In the present project, the study of the behavior of large landslides encompasses the definition and implementation of monitoring and alert systems at three pilot sites within the POCTEFA territory: Canillo (Andorra), Viella - Hautes Pyrénées (France) and Gourette - Pyrénées Atlantiques (France). The information derived from these systems will be used by communities and crisis management agents to reduce the exposure of people in these areas to the risk of landslides (around 35,000 people).
Description of the pilot sites:
In the eastern sector of Canillo (Canillo parish, Principality of Andorra) there are two major slope instabilities of significant dimensions: the "Forn" movement and the lateral expansion of the "Encampadana"; which have lateral continuity on the "Ribaescorjada" slope (northern location). The sliding mass extends from the valley of the "Valira de Oriente" (1.450 m) to the "Planada de Maians" on the North (2.338 m) and the "Cap del Rep" on the South (2.316 m), representing an area of approximately 2,7 Km2 (Figure 1).
Figure 1. Geomorphological diagram of the Canillo landslide (Source: Modified from Planas, X. et al., 2011)
Geologically the "Forn" is a rocky massif formed by Silurian black slates and Devonian chalcophyllites. Since the Upper Pleistocene this slope has been destabilized at least three times leading to the development of a large landslide with rototranslational scars at the head and flow at the foot. The first major landslide event - Phase 1 - predates 30.000 BP, the second - Phase 2 - was after 21.000 BP and the third - Phase 3 - took place in the Postglacial, around 8.700 BP. The interpretation of the deposits found suggests that the first movement cutt off the "Valira d'Orient" valley. Afterwars, the "Valira d'Orient" glacier advanced on top of the colluvial deposits linked to the first landslise, partially reworking them and generating a series of lateral moraine deposits as well as a possible juxtaglacial lake in the area of the "Obaga del Cultiar". Thes resulting formations were covered by the second landslide deposits. Thanks to the dating of the moraine deposits and the coal in sealing laminites found in the "Sella" area, it is interpreted that after 11.000 BP the "Valira d’Orient" glacier no longer reached this area. Human settlement on top of the large landslide in Canillo has been represented prehistorically since the Middle Bronze Age, with the discovery of a Prats pit. In historical times, the town of Prats was documented for the first time during the bishop’s visit in 1.312 to the Romanesque church of Sant Miquel. Currently, several urbanizations have also been built on top of the landslide body and since 1.934 a hydraulic gallery of the Electric Forces of Andorra has been buried there. Thanks to the monitoring and auscultation of the Forn carried out by the Government of Andorra, it has been possible to differentiate and quantify recent local movements –Phase 4– which have happen to have a higher speed within the landslide mass (for example the Cal Ponet area- Cal Borronet, the Clots Fondos and a sector located under the town of Prats) (Figure 2).
Figure 2. Interpretation over photograph of the different landslide phases identified in the Canillo area (Source:Xavier Planas)
Viella is a municipality located in Hautes-Pyrénées (Occitanie), west of Luz-Saint-Sauveur in the Bastan valley (right bank of the Gave de Gavarnie river). From a geological point of view, the study area belongs to the upper axial primary chain of the Pyrenees. The present geological formations are related to the sedimentary cover of the Cauterets and Neouvial crystalline massifs (Devonian shales, limestones and sandstones), these are crowned by imbricated slipped moraine deposits resulting from the post-glacial decompression and destabilization. The instabilities identified the slope area affect a total of 80 ha, and rise to altitudes of 1450 m.a.s.l. at the level of the Couret ridge and 780 m.a.s.l. in Bastan ridge, while the town is locatedabout 850 m.a.s.l. The landslides have affected the entire town, as well as different supply networks (electricity, sewerage, etc.) and various road infrastructures, in particular RD918 that leads to the Col du Tourmalet (a place whith lots of tourism). The most recent activity observed in Viella (approximately since the mid-19th century) has coincided with various episodes of major floods and is characterized by large-scale landslides and minor landslides on slopes with varying consequences on the population.
In particular, two notable episodes can be highlighted:
- In 1898, there was a significant collapse of rocks on the north face of the Crête de Couret, forming a scree cone of between 5 and 6 hectares that extended to about 950 m.a.s.l. The volume of material mobilized was estimated between 600,000 and 1 million m3. After this event, signs of landslides were observed in the area between the scree cone and the town, with particular affectation in the adjacent buildings.
- Although the 20th century the area has been characterized by relative stability, in 2018 massive sequences of landslides occurred. In February 2018, after a winter of heavy rains, 250,000 m3 of shale slipped, partially covering the scree cone. The landslide reactivation again affected the lower part of the slope and caused problems in constructions and roads (Figure 3). Note that a major flood of Bastan in June 2013 had strongly eroded the foot of the slope, which could have conditioned its stability against the rains occurred in 2018.
Figure 3. a) Town view of the Viella's landslide occurred in x , b) landslide's view from the upper part of the screecone, c) atructural damage of the buildings adjacent to the landslide (Source: BRGM ).
In the last two years, numerous studies (geotechnical, geophysical, piezometric, etc.) have been carried out and monitoring means (Lidar, topography, etc.) have been implemented to improve knowledge of the factors that control the dynamics and the kinematics of the Viella's landslide. Results to date suggest that:
- The slope shows areas of variable activity where the fault surfaces reach depths of between 10 and 54 m (the maximum depth is to the right of the town center).
- The lithology of the slope consists of unstructured shale blocks corresponding to the cone of slipped material. The bedrock has not been altered by recent landslides.
- Areas of preferential water circulation have been detected, generally coinciding with the fault surface at great depth (54 m).
- Landslide speed do not appear to be directly related to rain.
- Massive rock collapses appear to be an aggravating factor opposed to the landslide dynamics.
The Laruns-Gourette site is located in the middle of the Ossau Valley and the Valentin catchment (France). The lithology of this area is dominated by Paleozoic limestone and schists (from the Devonian to the Carboniferous), with some intercalations of Triassic rocks in the fault zones. The different geological formations are affected by the Hercynian (360-250 Ma) and Alpine (83-20 Ma) orogenic phases. As a result, characteristic kink folds bands have developed at different scales (km to cm). Although the Alpine orogeny ceased in the early Miocene (Mouthereau et al., 2015), the Northern Pyrenees still suffer from seismic activity. In fact, recent earthquakes of magnitudes up to 5.5 have been recorded (with focus mechanisms rooted at low depth, between 2-15 km). In particular, the Laruns-Gourette site coincides with the most seismically active area in the Pyrenees (for example, in 1980, the Arudy earthquake with a magnitude of 5.1 took place in the Ossau valley at 5 ± 1 km depth). This event was followed by ~ 1,200 aftershocks. This area was also heavily remodeled by the successive glacial phases of the last 800 ka. However, only a few moorish deposits and fluvial-glacial formations near Laruns and Eaux-Bonnes are testimony to the passage of glaciers (which explains their rapid removal).
The current geomorphology of the Laruns-Gourette valley is the result of its complex geology combined with the seismicity of the area and the erosion triggered by the removal of the last glacier. This geomorphology is characterized by the instability of its slopes. In fact, throughout history several slope instabilities have been identified in the area, from very deep landslides to shallow landslides. For example, gravitational slope deformations, rock avalanches and debris, slow mudflows, and translational landslides have been observed. Relict, latent and active events are distinguished between these landslides. In particular, the active landslides of Pleysse (Figure 4), triggered in 1982 near Eaux-Bonne, the Gourette landslide or the Listo landslide have caused damage to nearby infrastructure (buildings and roads). In addition, debris slides triggered annually by the flushing moraine deposits during heavy rains threaten the area's communication networks
Figure 4. Comparison of orthophotos from the years 1959 and 2010 in which the mass of slipped material in Pleysse is identified (Source: Institut Géographique National)