The island of Majorca, located in western Mediterranean, has a variety of different geomorphological domains, most prominently the Tramuntana Range (1,100 km2) in the north-western part of the island (Fig. 1). The steep topography of this chain, which is linked to its geological complexity and Mediterranean climate, determines intense slope dynamics with the consequent movements of all categories. The main income of the island of Majorca comes from tourism (83% of its GDP), as it welcomes 11 million visitors each year. The urban development that the Tramuntana region has undergone in the past 30 years has considerably increased the risk originating from mass movements.
Practically all the slope movements recorded on Majorca have taken place in the Tramuntana Range. The variety of lithologies cropping out in this mountain chain determines a wide range of slope movements. Landslides and earth flows are frequent phenomena, primarily affecting soft sediments from the Late Triassic (Keuper), made up of clays with gypsum, as well as an entire series of loamy materials from the Palaeogene and Neogene that occasionally outcrop along the mountain range. The most prominent movements include the Fornalutx landslide which took place on the 17 December 1924, affecting an area of around 150,000 m2. However, the most important mass movement in the Balearic Islands, because of both its dimensions and the damage it caused, was the Biniarroi landslide in spring 1721, with later local reactivations in 1816, 1857 and 1943. This landslide affected an area measuring around 300,000 m2 and totally modified the original topography in the region.
Rockfalls are the most frequent slope movements in the Tramuntana Range (69% of the events) due to the predominance of Jurassic rocky massifs made up of limestone and dolostone. Historically, there are records of several major rockfalls. On the 16 March 1857, a huge rockfall on the Valldemossa area razed and buried a large extension of cropland, leaving reports in the daily news. More recently, numerous rockfalls have made the news as well, such as the one in Cala de Banyalbufar (1993), which affected several fishing huts and the rockfall in Son Matge (Valldemossa) in 2005, in which one of the most important archaeological sites from Majorca’s prehistory was buried. The main traffic arteries in the mountain range, both road and rail, have often been intercepted by slope movements, triggering serious circulation problems as well as major economic losses. The historical compilation of the slope movements on the island, as well as the record of those that have occurred more recently, reveal that all processes have taken place after short-intense and/or continuous rainfall. Between 2008 and 2010, the island of Majorca experienced the coldest and wettest winters of the last 40 years. Accumulated rainfall was twice the average and values of intense rainfall up to 296 mm /24 h were recorded. Additionally, high precipitation coincided with anomalous, low temperatures and freezing in the highest zones of the Tramuntana range. As a result, 34 mass movements were recorded (Fig. 1): 14 rockfalls, 1 rock avalanche, 15 landslides and 4 karstic collapses. Fortunately, there were no deaths but there were numerous cases of damage to dwellings, holiday apartment blocks, barns and power stations, and especially the road network in the range, most significantly the numerous blockages on the Ma-10 road (Figs. 2 and 3), which caused significant economic losses in the different tourist resorts. On the southern coast of the range, 17 holiday homes have been evacuated recently due to the impending risk of a large rockfall. Total economic losses are valued at approximately 11M Euro, which represents 0.042% of the Balearic Autonomous Region GDP.
Figure 1: Location of the island of Majorca in the western Mediterranean and the Tramuntana Range on the northwest extreme of the island. The location of the 34 mass movements registered during the rainy and cold period 2008-2010.
Figure 2: The Gorg-Blau rockfall (31/12/2009) which blocked the main road of the range during 3,5 months.
Figure 3: The Estellencs landslide (8/3/2010) which blocked the main road of the range (Ma-10) during several months in 2010.
DORIS- Data and Technologies
For DORIS in the Tramuntana range test-site, radar SAR images acquired by distinct satellite systems in L, C and X band in different time intervals were processed, using the Stable Point Network (SPN) technique. The SPN is an advanced differential interferometric processing technique that includes both the PS (Persistent Scatterers) and SB (Small Baseline) approaches (Ferretti et al., 2001; Arnaud et al., 2003). In Table I the main characteristics of SAR datasets used in this work are shown.
ALOS ENVISAT ERS 1/2 CSK Band L C C X Geometry Ascending Ascending Descending Descending Descending Temporal interval 1/1/2007-28/6/2010 22/11/2003- 19/5/2010 19/08/2003-19/5/2009 13/6/1992-7/11/2000 18/5/2012-30/8/2012 Incidence angle 39º 22º 22º 23º 30º Track angle 346 345 194 194 193 Cycle (days) 46 35 35 35 8
Table I: SAR-images dataset using in the Tramuntana range for DORIS project
In order to homogenize PS data derived from different satellite sensors and from different acquisition orbits (i.e. ascending and descending), we introduce three procedures applicable in mountainous environments. The first one can be applied when only a single orbit of a satellite is available and leads to the re-projection of PS LOS velocities along the slope direction (VSlope, Fig.4). The second one is applied when the two acquisition geometries (i.e. ascending and descending) are available for the same satellite sensor and allows combining velocities obtained from the two orbits in order to decompose the displacement into its horizontal and vertical motion components, hereafter Synthetic PS. Finally,a further homogenization of the radar data is carried out for the PS velocity classification and stability threshold determination.
Figure 4: Vslope (mm/year) in the Tramuntana range by processing ALOS ascending radar images (L band).
Exploiting the “In-Points” which are PS included within landslide boundaries, we evaluate the spatial distribution and displacement velocity of phenomena and we deliver a landslide activity map, based on each radar dataset (Fig. 5). The superposition of the PSI data with the landslides-inventory (215 events) reveals that 36% of the landslides have no sufficient PS, 18% without any information; 5 new landslides (2%) have been identified and 5 (2%) of them enlarged. The 42% of pre-existing landslide have enough PS for estimate their activity (Fig.6). L-band (ALOS) data show the best performance for detecting landslides in the study area thanks to the good resolution and higher penetration capacity of the radar signal that increases the detected PS density.
Figure 5: Landslide activity map in the central and coastal fringe of the Tramuntana range for different periods of time and radar datasets.
Figure 6: Updated landslide inventory map. 46% of the landslides inventoried in the test-site have enough PS for estimate their activity.
Steep relief and geological diversity.
Rock-falls, rock-slides, earth slides and karstic collapses.