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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.

In recent years, stage / velocity radar stations have been installed in France. This type of stations provides measurements of water level and surface velocity at a fixed point of the cross-section. Discharge is calculated as the product of the wetted area and cross-sectional average velocity; respectively obtained using a stage-area scale S(h) established from a bathymetry profile, and a relation between the index velocity measured by the radar (Vi) and the cross-sectional average velocity (Vmean), established with gaugings. This is the «index velocity method ».As discussed through various examples, the index velocity method has advantages over traditional stage-discharge rating curves: - Flood extrapolations are better known: velocity varies more slowly than water discharge with stage and the stage-area relation can be predicted. - Managing bed evolution is easier because the Vmean(Vi) relation changes little and the S(h) relation is easy to reset with a bathymetry survey. - Monitoring the Vi (h) measured relation in real time allows detecting rating changes of the station. The wider use of the method and the development of measurement instruments will allow the future assessment of the uncertainty related to the calculation of water discharge. Perspectives for the bayesian analysis of index velocity ratings are discussed. / Depuis quelques années, des stations radar hauteur-vitesse ont été mises en service en France. Ce type de station fournit une mesure de hauteur d'eau et de vitesse de surface en un point fixe de la section. Le débit est calculé comme le produit de la surface mouillée et de la vitesse débitante. La surface mouillée est obtenue via une relation hauteur-surface S(h) établie à partir d'un profil bathymétrique ; et la vitesse débitante est calculée via une relation entre la vitesse mesurée par radar (Vi) et la vitesse moyenne dans la section (Vmoy), établie à partir de jaugeages. Il s'agit de la «méthode de la vitesse témoin» ou «index velocity method». Comme nous le verrons au travers d'exemples sur différentes stations, la méthode présente des avantages par rapport aux courbes de tarage hauteur-débit : - les extrapolations en crue sont mieux documentées, la vitesse évoluant plus faiblement que le débit avec la hauteur et la relation hauteur-surface étant connue par ailleurs. - la gestion des détarages est plus facile car la relation Vmoy(Vi) évolue peu et la relation S(h) est facile à recaler avec un seul levé bathymétrique. - l'évolution de la relation Vi(h), mesurée en temps réel, permet de détecter les anomalies, signes de détarages potentiels de la station. L'utilisation plus large de la méthode et le développement des instruments de mesure permettra à l'avenir de mieux évaluer l'incertitude liée au calcul du débit. Des perspectives pour l'analyse bayésienne des stations vitesse témoin sont discutées.

A simple rating curve model using two water level measurements is proposed for tidal rivers. It is based on the Manning-Strickler equation adapted for possible negative slopes resulting from a negative discharge (upstream directed). The model calibration can be achieved thanks to two parameters only: the Strickler coefficient and the temporal lag between the measured slope and the slope to be applied at the reference section. The model has been applied to two tidal rivers for which ADCP gaugings have been achieved during two tidal cycles: the Saigon River in Vietnam and the Seine River in France. Results are encouraging for the Saigon River where water level variations remain mild. For the Seine River case, results are not as good due to a sharp variation of the water level and discharge during the flow tide that makes the calibration difficult. Nevertheless, the proposed model appears to be valid for tidal rivers as soon as the discharge temporal gradient during the tide flow remains moderate.

International audience; This article presents the highlights of the Hydrométrie 2017 conference: origin of the participants, main advances on the three themes of the conference (issues related to measurement networks, measurement and methods in rivers and urban networks, qualification and valuation of data, analysis of uncertainty), scientific awarding (best poster, SHF Grand Prize). A Hydrometry section, based on the French Doppler Hydrometry group, has just been created at the SHF.; Cet article présente les éléments marquants du colloque Hydrométrie 2017 : origine des participants, principales avancées sur les trois thèmes de la conférence (enjeux liés aux réseaux de mesure, mesure et méthodes en rivières et réseaux urbains, qualification et valorisation des données, analyses d’incertitude), attribution de prix scientifiques (meilleur poster, grand prix de la SHF). Une section Hydrométrie, basée sur le groupe francophone Doppler Hydrométrie, vient d'être créé à la SHF.

Normalization allows to define a common language among the actors in a particular field, to clarify, to harmonize practices and to define the level of quality, safety and less environmental impact of practices. In France, in the field of hydrometry, the normalization work is carried out by the AFNOR / X10C commission. After several years of inactivity, the commission was recently reactivated two years ago, under the impulse of a few motivated actors. The objective of this paper is to highlight the added value of standardization in the hydrometry activity, through a few concrete examples of standards. The organization of standardization at national and international level may seem complex. A presentation of the different standardization structures is therefore proposed, as well as the process of developing a standard.