Is the response of hill slope debris flows to recent climate change univocal? A case study in the Massif des Ecrins (French Alps)

To assess the impacts of future climatic change, we need a better understanding of the relationships between debris flows dynamic and the climate. The subject of this paper is the variability in the response of debris flows to climatic change in recent decades in the Massif des Ecrins (French Alps). The climatic trend observed in the French Alps was characterized by analyzing data on extreme summer rainfall events recorded daily at nine stations located all over the Massif des Ecrins since 1960. According to the generalized Pareto law (GPD) our results showed that extreme summer rainfalls have increased significantly in the Massif des Ecrins since the 1980s. Statistical tests showed a significant increase in average annual air temperature. The response of 647 hill slope debris flows to this climatic change was investigated. Different types of hill slope debris flows were analyzed as a function of their lithology or the nature of the accumulated debris. A number of logistic regression based models were developed to characterize the relationship between climate and the occurrence of debris flows in a specific geomorphological context. Analysis of frequency and return period over the last two decades showed different patterns depending on the type of debris flow concerned. Hence, the response of hill slope debris flows to climatic change is not the same everywhere in the Massif des Ecrins.