Nine years of spatial and temporal evolution of the La Valette landslide observed by SAR interferometry

The La Valette landslide located in the Ubaye valley (southern French Alps) has been investigated using 15 differential interferograms realized from ERS-1 and ERS-2 satellite radar images acquired between 1991 and 1999, both in 3-day cycle and TANDEM phase. Displacement values of the landslide have been deduced from the Synthetic Apenture Radan (SAR) interferometric products interpretation and compared with ground laser measurements. Four domains characterized by different velocity fields have been detected. Three of them can be distinguished from aerial photographs and field analysis. The slow velocity of a resistant bar located in the eastern side of the landslide has been detected on SAR interferograms. Between 1991 and 1996, changes in the landslide limits have been observed both in the upper and in the lower part; the changes have been caused by a retrogression of the main scarp and a down slope progression of the main body of the landslide, respectively. The average daily velocity of the landslide between 1991 and 1999 derived by interferometric analysis decreased from 1 to 0.4 cm/day, in agreement with ground-based measurements. A peak velocity of around 2 cm/day was observed in 1996.