Impacts des aménagements en montagne sur les processus hydrologiques et l’évolution géodynamique des versants (Les Arcs, Savoie, France)

Résumé

Aujourd’hui, l’homme, par les progrès techniques qu’il développe, défie les limites autrefois imposées par la nature, en colonisant des milieux parfois extrêmes. En outre, il accroît sa vulnérabilité en s’exposant plus directement aux aléas naturels, mais surtout, il génère une charge croissante sur son environnement, et dans une certaine mesure, amplifie l’intensité du risque pour lui-même. Cette étude a pour objectif d’identifier les facteurs et processus à l’origine du déclenchement des laves torrentielles sur le versant des Arcs et, plus particulièrement, de mettre en rapport le développement de l’activité anthropique et l’occurrence de ces phénomènes destructeurs. Compte tenu des phénomènes mis en jeu nous optons pour une approche progressive de l’aléa en étudiant successivement les contextes géologique, géomorphologique, hydrogéologique et climatique du site. Nous complétons cette analyse par l’étude des transformations induites par l’homme sur le milieu. En particulier, leur influence sur l’hydrologie des torrents est mise en évidence au moyen d’une modélisation.

Abstract

This work focuses on the interaction between human activity in mountainous areas and slope movement hazards. The paper considers the triggering factors and processes of instability in the urbanized area of Les Arcs, an area exploited for mass ski runs and where many debris flows have occurred over the last 40 years. A progressive survey of the geological, geomorphological, hydrological, hydrogeological and climatic contexts is necessary to tackle the complexity of the phenomena. The human impact is considered as well. From the analysis, the geology associated with a specific climate was determined to be the main triggering factor for debris flow hazards. The influence of the glacial and postglacial period on the geomorphological aspects (e.g. steep slopes) and on the mechanical component of the superficial formations is the main factor. Furthermore, an analysis of regional meteorological data over the last 5 decades has emphasized a significant evolution of the climate since the 1960s which resulted in a general increase in temperature, rainfall (with seasonal variations) and the occurrence of extreme weather events, with long term implications for erosion and on water balance. This seems to be exacerbated by recent human activities which have significantly changed the natural environment: deforestation, changes in vegetation, the development of impervious surfaces such as roofs, roads and car parks, the creation of ski runs and the intensive exploitation of the catchment, all of which may have an influence on runoff and erosion affecting the natural surface structure. These changes and their effects on such factors as runoff and erosion have been reviewed and analyzed. In particular, the effects on hydrology have been highlighted by a hydrological modeling created using PCRaster Environmental Software. The hydrology of the Ravoire torrent before the creation of the ski resort was compared with the present configuration of the catchment. The annual water balance model shows that the ski resort generates a significant increase in the river flow compared to that before the change in land use. The parametric analyses undertaken revealed that the deforestation, the development of ski slopes and impervious areas are responsible for the reduction in infiltration inducing runoff. The maximum increase in water flow occurs during periods of natural high water flow (during snow melt) when the risk of debris flows is naturally high. For the last 20 years, this phenomenon has been amplified by the increasing production of artificial snow on the ski runs. Finally, a complementary model was created to consider the effects over a short time scale. The results of this rain event model emphasize the change in the hydrological response of the rivers after rainfall as a consequence of the change in land use. In particular this indicated a rise in the peak flow towards the resort, which may activate erosive processes above the debris flow source areas.