Flood frequency estimation in ungauged sites based on simulation : regionalisation of the simulation-based method SHYREG

L’estimation de l’aléa hydrologique en sites non jaugés présente un enjeu important pour la gestion des risques. La complexité du phénomène réside à la fois dans la nécessité d’avoir une approche multivariée (estimation de caractéristiques multiples des crues : durées, périodes de retour) qui propose une extrapolation raisonnable des événements. SHYREG est une méthode basée sur la simulation de scénarios de crues, qui présente ces avantages. Évaluée lors du projet ANR ExtraFlo, elle présente de bonnes performances en justesse et en stabilité lorsqu'elle est calée sur des données locales de débits. Cette méthode vise à estimer des débits de crue en tout point du territoire. Elle doit donc pouvoir être appliquée en site non jaugés.Le travail de thèse présenté ici se focalise sur le transfert de la méthode vers le non jaugé en s’intéressant aux valeurs des débits simulés mais aussi à leur cohérence. Tout d’abord, une révision du calage a permis de s’assurer de la cohérence des débits simulés le long d’un cours d’eau. Ensuite, l’application d’un large panel de méthodes de régionalisation a permis de déterminer que la régionalisation devait s’appuyer à la fois sur la structure spatiale et sur les caractéristiques physiographiques des bassins. Finalement, une méthode qui régionalise SHYREG simultanément à son calage a été retenue. Une comparaison avec d’autres approches régionalisées a mis en évidence la qualité du modèle développé.Flood hazard estimation in ungauged sites presents a major challenge for risk management. The complexity of the phenomenon arises from both the need for a multivariate approach (estimation of different flood characteristics: peak flow, volume, duration ...), and for an approach which offers a reasonable extrapolation of extreme events. The SHYREG method is based on the simulation of flood scenarios and presents these benefits. It has been evaluated during the ANR ExtraFlo project. It showed good performance in both accuracy and stability when calibrated against local discharge data. However, weaknesses have been identified when implemented in ungauged sites.The objective of the present thesis is to develop the method in order to improve the SHYREG performances in ungauged sites. Two kinds of modifications were implemented. First, the calibration of the method in gauged sites was reviewed. The main idea was to integrate more data and to take more into account the coherence between simulated discharges in different sites. Then, diverse regionalisation schemes extracted from the scientific literature were considered. Their application demonstrated the necessity to exploit information from nearby sites and the physical properties of the catchments. Finally, a version which realises the regionalisation simultaneously to the calibration has been selected. Its comparison with other method showed the quality of this new version of SHYREG

Comparaison de méthodes d'analyse fréquentielle des crues sur des bassins français non jaugés

International audienceThe objective of flood frequency analysis (FFA) is to associate flood intensity with a probability of exceedance. Many methods are currently employed for this, ranging from statistical distribution fitting to simulation approaches. In many cases the site of interest is actually ungauged, and a regionalisation scheme has to be associated with the FFA method, leading to a multiplication of the number of possible methods available. This paper presents the results of a wide-range comparison of FFA methods from statistical and simulation families associated with different regionalisation schemes based on regression, or spatial or physical proximity. The methods are applied to a set of 1535 French catchments, and a k-fold cross-validation procedure is used to consider the ungauged configuration. The results suggest that FFA from the statistical family largely relies on the regionalisation step, whereas the simulation-based method is more stable regarding regionalisation. This conclusion emphasises the difficulty of the regionalisation process. The results are also contrasted depending on the type of climate: the Mediterranean catchments tend to aggravate the differences between the methods

SHYREG, a national database of flood frequency estimation

SHYREG method is a regionalized method for rainfall and flood frequency analysis (FFA). It is based on processes simulation. It couples an hourly rainfall generator with a rainfall-runoff model, simplified enough to be regionalized. The method has been calibrated using all hydro meteorological data available at the national level. In France, that represents about 2800 raingauges of the French Weather Service network and about 1800 stations of the hydrometric National Bank network. Then, the method has been regionalized to provide a rainfall and flow quantiles database. An evaluation of the method was carried out during different thesis works and more recently during the ANR project Extraflo, with the aim of comparing different FFA approaches. The accuracy of the method in estimating rainfall and flow quantiles has been proved, as well as its stability due to a parameterization based on average values. The link with rainfall seems preferable to extrapolation based solely on the flow. Thus, another interest of the method is to take into account extreme flood behaviour with help of rainfall frequency estimation. In addition, the approach is implicitly multi-durational, and only one regionalization meets all the needs in terms hydrological hazards characterisation. For engineering needs and to avoid repeating the method implementation, this method has been applied throughout a 50 meters resolution mesh to provide a complete flood quantiles database over the French territory providing regional information on hydrological hazards. However, it is subject to restrictions related to the nature of the method: the SHYREG flows are “natural”, and do not take into account specific cases like the basins highly influenced by presence of hydraulic works, flood expansion areas, high snowmelt or karsts. Information about these restrictions and uncertainty estimation is provided with this database, which can be consulted via web access

SHYREG, a national database of flood frequency estimation

SHYREG method is a regionalized method for rainfall and flood frequency analysis (FFA). It is based on processes simulation. It couples an hourly rainfall generator with a rainfall-runoff model, simplified enough to be regionalized. The method has been calibrated using all hydro meteorological data available at the national level. In France, that represents about 2800 raingauges of the French Weather Service network and about 1800 stations of the hydrometric National Bank network. Then, the method has been regionalized to provide a rainfall and flow quantiles database. An evaluation of the method was carried out during different thesis works and more recently during the ANR project Extraflo, with the aim of comparing different FFA approaches. The accuracy of the method in estimating rainfall and flow quantiles has been proved, as well as its stability due to a parameterization based on average values. The link with rainfall seems preferable to extrapolation based solely on the flow. Thus, another interest of the method is to take into account extreme flood behaviour with help of rainfall frequency estimation. In addition, the approach is implicitly multi-durational, and only one regionalization meets all the needs in terms hydrological hazards characterisation. For engineering needs and to avoid repeating the method implementation, this method has been applied throughout a 50 meters resolution mesh to provide a complete flood quantiles database over the French territory providing regional information on hydrological hazards. However, it is subject to restrictions related to the nature of the method: the SHYREG flows are “natural”, and do not take into account specific cases like the basins highly influenced by presence of hydraulic works, flood expansion areas, high snowmelt or karsts. Information about these restrictions and uncertainty estimation is provided with this database, which can be consulted via web access

Santé et Education

Ce numéro 3, coordonné par un collectif de chercheurs, présente différentes recherches en sciences de l'éducation dans le champ de la santé. On observe comment "la santé" peut être mobilisée par les acteurs de la recherche en sciences de l'éducation qui exploitent différentes disciplines de la discipline pour tenter d'organiser leurs pratiques de recherches en absence d'épistémologie propre. L'objet santé en sciences de l'éducation fait naître un nexus permettant de saisir ce qui constitue l'acceptabilité d'un système. La méthode de recherche des contributeurs consiste alors non pas à établir quelles sont les conditions de constitution et de légitimité de la santé dans les recherches en sciences de l'éducation comme une connaissance possible, mais à désigner des entités propres aux recherches qui opèrent une neutralisation quant aux effets de légitimité et une mise en lumière de ce qui les rend, à une certaine époque, acceptables et acceptés

The challenge of mapping the human connectome based on diffusion tractography

Tractography based on non-invasive diffusion imaging is central to the study of human brain connectivity. To date, the approach has not been systematically validated in ground truth studies. Based on a simulated human brain data set with ground truth tracts, we organized an open international tractography challenge, which resulted in 96 distinct submissions from 20 research groups. Here, we report the encouraging finding that most state-of-the-art algorithms produce tractograms containing 90% of the ground truth bundles (to at least some extent). However, the same tractograms contain many more invalid than valid bundles, and half of these invalid bundles occur systematically across research groups. Taken together, our results demonstrate and confirm fundamental ambiguities inherent in tract reconstruction based on orientation information alone, which need to be considered when interpreting tractography and connectivity results. Our approach provides a novel framework for estimating reliability of tractography and encourages innovation to address its current limitations

The challenge of mapping the human connectome based on diffusion tractography

Tractography based on non-invasive diffusion imaging is central to the study of human brain connectivity. To date, the approach has not been systematically validated in ground truth studies. Based on a simulated human brain data set with ground truth tracts, we organized an open international tractography challenge, which resulted in 96 distinct submissions from 20 research groups. Here, we report the encouraging finding that most state-of-the-art algorithms produce tractograms containing 90% of the ground truth bundles (to at least some extent). However, the same tractograms contain many more invalid than valid bundles, and half of these invalid bundles occur systematically across research groups. Taken together, our results demonstrate and confirm fundamental ambiguities inherent in tract reconstruction based on orientation information alone, which need to be considered when interpreting tractography and connectivity results. Our approach provides a novel framework for estimating reliability of tractography and encourages innovation to address its current limitations