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Perrochet, Pierre

Nom
Perrochet, Pierre
Affiliation principale
Fonction
Professeur ordinaire
Email
pierre.perrochet@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/144

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  • Publication
    Accès libre
    Hydraulics and Turbidity Generation in the Milandre Cave (Switzerland)
    (2021)
    Vuilleumier, Cécile 
    ;
    Jeannin, Pierre-Yves 
    ;
    Marc Hessenauer
    ;
    Perrochet, Pierre 
    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
    Conditioning Multi-Gaussian Groundwater Flow Parameters to Transient Hydraulic Head and Flowrate Data With Iterative Ensemble Smoothers: A Synthetic Case Study
    (2020-6)
    Lam, Dan-Thuy 
    ;
    Kerrou, Jaouhar 
    ;
    Renard, Philippe 
    ;
    Benabderrahmane, Hakim
    ;
    Perrochet, Pierre 
    Over the last decade, data assimilation methods based on the ensemble Kalman filter (EnKF) have been particularly explored in various geoscience fields to solve inverse problems. Although this type of ensemble methods can handle high-dimensional systems, they assume that the errors coming from whether the observations or the numerical model are multivariate Gaussian. To handle existing non-linearities between the observations and the variables to estimate, iterative methods have been proposed. In this paper, we investigate the feasibility of using the ensemble smoother and two iterative variants for the calibration of a synthetic 2D groundwater model inspired by a real nuclear storage problem in France. Using the same set of sparse and transient flow data, we compare the results of each method when employing them to condition an ensemble of multi-Gaussian groundwater flow parameter fields. In particular, we explore the benefit of transforming the state observations to improve the parameter identification performed by one of the two iterative algorithms tested. Despite the favorable case of a multi-Gaussian parameter distribution addressed, we show the importance of defining an ensemble size of at least 200 to obtain sufficiently accurate parameter and uncertainty estimates for the groundwater flow inverse problem considered.
  • Publication
    Accès libre
    Rapid identification of transience in streambed conductance by inversion of floodwave responses
    (2016)
    Gianni, Guillaume 
    ;
    Richon, Julien
    ;
    Perrochet, Pierre 
    ;
    Vogel, Alexandre
    ;
    Brunner, Philip 
    Streambed conductance controls the interaction between surface and groundwater. However, the streambed conductance is often subject to transience. Directly measuring hydraulic properties in a river yields only point values, is time-consuming and therefore not suited to detect transience of physical properties. Here, we present a method to continuously monitor transience in streambed conductance. Input data are time series of stream stage and near stream hydraulic head. The method is based on the inversion of floodwave responses. The analytical model consists of three parameters: x, the distance between streambank and an observation well, a, the aquifer diffusivity, and a the retardation coefficient that is inversely proportional to the streambed conductance. Estimation of a is carried out over successive time steps in order to identify transience in streambed conductance. The method is tested using synthetic data and is applied to field data from the Rh^one River and its alluvial aquifer (Switzerland). The synthetic method demonstrated the robustness of the proposed methodology. Application of the method to the field data allowed identifying transience in streambed properties, following flood events in the Rh^one. This method requires transience in the surface water, and the river should not change its width significantly with a rising water level. If these conditions are fulfilled, this method allows for a rapid and effective identification of transience in streambed conductance.
  • Publication
    Accès libre
    Assessing groundwater quality trends in pumping wells using spatially
    (2015-7)
    Baillieux, Antoine 
    ;
    Moeck, Christian 
    ;
    Perrochet, Pierre 
    ;
    Hunkeler, Daniel 
    When implementing remediation programs to mitigate diffuse-source contamination of aquifers, tools are required to anticipate if the measures are sufficient to meet groundwater quality objectives and, if so, in what time frame. Transfer function methods are an attractive approach, as they are easier to implement than numerical groundwater models. However, transfer function approaches as commonly applied in environmental tracer studies are limited to a homogenous input of solute across the catchment area and a unique transfer compartment. The objective of this study was to develop and test an original approach suitable for the transfer of spatially varying inputs across multiple compartments (e.g. unsaturated and saturated zone). The method makes use of a double convolution equation accounting for transfer across two compartments separately. The modified transfer function approach was applied to the Wohlenschwil aquifer (Switzerland), using a formulation of the exponential model of solute transfer for application to subareas of aquifer catchments. A minimum of information was required: (1) delimitation of the capture zone of the outlet of interest; (2) spatial distribution of historical and future pollution input within the capture zone; (3) contribution of each subarea of the recharge zone to the flow at the outlet; (4) transfer functions of the pollutant in the aquifer. A good fit to historical nitrate concentrations at the pumping well was obtained. This suggests that the modified transfer function approach is suitable to explore the effect of environmental projects on groundwater concentration trends, especially at an early screening stage.
  • Publication
    Accès libre
    Regional Flow Simulation in Fractured Aquifers Using Stress-Dependent Parameters
    (2011)
    Preisig, Giona 
    ;
    Cornaton, Fabien Joel
    ;
    Perrochet, Pierre 
    A model function relating effective stress to fracture permeability is developed from Hooke's law, implemented in the tensorial form of Darcy's law, and used to evaluate discharge rates and pressure distributions at regional scales. The model takes into account elastic and statistical fracture parameters, and is able to simulate real stress-dependent permeabilities from laboratory to field studies. This modeling approach gains in phenomenology in comparison to the classical ones because the permeability tensors may vary in both strength and principal directions according to effective stresses. Moreover this method allows evaluation of the fracture porosity changes, which are then translated into consolidation of the medium..
  • Publication
    Accès libre
    A quantitative approach to spring hydrograph decomposition
    (2008)
    Kovács, Attila
    ;
    Perrochet, Pierre 
    A combined analytical–numerical study for the characterization of spring hydrographs is presented. Two-dimensional analytical solutions for diffusive flux from rectangular blocks of arbitrary shape facilitate a quantitative characterization of exponential hydrograph components. Together with analytical solutions for block discharge, a systematic analysis of numerically simulated spring hydrographs of synthetic karst systems provides an insight into karst hydrodynamics.
    Different hydrograph components do not represent different classes of rock permeability. Hydrographs of individual homogeneous blocks can be decomposed into several exponential components. Discharge hydrographs of symmetric rectangular blocks can be reconstructed by the sum of only three exponential components. Increasing block asymmetry results in an increasing number of exponential components contributing significantly to total discharge.
    Spring hydrographs represent a sum of individual block discharges originating from diffuse infiltration and conduit discharge originating from concentrated recharge. Beyond the inflection of the recession limb, a spring hydrograph can be decomposed in a similar manner to that of individual homogeneous blocks. The presented hydrograph analytical method facilitates the estimation of hydraulic and geometric parameters of karst hydrogeological systems.
  • Publication
    Accès libre
    Simultaneous identification of a single pollution point-source location and contamination time under known flow field conditions
    (2007)
    Milnes, Ellen 
    ;
    Perrochet, Pierre 
    A theoretical framework is presented that allows direct identification of a single point-source pollution location and time in heterogeneous multidimensional systems under known flow field conditions. Based on the concept of the transfer function theory, it is shown that an observed pollution plume contains all the necessary information to predict the concentration at the unknown pollution source when a reversed flow field transport simulation is performed. This target concentration C0 is obtained from a quadratic integral of the observed pollution plume itself. Backwards simulation of the pollution plume leads to shrinkage of the C0-contour due to dispersion. When the C0-contour reduces to a singular point, i.e. becomes a concentration maximum, the position of the pollution source is identified and the backward simulation time indicates the time elapsed since the contaminant release. The theoretical basis of the method is first developed for the ideal case that the pollution plume is entirely known and is illustrated using a synthetic heterogeneous 2D example where all the hydro-dispersive parameters are known. The same example is then used to illustrate the procedure for a more realistic case, i.e. where only few observation points exist.
  • Publication
    Accès libre
    Reply to “Comment on groundwater age, life expectancy and transit time distributions in advective–dispersive systems : 1. Generalized reservoir theory” by Timothy R. Ginn
    (2007)
    Cornaton, Fabien 
    ;
    Perrochet, Pierre 
    We thank T.R. Ginn for his interest in our recently published article (1) on the subject of groundwater age modelling and reservoir theory. In his previous comment (2), T.R. Ginn expresses concern about some conceptual inconsistencies in the formulations presented in our work. We basically agree with the fundaments of his comments, and we wish to continue the discussion. (1) F. Cornaton and P. Perrochet, Groundwater age, life expectancy and transit time distributions in advective–dispersive systems: 1. Generalized reservoir theory, Adv Water Res 29 (2006), pp. 1267–1291, doi :10.1016/j.advwatres.2005.10.009 (2) Ginn TR. Comment on “Groundwater age, life expectancy and transit time distributions in advective–dispersive systems: 1. Generalized reservoir theory”, by F. Cornaton and P. Perrochet. Adv Water Res, in press, doi :10.1016/j.advwatres.2006.09.005.