Hillslope debris flows frequency since the beginning of the 20th century in the Massif des Ecrins (French Alps) V. Jomelli & D. Brunstein CNRS, UMR 8591, Meudon-bellevue, France C. Chochillon & P. Pech Université Paris 1 Panthéon-Sorbonne, Paris, France Keywords: frequency, dating methods, logistic regression, French alps ABSTRACT: The aim of this paper is firstly to characterise the debris flow frequency since the beginning of the 20 th century in 6 valleys located on the eastern part of the Massif des Ecrins (French Alps). This analysis is based on the dating of 142 debris flow deposits by lichenometry, dendrochronology, aerial photographs analysis from 1952 and old documents. Results show that it is very difficult to compare the two periods of 1900-1950 and 1950-2000 because the number of debris flows over the first half of the century is probably underestimated. Over the second pe- riod, the frequency of debris flow has decreased significantly for the 200-399 m class lengths. We have also observed a decrease in the number of debris flows between 1950-1975 and 1975- 2000. We have made two logistic regression models in order to determine an frequency probabil- ity according to geomorphological and climatic parameters. The first model shows that the fre- quency probability depends on the debris flow triggering zone altitude and the surface of the rock face, while the second model shows that the frequency probability depends on rainy events greater than 30 mm/day and also on the cumulated number of freezing days. 1 INTRODUCTION Hillslope debris flow can be defined as a rapid mass movement of a fast flowing mixture of sedi- ment and water, which is a very common phenomenon in the Alpine environment. Over the last few decades special attention has been focused on the magnitude/frequency variations of this proc- ess (Innes 1985, Rapp 1995). In the Alps Van Steijn (1991, 1999) and Blijenberg (1998) have ob- served in the Bachelard Valley (south of France) that activity has been vigorous since 1980 (espe- cially in 1987) but results do not necessarily point to a recent increase in debris flow activity. In the Dolomites, Strunk (1992) says:“there is no evidence of any trend towards an increase or a decrease in the frequency since the end of the last century”. On the opposite Rebetez et al. (1997) have ob- served that the frequency of large debris flows has increased since the late 1980s which could be due to an increase in the number of intense rainfall events. Other researchers (Haeberli et al. 1990; Zimmerman & Haeberli 1992) show a relationship between the triggering of debris flows and gla- cier retreat and permafrost degradation due to an increase in temperatures since the middle of the 19 th century, a relationship which can explain the great number of triggering in 1987 (Rickenmann & Zimmerman 1993). In spite of these numerous studies, additional observations made from a wide sample of debris flows in other areas of the Alps seem necessary in order to confirm or refute pre- liminary predictions especially for places which have not been covered by glaciers recently. 127 Debris-Flow Hazards Mitigation: Mechanics, Prediction, and Assessment, Rickenmann & Chen (eds) © 2003 Millpress, Rotterdam, ISBN 90 77017 78 X