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mi\ UNIVERSITÉ DI: NEUCHÂTEL FACULTÉ DES SCIENCES INSTITUT DE GÉOLOGIE CENTRE D

mi\ UNIVERSITÉ DI: NEUCHÂTEL FACULTÉ DES SCIENCES INSTITUT DE GÉOLOGIE CENTRE D'HYDROGÉOLOGIE Structure et comportement hydraulique des aquifères karstiques Pierre-Yves Jeannin Thèse de doctoral soutenue le 8 juillet 1996 devant un jury constitue des personnes suivantes: Professeur Las/l<> Kiraly, Université de Neuchâtel. Suisse Dr. Alain Mangili, CNRS-Moulis. France Professeur Jacques Mudry. Besançon. Prance Professeur Peler I Smart. Université de Bristol, Grande-Brelu: Professeur François Zwahlen. Université de Neuchâtel. Suisse UNIVERSITE DE NEUCHATEL INSTITUT DE GEOLOGIE FACULTE DES SCIENCES CENTRE D'HYDROGEOLOGIE THESE Soutenue le 8 juillet 1996 à la faculté des Sciences de l'Université de Neuchâtel pour obtenir le titre de Docteur es Sciences par Pierre-Yves Jeannin Structure et comportement hydraulique des aquifères karstiques- devant un jury constitué des personnes suivantes : Professeur Lâszlo Kiraly, Université de Neuchâtel Dr. Alain Mangin, CNRS-Moulis, France Professeur Jacques Mudry, Besançon Professeur Peter L. Smart, Université de Bristol, Grande-Bretagne Professeur François Zwahlen, Université de Neuchâtel IMPRIMATUR POUR LATHESE Structure et comportement hydraulique des aquifères karstiques de M. Pierre-Yves Jeannin UNIVERSITE DE NEUCHATEL FACULTÉ DES SCIENCES La Faculté des sciences de l'Université de Neuchâtel sur le rapport des membres du jury, MM. F. Zwahlen (directeur de thèse), L. Kiraly, J. Mudry (Besançon), A. Mangin (Moulis, France) et P. Smart (Bristol, France) autorise l'impression de la présente thèse. Neuchâtel, le 2 septembre 1998 Le doyen: F. Stoeckli Page de couverture: Image des galeries de la partie profonde labyrinthique du K2-lnnerberglL Image réalisée à l'aide des logiciels TOPOROBOT (M. Heller) et NVELOPE (J. Farine). © 1998, Copyright by P.-Y. Jeannin Speleo Projects, Basel ISBN 3-908495-08-3 Printed in Switzerland A Isabelle A mes parents En souvenir de Philippe, Tarn, Steve et Pascal En souvenir de Thomas Bìtterli et Maja iCoppel décédés au Faustloch quelques jours avant l'impression de ce manuscript. Sommaire Abstract HI Résumé IV Chapitre 1. Introduction et buts 1.1. Introduction générale 3 1.2. Position des problèmes et buts de cette thèse 7 Chapitre 2. Comportement hydrodynamique Introduction 15 2.1. Action COST-65 Suisse, Projets Bure et Hölloch : Cadre théorique, position des problèmes, présentation des sites étudiés et des données disponibles 19 2.2. Estimation des infiltrations efficaces journalières sur le bassin karstique de la Milandrine (Ajoie, JU) 49 2.3. Recharge respective des volumes de roche peu perméable et des conduits karstiques, rôle de l'épikarst 61 2.4. Comportement hydraulique mutuel des volumes de roche peu perméables et des conduits karstiques : conséquences sur l'étude des aquifères karstiques 79 2.5. Lois de pertes de charge dans les conduits karstiques : base théorique et observations 117 2.6. Modélisation des écoulements dans le réseau du Hölloch (Muotathal, Schwyz) 145 2.7. Dispersion and tailing of tracer plumes in a karstic system (Milandre, JU, Switzerland) 155 2.8. Conclusion concernant le comportement hydrodynamique 159 Chapitre 3. Géométrie des réseaux de conduits karstiques 3.1. Position du problème 169 3.2. Géométrie et genèse d'un grand réseau spéléologique : l'exemple du réseau du nord du Lac de Thoune (canton de Berne, Suisse) 173 3.3. Résumé de quelques approches possibles de la géométrie des réseaux de conduits karstiques et comparaison avec nos sites d'étude 189 3.4. Conclusion sur la géométrie des réseaux karstiques 231 Chapitre 4. Conclusion générale 233 Annexes Annexe 1 : Le programme « Modulus » A1 Annexe 2 : Le programme « cheminfractal » A4 Table des matières A7 I Abstract This thesis aims to provide a better knowledge of karst flow systems, from a functional point of view (behaviour with time), as well as from a structural one (behaviour in space). The first part of the thesis deals with the hydrodynamic behaviour of karst systems, and the second part with the geometry of karsttc networks, which is a strong conditioning factor for the hydrodynamic behaviour. Many models have been developed in the past for describing the hydrodynamic behaviour of karst hydrogeological systems. They usually aim to provide a tool to extrapolate, in time and/or space, some characteristics of the flow fields, which can only be measured at a few points. Such models often provide a new understanding of the systems, beyond what can be observed directly in the field. Only special field measurements can verity such hypotheses based on numerical models. This is an significant part of this work. For this purpose, two experimental sites have been equipped and measured: Bure site or Milandrine, Ajoie, Switzerland, and Hölloch site, Muotathal, Schwyz, Switzerland. These sites gave us this opportunity of simultaneously observe hydrodynamic parameters within the conduit network and, in drillholes, the low permeability volumes" (LPV) surrounding the conduits. These observations clearly show the existence of a flow circulation across the low permeability volumes. This flow may represent about 50% of the infiltrated water in the Bure test-field. The epikarst appears to play an important role into the allotment of the infiltrated waters: Part of the infiltrated water is stored at the bottom of the epikarst and slowly flows through the low permeability volumes (LPV) contribu- ting to base flow. When infiltration is significant enough the other part of the water exceeds the storage capacity and flows quickly into the conduit network (quick flow). For the ptireatic zone, observations and models show that the fol- lowing scheme is adequate to describe the flow behaviour: a network of high permeability conduits, of low volume, leading to the spring, is surrounded by a large volume of low permeability fissured rock (LPV), which is hydraulically connected to the conduits. Due to the strong difference in hydraulic conductivity between conduits and LPV, hydraulic heads and their variations in time and space are strongly heterogeneous. This makes the use of piezometric maps in karst very questionable. Flow in LPV can be considered as similar to flow in fractured rocks (laminar flow within joints and joints intersections). At a catchment scale, they can be effectively considered as an equivalent porous media with a hydraulic conductivity of about 10* to 10"7 m/s. Flow in conduits is turbulent and loss of head has to be calculated with appropriate formulas, if wanting any quantitative results. Our observations permitted us to determine the turbulent hydraulic conduc- tivity of some simple karst conduits (k'.turbulent flow), which ranges from 0.2 to 11 m/s. Examples also show that the structure of the conduit network plays a significant role on the spatial distribution of hydraulic heads. Particularity hydraulic transmissivity of the aquifer varies with respect to hydrological conditions, because of the pre- sence of overflow conduits located wfthin the epiphreatic zone. This makes the relation between head and discharge not quadratic as would be expected from a (too) simple model (with only one single conduit). The model applied to the downstream part of Hölloch is a good illustration of this phenomena. The flow velocity strongly varies along the length of karst conduits, as shown by tracer experiments. Also, changes in the conduit cross- section produce changes in the flow velocity profile. Such heteroge- neous flow-field plays a significant role in the shape of the break- through curves of tracer experiments. It is empirically demonstrated that conduit enlargements induce retardation of the breakthrough curve. If there are several enlargements one after the other, an increase of the apparent dispersivity will result, although no diffusion with the rock matrix or immobile water is present. This produces a scale effect (increase of the apparent dispersivity with observation scale). Such observations can easily be simulated by deterministic and/or black box models. The structure of karst conduit networks, especially within the phreatic zone, plays an important role not only on the spatial distribution of the hydraulic heads in the conduits themselves, but in the LPV as wed. Study of the network geometry is therefore useful for assessing the shape of the flow systems. We further suggest that any hydrogeo- logical study aiming to assess the major characteristics of a flow system should start with a preliminary estimation of the conduit net- work geometry. Theories and examples presented show that the geo- metry of karst conduits mainly depends on boundary conditions and the permeability field at the initial stage of the karst genesis. The most significant boundary conditions are: the geometry of the imper- vious boundaries, infiltration and exfirtration conditions (spring). The initial permeability field is mainly determined by discontinuities (frac- tures and bedding planes). Today's knowledge allows us to approxi- mate the geometry of a karst network by studying these parameters (impervious boundaries, infiltration, exfirtration, discontinuity field). Analogs and recently developed numerical models help to qualitatively evaluate the sensitivity of the geometry to these parameters. Within the near future, new numerical tools will be developed and will help more closely to address this difficult problem. This development will only be possible if speleological networks can be sufficiently explored and used to calibrate models. Images provided by speleologists to date are and will for a long time be the only data which can adequately portray the conduit networks in karst systems. This is helpful to hydrogeologists. The reason that we present the example of the Lake Thun karst system is that it illustrates the geometry of such conduits networks. Unfortunately, these networks are three- dimensional and their uploads/Litterature/ 2-these-jeanninpy.pdf