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

Abstract

Springer-Verlag 2008 ResumeAujourd'hui, l'homme, par les progres tech- niques qu'il developpe, defie les limites autrefois imposees par la nature, en colonisant des milieux parfois extremes. En outre, il accroo ˆt sa vulnerabiliteen s'exposant plus directement aux aleas naturels, mais surtout, il genere une charge croissante sur son environnement, et dans une cer- taine mesure, amplifie l'intensitedu risque pour lui-meme. Cette etude a pour objectif d'identifier les facteurs et pro- cessus a l'origine du declenchement des laves torrentielles sur le versant des Arcs et, plus particulierement, de mettre en rapport le developpement de l'activiteanthropique et l'occurrence de ces phenomenes destructeurs. Compte tenu des phenomenes mis en jeu nous optons pour une approche progressive de l'aleae n etudiant successivement les con- textes geologique, geomorphologique, hydrogeologique et climatique du site. Nous completons cette analyse par l'etude des transformations induites par l'homme sur le milieu. En particulier, leur influence sur l'hydrologie des torrents est mise en evidence au moyen d'une modelisation. 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, hydrogeo- logical and climatic contexts is necessary to tackle the complexity of the phenomena. The human impact is con- sidered as well. From the analysis, the geology associated with a specific climate was determined to be the main trig- gering 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 cli- mate 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 sig- nificantly changed the natural environment: deforestation, changes in vegetation, the development of impervious sur- faces 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