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  • Publication
    Accès libre
    Hydraulics and Turbidity Generation in the Milandre Cave (Switzerland)
    AbstractKarst aquifers may convey significant sediment fluxes, as displayed by the intense turbidity peaks commonly observed at karst springs. The understanding of the origin of the suspended solids discharged at springs is key in assessing spring vulnerability and securing drinking water quality. The mechanisms for turbidity generation and sediment transport in karst are however difficult to investigate because of the general lack of access to the karst conduits. These processes have been examined in the Milandre Cave, which hosts a karst drain of regional importance, for more than 10 years by means of turbidity monitoring both inside and at the outlets of this karst system. Additionally, the composition of the suspended load (particle‐size distribution and Escherichia coli content) has been monitored over the course of a flood event. These data are compared against a numerical simulation of the mean boundary shear stress inside the conduit network. The following conceptual model for sediment transport through the system is derived: during minor flood events, most of the turbidity comes from underground sediment remobilization, while during medium to intense flood events, soil‐derived turbidity also reaches the spring. Hydraulics in the epiphreatic zone is tightly linked with autochthonous turbidity generation (mostly during the flooding and the flushing of conduits). In comparison, allochthonous turbidity is associated with finer particles, higher E. coli, and higher UV fluorescence. This improves the overall understanding of turbidity generation and could help the monitoring and forecast of pollution events at drinking water supplies.
  • Publication
    Accès libre
    Hydrogeological characterization of karst aquifers in Switzerland using a pragmatic approach
    (2018)
    Malard, Arnauld
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    Des études récentes révèlent que les aquifères karstiques représentent une part importante des réserves en eau souterraine de la Suisse (120 km3) et des ressources renouvelables (8,4 km3/an), bien qu'ils ne s'étendent que sur 20% du territoire. D'une part, les taux d'infiltration élevés et les grandes perméabilités des roches karstifiées rendent les aquifères karstiques très intéressants pour la gestion de l'eau. D'autre part, les systèmes d'écoulement karstiques sont caractérisés par une structure très hétérogène à composantes d’écoulement rapides et lentes (réseau de conduits, stockage phréatique et épikarst) qui conduisent à d'importantes variations hydrodynamiques et à des dynamiques d'écoulement complexes qui ne peuvent être résolues à l'aide d'outils hydrogéologiques standard. Enfin, les aquifères karstiques sont également très vulnérables à la contamination et nécessitent une attention particulière pour leur protection. Par conséquent, malgré des ressources en eaux souterraines intéressantes, les aquifères karstiques sont souvent ignorés, la dynamique de l'écoulement est mal connue et la gestion des eaux souterraines est loin d'être optimale.
    C'est pour ces raisons que l'ISSKA a décidé de présenter le projet Swisskarst dans le cadre du Programme National Suisse de Recherche 61 consacré à la gestion durable de l'eau en Suisse (janvier 2010 - décembre 2013). La motivation du projet Swisskarst était de développer une approche conceptuelle 3D (KARSYS) pour améliorer la caractérisation hydrogéologique des aquifères karstiques. Cette thèse est directement liée au projet Swisskarst et à l'approche KARSYS.
    Tout au long du projet, la forme existante de l'approche KARSYS a été testée sur différents sites en Suisse et à l'étranger afin (i) de tester l'applicabilité sur des sites réels, (ii) de formaliser les étapes méthodologiques et (iii) d'améliorer les opérations standards. Par rapport à la forme initiale de KARSYS (telle que publiée en 2013), des procédures semi-automatisées ont été développées pour générer un réseau de conduits et pour délimiter les bassins d’alimentation en surface. Les applications de KARSYS à de nombreuses études de cas ont montré que l'approche se révèle extrêmement efficace pour documenter les aquifères karstiques épigéniques où les processus karstiques sont en équilibre avec le niveau de base hydrologique, où les contrastes des lithologies permettent d'identifier les roches karstifiées à partir de roches non karstifiées et où les zones phréatiques sont d'extension modérée ou compartimentées en plusieurs unités distinctes. En ce qui concerne les aquifères purement confinés ou lorsque les contrastes lithologiques rendent difficile la distinction entre les roches karstifiées et les roches non karstifiées, l’approche KARSYS reste applicable mais moins performante. Les principales limites à l'applicabilité de KARSYS concernent la précision des données géologiques et des indications hydrologiques concernant les sources karstiques (activité, débit moyen, etc.).
    Comme KARSYS est une approche conceptuelle, des approches numériques de simulation ont été développées en tant qu'extensions. Deux types de modèles de simulation ont été conçus pour la recharge des eaux souterraines : l'un pour les régions alpines où la recharge est dominée par les contrastes de relief, la fonte des neiges et des glaciers et l'autre pour les régions basses où la recharge est dominée par la végétation et les processus sol/épikarst. Les applications de ces modèles permettent de distinguer toutes les composantes des processus de recharge (précipitations, RET, etc., à l'exception de la condensation) dans les différents compartiments des aquifères (stockage dans les sols, épikarst, volumes faiblement perméables, etc.). En plus de ces modèles de recharge, un modèle hydraulique de simulation des débits dans le réseau de conduits a été développé. Ce modèle utilise le réseau de conduits généré et la recharge simulée comme entrées pour reproduire la décharge à chaque exutoire du système d'écoulement. Les applications de ces modèles en interaction constante avec le modèle conceptuel 3D permettent de déduire des propriétés supplémentaires des systèmes d'écoulement (conduits perchés, seuils, etc.). Ces modèles peuvent maintenant aborder diverses questions relatives à l'hydrologie karstique (stockage, impacts de la construction, risques d'inondation, etc.).
    Une autre extension a été développée sous la forme de lignes directrices pour la cartographie de l'information hydrogéologique résultant de l'application de KARSYS. Ces lignes directrices favorisent trois types de cartes hydrogéologiques karstiques selon l'échelle et les enjeux : la carte des eaux souterraines karstiques, la carte des aquifères karstiques et la carte du système d'écoulement karstique.
    Enfin, ce projet a également été l'occasion d'aborder des questions générales sur les eaux souterraines des aquifères karstiques à l'échelle suisse : la recharge annuelle, le stockage minimal à faible débit et le stockage saisonnier et l'évolution attendue des ressources en eaux souterraines avec les changements climatiques. Ces travaux ont permis de fournir des aperçus, des valeurs clés ou des recommandations concernant la dynamique actuelle des eaux souterraines et leur évolution prévue dans les décennies à venir. Ils contribueront à appuyer les décisions concernant les stratégies futures de gestion des eaux souterraines karstiques.
    Les approches et extensions développées dans cette thèse contribuent à améliorer les connaissances sur les aquifères karstiques dans la perspective d’une gestion durable des ressources., Recent studies reveal that karst aquifers represent a significant part of the Swiss groundwater reserve (120 km3) and resource (8.4 km3/year), although they only extend over 20% of the territory. On the one hand, high infiltration rates and large permeabilities of karstified rocks make karst aquifers highly interesting for water management. On the other hand, karst groundwater flow-systems are characterized by a highly heterogeneous structure including quick- and slow-flow components (conduit network, phreatic and epikarst storage) which lead to important hydrodynamic variabilities and complex flow dynamics which cannot be solved by the mean of standard hydrogeological tools. Finally, karst aquifers are also highly vulnerable to contamination and require specific attention for protection. Consequently, in spite of interesting groundwater resources, karst aquifers are often disregarded, flow-dynamics are poorly known and groundwater management is far from being optimal.
    These are the reasons which motivated SISKA to submit the Swisskarst project as part of the Swiss National Research Program 61 dedicated to sustainable water management in Switzerland (Jan. 2010 – Dec. 2013). The motivation of the Swisskarst project was to develop a 3D conceptualization approach (KARSYS) for improving the hydrogeological characterization of karst aquifers. This dissertation is directly related to the Swisskarst project and to the KARSYS approach.
    All along the project, the existing form of the KARSYS approach has been tested on various sites in Switzerland and abroad in order (i) to test the applicability on real sites, (ii) to formalize methodological steps and (iii) to improve standard operations. Compared to the initial form of KARSYS (as published in 2013), semi-automatized procedures have been developed for generating conduit network and for delineating the systems catchment over the ground surface. Applications of KARSYS to numerous case studies showed that the approach reveals extremely efficient for documenting epigenic karst aquifers where karst processes are in equilibrium with hydrological base level, where contrasts of lithologies make it possible to identify karstified rocks from non-karstified rocks and where phreatic zones are of moderate extension or compartmentalized into several distinct units. For pure confined aquifers or where lithological contrasts make difficult to distinguish karstified from non-karstified rocks, KARSYS remains applicable but less fruitful. Main limitations in the applicability of KARSYS concern the precision of geological data and hydrological indications regarding karst springs (activity, mean discharge, etc.).
    As KARSYS is a conceptual approach, numerical approaches of simulation have been developed as extensions. Two types of simulation models have been designed for groundwater recharge: one for alpine regions where recharge is dominated by relief-contrasts, snow and glacier melts and one other for low-land regions where recharge are dominated by vegetation and soils/epikarst processes. Applications of these models make it possible to distinguish all the components of the recharge processes (precipitation, RET, etc., with the exception of the condensation) in the different compartments of the aquifers (storage in soils, epikarst, low permeable volumes, etc.). In addition to these recharge model, a hydraulic model for simulating flows in the conduit network has been developed. This model uses the generated conduit network and the simulated recharge as inputs to reproduce the discharge for each outlets of the flow-system. Applications of these models with a constant interaction with the 3D conceptual model of the karst aquifers make it possible to infer additional properties of flow-systems (perched conduits, thresholds, etc.). These models may now address various issues in karst hydrology (storage, impacts of construction, flood hazards, etc.).
    Another extension has been developed in the form of guidelines for mapping hydrogeological information resulting from the application of KARSYS. These guidelines promote three types of karst hydrogeological maps depending on the scale and on the issues: the karst groundwater map, the karst aquifer map and the karst flow-system map.
    Finally, this project was also the opportunity to address general questions on karst groundwater at Swiss scale: the annual recharge, the minimal low-flow storage and the seasonal storage and the expected evolution of groundwater resources with the climate changes. These works made it possible to provide insights, key-values or recommendations regarding the current dynamics of karst aquifers and their expected evolution in the coming decades. They will contribute to support decision regarding future strategies for karst groundwater management.
    Approaches and extensions which have been developed in this dissertation contribute to improve knowledge on karst aquifers in the scope of improving the sustainable management of groundwater in Switzerland.
  • Publication
    Accès libre
    Hydraulics and sedimentary processes in the karst aquifer of Milandre (Jura Mountains, Switzerland)
    (2017)
    Vuilleumier, Cécile
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    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.
  • Publication
    Accès libre
    The role of tributary mixing in chemical variations at a karst spring, Milandre, Switzerland
    (2007-01-01)
    Perrin, J.
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    Solute concentration variations during flood events were investigated in a karst aquifer of the Swiss Jura. Observations were made at the spring, and at the three main subterraneous tributaries feeding the spring. A simple transient flow and transport numerical model was able to reproduce chemographs and hydrographs observed at the spring, as a result of a mixing of the concentration and discharge of the respective tributaries. Sensitivity analysis carried out with the model showed that it is possible to produce chemical variations at the spring even if all tributaries have constant (but different for each of them) solute concentrations. This process is called tributary mixing. The good match between observed and modelled curves indicate that, in the phreatic zone, tributary mixing is probably an important process that shapes spring chemographs. Chemical reactions and other mixing components (e.g. from low permeability volumes) have a limited influence.
    Dissolution-related (calcium, bicarbonate, specific conductance) and pollution-related parameters (nitrate, chloride, potassium) displayed slightly different behaviours: during moderate flood events, the former showed limited variations compared to the latter. During large flood events, both presented chemographs with significant changes. No significant event water participates in moderate flood events and tributary mixing will be the major process shaping chemographs. Variations are greater for parameters with higher spatial variability (e.g. pollution-related). Whereas for large flood events, the contribution of event water becomes significant and influences the chemographs of all the parameters. As a result, spring water vulnerability to an accidental pollution is low during moderate flood events and under base flow conditions. It strongly increases during large flood events, because event water contributes to the spring discharge.
  • Publication
    Accès libre
    A quantitative method for the characterisation of karst aquifers based on spring hydrograph analysis
    This paper presents a method for characterizing flow systems in karst aquifers by acquiring quantitative information about the geometric and hydraulic aquifer parameters from spring hydrograph analysis. Numerical sensitivity analyses identified two fundamentally different flow domains, depending on the overall configuration of aquifer parameters. These two domains have been quantitatively characterized by deducing analytical solutions for the global hydraulic response of simple two-dimensional model geometries.

    During the baseflow recession of mature karst systems, the hydraulic parameters of karst conduits do not influence the drainage of the low-permeability matrix. In this case the drainage process is influenced by the size and hydraulic parameters of the low-permeability blocks alone. This flow condition has been defined as matrix-restrained flow regime (MRFR). During the baseflow recession of early karst systems and fissured systems, as well as the flood recession of mature systems, the recession process depends on the hydraulic parameters and the size of the low-permeability blocks, conduit conductivity and the total extent of the aquifer. This flow condition has been defined as conduit-influenced flow regime (CIFR).

    Analytical formulae demonstrated the limitations of equivalent models. While equivalent discrete-continuum models of early karst systems may reflect their real hydraulic response, there is only one adequate parameter configuration for mature systems that yields appropriate recession coefficient. Consequently, equivalent discrete-continuum models are inadequate for simulating global response of mature karst systems. The recession coefficient of equivalent porous medium models corresponds to the transition between matrix-restrained and conduit-influenced flow. Consequently, equivalent porous medium models yield corrupted hydrographs both in mature and early systems, and this approach is basically inadequate for modelling global response of karst aquifers.
  • Publication
    Accès libre
    L’eau, sculptrice sur pierre
    (2004-08-25)
    Huber, Nicolas
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    Non, l’intérêt géologique du canton de Neuchâtel ne se limite pas au Creux-du-Van ! Nouvelle preuve avec le quatrième volet de nos cinq découvertes aux côtés d’un spécialiste. Aujourd’hui, sur les traces de l’eau avec l’hydrogéologue Pierre-Yves Jeannin, directeur de l’Institut suisse de spéléologie et de karstologie (Isska).
  • Publication
    Accès libre
  • Publication
    Accès libre
    Vulnerability assessment in karstic areas: validation by field experiments
    (2004)
    Perrin, J.
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    Pochon, Alain
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    Several methods have been developed for vulnerability mapping in karstic areas. These methods need additional validation by field experiments. Several tests have been carried out in the Swiss Jura with natural and artificial tracers. The protective role of some intrinsic properties of the system, such as glacial deposits covering karst, epikarst storage and system dilution effect, have been clearly demonstrated. Use of three tracers in parallel showed the reactivity of the epikarst: all tracers arrived at the same time, but their relative concentration stayed clearly different. A classification of contamination scenarios into four classes is proposed. It is shown that the relevance of some intrinsic properties depends on the considered scenario class. The hydrodynamic state of the aquifer influences greatly flow velocities and can strongly modify contaminant concentrations at the output of the system. The spatial repartition (point vs diffuse) and the quantity of contaminant entering the system will also influence the output response. Hence, results from tracing experiments cannot be used straightforward for obtaining a representative value of flow velocity, dispersion or recovery rate.
  • Publication
    Métadonnées seulement
    Epikarst storage in a karst aquifer: a conceptual model based on isotopic data, Milandre test site, Switzerland
    (2003)
    Perrin, Jérôme
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    The Milandre test site is a karst aquifer characterized by diffuse infiltration, a well developed conduit network, and several tributaries feeding an underground river. Field data include discharge rate measurements, stable isotopes, weekly rainfall and spring-water isotope sampling, and detailed isotope sampling during three flood events. Flood sampling was carried out at several tributaries corresponding to conduit flow, vadose flow and seepage flow. Weekly sampling showed a strong buffering of the rainfall isotopic signal at the spring. This attenuation suggests an important mixing reservoir in the system. Flood events showed highly peaking hydraulic responses but buffered rain isotope responses. These results indicate that the soil and epikarst sub-systems have an important storage capacity. A conceptual model of flow and transport in the soil and epikarst zone is proposed: Soil plays an important role in mixing due to the presence of capillary water storage. Consequently dampened concentrations reach the epikarst despite a rapid hydraulic response. The epikarst acts as the storage element and distributes water as either a base flow component or a quick flow component. When recharge exceeds a given threshold, excess infiltrated water bypasses the soil and epikarst and reaches the saturated zone as fresh flow. Based on this model, the significance of phreatic storage is thought to be limited, at least in Milandre test site. Hence the saturated zone is seen mainly as a transmissive zone through its well developed conduit network. (C) 2003 Elsevier B.V. All rights reserved.