Une méthodologie d'analyse et de cartographie de l'aléa "initiation de laves torrentielles" – application au torrent du Bragousse (France)

Résumé

L'occurrence des laves torrentielles résulte de la conjonction de multiples facteurs de prédisposition, en rapport notamment avec la géologie, la géomorphologie et l'hydrogéologie, le facteur déclenchant étant le plus souvent l'eau apportée par de violents orages. L'étude de nombreux dépôts de laves torrentielles de provenances variées dans les Alpes françaises a permis de déterminer l'existence de deux familles de laves torrentielles, les laves à matrice cohésive et les laves à matrice granulaire, dont la distinction est basée sur plusieurs critères granulométriques définis sur le matériau écrêtéà 20 mm. L'étude des zones sources a permis d'identifier les facteurs de prédisposition à l'initiation des laves torrentielles: pente topographique, types de formations géologiques, érodabilité et mouvements de masse, granularité et volumes mobilisables. La cartographie de chacun de ces facteurs conduit à l'élaboration d'une carte de synthèse ou carte "d'aptitude à l'initiation des laves torrentielles".

Abstract

 Debris flows may result in personal injury or loss of life as well as damage to constructions. The economic consequences of such events are important, and likely to become more so with increasing urbanisation. In this paper, "debris flow" is defined as a viscous flow of saturated materials at high velocities of up to 20 m/s in channels. These materials have a high concentration of solids such that their dynamics may be considered at the interface between mechanical and hydraulic flow. The occurrence of a debris flow results from a number of interrelated factors including topography, geology, geomorphology and hydrogeology. The triggering factor is most often water, frequently a violent rainstorm. In order to identify the initiation mechanisms, it is necessary to have a knowledge of the environmental situation in the particular catchment area. Clearly, this will involve an understanding of: (1) the catchment area and its geological conditions; (2) the climatic characteristics of the locality and the nature of the materials in the source area; (3) the pattern of such events in the past. In order to obtain such an understanding, a dual approach of field and laboratory work was considered appropriate. Twelve catchment areas in the northern and southern French Alps were studied to assess the significance of the three aspects mentioned above. The study of numerous debris flow deposits sampled in these catchment areas suggested two types of debris flows: those with a cohesive matrix and those with a granular matrix. In the field, they were categorised on the basis of a morphological consideration of the area as well as according to the texture and grain size of the material in the flow. In the laboratory, granulometric tests were used. In addition, the granulometric and geotechnical characteristics of the source areas were compared with those of the debris flow materials. The grain size distribution appears to be an important factor in determining the likelihood that the source area materials would be moved by violent storms and from a debris flow. From tests undertaken on the <20 mm fraction, superficial deposits with between 16 and 40% of the particles of <50 μm seemed to be most likely to result in debris flows. This paper discusses the fieldwork undertaken from the upper catchment area down to the channel of a debris flow. Each mechanism is described with an analysis of the geological and topographical contexts, the extent of the movement and the hydrological processes that lead to the initiation of a debris flow. On the basis of this, maps have been produced to show the potential for the initiation of a debris flow. Five criteria were chosen to define the debris-flow hazard: (1) slope angle; (2) nature of the geological formation and hydrogeological characteristics; (3) slope erodability and instability; (4) grain size; (5) available volumes of superficial deposits present in the source area. These five indices form the global index. The maps show the spatial distribution of the hazard and can be useful in determining the most appropriate remedial/protective works to be undertaken in zones of the greatest risk.