Hdr Année : 2025

Sedimentation and erosion of fluid-solid systems

Résumé

This manuscript synthesizes the main research investigations that I developed since my arrival as associate professor at the University of Tours. These activities are mainly devoted to the physical modeling of extreme events (pyroclastic flows, lahars), involving particulate suspensions, while seeking to understand how sediments transported in these flows settle, and later in their course, over a broader time and space scale, how these sedimentary reservoirs rearrange when exposed to erosion processes during floods. The hazardous nature of these phenomena may often limit their direct observation through video footages, then requiring the implementation of indirect observation methods as well as the description of the physical processes involved through laboratory experiments to overcome the persistent locks of the literature. The first research project presents the description of the fluidization and sedimentation of particles (of variable size, density, and shape) homogeneously suspended into a viscous fluid (of variable density and viscosity). Experiments of gas-solid and liquid-solid suspensions generated in a non-confined reservoir allowed us to propose a general expression of the particle sedimentation velocity from that determined for an isolated particle falling into a pure fluid at rest to which a density and a viscosity correction are required, such as considering the suspension as an equivalent fluid of similar properties. This universal law, determined in the Stokes flow regime, allowed us to describe the effect of the solid concentration as well as that of the particle inertia on the settling velocity. The particle inertia was also observed to control the minimum concentration of the mixture and thus the limit of the bed stability. The investigation of the bed surface fluctuations allowed us to highlight the propagation of concentration waves within the suspension, traveling from the base to the surface, whose wavelengths seem indicating that the bed becomes unstable when the size of heterogeneities may become of the order of that of the reservoir. The second research project presents the transport and sedimentation behavior of the suspensions once released down the dam-break flume. To describe such experimental free-surface flows, made with gas-solid and liquid-solid suspensions taken at different concentrations, we proposed a physical model in which the homogeneous mixture travels and settles independently, at constant speed, while ultimately forming a deposit of constant slope which can be predicted from the two characteristic velocities. Numerical simulations indicate that both the kinematics and deposits geometry are satisfyingly captured by the model, provided that the mixture agitation does not disturb the particle deposition considered similar to that developed in a static suspension of same concentration. Once accumulated down the deltaic plain, the sedimentary reservoir is likely to be confronted to reworking during upcoming events of floods and thus to be affected by erosion through internal seepage flows where a part of the soil, especially the fine matrice, can be ultimately transported towards the surface and gives rise to signatures around the diked river. The third research project presents geophysical observations of the river paleo-environment which provides an overview of the geomorphological situation and allowed us to understand the origin of erosion signatures, while representing the starting point for geomechanical simulations. Numerical predictions of the soil erosion, located beneath the river bed and dikes, developed after repeated episodes of floods, allowed us to explain the distribution of leaks, sand-boils, sinkholes along the protected plain and to characterize situations which may question the safety of the hydraulic facility.

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Dates et versions

tel-05249101 , version 1 (10-09-2025)

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  • HAL Id : tel-05249101 , version 1

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Laurence Girolami. Sedimentation and erosion of fluid-solid systems. Earth Sciences. University of Tours, 2025. ⟨tel-05249101⟩
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