Slope Activity Analysis in the Rogun Catchment Area, Tajikistan, using Remote Sensing Techniques

The stability and lifetime of construction projects in mountain areas are strongly dependent on local slope activity. Hydropower projects in particular are often affected and endangered by landslide damming and flood wave generation due to slope failures, and thus extensive studies of ground surface instability are vital to assess associated hazards. The Rogun Hydropower Project HPP located in Tajikistan in the Vakhsh – Surkhob River network is currently under construction. The site lies within the seismically active Tien Shan and Pamir Mountain ranges of Central Asia and in particular the Peter the First Range. This region is marked by extreme topography, steep slopes and extensive valley networks and has experienced large and catastrophic slope failures in the past, of which a multitude have been triggered by earthquakes of magnitude M≥4. Co-seismic failures are thus common in the area and present a high geotechnical hazard; however, to date no information on active slope instabilities in its catchment area exists.

Here we present an inventory of slope instabilities in the Rogun Dam catchment area based on optical and synthetic aperture radar differential interferometry (DInSAR) remote sensing techniques. Sentinel-1 multi–temporal differential interferograms are generated for summer periods of 2016 – 2018 to detect surface displacements. Slope velocities are estimated based on a comparison between differential interferograms, while landslide types are identified based on a geomorphological classification. A likelihood analysis is developed to understand the state of activity of slopes and provide a semi-quantitative confidence thereof. The collected data is subsequently integrated to perform spatial and statistical analyses in order to perform a proximity analysis, assess a co-seismic link and evaluate the damming hazard potential to the Rogun HPP. Results show that a clear majority of detected features are located within 10 km of major faults and in zones of high peak ground acceleration, indicating a potential seismic influence or triggering. Some active slopes show an increase in surface displacement after a particular earthquake event and equally suggest a potential link. Moreover, we developed a damming hazard analysis for slopes detected as active in Sentinel-1 differential interferograms, considering the likelihood of movements, their distance to rivers and faults, as well as estimated volume and velocity per year. The results indicate that a total of 29.6 % of all features constitute a high damming hazard potential in case of catastrophic failure, with 4.5 % located within 1 km of the Rogun Dam reservoir. Although many potential sites are not directly on the slopes rising above the future reservoir, hazardous locations in the catchment upstream pose a threat due to possibility of significant outburst floods in case of the dammed lake outburst.