Groundwater-level rise plays an important role in the activation or reactivation of deep-seated landslides and so hydromechanical studies require a good knowledge of groundwater flows. Anisotropic and heterogeneous media combined with landslide deformation make classical hydrogeological investigations difficult. Hydrogeological investigations have recently focused on indirect hydrochemistry methods. This study aims at determining the groundwater conceptual model of the Séchilienne landslide and its hosting massif in the western Alps (France). The hydrogeological investigation is streamlined by combining three approaches: a one-time multitracer test survey during high-flow periods, a seasonal monitoring of the water stable-isotope content and electrical conductivity, and a hydrochemical survey during low-flow periods. The complexity of the hydrogeological setting of the Séchilienne massif leads to development of an original method to estimate the elevations of the spring recharge areas, based on topographical analyses and water stableisotope contents of springs and precipitation. This study shows that the massif supporting the Séchilienne landslide is characterized by a dual-permeability behaviour typical of fractured-rock aquifers where conductive fractures play a major role in the drainage. There is a permeability contrast between the unstable zone and the intact rock mass supporting the landslide. This contrast leads to the definition of a shallow perched aquifer in the unstable zone and a deep aquifer in the intact massif hosting the landslide. The perched aquifer in the landslide is temporary, mainly discontinuous, and its extent and connectivity fluctuate according to the seasonal recharge.
International audienceWe propose an approach to study the hydro-mechanical behaviour and evolution of rainfall-induced deep-seated landslides subjected to creep deformation by combining signal processing and modelling. The method is applied to the Séchilienne landslide in the French Alps, where precipitation and displacement have been monitored for 20 years. Wavelet analysis is first applied on precipitation and recharge as inputs and then on displacement time-series decomposed into trend and detrended signals as outputs. Results show that the detrended displacement is better linked to the recharge signal than to the total precipitation signal. The infra-annual detrended displacement is generated by high precipitation events, whereas annual and multi-annual variations are rather linked to recharge variations and thus to groundwater processes. This leads to conceptualise the system into a twolayer aquifer constituted of a perched aquifer (reactive aquifer responsible of high-frequency displacements) and a deep aquifer(inertial aquifer responsible of low-frequency displacements). In a second step, a new lumped model (GLIDE) coupling groundwater and a creep deformation model is applied to simulate displacement on three extensometer stations. The application of the GLIDE model gives good performance, validating most of the preliminary functioning hypotheses. Our results show that groundwater fluctuations can explain the displacement periodic variations as well as the long-term creep exponential trend. In the case of deep-seated landslides, this displacement trend is interpreted as the consequence of the weakening of the rock mechanical properties due to repeated actions of the groundwater pressure
Abstract. Pore water pressure build-up by recharge of underground hydrosystems is one of the main triggering factors of deep-seated landslides. In most deep-seated landslides, pore water pressure data are not available since piezometers, if any, have a very short lifespan because of slope movements. As a consequence, indirect parameters, such as the calculated recharge, are the only data which enable understanding landslide hydrodynamic behaviour. However, in landslide studies, methods and recharge-area parameters used to determine the groundwater recharge are rarely detailed. In this study, the groundwater recharge is estimated with a soil-water balance based on characterisation of evapotranspiration and parameters characterising the recharge area (soil available water capacity, runoff and vegetation coefficient). A workflow to compute daily groundwater recharge is developed. This workflow requires the records of precipitation, air temperature, relative humidity, solar radiation and wind speed within or close to the landslide area. The determination of the parameters of the recharge area is based on a spatial analysis requiring field observations and spatial data sets (digital elevation models, aerial photographs and geological maps). This study demonstrates that the performance of the correlation with landslide displacement velocity data is significantly improved using the recharge estimated with the proposed workflow. The coefficient of determination obtained with the recharge estimated with the proposed workflow is 78 % higher on average than that obtained with precipitation, and is 38 % higher on average than that obtained with recharge computed with a commonly used simplification in landslide studies (recharge = precipitation minus non-calibrated evapotranspiration method).
Abstract. Pore water pressure, build up by recharge of hydrosystems, is one of the main triggering factors of deep seated landslides. Effective rainfall, which is the part of the rainfall which recharges the aquifer, is a significant parameter. Soil-water balance is an accurate way to estimate effective rainfall. Nevertheless this approach requires evapotranspiration, soil water storage and runoff characterization. Available soil storage and runoff were deduced from field observations whereas evapotranspiration computation is a highly demanding method requiring significant input of meteorological data. Most of the landslide sites used weather stations with limited datasets. A workflow method was developed to compute effective rainfall requiring only temperature and rainfall as inputs. Two solar radiation and five commonly used evapotranspiration equations were tested at Séchilienne. The method was developed to be as general as possible in order to be able to be applied to other landslides. This study demonstrated that, for the Séchilienne unstable slope, the displacement data correlation performance (coefficient of determination) is significantly enhanced with effective rainfall (0.633) compared to results obtained with raw rainfall (0.436) data. The proposed method for estimation of effective rainfall was developed to be sufficiently simple to be used by any non-hydro specialist who intends to characterize the relationship of rainfall to landslide displacements.
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