Présentations Colloques

    Oral Presentation
    Session 8.05: Karst aquifers
    Vuilleumier Cécile
    A hydrodynamical approach to understand sediment fluxes in a karstic aquifer
    High flows at karstic springs are often associated with strong turbidity peaks. The observed turbidity signal results from the contribution of two potential sources of suspended solids- intra-karstic sediments (autochtonous) and soil particles (allochthonous), as well as from the integration of different processes- erosion, transport and deposition of particles. This study aims at characterizing particle transport in a karstic aquifer by confronting a hydrodynamic model of the system with observations of sedimentation processes in the cave and at the springs.****The Milandre karstic catchment, located in the Swiss Tabular Jura, hosts a major speleological network. The hydrodynamics of the system are investigated by monitoring hydraulic heads in different cave passages and building a variably saturated pipe-flow model. The sedimentation processes have been monitored using a set of sedimentation plates that are distributed at different heights along a section of the stream conduit. At the springs, bacteriological and particle-size distribution analyses were performed to characterize the suspended solids content within the course of a flood event.****To gain more insights into the sedimentation processes, flow velocities and conduit saturation computed by the model are analyzed. In the first stage of a flood event, the velocity in the stream conduit shows a steep increase. Due to the morphology of the conduit cross-section, in a second stage the computed velocity decreases as flow further increases and the conduit gets filled up. These results are consistent with the sedimentation plate observations- erosion is recorded only in the lower part of the conduit, and sediment accumulation is observed in the upper part of the conduit. At the main outlet, a first peak of larger particles is observed, followed by a second peak characterized by smaller particles and a simultaneous increase of E.coli content. These results converge towards a conceptual model of transitioning contributions of sources and of processes- erosion of autochtonous sediments in the early stages of the flood, followed by the transport and deposition of allochthonous particles.


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