Smith model, that slightly overestimates this parameter. The regulation of ponds’ water volumes by managers, the increased duration of bungs closure and the intermittence of precipitations in recent years exacerbate the reduction of direct inputs to ponds and the aggravates the impacts of a changing climate. Under the effect of increasing air temperatures, losses by evaporation will also increase significantly. If we generalise the results obtained to all of the Brenne Park water bodies (4500 ponds of the park), losses by evaporation will lead to a significant water deficit of the Loire basin. From this study, the use of deep learning ensemble models was found to provide better short-term predictions (RMSE between 0.003 and 0.006 for all methods), thus confirming the effectiveness of these methods for similar applications.

International audience; A number of evacuation models exist in literature, but investigations of their operational aspects and efficient implementation are rare. The purpose of this paper is to critically assess the operational aspect of pedestrian evacuation models for flash floods mitigation. Through a detailed real-life case study in Ras-Baalbek village-Lebanon, the efficiency of evacuation models when it comes to saving lives was examined. Potential evacuation times were established using an anisotropic least-cost-distance (LCD) approach combined with an exploratory geospatial analysis. The next step was a field verification of the obtained results and accordingly, a detailed evacuation plan that would allow evacuees for a quick departure before flash floods arrival was established. Based on the modeling results, evacuation signs were proposed to be installed on roads along with building vertical-evacuation structures to enhance the evacuation time. The obtained evacuation-modeling outcomes showcased to have multiple operational implications: flood risk outreach, educating and training (preparedness and response), reducing risk and casualties, long-term land use/land cover zoning and, ultimately, improving communities’ resilience to floods.

In mountainous areas in recent decades urbanisation has expanded to areas where low ground adjoins mountainsides that are unstable in a number of respects. Periurbanisation in mountain basins with unstable sides poses specific problems that local players have to address. The Lavanchon basin (southeast of Grenoble), which is subject to very rapid urban growth combined with particularly dynamic mountainsides, is representative of the way activity is being brought into closer contact with potential hazards. A diachronic study of changes in land use between 1956 and 2001 shows how valley infrastructures at the bottom of mountainsides have become increasingly dense. In this context, a survey was carried out among a number of residents in the Lavanchon basin in an attempt to evaluate the degree of awareness that the population has of the natural hazards to which it is exposed. The results show that slightly more than half of the population surveyed was aware of the problem of natural hazards being present in the area, with most inhabitants being more concerned about industrial and pollution hazards. New residents were unaware of or were unwilling to accept the reality of hazards. The low incidence of significant natural events, the effectiveness of the protective structures built, the absence of information provided by the public authorities and the division of the basin between several management bodies appear to have engendered a feeling of safety from natural phenomena. The geographical distribution of appreciation of the hazard clearly shows a distinction between those inhabitants living on the low ground and those at the bottom of the mountainsides, and this corresponds fairly closely with the historical and current location of the main potentially hazardous events that have occurred.Dans les territoires de montagne, les dernières décennies ont vu l’expansion de l’urbanisation vers les zones de contact entre la plaine et les versants, lieux d’instabilités multiples. La périurbanisation au sein de bassins montagnards aux versants instables pose des problèmes spécifiques auxquels les acteurs locaux tentent de faire face. Le bassin du Lavanchon (sud-est de Grenoble), qui combine un accroissement urbain très rapide et des versants particulièrement dynamiques est représentatif de ce rapprochement entre les aléas et les activités. L’étude diachronique de l’évolution de l’utilisation du sol entre 1956 et 2001 montre la densification des infrastructures dans la vallée et au bas des versants. Dans ce contexte, une enquête a été réalisée auprès d’un certain nombre de résidents du bassin du Lavanchon dans le but l’évaluer le degré de conscience que les populations ont des risques naturels auxquels ils sont exposés. Les résultats montrent qu’un peu plus de la moitié de la population interrogée a conscience de la problématique des risques naturels sur ce territoire, plutôt marquée selon la plupart des habitants par les risques industriels et de pollution. Les nouveaux résidants ignorent ou occultent la réalité des risques. La faible fréquence d’événements naturels marquants, l’efficacité des ouvrages de protection réalisés, l’absence d’informations de la part des pouvoirs publics et le morcellement du bassin entre plusieurs gestionnaires semble avoir généré un sentiment de sécurité par rapport aux phénomènes naturels. La répartition géographique de cette appréhension du risque montre clairement une distinction entre les habitants de la plaine et ceux des bas de versants, qui correspond assez bien à la localisation historique et actuelle des principaux aléas.

This study assessed soil erosion risks of two basins representing different geographical, topographical, climatological and land occupation/management settings. A comparison and an evaluation of site-specific factors influencing erosion in the French Claise and the Lebanese Nahr Ibrahim basins were performed. The Claise corresponds to a natural park with a flat area and an oceanic climate, and is characterized by the presence of 2179 waterbodies (mostly ponds) considered as hydro-sedimentary alternating structures, while Nahr Ibrahim represents an orographic Mediterranean basin characterized by a random unequal land occupation distribution. The Claise was found to be under 12.48% no erosion (attributed to the dense pond network), 65.66% low, 21.68% moderate and 0.18% high erosion risks; while Nahr Ibrahim was found to be under 4, 39.5 and 56.4%, low, moderate and high erosion risks, along with 66% land degradation determined from the intersection of land capability and land occupation maps. Under the alternative scenario for the Claise where ponds were considered dried, erosion risks became 1.12, 0.52, 76.8 and 21.56%, no erosion, low, moderate and high risks, respectively. For Nahr Ibrahim, and following the Land Degradation Neutrality intervention, high erosion risks decreased by 13.9%, while low and moderate risks increased by 3 and 10.8%.

International audience; Total mercury (THg) and methylmercury (MeHg) concentrations were measured in the muscle of Arctic charr (Salvelinus alpinus) and in the water column of 4 lakes that are located in the French Alps. Watershed characteristics were determined (6 coverage classes) for each lake in order to evaluate the influence of watershed composition on mercury and methylmercury concentrations in fish muscle and in the water column. THg and MeHg concentrations in surface water were relatively low and similar among lakes and watershed characteristics play a major role in determining water column Hg and MeHg levels. THg muscle concentrations for fish with either a standardized length of 220 mm, a standardized age of 5 years or for individualuals did not exceed the 0.5 mg kg(-1) fish consumption advisory limit established for Hg by the World Health Organization (WHO, 1990). These relatively low THg concentrations can be explained by watershed characteristics, which lead to short Hg residence time in the water column, and also by the short trophic chain that is characteristic of mountain lakes. Growth rate did not seem to influence THg concentrations in fish muscles of these lakes and we observed no relationship between fish Hg concentrations and altitude. This study shows that in the French Alps, high altitude lakes have relatively low THg and MeHg concentrations in both the water column and in Arctic charr populations. Therefore, Hg does not appear to present a danger for local populations and the fishermen of these lakes.