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In North Tanzania, magmatic processes interact with tectonic to give rise to recurrent seismicity and volcanic eruptions, leading to spare and heterogeneous hazards. North Tan- zania is also experiencing a growing economy and an extremely rapid population expansion. The presence of protected areas, natural parks and an active volcano (Oldoynio Lengai) drives a lot of tourism and increases the stakes in the region. The purpose of this work is first to estimate the seismic and volcanic hazards in the area. It implies understanding the interaction between deep and surficial processes. For this purpose, we combine geophysical imaging (seismology, gravimetry) with petrophysical analysis of rocks and lavas. It helps us to reach different scales in space and time and to obtain constraints for crust and mantlestructure. Second, we intend to map the structural vulnerabilities to conduct a diagnosis of the geological risk. We assess a priori and human vulnerability by questioning the popu- lations’ volcanic and seismic risk perception. The people in North Tanzania are differently exposed to natural hazards. The proximity to volcanic edifices or seismic crises experience can strongly influence a population’s naive knowledge. We thus aim to understand the dy- namics between the na ̈ıve knowledge that people can get from their experience and culture and the scientific knowledge obtained from education. Finally, we will use formal modelling approaches to model the ecosystem dynamics in light of our results. With these models, we identify the components and processes favouring a resilient system behaviour or a more dangerous one.

In view of the high vulnerability of the small islands of the Lesser Antilles to cyclonic hazards, realistic very fine scale numerical simulation of hurricane-induced winds is essential to prevent and manage risks. The present innovative modeling aims at combining the most realistically simulated strongest gusts driven by tornado-scale vortices within the eyewall and the most realistic complex terrain effects. The Weather Research and Forecasting (WRF) model with the nonlinear backscatter and anisotropy (NBA) large eddy simulation (LES) configuration was used to reconstruct the devastating landfall of category 5 Hurricane Irma (2017) on Saint Barthélemy and Saint Martin. The results pointed out that the 30 m scale seems necessary to simulate structures of multiple subtornadic-scale vortices leading to extreme peak gusts of 132 m s−1 over the sea. Based on the literature, such extreme gust values have already been observed and are expected for category 5 hurricanes like Irma. Risk areas associated with terrain gust speed-up factors greater than 1 have been identified for the two islands. The comparison between the simulated gusts and the remote sensing building damage highlighted the major role of structure strength linked with the socio-economic development of the territory. The present modeling method could be easily extended to other small mountainous islands to improve the understanding of observed past damage and to develop safer urban management and appropriate building standards.

Proof; International audience; This paper investigates the resilience of traditional societies from Small Island Developing States in the face of disaster. It focuses on tropical cyclone Pam that impacted Vanuatu in 2015. Drawing upon focus group discussions using participatory tools in disaster-affected villages between 2015 and 2017, it examines both the recovery challenges faced and mechanisms developed by traditional societies from Tanna Island post-disaster. The study emphasizes that traditional societies' resilience goes beyond absorbing and recovering from a large event but also relates to recurrent small-scale hazards. The paper further highlights traditional societies' capacities in the face of disaster and stresses the need to strengthen or reintroduce traditional practices while supporting newly adopted mechanisms such as transnational kinship networks and remittances. It also points out the role of external aid intervention post-disaster that, if top-down and delivered disproportionally can compete with local initiatives and have negative effects on traditional societies' resilience. The paper concludes that traditional societies should themselves define and assess their own resilience and ways to strengthen it.

International audience

Ce dossier d’EchoGéo est centré sur les conséquences environnementales, sociales, économiques et politiques des cyclones sur les territoires, et leur gestion. Le néologisme « cycloné » est apparu sur les réseaux sociaux en 2017 après le passage du cyclone Irma aux Antilles. Il renvoie à tout type de destruction causée par des cyclones de catégorie 4 ou 5, mais aussi à l’expérience humaine éprouvante vécue par les sinistrés. Ces événements posent question en termes d’évaluation ex ante et ex p...

This study aims to better understand coastal processes associated with extreme cyclonic events through the study of the coastal changes, flooding and damage that resulted from the passage of a category 5 hurricane (Irma) on 6 September 2017 over the islands of Saint-Martin and Saint-Barth&eacute, lemy in the Lesser Antilles. Hurricane Irma was contextualized from tropical cyclone track data and local weather observations collected by M&eacute, t&eacute, o-France, as well as high-resolution numerical modelling. Field work involved the study of accretion coasts through qualitative observations, topo-morphological and sedimentary surveys, as well as image acquisition with Unmanned Aerial Vehicle (UAV) surveys during two trips that were made 2 and 8 months after the catastrophe. Wave propagation and flood numerical models are presented and compared to field data. Our field analysis also reports on the devastating impacts of storm surges and waves, which reached 4 and 10 meters height, respectively, especially along east-facing shores. The approaches reveal a variety of morpho-sedimentary responses over both natural and highly urbanized coasts. The analysis shows the effects of coastal structures and streets on flow channeling, on the amplification of some erosion types, and on water level increase. Positive spatial correlation is found between damage intensity and marine flood depth. The signatures of ocean-induced damage are clear and tend to validate the relevance of the intensity scale used in this study.

International audience; The 2017 Atlantic hurricane season is undoubtedly one of the most active and devastating ever recorded. Hurricane Maria ranks as the deadliest of the six strongest hurricanes of the season causing about 3,000 fatalities as recently reported. It started as a tropical wave off the West African coast before turning into a category 5 hurricane in less than 18 hours just before making landfall on the island of Dominica in the evening of the 18th of September 2017. It stroke the island going from the south-east to the north-west and wrecked considerably this small island developing state (1.3 billion American Dollars loss or 224% of the 2016 GDP). It affected seriously a population already made vulnerable by the passing of Hurricane Erika in 2015. Analysing the physical impact of Hurricane Maria on the Dominican coastline is relevant to the TIREX 2017 research project funded by the ANR OURAGANS - a national grant devoted to the study of hurricanes. TIREX refers to the use of scientific feedback to encourage better response and adjustment from the civilian population as well as the local and national authorities in a context of climate change ; it focuses on the north area of the West Indies during 2017 hurricane season. Physical impact is studied through a comparative and diachronic approach on several sediment cells and watersheds displaying different signs of anthropisation. Their resilience is assessed a year and a half after the event. The use of aerial footage and topomorphological data analysis made on field are put forward thanks to a cartographic translation in order to emphasize morphosedimentary response at different moments in time. Working on the ability to adjust as well as the physical (geomorphological changes) and social recovery (through reconstruction) is a priority when taking into account the territorial vulnerality at stake in Dominica in a context of climate change. Our results show that coast evolution is mainly due to fluvial processes rather than marine dynamics. Heavy rain caused major landslides and flooding while enabling organic materials and debris to flow downstream towards river mouths. This phenomenon of extreme sedimentation accounts for major morphological alterations such as land elevation, sedimentary progradation, and river alvulsion. Contrary to the idea according to which hurricanes generally make coastlines recede, in the case of Dominica island, large amounts of input materials of heterogenous granulometry have generally contributed to a sustained fattening/growth/development (t + 1 year) of sediment cells. The aggradation of the lower coastal area produced by the sudden dumping of waste materials brought about the destruction of most of the water crossing structures ; it also affected buildings located in the flood plain being completely carried away or buried. Large amounts of woody debris washed away into the sea and reshaped by the cyclonic swell intensified the impact of the waves on the coastline. Identifying Dominican specificities about hurricane Maria event should allow us to develop adjustment measures to reduce the cyclonic risk and improve the resilience of the island coastal systems.

International audience; Les oueds du Haut Atlas de Marrakech ont la triste réputation d’être imprévisibles et redoutables parleurs crues éclairs et leurs inondations répétées des dernières décennies (1989, 1995, 1999, 2006,2009, 2014). Le bassin versant de la Gheraya (225 km²) ne sort pas de la règle. Ses fortes cruespeuvent inonder des zones agricoles, des zones de fréquentation touristique et les voies decommunication. A cet effet, ce travail consiste à caractériser l’aléa de crue en simulant les hauteursdes eaux de l’oued et les extensions latérales des crues. Après une analyse fréquentielle des débitsde pointe, et à l’aide d’un modèle numérique de terrain (MNT) à bonne résolution (4 mètres) et desoutils de modélisation hydraulique, nous avons simulé l’étendue des crues de différentes périodes deretour sur les berges de l’oued. Le choix des secteurs d’application est motivé par deux types d’enjeu: une zone touristique habituellement fréquentée par les visiteurs (Rha de Moulay Brahim) où sontexposés des bâtiments de restauration, la route régionale 203 et évidemment les vies humaines. Laseconde zone est un secteur d’activité agricole sur les terrasses alluviales au niveau de la ville deTahannaout où les champs et quelques aménagements anthropiques sont vulnérables. Lacartographie des zones inondables attire l’attention sur le risque des débordements de la Gherayasur ces zones d’activités humaines ; notamment par l’inondabilité des champs cultivés, de certainsbâtiments et de la route qui désenclave la haute vallée d’Imlil et d’Asni.

International audience

International audience

Road accessibility is highly strategic for the maintenance of economic activities but also for the emergency services. In mountains, snow avalanches are a particularly strong threat because, in addition to the victims and direct damage, they cause a loss of accessibility more or less prolonged when the networks are already strongly altered by seasonal closures. Specifically, risk to traffic roads caused by snow avalanches has been very rarely assessed at a regional scale. To assess the physical, human and functional vulnerabilities of road networks in three Alpine departments (Alpes-de-Haute-Provence, Hautes-Alpes, Alpes-Maritimes), the first step of this research was to geo-locate and harmonize within a GIS all information sources about the consequences of avalanches on roads. This allowed identifying the road impacts of avalanches since 1937, to characterize the intensity and typology of damages and to evaluate the functional vulnerability of networks. The second step was to produce simple risk indexes of dysfunction and isolation at this regional scale. These indicators were modeled using the graph theory in a GIS framework, integrating avalanche activity indicators derived from the past activity with the road network. The obtained output maps should facilitate the decision support for crisis management and a comparative spatial analysis at the regional scale.

L’accessibilité routière revêt un caractère particulièrement stratégique pour le maintien des activités économiques, mais également pour les interventions de secours. En montagne, les avalanches représentent une menace particulièrement forte car, outre les victimes et dommages directs qu’elles peuvent causer, elles entraînent très souvent une perte d’accessibilité plus ou moins prolongée à une époque de l’année où les réseaux sont déjà fortement altérés par les fermetures saisonnières. Pourtant ces incidences sont rarement étudiées à l’échelle régionale. Afin d’évaluer la vulnérabilité physique, fonctionnelle et humaine des réseaux routiers, la première étape de notre étude a consisté à géo-référencer et harmoniser au sein d’un SIG les informations provenant des principales bases de données disponibles sur les conséquences des avalanches pour trois départements alpins (Alpes-de-Haute-Provence, Hautes-Alpes, Alpes-Maritimes). Cela a permis d’identifier les impacts avalancheux sur les routes depuis 1937, de dresser la typologie et l’intensité des dommages, et d’évaluer la vulnérabilité fonctionnelle du réseau. Une seconde étape a consisté à produire des indices de risque de dysfonctionnement et d’isolement à l’échelle régionale. Ces indices intègrent les données historiques sur les dommages aux routes et reposent sur des calculs d’accessibilité utilisant la théorie des graphes dans un environnement SIG. Les cartographies produites présentent une valeur ajoutée pour l’aide à la décision en cas de crise et l’analyse spatiale comparative à l’échelle régionale.

International audience

International audience

International audience

International audience; Coastal communities in the Atlantic marine limit of Morocco risk of inundation by regional and local tsunamis generated in the SW Iberia zone. Tsunami catalogs indicate that this area was the place of several tsunami events since historical time. Among them, the 1755 tsunami remains the largest eye witnessed historical event in the North East Atlantic area. Historical documents described, in some details, the generated waves along the coasts of Portugal, Spain and Morocco. They mentioned that the tsunami run-up has reached 15 m and the wave amplitude was as high as 24 m in some locations. However, these wave heights, run-up values and inundated areas may be (in some cases) overestimated as it was revealed in the recently published studies on historical documents focused on the Gulf of Cadiz area (Blanc, 2009). One of the coastal segments where the reported historical data, related to the 1755 event, are uncertain is the city of Mazagão, actually El-Jadida, located at the SW of Moroccan Atlantic coast. The present study seeks to numerically evaluate the tsunami impact along the El-Jadida coastal segment in order to clarify the uncertainties of the historical reports. A detailed numerical modeling of the tsunami waves evolution onshore and offshore El-Jadida site has been conducted. The digital terrain model (DTM) considered in this study is a reconstruction of the paleo-DTM of El-Jadida site in the 1750s, that we have computed from the paleo-bathymetric/topographic charts available before 1755. Earthquake scenario of magnitude~8.5 has been considered to represent a 1755-like event. Results in term of wave heights, maximum run-ups, high resolution inundation maps and flow limits for the study area are presented for the considered tsunami scenario. Reliability and/or interpretation of historical reports is discussed in light of the comparison of these reports with the worst tsunami impact in El-Jadida obtained from numerical modeling. The results will be incorporated in the future tsunami warning system in the North East Atlantic area.

International audience

International audience; The study develops an innovative GIS-based integrated approach for the assessment of a comprehensive volcanic risk. This is based on a four-step methodology which has been developed at Mt. Pelée volcano, in Martinique, a French island in the Lesser Antilles. The first stage of the methodological framework integrates the spatial extent of volcanic hazards for a maximal credible eruptive scenario. The second stage covers an assessment of the elements (especially the buildings) that may be affected in the event of an eruption and defines their level of damages by given volcanic hazards. A computerized analysis leads to the definition of an index for the risk of total loss in a 500-meter grid. The third stage consists of designing regulative land use maps which should orient and limit the occupation of areas exposed to severe hazards. Finally, the fourth stage, based on survey data, includes a spatial assessment of collective representations for a future eruption by the surrounding communities. These maps turn out to be powerful communication tools which help planners and disaster authorities in mitigating damages from volcanic hazards. They can contribute to enhance the perception of volcanic risk in Martinique.

National audience

International audience; This study aims at assessing the vulnerability of populations in the Pasig-Potrero River basin, who are threatened by the recurrent lahars of Mt Pinatubo volcano (Philippines) in the aftermath of its 1991 eruption. A survey, carried out between February and March 1998, enabled us to define and to map different levels of risk perception, the quality of potential behavioural patterns in the event of further lahars, and attitudes in the event of an evacuation order. It was then possible to work out a classification and to map the patterns of human vulnerability in the study area. This study points out that, while risk perception is quite high, some problems remain, notably concerning the warning systems and the behaviour of the population in the case of an evacuation. Theses results should assist in elaborating risk and disaster scenarios. They are intended to be used for operational purposes in crisis management such as the organization of disaster relief, and in land use planning.

International audience; Mount Pinatubo volcano erupted in June 1991 in the main island of Luzon belonging to the Philippines archipelago. Huge economic losses and population exodus have followed. This major crisis has been relayed with other crises due to rainfed lahars which have been supplied with eruption deposits. These lahars have occurred every year since 1991 during the rainy season. They will probably last until 2005. After a brief presentation of the Philippine official response system to disasters, this paper draws up a critical analysis of the different kinds of institutional and social responses deployed to manage the different crisis and post-crisis phases of this event. Based on three viewpoints: from population, media and other actors, this analysis attempts to point out the strengths and weaknesses of the official management system, especially by studying the efficiency and the range of the solutions taken. So, it appears that the management of the June 1991 main crisis (eruption) was a success. On the other hand, difficulties have occurred with lahars risk management. Indeed, these lahars have obliged the authorities to protect and relocate thousands of people. In spite of persistent problems, the management system (monitoring/warning/evacuation) of lahar crises improves year after year. Failures appear especially within the rehabilitation program (protection/rehousing). Many direct (lack of means, preparedness, coordination, dialog,etc.) and indirect (politico-administrative, socio-economic, cultural contexts) factors come together to lock the wheels ofthe institutional response system. They defer the socio-economic start of this vital northern Philippines area.

L’augmentation du volume des débris, les impératifs de protection de l’environnement, le coût de la collecte et du traitement pour les collectivités territoriales ou les entreprises sont des défis de plus en plus complexes dans les territoires soumis à des catastrophes naturelles, technologiques ou géopolitiques (conflits) destructrices. Le passage des cyclones Irma, et dans une moindre mesure José, a généré environ 60 000 tonnes de débris sur la partie française de l’île Saint-Martin, soit 1,7 tonne par habitant ou l’équivalent de trois ans de collecte normale de déchets. La collecte a nécessité quatre mois alors que le traitement et la valorisation étaient toujours en cours en novembre 2018 soit plus d’un an après le cyclone. Les opérations ont été entravées par le cyclone lui-même (destruction des infrastructures et des moyens matériels, pénurie de main d’œuvre) et par les contraintes de l’insularité (faibles volumes à valoriser, manque d’aire de stockage…). En revanche, Saint-Martin bénéficiait d’une part d’un opérateur public unique (la collectivité territoriale de Saint-Martin), d’autre part d’un lieu de stockage et de traitement appelé Écosite qui monte en compétence depuis une dizaine d’années dans la valorisation des déchets malgré des moyens modestes. L’exemple de Saint-Martin montre toutes les difficultés du traitement des débris post-cycloniques et souligne la dépendance de la production de débris et de leur traitement aux caractéristiques du territoire. In the aftermath of a disaster, the debris caused by the destruction of property or vegetation become a crucial problem. The increase in the volume of debris, the growing requirements for environmental protection, the cost of waste collection and treatment for local authorities or companies are increasingly complex challenges for lands subject to natural, technological or political disasters (conflicts). The landfall of the cat 5 hurricane Irma and, to a lesser extent Jose, generated an amount of debris estimated up to 60 000 metric tons only for the French part of the island of St. Martin, i.e. 1.7 metric ton per capita. The waste collection and treatment represented a huge challenge. The waste collection took four months whereas the full process (treatment and recycling) was still going on in November 2018. Operations have been hampered by the cyclone itself (destruction of infrastructure and material resources, lack of laborers…) and by the constraints of insularity (low volume to value, lack of storage areas...). On the other hand, Saint-Martin benefits from a single public operator (the collectivity of Saint-Martin) and a recycling and landfill site called “Écosite” with rising skills in waste valorization despite modest funds. The example of Saint-Martin shows all the difficulties of post-cyclonic debris management and emphasizes the dependence of the production of debris and their treatment to the characteristics of the island. El aumento del volumen de escombros, su costo de recolección y tratamiento por parte de las autoridades locales o empresas, vinculado a una necesidad imperiosa de protección al medio ambiente, implica desafíos cada vez más complejos en territorios expuestos a desastres naturales, tecnológicos o geopolíticos destructivos (conflictos). El paso del ciclón Irma y en menor medida José, generaron alrededor de 60.000 toneladas de escombros en la parte francesa de la isla de Saint-Martin, es decir, 1,7 toneladas por habitante, lo que equivale a tres años de residuos en un contexto normal. La recolección tomó cuatro meses, mientras que el tratamiento y recuperación aún estaban en desarrollo en noviembre del 2018, es decir más de un año después del ciclón. Las operaciones se vieron obstaculizadas por el propio ciclón (destrucción de infraestructura y recursos materiales, escasez de mano de obra) y por las limitaciones del territorio insular (bajos volúmenes de recuperación, falta de espacio de almacenamiento, etc.). Sin embargo, Saint-Martin se benefició, por un lado, de un único operador público (la autoridad local de Saint-Martin) y por otro, de un lugar de almacenamiento y tratamiento llamado Écosite, el cual ha tomado relevancia los últimos diez años en la recuperación de residuos a pesar de sus medios. El caso de Saint-Martin, expresa las dificultades en el tratamiento de escombros posterior a un ciclón y subraya la dependencia de la producción y tratamiento de escombros en el territorio.