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Perrochet, Pierre
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Perrochet, Pierre
Affiliation principale
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Professeur ordinaire
Email
pierre.perrochet@unine.ch
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Voici les éléments 1 - 10 sur 16
- PublicationAccès libreNumerical simulations and uncertainty analysis for assessing spatial and temporal dynamics in alluvial river-aquifer systems: an application in the context of the 3rd Rhône River Correction(2017)
; La caractérisation de l’interaction entre les rivières et les eaux souterraines pour des aquifères alluviaux est essentielle afin d’anticiper l’impact des stratégies de gestion de rivière, comme le sont les restaurations de rivières, sur l’ensemble de la ressource en eau et sur sa dynamique. De ce fait, le développement de méthodes et d’approches de calibration permettant une meilleure caractérisation et estimation des paramètres hydrauliques des aquifères et des lits de rivière est nécessaire. De plus, l’analyse d’hypothèses communes, telles que la constance des propriétés hydrauliques des lits de rivière, l’isotropie de la conductivité hydraulique de l’aquifère ou encore la calibration en régime permanent, est indispensable afin d’identifier des biais potentiels dans les prévisions et leurs incertitudes.
Le chapitre 1 introduit les concepts pertinents, les méthodes, le cadre de recherche, c.-à-d. la 3e Correction du Rhône, ainsi que le but de la thèse doctorale.
Le chapitre 2 présente une méthode qui permet d’évaluer le caractère transitoire des propriétés hydrauliques de lits de rivière. La méthode est basée sur l’inversion d’une convolution numérique entre les variations de niveau de la rivière et la réponse unitaire, en terme de variation de la surface piézométrique, de l’aquifère. L’estimation du caractère transitoire des propriétés hydrauliques du lit de la rivière est obtenue par des calibrations utilisant des séries temporelles successives de charges hydrauliques. Une analyse synthétique a démontré la fiabilité de la méthode et son application à des données réelles de terrain a permis d’indiquer l’influence d’événements climatiques sur le caractère transitoire des propriétés hydrauliques.
Le chapitre 3 analyse les conséquences sur l’estimation de l’incertitude de la prévision de l’hypothèse communément faite sur l’isotropie de la conductivité hydraulique de l’aquifère. Il est démontré que les présupposés sur l’isotropie de l’aquifère peuvent causer une sous-estimation de l’incertitude des prévisions sur l’élévation de la surface piézométrique. De plus, il est démontré que la calibration transitoire, en comparaison à celle en régime permanent, permet une meilleure estimation des paramètres hydrauliques de l’aquifère et du lit de la rivière et ainsi de réduire l’incertitude de la prévision.
Le chapitre 4 présente les prévisions sur l’élévation de la surface piézométrique et leurs incertitudes dans la région de Sion (Suisse) dans le cadre des modifications projetées par la 3e Correction du Rhône. Ce faisant, l’incertitude de la prévision liée à la calibration du modèle numérique hydrogéologique est complétée par la prise en compte, via la modélisation de scénarios, de l’incertitude sur les futures propriétés hydrauliques et géomorphologiques du lit du Rhône. Les résultats montrent que bien que le processus de calibration réduit de manière importante l’incertitude de la prévision, l’incertitude sur l’élévation future de la surface piézométrique, liée au potentielles variations des propriétés hydrauliques du lit du Rhône reste importante.
Le chapitre 5 résume les résultats des études présentées, fournis des recommandations quant aux approches de modélisations hydrogéologiques d’une manière générale et dans le cadre de la 3e Correction du Rhône., Characterizing river-groundwater interactions in alluvial aquifers is essential when forecasting the impact of river management strategies, such as river restorations, on the overall water resources distribution and dynamics. Therefore, the development of methods and calibration approaches that allow for better identification of the spatial and temporal characteristics of the hydraulic properties of the aquifer and the riverbed are required. Moreover, the analysis of commonly made assumptions, such as constant hydraulic properties of the streambed, isotropy of the aquifer hydraulic conductivity and steady-state calibration, is important in order to identify potential biases in predictions and related uncertainties.
Chapter 1 introduces the relevant concepts and methods as well as the framework, i.e. the 3rd Rhône River Correction, and the purpose of the Ph.D. thesis.
Chapter 2 presents a method that assesses the temporal variations of the hydraulic properties of the riverbed. The method is based on the inversion of a numerical convolution between an aquifer unit step response and stream stage variations. Calibrations against successive time series of observed water table variations allow to estimate the transience in the riverbed properties. A synthetic analysis demonstrated the robustness of the method and its application to field data pointed out the influence of climatic events on the transience in riverbed hydraulic properties.
Chapter 3 aims at understanding how simplifications in modeling practice regarding horizontal isotropy of the aquifer hydraulic conductivity affect the estimated uncertainty of predictions. It is demonstrated that assuming isotropy or fixed anisotropy may cause the predictive uncertainty of the water table elevation to be underestimated. Then, by taking into account the uncertainty in aquifer anisotropy, it is shown that calibration against transient data allows to achieve a better estimation of the aquifer and riverbed hydraulic parameters and to reduce the predictive uncertainty of water table elevations.
Chapter 4 presents the model forecasting and related uncertainty of the water table elevation in the area of Sion (Switzerland) in the framework of the modifications projected by the 3rd Rhône River Correction. Furthermore, the predictive uncertainty related to model calibration is complemented by scenario modeling taking into account the uncertainties in the future state of the Rhône riverbed hydraulic properties and geomorphologies. The results show that although the calibration process can significantly reduce the predictive uncertainty, the uncertainty in the future elevation of the water table, related to potential variations in the hydraulic properties of the Rhône riverbed, remains important.
Chapter 5 summarizes the results of the studies and provides recommendations and perspectives regarding hydrogeological modeling approaches in general and in the framework of 3rd Rhône River Correction. - PublicationAccès libreHydraulics and sedimentary processes in the karst aquifer of Milandre (Jura Mountains, Switzerland)(2017)
; Cette thèse vise à caractériser les processus sédimentaires s’opérant dans l’aquifère karstique de Milandre (Suisse), à la fois grâce à des observations de terrain à l’intérieur du réseau de conduits et à l’aide de la modélisation numérique. Un modèle de tuyaux, qui reproduit la physique du système, est développé sur la base de mesures de charges hydrauliques, de débits et de vitesses d’écoulement mesurés dans le réseau spéléologique. Les simulations d’écoulement permettent de calculer la contrainte de cisaillement limite moyenne et la vitesse de cisaillement dans les conduits, qui sont utilisées pour évaluer où et quand l’érosion et le dépôt de sédiment sont probables. Les prédictions du modèle sont comparées à des observations de terrain variées. Dans la rivière souterraine de Milandre, une station de surveillance de la sédimentation a été en fonction pendant 11 ans. La turbidité et la composition des sédiments en suspension ont été surveillées à trois sites dans le système souterrain et à l’exutoire pérenne au cours de la même période. De plus, l’évolution de la granulométrie et de la teneur en bactéries fécales de la charge sédimentaire à la source a été analysée au cours d’un événement de crue. Les simulations sont en ligne avec les observations disponibles. Le modèle conceptuel suivant est proposé : lors d’événements de faible intensité (débit maximal aux exutoires d’environ 400 L·s-1), l’essentiel de la turbidité observée aux sources provient de la remobilisation de sédiments karstiques (turbidité autochtone). La turbidité provenant de la surface (allochtone) peut atteindre la zone saturée avec un délai allant jusqu’à 3 jours, mais dans la plupart des cas elle n’est pas détectée à la source. Quand le pic de débit augmente, le délai entre le pic de crue et le pic de turbidité allochtone diminue, alors que l’intensité du pic de turbidité allochtone augmente. Les événements de crue d’intensité modérée à forte induisent donc une réponse mixte, à la fois autochtone et allochtone aux sources, alors que le signal de turbidité est fortement influencé par les processus de remobilisation de sédiment dans le système karstique. Le modèle a mis en évidence le fait que la contrainte de cisaillement limite est maximale durant le remplissage et la vidange des différents niveaux de conduits du réseau karstique. Cet effet conduit à la génération de pics secondaires de turbidité durant la phase de récession de la crue. Ces pics secondaires de turbidité ont été observés tant dans la rivière souterraine qu’aux exutoires du système. À moyen terme, le modèle prédit que les processus d’érosion et d’accumulation de sédiment sont tous les deux fréquents aux abords de la rivière souterraine. Par contre, dans les galeries épiphréatiques les plus hautes, une accumulation nette de sédiment est prévue par le modèle. D’après les observations, les flux sédimentaires sont principalement contrôlés par l’hydrodynamique du système karstique. Cependant, une composante saisonnière apparaît dans les variations de la concentration de sédiment à la source. Ce cycle annuel est attribué à une disponibilité accrue de sédiments de surface durant l’automne. En terme de composition, une augmentation pluriannuelle de la teneur en phyllosilicate dans les sédiments en suspension est observée. La concentration en phyllosilicate apparaît bien corrélée avec la température des eaux souterraines, autant à l’échelle saisonnière qu’à l’échelle pluriannuelle., This thesis aims at characterizing the sedimentary processes taking place in the karst aquifer of Milandre (Switzerland) both by direct observation inside the conduit network and through numerical modeling. A physics based pipe flow model of the downstream part of the karst system is developed on the basis of measurements of hydraulic heads, flow rates and flow velocities performed in the speleological network. The flow simulations allow to compute the mean boundary shear stress and the shear velocity in the conduits, which are used to assess when and where erosion and deposition of sediments are likely to occur. The model predictions are compared to various field observations. In the Milandre cave stream, a sedimentation monitoring station has been in operation for 11 years. The turbidity and the suspended sediment composition have been monitored at three locations in the underground system and at the perennial outlet over the same time period. Furthermore, the evolution of the grain size and fecal bacteria content of the suspended solids discharged at the spring has been analyzed over the course of a flood event. The simulations are in good agreement with the available data. Overall, the following conceptual model of sedimentary fluxes in the Milandre system is proposed: during low intensity flood events (maximum discharge at the outlets of ∼400 L·s-1), the bulk of the turbidity observed at the springs comes from the remobilization of karstic sediments (autochthonous turbidity). Soil derived (allochthonous) turbidity may reach the saturated zone with a delay of up to 3 days, but is often not detected at the spring. As the peak discharge of the event increases, the delay between the flood peak and the allochthonous turbidity peak shortens and the intensity of the allochthonous turbidity peak increases. Moderate to intense flood events thus yield a mixed autochthonous and allochthonous turbidity response at the springs, while the turbidity signal is mostly shaped by the processes of sediment remobilization in the karst system. The model highlights the fact that the mean boundary shear stress reaches a maximum during the flooding and the emptying of the different levels of conduits of the karst network. This leads to the generation of secondary turbidity peaks during flow recession, which are observed both in the cave stream and at the outlets of the system. In the medium term, the model suggests that both the accumulation and the erosion of sediments are frequent along the cave stream. In contrast, the uppermost epiphreatic passages are predicted to act as effective sediment traps. From the observed data, it appears that the sediment fluxes are mainly controlled by the hydrodynamics of the karst system. There is however a seasonal component in the variations of the sediment concentration at the spring. This annual cycle is attributed to an enhanced soil sediment availability during fall. In terms of composition, there is a pluriannual increase in the phyllosilicate content in the suspended sediment. The phyllosilicate concentration was found to be well correlated with groundwater temperature, both on a seasonal and on a pluriannual scale. - PublicationAccès libreRecharge quantification and continental freshwater lens dynamics in arid regions: application to the Merti aquifer (eastern Kenya)(2015)
; ; Avec une population de près de 400'000 personnes, le camp de réfugiés de Dadaab est le plus grand au monde. Il est situé dans la région semi-aride de l’est du Kenya, proche de la frontière somalienne. L’unique ressource permanente en eau potable pour les réfugiés et les communautés locales provient de l’aquifère de Merti, qui consiste en une large lentille d’eau douce souterraine de 250 km sur 50 km, et qui est entourée par de l’eau salée. L’augmentation des volumes pompés, des signes d’augmentation de la salinité de l’eau et l’incertitude sur la diminution du niveau de l’eau souterraine ont fait prendre conscience du risque d’épuisement de la ressource. Ces observations ont abouti à la nécessité de mieux caractériser la recharge de l’aquifère ainsi que la dynamique entre la lentille d’eau douce et l’eau salée environnante.
Dans un premier temps, une nouvelle méthodologie a été développée pour quantifier la recharge concentrée à travers un modèle numérique, basé sur la physique des écoulements, couplant l’eau souterraine et l’eau de surface. Ce modèle a eu pour but de reproduire les surfaces inondées durant les crues et a été calibré à l’aide d’images satellites. Deuxièmement, la dynamique de la lentille d’eau douce a été investiguée par une série de modèles numériques synthétiques. Ces modèles ont permis d’analyser les effets des taux de recharge et leurs mécanismes (pluie, recharge concentrée) sur la géométrie de la lentille et de comparer ces géométries simulées avec la géométrie réelle de la lentille de l’aquifère de Merti. Ces deux approches ont été contre-validées à l’aide d’un réseau de monitoring de l’eau souterraine composé de vingt stations de mesures à haute résolution temporelle (15 min), installé sur toute la lentille en septembre 2013. Finalement, les résultats de ces trois axes de recherche ont été combinés dans un modèle numérique régional.
L’approche développée dans cette étude a permis de quantifier une recharge concentrée entre 195 et 329 x 106 m3/a. La recharge diffuse sur la lentille d’eau douce est quant à elle estimée entre 12 et 62 x 106 m3/a. Comparée à ces taux de recharge, l’extraction courante de l’eau souterraine (environ 4.8 x 106 m3/a) est considérée comme durable à l’échelle de l’aquifère. Cependant, la recharge totale est environ 50 à 100 fois plus grande que l’inféroflux traversant le niveau exploité, calculé à l’aide du gradient hydraulique de l’aquifère et des valeurs de transmissivité issues des essais de pompage. Cet écart a mené à postuler la présence d’un aquifère multi-couche considérablement plus épais qu’uniquement l’horizon couramment exploité, mais avec une importante incertitude sur son épaisseur et sur le gradient vertical de salinité entre la lentille d’eau douce et l’eau salée sous-jacente.
Les modèles numériques ont révélé la très grande inertie de l’aquifère et ont également confirmé que la recharge a principalement lieu de manière concentrée lors des évènements de crue, juste en amont de la lentille d’eau douce. La grande inertie de l’aquifère est cohérente avec les faibles variations des niveaux d’eau et de la conductivité électrique (salinité) observées avec le réseau de monitoring.
En conclusion, ce travail ouvre de nouvelles perspectives pour la quantification de la recharge en milieu aride à semi-aride lors d’évènements de crue. Il a également permis de mettre en avant la nécessité de poursuivre le monitoring de l’aquifère et de mener de nouvelles investigations sur l’épaisseur de l’aquifère afin de confirmer les résultats de la recharge., The Dadaab refugee camp, the largest refugee camp in the world with a population of approximately 400’000 persons, is located in the arid to semi-arid eastern Kenya, close to the Somali border. The only permanent water resource for the refugees and the host communities comes from the Merti aquifer which consists in a large continental freshwater lens of 250 km by 50 km surrounded by salty water. The increasing groundwater abstractions as well as signs of increasing salinity and uncertainty on the water level depletion led to the necessity to better characterise the aquifer recharge and the dynamics between the freshwater lens and the surrounding salty water.
Firstly, a new methodology was developed for quantifying the concentrated groundwater recharge through a physically-based coupled surface/groundwater numerical model reproducing inundated surfaces during flood events which is calibrated with inundated surfaces derived from satellite images. Secondly, the dynamics of freshwater lenses are investigated with a series of synthetic numerical models. These models aim to analyse the effect of the recharge rates and mechanisms (rainfall, concentrated recharge) on the freshwater lens geometries and to compare these geometries with the observed geometry of the freshwater lens of the Merti aquifer. These two approaches are cross-validated owing to a groundwater monitoring network of twenty high time-resolution devices installed over the whole freshwater lens in September 2013. Finally, results from these three axes are combined in a regional numerical model.
The approaches developed in this study allowed to quantify a concentrated groundwater recharge to be between 195 and 329 x 106 m3/y. Diffuse recharge contributing to the freshwater lens is estimated to be between 12 and 62 x 106 m3/y. Compared to these recharge rates, the current groundwater extraction (about 4.8 x 106 m3/y), is considered as sustainable on the regional scale. However, this recharge is about 50 to 100 higher than the axial flow estimated with the gradient and the transmissivities obtained with pumping tests. This discrepancy led to postulate the presence of a multi-layer aquifer much thicker than the currently exploited horizon but with uncertainties on its thickness and the vertical salinity gradient.
The synthetic numerical model revealed a very high inertia of the Merti aquifer and confirmed that the recharge of the aquifer is mainly controlled by concentrated recharge on flood plains in the upstream area of the freshwater lens. The high inertia of the aquifer is consistent with the very small groundwater level and electrical conductivity variations observed with the monitoring network.
As conclusion, this work opens new perspectives for the quantification of groundwater recharge in arid to semi-arid areas occurring during large scale flood events. However, it also showed the necessity to continue the monitoring of the aquifer and to carry out further investigations on the aquifer thickness if further exploitations are foreseen. - PublicationAccès libreUncertainty propagation and global sensitivity analysis in multi-layered hydrogeological models of flow and lifetime expectancy(2015)
; The main focus of this thesis is the uncertainty propagation (UP) and global sensitivity analysis (GSA) in complex hydrogeological numerical models. Various methods are presented with applications on numerical models for the groundwater flow and mean lifetime expectancy (MLE) in the scope of Andra's (French National Radioactive Waste Management Agency) project for the geological disposal of high-level and intermediate-level long-lived radioactive wastes in a highly impermeable layer from Callovo-Oxfordian age (COX) in France.
A state of the art is provided for the theory of uncertainty propagation and for the methodologies of sensitivity analyses in a broad sense. Methods for UP are provided from 2-levels Factorial Designs to quasi-random samplings. GSA techniques encompass screening methods such as the Morris Measures and the Derivative-based Global Sensitivity Measures (DGSM), and also the so-called Sobol' indices based upon the variance decomposition of the response of interest. Meta-modelling techniques such as polynomial regression and Polynomial Chaos Expansions (PCE) meta-models are employed as surrogate models for UP and GSA purpose at negligible computational-costs. A comparison of GSA techniques upon various complex analytical test-functions was undertaken with the purpose of determining a relevant method to be employed in the context of “screening” out unimportant variables in computer-intensive, high-dimensional models.
A numerical model of groundwater flow and lifetime expectancy is employed for assessing the effect of uncertain advection-dispersion parameters and their spatial distributions upon the MLE from a target zone inside the domain. The model is a 2-dimensions synthetic cross-section of the eastern region of the Paris Basin (Meuse/Haute-Marne sector). This model was used as an exploratory tool for sensitivity analysis methods applied upon numerous sets of uncertain hydrodynamic and dispersion parameters in 15 layers. The uncertainty characterizing the permeability-porosity values in aquifer formations encompassing the COX have proved to add much of variability to the MLE calculated from the target zone. The model also served at exploring the effect of the spatial variability of permeability-porosity parameters in two main aquifer sequences on the groundwater flow rates and MLE in the model. The variabilities of the output responses are mainly due to the uncertainty upon the means and variances of the permeability-porosity distributions, as well as the longitudinal correlation lengths, in each sequence.
Then, a 3-dimensions high-definition hydrogeological model representing the Meuse/Haute-Marne sector in the eastern region of the Paris Basin is a more comprehensive numerical model incorporating realistic geometries, fractures, heterogeneities and discontinuities encountered on field. A sensitivity analysis of the MLE from a given zone located in the middle of the COX layer was performed by perturbing the hydraulic conductivities and porosities values in fourteen hydrogeological formations. The uncertain permeability-porosity parameters in the aquifer formations from Bathonian in the Dogger sequence, and Rauracian-Sequanian in the Oxfordian sequence, have strong and rather non-linear effects on the variability of the output response of interest.
The methodologies for UP and GSA employed in the present thesis have proved to be very efficient when applied to large hydrogeological models of groundwater flow and MLE. In particular, quasi-random sampling methods offer a flexible frame for providing the uncertainty distribution of the output response of interest at low computational costs. Screening techniques provide a fast estimation for the overall contribution of each input variable to the variability of the output. Meta-modelling techniques such as PCE proved highly accurate in individualising the low- and high-order effects of each input variable upon the output response of interest. - PublicationAccès libreSondages géoélectriques "null-arrays" pour la caractérisation des structures de subsurface(2013)
; Le terme "géoélectrique null-array" est utilisé pour les configurations géoélectriques qui mesurent une différence de potentiel nulle sur un espace homogène. Ce papier présente une étude du comportement de trois de ces null-arrays - le midpoint null-array, le Wenner-γ null-array et le Schlumberger null-array - sur différentes structures verticales, en mesure en profil et en mesure azimutale. Une analyse numérique est présentée avec une approche qui utilise un logiciel de calcul hydrodynamique, en utilisant l'analogie entre les théories hydrodynamiques et électriques. Cette analyse numérique concorde avec les mesures de terrain effectuées avec les méthodes géoélectriques classiques et les null-arrays. Une campagne de terrain a été effectuée sur une carrière aux Breuleux (Jura, Suisse) afin de détecter les fractures et failles verticales ainsi que leurs orientations, qui pouvaient aussi être déterminées et mesurées sur le mur de la carrière. Une autre campagne a été menée à Vers chez le Brandt (Neuchâtel, Suisse) pour localiser la grotte et déterminer son orientation. Une dernière campagne à été menée sur la plaine de Buix (Jura, Suisse) dans le but de détecter et caractériser la position et l'orientation de chenaux. Cette étude a montré que certains null-arrays donnaient des résultats plus précis et sont mieux adaptés pour détecter et caractériser précisément ces structures verticales que d'autres méthodes classiques. Le Wenner-γ null-array a montré qu'il est le plus efficient pour la détection de structures verticales et plus sensible que les autres null-arrays. Le Schlumberger null-array donne aussi de bons résultats, mais sa sensibilité est légèrement plus faible que pour le premier. De plus, cette configuration est nettement moins rapide à mettre en oeuvre que les autres. Le midpoint null-array est le plus efficace pour la détermination de l'orientation des structures, alors que le Schlumberger null-array permet d'ajouter de la précision à la mesure. Finalement, cette étude a mené à l'élaboration de conseils d'application de ces méthodes, comme le choix des configurations ou les dimensions à utiliser en pratique., The term "geoelectric null-array" is used for those geoelectrical configurations measuring a zero potential difference above a homogenous half-space. This thesis presents a study of the behaviour of three of these null-arrays - the midpoint null-array, the Wenner-γ null-array and the Schlumberger null-array - above different vertical structures, using profiling and azimuthal measurements. A numerical analysis is presented with an approach which uses a hydrodynamic software based on the analogy between hydrodynamic and electric theories. This numerical analysis provides similar results to the field data acquired with the classical and null-array geoelectrical methods. Field measurements have been carried out on a quarry located in Les Breuleux (Jura, Switzerland) with the aim to detect vertical fractures and faults as well as to determine their orientations. Other measurements were performed in Vers chez le Brandt (Neuchâtel, Switzerland) with the aim to localise a cave and determine the orientation. A last data set was acquired at Buix (Jura, Swizerland) with the aim to detect and characterise the position and orientation of channels. These studies showed that some of these null-arrays provided more accurate results and are more capable of detecting and characterising precisely these vertical structures than other classical methods. The Wenner-γ null-array was the most efficient for the detection of vertical structures and the most sensitive of the three null-arrays. The Schlumberger null-array provided good results, but the sensitivity was slightly lower. Moreover, this configuration was clearly more time-consuming than the others. The midpoint null-array is the best to detect orientation of structures, while the Schlumberger null-array added accuracy to the measurements. Finally, this thesis led to the elaboration of advices for a field application of the methods, as the choice of the array or of the array size. - PublicationAccès libreRegional simulation of coupled hydromechanical processes in fractured and granular porous aquifer using effective stress-dependent parameters(2013)
; Field observations and laboratory experiments have clearly demonstrated that heavily perturbed / exploited aquifers are subject to 3D deformations, which may cause significant socio-economic impacts at regional scale. Most common examples include: (1) excessive pumping of groundwater from deep aquifers leading to land subsidence; (2) deep excavation of tunnels in permeable geological units resulting in dangerous differential consolidation, especially for dams; and (3) fluid injection into deep reservoirs causing ground uplift and microsismicity. These manifestations are due to a substantial modification of water pressures within the aquifer, leading to effective stress variations, and deformations. Moreover, such deformations modify hydrodynamic parameters, i.e., hydraulic conductivity, porosity and storage coefficient. In confined or deep aquifer systems, hydrodynamic parameters have to be considered as effective stress-dependent variables. In such environments the assumption of constant parameters can lead to significant quantitative errors. The afore-mentioned fluid-to-solid hydromechanical processes seem to be essentially governed by hydrogeological, geomechanical and structural properties of the aquifer system. In order to take into account the major processes, as well as their principal properties, regional coupled hydromechanical simulation necessarily requires simplification of the governing equations to be operational in real, large scale, hydrogeological systems.
In the present thesis, model functions relating effective stress to hydrodynamic parameters are developed from fundamental hydrogeological and physical concepts, and implemented in the groundwater flow equation. Proposed stress-dependent equations are verified by a comparison with laboratory and field data. This is carried out for (1) fractured aquifers, i.e., consolidated rocks whose porosity results principally from the presence of fractures, cracks, joints and faults, and (2) granular porous aquifers, i.e., unconsolidated rocks whose porosity results from voids between solid grains. The relation between porosity and stress is also used to elaborate a deformation model for solving aquifer vertical volume change, i.e., ground settlement / uplift. A modelling approach is proposed in order to solve fluid-to-solid hydromechanical processes at regional scale, considering detailed geological structures. In this numerical method, hydrodynamic parameters are considered as stress-dependent variables.
Exact analytical solutions solving flow in a media under stress are developed in order to verify the numerical method. The proposed approach is then applied to the analysis of real case studies. In particular, to (1) the abnormal deformation of the Zeuzier arch dam (Wallis, Switzerland) due to the drainage of an unexpected confined aquifer by the Rawyl exploratory adit; (2) the problematic of water inflow into tunnels based on the geological investigations undertaken by the Lyon-Turin railway project for the 57 km basis tunnel; as well as (3) the anthropogenic land subsidence affecting the Mexico City basin.
Quantitative studies of deep aquifer systems considering constant hydrodynamic parameters result in non-accurate volumetric discharge rates and pressure head fields. On one hand, increasing effective stress leads to decreasing hydrodynamic parameters. This results in a diminution of volumetric flow rates through a deep reservoir or in a deep excavation. Moreover, the decrease of water pressure is slowed down due to the decrease of hydraulic conductivity. This has repercussion on consolidation time. On the other hand, if - and only if - the rock is elastic, decreasing effective stress can lead to increasing hydrodynamic parameters and volumetric discharge rates.
For analytical solutions of volumetric discharge rate in deep wells or into a tunnel, the dependency of hydrodynamic parameters on effective stress can be taken into account by using a factor allowing stress consideration; whereas, in numerical analysis, such a process can be considered by implementing stress-dependent parameters in the groundwater flow and aquifer deformation models.
The proposed numerical approach for fluid-to-solid coupled hydromechanical processes, is computationally simple, based on few unknowns, and efficiently reproduces regional consolidations in geologically oriented 3D meshes. This method is critical for hydrogeological and geomechanical quantitative studies investigating the sensitivity of deep aquifers on decreasing / increasing effective stresses. In particular for regional scale projects where water pressure may be subject to substantial modifications, such as dam construction or tunnel excavation, geologic radioactive waste repositories, deep reservoir exploitation for CO2 sequestration, geothermal energy production, as well as extraction of groundwater and / or hydrocarbons. - PublicationAccès libreInverse modeling of groundwater flow in the Rhône alluvial aquifer - impact of the Third Rhône correction(2013)
; ; Le principal objectif de cette thèse est l’évaluation quantitative de l’impact de la troisième correction du Rhône (PR3) sur l’aquifère alluvial de la plaine du Rhône. Le PR3 est un projet d’aménagement, dont le but est d’une part l’amélioration de la protection contre les inondations, et d’autre part la renaturalisation du lit de la rivière Rhône (Valais, Suisse). L’approche considérée pour améliorer la capacité de drainage du Rhône est l’élargissement du lit de la rivière, ce qui causera un abaissement du niveau de la rivière (d’environ 0.9m en moyenne).
L’impact du PR3 sur les eaux souterraines a été estimé avec un modèle numérique d’écoulement permanent. Ce modèle a été calibré avec des données de charge hydraulique. Le modèle numérique permet d’intégrer les données existantes, et d’étudier ensuite le comportement possible du système lorsqu’il sera soumis à des conditions qui diffèrent de celles investiguées actuellement (état restauré). Quand le modèle est utilisé pour simuler l’état futur du système, ses prévisions contiennent nécessairement un certain degré d’incertitude. Deux sources d’incertitude ont été considérées dans ce travail : d’une part la non-unicité de la solution du problème inverse, qui consiste à estimer les paramètres du modèle sur la base des mesures disponibles de charge hydraulique; d’autre part l’incertitude concernant l’évolution de la conductivité hydraulique du lit du Rhône suite à la restauration. L’incertitude sur les prévisions découlant de la non-unicité du problème inverse a été traitée grâce à la méthode du “null Space Monte Carlo” (NSMC) implémentée dans PEST. L’incertitude induite par l’état futur incertain du lit du Rhône a été traitée avec une analyse de sensibilité des prévisions par rapport à la conductivité du lit de la rivière.
Le modèle est basé sur une compréhension conceptuelle du système, acquise grâce à l’interprétation des données disponibles. Ces données suggèrent clairement que le système est fortement contrôlé par des paramètres à l’échelle régionale, et que les interactions de l’eau souterraine avec le Rhône, ainsi qu’avec les canaux, joue un rôle clé dans le fonctionnement de l’aquifère.
Le modèle est cohérent avec les données mesurées sur le terrain (charge hydraulique, zones d’échanges entre eau de surface et nappe phréatique, flux d’échange dans les canaux). Un résultat inattendu du modèle est que les zones d’échanges importants entre eaux superficielles et souterraines se produit sur de brefs tronçon du Rhône et à proximité des gravières et des carrières dans le lit de la rivière.
En supposant que les propriétés du fond du lit du Rhône vont revenir à un état stable et identique à l’état actuel (ce qui implique le développement d’une couche de colmatation), le modèle prévoit que, à terme, le niveau d’eau s’abaisse à cause du PR3. Les résultats de modélisation suggèrent aussi que les canaux de drainage vont considérablement atténuer l’influence de la restauration du Rhône, et réduisent de plus l’incertitude sur les prévisions.
L’incertitude des prévisions découlant de la non-unicité du problème inverse est faibles (une gamme de 0.07m en moyenne entre les quantiles q90 et q10), ce qui indiquent que les prévisions du modèle calibré semblent très fiables dans la majeure partie du domaine. Toutefois, cette faible incertitude doit être interprétée avec précaution : l’étude d’un exemple synthétique simple a montré que dans un modèle non-linéaire, la méthode NSMC peut sous-estimer l’incertitude. De plus, l’incertitude sur les paramètres découlant de la non-unicité du problème inverse, et calculée avec la méthode NSMC, est considérée comme faible en comparaison de l’évolution incertaine des propriétés du lit de la rivière dans l’état restauré. Dans la partie amont du modèle, il a été remarqué que les prévisions dépendent fortement de la conductivité hydraulique du lit de la rivière à un tel point que, en dépit de l’abaissement du niveau d’eau de la rivière, un léger accroissement de la conductivité du lit de la rivière produit une hausse du niveau piézométrique dans la nappe., The main objective of this thesis is the quantitative evaluation of the impact of the Third Rhône correction (PR3) on groundwater within the Rhône alluvial aquifer. PR3 is a river restoration project which aims at improving flood protection and at upgrading the ecologically deficient streambed of the Rhône River (Valais, Switzerland). The approach taken to extend the drainage capacity of the Rhône River is a widening of the streambed, which causes at large a lowering of river stages (of about 0.9 m on average).
The impact of PR3 on groundwater was estimated using a steady state numerical flow model that was calibrated against average hydraulic head. The numerical model allows integrating data and can be used to simulate the system behavior under conditions which differ from the ones of the investigated state. When the model is used to simulate future system behavior, its predictions contain necessarily a certain degree of uncertainty. Two sources of predictive uncertainty were explored herein. On the one hand the non-uniqueness of the solution of the inverse problem, i.e. the estimation of model parameters based on available measurements of hydraulic head; on the other hand the uncertainty as to how the streambed conductance of the Rhône River will be altered as a consequence of the restoration. Predictive uncertainty arising from the non-uniqueness of the inverse problem was assessed by the “null space Monte Carlo” (NSMC) method implemented in PEST; predictive uncertainty associated with the uncertain evolution of the streambed conductance was estimated by a sensitivity analysis of model predictions to streambed conductance.
The model is based upon a conceptual understanding of the system gained through interpretation of available data. Those data clearly suggests that the system is strongly driven by regional controls and that the interaction of groundwater with the Rhône River, as well as with ditches, plays a key role.
The model is consistent with field measurements (head measurements, patterns of SW-GW interaction, exchange flow at ditches). One unexpected result of the model is that high SW-GW exchange flows occur on short stretches of the Rhône River close to gravel pits and at in-stream quarries.
Assuming that streambed properties will return to a stable state identical to the current state (involving the development of a clogging layer), the model predicts that the groundwater table will lower at large in response to the PR3 in the long-term. Modeling results suggest also that drainage ditches considerably attenuate the influence of the restoration, and that furthermore they reduce predictive uncertainty.
The predictive uncertainty arising from the non-uniqueness of the inverse problem is small (q90–q10 inter-percentile range of 0.07 m on average), indicating that the predictions made using the calibrated model seem to be very reliable in most parts of the model. However, this low uncertainty has to be interpreted with care: a simple synthetic example showed that in the context of a nonlinear model, the NSMC methodology potentially underestimates uncertainty. Moreover, parameter uncertainties arising from the non-uniqueness of the inverse problem are considered as low as compared to the uncertain evolution of streambed properties in the restored state. In the upstream part of the model, predictions were found to depend strongly on streambed conductance, to the point that despite the lowering of river stages, already a slight increase of the streambed conductance produces a rising of the groundwater table., Das Hauptziel dieser Doktorarbeit ist es, den Einfluss der 3. Rhonekorrektion (PR3) auf das Grundwasser des alluvialen Rhonetal-Aquifers quantitativ abzuschätzen. PR3 ist ein Projekt zur Flussrestaurierung der Rhone (Wallis, Schweiz) mit den Hauptzielen, den Hochwasserschutz zu verbessern und den Fluss ökologisch aufzuwerten. Das Projekt verfolgt den Ansatz, die Abflusskapazität des Flusses mittels einer Aufweitung des Flussbettes zu erhöhen; generell bewirkt dies eine Absenkung des Wasserspiegels der Rhone (um ca. 0.9 m im Mittel).
Der Einfluss des PR3 auf das Grundwasser wurde mit Hilfe eines stationären numerischen Grundwasserfliessmodelles abgeschätzt, welches unter Verwendung von Mittelwerten des hydraulischen Potentials im Aquifer kalibriert wurde. Das numerische Modell eignet sich gut zur Integration vorhandener Daten und kann dazu verwendet werden, ein System unter veränderten Bedingungen zu untersuchen. Modellvoraussagen über das Verhalten des Systems in der Zukunft sind notwendigerweise mit einer gewissen Unsicherheit behaftet. In dieser Arbeit wurden zwei mögliche Quellen, aus denen sich Unsicherheiten bezüglich der Modellvoraussagen ergeben, berücksichtigt: einerseits die Unmöglichkeit, die Parameter des Fliessmodelles aus den vorhandenen Messungen des hydraulischen Potentials im Aquifer eindeutig zu bestimmen; andererseits die Ungewissheit darüber, wie die Durchlässigkeit der Rhonesohle sich im Zuge des PR3 verändert. Die Schätzunsicherheit, welche aus der fehlenden Eindeutigkeit des Inversen Problems entsteht, wurde mittels der „null-space-Monte-Carlo-Methode“ (NSMC) in PEST bemessen; die Unsicherheit, welche sich aus der Abhängigkeit der Modellvorhersagen von der unsicheren Entwicklung der Durchlässigkeit der Flusssohle ergibt, wurde mittels einer Sensitivitätsanalyse abgeschätzt.
Das Modell basiert auf einem konzeptuellen Verständnis des Systems, welches durch die Interpretation der vorhandenen Daten gewonnen wurde. Diese Daten deuten eindeutig an, dass das System zu einem bedeutenden Teil durch Prozesse auf regionalem Massstab gesteuert wird und dass die Wechselwirkung von Grundwasser mit der Rhone sowie mit Kanälen eine Schlüsselrolle einnimmt für das Funktionieren des Aquifers.
Das Modell ist konsistent mit Feldmessungen (Messungen des hydraulischen Potentials, Muster der Wechselwirkung zwischen Oberflächengewässer und Grundwasser, Austauschraten an Kanälen). Ein unerwartetes Modellresultat ist das Auftreten von hohen Austauschraten zwischen der Rhone und dem Grundwasser auf kurzen Abschnitten der Rhone in der Nähe von Baggerseen und an Stellen mit Kiesentnahmen aus dem Rhonebett.
Unter der Voraussetzung, dass die Eigenschaften des Rhonebettes langfristig den heutigen entsprechen (was die Entwicklung einer Kolmationsschicht mit einschliesst), sagt das Modell als Folge des PR3 langfristig eine Absenkung des Grundwasserspiegels voraus. Die Modellresultate deuten auch an, dass die Drainagekanäle die Auswirkungen der Restaurierung merklich abmindern und dass deren Präsenz ausserdem die Unsicherheit der Modellvoraussagen reduziert.
Die Unsicherheit der Voraussagen, die aus der Schätzunsicherheit der Parameter resultiert, scheinen die Voraussagen des kalibrierten Modelles an den meisten Stellen des Modells sehr verlässlich (durchschnittlicher x90–x10 Interperzentilbereich von 0.07 m). Diese geringe Unsicherheit muss jedoch mit Vorsicht gedeutet werden: ein einfaches synthetisches Beispiel verdeutlichte, dass die NSMC-Methode in nichtlinearen Modellen die Unsicherheit potentiell unterschätzt. Ausserdem wurde die mit der NSMC-Methode ermittelte Schätzunsicherheit als klein eingestuft im Vergleich zur Unsicherheit, die im Zusammenhang mit der ungewissen Entwicklung der hydraulischen Durchlässigkeit der Rhonesohle im restaurierten Zustand besteht. Es zeigte sich, dass die Modellvorhersagen im obstromseitigen Teil des Modells stark von der Durchlässigkeit der Rhonesohle abhängen; so sehr, dass in diesem Bereich schon eine geringfügige Zunahme der Durchlässigkeit der Rhonesohle trotz einer projektbedingten Absenkung des Wasserspiegels der Rhone einen Anstieg des Grundwassers nach sich zieht. - PublicationAccès libreGroundwater 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)
; 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. - PublicationAccès libreEstimation et cartographie de la vulnérabilité des eaux souterraines en milieu urbain(2010)
; ; - PublicationAccès libreField experimentation and modelling of salt transfer in Korba coastal plain: impact of seawater intrusion and irrigation practices(2010)
; Bouhlila, R.Seawater intrusion and agricultural pollution are the major sources of groundwater contamination and salinization in coastal irrigated areas. The Korba coastal aquifer, located in the Cap Bon peninsula, North-East of Tunisia, has been intensively exploited since the seventies. Several studies reported the sharp piezometric drop and groundwater salinization. Seawater intrusion was presented as the major source of salinity and irrigation return flow was rarely considered.
In this study, we are proposing to identify soil and groundwater salinization sources and processes. Besides, an endeavour to quantify solute irrigation return fluxes is presented. To fulfil these objectives experimental and numerical studies were performed.
Two transects perpendicular to the shoreline and following two flow paths (the first landward and the second seaward) were selected. Soil and groundwater investigations were periodically performed along theses transects. They involved soil and groundwater sampling, piezometric measurements and Electrical Resistivity Tomography Imaging. Various geochemical methods were applied to analyse the collected data. Besides, multivariate statistical analysis techniques were performed to assess salinity sources and processes. Modelling of the seawater/freshwater mixing problem was also carried out using PHREEQC code. HYDRUS1D was used to simulate long term salt transfer towards groundwater.
The results show that seawater intrusion and agricultural pollution are the major sources of salinity. Seawater intrusion was controlled by cation exchange, carbonate weathering and redox processes. Agricultural pollution is enriching the groundwater with NO3, Ca, SO4, and K elements. Geochemical and multivariate statistics techniques estimated the seawater penetration length of about 4000 m.
The soil survey confirms the latter result. It shows that the soil solution gets concentrated in the dry season with possible calcite and gypsum precipitation. Rainfall, afterwards, leaches the soil and reduces its salinity.
The unsaturated modelling revealed that the solute flux considerably depended on the irrigation amounts and the vadose zone thickness. It also demonstrated the important role of stormy episode in the leaching process.