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Contribution des processus hydrologiques et hydrogéologiques aux glissements de terrain de grande ampleur : application au contexte tropical de la Réunion

Abstract : The landslide activity is commonly controlled by the variation of hydraulic head inside the instable mass. Thus, the hydrogeological study of landslides is an essential step to predict landslide dynamic, and for the remediation choices. However, the aquifers developed in landslide are generally complexes. Actually, the comprehension of landslide hydrogeological functioning is a major problematic, especially for the deep-seated landslides.Under humid tropical climate, the Salazie landslides (Reunion Island) allow to implement an interesting study to characterized deep-seated landslide hydrogeology and functioning. In this study, we performed a multidisciplinary approach, combining geology, hydrochemistry and numerical modeling (global and deterministic);The geological study allows the construction of the deep seated landslide geological model (Grand Ilet sector). The Grand Ilet landslide, corresponding to 175 Mm3, is the most active part of deep seated instability whose the total volume is estimated at 370 Mm3. The volcano-detritic lithologies (i.e basic breccia) constitute the main geological formation of the unstable mass. This breccia is a present-day reactivation of an old destabilization (> 350 Ma). Furthermore, the extensive deformation generated by the landslide activity allows the formation of decompressed zones.Inside breccias, a continuous aquifer is identified. Here, the groundwater flows are controlled by the geometry of the landslide base. Natural land cover, soils properties, unsaturated zone thickness and warm temperature limit the groundwater recharge. Only the intense rainfall episode (80 mm/event) can generated the recharge of landslide aquifer. During these events, rapid transfers circulating inside the decompressed zones have a significant effect on hydraulic charge variations. A slow component is infiltrated inside the porous medium of breccias.The inverse modeling methods with bimodal Gaussian-Exponential transfer function is applied to study the Salazie landslides dynamic. The results show that landslide speed variations are directly controlled by groundwater level variations during the hydrologic cycle (recharge and recession stage). Consequently, the inverse model is a powerful tool for predicting deep-seated landslide movements and for studying how they function.
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Pierre Belle. Contribution des processus hydrologiques et hydrogéologiques aux glissements de terrain de grande ampleur : application au contexte tropical de la Réunion. Géologie appliquée. Université de la Réunion, 2014. Français. ⟨NNT : 2014LARE0022⟩. ⟨tel-01155266⟩

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