Combination of high frequency (Sentinel-2) and high resolution (Pl&#233;iades) satellite images for the monitoring of clayey landslide reactivations, application to the Harmali&#232;re landslide (French Alps).
Authors: Jongmans Denis , Fiolleau Sylvain , Bièvre Gregory , Chambon Guillaume , Lacroix Pascal 
Published: March 2020

&lt;p&gt;Many regions of the world are exposed to landslides in clay deposits, which poses major problems for land management and population safety. In recent years, optical satellite imaging has emerged as a major and inexpensive tool for understanding and monitoring the kinematics of slow moving landslides, such as earthflows/earthslides, through easy access of data and reliable calibration.&lt;/p&gt;&lt;p&gt;The Sentinel-2 optical satellites provide a global coverage of land surfaces with a 5-day revisit time at the Equator. We studied the ability of these freely available optical images to detect landslide reactivations in a zone of 25 km&lt;sup&gt;2&lt;/sup&gt; around the Harmali&amp;#232;re landslide in the Tri&amp;#232;ves area (western Alps, France). This area is characterized by the presence of a thick lacustrine clay layer that is affected by numerous landslides. Using a 9-month time-series of displacement derived from Sentinel-2 data, Lacroix et al. 2018 recently evidenced a precursor displacement of a major reactivation of the Harmali&amp;#232;re landslide that occurred in June 2016.&lt;/p&gt;&lt;p&gt;In this study, we attempted to detect following reactivations using the medium resolution high frequency satellite images (Sentinel 2) coupled with high resolution images (Pl&amp;#233;iades) over a longer period (2016- 2019). We used an inversion strategy of redundant cross-correlation images to produce a robust time-series of displacement from Sentinel 2 data (Bontemps et al. 2018). By applying this technique, we were able to identify a reactivation of the same order of magnitude as the previous one, which affected the headscarp in January 2017. The reactivation signal is validated by the cross-correlation of Pl&amp;#233;iades images taken at 2 years interval. We quantified this reactivation in time and space. We have also identified an area of 30x10&lt;sup&gt;3&lt;/sup&gt; m2 located at the foot of the landslide, which was simultaneously accelerated by 10 m/month during this event. This information contributes to better understand the dynamics of the landslide that evolves from a solid to fluid behavior from the headscarp to the toe. However, a smaller slide that occurred in January 2018 at the headscarp was not detected by this method despite its significant size (10x10&lt;sup&gt;3&lt;/sup&gt; m&lt;sup&gt;2&lt;/sup&gt;). We attribute this non-detection to a major reshaping of the surface following reactivation.&lt;/p&gt;&lt;p&gt;This study identified the possibilities and limitations of the proposed treatment method to detect and monitor landslides on a low-slope area located in clayey soils in a temperate climate.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Bontemps, N., Lacroix, P. &amp; Doin, M.-P. (2018) Inversion of deformation fields time-series from optical images, and application to the long term kinematics of slow-moving landslides in Peru. Remote Sensing of Environment, &lt;strong&gt;210&lt;/strong&gt;, 144&amp;#8211;158. doi:10.1016/j.rse.2018.02.023&lt;/p&gt;&lt;p&gt;Lacroix, P., Bi&amp;#232;vre, G., Pathier, E., Kniess, U. &amp; Jongmans, D. (2018) Use of Sentinel-2 images for the detection of precursory motions before landslide failures. Remote Sensing of Environment, &lt;strong&gt;215&lt;/strong&gt;, 507&amp;#8211;516. doi:10.1016/j.rse.2018.03.042&lt;/p&gt;