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[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).

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.

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.

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.

National audience; La mesure du débit des rivières en crue est très complexe, et les techniques traditionnelles (ADCP, moulinet) sont limitées par les vitesses trop importantes et le transport solide en surface ou en suspension. Depuis 2005, l'équipe métrologie du Cemagref de Lyon travaille avec différents partenaires sur le développement des méthodes de mesure du débit sans contact. Les deux techniques présentées et testées dans cette étude permettent des mesures de vitesse de surface sans contact par analyse de séquences d'images (LSPIV) et par radar. Les chaînes de mesure sans contact présentées dans cette étude offrent de nouvelles solutions opérationnelles pour mesurer les débits, et permettent de réduire l'incertitude considérable qui affecte souvent les hydrogrammes de crue.

International audience; La mesure du débit des torrents et rivières torrentielles est très complexe : en étiage il est difficile de trouver des sections de mesure bien adaptées aux techniques traditionnelles (moulinets, ADCP...) et en crue, ces mêmes techniques sont limitées par les vitesses trop importantes et le transport solide en surface ou en suspension. Depuis 2005, l'équipe métrologie du Cemagref de Lyon étudie les chasses de la chaîne de barrages EDF sur l'Arc en Maurienne. Ces évènements contrôlés et leur suivi sont particulièrement adaptés au développement et à la validation de nouvelles techniques de mesure du débit, pour être ensuite déployées en crue. Les deux techniques présentées et testées dans cette étude sont des mesures de vitesse de surface sans contact par analyse d'image et par radar. A partir des jeux de données de vitesses de surface et de mesures complémentaires (section mouillée, mesures de profils de vitesse), un coefficient de vitesse est déterminé pour évaluer la vitesse moyenne dans la section instrumentée. La vitesse moyenne et la section mouillée nous permettent ensuite de calculer le débit. L'évenement étudié permet de valider ces méthodes de mesure. Ces techniques permettent de réduire l'incertitude considérable qui affecte souvent les hydrogrammes de crue sur les torrents et rivières torrentielles. / Discharge measurement in mountain streams is difficult : during low water it isn't easy to find good gaging sections and during floods traditional methods are limited by high velocities, floating objects and suspended load sediments. Since 2005, the hydrometric team from Cemagref Lyon has been studying an annual dam flush on a mountain stream in the French Alps. This kind of flow controlled events are very interesting for trying and improving new discharge measurements techniques. Techniques presented in this study are non-contact surface velocity measurement solutions : LSPIV, an image based method, and a microwave radar method. Other measurements are needed for discharge computing (bathymetry, velocity profiles), a velocity coefficient is calculated for each site. Mean velocity and surface velocity are linked with this coefficient. The measurement of the wetted area is the last step before computing the flow. Both non-contact methods presented in this paper increase the knowledge of high flows in torrents and could reduce the uncertainty on flood hydrograph.

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).

Depuis une quinzaine d'années, les profileurs de vitesse acoustiques à effet Doppler (aDcp) sont de plus en plus employés par les équipes hydrométriques pour le jaugeage des cours d'eau. Ces appareils offrent la possibilité d'investiguer la bathymétrie et les vitesses d'écoulement tridimensionnelles rapidement, de manière peu intrusive, sur de vastes domaines et pour une large gamme de conditions de terrain. Le principe de fonctionnement, les stratégies de déploiement et de traitement des données mises en oeuvre sont exposés en s'appuyant sur un cas d'étude expérimental : la station hydrométrique "Saint-Georges" sur la Saône à Lyon. / For about fifteen years, the acoustic Doppler current profilers (aDcp) have been increasingly used by hydrometric teams to measure river discharges. Such devices offer opportunities for investigating bed topography and 3-dimensional water velocities: the fast, slightly intrusive measurement procedure can be achieved in large areas, for a wide range of field conditions. This paper reports the principle of operation and the deployment and data post-processing strategies carried out to study an experimental case: the Saint-Georges gauging station (Saône river in Lyon, France).