HAL-Ecole des Ponts ParisTech
Not a member yet
42743 research outputs found
Sort by
Modélisation numérique de la dynamique des dunes marines dans une mer peu profonde dans le contexte d'un parc éolien
No full textMarine dunes, also known as sand waves, are large sedimentary bedforms found on the seabed of shallow shelf seas. They migrate under the combined influence of tidal currents, winds, and waves, moving at rates of up to tens of metres per year. This dynamic behaviour is relevant to human activities, such as renewable energy projects. However, the morphology and dynamics of dunes in open marine environments are not well understood. Most modelling research to date has relied on two-dimensional vertical representations, ignoring lateral variations along the dune crests. Previous studies have largely used simplified hydrometeorological forcing, such as representative tidal currents and winds.The present research aims to extend our knowledge of marine dune dynamics and modelling over time and space scales ranging from days to years and metres to kilometres. It also seeks to advance our understanding of the interactions between marine dunes and offshore wind farm (OWF) foundations. To achieve this, we have developed a three-dimensional (3D) coastal area model using the openTELEMAC system for an application offshore Dunkirk, France. The area, proposed site for a 600 MW offshore wind project, is characterised by large marine dunes.Extensive site-specific surveys have been conducted, providing data to characterise the meteorological forcing, hydrodynamics, sedimentology, and morphodynamics of the area. The region is influenced by strong tidal flows, with near-bed currents up to 0.9 m/s, and winds predominantly from the southwest and, to a lesser extent, from the north. The seabed is mainly composed of reasonably well sorted gravelly sands with median grain sizes between 240 µm and 450 µm. There is little evidence of spatial or seasonal trends in the sediment data. The surveys document the evolution of large marine dunes with heights between 2 m and 3 m, travelling at a net rate of approximately 30 m/year.Our 3D numerical model, validated with the in situ data, was shown to accurately simulate hydrodynamics and dune dynamics. Simplifying assumptions excluded wave effects and focused on bedload transport. Two sediment transport predictors were evaluated in detail: the Soulsby and van Rijn predictor (SvR97), which was noted for its robustness and suitability for long-term modelling, and the Soulsby and Damgaard predictor (SD05), which accurately reflected the morphology of the dune but produced instabilities in the long term. Therefore, the SvR97 predictor was used in our research.To determine the contribution of wind-induced currents compared to tide-induced currents to the dynamics of large marine dunes, a 4-month period was simulated, with and without meteorological effects included in the numerical model. The results show that the migration of marine dunes is closely related to the amplitude and direction of the residual flow, which are influenced by wind conditions. Therefore, meteorological forcing is necessary to accurately represent seabed changes over time. Although deviations in wind direction from the prevailing pattern have the potential to temporarily reverse the general migration trend, they are too sporadic to fundamentally alter the sediment dynamics in the study area.To understand the impact of OWF foundations on the dynamics of large marine dunes, exploratory scenarios with a single monopile were run for a period of 2 years. These scenarios were later extended to a realistic scenario consisting of 46 wind turbines within the proposed Dunkirk OWF footprint. Our results show that monopiles significantly affect their environment, causing dunes to converge towards them along the tidal flow axis, which we linked to the ‘wake effect’. This effect accelerates dune propagation, highlighting the need for careful siting of wind turbines to mitigate impacts on dune fields. These findings bridge the gap between studies at the OWF level and those at the monopile level.Les dunes marines sont de grandes formes sédimentaires présentes au fond des mers épicontinentales peu profondes. Sous l'influence des courants de marée, vents et vagues, elles peuvent migrer jusqu’à des dizaines de mètres par an, et impacter les projets d'énergie renouvelable. Cependant, la morphologie et la dynamique des dunes dans les environnements marins ouverts ne sont pas bien comprises. Les recherches menées jusqu'à présent en modélisation se sont appuyées sur des représentations verticales bidimensionnelles, ignorant les variations latérales le long des crêtes de dunes, et utilisant des forçages hydrométéorologiques simplifiés.La contribution présente vise à étendre notre connaissance de la dynamique et de la modélisation des dunes marines sur des échelles de temps de jours à années et d'espace de mètres à kilomètres, et à améliorer notre compréhension des interactions entre les dunes marines et les fondations de parcs éoliens en mer. Pour ce faire, nous avons développé un modèle tridimensionnel (3D) avec le système openTELEMAC pour une application dans le sud de la mer du Nord, au large de Dunkerque, France. Cette zone, proposée pour un projet éolien en mer de 600 MW, est caractérisée par de grandes dunes marines.Le forçage météorologique, l'hydrodynamique, la sédimentologie et la morphodynamique de la zone ont été caractérisés grâce à des données spécifiques au site. La région est influencée par de forts courants de marée, pouvant atteindre 0,9 m/s près du fond, et des vents provenant principalement du sud-ouest et, dans une moindre mesure, du nord. Les fonds marins sont principalement composés de sables graveleux raisonnablement bien triés, dont la taille médiane des grains se situe entre 240 µm et 450 µm, sans tendances spatiales ou saisonnières. De grandes dunes marines d'une hauteur comprise entre 2 et 3 m se déplacent à une vitesse nette d'environ 30 m/an.Notre modèle numérique 3D, validé avec ces données in situ, simule avec précision l'hydrodynamique et la dynamique des dunes. Les effets des vagues sont exclus. Deux prédicteurs de transport de sédiments ont été évalués en détail : le prédicteur de Soulsby et van Rijn (SvR97), qui a été noté pour sa robustesse et son aptitude à la modélisation à long terme, et le prédicteur de Soulsby et Damgaard (SD05), qui a reflété avec précision la morphologie des dunes mais provoque des instabilités à long terme. C'est donc le prédicteur SvR97 qui a été utilisé dans notre recherche.Afin de déterminer la contribution à la dynamique des dunes marines des courants induits par le vent par rapport aux courants de marées, une période de 4 mois a été simulée, avec et sans effets météorologiques. Les résultats montrent que la migration des dunes est étroitement liée à l'amplitude et à la direction du courant résiduel, qui sont influencées par les conditions de vent. Par conséquent, le forçage météorologique est nécessaire pour représenter avec précision les changements du fond marin au fil du temps. Bien que des déviations de la direction du vent par rapport à la tendance dominante puissent temporairement inverser la direction de migration, elles sont trop sporadiques pour modifier fondamentalement la dynamique sédimentaire de la région.Pour comprendre l'impact des fondations des éoliennes sur la dynamique des dunes marines, des scénarios exploratoires avec un seul monopieu ont été exécutés sur une période de 2 ans. Ces scénarios ont ensuite été étendus à un scénario réaliste comprenant 46 éoliennes dans le parc de Dunkerque. Nos résultats montrent que les monopieux ont un impact significatif sur leur environnement, provoquant la convergence des dunes le long de l'axe des courants de marée, ce que nous avons lié à l'« effet de sillage », qui accélère la propagation des dunes. Une implantation soigneusement étudiée des éoliennes est donc nécessaire pour atténuer les impacts sur les champs de dunes
OpenStreetView-5M: The Many Roads to Global Visual Geolocation
No full textInternational audienceDetermining the location of an image anywhere on Earth is a complex visual task, which makes it particularly relevant for evaluating computer vision algorithms. Yet, the absence of standard, large-scale, open-access datasets with reliably localizable images has limited its potential. To address this issue, we introduce OpenStreetView-5M, a large-scale, open-access dataset comprising over 5.1 million geo-referenced street view images, covering 225 countries and territories. In contrast to existing benchmarks, we enforce a strict train/test separation, allowing us to evaluate the relevance of learned geographical features beyond mere memorization. To demonstrate the utility of our dataset, we conduct an extensive benchmark of various state-of-the-art image encoders, spatial representations, and training strategies. All associated codes and models can be found at https://github.com/gastruc/osv5m
Overview of microbial communities in the surface water of the Seine River to understand their response to climate change and human activities
No full textInternational audienceDiverse microbial communities play a crucial role in maintaining the proper functioning of river ecosystems and are considered important indicators of river water quality. Although the Seine River being heavily impacted by human activities, little is known about the microbial communities in its surface waters. In order to monitor water quality and promote sustainable use, we studied its microbial diversity using high-throughput sequencing technology, and explored their relationships with physico-chemical properties. The dominant phyla identified were Proteobacteria , Actinobacteriota , Bacteriodota , and Cyanobacteria . The presence of the Alphaproteobacteria and Gammaproteobacteria indicates that the Seine River water nutrient profile is mainly determined by the recalcitrant organic compounds present in WWTP effluents. Bacterial diversity showed significant temporal variability with a highly significant difference in bacterial composition between 2020 and 2021, probably due to variations in water flow favoring Cyanobacteria growth. Summer displayed higher microbial activity and abundance than autumn, attributed to temperature and orthophosphate content. Spatial variation in bacterial composition was observed between sites upstream and downstream of Paris, as well as before and after of the Seine Valenton-WWTP, subject to an accumulation phenomenon and impacted by wastewater treatment. Further assessment of emerging contaminants and other pollutants is required to better understand these variations. These results provide a basic understanding of the microbial community in the Seine River, serving as a reference for assessing the impact of implementing new wastewater disinfection techniques in the near future
Central limit theorem over non-linear functionals of empirical measures: beyond the iid setting
No full textInternational audienceThe central limit theorem is, with the strong law of large numbers, one of the two fundamental limit theorems in probability theory. Benjamin Jourdain and Alvin Tse have extended to non-linear functionals of the empirical measure of independent and identically distributed random vectors the central limit theorem which is well known for linear functionals. The main tool permitting this extension is the linear functional derivative, one of the notions of derivation on the Wasserstein space of probability measures that have recently been developed. The purpose of this work is to relax first the equal distribution assumption made by Jourdain and Tse and then the independence property to be able to deal with the successive values of an ergodic Markov chain
Geography versus income: the heterogeneous effects of carbon taxation
No full textDistributive effects of carbon taxation are key for its political acceptability. We introduce geographical heterogeneity into a calibrated dynamic general equilibrium heterogeneous-agent model, where energy is both a consumption good and an intermediate input. We evaluate the aggregate and distributive effects of carbon taxation and obtain three key results. First, the distributive effects of carbon taxation are driven by geography more than income, with rural households suffering larger welfare losses. Second, taxing households’ direct emissions is regressive, while taxing firms’ direct emissions is progressive. Third, we simulate various revenue-recycling policies using targeted transfers. We find that it is possible to reduce emissions and mitigate welfare losses associated with the green transition
Vietnam a model of green development?
No full textPublished in Vietnamese, translated by Hảo Linh.As Vietnam pursues its ambitious goal of transitioning from a middle-income to a higher-income country while simultaneously moving towards a zero-carbon society, it faces a complex set of challenges. This article delves into these challenges, focusing on the technological, infrastructural, financial and collaborative needs of this transition
L’économie face à la nature. De la prédation à la coévolution
No full textInternational audienceÀ l'aube de la sixième crise d'extinction du vivant, provoquée par un modèle économique insoutenable et encore largement aveugle à ses propres dégâts, il y a urgence à transformer radicalement nos façons de produire et de consommer, nos conceptions du monde, nos institutions, voire notre contrat social
Vaccinating according to the maximal endemic equilibrium achieves herd immunity
No full textarXiv admin note: substantial text overlap with arXiv:2103.10330We consider the simple epidemiological SIS model for a general heterogeneous population introduced by Lajmanovich and Yorke (1976) in finite dimension, and its infinite dimensional generalization we introduced in previous works. In this model the basic reproducing number is given by the spectral radius of an integral operator. If , then there exists a maximal endemic equilibrium. In this very general heterogeneous SIS model, we prove that vaccinating according to the profile of this maximal endemic equilibrium ensures herd immunity. Moreover, this vaccination strategy is critical: the resulting effective reproduction number is exactly equal to one. As an application, we estimate that if in an age-structured community with mixing rates fitted to social activity, applying this strategy would require approximately 29% less vaccine doses than the strategy which consists in vaccinating uniformly a proportion of the population. From a dynamical systems point of view, we prove that the non-maximality of an equilibrium is equivalent to its linear instability in the original dynamics, and to the linear instability of the disease-free state in the modified dynamics where we vaccinate according to
Analyse multi-échelle de l’accessibilité aux aménités urbaines : le cas de l’agglomération Rabat-Salé-Témara au Maroc
No full textInternational audienceAs part of current thinking on amenity accessibility in academic and operational circles, this study focuses on an application of the Fractalopolis multiscale analysis software to the territory of the Rabat-Salé-Témara conurbation in Morocco. This study area is unique in that it boasts a major rail, tramway and bus network serving the coastline (Zehmed and Jawab, 2021). Despite heavy urbanization, with a great diversity of urban forms inherited from history, the territory has many natural spaces and a green belt to preserve from urban sprawl. In this study, we will focus on proximity to shops, services and green spaces. The region's prime importance within the Moroccan urban fabric has led to a strong growth in its commercial offer, with an increase in the number of shopping malls and supermarkets (Gallouj, 2023). As the Kingdom's capital, Rabat also boasts several high-level administrative and educational services. Added to this is a certain dynamism in tourism, which is conducive to the preservation of a diversified craft industry. All these amenities generate a large number of travel flows and are at the heart of major accessibility issues for the population. Through this analysis, we aim to provide an overall understanding of these development issues. An initial diagnostic phase, based on scientific literature and urban planning documents, will enable us to build the spatial system required for accessibility measurements in the Fractalopolis software (Frankhauser, 2021). In addition to these initial results, we will be proposing a more detailed socio-economic focus through a gendered reading of the situation and an analysis of the tramway network's contribution to the conurbation's accessibility.Dans le cadre des réflexions actuelles sur l’accessibilité aux aménités dans les milieux académiques et opérationnels, cette étude porte sur une application du logiciel d’analyse multi-échelle Fractalopolis au territoire de l’agglomération de Rabat-Salé-Témara au Maroc. Cette zone d’étude a la particularité de disposer d’un important réseau ferré, de tramway et de bus desservant le littoral (Zehmed et Jawab, 2021). Malgré une forte urbanisation, avec une grande diversité de formes urbaines héritées de l’histoire, le territoire dispose de nombreux espaces naturels et d’une ceinture verte à préserver de l’étalement urbain. Pour ce travail, nous nous concentrerons sur la proximité aux commerces, services et espaces verts. De par son importance de premier rang au sein de l’armature urbaine marocaine, la région connait une forte croissance de son offre commerciale, avec une multiplication des centre-commerciaux et de la grande distribution (Gallouj, 2023). En tant que capitale du Royaume, Rabat dispose également de nombreux services de niveaux supérieurs dans les secteurs de l’administration et de l’enseignement. A cela s’ajoute un certain dynamisme touristique, favorable à la sauvegarde d’un tissu artisanal diversifié. L’ensemble de ces aménités est donc générateur de nombreux flux de déplacement et sont au cœur d’importants enjeux d’accessibilité pour la population. A travers cette analyse, nous souhaitons ainsi fournir une compréhension globale de ces problématiques d’aménagement. Une première étape de diagnostic, reposant sur la littérature scientifique et les documents d’urbanisme, nous permettra de construire le système spatial nécessaire aux mesures d’accessibilité du logiciel Fractalopolis (Frankhauser, 2021). En complément de ces premiers résultats, nous proposerons un focus socio-économique plus détaillé à travers une lecture genrée de la situation et une analyse de l’apport du réseau de tramway sur l’accessibilité de l’agglomération. Enfin, cette étude n’aurait pu voir le jour sans l’apparition récente de nombreuses bases de données ouvertes et mondiales, par l’action d’importants acteurs publics et privés. Références : 1) Frankhauser, P., 2021. Fractalopolis : a fractal concept for the sustainable development of metropolitean areas. Complex Systems, Smart Territories and Mobility, Springer International Publishing, pp.15-50. 2) Gallouj, C., 2023. Grande distribution et résilience du petit commerce dans les pays émergents : le cas du Maroc. Recherche en Sciences de Gestion, 157, 41-64. https://doi.org/10.3917/resg.157.0041 3) Zehmed, K., Jawab, F., 2021. The performance of tramway service from the users’ viewpoint : A comparative analysis between two Moroccan cities. Archives of Transport, 60(4), 7
How to choose a good dissertation topic ? In 10 steps
No full textMasterThese slides summarize the key points from a webinar aimed at guiding Master's students through the process of choosing a dissertation topic. They outline a 10-steps approach that interlaces four critical factors: the student's capabilities and interests, the innovative edge of the research, the supervisor's academic expertise, and the availability of financial resources. They further outline a practical approach to developing the dissertation topic within the larger research article production cycle, and discusses essential timelines for both funded research projects and academic Ph.D. funding opportunities for Master's students