SAR interferometry for landslides risk assessment at local scale: the case study of Castagnola (Northern Apennines, Italy)

Landslide risk assessment at local scale implies the full knowledge of the landslide behaviour and its state of activity. This is particularly important in large-size landslides or complex slope movements, which are characterized by different displacement patterns in different portions of the failing mass. Generally the conventional monitoring techniques - both geotechnical and topographic - only provide accurate information on a given number of points on the slopes affected by instability. In this case single-point data cannot be considered representative of the behaviour of the whole landslide mass. Remote sensing monitoring systems can overcome most of these spatial limitations. In this work we describe the application of remote monitoring techniques based on radar interferometry (PSI and GB-InSAR) to assess the state of activity and the associated risks of the Castagnola landslide located in Northern Apennines (Liguria, Italy). The ongoing slope instability affecting the Castagnola area is a complex and still poorly understood phenomenon which in the last few decades has undergone numerous reactivations leading to damage to buildings and civil infrastructures. Following a geomorphological field characterization and an aerial photo interpretation of the site, we have analyzed and compared the past long term evolution of the ground movements using Persistent Scatterers Interferometry technique (PSI) from a set of ERS 1/ERS2 space-borne data acquired in 1992-2001 and the recent landslide displacements obtained from Ground-Based Interferometric Synthetic Aperture Radar (GB-InSAR) surveys. The results of the space-borne inteferometric data analysis have been compared both with the deformation measured at the head of 10 inclinometric tubes and with the openings of some cracks on buildings. The GB-InSAR monitoring has been carried out in three distinct campaigns from October 2008 to March 2009. The interpretation of the results has allowed to derive a multi-temporal deformation maps of the landslide, showing the up-to-date displacement field and the mean landslide velocity. In particular, the results of the GB-InSAR surveys, have shown that the E and the SE landslide sectors during February and March 2009, have been characterized by maximum deformation rates, one order of magnitude higher than the mean deformation rates recorded with the PSI technique in the period 1992-2001. On the basis of the outcomes of GB-InSAR monitoring, the pre-existent boundaries of the Castagnola landslide (Casagli et al., 2008) have been updated. The new landslide boundaries obtained could be directly exploited as a new tool for risk assessment and territorial planning of the Castagnola area. This example of landslide “back-monitoring” demonstrates that the integration of PSI data, which provide long deformation time series, in-situ investigations, which provide information on the ground conditions at depth, and GB-InSAR monitoring represent a useful tool for landslides risk assessment purposes. This type of analysis gives information on spatial and temporal deformation pattern and is fundamental for evaluating the possible landslide evolution, defining deformation thresholds, and setting real time monitoring systems for early warning purpose. In this framework the GB-InSAR monitoring system can be hence considered as an effective early warning tool for risk management, able to provide near real time information which can be employed in civil protection actions and plans.