Barcelonnette area

Favourable predisposing factors to slope movements 

Location

The Barcelonnette Basin is representative of climatic, lithological, geomorphological and landcover conditions common to several regions of the South French Alps, and is highly affected by landslide hazard.  It  is located in the department  'Alpes-de-Haute-Provence', on the medium path of the Ubaye River, at a 1130 m average elevation. The basin extends over an area of 200 km², with a length of 22 km (from Jausiers to the East to Les Thuiles to the West), and a maximum width of 10 km. High crests, reaching altitudes from 2800 m to about 3100 m enclose this basin.            

    

Location of the Barcelonnette Basin in the South French Alps

Geology

The Barcelonnette Basin is a geological window in two Eocene sheet thrusts (Autapie and Parpaillon), made of strong limestones or sandstones, and overlaying an autochtonous bedrock of soft autochtonous Oxfordian-Callovian black marls. The thickness of the black marls reaches 250–300 m.  Three subsets can be distinguished: 
- the Lower Callovian black marl with detrital plates (50–60 m thick)
- the Middle and Upper Oxfordian black marl (150–250 m thick), with centimetre argillaceous-limestone banks
- some rare outcrops of Argovian black marl (15–20 m thick). 

Due to this predisposing geological structure, the hillslopes are affected by severe gullying, shallow landslides, and deep-seated large landslides, some of which directly affecting the local population. 

Geological and geomorphological cross-section of the Barcelonnette Basin

Geomorphology

The geomorphological setting is complex, and has a strong control on landslide distribution. In addition to the erosion activity of the glaciers, torrential erosion by the Ubaye river has progressively carved out the landscape.  
   

          
Simplified geomorphological map of the Barcelonnette Basin 
and location of the main relict and active landslides 

Hillslopes are characterized by very steep slopes (slope gradient ranging between 30° and 70°) and more gentle slopes (slope gradient ranging between 10° and 20°). The relief is irregular, and associates alternating steep convex slopes, regular planar slopes and hummocky slopes:
- the steepest convex slopes (> 35°) are carved in the black marl outcrops and are very often gullied in badlands or affected by rock-block slides or complex slides;
- the regular planar slopes (10°-20°) are made of thick morainic deposits (between 10 and 20 m), are very often cultivated and affected by rotational or translational slides;
- the hummocky slopes are generally covered by forests and/or by natural grasslands. 

Consequently, the main landforms are characterized by large relict landslides in moraine, badlands in black marls, deep-seated mudslides in black marls, and shallow slumps or debris slides in the moraine or in the colluvial cover.

Typical landscape of the Barcelonnette Basin for the South-facing and the North-facing hillslopes

Climate

The Barcelonnette Basin has a dry and mountainous Mediterranean climate with strong interannual rainfall variability (733±412 mm over the period 1928–2002), strong storm intensities during summer and autumn (over 50 mm h-1) and ca. 130 days of freezing per year. These characteristics are favourable to the triggering of mass movements. 

However, landslides are not controlled only by climatic conditions; instability can also occur during relatively dry periods whether or not they were preceded by heavy rainfall. This reveals that landuse changes control also the location of landslides.

Climate - Landslide relationships for the period 1900-2005

Landcover and landuse

Agricultural activities started at the Roman period, and reached their maximum in the second half of the last century, so much that the slopes had been almost completely cleared for cultivation and that more and more frequent flooding, landslides and debris flows caused catastrophic damages. To reduce them, the “Restauration des Terrains en Montagne” (RTM Office) starts a vast operation of reforestation and of construction of dams along the torrents. The occurrence and intensity of the hazards has been reduced, aznd is now more or less controlled, but the risks still exist, and, even, increase, because of the development of industrial and tourist activities, ski resorts and housing. Indeed, Barcelonnette is a small municipality but plays a important administrative, commercial, and communication role with the presence of the sub-prefecture, and of many commercial activities, hotels, hospital, industrial parks, and a main road ensuring relation with Italy and deserving several market towns.

Landcover map of the Barcelonnette Basin

The Barcelonnette area has been the subject of many research studies at the regional scale and several PhD or post-doc projects.

PhD Thesis completed

• Thiery, Y. 2007. Susceptibilité du Bassin de Barcelonnette (Alpes du sud, France) aux 'mouvements de versant' : cartographie morphodynamique, analyse spatiale et modélisation probabiliste. Thèse de Doctorat, Université de Caen, 445 p. [pdf]
• Remaître, A. 2006. Morphologie et dynamique des laves torrentielles : application aux torrents des Terres Noires du bassin de Barcelonnette (Alpes du Sud). Thèse de Doctorat, Université de Caen, 487 p. [pdf]

Phd Thesis in progress

• Kappes Melanie 

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