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Documenting landslide activity over long periods and monitoring standards (sensors, acquisition rates, quality-control) is critical for understanding the landslide forcing factors, develop process-based models, identify the effect of climate change on their behavior, and ultimately define warning thresholds.The French Landslide Observatory (Observatoire Multi-Disciplinaire des Instabilités de Versants) OMIV is the service of the French Research Institute (CNRS) in charge of deploying, acquiring, exploiting and disseminating multi-parametric sensor data over several large landslides in France. OMIV has developed, since more than 15 years, standards in terms of sensor types, using both high-grade and low-cost sensing in order to construct reference and spatially dense monitoring time series. The service provides open access to records of landslide kinematics, landslide micro-seismicity, landslide hydro-meteorology and landslide hydro-geophysics. Combined, these four categories of observations are unique worldwide for long-term landslide observations. OMIV is currently supervisizing the acquisition and dissemination of sensor data on 8 permanent unstable slopes (Avignonet/Harmallière, La Clapière, Séchilienne, Super-Sauze/La Valette, St-Eynard, Pégairolles, Vence, Villerville) and on unstable slopes currently experiencing gravitational crises (La Clape, Viella, Marie-sur-Tinée, Aiguilles). The service is organized around the dissemination of qualified data (in international reference file format) and products for 5 categories of observation (Geodesy, Seismology, Hydrology, Meteorology, Hydrogeophysics). For each categories of observation, specific FAIR data repository and access portals have been developed and automated processing methods have been proposed to meet the needs of the landslide research community. The products being generated are time series of GNSS and total station positions, catalogue of endogeneous landslide micro-seismicity, resistivity tomography datasets, and hydro-meterological parameters).OMIV provides consistent and harmonized landslide monitoring data in order to identify the physical processes that control the landslide dynamics, both for slopes affected by slow-moving slides and cliffs affected by rockfalls, use these datasets to develop and validate landslide deformation/propagation models, extract (from the long-term observations) the patterns that may characterize changes in the landslide dynamics (annual, seasonal, event) and propose possible forerunners. The OMIV observations aim at contributing at identifying the key controlling parameters of different landslide types (e.g. soft/hard rock, cohesion/friction, slip/fracture, localized/diffuse damage) and at monitoring their evolution in time and space (deceleration or acceleration according to the triggering factors, sliding- flowing transition).The objectives are to present the OMIV datasets, sensing standards and automated processing methods that has been developed, both for the science community and for operational partners in charge of landslide risk management (ONF-RTM, BRGM, CEREMA), for some of the monitored landslides. The objectives are also to present the future directions of the service with a focus on the modelling of the landslide processes using both process-based and machine learning approaches.

Remote sensing techniques are now widely spread for the early detection of ground deformation, implementation of warning systems in case of imminent landslide triggering, and medium- and long-term slope instability monitoring. The large breadth of data available to the scientific community, associated with processing techniques improved as the data volume was increasing, has led to noticeable developments in the field of remote sensing data processing, using machine learning algorithms and more particularly deep neural networks. This arsenal of data and techniques is necessary for the present scientific challenges the community of researchers on landslides still have to meet. As landslides can be complex, for risk management and disaster mitigation strategies, it is necessary to have a precise idea of their location, shape, and size to be studied and monitored. The challenge aims to automate landslide detection and mapping, especially through learning methods. Machine learning methods based on Deep Neural Networks have recently been employed for landslide studies and provide promising efficient results for landslide detection [1]. In this study, we propose an original neural network for landslide detection. More precisely, we exploit a fusion network [1] dealing with optical images on the one hand and Digital Elevation Models on the other hand. To improve the results, attention layers [3] (able to stabilize the training and more precise results) as well as mix up techniques [4] (able to generalize more efficiently) are exploited. The model was trained and tested on the open Bijie landslide dataset. Keywords: Remote sensing for landslide monitoring and detection, landslide detection, deep neural networks, attention [1] Ji, S., Yu, D., Shen, C., Li, W., & Xu, Q. (2020). Landslide detection from an open satellite imagery and digital elevation model dataset using attention-boosted convolutional neural networks. Landslides, 17(6), 1337-1352. [2] Song, W., Li, S., Fang, L., & Lu, T. (2018). Hyperspectral image classification with deep feature fusion network. IEEE Transactions on Geoscience and Remote Sensing, 56(6), 3173-3184. [3] Niu, Z., Zhong, G., & Yu, H. (2021). A review on the attention mechanism of deep learning. Neurocomputing, 452, 48-62. [4] Thulasidasan, S., Chennupati, G., Bilmes, J. A., Bhattacharya, T., & Michalak, S. (2019). On mixup training: Improved calibration and predictive uncertainty for deep neural networks. Advances in Neural Information Processing Systems, 32.

In recent years, heavy rainfall leading to floods, landslides and debris-flow hazards have had increasing impacts on communities in Japan, because of climate change and structural immobilism in a changing and ageing society. Decreasing rural population lowers the human vulnerability in mountains, but hazards can still leave the mountain to the plains and sea, potentially carrying drifted-wood. The aim of the paper is to measure the distribution of wood-debris deposits created by the 2017 Asakura disaster and to rethink the distribution and spatial extension of associated disaster-risk zoning. For this purpose, the authors: (1) digitized and measured the distribution of drifted-wood, (2) statistically analyzed its distribution and (3) calculated the potential impact force of individual drifted timber as a minimal value. The results have shown that there is a shortening of the wood debris as they travel downstream and that the geomorphology has an important control over deposition zones. The result of momentum calculation for different stems’ length show spatially differentiated hazard-zones, which limit different disaster-risk potentials. From the present finding, we can state that we (1) need to develop separate strategies for sediments and wood debris (2) and for wood hazards, zonations can be generated depending on the location and the size of the deposited trees that differs spatially in a watershed.

This article presents a review of the use of unmanned aerial vehicles (UAVs) in the context of geohazards. The pluri-disciplinary role of UAVs is outlined in numerous studies associated with mass earth movements, volcanology, flooding events and earthquakes. Scientific advances and innovations of several research teams around the world are presented from pre-events investigations to crisis management. More particularly, we emphasize the actual status of technology, methodologies and different applications that have emerged with the use of UAVs for each domain. It is shown that the deployment of UAVs in the geohazards context has experienced a tremendous increase during the last 10 years, with the development of more and more miniaturized, flexible and reliable systems. The use of such technology (UAV platform, instrumentation, methodologies) is different for each domain, depending on the spatial extent and the time scale of the observed phenomenon, but also on the practical constraints associated with the civil aviation agencies regulations (outside or within urban areas, before or during a crisis…). This paper also highlights the use of recent methodologies associated with semi-automatic/automatic segmentation or deep learning for the processing of important amounts of data provided by UAVs. Finally, although still sparse, the joint use of UAVs and satellite data is progressing and remains a challenge for future studies in the context of geohazards.

International audience

The economic downturn at the start of the 21st Century combined with the rise of new economies have helped propelling the need for cheaper, rapid and yet accurate data acquisition methods, including point-cloud producing technologies. As population is ageing in countries like Japan or Taiwan, and as climate change is bound to increase the frequency and number of landslides (at least until sediment stocks are depleted), the need of rapid and low-manpower data acquisition and processing has become increasingly essential. It is within this framework that the present contribution aims (1) to present research on present technologies for landslide monitoring, as well as emerging systems, and (2) to propose “new” ideas for point-cloud processing and data usage without having to grid or interpolate data. The authors thus use a variety of field locations around the pacific and in France using as a method ALS (Airborne Laser Scanning), TLS (Terrestrial Laser Scanning) and SfM-MVS (Structure from Motion—Multiple View Stereophotogrammetry). Then the authors present recent advances in landslide monitoring technology with the YellowScan UAV-ALS. As the resulting mounting amount of data presents constrains to the data processing steps, the authors also present ideas to progress processing: (1) developing a point-cloud signature to avoid gridding, and (2) a conceptual algorithm to process the point-clouds as vectors between a machine and a reflective object, recording the free space and the objects. In this way catastrophic landslides can be characterized from the signature of the data and the distribution of fill and voids without going through the traditional analysis on a Cartesian grid. The authors have worked on those techniques to provide fast and accurate data for landslides hazards and disaster risk.

Multi-platform remote sensing using space-, airborne and ground-based sensors has become essential tools for landslide assessment and disaster-risk prevention. Over the last 30 years, the multiplicity of Earth Observation satellites mission ensures uninterrupted optical and radar imagery archives. With the popularization of Unmanned Aerial Vehicles, free optical and radar imagery with high revisiting time, ground and aerial possibilities to perform high-resolution 3D point clouds and derived digital elevation models, it can make it difficult to choose the appropriate method for risk assessment. The aim of this paper is to review the mainstream remote-sensing methods commonly employed for landslide assessment, as well as processing. The purpose is to understand how remote-sensing techniques can be useful for landslide hazard detection and monitoring taking into consideration several constraints such as field location or costs of surveys. First we focus on the suitability of terrestrial, aerial and spaceborne systems that have been widely used for landslide assessment to underline their benefits and drawbacks for data acquisition, processing and interpretation. Several examples of application are presented such as Interferometry Synthetic Aperture Radar (InSAR), lasergrammetry, Terrestrial Optical Photogrammetry. Some of these techniques are unsuitable for slow moving landslides, others limited to large areas and others to local investigations. It can be complicated to select the most appropriate system. Today, the key for understanding landslides is the complementarity of methods and the automation of the data processing. All the mentioned approaches can be coupled (from field monitoring to satellite images analysis) to improve risk management, and the real challenge is to improve automatic solution for landslide recognition and monitoring for the implementation of near real-time emergency systems.

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Des investigations géomorphologiques, géologiques et géophysiques ont été réalisées afin de localiser et d'estimer les volumes sédimentaires stockés dans le bassin-versant du Peynin d’une superficie d’environ 15 km², situé dans les Alpes françaises du Sud. Suite à un travail de terrain et de cartographie ayant permis l’identification et la caractérisation des stocks sédimentaires, deux modélisations statistiques des épaisseurs de sédiments sont proposées. La première modélisation repose sur l’estimation du toit supposé du substratum rocheux à l’aide de fonctions polynomiales. La seconde modélisation s’appuie sur les observations de terrain par l’analyse de 46 coupes topographiques transversales et longitudinales couplée à une expertise géologique et géomorphologique des formations superficielles et des couches sous-jacentes. Pour contraindre les deux modèles, des mesures géophysiques (profils de sismique réfraction) réalisées durant l’été 2014 ont été intégrées pour caler et valider les modèles d’épaisseur. Les volumes de sédiments stockés dans le bassin-versant du Peynin ont été estimés à respectivement 0,46 x 10³ m³ (estimation haute) et 0,13 x 10³ m³ (estimation basse). Rapportés à la surface du bassin-versant, ces volumes correspondent à des épaisseurs moyennes de sédiments de l’ordre de ~30 m pour la première méthode et ~9 m pour la seconde. Dans les deux cas, les dépôts d’anciens mouvements de terrain constituent le principal stock sédimentaire (~80 % des sédiments stockés). Les autres stocks notables sont constitués par les dépôts d’éboulements, les tabliers d’éboulis et les cônes torrentiels. Geomorphological, geological and geophysical investigations were carried out to estimate the amount of sediments stored in the Peynin catchment of about 15 km², situated on the Queyras valley, on the Southern French Alps. Sediment storages were identified, characterized and mapped on GIS from extensive fieldwork. Based on these data, two models of sediment thickness are proposed. The first model is based on the modelling of the bedrock roof by using statistical functions (high estimate). The second model is field-based and results from the geological and geomorphological analyses of 46 cross-sections (low estimate). To reduce the margins of error of these estimates, three seismic refraction profiles made during summer 2014 were integrated into the models. Volumes of sediments stored in the Peynin catchment are respectively estimated at 0.46 x 10³.m³ (high estimate) and 0.13 x 10³.m³ (low estimate). They correspond to sediment thickness averages of ~30 m and ~9 m, respectively. In both cases, landslide material is the major sediment storage, representing more than 80% of the sediment volume stored in the catchment. Other notable sediment storages are collapse deposits, talus screes and torrential fans.

1. A trendy concept for old questions 1.1. A recent and spectacular scientific upturn “Old wine in new bottle”, here is Bracken's skepticism (Bracken et al., 2015) regarding the concept of hydro-sedimentary connectivity at the conclusion of a special issue of Earth Surface Processes and Landforms, dealing specifically with connectivity. This skepticism is due to the recent and spectacular infatuation of geomorphologists for this concept, particularly perceptible in the rhythm of publications ...

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The Peynin catchment (15 km2) is prone to catastrophic floods (June 1957 – Recurrence Interval R.I.>100 yr), June 2000 (R.I.-30 yr) with serious damages to infrastructure and buildings located at the outlet. In this paper, PIT tags tracers and Vensim modelling software are used to better assess the sediment delivery unsteadiness, and more specifically to evaluate the respective role of geomorphological processes on sediment supply during flood event. For the last 20 years, our results highlight a significant variability in sediment delivery from a tributary to another one. According to our studies, we suppose that two torrential tributaries of the Peynin river, the Peyronnelle and Three Arbres subcatchments (

Many authors focus on the concept of sediment connectivity to predict the sedimentary signal delivered at catchment outlets. In this framework, the sedimentary signal is seen as an emergent aggregation of local links and interactions. The challenge is then to open black boxes that remain within a sediment cascade, which requires both accurate geomorphic investigations in the field (reconstruction of sequences of geomorphic evolution and description of sediment routes) and the development of tools dedicated to the modeling of sediment cascades. On the basis of study cases in various environmental regimes (high‐energy mountainous environment and agricultural lowland catchments), in this paper, we (a) exhibit some spatial and temporal paradoxes in terms of sediment delivery and (b) develop various modeling procedures to test some hypothesis of interpretations. These modeling approaches explore different components of sediment connectivity at the catchment scale, including graph theory, agent‐based modeling, and differential equations. Each protocol is chosen according to the scientific objective and how the geomorphological system is simplified. Collectively, the results show that connectivity is an efficient conceptual framework with which to predict how a sediment cascade may transmit (or not) a perturbation throughout the system, including local perturbations (local sediment input, removal of a reservoir, etc.) and perturbations due to external‐boundary forces.

Du fait d’un certain nombre de facteurs prédisposants (lithologie schisteuse, forte connectivité versant – chenal), le bassin versant du Haut-Guil (317 km²) est particulièrement sensible au risque de crues torrentielles. Liées à des épisodes météorologiques de haute magnitude/basse fréquence, ces crues entraînent d’importants remaniements de stocks sédimentaires dans les drains élémentaires. Au cours de la crue trentennale de juin 2000, l’un des principaux affluents du Guil, le Peynin (15 km²), a fourni jusqu’à 22 500 m³ de matériaux déplacés par les laves torrentielles. Afin d’améliorer les connaissances sur la dynamique de ces stocks sédimentaires, 320 transpondeurs passifs RFID (PIT tags) ont été implantés dans les galets et blocs du lit du torrent de la Peyronnelle. Depuis deux ans, ces galets et blocs équipés ont fait l’objet d’une surveillance régulière. Le 9 août 2015, un orage estival (14 mm en 3 h) a déclenché des laves torrentielles dans deux sous-bassins du Peynin (Ravin de la Peyronnelle ; Ravin des Trois arbres), mettant en mouvement plus de 90 % des galets et blocs équipés. Les distances maximales parcourues ont dépassé 700 m. Pour cet épisode, un volume total de 9 000 m³ de sédiments a été mobilisé le long du bassin de la Peyronnelle et plus d'un tiers des galets et blocs équipés a rejoint le Peynin. Le taux de retour (~30 %) des galets et blocs équipés ainsi que les distances parcourues mettent en avant la rapide recharge sédimentaire des drains de premier ordre dans les bassins élémentaires. The upper Guil watershed (317 km²) in the Queyras is characterized by a lithology composed of 90‑95% of schists, supplying a high, fine and coarse sediment load to the active channel. The Guil River is subject to high magnitude/low frequency, flash-flood events. Most of the sediment load in the main channel of the Guil River comes from small torrential tributaries, such as the Peynin watershed. Transported by debris flows, the sediment volumes deposited during June 2000 have been estimated up to 12,000 m³ aggraded on the Peyronnelle fan. We intend to highlight the sediment dynamics on small torrential channels and its connection with gravel-bed streams. Thus, we set up 320 PIT tags equipped pebbles and boulders on the Peyronnelle watershed; distances and trajectories of tracers have been monitored since 2013 during summer periods. On August 9, 2015, a thunderstorm occurred in the Peynin watershed and triggered debris flows in the most active sub-watersheds. This event has mobilized 90% of the PIT tags equipped pebbles and boulders installed in the Peyronnelle watershed. On the basis of a 32% return rate, our results indicate a > 700 m maximum distance transfer and the mobilization of 36% of PIT tags equipped pebbles and boulders out of the watershed. Deposited volumes on the torrential fans have been estimated up to 9,000 m³ in the Peynin watershed. These results highlight the pulsating character of sediment fluxes associated with moderate magnitude and frequency events and indicate the strongest functionality of debris-flow dominated channels in the Peynin watershed.

(IF 3.07 [2018]; Q1); International audience; In risk analyses, two components are taken into account: (1) hazard analysis, including susceptibility and temporal occurrence, and (2) consequence analysis, including characterization of elements at risk (EaRs) and their vulnerability. This study focused on characterization of EaRs, in which items are spatially displayed and impacted by natural events. Several methods can assess these EaRs through expert or engineering approaches. Among the expert approaches, the multicriteria method is more flexible and allows integration of a wide range of information in order to characterize and discretize different EaRs. Traditionally, the mapping and criteria accuracy of EaRs is the same at all spatial analysis scales, while the hazard accuracy changes according to the spatial scales. Therefore, we propose an approach based on the selection of different information/criteria among several private or open access multiple geographical databases to adapt the mapping and criteria accuracy of each EaR according to the hazard analysis spatial scale. After harmonizing the different databases and merging them under GIS, a single database per work scale is created through a specific procedure with interoperability of results between scales. Thus, the number of criteria used to describe these EaRs will depend on the scale of work and the spatial scale of the analysis. To develop and test the transposability of this method, three experimental coastal study sites subject to several hazards (multirisk) have been selected in Normandy (France) with error estimations ranging between 10% and 20%. Subsequently, these data can be integrated into risk and multirisk analyses.

International audience; Shallow landslides are among the most frequent natural hazards in the Pays d'Auge plateau (Normandy, France) but no study has yet focused on the functioning of these phenomena at a detailed scale. This study aims to identify the structure and mechanical properties of a representative case study in the region. The main objective is to understand landslide dynamics and behaviour in order to assess triggering conditions and quantify triggering thresholds. The results will help complement the regional landslide hazard mapping based on landslide statistical susceptibility mapping and quantification of triggering thresholds. The landslide morphology and internal structure were identified using a multi-method approach. A morphodynamic map was produced in the field using car-tographic GPS to depict the surface morphology and map the estimated landslide activity. These field measurements were completed by an analysis of all available aerial-photo images from the French National Geographic Institute (IGN) to identify the occurrence dates and possible landslide reactivations. The landslide structure was defined using multiple electrical tomography profiles, boreholes, augerings and penetration tests. Despite the overall low electrical resistivity of the landslide materials (i.e. ± b 80 Ω·m), the electrical profiles showed good agreement with the interpreted structure based on direct observations (augerings and penetration tests). The landslide slip surface, internal morphology and palaeotopography were identified. A finite slope model was used to calculate the landslide safety factor based on the internal structure and geotechnical data. The evolution of this safety factor according to the rainfall and the groundwater levels shows that the landslides are more likely to occur after long episodes of high cumulative precipitations with an important role being played by the preliminary conditions and the rise of the surficial groundwater table level.

In the upper reaches of Alpine valleys, the shaping of active channels appears mostly dependent upon hydroclimatic variability, as illustrated by the upper Guil River. In this valley, recent land-use changes have reduced the channel capacity of the Guil River, and increased the vulnerability of human installations to damage. In the frame of the implementation of a River Scheme and a Flood Prevention Action Program, we try to improve our knowledge of sediment transfers at the origin of disasters. Our analysis was carried after two high magnitude low frequency hydroclimatic events (1957, 2000). It is based on desk and field data, account for active-channel width and area, sinuosity and braiding indexes, engineering structures and their variation in space and time. We show the major role played by the Guil tributaries in supplying debris to zones with important assets. Finally, we discuss about the difficulty to implement efficient management tools at a local scale.

International audience; Landslide analysis is increasingly based on the combination of multiple information sources at different spatial and temporal resolutions, spatial coverage, accuracy and acquired using various airborne and terrestrial platforms and geomorphological, geophysical, and geotechnical methods. Morphological mapping and morphostructure characterization are preliminary steps to analyze the observed and future landslide distributions. The objective of this work is to propose a morpho-structural model of an active landslide complex on the Normandy coast (north-west France) by combining different data sources. The methodology associates field investigation with interpretation of high-resolution topographic and geophysical images. The proposed morpho-structural model highlights the division of the landslide into several morphological features. This division may explain the spatial variability and temporal variability of the slope dynamics.

27 pages; International audience; The coastal slopes of the Pays d'Auge plateau (Calvados) are regularly affected by landslides. The displacements are small (5-10 cm.yr-1) but some important accelerations (with several meters of displacements) have been observed in 1982, 1988, 1995 and 2001. The processes have caused direct damages to infrastructures and to effects on the local and regional economical activity. To analyze the historical and morphological evolution of the slopes, we propose a diachronical method to quantify the displacements and the change of the coastline position since 1808. The analysis is based on numerous historical documents (archives) and spatial geodata (maps, cadastral surveys, aerial photographs, satellite images,) which uncertainties have been quantified.; Les versants littoraux du plateau du Pays d'Auge (Calvados) sont régulièrement affectés par des glissements de terrain. Les déplacements observés sont généralement lents (5-10 cm.an-1) mais certaines années des accélérations importantes ont provoqué des déplacements métriques (1982, 1988, 1995, 2001). Ces phénomènes induisent des dégâts directs aux infrastructures et des effets sur l'économie locale et régionale. Afin de caractériser l'évolution morphologique historique des versants, nous proposons une méthode d'analyse diachronique qui permet de quantifier les déplacements et la mobilité du trait de côte depuis 1808. Elle s'appuie sur l'analyse de documents historiques (archives) et de données géospatiales (cartes, cadastres, photographies aériennes, images satellitaires) dont les incertitudes ont été quantifiées.

The objective of this work is to present a methodology for the near-real time characterization of displacements using permanent GPS stations on landslides. In France, several GPS receivers have been installed on active landslides since a few years. These landslides show very different displacement rates and kinematic regimes. For the monitoring of landslides where the required degree of accuracy is millimetric, GPS has been mainly used for repeated measurements, as a complement to conventional geodetic methods. Permanent monitoring is still not usually performed operationally.

International audience; In recent years the interest for coastal hazards has increased significantly due to extreme events, sometimes related to climate change, that frequently occur in different parts of the world, often inducing high risk situations. This paper outlines the objectives, perspectives and preliminary results of a research project entitled “Coastline at risk: Methods for multi-hazard assessment” which aims at providing useful knowledge for the mitigation of coastal instability. The project is managed by the European Centre on Geomorphological Hazards (Centre Européen sur les Risques Geomorphologiques, CERG) and funded by the EUR-OPA Major Hazards Agreement. Investigations are focused on coastal landslides in three different morpho-climatic European environments: Malta (Mediterranean coastline), Lower Normandy (Channel coastline) and Central Portugal (Atlantic coastline). The research outputs are expected to provide a significant opportunity for scientific discussion based on the comparison of data regarding instability situations in the context of multi-hazards assessment. The latter has until now dealt slightly with the coasts of Malta, Normandy and Portugal on which the investigations will be focused, despite significant risk issues present there, as evidenced from a series of accidents and damages recorded after landslide events. The aims of the project will be pursued through multidisciplinary investigations which foresee geomorphological and engineering-geological approaches. Integrated avant-garde research techniques, both traditional and innovative, will be applied with special reference to mapping, monitoring and modelling of coastal instability phenomena. The final objective is to propose a method for coastal multi-hazard assessment that can be used to face and manage coastal hazards.; Au cours des dernières années, l'intérêt pour les risques côtiers a augmenté de manière significative en raison d'événements extrêmes, parfois liés au changement climatique, qui se produisent fréquemment dans différentes parties du monde, induisant souvent des situations à haut risque. Le présent document décrit les objectifs, les perspectives et les résultats préliminaires d'un projet de recherche intitulé "Littoral en danger: Méthodes d'évaluation multi-dangers" qui vise à fournir des connaissances utiles à l'atténuation de l'instabilité côtière. Le projet est géré par le Centre européen sur les risques géomorphologiques (Centre Européen sur les Risques Geomorphologiques, CERG) et financé par l'Accord sur les risques majeurs EUR-OPA. Les enquêtes sont axées sur les glissements de terrain côtiers en trois morpho-climatiques environnements européens différents: Malte (côte méditerranéenne), Basse-Normandie (côte de la Manche) et le Portugal (Central côte atlantique). Les résultats de la recherche sont censés fournir une occasion importante de discussion scientifique basée sur la comparaison des données concernant les situations d'instabilité dans le contexte de l'évaluation multi-risques. Ce dernier a jusqu'à présent traité légèrement avec les côtes de Malte, de la Normandie et au Portugal où les enquêtes seront axés, en dépit des problèmes importants de risque présents là-bas, comme en témoigne une série d'accidents et de dommages enregistrées après des glissements de terrain. Les objectifs du projet seront poursuivis à travers des enquêtes multidisciplinaires qui prévoient des approches géomorphologiques et géotechniques. Techniques de recherche d'avant-garde intégrés, à la fois traditionnels et innovants, seront appliquées avec une référence particulière à la cartographie, la surveillance et la modélisation des phénomènes d'instabilité côtières. L'objectif final est de proposer une méthode d'évaluation multi-risques côtiers qui peut être utilisé pour faire face et gérer les risques côtiers.

International audience

International audience; Coastal environment with high interaction between nature and societies (land-sea interface) are subject to multi-hazards interaction such as landslides, flood or cliff retreat. These territories are characterized by numerous Element at Risk (EaR) located in valley bottoms, front sea, at the outlets of small dry-watershed or on landslide areas. The aim is to quantify the potential consequences of EaR by integrating multiple hazards interaction at various scale analyses. The experimental study site is located in the margin of the Parisian Basin. It is characterized by alternating sequences of plateaus and valleys between Houlgate and Honfleur (Normandy—France). To identify and quantify these EaR, three scales analyses have been used from medium scale (1:50,000 – 1:25,000), to local scale (1:10,000 – 1:2,000) by using multi-criteria approach. To quantify the EaR, we adopted three steps approach: 1) Definition of an initial index value of EaR according to the rank order weighting without considering the hazards (proximity, location, etc.). This initial value is attributed at three different scales. (a) At medium scale, a global value of EaR is defined by integrating all components of the study site (such as built-up areas, agricultural areas, etc.). (b) At medium scale, the infrastructure value (buildings, roads and lifelines) is defined from physical injuries and both functional and structural components. (c) At local scale, the index value is defined according to the variations of intrinsic structural component of building. 2) Considering environment constraint (hazard location) to define a second index value to EaR. It means that we consider the location of EaR in relation to flood, landslide area or both areas. This new value is computed to the initial index value (step 1) to take into consideration these different hazards. Consequently, it is necessary to firstly define boundaries of the hazard (flood envelope, landslide limits) by GIS-approach. These boundaries are defined according to multiple criteria such as distance to the river, low-lying areas or morphology (alluvial plain) for flood area. Potential landslide areas are defined according to parameters as slope degree, curvature, aspect and historical density of landslides. Multi-hazards areas are the conjunction of both flood and landslide areas (including cliff retreat). 3) The initial value of EaRs is combined with hazards and multi-hazards index value by fuzzy logic system. A series of conditions defined by different operators (such as Max, Min, And, Or, etc.) take into consideration the evolution initial value (step 1) of EaR according to the type of hazard (step 2). Two main results emerged from this study. In first, we have quantified potential consequences at three different spatial scale analyses (from medium to local scale analysis) without considering hazards. This provides a first information of sensitive EaRs on the territory even in case of alteration of hazard areas (climate change). However, this first information should be completed with multi-hazards integration in order to highlighted potential consequences to different hazards. Consequently, this multiple scale analysis by integrating consequences to multiple hazard provide a better assessment of EaRs for the purpose of improve the disaster risk reduction.

National audience

Over the last decade, the use of UAVs in the field of environmental risks has been booming. The interest of this research paper lies in the use of UAVs on different scales applied to hydro-gravity risks in Japan. The data used are derived from post-crisis hydro-gravity UAV flights and produced by SfM-MVS photogrammetry. On a regional scale, landslide mapping and inventory are based on 2 methods: the threshold approach and the object-oriented approach (OBIA) in the eCognition software. On a local scale, the purpuse is to determine the possibilities offered by the UAV in terms of spatial resolution and precision of geographical positioning for the characterisation of hazards. For the regional inventory, the object-oriented method has enabled to obtain a success rate of about 70% against 50% for the thresholding method. The local scale protocol allowed, thanks to the use of 6 control points arranged in a hexagonal manner, to obtain a decimetric positioning accuracy (RMSE XY = 18cm, RMSE Z = 38cm and total RMSE = 43cm). However, there is a threshold effect linked to the surface and spatial configuration of the study site from which the addition of control points does not bring any significant improvement. The techniques and methodologies used can be transposed to different study objects such as flood areas or deposition zones in torrential beds.; Depuis une dizaine d’années, les utilisations du drone dans le domaine des risques environnementaux connaissent un essor fulgurant. L’intérêt de ce mémoire de recherche porte sur les utilisations du drone à différentes échelles appliquées aux risques hydro-gravitaires au Japon. Les données utilisées sont dérivées de vols drones post-crise hydro-gravitaire et produites par photogrammétrie SfM-MVS. À l’échelle régionale, la cartographie et l’inventaire de glissements de terrain sont basés sur 2 méthodes : l’approche par seuillage et l’approche orientée-objet (OBIA) dans le logiciel eCognition. À l’échelle locale l’objectif est de déterminer les possibilités qu’offre le drone en termes de résolution spatiale et de précision du positionnement géographique pour la caractérisation des aléas. Pour l’inventaire régional, la méthode orientée-objet a permis d’obtenir un taux de réussite d’environ 70% contre 50% pour la méthode par seuillage. Le protocole à l’échelle locale a permis grâce à l’utilisation de 6 points de contrôle disposés de manière hexagonale d’obtenir une précision de positionnement décimétrique (RMSE XY = 18cm, RMSE Z = 38cm et RMSE totale = 43cm). Cependant, on observe un effet de seuil lié à la surface et à la configuration spatiale du site d’étude à partir duquel l’ajout de points de contrôle n’apporte pas d’amélioration significative. Les techniques et méthodologies employées peuvent être transposées à différents objets d’étude tels que les zones d’inondations ou bien les zones de dépôt dans les lits torrentiels.

This memory subject is on the importance of vulnerability factors in the analysis of natural risk, more complex than just the natural causes. For this, the vulnerability of the Mayotte territory is used as a case study. Because of the many possible definitions, our work essentially studies the poverty-related factors that make some people more vulnerable than others. Mayotte is really in an unstable socialn political and environmental situation where the main problem is the massive immigration. These marginalized populations are therefore the most exposed to diferent hazards. So it is interesting to analyze the risk management policy that appears to be unsuitable in the territory of Mayotte.; Ce mémoire porte sur l'importance des facteurs de vulnérabilité dans l'analyse des risques naturels, plus complexes que les seules causes naturelles. Pour cela, la vulnérabilité du territoire de Mayotte est utilisée comme étude de cas. De par les nombreuses définitions possibles, notre travail examine essentiellement les facteurs liés à la pauvreté qui conditionnent certaines personnes à être plus vulnérables que d’autres. Mayotte est en effet dans une situation sociale, politique et environnementale instable où le principal problème est la forte immigration. Ces populations marginalisées sont donc les plus exposées aux différents aléas. Il est donc intéressant d'analyser ensuite la politique de gestion des risques qui apparaît comme inadaptée sur le territoire de Mayotte.

The decision to erase the two large dams in Vezins and La Roche-qui-Boit on the Sélune river was formalized in 2017. It led to the implementation of multi-scalar and multi-dimensional monitoring of this operation. This thesis is part of this scientific program and concerns the resilience capacities of this South Manche river flowing into the Bay of Mont-Saint-Michel. The objective is to determine if this river system has the ability to readjust its hydro-morphological conditions upstream of the Vezins dam following the disturbance materialized by its down-leveling. The collection of morphological and geometric data on- site and their processing made it possible to assess the geodynamic score of the Sélune as well as the potential constraints to theoretically possible readjustments. Thus, the consequences of the operation on the re-diversification of aquatic habitats, especially fish habitats, have also been studied. The results of this work show that, the Sélune has relatively limited resilience capacities in accordance to its topographic, geological and hydro-climatic contexts. In addition, its ancient anthropogenization could be the source of hydro morphological blockages and could prevent the return of undisturbed fluvial dynamics and the homogenization of the flow facies. The objectives of restoring the ecological state of the river - set by the Water Framework Directive (WFD) - have therefore not yet been achieved and passive restoration of the hydro system today seems difficult to envisage.; La décision d’effacement des deux grands barrages de Vezins et de la Roche-qui-Boit sur la Sélune a été officialisée en 2017, entraînant la mise en place d’un suivi multiscalaire et multidimensionnel de cette opération. Ce mémoire s’inscrit dans ce programme scientifique et concerne les capacités de résilience de ce fleuve du Sud-Manche se jetant dans la Baie du Mont-Saint-Michel. L’objectif et de déterminer si ce système fluvial possède la faculté de réajuster ses conditions hydromorphologiques en amont du barrage de Vezins suite à la perturbation matérialisée par sondérasement. La collecte des données morphologiques et géométriques sur le terrain et leur traitement ont ainsi permis d’évaluer le score géodynamique de la Sélune ainsi que les contraintes potentielles aux réajustements théoriquement envisageables. Les conséquences de l’opération sur la rediversification des habitats aquatiques, et notamment piscicoles, ont également été étudiées. Les résultats de ce travail montrent que, fidèle à son contexte topographique, géologique et hydro-climatique, la Sélune possède des capacités de résilience relativement limitées. De plus, son anthropisation ancienne pourrait être à l’origine de blocages hydromorphologiques et empêcher le retour d’une dynamique fluviale non perturbée et une homogénéisation des faciès d’écoulement. Les objectifs de restauration de l’état écologique du fleuve, fixés par la Directive Cadre sur l’Eau (DCE), ne sont donc pas encore atteints et une restauration passive de l’hydrosystème paraît aujourd’hui difficilement envisageable.