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River Flow 2020 -10th Conference on Fluvial Hydraulics, DELFT, PAYS-BAS, 07-/07/2020 - 10/07/2020; As a result of climate change, we believe that extreme floods will become more frequent and more intense. The flows through obstacles representing an urbanized floodplain is realised in a laboratory at Re = 4766 and 8580. The purpose of the present study is first to validate a numerical model using Large Eddy Simulation (LES) against experimental data regarding the average longitudinal velocity and the secondary vortices formation. Then, additional data can be obtained and quantified. Therefore the geometrical parameters can be changed and their influence analysed and compared to the literature. A good agreement was found between simulated and experimental vertical profiles of time averaged velocity for the flow through emerged or slightly submerged obstacles. The additional simulations highlight that the flow structure is mostly driven by the transverse aspect ratio using the distance between two adjacent obstacles.

International audience; Urban flood laboratory experiments usually consider that the flow is mostly concentrated in streets and does not enter the building areas. However, urban blocks might modify the flow patterns at local scale due to the inflows or/and outflows through openings such as gates, windows, doors, etc. To investigate the influence of urban block porosity on flow distribution, flow depth and velocity in the surrounding streets, experiments were performed on a physical model. The results show that the urban block porosity does not substantially affect the discharge distribution in the streets located downstream of the urban block. However, flow depth, velocity and flow rate in the surrounding streets are more strongly modified.

International audience; Steady uniform and non-uniform flows through an array of emergent and slightly submerged square cylinders are experimentally investigated with a specific focus on transverse seiche waves induced by vortex shedding. The study is first and foremost aimed at assessing the effect of streamwise flow non-uniformity on seiche waves. Its secondary purpose is to investigate the change in seiche magnitude, when initially emerged cylinders become slightly submerged. Thirdly and lastly, the effect of seiche waves on mean velocities and velocity fluctuations is quantified. The lock-in process between waves and vortex shedding is unaltered by flow non-uniformity and by a change from cylinder emergence to submergence. For non-uniform flows, this results in the coexistence of two differently oscillating transverse waves close to each other. Relative wave amplitude is found to be mainly influenced by relative submergence in the case of submerged cylinders, and by Froude number and oscillation mode in the case of emergent cylinders. Finally, seiche waves modify the streamwise mean velocity, when cylinders are emergent.

International audience; The structure of free-surface flows in a straight compound channel was investigated in a laboratory flume, consisting of a central smooth-bed main channel (MC) and two adjacent rough-surface floodplains (FPs). The experiments covered both uniform and non-uniform flow conditions, with the latter generated by imposing an imbalance in the discharge distribution between MC and FPs at the flume entrance. The non-uniform cases involved transverse currents directed from MC to FPs and vice versa. The focus of the study was on assessing the effects of transverse currents on: (i) transverse shear layer and horizontal Kelvin-Helmholtz-type coherent structures (KHCSs) forming at the interfaces between MC and FPs; (ii) helical secondary currents (SCs) developing across the channel due to topography-induced flow heterogeneity; and (iii) turbulent large- and very-large-scale motions (VLSMs). Transverse currents can entirely displace the shear layer over FP or in MC, but they do not alter the KHCSs to the same degree, resulting in a mismatch between shear layer extent and KHCS length scales. KHCSs emerge once dimensionless velocity shear exceeds a critical value above which KHCS length scales increase with the shear. Three well-established SC cells, which are induced by turbulence anisotropy, are observed in uniform flow and non-uniform flow with transverse currents towards FP. They are replaced by a single cell in the presence of a transverse mean flow towards MC. The spectral signatures of VLSMs are visible at the upstream section of the flume but they quickly disappear along the flow, being suppressed by simultaneous development of KHCSs and SCs.

International audience; The study of flooding in urban environments has some complexity, mainly due to the presence of buildings, among others (e.g., vehicles, street furniture, drainage system, underground transportation system). These buildings and their facilities are grouped into blocks that are normally assumed to be impervious in urban flood studies. However, water enters these blocks through different openings such as e.g., windows, doors, gates, fences, and these lateral flow exchanges between streets and blocks can locally alter the flow pattern. In order to study these exchange processes and their influence on both flow depth and velocity in the streets during flood events, experiments were conducted on a devoted physical model termed MURI (Urban Model for the study of Inundation Risk), at Irstea, Lyon-Villeurbanne, France. The present study focused on two geometric configurations, a straight street flanked by two lateral cavities, and a straight street flanked by one-side opening. These flows were also compared to a quasi-uniform flow (reference flow without openings). The experiments have shown that a detailed description of the building blocks is required if flow depths and velocities (flood hazard) has to be locally estimated.

International audience; The study of flooding in urban environments has some complexity, mainly due to the presence of buildings, among others (e.g., vehicles, street furniture, drainage system, underground transportation system). These buildings and their facilities are grouped into blocks that are normally assumed to be impervious in urban flood studies. However, water enters these blocks through different openings such as e.g., windows, doors, gates, fences, and these lateral flow exchanges between streets and blocks can locally alter the flow pattern. In order to study these exchange processes and their influence on both flow depth and velocity in the streets during flood events, experiments were conducted on a devoted physical model termed MURI (Urban Model for the study of Inundation Risk), at Irstea, Lyon-Villeurbanne, France. The present study focused on two geometric configurations, a straight street flanked by two lateral cavities, and a straight street flanked by one-side opening. These flows were also compared to a quasi-uniform flow (reference flow without openings). The experiments have shown that a detailed description of the building blocks is required if flow depths and velocities (flood hazard) has to be locally estimated.

International audience; This paper investigates the physical and numerical modellings of unsteady overbank flows in a compound open-channel, which consists in a main channel (MC) and one adjacent floodplain (FP). The experiments were performed in an 18 m long and 2 m wide flume. Simulations using the 1D code MAGE coupled with an improved one-dimensional (denoted as 1D+) method termed ISM (Independent Subsections Method, Proust et al. 2009) are compared to the experimental data. The originality of the ISM lies in its solving of the momentum conservation equation in each of the channel sub-sections (MC and FP). The ISM explicitly models the depth-averaged Reynolds stress at the MC/FP interface, and the transverse exchanges of mass and momentum by the mean flow between MC and FP. It also accounts for the upstream discharge distribution between MC and FP. As this method has been validated only for steady non-uniform flows, the present study aims at validating it under unsteady flow conditions. Hydrographs are injected at the flume entrance in the MC and FP, with 100 runs in order to compute ensemble averages of the flow parameters. The ratio of FP discharge to total discharge equals to 7 % at baseflow and 15 % at peakflow, while the ratio of FP flow depth to MC flow depth ranges from 0.14 to 0.30. It was found that the ISM could accurately predict the temporal variations in: (i) the flow depth along the flume; (ii) the depth-averaged streamwise and transverse velocities at the MC/FP interface; and subsequently (iii) the interfacial lateral discharge per unit length.

Steady uniform and non-uniform flows through an array of emergent and slightly submerged square cylinders are experimentally investigated with a specific focus on transverse seiche waves induced by vortex shedding. The study is first and foremost aimed at assessing the effect of streamwise flow non-uniformity on seiche waves. Its secondary purpose is to investigate the change in seiche magnitude, when initially emerged cylinders become slightly submerged. Thirdly and lastly, the effect of seiche waves on mean velocities and velocity fluctuations is quantified. The lock-in process between waves and vortex shedding is unaltered by flow non-uniformity and by a change from cylinder emergence to submergence. For non-uniform flows, this results in the co-existence of two differently oscillating transverse waves close to each other. Relative wave amplitude is found to be mainly influenced by relative submergence in the case of submerged cylinders, and by Froude number and oscillation mode in the case of emergent cylinders. Finally, seiche waves modify the streamwise mean velocity, when cylinders are emergent.

International audience; The simulation of open-channel flows through emergent rigid vegetation is carried out, using the open-source TELEMAC-2D system. The aim of this study is to find an optimal representation of the flow resistance caused by emergent rigid vegetation for a wide range of flow rates in absence of calibration data for the highest flows. Two strategies of flow resistance representation are compared: (1) the vegetation, in addition to the bed friction, is implicitly considered in a flow resistance coefficient; or (2) the vegetation is explicitly considered by being excluded from the computation domain. Results emphasize that the explicit representation is the more accurate solution for computing both flow depth and velocity, but field data are generally not available. The uncertainty observed on the water depth with the implicit representation is higher than for the explicit representation, but it can be reduced by using more evolved resistance coefficient.

International audience; no abstract

International audience; Convergent hydraulic structures are found in different open-channel flow configurations, particularly in the courier of the surface spillway and in some stilling basins. In some cases, the convergent transition is followed by a prismatic part with the same slope and steps with a different slope. The work that is presented here focuses on the physical modeling of this flow configuration type under steady flow conditions. It has been performed in a laboratory flume, located in the hydraulic laboratory of the National Polytechnic School of Algiers. As far as we are concerned, there has not been any systematic scientific research for the conception of this configuration type so far. In particular, the slope change effect between the convergent transition and the stepped spillway has not been explored yet. According to the slope change and the flowrate, we are interested in the transverse wave propagation that arises at the convergent downhill and the maximum height of the transverse wave, We have measured water heights and instantaneous velocities (transverse and longitudinal profiles) by varying the flowrate from 2 to 20 l/s, the main channel slope (convergent transition and prismatic part) from 0° to 24.4° and the stairs slope from 0° to 45°.

3rd European Conference on Flood Risk Management, Lyon, FRA, 17-/10/2016 - 21/10/2016; International audience; Flood hazards (flow depth and velocity) must be accurately assessed in high-risk areas during extreme flood events. However, the prediction of the very high flows is not an easy task due to the lack of field data and to the strong link between flow resistance and the land occupation of the floodplain. Confinement and inhomogeneity in lateral and longitudinal directions of hydraulic roughness strongly vary with return period T. The physical processes are complex, some still largely unexplored, and the assumptions linked to numerical modelling cannot be validated without field data. The FlowRes project (2015-2018), funded by the French National Research Agency (ANR), aims at improving the flood hazard assessment in floodplains in: 1) investigating in laboratory the hydrodynamic structure associated with extreme flood flows for various land occupations and flow discharge magnitudes; 2) assessing if the existing numerical modelling practices used for T ~ 100 years are still valid for extreme events with T > 1000 years, relying on the experimental

My research works focus on the physical and numerical modeling of river floods, in the case of streamwise non-uniform overbank flows in compound open channels. These flows are characterized by longitudinal changes in flow depth and by a lateral time-averaged discharge between the main channel of the river and the adjacent floodplains. The longitudinal non-uniformity of flow is mostly caused by the changes in topography and land occupation of floodplains. Non-uniformity can also be related to the influence of the boundary conditions associated with the reach under consideration. These various sources of non-uniformity have been investigated throughout this work. The physical processes induced by flow non-uniformity have been studied in laboratory flumes. The analysis is carried out at the reach scale, but also at a smaller scale, with a three dimensional description of the mean flow and turbulent quantities, especially in the mixing layer developing at the junction between the main channel and the floodplains. The numerical modeling work is focused on the approaches 1D, 2D and 1D+ (development of a new method). In particular, we assess their ability to take into account the physical processes that are specific to non-uniforms flows, and eventually, to predict the flow depth and velocity in the floodplains. This is done for small overbank flows, but also for very high flows associated with extreme flood events.; Mes travaux de recherche portent sur la modélisation physique et numérique du débordement des rivières en régime non-uniforme. Ce régime est caractérisé par des variations longitudinales de hauteur d'eau, et par un débit d'échange latéral entre le lit mineur de la rivière et les plaines d'inondations adjacentes. La non-uniformité de l'écoulement est principalement due aux variations de topographie et d'occupation du sol des plaines d'inondation, mais également à l'influence des conditions limites du bief de rivière considéré. Ces différentes sources de non-uniformité ont été explorées tout au long de ces travaux. Les processus physiques induits par la non-uniformité de l'écoulement ont été étudiés dans des canaux de laboratoire. L'analyse est faite à l'échelle du bief, mais également à une échelle plus fine (description tridimensionnelle du champ de vitesse moyenne et des quantités turbulentes, notamment dans la couche de mélange entre lit mineur et lit majeur). Le travail de modélisation numérique porte sur les approches numériques 1D, 2D et 1D+ (développement d'une nouvelle méthode). En particulier, on évalue leurs capacités à prendre en compte les phénomènes physiques spécifiques aux écoulements non-uniformes, et in fine, à simuler hauteur et vitesse de l'écoulement en plaine d'inondation, pour des faibles débordements, comme pour des crues extrêmes.

When dealing with flood issues, compound channels flows are often encountered in the field. This paper investigates the errors that can be expected when using 2-D modelling for compound channel flows, comparing the simulations with experiments. Three flow configurations are analyzed: uniform, gradually varied and rapidly varied flows. The last configuration is obtained by setting a transverse embankment on the flood plain. Errors are estimated on the sub-section mean velocity, discharge andwater depth, on the mixing layer width and on the depth-averaged stream-wise velocity and lateral shear stress. Depending on the flow configuration and on the studied parameter, relative errors significantly vary from nearly zero to 50%. The influence of the 0th order turbulence closure on the mean flow and the dimensions of the recirculation zone behind the embankment is also investigated, using either constant eddy viscosity or Elder’s model., Pour le cas des inondations, les écoulements en canal composé sont fréquents. Cet article recherche les erreurs faites lors d'une modélisation 2D en comparant expériences en laboratoire et résultats numériques. Trois configurations sont analysées : régimes uniforme, graduellement varié et fortement varié. les erreurs sont estimées sur les variables hydrauliques de base. les erreurs peuvent aller jusqu'à 50%. L'effet du terme de fermeture turbulente (coefficient constant ou modèle de Elder) est analysé sur les dimensions de la recirculation et l'écoulement moyen.

International audience; Under uniform flow conditions, compound channel flows are characterised by an horizontal mixing layer developing at the junction between the flows in the main channel (deeper and faster) and in the floodplain (shallower and slower), e.g. Shiono & Knight (1991). Maintaining the total discharge corresponding to the uniform flow and imposing a disequilibrium in the upstream discharge distribution generates a non-uniform flow where lateral mass exchange occurs between channels along the flume. Two cases are distinguished: an excess and a deficit in the inlet floodplain discharge. The over- and under-feeding of the floodplain reduces and increases, respectively, the difference in velocity between both channels at the inlet, compared to uniform flow conditions. This paper investigates the effect of the magnitude of the upstream disequilibrium on the turbulent shear layer. The influence of lateral mass exchange on three-dimensional velocity field and on Reynolds stresses is assessed.

International audience; In May 2008, a 10-year return period flood occurred in the Arc River, France, with a maximum discharge over 500 m3/s. During this event, large standing waves (approximately 1 to 2 meter high and 5 to 10 meter long) were observed at several locations in the river and for a given discharge range (approximately 250 m3/s, for both rising and falling flood limbs). These standing waves appeared to be unstable (lasted 15 to 30 min), and induced large bedforms located at the edge of a gravel bar. These bedforms form typically an antidune system. Two other assumptions were proposed in order to explain the formation of these standing waves in a compound channel flow. First, the waves may be the consequence of the formation of large eddies in the interfacial zone between the main channel and the floodplain. Secondly, the waves may result from an instability due to the juxtaposition of a supercritical flow in the floodplain and of a subcritical flow in the main channel, which induces a small surge at the interface. Finally, a discussion is provided on the consequences of these standing waves on the bed morphology. First, these standing waves are directly associated with bedforms observed later on in the side of a gravel bar. Also, the location of transverse channels on the gravel bar appeared to be strongly influenced by the location of these bedforms during the ebb flow.

International audience; There has been recently increasing concern about the uniformity of flow at measuring sections in straight compound channel experiments. Experiments have been usually carried out with the same total head at the inlet of the main channel and floodplain and as a result the velocities entering into the main channel and onto the floodplain are also the same. As flow enters onto the floodplain, the floodplain discharge exceeds the discharge for a uniform flow condition, hence mass transfer towards the main channel progressively occurs along the flume until flow becomes uniform for both main channel and floodplain. This paper investigates the influence of relative depth on the length required for a uniform flow condition to be achieved in compound channel flumes. The quasi one-dimensional model, the Independent Sub-Sections Method (ISM), the two-dimensional model Telemac 2D and a three-dimensional numerical model have been used to simulate the experiments conducted at LMFA (France) and at UCL (Belgium). The ISM has been subsequently used to investigate more upstream conditions at the LMFA and their resulting flow developments in relation to the flume length. The results show that as the relative depth increases, the length required for uniform flow condition increases and can even exceed the actual length of some of the experimental flumes studied in literature.

International audience; This paper investigates flow structures in a relatively straight compound channel of a river for overbank flows based on field measurements. Field measurements of instantaneous velocity were carried out using Acoustic Doppler Current Profiler (ADCP) at two cross sections of the River Rhône, at Pierre-Bénite near Lyon in France, for two flood events of 1600m3/s and 2380m3/s in April 2006. The study reach can be approximated as a straight trapezoidal compound channel, whose floodplain is vegetated with a relatively uniform density of mature trees. With the ADCP velocity data and bathymetry, the data analyses were carried out to estimate bed shear stress using the log-law and to identify secondary flows using the vorticity equation. Higher secondary currents and vorticity values are observed near the banks, especially on the side slope, which is similar to laboratory experimental results of the compound channel with vegetation in the literature. Since the flows were relatively steady and uniform in straight compound channel, a modified Shiono and Knight Method (SKM) was applied to identify the significance of bed friction, weight component, transverse Reynolds stress, secondary flow and drag force in two flood events. The drag force was included as an additional momentum sink term in the Navier-Stokes equation to account for the vegetation on the floodplain. The relative weights of the different terms in the modified SKM and their importance for the analyzed flood events are assessed.; L'article étudie les structures d'écoulement dans une section de rivière en forme de lit composé en se basant sur des mesures de terrain. Les mesures de vitesse ont été effectuées avec un ADCP dans deux sections du Rhône à Pierre Bénite près de Lyon pour deux crues de débit de 1600 et 2380 m3/s en avril 2006. Le bief peut être considéré comme trapézoïdal jusqu'au débordement où le lit majeur est végétalisé avec une densité quasiment uniforme d'arbres. L'analyse des données a permis d'estimer le cisaillement au fond en utilisant un profil de vitesses logarithmique et les courants secondaires ont été mis en évidence en utilisant la vorticité. Les plus fortes valeurs de courants secondaires et de vorticité sont observées près des berges sur la partie en pente, ce qui est similaire aux résultats en laboratoire. Puisque l'écoulement est presque uniforme, la méthode de Shiono et Knight (SKM) est utilisée pour identifier l'importance du frottement, du poids, du cisaillement de Reynolds transversal et des écoulements secondaires pendant les deux crues. Le modèle inclut la traînée comme source de moment additionnel dans l'équation de Navier Stokes en raison de la végétation.

International audience; This paper focuses on both physical and numerical modeling of flow in the vicinity of a groyne, blocking off the floodplain of an asymmetric compound channel, and perpendicular to the main flow direction. Experiments were conducted in a laboratory flume and in a small-scale model. Various lengths of groyne d were used in both the laboratory flume and the small-scale model. Water levels, ve-locity field and spreading of the recirculation zone behind the obstacle were measured for five flow configurations. The influence on the flow parameters of aspect ratio d/B (where B is the total width of the channel), of total discharge Q and of relative depth ratio hr were examined. Particularly, a hydraulic disjunction may occur between the subcritical flow and the supercritical flow respectively located up-stream and immediately downstream the obstacle, depending on the values of discharge Q and aspect ratio d/B. Flows were simulated with the 2D-H program RUBAR 20 (Cemagref). Two simple turbu-lence closures that are often used by engineers were tested: a constant eddy viscosity and the Elder's model u*, h. Simulations are compared to experimental measurements, focusing on the streamwise profiles of water level and discharge distribution across the channel, and on the spreading of the recir-culation zone. The influence of the turbulence model on flow parameters is then investigated.

International audience; Recirculation zones are of practical importance in natural streams. They create slack flows where silting can occur. Their boundary replaces the actual geometry of the channel and modifies the discharge distribution in the cross-sections. It is thus important to know the size and length of the recirculation zones. Studies reported in the literature show that the behaviour of a recirculation downstream an expansion is affected by the so-called confinement effect which compares vertical and transverse scales of the open channel flows. Literature showed that it can be accounted for by a dimensionless bed friction number, defining two asymptotic regimes: "shallow" or "frictional" and "deep" or "non-frictional". The present work studies experimentally the recirculation length for other experimental conditions, and notably with aspect ratios of the expansion different from 2. The results confirm the influence of the bed friction number but show that other flow parameters can be of primary importance. The Froude number appears to have no action, at least when the flow remains subcritical. Conversely, the aspect ratio of the expansion is shown to have a noticeable influence on the recirculation.

International audience; The aim of this paper is to evaluate the relative weights of three sources of energy loss for non-uniform flows in compound channel: (1) the bed friction; (2) the momentum transfer due to turbulent exchange between the main channel and the floodplains; and (3) the momentum transfer due to mass exchange between the subsections. Energy losses are resolved using a quasi-1D model, namely the Independent Subsections Method (ISM). The ISM computes the water level and the subsection-averaged velocities in the main channel and in the floodplains, simultaneously. This method was tested and validated against experimental measurements in three straight geometries and in eight non-prismatic geometries (skewed floodplains, diverging and converging compound channels). Using the ISM simulations of 46 flows, energy losses due to turbulent exchanges (St), to bed friction (Sf), and to mass exchanges (Sm) are estimated. The relative weights of the dissipation terms at the interfaces between the main channel and the floodplains (St and Sm) and of the mass conservation terms (denoted Ma) are then compared. The results show to what extent the mass conservation and the momentum transfer control the flow depth and the discharge in the floodplain.

International audience; The aim of this paper is to evaluate the relative weights of three sources of energy loss for non-uniform flows in compound channel: (1) the bed friction; (2) the momentum transfer due to turbulent exchange between the main channel and the floodplains; and (3) the momentum transfer due to mass exchange between the subsections. Energy losses are resolved using a quasi-1D model, namely the Independent Subsections Method (ISM). The ISM computes the water level and the subsection-averaged velocities in the main channel and in the floodplains, simultaneously. This method was tested and validated against experimental measurements in three straight geometries and in eight non-prismatic geometries (skewed floodplains, diverging and converging compound channels). Using the ISM simulations of 46 flows, energy losses due to turbulent exchanges (St), to bed friction (Sf), and to mass exchanges (Sm) are estimated. The relative weights of the dissipation terms at the interfaces between the main channel and the floodplains (St and Sm) and of the mass conservation terms (denoted Ma) are then compared. The results show to what extent the mass conservation and the momentum transfer control the flow depth and the discharge in the floodplain.

International audience; This paper focuses on both physical and numerical modeling of flow in the vicinity of a groyne, blocking off the floodplain of an asymmetric compound channel, and perpendicular to the main flow direction. Experiments were conducted in a laboratory flume and in a small-scale model. Various lengths of groyne d were used in both the laboratory flume and the small-scale model. Water levels, ve-locity field and spreading of the recirculation zone behind the obstacle were measured for five flow configurations. The influence on the flow parameters of aspect ratio d/B (where B is the total width of the channel), of total discharge Q and of relative depth ratio hr were examined. Particularly, a hydraulic disjunction may occur between the subcritical flow and the supercritical flow respectively located up-stream and immediately downstream the obstacle, depending on the values of discharge Q and aspect ratio d/B. Flows were simulated with the 2D-H program RUBAR 20 (Cemagref). Two simple turbu-lence closures that are often used by engineers were tested: a constant eddy viscosity and the Elder's model u*, h. Simulations are compared to experimental measurements, focusing on the streamwise profiles of water level and discharge distribution across the channel, and on the spreading of the recir-culation zone. The influence of the turbulence model on flow parameters is then investigated.

[Departement_IRSTEA]GMA [TR1_IRSTEA]11 - VERSEAU / TRANSFEAU; Hydraulic structures such as groynes or bridge piles in the main bed or buildings in the flood plain influence flows in rivers before and after overflowing. An experiment was performed in a rectangular channel in which a groyne created a recirculating zone that was limited by the downstream boundary. 3D velocity field was measured by LDA (Laser Doppler Anenometry) and water levels were determined using point gauges. Simple 2D codes solving shallow water equations were used to model this experiment; various meshes and downstream boundary conditions were applied. The comparison between the measurements and the results of the various 2D models was constituted by longitudinal profiles of water levels, of longitudinal velocities and of cross velocities as well as a map of the differences in horizontal velocity intensities. The observed differences emphasize the difficulties to simulate the processes (determined from free water surface and velocity field) from an accurate topography using any model. However, as experimental and calculated flow patterns are very similar, corrections could be applied to reach engineering purposes.; Les ouvrages hydrauliques tels que les épis ou les piles de pont dans le lit mineur ou les bâtiments dans le lit majeur influe sur les écoulements en rivière avant et après le débordement. Une expérience a été effectuée dans un canal rectangulaire dans lequel un épi a créé une zone de recirculation qui était limitée par la limite aval. Un champ de vitesses 3D a été mesuré par anémométrie laser (LDA) et les niveaux d'eau étaient déterminés en utilisant des sondes à pointé. Des codes 2D simples résolvant les équations de Saint Venant ont été utilisés pour modéliser cette expérience; plusieurs maillages et conditions aval ont été appliquées. La comparaison entre les mesures et les résultats des divers modèles 2D a été effectuée à partir de profils en long des niveaux d'eau, des vitesses longitudinales et des vitesses transversales ainsi que d'une carte des différences entre modules des vitesses horizontales. Les différences observées soulignent les difficultés à simuler les processus (déterminés par la surface libre et le champ de vitesses) à partir d'une topographie précise par n'importe lequel des modèles. Toutefois, comme les structures d'écoulements expérimentales et calculées sont très semblables, des corrections pourraient être appliquées pour atteindre des buts d'ingénierie.

Experiments in an asymmetric compound channel with a strongly converging floodplain are presented. The results are compared to predictions given by two different 1D programmes. Each programme uses a particular method to account for the momentum transfer that occurs in compound channels, due to the velocity gradient between the main channel and the floodplain. The first one is based on the DEBORD formula, which is an empirical correction of the Divided Channel Method. The other one is based on the Exchange Discharge Model, which computes momentum and discharge exchanges at the interface between the two subsections. In the studied situation, where significant three-dimensional phenomena are detected, both numerical models exhibit some weaknesses. Particularly, it is difficult to get simultaneously correct results for the water depth, the discharge distribution in the compound section, and the subsection mean velocities. The analysis of the momentum balance of the flow provides information on the additional phenomena that exist in the experimental situation. These phenomena should be taken into account in the 1D models when such strong conveyance decrease is considered.

The paper focuses on the use of 2D codes in the context of these large rivers and on this first question of how far such models can be trusted.In the mainframe of a common French-Slovenian project (program PROTEUS), the results of two codes were tested and compared with measurements. The first case was the flow around an obstacle in a laboratory channel. The objective was to test the capacity of a 2-D code to represent the recirculating flow downstream the obstacle and the change in the water levels. The second case was a flood entering the Ptuj Lake in Slovenia in which the representation of the velocity field aimed at foreseeing the location of the deposits., L'article est centré sur l'utilisation de codes 2D dans le contexte de ces grandes rivières et sur la première question de la confiance qu'on peut y porter. Dans le cadre d'un projet commun franco-slovène (programme Proteus), les résultats de deux codes ont été testés et comparé aux mesures. Le premier cas est l'écoulement autour d'un obstacle dans un canal de laboratoire. L'objectif est de tester la capacité d'un code 2D à représenter la recirculation à l'aval de l'obstacle et la modification des niveaux d'eau. Le second cas est une crue entrant dans le réservoir Ptuj en Slovénie dans lequel la représentation du champ de vitesses avait pour objectif la localisation de dépôts.

The modeling of natural streams overflowing is of primary importance when we have to identify flooded areas from predicted discharges, perform flood routing in real time or estimate the impact of a mitigation scheme. To face those modeling needs, a few authors conducted experiments in compound flumes so as to estimate the discharge capacity in that specific geometries, and to elaborate stage-discharge curves and water profile computations. In that context, they also studied the boundary shear stress distribution, specially in the floodplain, indispensable element to understand solid transport phenomenae in natural streams. We have thus chosen to present different tools involved in the hydraulic parameters computation for compound channels flows, so different methods of the bed shear stress distribution description. Our aim is not so to review them, than to elaborate a synthesis which highlights limits and complementarities of the different approaches., La modélisation du débordement des cours d'eau naturels hors de leurs lits mineurs est de première importance lorsque l'on doit identifier les zones d'expansion des crues à débit connu, évaluer l'efficacité d'un schéma d'écrêtement des crues ou estimer leur évolution spatiale et temporelle. Pour répondre à ces besoins de modélisation, de nombreux auteurs ont eu recours à des expériences en canaux composés afin d'estimer la capacité d'écoulement de ces géométries particulières, et d'élaborer des méthodes de calcul de courbes de tarage et de ligne d'eaux spécifiques à ce type de canaux. Dans ce contexte, ils se sont également penchés sur la modélisation de la répartition des contraintes à la paroi, notamment dans la plaine d'inondation, élément indispensable à la compréhension des phénomènes de transport solide en rivière. Nous avons, dès lors, choisi de présenter différents outils de calcul des paramètres hydrauliques d'écoulement en canaux composés, ainsi que différentes méthodes de description de la répartition de la contrainte au fond. Notre objectif n'est pas tant de les " passer en revue " que d'élaborer une synthèse mettant en exergue les limites et les complémentarités des différentes approches.

Mudflows are natural, highly concentrated water-clay-grain mixtures that flow in mountain streams after long or intense rainy periods and may cause considerable damage if they overflow on the alluvial fan. The possiblility of predicting the extent of these flows on the basis of material and flow parameters is examined. The simplest realistic case of a yield stress mudflow moving through a narrow open channel followed by a wide, long plane is considered. It is demonstrated that the unconfined flow of a yield stress fluide over an inclined plane cannot be uniform even in steady state the flow equation on the basis of the long-wave approximation, including an appropriate three-dimensional expression for the constitutive equation, makes it possible to establish a system of equations from which flow characteristics at any point can be deduced. In particular, for a Herschel-Bulkley fluide with a flow index of 1/3 it is found that the lateral extent will increase proportionally to the distance from the channel exit to the power 9/20 and that, in the sheared part, the fluid depth in cross section will have a parabolic distribution. Experiments have been carried out with fine mud suspensions whose rheological behavior is similar to that of natural mudflows. The theory is in fair agreement with experimental data concerning fluide depth distribution but systematically overrestimates lateral extent. This is certainly due to he fact that the assumption of lateral extent much smaller than flow length is not respected in our tests., Les laves torrentielles sont des mélanges très concentrés d'eau, d'argile et de matériaux solides qui s'écoulent dans le torrent et peuvent créer des dommages considerables s'ils debordent sur les cônes de déjection. On étudie la possibilité de prédire l'extension de ces écoulements à partir de leurs caractéristiques. On se limite au cas le plus simplle mais réaliste d'une lave torrentielle avec seuil de contrainte passant d'un chenal étroit à un plan infini non limité latéralement. On démontre que l'écoulemnt sur le plan infini ne peut être uniforme : même en écoulement pemanent, la largeur de l'écoulement augmente indéfiniment à partir de l'exutoire du chenal. En traitant de manière complète l'équation de l'écoulement sur la base de l'onde solitaire, et avec une expression appropié tri-dimensionnelle des équations du comportement rhéologique, on a pu exprimer les caractéristiques de l'écoulement en chaque point. En particulière, avec un fluide Herschel-Bulkley ayant un indice d'écoulement de 1/3, on trouve que l'extension latérale augmente proportionnellement à la distance à l'exutoire à la puissance 9/20 et que dans la partie cisaillée, la hauteur d'écoulement dans une section en travers a une distribution parabolique. On a réalisé des experiences à différents concentrations avec des suspensions de matériaux fins dont le comportement rhéologique est semblable à celui des laves torrentielles naturelles. La théorie est en bon accord avec les résultats experimentaux pour la répartition des hauteurs mais surestime systématiquement l'extesion latérale ( de 30%). Ceci provient sans doute de l'hypothèse faite sur le rapport largeur/longueur de l'étalement, faible, qui est mal respectée dans nos essais.

Flood hazards (flow depth and velocity) must be accurately assessed in high-risk areas during extreme flood events. However, the prediction of the very high flows is not an easy task due to the lack of field data and to the strong link between flow resistance and the land occupation of the floodplain. Confinement and inhomogeneity in lateral and longitudinal directions of hydraulic roughness strongly vary with return period T. The physical processes are complex, some still largely unexplored, and the assumptions linked to numerical modelling cannot be validated without field data. The FlowRes project (2015-2018), funded by the French National Research Agency (ANR), aims at improving the flood hazard assessment in floodplains in: 1) investigating in laboratory the hydrodynamic structure associated with extreme flood flows for various land occupations and flow discharge magnitudes; 2) assessing if the existing numerical modelling practices used for T ~ 100 years are still valid for extreme events with T > 1000 years, relying on the experimental data and on one field case. This paper reports some results obtained during the first year of the project., E3S Web of Conferences, 7, ISSN:2267-1242, 3rd European Conference on Flood Risk Management (FLOODrisk 2016)

International audience; We report here a laboratory study on the effects of bed roughness and emergent rigid veg-etation on the unsteady flows associated with flash floods and extreme flood events. The flow structure of unsteady open-channel flows over rough bed with and without emergent rigid vegetation has been investigated in an 18m long and 3m wide laboratory flume. Steady uniform flows were also studied and served as reference flows. For both steady and unsteady flows, two geometries are tested: (1) uniform bed roughness (uniform dense synthetic grass modelling meadow) along the flume and (2) a uniform staggered distribu-tion of emergent wooden circular cylinders (model of rigid vegetation) set on the bed roughness. Transient flow depths are simultaneously measured at six longitudinal positions using ultra-sonic sensors. Transient velocities are measured at one longitudinal position over the water column using a side looking ADV probe to estimate depth-averaged veloci-ty. In order to compute ensemble averages of the flow parameters, 109 runs of the same hydrograph are carried out repeatedly at the flume entrance. Two consecutive runs are separated by a base flow. The ensemble averages of the measured discharge, flow depths and velocity are found to be converged after 72 runs. For the two geometries: at the peak flow of the hydrograph, the vertical profiles of mean streamwise velocity and horizontal Reynolds shear stress are comparable to the profiles obtained for steady uniform cases of same flow depth; and accelerated and decelerated velocity profiles are obtained during the rising limb and falling limb of the hydrograph, respectively. Reynolds shear stresses are also found to be higher during the rising limb than during the falling limb for a fixed flow depth. The hysteresis in the depth-averaged velocity / flow depth relationships is compa-rable in shape and size for the two geometries highlighting the weak effect of the rigid vegetation compared to the effects of un-stationarity.

International audience; Free-surface shallow mixing layers behind a splitter plate were studied in a flume of rectangular cross-section consisting of a bed and sidewalls made of glass. The focus is on the effects on the emergence of horizontal Kelvin-Helmholtz-type coherent structures (KHCSs) of vertical flow confinement (i.e., flow depth) and dimensionless velocity shear λ = (Ux2 − Ux1)/(Ux2+Ux1), where Ux1 and Ux2 are two flow velocities characterizing the low-speed stream and high-speed stream, respectively. Two flow depths were tested, with the λ-value at the flume entrance varying between 0.1 to 0.8, with a threshold value λt = 0.3 needed for the emergence of KHCSs in compound open-channel flows being inside this range. This paper reports new data that provide a better insight in the interplay between the constraining effect of flow depth, damping effect of three-dimensional bed-induced turbulence, and triggering effect of dimensionless shear on the formation of KHCSs.