Marie Chenet

Are Debris Floods and Debris Avalanches Responding Univocally to Recent Climatic Change – A Case Study in the French Alps

Debris flow is a dominant mass movement process in mountain areas all over the world and is a significant natural hazard. A classical distinction is made between a debris flood (DF) corresponding to a rapid, surging flow of water, heavily charged with debris in a steep channel, and a debris avalanche (DA) corresponding to a rapid or extremely rapid shallow flow of partially or fully saturated debris on a steep slope without confinement in an established channel (Hungr, 2005). In mountain areas like the Alps, the increase in human activity has resulted in increased risks of natural hazards such as debris flows. There is thus a growing demand for hazard zoning and debris flow protection. However, debris flows are caused by complex interactions between local topography, weather and sediment properties, making the understanding of debris flow activity very difficult. Because anticipated changes in climate may alter the dynamics of slope processes and the frequency or magnitude of extreme events, understanding the mechanisms that link climate and debris flow activity is the first step in any attempt at forecasting. Consequently, many studies have focused on the meteorological conditions that trigger debris flows in different environmental conditions in northern Europe (Innes, 1985; Rapp, 1995; Nyberg and Rapp, 1998) and in the Alps (Haeberli et al., 1990; Zimmerman & Haeberli, 1992; Rebetez et al., 1997). Triggering thresholds based on analyses of intense rainy events or long duration precipitation have been proposed for different spatial scales (Caine 1980; Guzzetti et al., 2008). An increase in temperatures and changes in the amount and frequency of rainfall have been observed in different mountain regions in the last few decades. Such changes in climate conditions could have an impact on the intensity and/or frequency of debris flows. However, only a few authors conducted detailed analyses of the impacts of climate change on DF activity to check the validity of this hypothesis. In British Columbia, Canada, Jakob & Lambert (2009) predicted an increase in the total number of debris flows by the end of the century due to increases in precipitation. From tree-ring series Stoffel & Beniston (2006) clearly show that the debris-flow frequency at Ritigraben (Swiss Alps) increased in the 1866–1895 period that followed the maximum extent of LIA glaciers and that events occurred most often in the early decades of the 20th century.