Large landslides in the Pyrenees

Large landslides are characterized by a deep fault surface (typically more than 10m deep) and involve earth volumes ranging from a few hundred thousand to several hundred million cubic meters. Large landslides are recognized as one of the main erosive agents in mountain ranges and also responsible for a significant proportion of economic losses and lives. Particularly in the Pyrenees, large landslides include complete slopes that involve towns, infrastructures and activity on their surface and that are capable of accelerating to lead to catastrophic events. However, (i) their geographic distribution at a regional level is poorly understood; (ii) their geological and geomorphological control factors have only been preliminarily studied; and (iii) their activity status and stability conditions are unknown in most cases.

Some examples of recent large landslides in the Pyrenees andthat were highly catastrophic are:

  • Puigcercós (Pallars Jusà, Lérida, Spain). This landslide, with a volume of around 1 Mm3, occurred in 1881, destroyed several houses and led to the abandonment of the town of Puigcercós. ​​
  • Pont de Bar (La Cerdanya, Lérida, Spain). This landslide, with a volume of around 10 Mm3, is an old landslide reactivated by the erosion of the Segre river during the catastrophic floods of November 1982. The landslide ruined several houses in the town of Pont de Bar, which was finally abandoned. The landslide also threatened the town of Toloriu, which was temporarily evacuated and destroyed a 300 m long section of the national highway between Puigcerdà and La Seu d’Urgell (Figure 1). Landslides in the following 5 years after the reactivation posed a significant risk due to potential damage to the Segre's river dam.

Figure 1. On the left, area affected by the Pont de Bar landslide, on the right, complete destruction of a a 300 m length section of the national road N-260. References?

  • Salinas de Jaca (Huesca, Spain). This landslide, with a volume of aproximately 0.3 Mm3, triggered the abandonment of the town of Jaca in the 1940s.​
  • Tarteres-Hortells (Sant Julià de Lòria, Principality of Andorra). The south-eastern sector of the parish of Sant Julià de Lòria is characterized by a system of progressively staggered blocks, associated with a deep gravitational deformation linked to the Urgellet and Cerdanya pits. Related to this structural morphology, and as a consequence of the incision of the Valira valley, the Tarteres-Hortells sector was affected by a large ancient landslide (Figure 2).

Figure 2. Structural diagram of the south-eastern sector of Sant Julià de Lòria (Principality of Andorra) and the landslides observed in the territory (Source: Xavier Planas)

The Tarteres landslide seems to show an old scar located at the foot of the Hortells landslide; which could explain its occurence as a reactivation of the lower section of the ancient Hortells landslide. In the intermediate sector another landslide lobe (Comabella) can also be differentiated. The set of three landslide bodies expand from elevation 885m to 1360m and occupies an area of ​​approximately 0.23 Km2 (Figure 3).

Figure 3. Interpretation of the limits of the Tarteres-Hortells landslide using a photography of the territory (Source: Xavier Planas)

The lower sector of the Tarteres landslide was reactivated in 1994 as a result of the widening works on the national road (CG-1). Since then, the landslide movement has been ausculted. Its measured activity determined centimetric to milimetric movements with a certain and prolonged activity to a depth of about 15 meters. In 1994-1995, work was carried out to stabilize the landslide by means of deep drains on the northern sector of the movement's foot. These drains were clearly effective and markedly decreased the movement's activity.

  • Baillanouse (Vallespir, France). This landslide took place on the 18th of October 1940 due to extreme rains. This landslide mobilized between 6 and 7 Mm3 of material that rushed over the Tec's river channel. Although the landslide occurred in full flood, it was able to block the river flow and generate a dam of more than 60m in height, which was dismantled by water erosion in a matter of about five hours (Corominas, 1985).​
  • + French cases with pictures

Currently, many of the large landslides existing in the Pyrenees seem to be inactive. These are usually very old and probably occurred under conditions different from the current ones (relict landslides), which means that they represent a relatively low danger if their stability conditions remain unchanged (i.e., if the landslide is not affected by road excavation work or loaded by new settlements). However, other large landslides are still active in the Pyrenees (i.e., they are moving) and affect existing towns and infrastructures. The landslides of Canillo (Andorra, Planas et al., 2011) (Figure 4), El Portalet (Spain, Fernández-Merodo et al., 2014), La Escarrilla (Spain, García-Ruiz et al., 2004) , Vallcebre (Spain, Corominas et al., 2005), Arguisal (Spain, García-Ruiz et al., 2004), Canelles (Spain, Pinyol et al., 2011), the Yesa reservoir (Spain, Gutiérrez et al. , 2011) and Gourette (France, Largillier, J., 1985) are some examples of large landslides in the Pyrenees that are currently registering movement.

Figure 4. General view of Canillo landslide (PyrMove network, Regional analysis of landslide risk in the Pyrenees -2014 CTP 00051-)