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International audience; Transverse mixing coefficients are key parameters for predicting pollutant dispersion in rivers. While most studies deal with river reaches considered homogeneous, the literature review indi-cates that shallow, gravel-bedded rivers with a longitudinally-varied bed morphology are still poorly documented. The motivation of the present work is that the transverse mixing coeffi-cient in rivers with varied hydraulic parameters could be estimated with a succession of coeffi-cients of more homogeneous sub-reaches. This work applies a 1D-diffusion model to estimate these local transverse mixing coefficients from several local injections. Slug injections of Rho-damine WT are performed at various locations of the Durance River, France. Cross-profiles of tracer concentration at selected cross-sections delineating as homogeneous as possible sub-reaches are measured using fluorometers located near the bed. In addition, hydraulic data are derived from ADCP measurements. Best-fitting solutions of the 1D-diffusion model against the measurements suggest substantial variations of the transverse mixing coefficient between con-secutive sub-reaches.

International audience; Il s'agit de la présentation du protocole expérimental ainsi que de quelques résultats de campagnes de traçages réalisées pendant la première année de thèse de Lorris Gond (campagne sur le Drac (mai 2019) et campagne sur la Durance (sept 2019))

[Departement_IRSTEA]Eaux [ADD1_IRSTEA]Hydrosystèmes et risques naturels; International audience; Several research groups have been developing algorithms to calculate river discharges using satellite measurements only. However, the use of ground measurements is useful to greatly reduce the uncertainties. We have developed and tested an original Bayesian method to estimate streamflow at hydrometric stations using satellite measurements of water surface elevation (stage), width and slope as input data. The tests were conducted using an existing set of synthetic data defined in the context of the future SWOT satellite mission for a range of rivers. Typical sets of ground discharge measurements (1-24 gaugings) were created by sampling these synthetic data and adding measurement errors. We found that the reach-averaged cross-sectional geometry of each site can usually be approximated by a single rectangular, trapezoidal, parabolic or triangular shape, or a combination of two shapes for low and high flows. Based on such cross-sectional shapes, simple stage-width-slope-discharge models can be derived based on the Manning equation for fairly steady and uniform flows. A few gaugings in each flow segment of the model are necessary to estimate the parameters and produce discharge records with acceptable uncertainty. Our results show that it is possible to apply methods used for ground hydrometric stations to virtual stations created by satellite remote sensing throughout large river networks. Fewer ground discharge measurements are required because the satellite provides the water surface width and slope in addition to the stage. The proposed Bayesian approach bridges the gap between traditional ground-based hydrometry and river hydraulics remote sensing, and observational strategy can be optimized based on uncertainty quantification. Figure: Estimates of a parameter of the model: the mean elevation h0 of the Sacramento River bed using 2-24 gaugings. Boxplots show 95% probability intervals and the medians. Prior distribution is shown in white (0 gauging).

River Flow 2018: 9th International Conference on Fluvial Hydraulics, Lyon, FRA, 05-/09/2018 - 08/09/2018; International audience; SSC (Suspended Solid Concentration) measurements in rivers are a complex scientific issue. Many questions arise on the spatial and temporal distribution of particles throughout a cross-section, on the properties of particles and grain-size, and also on the sediment transport capacity of streams and rivers. In this study, we focused on the SSC and grain size distribution measured from river samples, automatically or manually acquired. Many agencies suggested slightly different methods for measuring SSC: The European standard NF EN 872, which related to the US EPA 160.2 requires sub-sampling using shake-and-pour aliquot selection. The APHA 2540D requires sub-sampling by pipetting at mid-depth in the original sample shaken with a magnetic stirrer. These methods lead to significant uncertainty when particles larger than 63 µm are present in the samples. The ASTM D3977 analysis method, endorsed by the USGS is more accurate to capture and quantify particles larger than 63 µm. In this study we confirm the sub-sampling problem in a large concentration range using a set of samples from an alpine river.

River Flow 2018: 9th International Conference on Fluvial Hydraulics, Lyon, FRA, 05-/09/2018 -; International audience; There has been a growing interest in the last decade in extracting information on Suspended Sediment Concentration (SSC) from acoustic backscatter in rivers. Quantitative techniques are not yet effective, but acoustic backscatter already provides qualitative information on suspended sediments. In particular, in the common case of a bi-modal sediment size distribution, corrected acoustic backscatter can be used to look for sand particles in suspension and provide spatial information on their distribution throughout a river crosssection. This paper presents a case-study where these techniques have been applied.

International audience; Monitoring hydrophobic contaminants in surface freshwaters requires measuring contaminant concentrations in the particulate fraction (sediment or suspended particulate matter, SPM) of the water column. Particle traps (PTs) have been recently developed to sample SPM as cost-efficient, easy to operate and time-integrative tools. But the representativeness of SPM collected with PTs is not fully understood, notably in terms of grain size distribution and particulate organic carbon (POC) content, which could both skew particulate contaminant concentrations. The aim of this study was to evaluate the representativeness of SPM characteristics (i.e. grain size distribution and POC content) and associated contaminants (i.e. polychlorinated biphenyls, PCBs; mercury, Hg) in samples collected in a large river using PTs for differing hydrological conditions. Samples collected using PTs (n = 74) were compared with samples collected during the same time period by continuous flow centrifugation (CFC). The grain size distribution of PT samples shifted with increasing water discharge: the proportion of very fine silts (2-6 µm) decreased while that of coarse silts (27-74 µm) increased. Regardless of water discharge, POC contents were different likely due to integration by PT of high POC-content phytoplankton blooms or low POC-content flood events. Differences in PCBs and Hg concentrations were usually within the range of analytical uncertainties and could not be related to grain size or POC content shifts. Occasional Hg-enriched inputs may have led to higher Hg concentrations in a few PT samples (n = 4) which highlights the time-integrative capacity of the PTs. The differences of annual Hg and PCB fluxes calculated either from PT samples or CFC samples were generally below 20%. Despite some inherent limitations (e.g. grain size distribution bias), our findings suggest that PT sampling is a valuable technique to assess reliable spatial and temporal trends of particulate contaminants such as PCBs and Hg within a river monitoring network.

Underwater Acoustics Conference and Exhibition 2017, Skiathos, GRC, 03-/09/2017 - 08/09/2017; International audience; Following the success of Acoustic Doppler Current Profiler (ADCP) technology for measuring river discharge, there has been a growing interest in the last decade in extracting information on river suspended sediment fluxes from acoustic backscatter data. Despite the efforts to find a relationship between suspended sediment concentration (SSC) and backscatter, an inversion technique applicable to vertical river backscatter profiles is still missing. The theoretical and empirical bases of such techniques have been originally developed for ocean bottom suspended sediment monitoring. As a first step in our attempt to adapt these models to rivers, we measured the acoustic response of a usual glass beads suspension in a tank. Unsurprisingly, the acoustic model agrees quite well with the measurements as soon as the effect of air micro-bubbles is limited. As a second step, we deployed an acoustic backscatter profiler in the Rhône River (France) in very clear water conditions (SSC < 10 mg/l). The recorded acoustic intensities were stronger than expected at low frequency, suggesting that other scatters may contribute to the recorded echo. Typical river sediment suspensions are expected to produce weak backscatter signal, especially at common ADCP frequencies, due to small particle sizes (Rayleigh regime) and relatively low concentrations. In such conditions, the impact of air micro-bubbles - generally neglected at the bottom of the ocean - could be relevant in rivers. This preliminary work calls for further investigation to assess the potential impact of nonsediment scatterers on acoustic backscatter when trying to measure SSC with sonar technologies in rivers.

HMEM, Durham, New Hampshire, USA, 09-/07/2017 - 12/07/2017; International audience; Stage-discharge rating curves are used to relate streamflow discharge to continuously measured river stage readings to create a continuous record of streamflow discharge. The stage-discharge relationship is estimated and refined using discrete streamflow measurements over time, during which both the discharge and stage are measured. There is uncertainty in the resulting rating curve due to multiple factors including the curve-fitting process, assumptions on the form of the model used, fluvial geomorphology of natural channels, and the approaches used to extrapolate the rating equation beyond available observations. This rating curve uncertainty leads to uncertainty in the streamflow timeseries, and therefore to uncertainty in predictive models that use the streamflow data. Many different methods have been proposed in the literature for estimating rating curve uncertainty, differing in mathematical rigor, in the assumptions made about the component errors, and in the information required to implement the method at any given site. This study describes the results of an international experiment to test and compare streamflow uncertainty estimation methods from 7 research groups across 9 institutions. The methods range from simple LOWESS fits to more complicated Bayesian methods that consider hydraulic principles directly. We evaluate these different methods when applied to three diverse gauging stations using standardized information (channel characteristics, hydrographs, and streamflow measurements). Our results quantify the resultant spread of the stage-discharge Quantifying the uncertainty of discharge measurements (or "gaugings") is a challenge in the hydrometric community. A useful tool to empirically estimate the uncertainty of a gauging method is the field inter-laboratory experiment (Le Coz et al., 2016). Previous inter-laboratory experiments conducted in France (in 2009, 2010, 2011 and 2012) showed that the expanded uncertainty (with a probability level of 95%) of an ADCP gauging made of six successive transects is typically around 5% under optimum site conditions (straight reach, uniform and smooth streambed cross-section, homogeneous flow, etc.) and may be twice higher under poorer site conditions. In practice, the selected cross-section does not always match all quality requirements which may result in larger uncertainty. However, the uncertainty due to site selection is very difficult to estimate with predictive equations. From 9 to 10 November 2016, 50 teams from 8 different countries, using 50 ADCPs simultaneously, conducted more than 600 discharge measurements in steady flow conditions (~14 m3/s released by a dam). 26 cross-sections with various shapes and flow conditions were distributed over 500 meters along the Taurion River at Saint-Priest-de-Taurion, France. A specific experiment protocol, which consisted of circulating every team over half of the cross-sections, was implemented in order to quantify the impact of site selection on the discharge measurement uncertainty. Beyond the description of the experiments, uncertainty estimates are presented. The overall expanded uncertainty of a 6-transect ADCP gaugings (duration around 720 seconds) is estimated to be around 6%.The uncertainty of the discharge measurements varies among the cross-sections. These variations are well correlated to the expert judgment on the cross-section quality made by each team. First results seem to highlight a relation between uncertainty computed for each cross-section and criteria such as flow shallowness and measured discharge ratio. Further investigations are necessary to identify the criteria related to error sources that are possibly meaningful for categorizing measurement conditions and site selection. Moreover, experimental uncertainty and the uncertainty predicted by analytical methods such as QRev, QUant, OURSIN, RiverFlowUA or QMSys software will be compared.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]ARCEAUFLOODrisk 2016, 3rd European Conference on Flood Risk Management, Innovation, Implementation, Integration, Lyon, FRA, 18-/10/2016 - 20/10/2016; International audience; The Mediterranean area is prone to intense rainfall events triggering flash floods, characterized by very short response times that sometimes lead to dramatic consequences in terms of casualties and damages. These events can affect large territories, but their impact may be very local in catchments that are generally ungauged. These events remain difficult to predict and the processes leading to their generation still need to be clarified. The HyMeX initiative (Hydrological Cycle in the Mediterranean Experiment, 2010-2020) aims at increasing our understanding of the water cycle in the Mediterranean basin, in particular in terms of extreme events. In order to better understand processes leading to flash floods, a four-year experiment (2012-2015) was conducted in the Cévennes region (South-East) France as part of the FloodScale project. Both continuous and opportunistic measurements during floods were conducted in two large catchments (Ardèche and Gard rivers) with nested instrumentation from the hillslopes to catchments of about 1, 10, 100 to 1000 km2 covering contrasted geology and land use. Continuous measurements include distributed rainfall, stream water level, discharge, water temperature and conductivity and soil moisture measurements. Opportunistic measurements include surface soil moisture and geochemistry sampling during events and gauging of floods using non-contact methods: portable radars to measure surface water velocity or image sequence analysis using LS-PIV (Large Scale Particle Image Velocimetry). During the period 2012-2014, and in particular during autumn 2014, several intense events affected the catchments and provided very rich data sets. Data collection was complemented with modelling activity aiming at simulating observed processes. The modelling strategy was setup through a wide range of scales, in order to test hypotheses about physical processes at the smallest scales, and aggregated functioning hypothesis at the largest scales. During the project, a focus was also put on the improvement of rainfall fields characterization both in terms of spatial and temporal variability and in terms of uncertainty quantification. Rainfall reanalyses combining radar and rain gauges were developed. Rainfall simulation using a stochastic generator was also performed. Another effort was dedicated to the improvement of discharge estimation during floods and the quantification of streamflow uncertainties using Bayesian techniques. The paper summarizes the main results gained from the observations and the subsequent modelling activity in terms of flash flood process understanding at the various scales. It concludes on how the new acquired knowledge can be used for prevention and management of flash floods.

FloodRisk 2016, Lyon, FRA, 17-/10/2016 - 21/10/2016; International audience; New communication and digital image technologies have enabled the public to produce and share large quantities of flood observations. Valuable hydraulic data such as water levels, flow rates, inundated areas, etc., can be extracted from photos and movies taken by citizens and help improve the analysis and modelling of flood hazard. We introduce recent citizen science initiatives which have been launched independently by research organisations to document floods in some catchments and urban areas of France, Argentina and New Zealand. Key drivers for success appear to be: a clear and simple procedure, suitable tools for data collecting and processing, an efficient communication plan, the support of local stakeholders, and the public awareness of natural hazards.

International audience

International audience; The applicability of a portable, commercially available surface velocity radar (SVR) for noncontact stream gauging was evaluated through a series of field-scale experiments carried out in a variety of sites and deployment conditions. Comparisons with various concurrent techniques showed acceptable agreement with velocity profiles, with larger uncertainties close to the banks. In addition to discharge error sources shared with intrusive velocity-area techniques, SVR discharge estimates are affected by flood-induced changes in the bed profile and by the selection of a depth-averaged to surface velocity ratio, or velocity coefficient (a). Cross-sectional averaged velocity coefficients showed smaller fluctuations and closer agreement with theoretical values than those computed on individual verticals, especially in channels with high relative roughness. Our findings confirm that a 0.85 is a valid default value, with a preferred site-specific calibration to avoid underestimation of discharge in very smooth channels (relative roughness < 0.001) and overestimation in very rough channels (relative roughness>0.05). Theoretically derived and site-calibrated values of a also give accurate SVR-based discharge estimates (within 10%) for low and intermediate roughness flows (relative roughness 0.001 to 0.05). Moreover, discharge uncertainty does not exceed 10% even for a limited number of SVR positions along the cross section (particularly advantageous to gauge unsteady flood flows and very large floods), thereby extending the range of validity of rating curves.

International audience; Movies taken by witnesses of extreme flood events are increasingly available on video sharing websites. They potentially provide highly valuable information on flow velocities and hydraulic processes that can help improve the post-flood determination of discharges in streams and flooded areas. We investigated the troubles and potential of applying the now mature large-scale particle image velocimetry (LSPIV) technique to such flood movies that are recorded under non-ideal conditions. Processing was performed using user-friendly, free software only, such as Fudaa-LSPIV. Typical issues related to the image processing and to the hydrological analysis are illustrated using a selected example of a pulsed flash-flood flow filmed in a mountainous torrent. Simple corrections for lens distortion (fisheye) and limited incoherent camera movement (shake) were successfully applied, and the related errors were reduced to a few percents. Testing the different image resolution levels offered by YouTube showed that the difference in time-averaged longitudinal velocity was less than 5% compared with full resolution. A limited number of GRPs, typically 10, is required, but they must be adequately distributed around the area of interest. The indirect determination of the water level is the main source of uncertainty in the results, usually much more than errors because of the longitudinal slope and waviness of the free-surface of the flow. The image-based method yielded direct discharge estimates of the base flow between pulses, of the pulse waves, and of the time-averaged flow over a movie sequence including a series of five pulses. A comparison with traditional indirect determination methods showed that the critical-depth method may produce significantly biassed results for such a fast, unsteady flow, while the slope-area method seems to be more robust but would overestimate the time-averaged flow rate if applied to the high-water marks of a pulsed flow.

International audience; Streamflow time series provide baseline data for many hydrological investigations. Errors in the data mainly occur through uncertainty in gauging (measurement uncertainty) and uncertainty in the determination of the stage-discharge relationship based on gaugings (rating curve uncertainty). As the velocity-area method is the measurement technique typically used for gaugings, it is fundamental to estimate its level of uncertainty. Different methods are available in the literature (View the MathML sourceISO748,Q+, IVE ), all with their own limitations and drawbacks. Among the terms forming the combined relative uncertainty in measured discharge, the uncertainty component relating to the limited number of verticals often includes a large part of the relative uncertainty. It should therefore be estimated carefully. In View the MathML sourceISO748 standard, proposed values of this uncertainty component only depend on the number of verticals without considering their distribution with respect to the depth and velocity cross-sectional profiles. The Q+Q+ method is sensitive to a user-defined parameter while it is questionable whether the IVE method is applicable to stream-gaugings performed with a limited number of verticals. To address the limitations of existing methods, this paper presents a new methodology, called FLow Analog UnceRtainty Estimation (Flaure), to estimate the uncertainty component relating to the limited number of verticals. High-resolution reference gaugings (with 31 and more verticals) are used to assess the uncertainty component through a statistical analysis. Instead of subsampling purely randomly the verticals of these reference stream-gaugings, a subsampling method is developed in a way that mimicks the behavior of a hydrometric technician. A sampling quality index (SQI) is suggested and appears to be a more explanatory variable than the number of verticals. This index takes into account the spacing between verticals and the variation of unit flow between two verticals. To compute the uncertainty component for any routine gauging, the four most similar gaugings among the reference stream-gaugings dataset are selected using an analog approach, where analogy includes both riverbed shape and flow distribution complexity. This new method was applied to 3185 stream-gaugings with various flow conditions and compared with the other methods (View the MathML sourceISO748, IVE , Q+Q+ with a simple automated parametrization). Results show that Flaure is overall consistent with the Q+Q+ method but not with View the MathML sourceISO748 and IVE methods, which produce clearly overestimated uncertainties for discharge measurements with less than 15 verticals. The Flaure approach therefore appears to be a consistent method. An advantage is the explicit link made between the estimation of cross-sectional interpolation errors and the study of high-resolution reference gaugings.

International audience; While the application of uncertainty propagation methods to hydrometry is still challenging, in situ collaborative interlaboratory experiments are a valuable tool for empirically estimating the uncertainty of streamgauging techniques in given measurement conditions. We propose a simple procedure for organizing such experiments and processing the results according to the authoritative ISO standards related to interlaboratory experiments, which are of common practice in many metrological fields. Beyond the computation and interpretation of the results, some issues are discussed as regards: the estimation of the streamgauging technique bias in the absence of accurate enough discharge references in rivers; the uncertainty of the uncertainty estimates, according to the number of participants and repeated measurements; the criteria related to error sources which are possibly meaningful for categorizing measurement conditions. The interest and limitations of the in situ collaborative interlaboratory experiments are exemplified by an application to the hydro-acoustic profiler (ADCP) streamgauging technique conducted in 2010 at two different sites downstream of Génissiat hydropower plant in the Rhône river, France. Typically, the expanded uncertainty (with a probability level of 95%) of the average discharge over 6 successive transects varied from +/-5% at one site with favourable conditions to +/-9% at the other site due to unstable flow conditions.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]ARCEAU; International audience

International audience

International audience; Several formal mistakes can be found in the research paper published by Le Coz et al. [2012] in Flow Measurement and Instrumentation. Their conclusions on the introduced method are still valid but the following errors may be detrimental to its correct implementation by others.; Plusieurs erreurs formelles peuvent être trouvées dans l'article scientifique publié par Le Coz et al. [2012] dans Flow Measurement and Instrumentation. Leurs conclusions sur la méthode introduite sont toujours valables mais les erreurs suivantes peuvent nuire à sa bonne mise en ½oeuvre par d'autres.

International audience; The objective of this article is to investigate the major issues associated with the calibration of the pollutant dispersion in 1-D hydraulic models applied to river networks, especially large, complex, artificializedones where ecological and socio-economical threats are important. Such issues are illustrated and discussedusing the results of five fluorescent tracer experiments conducted in contrasted open-channel systems,ranging from a simple trapezoidal canal to a more complex river network. Experimental dispersion values were quantified using both the change of moment method and a simple fit-by-eye procedure for eight river reaches with homogeneous hydraulic conditions and an achieved tracer mixing and dispersive equilibrium. Since dispersion coefficient values depend on the assumed dispersion model, ideally they should be calibrated using the same model in which they are to be used, as was done in this study. Wealso derived concurrent longitudinal dispersion values using the velocity field measured by hydro-acoustic profilers (ADCP), which appears as a promising and cost-efficient technique for documenting dispersion in large river systems. It appears that the formulae for which the fit was mainly based on the cross-sectional aspect ratio are generally more appropriate for field data than those which are sensitive to the velocity to shear velocity ratio. The interpretation of complex dispersion and mixing processes, along with the selection of relevant dispersion coefficient predictors are key to minimizing errors in the numerical simulation of pollution dynamics in river networks.

L'objectif du projet GSM Moteur de Synthèse à Allumage Commandéest de concevoir un moteur fonctionnant en mélange pauvre à la fois dépollué et économe. La première phase analyse finement sur moteur monocylindre les interactions entre l'aérodynamique et la combustion qui déterminent l'aptitude au fonctionnement en mélange pauvre ou dilué. La procédure employée présente la particularité d'associer des outils complémentaires tels que code de calcul tridimensionnel, diagnostics optiques (anémométrie laser et ombroscopie) et mesures classiques sur banc monocylindre. La modélisation tridimensionnelle est utilisée comme moyen efficace de sélection et de prédiction de l'aérodynamique interne. Les paramètres les plus influents sur la stabilité de l'initiation de la combustion sont la direction et l'intensité de la vitesse au point d'allumage et le niveau de turbulence. Le meilleur compromis favorable au fonctionnement en mélange pauvre est constitué par une culasse possédant une chambre de combustion en toit avec une seule soupape d'admission. Son aérodynamique interne est caractérisée par la combinaison d'un mouvement de rotation d'axe vertical (swirl) avec un tourbillon d'axe horizontal balayant l'arête du toit (tumble). Son niveau de turbulence est ajusté de manière à accroître la vitesse de combustion en limitant les instabilités cycle-à-cycle. La deuxième phase est consacrée à la transposition de cette solution sur un moteur multicylindre. Les principales difficultés rencontrées sont liées aux disparités de comportement entre cylindres accentuées par le fonctionnement en mélange pauvre. Seul un contrôle individuel des paramètres de combustion de chaque cylindre (avance, injection, richesse) associé à un écartement d'électrodes de bougie accru permet de re-trouver des résultats proches de ceux acquis sur monocylindre. Dans ces conditions, les limites pauvres se situent à des richesses comprises entre 0,60 et 0,70 suivant la charge et le régime. La diminution des émissions d'oxydes d'azote qui en résulte atteint en moyenne sur des points représentatifs du cycle ECE15 74 % par rapport au fonctionnement à la stoechiométrie. Enfin, des essais avec recyclage des gaz d'échappement ont démontré le potentiel de la solution optimisée d'un point de vue aérodynamique à tout mode de fonctionnement en mélange dilué. The constraints that must be accounted for inbuilding a new engine are increasing. It is necessary to contend with user demands for performance, while improving efficiency, limiting pollutant emissions and reducing knock sensitivity. Given the number of parameters to be taken into account, the response to these demands would require large investments in time and resources if conventional development methods (which usually follow an empirical approach) were applied. The purpose of this research supported by Groupement Scientifique Moteur (GSM) was to design a lean-burn sparkignition engine with the object of reducing pollutant emissions and fuel consumption. We try to develop a promising new approach that follows a scientific methodology based in particular on the use of predictive computer codes. The objective of this project is to design an engine that runs on a lean or possibly dilute mixture that will meet European pollution regulations. This lean-burn solution, combined with oxidation catalysis, is considered as an alternative to threeway catalysis, which imposes operation at stoichiometry. The original configuration is a 4-valve engine. One of the advantages this engine offers is great flexibility in changing the inlet conditions. This provides a way of optimizing internal fluid motion, which turns out to be a determining factor in the ability to operate with a lean or dilute mixture. The operation of an engine with a lean or dilute mixture results in substantially reducing nitrogen oxide (NOx) emissions, virtually eliminating carbon dioxide (CO) emissions, and in lower specific fuel consumption. On the other hand, unburnt hydrocarbon (HC) emissions generally increase, which implies the use of an oxidation catalyst if the antipollution standards become too severe. The first phase was to analyze the interactions between fluid dynamics and combustion, which determine the capability of this engine to run with a lean or dilute mixture. The methodology relies on complementary means :(a) Three-dimensional computer code (KIVA). (b) Optical diagnostics (Laser Doppler Velocimetry). (c) Single-cylinder engine equipped with conventional measurement systems. Three dimensional modeling is used to predict and to optimize fluid motion in the cylinder for different intake configurations. The most important parameters influencing the stability of initial combustion are found to be the direction and magnitude of the mean velocity at the spark location, and the turbulence level. We should note that this flow field optimization is also applicable for operation with any dilute mixture (diluted by exhaust gases for example). The question of the minimization of the cyclic variability remains. The most favorable configuration for lean-burn operation was a pent-roof combustion chamber with a single operating intake valve. Fluid motion in this engine is characterized by the combination of a swirling and a tumbling motion and can be described as an inclined tumble. This motion leads to a flow at the spark plug location directed along the edge of the cylinder head. Moreover, the turbulence level is optimal for a high burning rate and low cycleto-cycle instability. The second phase was to apply this solution to a multicylinder system. The main difficulties came from the variability between cylinders, which was amplified during lean-burn operation. Each cylinder must be independently controlled (spark timing, sequential injection, fuel-air ratio, etc. ). Moreover, an increased spark gap is needed in order to reproduce the performance (i. e. efficiency) obtained with the single-cylinder. For these conditions, the minimum fuelair equivalence ratio is between 0. 60 and 0. 70, depending on the engine load and speed. Nitrogen oxide emissions are then reduced from about 74% on the average (compared with stoichiometric emissions) at several selected running conditions representative of the ECE 15 cycle. Finally, the optimized solution was proven to be capable of accommodating any dilute mixture. This was demonstrated via tests using exhaust gases for dilution.

International audience; This paper presents a coupled observation and modelling strategy aiming at improving the understanding of processes triggering flash floods. This strategy is illustrated for the Mediterranean area using two French catchments (Gard and Ardèche) larger than 2000 km2. The approach is based on the monitoring of nested spatial scales: (1) the hillslope scale, where processes influencing the runoff generation and its concentration can be tackled; (2) the small to medium catchment scale (1–100 km2), where the impact of the network structure and of the spatial variability of rainfall, landscape and initial soil moisture can be quantified; (3) the larger scale (100–1000 km2), where the river routing and flooding processes become important. These observations are part of the HyMeX (HYdrological cycle in the Mediterranean EXperiment) enhanced observation period (EOP), which will last 4 years (2012–2015). In terms of hydrological modelling, the objective is to set up regional-scale models, while addressing small and generally ungauged catchments, which represent the scale of interest for flood risk assessment. Topdown and bottom-up approaches are combined and the models are used as “hypothesis testing” tools by coupling model development with data analyses in order to incrementally evaluate the validity of model hypotheses. The paper first presents the rationale behind the experimental set-up and the instrumentation itself. Second, we discuss the associated modelling strategy. Results illustrate the potential of the approach in advancing our understanding of flash flood processes on various scales.

International audience

National audience; Les contaminants hydrophobes sont majoritairement transportés par les matières en suspension (MES). L'évaluation des flux de contaminants particulaires présente donc un intérêt pour la compréhension de la dynamique, du transport et du devenir des PCB et du mercure au sein du réseau hydrographique. Les objectifs de cette étude sont de classifier les principaux affluents en termes d'apports de MES et de contaminants, d'étudier la variabilité spatio-temporelle des flux et d'évaluer les incertitudes associées. Le flux d'un contaminant particulaire à travers une section du fleuve est le produit du débit, de la concentration en MES et de la teneur en contaminant adsorbé sur les particules. Les variations temporelles aussi bien que spatiales de ces trois paramètres doivent être considérées pour calculer des flux annuels avec une incertitude minimale. Les stations de jaugeage sont en générales bien développées et permettent d'enregistrer en continu les débits de la plupart des cours d'eau. En revanche, les données de concentration en MES et en contaminants proviennent souvent de réseaux de suivi de la qualité de l'eau, basés sur des mesures ponctuelles. Un réseau dédié au calcul de flux sur le bassin versant du Haut-Rhône a été développé depuis septembre 2011 et permet d'étudier la dynamique sédimentaire et des contaminants particulaires. La stratégie de suivi repose sur des mesures effectuées en différents points le long du Haut-Rhône et à l'exutoire de ses principaux affluents pendant des crues et les opérations de chasse des barrages. Un suivi à pas de temps fin est également mené à une station située à l'aval du bassin versant pour mesurer les concentrations en MES et les teneurs en contaminants en régime de base et au cours des crues. Premièrement, à partir des enregistrements continus de débit disponible, nous avons déterminé les débits et les flux moyens annuels du Haut Rhône et de ses affluents. Nous avons vérifié que les bilans d'eau annuels entre les entrées et les sorties du réseau sont équilibrés à -6% sur une période de 10 ans. Ces -6% correspondent aux petits bassins affluents qui ne sont pas jaugés. Deuxièmement, nous avons développé des stratégies pour calculer les flux moyens annuels de MES pour chaque affluent, en utilisant les données ponctuelles ou les courtes chroniques de MES disponibles. Trois méthodes pour extrapoler de longues séries temporelles de concentration en MES sont proposées : i) la calibration des enregistrements de turbidité avec des mesures de concentrations en MES, ii) la modélisation en cherchant des relations entre la concentration en MES et le débit et iii) des estimations par analogie avec les bassins-versants voisins ou par analyse spatiale dans le cas d'affluents pour lesquels aucune donnée de concentrations en MES n'est disponible. Ces méthodes permettent d'estimer la contribution moyenne de chaque affluent au flux de MES et d'étudier la dynamique sédimentaire locale à l'échelle d'un événement de crue. Troisièmement, les données de teneurs en contaminants obtenues pendant les périodes de régime de base et de crue montrent des différences significatives de taux de contamination entre les différents affluents. La stabilité ou la variabilité temporelle des teneurs en contaminant à chaque station doit être confirmée pour établir des moyennes robustes permettant la quantification des flux. Le suivi doit être poursuivi, particulièrement pendant les crues. Une perspective de ce travail est l'utilisation d'un modèle numérique pour calculer les flux d'eau, de MES et de contaminants particulaires à travers le réseau hydrographique et pour établir des bilans à l'échelle de l'événement.

International audience; A reservoir flushing was completed on September 2008 in the Longefan reservoir (Arc en Maurienne River, France). This reservoir (approximately 300m×500m) is part of the EDF (French Electricity company) hydropower system. It is mainly used for the sedimentation of the water released from the upstream hydropower plant and for temporary storage for the next plant. LSPIV measurements were completed during the flushing event using three cameras covering the main zones of interest (entrance, main channel and exit). For each movie, biologically degradable chips were injected to produce visible patterns on the surface flow and facilitate the LSPIV processing. Two 2DH hy drodynamic models (Rubar20 and Telemac2D) were applied to the system and their results were compared to the experimental data. Advantages and limits of these models were discussed. In particular, the effects of water level and wind were studied as well as the direction of the jet entering the reservoir.

International audience; The 45 km-long reach of the Rhine River from Kembs to Breisach has been heavily impacted by engineering works during the last two centuries. The Kembs dam and the lateral “Grand Canal d’Alsace” achieved in 1959 induced significant decrease in sediment transport and diversion of most of the flow in the lateral canal so that the by-passed “Old Rhine” runs now a minimum flow (3% of the mean annual discharge during 300 days per year). Two combined approaches were performed to understand the Old Rhine morphological response to “dewatering”, peak flow reduction and sediment transport disruption in particular the vegetation expansion process and its potential feedback impacts on the channel hydraulics and the sediment transport: i) a space-time approach using series of aerial photographs and historical cross-sections and ii) 1D morphodynamic simulations (see the joined communication of Béraud et al.). Extents of water bodies, gravel bars, riparian vegetation (grasslands, shrubs, trees identified using differences in colour, texture and structure) and anthropogenic features were mapped every 10 years since the 1950s. Results show a strong channel narrowing and associated vegetation expansion until 1982. Tests conducted on the different dates underlined homogeneous segments and downstream progression of the processes through time, demonstrating different patterns of adjustments. Cross-sections of 1950 and 1990 were overlaid with land cover layers, demonstrating that incision and sedimentation areas are spatially linked with the planform changes. From these findings, we hypothesised that vegetation encroachment modified hydraulic conditions, enabling sediment transport, winnowing and deposition processes despite of peak flow reduction and armour layer development due to intense incision following channelization works. In order to test these causality hypothesis the mean sediment mobility along the homogeneous segments was assessed using 1D hydromorphodynamical simulations.

International audience

International audience; Passive Integrated Transducers (PIT) tag method to trace gravel particle is increasingly used to estimate gravel movements in mountainous rivers. Reported recovery percentages after one flood are often over 80%, demonstrating the effectiveness of this technique for small mountainous rivers. We used this technique to get detailed spatial information on particle movements on a gravel bar in an alpine river (Arc en Maurienne, France). Six patches have been placed on the gravel bar (three on the bar head in August 2008 and three along the secondary channel in June 2009). During this period, the main events correspond to flushing events of the upstream dams and reservoirs by dam managers, as well as short high flow periods in May 2009 due to snow melt. Only these events have an impact on the gravel bar since it is dry for lower flows. First results with PIT-tag technique indicate a very strong erosion of the head of the bar, especially on the side of the main channel. Two sample plots have been totally eroded and marked particles have moved to the main channel after the first and the second flushing events, respectively. The secondary channel shows lower dynamics. Recovery percentages were approximately 80% after each event until marked gravels reached the main channel through the first connecting cross-channel. Travel distances were independent of the particle diameter (40 to 300mm). The downstream part of the secondary channel and the tail of the gravel bar were not affected by the last flushing event in June 2009. Most of the surface of the downstream part of the gravel bar was dry during this event as a consequence of the May 2008 flood that significantly eroded the main channel. As a first conclusion, the PIT-tag method yields interesting results for the gravel bar dynamics but the investigation domain is limited in space (and time) for our study case. Indeed, we were not able to recover PIT-tag in the main channel and downstream.

International audience; In France, the national stream flow measurement network is ruled by the Ministry of Environment through Regional Environment Agencies (DIREN) and Flood Forecast Services (SPC/SCHAPI). Other organizations such as hydropower producers (EDF, CNR), the waterways office (VNF), research institutes (e.g. IRD in Brazil and Africa) also produce hydrometric data. The main gauging techniques are the velocity-area method (currentmeters), Doppler profilers (aDcp) and chemical dilution. Some efforts are needed to improve and quantify the quality of discharge measurements, and also to diversify the range of measuring techniques available to practitioners. In close connection with hydrometry services, research projects are being conducted on conventional and innovating instruments and techniques for flow measurement. In particular, the following issues are currently investigated: rating curves assessment and extension, uncertainties associated with various gauging methods (especially aDcp), evaluation of horizontal Doppler profilers (H-aDcp), video-based flash-flood monitoring (LSPIV).