Effets de la variation spatiale et temporelle des propriétés des terrains sur les défaillances des réseaux enterrés

Les interactions entre la nappe phréatique peu profonde et les réseaux d assainissement sont en général très complexes. Dans le but d étudier l impact des variations du niveau de la nappe phréatique sur les dégradations des conduites, un modèle hydrogéologique numérique tridimensionnel (3D) de la nappe phréatique ayant un aquifère multicouche au sein d une zone de 83 km2 de l agglomération bordelaise (Mérignac, Pessac, Talence) a été développé, calibré et validé. Pour ce faire, un modèle conceptuel de cet aquifère multicouche a dans un premier temps été défini sur la base des descriptions lithostratigraphiques des forages. Puis un travail géostatistique a été entrepris pour construire les limites des formations géologiques et les cartes piézométriques issues de campagnes de mesures de hauteurs d eau de la nappe phréatique. Nous avons par la suite développé une méthodologie originale de transformation de l information lithologique au droit des sondages en perméabilités, puis en champs de perméabilités en utilisant des méthodes géostatistiques. Dans un second temps, les conditions hydrogéologiques ont été simulées en utilisant le logiciel Visual Modflow . Les paramètres hydrogéologiques ont été calibrés manuellement et automatiquement sur la base des niveaux d eau mesurés lors des campagnes de juillet 2010, octobre 2010 et mars 2011. La validation du modèle qui s est effectuée en simulant les cotes piézométriques au pas mi-décadaire allant de juin 1993 à mai 2012 montre que les chroniques piézométriques observées sur les 35 piézomètres du domaine étudié sont correctement restituées et la dynamique de la nappe phréatique est reproduite. Dans un dernier temps, les distributions spatiales et temporelles des cotes piézométriques simulées ont permis d identifier les zones d ennoiement des réseaux d assainissements en comparant les cotes piézométriques aux cotes des réseaux. Ces zones d ennoiement ont été comparées aux principaux secteurs de casses de canalisations. Les secteurs de casses de canalisations sont localisés dans les zones à faible probabilité d ennoiement des conduites. L environnement naturel hydrogéologique pris comme paramètre isolé ne constitue donc pas un facteur explicatif au phénomène de casses des canalisations.The interactions between shallow groundwater and sewer network are generally complex. The aim of this research is to study the impact of the water table level variation on the damages of the buried pipes. For this purpose, a three-dimensional (3D) numerical groundwater flow model of the shallow multi-aquifer of the Bordeaux urban area, within a 83 km2 area (Mérignac, Pessac, Talence), is developed, calibrated and validated. In order to do this, in a first step, a conceptual model was built based on stratigraphic descriptions of the boreholes. A geostastistical study was carried out to build the limits of the geological layers and the piezometrical maps from the measurement campaigns of water table levels. Then, an original methodology to transform the description of lithological units to local hydraulic conductivity values is proposed and the reconstruction of hydraulic conductivity fields at the urban scale was performed based on geostatistical methods. In a second step, hydrogeological conditions were simulated using the finite-difference groundwater flow model MODFLOW-2005 (Visual Modflow ). The hydrogeological parameters were then calibrated manually and automatically based on water table elevation data measured in July 2010, October 2010 and March 2010. The model validation covered 6890 days, from June 1993 to may 2012. The results showed that the simulated heads are quite accurate and reproduce the main dynamics of the system at 35 piezometers. The spatial and temporal distribution of the heads of the piezometers simulated permitted to identify the flooding zones of the sewer network by comparing the piezometric head with the altitude of the networks. The sectors where the pipes are damaged were localized in the zone wherein there is low probability of flooding. Additionally, the natural hydrologic environment, taken as a sole factor, cannot explain the pipe breakage phenomena.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Groundwater-flow characterization in a multilayered karst aquifer on the edge of a sedimentary basin in western France

On the edge of sedimentary basins, burial of geological layers can involve the formation of confined karst aquifers. In some cases, relationships between confined and unconfined karst aquifers can exist and imply an increased difficulty in delineating contributive areas of karst system outlets with accuracy. This work aims to develop a multi-disciplinary approach to highlight groundwater exchanges between multilayered karst aquifers feeding a single spring.Toulon Springs, located in western France, provides the opportunity to study relationships between two multilayered karst aquifers. Hydrograph and chemograph analysis coupled with principal component analysis (PCA) are used to evaluate groundwater-flow origins on the northern edge of a sedimentary basin. Natural hydrodynamic responses (baseflow recession analysis, auto-cross correlation function) show an important volume of water located in a saturated zone of the Toulon karst system. Significant contrasts in water quality as a function of the hydrological regime (flood, recession, and baseflow regime) and comparison with shallow and deep water show the participation of a deep aquifer supporting the flow of Toulon Springs

Examining the Functioning of a Multilayer Karst System: The Case of Toulon Springs (Dordogne, France)

International audienceHydrodynamic and hydrochemical responses of Toulon springs are examined to identify the functioning of the karst system. First, analysis of the recession curve coupled to univariate and bivariate analyses is conducted to determine storage capacity and relationship between rainfall and discharge. Results show a complex system which could provide fast hydrodynamic responses to rainfall solicitation, but have an ineffective drainage network and then high storage capacity. In addition, temporal evolution of hydrochemical parameters leads to identify quick variation of hydrochemical responses during flood events. Furthermore, a principal component analysis (PCA) is conducted on 61 samples and confirms that springs are fed with waters from different origins

Identification des conditions d’écoulement dans la zone non saturée et dans la zone saturée de l’aquifère karstique - Apport du Carbone 13-CMTD dans l’interprétation des équilibres calco-carboniques

International audienc

Characterization of the Functionality of Karstic Systems Based on the Study of the SIc–Pco2 Relation

International audienceThis paper aims at characterizing the functionality of karstic systems. In order to do so, an extend of a method based on the saturated index with respect to calcite (SIc) and CO2 partial pressure (Pco2) is used. The initial method already applied by Peyraube et al. (2012) uses the Pco2 of water at equilibrium with atmosphere (Pco2_eq) and the Pco2 of water for a SIc equal to zero (Pco2_sat). The Pco2_sat variation gives information on flows conditions within the karstic system. It describes the degree of karstification of the system. Systems with developed saturated zone are characterized by under-saturated waters as well as saturated ones with variable Pco2_sat. At last, fissured system has Pco2_sat which vary only slightly. Moreover, variations of Pco2_sat show homogenization capacities of a karst

Decomposition of CO2 Signal in Lascaux Cave

International audienceAir CO2 partial pressure (Pco2) in one of the monitored parts in Lascaux cave (Hall of the Bulls, near the entrance) is variable. For the year 2009, three major components of Pco2 were identified: a seasonal component, a low-frequency component and a high-frequency component. As a first step to understand the presence of Pco2 in caves, this study aims to separate these components and quantify their contributions to the Pco2 signal. Seasonal component is calculated with a sinusoid curve fitted to the measured Pco2. This component is subtracted from the measured Pco2. Then, from the remaining value of Pco2, low- and high-frequency components were separated using Fourier transform, threshold filtering of the frequency and Inverse Fourier transform. Seasonal component is the main contributor (82 % on the average); high-frequency component participates 12 % on the average. Low-frequency component is not present all the year; the average participation is 6 %. Finally, to link these three components with other measured parameters such as temperature, water flow and atmospheric pressure, hypotheses were proposed

Influence de la zone non saturée sur le fonctionnement des systèmes karstiques. Mise en évidence dans les sources du Nord Vaucluse.

International audienc

A field abacus for the estimation of saturation index with respect to calcite

International audienc

Assessing cave internal aerology in understanding carbon dioxide (CO2) dynamics: implications on calcite mass variation on the wall of Lascaux Cave (France)

Carbon dioxide gas is a key component in dissolution and precipitation of carbonates in karst and cave systems. Therefore, characterizing the internal aerology of a cave is essential to obtain the spatiotemporal distribution of temperature and CO2 level. In this research, Lascaux Cave (France), an important adorned cavity, was studied. First, the spatiotemporal distribution of CO2 and temperatures were examined using continuous monitoring at a per minute basis. High-resolution spatial measurements (14 PCO2 locations and 27 locations for temperature) were carried out for a year in the epikarst and the cave (February 2015 to February 2016). The spatiotemporal analysis presents that air and rock temperatures vary for less than a degree Celsius (12.4–12.9 °C). These are controlled by the conduction of the external thermal waves through the overlying calcarenite massif. As a consequence, two seasonal internal aerologic regimes were identified: stratification and convection. These regimes govern the spatiotemporal distribution of the CO2 levels (1.1–3.7%), showing that this parameter is a good natural marker of the internal air movements. Second, a method was proposed to estimate the calcite mass potentially affected by condensation water (dissolution process) and exfiltration water (precipitation process). This method, based on numerical simulations, relies on CO2 and air and rock temperature spatiotemporal distributions in the cave. Third, the method was applied using the case of the left wall of the Hall of the Bulls (one of the most adorned part of the cave). Results showed that the calcite mass, possibly dissolved, varies from 0.0002 to 0.006 g when the mass potentially precipitated is higher (from 0.013 to 0.067 g) depending on the aerologic conditions. This method allows determining which alteration process (e.g., precipitation or dissolution) could eventually lead to the largest variation of calcite on the wall. The results can serve as useful data to the cave experts of the French Ministry of Culture and Communication in terms of Lascaux Cave management policies