PSI for landslide hazard assessment and monitoring: current issues, underexploited and future application opportunities

We discuss current and future opportunities and challenges of PSI applications to slope hazard assessment and monitoring with reference to the recent and upcoming radar satellite launches and the most recent literature (e.g. Bianchini et al., 2014; Wasowski and Bovenga, 2014a,b). In particular, it is envisioned that by offering regular globe-scale coverage, improved temporal resolution (weekly or better) and freely available imagery, new radar satellite missions such as the ESA’s Sentinel-1 will guarantee an ever increasing and more efficient use of PSI in landslide investigations. These background missions are necessary for long-term, systematic mapping of unstable or potentially unstable slopes and regional scale assessment of landslide processes. The initial wide-area (reconnaissance) approaches relying on medium resolution PSI (e.g. ENVISAT, Sentinel-1) can be suitably integrated with high resolution PSI relying on the new radar sensors (e.g. COSMO-SkyMed, TerraSAR-X, RADARSAT-2), thereby providing most valuable information for the spatial and temporal analyses of slope deformation and a sound basis for derived products ranging from individual landslide monitoring to regional hazard identification. The benefits of multi-sensor and multi-scale investigations (from regional to site-specific) are discussed by presenting PSI results concerning two regions (Central Europe and Western China) characterized by distinctly different topographic, climatic and vegetation conditions. It is shown that, with respect to the medium resolution PSI products (based on ERS and ENVISAT imagery in our case studies), the PSI products derived from high resolution imagery (Stripmap COSMO-SkyMed and TerraSAR-X) may not always lead to a significant improvement in terms of detection of unstable slopes or kinematic characterization of active landslides. Therefore, for its most effective use PSI will have to be tailored to the specific region/site conditions, landslide types, depending on the primary objectives of the investigation. For example, in case of critical facilities at risk such as dams or bridges, of urbanized landslides or potentially unstable slopes in urban/peri-urban areas, as well as of slopes traversed by critical lifelines and engineering structures, the cost of acquiring and processing high resolution radar data can easily be justified. Conversely, in wide-area regional investigations, the use of medium resolution imagery will be more appropriate and the most cost-effective. In general, thanks to the improving temporal and spatial resolutions of new generation radar sensors, significant breakthroughs are expected in detailed slope instability process modelling (e.g. kinematic and geotechnical models), as well as in the understanding of spatial and temporal patterns of landslide movement/activity and their relationships to causative or triggering factors (e.g. precipitation, seismic loading). The capability to provide, at regular intervals, long-term ground surface deformation trends offers an unprecedented opportunity for early detection and warning of potential slope instabilities, but further research is needed with focus on the integration of data from PSI and ground-based geotechnical monitoring. We consider this as one of the top applied research priorities. Finally, we stress that PSI-derived results have yet to be fully explored, in particular those based on high spatio-temporal resolution data. Some of the landslide research and application areas that may benefit more from PSI include: (i) Long-term behavior and climatic controls of very slow deeper landslides and deep-seated gravitational slope deformations; (ii) Numerical modeling of very slow persistent landslides and long term evolution of slopes; (iii) Post-earthquake landslide activity and evolution of slopes; (iv) Non-linear kinematics of landslides, maximum velocities and accumulated displacements.