Apports et enjeux de la modélisation hydraulique 3D pour la conception et la réhabilitation des ouvrages hydrauliques

Jusqu'à ces dernières années, les études détaillées des ouvrages hydrauliques étaient réalisées quasiment systématiquement par le biais de modèles physiques. Si l'utilisation de modèles physiques permet de bien appréhender le comportement hydraulique des ouvrages, ils sont en général coûteux, requièrent un temps de mise en œuvre important et ne peuvent représenter que les phénomènes hydrauliques pour lesquels ils ont été définis par similitude. De nos jours, grâce à la puissance de calcul toujours plus grande des ordinateurs et au développement des logiciels spécialisés, la modélisation numérique 3D est devenu un outil flexible et puissant pour accompagner et compléter la modélisation physique, voire même la remplacer dans certains cas. Cet article décrit les apports et enjeux de la modélisation hydraulique 3D pour la conception et la réhabilitation des ouvrages hydrauliques. Il présente, dans un premier temps, comment la modélisation numérique 3D a d'abord été validée sur une large gamme de phénomènes hydrauliques, puis comment elle est exploitée par Tractebel sur plusieurs grands projets, pour analyser et optimiser la conception ou l'exploitation des structures hydrauliques en appui des méthodes analytiques et modèles physiques. Des exemples d'applications complémentaires sont présentés tels que la modélisation de grands évacuateurs de crues, un canal d'amenée, une prise d'eau usinière, ou encore les phénomènes d'entraînement d'air et de dissipation d'énergie dans un ressaut hydraulique ou dans une fosse d'érosion. Les possibilités et les limites de la modélisation 3D sur ces divers sujets sont discutées. Des méthodes de modélisation simplifiées ont été développées pour répondre à des questions spécifiques de manière optimisée. Les procédures élaborées pour chaque cas particulier sont décrites de manière à fournir des processus indicatifs pour le déploiement de la modélisation hydraulique 3D dans des projets d'ingénierie, et ainsi y apporter une valeur ajoutée. En se basant sur l'expérience de Tractebel, les limites et défis qui restent à surmonter sont discutés, avec un accent mis sur la modélisation de l'entraînement d'air

Dissolution rate of a basalt glass in silica-rich solutions: Implications for long-term alteration

International audienc

Geographic variations in the slope of the δ2H–δ18O meteoric water line over Europe: A record of increasing continentality

International audienceδ2H and δ18O values of precipitations follow an empirical linear relationship at the global scale that is called the Global Meteoric Water Line (GMWL) and characterized by a slope of 8. However, Local Meteoric Water Lines (LMWLs) may have different slopes S depending on their geographic situation. Monthly δ2H and δ18O of precipitation have been compiled from European International Atomic Energy Agency (IAEA) stations. Those data allowed the calculation of the slopes S of the δ2H–δ18O LMWL determined for each station. S increases with longitude ϕ from c. 5 (Portugal) to c. 9 (Russia) – they are positively correlated with relative humidity (RH), negatively with temperature and positively with the mean intra-annual amplitude of temperatures, which is a proxy of continentality. Slopes of 5–6, recorded in SW Europe, reflect mean RH (70–75%) and sea surface temperatures (c. 25°C) of the Central Atlantic Ocean where the main flux of moisture is formed before being transported by the westerlies. In addition, falling water droplets within an air column with a high RH (>80%) and low temperature are expected to escape sub-cloud evaporation. Therefore, slopes with values close to 9 are considered to reflect isotopic equilibrium conditions during the condensation of water vapour in clouds

Seuil de réoxygénation du barrage de Lom Pangar

Lom Pangar is a new dam under construction in Cameroun. Its aim is mainly flow regulation of the Sanaga River in order to improve and secure hydropower production of downstream facilities. To reduce impact on water quality and especially on dissolved oxygen for 25 to 100 m3/s discharge, an aerating weir is going to be implemented downstream of the dam for at least the 10 first years of the dam exploitation. After site selection, preliminary studies have conducted to chose a rockfill stepped weir for technical reasons including availability of materials on the dam work site and asier decommissioning. According to literature, nappe flow is the flow regime providing most effective aeration for this type of structure and application of formulae to the discharge range and geometry of the project leads to choose 50 cm high steps to create this flow regime. The proposed solution has been tested using a 1:1 scale physical model, with the following geometry: 3 m high and 15° downstream slope. Dissolved oxygen concentration has been measured in the upstream reservoir, in the downstream collecting channel and on several steps. Aeration efficiency has been measured and the physical model has been able to validate and optimize the weir design in order to reach the water aeration objectives and limit hydraulic impacts. The works are planned beginning 2017 and water aeration will be monitored

Oxygen Isotope Composition Of Human Teeth And The Record Of Climate Changes In France (Lorraine) During The Last 1700 Years

International audienceIn order to study climate variations during the last 1700 years in eastern France, fifty-eight oxygen isotope compositions of phosphate were measured in human tooth enamel. The individuals, who lived in Lorraine, are assumed to have drunk local water derived directly from rainfall. According to previous work, drinking water is the main source of oxygen that sets the isotopic composition of phosphatic tissues in humans. The empirical fractionation equation determined from our data combined with those of Longinelli's one [Geochim. Cosmochim. Acta 48 (1984) 385] was used to calculate the oxygen isotope composition of meteoric waters. The mean air temperature was inferred from these isotope ratios and the Von Grafenstein et al.'s [Geochim. Cosmochim. Acta 60 (1996) 4025] relationship between δ18O and air temperature. Oxygen isotope composition of present-day individuals yields a mean air temperature of 9.9 ± 1.7 ◦C which is consistent with meteorological data. Application of this method to historical individuals results in mean air temperatures estimates 0 to 3 ◦C higher than present-day air temperature. These warm air temperatures are not realistic during the so-called Little Ice Age for which an air-cooling of about 0.5 to 2 ◦C has been documented. We propose that these relatively high δ18O values of human tooth enamel reflect higher mean δ18O values of meteoric water which can be attributed to an increased proportion of summer rainfall during the “Little Ice Age” time frame in Lorrain

Kinetic aspects of basaltic glass dissolution at 90°C: role of aqueous silicon and aluminium

International audienc

SUMMARY

the extended nite element metho

Energy dissipation on a stepped spillway downstream of a Piano Key Weir - Experimental study

peer reviewedPiano Key Weir (PKW) is a new type of weir showing appealing hydraulic capacities. Its specific geometric features create different interacting flows and jets downstream of the structure. This suggests that the use of a PKW to control the flow upstream of a stepped spillway may help in enhancing energy dissipation on the downstream channel. The first results of an experimental study carried out at the Laboratory of Engineering Hydraulics – HACH in the scope of a Coyne & Bellier – Tractebel Engineering project are presented in the paper. The goal of the study was to compare the energy dissipation on a stepped spillway downstream of a PKW with the one which takes place on the same spillway equipped with a standard ogee-crested weir. Despite similar global energy dissipation rates whatever the weir type due to the relative important height of the spillway, significant differences in the aeration of the flow along the spillway have been shown. This paves the way to further investigations