The study area (Monti Nebrodi e Giampilieri) is located along the north eastern part of Sicily (south Italy). In the period from October 2009 to February 2010, the area was highly affected by several landslide events that caused intense damages and casualties. The landslide events continue till today as testified by the event occurred during March 2011 in San Fratello where 12 peoples were evacuated.
The Monti Nebrodi ridge is 70 km long with ENE-WSW direction and is part of the Sicilian Apennines. The altitude of the area span from the sea level to 1847 m (Monte Soro). From a lithological point of view the western part is characterized by terrigenous formations (mainly marls and claystone), whereas the central by clay deposits. The eastern sector is also composed by terrigenous formation showing a coarser grain size with respect to the western deposits. Close to San Fratello (West) and Frazzanò (East) limestone and dolomite crop out. Intense and exceptional rainfall event was the main factor that combined with the steep slopes, triggered several slope movements along the Monti Nebrodi. In particular debris flow, rotational and translational slides, rock falls and shallow and deep-seated landslides.
Giampilieri is located on the eastern coast of Sicily, in the Peloritani Mountain Belt which represents a segment of the Apennine-Maghrebide Orogen. This area is characterized by the presence of the Kabilo-Calabride Units, formed by continental crust fragments deriving from the European margin and, in particular, by Hercinian crystalline rocks with their Mesozoic-Tertiary cover. The morphology of the area is characterized by steep slopes eroded by torrent-like straight watercourses which became very rich in solid material during the rainy season. The altitude span from the sea level to 1040 m a.s.l.. The investigated area, 75 km2 wide, is located in the coastal area south of the city of Messina. It is delimited by the Ionian sea on the East side and by the Peloritani ridge on the West side. Five municipal towns are included in this area: Alì, Alì Terme, Itala, Scaletta Zanclea and Messina. The main villages are scattered along the strongly urbanized coastal area. The hilly area opposite to the Ionian Sea is characterized by the presence of smaller settlements, connected with the populated coastal area by a single road axis climbing upward along very steep slopes. The geomorphology of the area is strongly influenced by the geo-structural conditions, by the crystalline competence of the outcropping rock (mainly medium grade metamorphic rocks) and by the recent tectonic activity. The coastal landscape is typical of the recently uplifted areas: steep slopes, narrow valleys and high relief energy are the main geomorphologic feature. The morphometric characteristics of the river basins, represented by a watercourse network having regular and parallel path, are influenced by the short distance separating the watershed from the coast. River catchments have a reduced widening with a significant transport of solid materials; incisions are short and deeply entrenched into V-shaped valleys, especially in the mountainous sector. Several small alluvial plains, formed where riverbeds become over-flooded, characterise the coastal area. The presence of the so-called "fiumare", straight, steep course, gravel-bed river draining mountain areas is typical of Mediterranean climate region. Their flow varies seasonally and their regime is torrential with catastrophic transport of solid materials following heavy rainfall, causing severe damages if flooding occurs close to populated centres. In this area the main landslides type occurred can be prevalently classified as debris flow and debris avalanches. More than 600 landslides in a day were triggered by exceptionally intense and localized rainfalls.
DORIS - Data and Technologies - Results
The 14th February 2010 landslide in San Fratello (Nebrodi study area, Sicily, Italy) was investigated using both space-borne and ground-based InSAR techniques. InSAR techniques, using ERS-1/2, ENVISAT, RADARSAT-1, and COSMO-SkyMed, were applied to analyse ground displacements during both the pre- and post-event phases. In order to monitor the town area displacements with high spatial accuracy during the post-event phase, a GB-InSAR system was installed collecting ground displacement maps for a period of about three years, until December 2012. Through the integration of space-born and ground based datasets ground deformation velocity maps were obtained, providing a more accurate delimitation of the 2010 landslide with respect to the carried out traditional geomorphological field observations (Fig. 1).
The integration of GB and PS-InSAR techniques proved to be very effective in landslide mapping and monitoring, and with special regards to the San Fratello test site, it represented a valid scientific support for local authorities and decision makers during the emergency management.
Fig. 1 - New boundary of the San Fratello landslide obtained through the integration of space-borne (left) and ground based (right) SAR data.
Pre-PSI and Post-PSI processing procedures were tested in order to evaluate PS detection suitability of all the available radar data with respect to the relief (Fig. 2, RI-index evaluation) and land use, and to obtain a more intelligible interpretation for landslide studies (i.e. downslope projection of LOS rates, Vslope).
Fig. 2 - Mean RI-maps of the area between Furiano stream and Rosmarino stream based on four PS datasets
Landslide activity maps (and ground motion activity maps, Fig. 3, referred to further unstable areas, i.e. “active clusters”) were updated and produced for each PS dataset dated up to 2012, as well as the confidence degree map of these products, based on the integration of radar data with ground truth information represented by damage assessment and field survey. Thus, updated landslide inventory maps were compared with radar velocities and field observations in order to evaluate a confidence degree of the provided product maps. It is worth to highlight that in the Nebrodi study area no ground truth measurements were available for the inventoried landslides, except that for San Fratello site (high confidence degree) where an accurate field survey was performed between November 2012 and January 2013.
Fig. 3 – Landslide Activity map based on Vslope rates of each PS dataset (left) and confidence degree determination (right).
In Sicily, the most recent available SAR (RADARSAT-1 and CSK) data were used to update the state of activity of the landslide inventory maps obtained scale through the interpretation of two sets of aerial photographs acquired in the 1954 and 2005. This update was carried out considering the mean velocities of each polygon that represents a landslide or a part of it. This methodology was applied to several municipalities located within the Nebrodi Mountain range (Fig. 4).
Fig. 4 - Updated landslide inventory map of Tortorici Municipality and the related state of activity percentages.
Damage assessment maps have been produced at local scale within San Fratello site exploiting satellite PSI data acquired in historical, recent and current time intervals (i.e. ERS, ENVISAT, RADARSAT, COSMOSky-Med PS data) and successfully compared with ground displacement radar data (Fig.5).
Field investigations have been carried out in winter 2012-2013, to confirm EO data, to provide useful in situ observations and to evaluate crack patterns and entity of damage to buildings and infrastructure (Fig.5).
Fig. 5 – Ground displacement velocity map and building velocity map through PS Cosmo-SkyMed data, Damage assessment map and examples of field survey.
Coastal chain drained by several narrow streams perpendicular by the coast.
Debris flow, rotational and translational slides, rock falls and shallow and deep-seated landslides.