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[Observatoire Gravitaire Géoazur] Hydrogeology

- Drainage network geometry versus tectonics in the Argentera Massif (French–Italian Alps)
A. Ribolini and M. Spagnolo

The Argentera Massif (French–Italian Alps), with its uniform lithology, was selected to evaluate how known Plio–Pleistocene tectonics have conditioned the drainage network geometry. The drainage network was automatically derived and ordered from a 10 m-resolution DEM. On hillshade images, alignments of morphological features were identified. The Massif was subdivided into 22 domains of 50 km2 within which the directions of every river channel segment and the direction of the aligned morphological features were compared and contrasted with the strike of tectonic structures measured in the field. Results suggest that the Argentera drainage system is variously controlled by recent tectonics, depending on the Massif sector taken into account. In the NW sector, the vertical uplift is less because the strain has been accommodated in an oblique direction along a lateral thrust. In the SE sector, strain in a predominantly vertical direction along a frontal thrust has resulted in a major vertical displacement. Accordingly, the NW sector is characterized by (i) a strong geometric relationship between the main tectonic structures and the directions of river channels, (ii) longitudinal main rivers bordering the Massif, and (iii) a general trellis pattern within the domains.

In the SE sector, the prolonged uplift has forced an original longitudinal drainage system to develop as a transverse system. This change has occurred by means of fluvial captures that have been identified by the presence of windgaps, fluvial elbows and knickpoints. At the domain scale, intense uplift of the SE sector has prompted the drainage pattern to evolve as a dendritic type with no clear influence of structure in the channel orientations.

Keywords: Drainage network pattern; Channel orientation; Tectonic geomorphology; Remote sensing; Argentera Massif
Geomorphology, Volume 93, Issues 3-4, 15 January 2008, Pages 253-266, doi:10.1016/j.geomorph.2007.02.016

- Experimental analysis of groundwater flow through a landslide slip surface using natural and artificial water chemical tracers
S. Binet, H. Jomard, T. Lebourg, Y. Guglielmi, E. Tric, C. Bertrand and J. Mudry

Artificial and natural tracer tests combined with high accurate electronic distancemeter measurements are conducted on a small landslide with a well known slip surface geometry. Outflow yields and chemical contents are monitored for all the experiment duration and they analyzed to estimate the slip surface hydraulic parameters. The main result is that the slip surface acts as a drain for groundwater flows that evacuates interstitial pressures in the slope and brings the sliding mass to be more stable one.

Keywords: Hydromechanics; Tracer test; Landslides; Slip surface; La Clapière
Hydrological Processes, Volume 21, Issue 25, 1 December 2007, Pages 3463-3472, doi:10.1002/hyp.6579

- Characterization of an internal slope movement structure by hydrogeophysical surveying
H. Jomard, T. Lebourg, S. Binet, E. Tric and M. Hernandez

A hydrogeophysical study was carried out by a water controlled injection within a landslide situated on an active part of the La Clapière landslide foot (Alpes Maritimes, France). Coupling of both real-time geophysical and hydrological follow ups allowed the representation and quantification of the surface water drainage in space and time within the slipped mass. Thus, 30% of the injected water is quickly drained by a complex slipping surface meanly situated at 10-m depth. The transit time between injection and outflow of the water allowed an overloading of about 10 m3 (i.e. 10 tons) comparable with classical rain events in the area. This weight and the associated interstitial pressures increase have not led to any movements asking for the origin of the water volumes which could induce destabilizations. This experiment enabled an accurate redefinition of the internal slope structure and the understanding of the dynamics of the slipped mass with a surface hydraulic request.
Terra Nova, Volume 19, Issue 1, February 2007, Pages 48-57, doi:10.1111/j.1365-3121.2006.00712.x

- Coupling between hydrogeology and deformation of mountainous rock slopes: Insights from La Clapière area (southern Alps, France)
Y. Guglielmi, F. Cappa and S. Binet

Meteoric infiltration influence on large mountainous rock slopes stability is investigated by comparing hydrogeologic and gravitational structures from detailed mapping of the ‘La Clapière’ slope. The slope infiltrated waters are trapped in a perched aquifer that is contained in deposits inside tensile cracks of the upper part of the slope. Flow rates of 0.4 to 0.8 l s−1 from the perched aquifer to the landslide cause landslide accelerations. Numerical modeling shows that a 0.75 l s−1 infiltration yield increases conditions for toppling with failure through tilting of large rock volumes from the perched aquifer bottom down to the foot of the slope.

Keywords: Toppling; Hydromechanical coupling; Gravitational; Modelling; Long-term
C. R. Geoscience, Volume 337, Issue 13, September-October 2005, Pages 1154-1163, doi:10.1016/j.crte.2005.04.016

- Hydrologic measurements in wells in the Aigion area (Corinth Gulf, Greece): Preliminary results
V. Léonardi and P. Gavrilenko

Two wells have been equipped in 2002, in order to provide complementary hydrological information within the framework of the European project devoted to the Corinth Rift Laboratory. In this distensive tectonic domain, temporal series of flow (Neratzes well) and of piezometric fluid level (Trapeza well) have been recorded in 2002. We present here a first analysis of the data. As far as Trapeza is concerned, we present the first results of the calibration of the well, using the tidal and barometric analyses. The strain sensitivity of the well, as well as the aquifer hydraulic conductivity, is derived from tides, whereas the specific storativity of the aquifer and its confinement degree are estimated from the barometric response. These preliminary results show that the aquifer has got properties that make possible the observation of coupled tectonic/hydrological processes. In addition, the time series let appear some fast and permanent variations of level. We analyse these phenomena and detail the various assumptions, which could explain these processes. As far as Neratzes is concerned, we present the first data measured from the flowmeter. Contrary to the piezometric data, the flow measurements are not conventional. At the end of this first year of measurement, we prove that we are able to record the tides effect using the flowmeter. A prospective analysis of all these results will be drawn.

Keywords: Rift Corinth Laboratory; Hydrological measurement; Wells calibration; Water level change
C. R. Geoscience, Volume 336, Issues 4-5, March 2004, Pages 385-393, doi:10.1016/j.crte.2003.11.018