Publications

2024

• Experimental study of unsteady flows in urbanised areas
  
  • Beteille Elisa
  , 2024. The failure of hydraulic structures can result in the propagation of flood waves across large domains containing rural and urban areas. Such events would cause rapid flooding with complex flows, leading to catastrophic consequences for downstream populations and facilities, and devastating natural environments and ecosystems. Unfortunately, climate change is expected to increase the probability of extreme weather events, and the rise of intense rainfall presents new challenges for managing hydraulic dams. Additionally, the rapid expansion of urban areas increases communities’ vulnerability to flooding.Therefore, it is crucial to accurately describe these complex phenomena and predict their evolution using numerical models. The validation and calibration of these models are performed using data from laboratory studies, as there are scarce observations and measurements from real-case event. Furthermore, the highly transient nature of these phenomena poses challenges for current metrology, contributing to a lack of precise and comprehensive data on these phenomenons within the scientific community.To advance our understanding of the physical processes involves and validate numerical models, an experimental setup was developed at the EDF Lab Chatou in France. This setup enables the study of dam-break wave propagation through a series of obstacles representing idealised urban areas. The objective is to characterise the flow dynamics and analyse how various urban parameters, such as building spacing and number of streets, influence the flood process. Wave sensors measure water levels upstream, within, and downstream of the urban area. Additionally, a high-speed camera positioned above the flow performs Large-Scale Particle Image Velocimetry (LSPIV) measurements to calculate the instantaneous velocity field of the free surface. Moreover, the interaction between the wave and obstacles is documented using a high-speed camera along the channel and the water elevation in this area is calculated using image analysis measurements. Finally, the global impact of the urban forms on the flow is analysed through energy head calculations downstream of them.Simultaneously, detailed numerical simulations of the observed flows are conducted using the open-source Computational Fluid Dynamics (CFD) solver code_saturne. The initial validation of the model is carried out on simple urban configurations, identifying areas for improvement. Finally, simulations of a selected number of urban forms are performed to extend the experimental dataset. Previously measured punctual tendencies of the energy head are confirmed through profile analysis, and head loss coefficients related to the studied urban parameters are extracted.Overall, the thesis results contribute to a new dataset on unsteady flows in urbanised areas, enhancing understanding of these complex phenomena and supporting the validation of numerical models. Moreover, with improved knowledge of these phenomena, communities can develop safer urban planning strategies and optimise the design of industrial safety measures. A novel method to assess the influence of various urban parameters on the flow using the spatially averaged energy head is proposed. This method could support the calibration of simplified numerical models to account for the effects of urban areas on large-scale flood studies. This work aims to protect both property and people within a changing environmental and societal context.
• Numerical modelling of marine dunes in a shallow shelf sea in an offshore wind farm context
  
  • Durand Noémie
  , 2024. Marine 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.
• Étude hydrodynamique des courses de natation en eau libre
  
  • Bolon Baptiste
  , 2024. Ce travail présente une étude physique des courses de natation en eau libre. Deux grands sujets sont abordés. D'abord, nous étudions les interactions entre nageurs (drafting), dans des configurations de nage à 2 athlètes et en peloton. Grâce à des mesures expérimentales et des calculs numériques, nous sommes capable d'évaluer l'impact de la position relative des nageurs sur la force de traînée qu'ils subissent. Ces résultats nous permettent de proposer une trajectoire optimale de dépassement. Ensuite, nous nous intéressons à l'impact des courants de la Seine sur la stratégie à mener lors de la course des Jeux Olympiques de Paris. Nos travaux montrent qu'il existe une trajectoire optimale le long du parcours, qui permet, en profitant des courants favorables et en évitant les courants les plus défavorables, de minimiser le temps de course d'un nageur. De même, il existe aussi une stratégie optimale de dépense énergétique qui permet à un nageur de minimiser son temps de course.
• Experimental analysis on the influence of urban forms on unsteady urban flooding
  
  • Beteille Elisa
  • Boyaval Sébastien
  • Larrarte Frédérique
  • Demay Eric
  , 2025. Unsteady urban flooding, such as dam-break waves, poses catastrophic risks to downstream populations and results in severe damage. To assess and mitigate these risks, it is essential to forecast the influence of urban forms on flooding severity. In this paper, datasets are provided from reduced-scale physical experiments of transient flow through idealized suburban districts. The experiments are conducted in a rectangular horizontal open channel, where flow conditions are achieved by rapidly opening a gate holding a volume of water. To evaluate the impact of urban forms on flow behavior, we investigated two urban parameters; the number and width of streets in the main direction of the flow. The experiments provide complete water hydrographs for thirteen urban forms. Conductive and acoustic gauges are positioned at different locations to track the wavefront and water depth variation. The results illustrate the impact of the two studied urban parameters on flow variables and provide valuable validation data for computational urban planning models.
• Spatial and temporal analysis of the sand dynamics in alpine rivers for 3D numerical modelling
  
  • Weit A
  • Camenen B
  • Dramais G
  • Bel C
  • Jodeau M
  • Langlais S
  • Lauters F
  , 2024. Hydraulic structures can impact the sediment continuity of the river and can create a sediment imbalance downstream of dams according to the different sediment classes (silt, sand, gravel). For operators, one important issue is the presence of sand (d>63 μm), which can lead to exploitation difficulties, issues for flushing or emptying operations as well as to ecological, in-dustrial or safety issues. It was recognized that the percentage of sand particles that is transported by suspension compared to bedload transport can often prevail. However, today it is still a real challenge to evaluate the spatial and temporal variability of these sand concentrations in gravel-bed rivers since sand dynamics are generally supply limited. Using 3D hydro-sedimentary modelling could help to better understand processes of sand transport in rivers. In this article, we will present results from experimental monitoring that will then be used for 3D numerical modelling (TELEMAC 3D + Gaia) on engineered rivers in the Northern French Alps.
• Wave–structure interaction by a two–way coupling between a fully nonlinear potential flow model and a Navier–Stokes solver
  
  • Landesman Paul
  • Harris Jeffrey
  • Peyrard Christophe
  • Benoit Michel
  Ocean Engineering, Elsevier, 2024, 308, pp.118209. A two-way domain decomposition coupling procedure between a fully nonlinear potential flow model and a Navier–Stokes solver capturing the free surface with a Volume of Fluid method is used to study wave–structure interaction applied to offshore wind turbines. Away from the structure, the large-scale inviscid wave field is modeled by the potential code. Wave generation and absorption in this 3D hybrid model take place in the outer potential domain. The codes exchange data in the region around their common boundaries. Through the two-way coupling, waves propagate in and out of the viscous subdomain, making the hybrid algorithm suitable to study wave diffraction on marine structures, while keeping the viscous subdomain small. Each code uses its own mesh and time step. Subdomains are overlapping, therefore continuity conditions on velocity and free surface have to be verified on two distinct coupling surfaces at any time. Parallel implementation with communications between the models relying on the Message Passing Interface library allows calculations on large spatial and temporal scales. The coupling algorithm is first tested for regular nonlinear waves and then applied to simulate wave loads exerted on a vertical monopile in 3D. Attention is paid to the high-order components of the horizontal force. (10.1016/j.oceaneng.2024.118209)
  DOI : 10.1016/j.oceaneng.2024.118209
• Experimental and numerical modeling of flow and sediment transport in shallow reservoirs
  
  • Chagdali El Mehdi
  , 2024. Shallow reservoirs are widely used in hydraulic engineering, whether for temporary water storage, or as settling basins. Many of these reservoirs have a relatively simple geometric configuration (i.e., rectangular), but exhibit complex flow phenomena (i.e., eddies, recirculation zones, attached or oscillating jets) and even instable flow patterns. This impacts the sediment transport and sedimentation (e.g., deposition rate, location), and thus the operational efficiency of reservoirs. The objective of this thesis is to enhance our understanding of the physical hydro-sedimentary processes in shallow reservoirs through experimentation and numerical modeling. Design of the laboratory setup was inspired by real basins and was carried out based on dimensional analysis and preliminary numerical simulations using the open source two-depth averaged model TELEMAC-2D (2D) and three-dimensional (3D) model TELEMAC-3D. A series of forty-two (42) experiments with clear water and fifteen (15) experiments with inlet sediment injection were conducted. Velocity fields were measured using the LSPIV (Large Scale Particle Image Velocimetry) technique as well as intrusive probes such as the ADV (Acoustic Doppler Velocimeter) and ADVP (Acoustic Doppler Velocimeter Profiler). This later technique allowed precise and advanced evaluation of average velocities and flow turbulent characteristics. Sedimentation within the reservoir was mapped using an echosounder, while the time-evolution of deposit pattern was analyzed through image processing. These experiments helped to clarify the impact of inlet boundary conditions (open channel vs pressurized jet), geometric (short vs long reservoirs), hydraulic and sediment conditions. Using the experimental data, the software code_saturne was used to assess the capability of turbulence models, both low and high Reynolds numbers, to observed replicate stable and unstable flow patterns. Finally, a real-world case was stimulated using TELEMAC-2D coupled with its sediment transport module GAIA.