127 research outputs found
Simulation numérique de la sédimentation dans les retenues de barrages : cas de la retenue de Zardezas, Algérie
La construction d'un modĂšle numĂ©rique destinĂ© Ă prĂ©dire la formation et l'Ă©volution de dĂ©pĂŽts de sĂ©diments Ă l'amont d'un barrage est prĂ©sentĂ©e. A partir d'informations sur les apports en eau et en sĂ©diments en provenance du bassin versant consolidĂ©es par une analyse hydrologique en QdF, un modĂšle hydraulique bidimensionnel horizontal couplant Ă©quations de Saint Venant et une Ă©quation de convection-diffusion est mis en Ćuvre. L'application de ce modĂšle sur la retenue de Zardezas de la rĂ©gion de Skikda (AlgĂ©rie) montre, Ă la fois, les difficultĂ©s pratiques rencontrĂ©es dans la mise en Ćuvre et l'apport possible d'une telle mĂ©thode pour la gestion des retenues algĂ©riennes.Sedimentation rates are often very high in Algeria, reaching about 1% of the reservoir volume per year in most cases. The management of existing reservoirs and the choice of location of new reservoirs may be improved by using a numerical model that simulates sediment deposition. The proposed method was developed on a selected case for which a convenient set of data had been gathered.Initially, the Zardezas reservoir had a capacity of 34 million m3, but presently, the capacity is only 17 million m3. Due to the levelling of two topographies in 1975 and 1986 and discharge data available from 1968 to 1993, the numerical model could be calibrated for the period 1975-1986.As the cross-distribution of sediments is thought to be a main factor for the reservoir deposition rate, a 2-D horizontal hydrodynamic model was selected. Sediments were modelled by a concentration that was calculated using an advection-diffusion equation. A source term determining the exchange rate between the flow and the bottom as proportional to an equilibrium concentration was used. Calculation of this source term followed a simplified version of the method developed by VAN RIJN (1984). The set of 4 equations ((8) + (9) + (10) + (11)) was solved by a second-order explicit finite volume scheme of the Godunov type, which allows the modelling of very unsteady flows (PAQUIER, 1998). The bottom elevation was modified at every time step by distributing the calculated deposits inside one cell among the neighbouring vertices.Globally, the proposed method should be carried out in two steps. The first step involved model calibration including a hydrological analysis in order to determine the inputs (water and sediments) during the calibration period and calculation of the features of the hydrological regime for the extrapolation periods. The second step involved use if the model to define management strategies. The hydrological scenarios are built from the hydrological regime and the 2-D model is used to calculate the sediment deposits for every scenario. This second step is not described in the present paper.The hydrological analysis involved building QdF (flood-duration-frequency) curves (JAVELLE et al., 2000) from the daily discharges and from the maximum discharges of the rarest floods. Some flood discharge hydrographs were considered and were used to determine the duration of typical floods. Results from this hydrological analysis are summarised by curves in V(d,T) (Table 2) (maximum mean stream flows during the duration d for a return period T) and Q(d,T) (Table 3) (maximum over-threshold during stream flows for T) which were built from the converging QdF model developed by JAVELLE et al. (1999). The main catchment parameters D (characteristic flood duration) and the instantaneous peak discharge over a return period of 10 years were respectively equal to 4 hours and 362 m3 /s. For the estimate of the curves over a return period of 10 years, the gradex of maximum 24 hour rainfalls (estimated to be 24.7 mm) was used. From Table 3 of Q(d,T), mono frequency synthetic discharge hydrographs (HSMF) can be built (e.g. Figure 4) using a rising time equal to D. These hydrographs can be used to define hydrological scenarios by fixing the successive return periods (of the HSMF).For the calibration period 1975 to 1986, the observed or reconstituted discharge hydrographs were used to be closer to real events (Table 4). Because concentrations were not registered precisely enough, simplified assumptions were used for the calibration period and should be kept for future scenarios (peak concentration was fixed to 100 kg/m3 and a linear relation between discharge and concentration was assumed during the flood (see Figure 5)). Only one class of sediment with a mean diameter of 0.1 mm was considered. The 2-D calculations were performed on a grid of 1005 cells (Figure 6) with a space step between 10 and 80 metres. Model calibration consisted of selecting a suitable coefficient a (in equation (12)), which is equivalent to the average distance required to reach the equilibrium concentration. For the period 1975-1986, the calculation provides 4 m thick deposits through the entire reservoir bottom (Figure 8). The discrepancies with measurements were mainly too few deposits near the dam and too much sediment accumulated on the banks of the reservoir (Figures 7 to 9). It can be concluded that the proposed method provides useful results although some improvements are required such as: sediment exchange relations between the flow and the bottom; refining the calculation grid and reducing the uncertainty about the inputs by accurately and regularly measuring both discharge and sediment concentrations. The method should be further validated on other existing reservoirs in the same hydroclimatic context
A multidating approach applied to historical slackwater flood deposits of the Gardon River, SE France
A multidating approach was carried out on slackwater flood deposits, preserved in valley side rock cave and terrace, of the Gardon River in Languedoc, southeast France. Lead-210, caesium-137, and geochemical analysis of mining-contaminated slackwater flood sediments have been used to reconstruct the history of these flood deposits. These age controls were combined with the continuous record of Gardon flow since 1890, and the combined records were then used to assign ages to slackwater deposits. The stratigraphic records of terrace GE and cave GG were excellent examples to illustrate the effects of erosion/preservation in a context of a progressively self-censoring, vertically accreting sequence. The sedimentary flood record of the terrace GE located at 10 m above the channel bed is complete for years post-1958 but incomplete before. During the 78-year period 1880-1958, 25 floods of a sufficient magnitude (> 1450 m3/s) have covered the terrace. Since 1958, however, the frequency of inundation of the deposits has been lower:only 5 or 6 floods in 52 years have been large enough to exceed the necessary threshold discharge (> 1700 m3/s). The progressive increase of threshold discharge and the reduced frequency of inundation at the terrace could allow stabilisation of the vegetation cover and improved protection against erosion from subsequent large magnitude flood events. The sedimentary flood record seems complete for cave GG located at 15 m above the channel bed. Here, the low frequency of events would have enabled a high degree of stabilisation of the sedimentary flood record, rendering the deposits less susceptible to erosion. Radiocarbon dating are used in this study and compared to the other dating techniques.Eighty percent of radiocarbon dates on charcoals were considerably older than those obtained by the other techniques in the terrace. On the other hand, radiocarbon dating on seeds provided better results. This discrepancy between radiocarbon dates on charcoal and seeds is explained by the nature of the dated material (permanent wood vs. annual production and resistance to degradation process). Finally, we showed in this study that although the most common dating technique used in paleoflood hydrology is radiocarbon dating, usually on charcoal preserved within slackwater flood sediments, this method did not permitus to define a coherent age model. Only the combined use of lead-210, caesium-137, and geochemical analysis of mining-contaminated sediments with the instrumental flood record can be applied to discriminate and date the recent slackwater deposits of the terrace GE and cave GG.Une approche par datations multiples a Ă©tĂ© menĂ©e sur les dĂ©pĂŽts de sĂ©diments de crue retrouvĂ©s dans les gorges du Gardon (Languedoc). Le plomb-210, le cĂ©sium-137 et lâanalyse gĂ©ochimique des sĂ©diments contaminĂ©s par les mines ont Ă©tĂ© utilisĂ©s pour reconstruire lâhistoire des dĂ©pĂŽts de crue. Cette approche a Ă©tĂ© combinĂ©e avec lâhistorique des crues du Gardon depuis 1890 pour affiner lâapproche gĂ©ochronologique. Au niveau de la cavitĂ© GG et de la terrasse GE les analyses ont permis dâillustrer les mĂ©canismes dâĂ©rosion-redĂ©position pouvant perturber les sĂ©quences de sĂ©diments. Les enregistrements des dĂ©pĂŽts de sĂ©diments de crue sont complets sur la terrasse GE situĂ©e Ă 10 m au-dessus du lit mineur Ă partir de 1958, et incomplets auparavant. Sur la pĂ©riode 1880-1958, 25 crues ont eu un dĂ©bit suffisant (>1450 m3/s) pour recouvrir la terrasse, alors que lâon recense seulement 5 Ă 6 crues depuis 1958 (pour un dĂ©bit de submersion supĂ©rieur Ă 1700 m3/s). Lâaugmentation progressive du dĂ©bit de submersion et la frĂ©quence moindre dâinondation de la terrasse ont pu permettre sur la pĂ©riode rĂ©cente la stabilisation de la couche de surface par la vĂ©gĂ©tation et limiter lâĂ©rosion lors de crues importantes. Les enregistrements semblent complets dans la cavitĂ© GG situĂ©e Ă 15 m au-dessus du lit mineur. La frĂ©quence moindre de submersion Ă ce niveau plus Ă©levĂ© peut expliquer une meilleure stabilisation des dĂ©pĂŽts de crue, rendus moins vulnĂ©rables Ă lâĂ©rosion. La datation au radiocarbone a Ă©tĂ© testĂ©e mĂȘme si elle ne permet pas de dater avec prĂ©cision les dĂ©pĂŽts de crue des derniers siĂšcles. Sur la terrasse GE, 80% des dates au carbone 14 sur charbon de bois sont beaucoup trop vieilles par rapport aux autres techniques de datation, ce qui peut sâexpliquer par une remobilisation/dĂ©stockage de charbons de bois anciens. Les datations radiocarbone sur graines donnent de meilleurs rĂ©sultats, car issues dâune production annuelle, plus facilement dĂ©composable et moins sujette Ă des dĂ©pĂŽts intermĂ©diaires. Au final, il apparaĂźt que la technique de datation par radiocarbone sur des charbons de bois retrouvĂ©s dans les sĂ©quences sĂ©dimentaires nâest pas fiable, alors quâelle est la plus pratiquĂ©e en palĂ©o-hydrologie. Lâutilisation combinĂ©e du plomb-210, du cĂ©sium-137, de lâanalyse gĂ©ochimique des sĂ©diments et des informations sur les crues historiques a permis de discriminer et de dater les diffĂ©rents dĂ©pĂŽts de crue de ce dernier siĂšcle dans la cavitĂ© GG et la terrasse GE
Attending school after treatment for a brain tumor: Experiences of children and key figures
Reintegration into school is a milestone for childhood brain tumor survivors, as well as for their parents, teachers, and healthcare providers. We explored their experiences following the school re-entry by conducting semi-structured interviews. Thematic analysis resulted in four main themes: "school performance," "psychosocial well-being," "support and approach," and "communication and collaboration." Children were pleased to return to school despite confrontation with adverse outcomes. Parents, teachers, and healthcare providers identified current and future concerns and challenges, as well as opportunities for academic and personal development. Their experiences highlight the importance of coordinated and systematic follow-up in close collaboration with healthcare providers
Short-term perspectives of parents and teachers on school reintegration of childhood brain tumour survivors
OBJECTIVE: To discover short-term changes in perspectives of parents and teachers of childhood brain tumour survivors on school reintegration, in order to reveal similarities and differences between them over time.
METHODS: Semi-structured interviews were conducted with parents and teachers of five children at the start and the end of a 1-year period following the child\u27s school re-entry.
RESULTS: Thematic analysis of data resulted in three main themes: \u27the child\u27s performance and wellbeing\u27, \u27the school\u27s attitude and approach\u27 and \u27communication and working together\u27. Parental concerns about child-specific changes and the school\u27s approach to the child could either decrease or increase over time. Teachers remained focused on assessing their pupil\u27s learning potential and finding ways of appropriate support. Their different perspectives on communication and working together became more pronounced.
CONCLUSIONS: This study emphasizes the importance of clear communication and collaboration, coordinated follow-up and availability of healthcare professionals during the child\u27s school reintegration
Assessment of Methods Used in 1D Models for Computing Bed-Load Transport in a Large River: The Danube River in Slovakia
Restauration morpho-dynamique et redynamisation de la section court-circuitée du Rhin en aval du barrage de Kembs (projet INTERREG / EDF)
National audienceThe Upper Rhine River has been heavily impacted by channelization for flood protection and navigation, and then by damming for hydropower generation. In normal non flooding conditions, most of the flows are diverted in a canalized section whereas the regulated âold Rhineâ bypassed reach runs a minimum flow. Between Huningue and Neuf-Brisach, engineering works induced simplification and stabilization of the channel pattern from a formerly braiding sector to a single incised channel, hydrological modifications, bottom armouring due to bedload decrease, and thus ecological alterations. Two complementary and interdisciplinary projects have been initiated to restore alluvial morphodynamics: i) the international âINTERREG IV - Redynamisation of the old Rhineâ project (2009-2012) coordinated by the Alsace region, France; ii) the left bank âcontrolled erosionâ project launched by ElectricitĂ© de France (EDF) within Kembs hydroelectric station relicensing process since 2003-2004. The purpose of these projects is to evaluate the feasibility of an important hydro-morphological and ecological restoration plan on a 45 km long reach, through both field testing of bank erosion techniques at favourable locations, and artificial sediments input from right bank excavations. This will help define possible long term prospective scenarios, in order to restore sustainable sediment transport, morphodynamics variability and associated ecological functions. The study will involve historical analysis, hydro-morphological / hydraulic physical and numerical modelling, physical and ecological monitoring, and sociological aspectsLe Rhin alsacien-allemand a enregistrĂ© de profondes modifications morphologiques et hydrologiques Ă la suite de sa correction et de sa rĂ©gularisation pour la protection contre les crues et la navigation, puis aprĂšs la construction de barrages hydro-Ă©lectriques. Les amĂ©nagements rĂ©alisĂ©s entre Huningue et Neuf-Brisach ont engendrĂ© une simplification et une stabilisation du style fluvial. Un fleuve en tresses a cĂ©dĂ© la place Ă un chenal unique incisĂ©. Le fond de chenal est devenu pavĂ© Ă cause dâune diminution des apports de charge de fond et des altĂ©rations Ă©cologiques ont Ă©tĂ© observĂ©es (simplification des habitats aquatiques et riverains). Deux projets complĂ©mentaires et interdisciplinaires ont Ă©tĂ© engagĂ©s afin de restaurer une dynamique des formes alluviales : i) le projet international INTERREG IV â Redynamisation du Vieux Rhin (2009-2012) sous lâimpulsion de la rĂ©gion Alsace ; ii) le projet dâĂ©rosion maitrisĂ©e des berges de la rive gauche conduit par ElectricitĂ© de France (EDF) dans le cadre du renouvellement de la concession de lâamĂ©nagement de Kembs. Lâobjectif des deux projets est de dĂ©finir un plan de restauration hydro-morphologique et Ă©cologique conduisant Ă la redynamisation dâun tronçon de 45 km. LâĂ©tude repose sur une analyse historique, lâexploitation de modĂšles Ă la fois physiques et numĂ©riques, et les suivis morphologiques in situ dâune recharge artificielle en sĂ©diments et dâĂ©rosions de berge contrĂŽlĂ©es. Ces Ă©tudes de faisabilitĂ© sont complĂ©tĂ©es par des analyses Ă©cologique et sociologique pour apprĂ©cier lâimpact socio-environnemental de ces projets
Flow structure in a compound channel: benchmarking 2D and 3D numerical models
The benchmarking test of 2D and 3D numerical models on a compound channel flow with a rectangular-shaped main channel and a rectangular-shaped floodplain was carried out by the IAHR Working Group on Compound Channels. The selected test case is the flume experiment by Nezu and Tominaga (1991). Nine depth-averaged 2D models and four 3D models participated in the benchmark. In the 2D models, the depth-averaged streamwise velocity profiles in the lateral direction were compared. In the 3D models, velocity components in three directions as well as the distribution of the turbulence kinetic energy in a cross-section were compared. Through the comparison, the applicability and limitations of each model are highlighted and discussed with regard to the model characteristics.Konferencija je odrĆŸana na daljinu (on-line), bez fiziÄkog prisustva i sav materijal se nalazi na web-stranici organizatora skupa
Shortened Version of the Token Test: Normative data for Spanish-speaking pediatric population
Use of a 2-D model for simulating the flooding of a plain
When simulating the flooding of a plain, more accurate results can be provided by solving 2-D shallow water equation. It is necessary to use suitable topographical data, a grid adapted to hydraulics and a numerical sheme that avoid spurious oscillations. The example of 100-year flood of Agly River illustrates such use with computation carried out by RUBAR 20 developped by Cemagref and, morever, desmonstrates the possibility of simulating the instantaneous braching of an embankment. / La résolution des équations de Saint-Venant bidimensionnelles amÚne à des résultats plus précis pour la simulation de l'inondation d'une plaine. Il est nécessaire d'utiliser des données topographiques adaptées, une grille construite pour l'hydraulique et un shéma numérique qui évite des oscillations parasites. L'exemple de l'inondation centennale le long de l'Agly illustres l'utilisation d'un tel calcul avec le logiciel RUBAR 20 développé par le Cemagref. En outre, il démontre la possibilité de simuler la rupture instantanée d'une digue de protection
Assessment of risks of flooding by use of a 2-D model
In case of rare floods, the rivers might go far away from their mean-water bed, submerging large areas, eventually with relatively high velocities. To simulate such events, 2-D models are required. In order to avoid the use of very small cells, the grid should be designed from a structured topographical model. The accuracy of the results is linked with a suitable definition of boundary conditions, embankments and hydraulic structures. Instantaneous breaching of dikes can be simulated when using the program RUBAR 20 developed by Cemagref. We complete the presentation of the method by describing the study of the 100-year flood in a coastal plain (Agly River) in Southern France / Dans le cas de crues rares, les riviĂšres peuvent aller bien loin de leur lit mineur, submergeant de vastes zones Ă©ventuellement avec des vitesses Ă©levĂ©es. Pour simuler de tels Ă©vĂ©nements, des modĂšles bidimensionnels sont nĂ©cessaires. Afin d'Ă©viter l'emploi de trĂšs petites mailles, le maillage doit ĂȘtre Ă©tabli Ă partir d'un modĂšle topographique structurĂ©. La prĂ©cision des rĂ©sultats est liĂ©e Ă la dĂ©finition convenable des conditions aux limites, des digues et des ouvrages hydrauliques. La rupture instantanĂ©e de digues peut ĂȘtre simulĂ©e en utilisant le programme RUBAR 20 dĂ©veloppĂ© par le Cemagref. Nous complĂ©terons la prĂ©sentation de la mĂ©thode en dĂ©crivant l'Ă©tude d'une inondation centennale dans une plaine cĂŽtiĂšre (Agly) dans le Sud de la France
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