Parcourir la collection par Sujet "numerical modelling"

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  • Publication
    Accès libre
    Grid-enabled Monte Carlo analysis of the impacts of uncertain discharge rates on seawater intrusion in the Korba aquifer (Tunisia)
    (2010)
    Kerrou, Jaouhar 
    ;
    Renard, Philippe 
    ;
    Lecca, Giuditta
    ;
    Tarhouni, Jamila
    L'aquifère de Korba, situé au nord de la Tunisie, est gravement touché par une salinisation du à l'intrusion marine. En 2000, l'aquifère a été exploité par plus de 9000 puits. Le problème, c'est qu'il n'y a pas d'information précise concernant les débits de pompage, leur répartition dans l'espace ainsi que leur évolution dans le temps. Dans cette étude, un modèle géostatistique des débits d'exploitation a été construit en se basant sur une régression multilinéaire combinant des données directes incomplètes ainsi que des données secondaires exhaustives. Les impacts de l'incertitude associée à la distribution spatiale des débits de pompage sur l'intrusion marine ont été évalués en utilisant un modèle tridimensionnel d'écoulement et de transport à densité variable. Pour contourner les difficultés liées à de longs temps de calcul, nécessaires pour résoudre des problèmes en régime transitoire, les simulations ont été réalisées en parallèle sur une grille informatique de calcul mise à disposition par le projet “Enabling Grid for E-Science in Europe”. Les résultats des simulations de Monte Carlo ont montré que 8.3% de la surface de l'aquifère est affectée par l'incertitude liée aux données d'entrée., The Korba aquifer, located in the north of Tunisia, suffers heavily from salinization due to seawater intrusion. In 2000, the aquifer was exploited from more than 9000 wells. The problem is that no precise information was recorded concerning the current extraction rates, their spatial distribution, or their evolution in time. In this study, a geostatistical model of the exploitation rates was constructed based on a multi-linear regression model combining incomplete direct data and exhaustive secondary information. The impacts of the uncertainty on the spatial distribution of the pumping rates on seawater intrusion were evaluated using a 3-D density-dependent groundwater model. To circumvent the large amount of computing time required to run transient models, the simulations were run in a parallel fashion on the Grid infrastructure provided by the Enabling Grid for E-Science in Europe project. Monte Carlo simulations results showed that 8.3% of the aquifer area is affected by input uncertainty.
  • Publication
    Accès libre
    Groundwater flow, heat and mass transport in geothermal systems of a Central Alpine Massif.: The cases of Lavey-les-Bains, Saint-Gervais-les-Bains, and Val d'Illiez
    (2010)
    Sonney, Romain
    ;
    Perrochet, Pierre 
    ;
    Vuataz, François 
    Groundwater flow, heat and mass transport in geothermal or hydrothermal systems locally occurs in the Alps Range where a series of precise conditions are met such as active downflow, permeability at depth, concentrated and fast upflow and favorable geomorphological surface patterns. Advanced studies of the regional and local geology, thermal water chemistry, mixing processes, infiltration area, water-rock interactions, circulation depth, uprising conditions, groundwater residence time, mineralchemical fingerprint, etc. allow the deep flow system to be understood. To carry out these studies, different methods can be employed and combined together such as geological, hydrogeological, geophysical, geochemical, isotopic and numerical investigations. Moreover, these investigations represent a scientific basis providing information required to better manage the short and long term exploitation of the discharged thermal water, qualitatively and quantitatively. They also provide crucial knowledge to prepare future geothermal projects with boreholes and to limit failures that may occur in subsurface and deep geothermal prospection. Due to the high costs for the implementation of a deep borehole, the risk of failure needs to be lowered as much as possible, i.e. it requires a good knowledge of the explored site. This is important for long term sustainable development of geothermal projects. The three selected hydrothermal sites are Lavey-les-Bains and Val d’Illiez in Switzerland and Saint- Gervais-les-Bains in France, where the uprising thermal waters are continuously exploited for various uses such as spa, heating buildings, medical care and cosmetics. Lavey-les-Bains and Saint-Gervaisles- Bains were selected due to several criteria: 1) deep flow systems in the basement, 2) similar chemical and isotopic properties, 3) presence of several end-members with mixing processes, 4) interesting properties of the geothermal reservoirs, 5) many data available and 6) on-going geothermal projects or planned. On the contrary, Val d’Illiez was selected for orther criteria: 1) deep flow system in the autochthonous sedimentary cover of the basement where the hydrothermal systems of Lavey-les-Bains and Saint-Gervais-les-Bains occur, 2) comparison of the chemical and isotopic properties of thermal waters from the cover with those of thermal waters flowing in the basement, 3) history of the site related to the Salanfe Lake and 4) many data available. These three studied sites are located around the Aiguilles Rouges Massif, one of the external crystalline massifs of the Western Alps. Lavey-les-Bains and Saint-Gervais-les-Bains represent the two low-elevation points of the Aiguilles Rouges basement, respectively on the north-eastern and southwestern sides. In contrast, Val d’Illiez is located out of the Aiguilles Rouges basement in a sedimentary domain belonging to the autochthonous cover which outcrops along the north-western edge of the basement. Firstly, the investigations the selected hydrothermal systems were used to study the geological setting and the fracturing conditions for each site. Then, a new sampling campaign was carried out with pumping tests aiming to define the chemical-mineral processes leading to the composition of groundwaters. Finally, two and threedimensional numerical models were established to validate the assumptions formulated from the geochemical investigations, and to represent the deep flow system, the geothermal anomalies and the mixing processes. Groundwater analyses since 1973 at Lavey-les-Bains have revealed a mixing process between a deep Na-SO4 and high-Cl thermal component circulating in the basement (68oC and TDS 1.4 g/L) and cold shallow water from the mountain slope and the Quaternary filling. The production rate of the new deep well P600, installed in 1997, has amplified this mixing process in the well P201, for which a decline in temperature and total dissolved solids has been observed. Numerical hydrogeological two and three-dimensional models of flow, heat and mass transport reproduced the deep flow system and forecasted the long-term exploitation potential of the geothermal resource. Computed temperature of the deep inferred reservoir (100-130oC) is in agreement with the geothermometers, whereas the simulated thermal water flux (5400-9000 m3/d) is probably underestimated. Different fluid production scenarios have been documenting the decline and stabilization phases of temperatures in the hydrothermal ield since 1997. For P201, the mixing ratio calculated before and during the exploitation of P600 is comparable with observed data; the computed temperature tends towards stabilization in P201 at 56oC after 10 to 15 years of production at P600. Another planned new well is likely to reduce the thermal output of the existing wells. The crystalline rocks are not directly outcropping at the Saint-Gervais-les-Bains spa but certainly exist beyond 300 m depth. Uprising waters are pumped from two different aquifers below the Quaternary deposits of the Bon Nant Valley. In the lower Trias-Permian aquifer crossed by De Mey boreholes (27-36oC), the ascending Na-SO4 and high-Cl thermal water from the basement (4.8 g/L) is mostly mixed by a Ca-SO4 and low-Cl cold water circulating in the autochthonous cover of the Aiguilles Rouges basement. The origin of the saline thermal water probably results from infiltration and circulation in the basement until it reaches deep thrust faults where there is leaching of residual brines or there are fluid inclusions at depth. The dissolution of Triassic halite is not possible at Saint-Gervais-les-Bains because the Triassic cold waters have a very low-Cl concentration (< 20 mg/L). For the De Mey Est borehole, gypsum dissolution is occurring with cationic exchanges involving Na, as well as low-temperature Mg dissolution from dolomite in the Triassic formations. The aquifer made of imbricated structures (upper-middle Trias) and crossed by the Lépinay well (39oC) contains thermal waters which are strongly mixed with a low-Cl water, where gypsum dissolution also occurs. The infiltration area for the thermal end-member is in the range of 1700-2100 meters, close to the Lavey-les-Bains hydrothermal system, and corresponds to the average elevation of the Aiguilles Rouges Massif. For the Ca-SO4 and low-Cl end-member, the infiltration area is lower (1100-1300 m), showing circulation from the Mont Joly Massif. The geothermometry method indicates a reservoir temperature of probably up to 65oC but not exceeding 100oC. The deep flow system leading to the thermal springs in the Val d’Illiez occurs at the bottom of the autochthonous cover of the Aiguilles Rouges basement, mainly inside the Triassic formations. The structure of the cover is a great recumbent anticline with an axial plane plunging towards the south-east which would be limited by a basal thrust fault related to the thrust system between the Aiguilles Rouges and Infa-Aiguilles Rouges basements. Mixing processes occur between a Ca-HCO3 cold groundwater and a Ca-SO4 and low-Cl thermal endmember, having a temperature and a total dissolved solids roughly of 30-31oC and 1.8 g/L respectively. The thermal component acquires its mineral composition from the dissolution of gypsum and dolomite occurring in the Triassic formations, in a different way compared to Lavey-les-Bains and Saint-Gervais-les-Bains. The thermal component has an infiltration area close to the elevation of the Salanfe Lake (1900 m) and the reservoir temperature of the deep flow system should probably not exceed 35-40oC at a depth of around 1 kilometre below the Val d’Illiez spring zone. Using the tritium data, the piston-flow model calculated an average residence time for the thermal end-member of around 5 years. A two-dimensional model of groundwater flow and heat transport was carried out to study the relation between the Salanfe Lake and the thermal springs. Four scenarios were tested with different values of water losses from the lake and a long-term cooling of the host rocks was simulated. The natural variations of parameters in the springs indicate that the lake probably does not act on the thermal regime as a single intake. It appears that temperature and chemistry of the thermal waters in the Val d’Illiez have a current steady evolution, while the flow rate seems to vary with the natural variation of the water level of the lake with a time lag of around 140 days. This should indicate that an upper aquifer is situated on the thermal aquifer, without mixing processes, adding a pressure which raises the flow rate of the thermal spring without changing its temperature and chemistry. The understanding gained through this study on groundwater flow, heat and mass transport in the Aiguilles Rouges Massif improves the knowledge for the other external crystalline massifs where hydrothermal sites are present at the front of them. The Mont Blanc Massif is also an area where a large amount of thermal water can discharge from its lowelevation points. The hydrothermal site of Saxon along the Rhone Valley in Switzerland is an interesting site to investigate for this reason. Generally, the density of hydrothermal sites is often higher in the Western Alps where the basement outcrops. Indeed, vertical faults in the basement facilitate the deep infiltration of water leading to deep flow systems, whereas in sedimentary domain the superposition of nappes tends to generate shallower flow systems. Consequently, it would be interesting to investigate other low-elevation zones bordering the external crystalline massifs where the Quaternary filling could mask areas of uprising thermal waters.
  • Publication
    Accès libre
    Hydrogeological and topographical controls on catchment dynamics and their implications for low flows
    (2018)
    Carlier, Claire
    ;
    Brunner, Philip 
    ;
    Hunkeler, Daniel 
    Même dans des régions relativement humides comme la Suisse, des périodes de sécheresses plus intenses et prolongées sont attendues à cause des changements climatiques. Afin d’appréhender la sensibilité des ressources en eaux aux sécheresses et d’identifier les régions à risque, une compréhension profonde des mécanismes gouvernant la dynamique des bassins versant en l’absence de précipitation est cruciale. Pendant les périodes de sécheresses, les rivières sont principalement alimentées par l’eau souterraine. Leur débit reflète donc la capacité du bassin versant à libérer de l’eau stockée lors de précédents événements pluvieux. Les caractéristiques des bassins versants qui influencent les processus hydrogéologiques sont ainsi inhérentes à leur dynamique de basses eaux. La sensibilité des bassins versants doit donc être évaluée d’une perspective hydrogéologique.
    Afin de développer des outils pour l’identification des ressources en eaux sensibles aux sécheresses, cette thèse de doctorat explore les influences des propriétés physiographiques sur la dynamique des bassins versants, en mettant l’accent sur leur comportement de basses eaux et sur le rôle de l’hydrogéologie. Les précédentes études consacrées au lien entre les propriétés physiques des bassins versant et leur dynamique négligent souvent leurs caractéristiques souterraines. De plus, l’identification des effets de chaque propriété physique sur le comportement hydrologique des bassins reste complexe. Afin de contrer ces limites, deux approches sont développées : (1) l’utilisation de modèles hydrogéologiques synthétiques permettant d’évaluer systématiquement l’influence des paramètres hydrogéologiques et topographiques sur les basses eaux, et (2) l’étude de la dynamique hydrologique de 22 bassins versants suisses avec la prise en considération détaillée de leurs caractéristiques géologiques et hydrogéologiques (roche en place ou cohérente – “bedrock” en anglais -- et dépôts quaternaires).
    Dans le cadre de la première approche, les propriétés hydrogéologiques et topographiques des bassins (conductivité hydraulique de la roche en place et de l'aquifère alluvial, pente des versants et de la rivière) sont variées systématiquement avec HydroGeoSphere. Ce modèle numérique et distribué simule de manière couplée et simultanée les flux souterrains et de surface. Ainsi, les processus hydrogéologiques sont considérés explicitement et l’impact de chaque propriété physique sur la dynamique des bassins versants peut être quantifié. Ces modèles synthétiques bénéficient grandement à la caractérisation: du lien entre dynamiques de basses eaux et de l’eau souterraine, de l’importance relative de la roche en place et des dépôts alluviaux, et de l’influence combinée de la conductivité hydraulique et de la topographie. En outre, le rôle de propriétés difficilement mesurables sur le terrain, comme la perméabilité de la roche en place (p.ex. la Molasse en Suisse), peut être étudié. Cette caractéristique est d’ailleurs la seule à exercer un effet global sur les basses eaux de tous les bassins synthétiques. Une conductivité relativement haute (p.ex. 10-4 to 10-5 m/s) de la roche en place garantit des débits de basses eaux importants. En fonction de cette valeur, la contribution de la roche en place aux basses eaux peut être favorisée par des versants raides ou diminuée par un relief limité. Lorsque la capacité de la roche en place à subvenir aux bas débits est limitée (quantifiée par le bedrock productivity index BPI), la contribution relative de l’aquifère alluvial peut devenir significative.
    Dans la seconde approche, les propriétés physiques des 22 bassins versants suisses sélectionnés (utilisation et types de sol, topographie, géologie et paramètres météorologiques) sont comparées à une multitude d’indicateurs hydrologiques décrivant toutes les gammes de débits sur 20 ans de mesure. Des indicateurs de débits absolus (p.ex. Q95 à Q5) ainsi que des indicateurs relatifs (p.ex. Q95 divisé par le débit moyen) sont utilisés. La normalisation des indicateurs de débit permet de filtrer l'effet des précipitations et donc de se concentrer sur l'influence des propriétés physiques du bassin sur sa dynamique. Ainsi, les effets de la précipitation et des paramètres physiques sur le comportement hydrologique deviennent distinguables. Les indicateurs absolus de débit, à part les bas débits, dépendent principalement de la météorologie. Les indicateurs relatifs, décrivant tout autant les bas que les hauts débits relatifs, sont en revanche uniquement corrélés aux paramètres géologiques et hydrogéologiques des bassins (% de grès, % de dépôts quaternaires productifs). La capacité d’un bassin versant d’”amortir” le signal de la précipitation peut donc être attribuée à ses caractéristiques géologiques et hydrogéologiques. Les résultats suggèrent que ce potentiel de “stabilisation” des débits, quantifié par exemple par le ratio Q95/Qmean, est favorisé par la présence de grès dans le bassin. De plus, des dépôts quaternaires importants semblent également exercer un effet positif sur les bas débits normalisés.
    Les deux approches sont complémentaires et permettent d’identifier des processus similaires, cruciaux pour la caractérisation de la dynamique générale et de basses eaux des bassins versants. Selon les deux lignes de recherche, une roche en place relativement perméable (p.ex. 10-5 m/s, du grès) est un prérequis pour des débits soutenus lors de périodes sèches. L’influence de dépôts productifs locaux sur la dynamique des bassins est soulignée par les deux approches. Sur la base de ces résultats, deux aides à l’évaluation de la sensibilité des rivières et des aquifères alluviaux aux sécheresses sont développées. Les méthodes dépendent du type et de la qualité des données disponibles. Si celles-ci sont suffisantes, l’estimation de la sensibilité peut être quantitative, alors qu’elle a une valeur qualitative si les données de débits ou de hauteurs piézométriques sont rares. Dans le second cas, des stratégies de surveillance des ressources en eaux peuvent notamment être établies sur la base des lignes directives proposées. En outre, celles-ci proposent un cadre de comparaison du comportement des bassins versants en période sèche., Periods with scarce precipitation will likely occur more frequently and last longer under changing climatic conditions, even in relatively humid regions like Switzerland. To assess the sensitivity of water resources to dry spells and to identify regions that might experience water scarcity issues, a thorough understanding of the mechanisms governing catchment dynamics in the absence of rain is essential. During dry periods, streamflow is mainly fed by groundwater reservoirs and thus reflects the ability of the catchment to release water that has been previously stored during precipitation events. Catchment characteristics that govern groundwater processes are consequently inherent to low-flow dynamics. The sensitivity of catchments to dry periods thus has to be assessed from a hydrogeological perspective.
    This PhD thesis, with the global aim of providing tools for the identification of catchments sensitive to dry conditions, explores the physiographic controls on catchment dynamics with emphasis on low flows and on the role of hydrogeological factors. Previous studies dedicated to the relationship between catchment properties and streamflow dynamics often disregard the subsurface characteristics. Moreover, unravelling the various physical controls on hydrological signatures is complex based on observed data. To cope with these limitations, two approaches are developed: (1) the use of hydrogeological synthetic models, which allow the systematic assessment of topographical and hydrogeological influence on low flows and groundwater storage, and (2) an investigation of streamflow dynamics of 22 Swiss catchments with the consideration of detailed geological and hydrogeological descriptors of both the general geological environment (bedrock lithologies) and alluvial quaternary aquifers.
    In the first approach, catchment hydrogeological and topographical features (bedrock and alluvial hydraulic conductivity, hillslope and river slope) are systematically varied using the numerical model HydroGeoSphere. This software simulates surface and subsurface flow in a fully coupled, distributed way. It thus allows the explicit consideration of groundwater processes and the quantification of the impact of each physical property on catchment dynamics. The synthetic models provide great insights on the relationship between low flows and groundwater processes, on the relative importance of the bedrock and the alluvial aquifer, and on the combined impact of hydraulic conductivity and slope gradients. Moreover, the role of catchment properties whose observation in the field is bound to high uncertainties, such as the hydraulic conductivity of the bedrock, can be explored with the synthetic models. The only catchment property exerting an overall impact on low flows is indeed the hydraulic conductivity of the bedrock. Relatively high hydraulic conductivities (e.g. 10-4 to 10-5 m/s) of the bedrock guarantee sustained low flows. Depending on this value, the contribution of the bedrock to low flows can be increased respectively diminished by steep respectively flat hillslopes. When the capacity of the bedrock to sustain the stream (quantified by the proposed bedrock productivity index BPI) is limited, the relative contribution of the alluvial aquifer can become significant.
    In the second approach, the catchment properties of the 22 selected catchments, encompassing land use, soil, topography and geology, as well as precipitation characteristics, are compared to numerous streamflow indicators describing the entire range of dynamics over 20 years. Absolute (e.g. Q95 to Q5) as well as relative indicators (e.g. Q95 divided by mean discharge) are used. The normalisation of the discharge indicators filters the influence of precipitation, which allows focusing on the impact of catchment properties on discharge dynamics. The meteorological and the catchment controls on hydrological signatures thus become distinguishable. The impact of precipitation is consequent on the absolute discharge indicators except for the low-flow range. The relative indicators, which describe both high and low normalised discharges, are however only correlated to the geological properties of the catchments (% of sandstone and % of productive quaternary deposits). The ability of the catchment to “buffer” the precipitation signal can thus be attributed to its geological and hydrogeological characteristics. The results suggest that this “stabilisation” effect on streamflow, quantified for instance by Q95/Qmean, is sustained by the presence of sandstone in the catchment. Moreover, productive quaternary deposits with a large extent or volume also seem to have a favourable effect on normalised low flows.
    The two approaches are complementary and enable to identify similar processes and governing mechanisms, which are of high relevance for the characterisation of catchment and of low-flow dynamics. According to both approaches, a relatively permeable bedrock (e.g. 10-5 m/s, sandstone) is a prerequisite for sustained streamflow during dry periods. The influence of local productive deposits on catchment dynamics is also highlighted by both methods. Based on these findings, two guidelines are developed to assess the sensitivity of rivers and alluvial aquifers to dry periods. The assessment can be quantitative if adequate time series and data describing the resource exist, whereas it has a qualitative value if scarce discharge or groundwater head data are available. In the latter case, monitoring strategies can for instance be established on the basis of this guideline. Furthermore, it provides a framework for catchment inter-comparison with regards to their behaviour under dry conditions.
  • Publication
    Accès libre
    Implication of density-dependent flow on numerical modelling of SW-GW interactions
    (: S. Sauvage, J. M. Sánchez-Pérez, A. E. Rizzoli, 2015-7)
    Alaghmand, Sina
    ;
    Brunner, Philip 
    ;
    Graf, Thomas
    ;
    Simmons, Craig T.
    With the growing interest in the last decades in the modelling of hydrogeological processes involved in the water resources management, it has been recognized that the assumption of constantproperties water is no longer adequate in the analysis and simulation of the flow considered in these cases. In recent years, many studies used simplistic approaches that may not represent the aquifer flow dynamics realistically by not accounting for changing fluid density. This study explore the importance of understanding the impact of density-dependent flow on SW-GW interactions. To this aim two synthetic models was developed at large and small scales and various scenarios were defined to explore the impact of density-dependent flow on drivers including river and aquifer salinity ratio, hydraulic gradient and river geometry. The results shows that simplifying by excluding density-dependent flow leads to overestimation of solute mass accumulation, and eventually groundwater salinity and limited freshwater lens. Also, the simulated model without density-dependent flow is not able to represent the unsaturated zone properly. However, these impacts are limited to the river banks. In the small scale, when simulated with density-dependent flow, large salinity ratio between river and aquifer can significantly influence both solute and flow dynamics. Moreover, mixed-convention was observed when hydraulic gradient was towards river. Overall, it was concluded that density-dependent flow play an essential role in SW-GW interaction and needs to be taken in to account where the river and aquifer have significant salinity difference, particularly at the vicinity of the river banks.
  • Publication
    Accès libre
    Simulation of flow in fractured rocks using effective stress-dependent parameters and aquifer consolidation
    Preisig, Giona 
    ;
    Cornaton, Fabien Joel
    ;
    Perrochet, Pierre 
    Effective stress plays an important role in aquifer dynamics, especially in those affected by high variations of water pressures. Increasing/decreasing effective stresses affect hydrogeological parameters, even in media of high stiffness, such as fractured rocks. This study presents a modelling approach of groundwater flow in fractured rocks and aquifer deformation taking into account the dependency of hydrogeological parameters on effective stress. This approach has been illustrated by modelling a fractured aquifer dynamic, the Zeuzier arch dam settlement. The calibrated model showed agreement with measured data. This simulation method could be used to study the sensitivity of aquifers to variations in effective stress due to water pressure.