Long-term kinematics of the active Poche landslide, South French Alps : sixty years of slope displacements revealed from the interpretation of multi-source data

Mathieu, A., Mazeau, A. Malet, J.-P.

Institut de Physique du Globe de Strasbourg, CNRS UMR 7516, Université de Strasbourg, 5 rue Descartes, F-67084 Strasbourg Cedex

Although landslides are the dominant erosion mechanism in many mountainous landscapes, estimates of long-term landslide-driven sediment flux remain elusive because landslide displacement data are typically limited to the recent time periods, when on-site monitoring instruments are used for the monitoring.
Combining high-resolution topography from airborne and terrestrial LiDAR (light detection and ranging), orthorectified historical aerial photographs, and morphological observations guided by geometrical modelling, more than sixty years of slope movement on a 1.5-km-long landslide in the Barcelonnette Basin, South French Alps, are analysed. Using these multi-source data, the long-term kinematics is analyzed by 1) tracking the displacement of well identified objects on the landslide body (rocks, gullies, cracks, etc.) and 2) characterizing the changes in the boundaries of the landslides at the scarp and the toe. A method to estimate the uncertainties associated to the range of resolution and accuracy of the different source of remote-sensing data is proposed. In the last decade, higher accuracy in the determination of the displacement direction and amplitude is observed, and a sediment budget is proposed to characterize the volume in transit by combing dGNSS cross-section analysis and terrestial LiDAR.
Using the detailled-LiDAR topography of year 2007, three distinct kinematic zones can be mapped : an upslope source area with an important retrogression of the main scarp, a long narrow transit zone in the main old gully, and a downslope compressional zone. From the kinematic analysis (1948–2010), average velocities are fastest in the upslope and transport zones (1 m.yr-1) and slowest in the downslope zone (<1 m.yr-1). The displacement regime is highly variable over time, as succession of acceleration and decceleration periods are observed in relation to inter-annual changes in the total rainfall amounts.