Geomorphic Variations of Debris Flows and Recent Climatic Changes in the French Alps

Geomorphic variations of debris flow in two areas of the French Alps were examined and related to climate change. Debris flows at the south of Devoluy and in the northern part of the Massif des Ecrins were observed using aerial photographs covering the period 1948 2000 and field observations made annually since 1995. Meteorological data were collected from stations covering very large areas in the region. Climatic change was indicated by an increase in temperatures and an increase in the number of daily rainfalls above 30 mm during the summer. In the Devoluy region, a reduction in the number of debris flows was observed since the 1980s, while there was an upward shift in the triggering of the debris flow zone in the Massif des Ecrins but a lack of significant variation in the number of debris flows. These two different trends-decreasing debris flows at low altitude and increasing debris flows at high altitude-were explained by a decrease in the number of freezing days related to the increase in temperatures.