Sedimentological reconstruction of hydrological extremes over the last millennium in the French Alps: Relation to climatic changes

An increase of the intensity of hydrological extremes is expected in the current context of the global warming. However, the lack of observations of torrential events and measurements of precipitation at high elevation areas does not allow supporting this theory. Past flood evolutions can be recorded in lake sediments and allow to explore the evolution of such events and the relationships to past climatic changes. This work aims at reconstructing the past evolution of flood frequency and intensity based on the study of sediment sequences of high elevation lakes of the French Alps. To assess the role of the temperature on this evolution, our study focused over the last millennium which includes thermal-contrasted periods such as the warm Medieval Climate Anomaly (MCA) and the cold Little Ice Age (LIA). Studied sites were selected along a north-south transect to investigate the regional variability of flood evolution in response to the same context of past climatic changes. High-resolution sedimentological and geochemical analyses allowed to (i) identify a large number of flood deposits, (ii) distinguish deposits resulting from floods from gravity processes and (iii) to assess a reliable flood intensity proxy. Some distinct dating methods were finally used to reduce age uncertainties on the age-depth relationships. An increase of the flood frequency at a multi-secular time-scale appeared over the entire French Alps during the LIA. However at a multi-decennial time-scale flood frequencies evolved differently between the north and the south of the French Alps. Maxima of flood frequency appear during the warmer periods in the northern Alps while maxima of flood frequency appear during negative phases of the North Atlantic Oscillations in the southern Alps. Finally the most extreme events occurred during the warmer periods in the northern Alps, especially during the MCA, while they occur during the LIA in the southern ones. Those results suggest thus distinct regional effects of the global warming on extreme flood events and intense precipitations and, especially, an increase of flood hazard in the northern French Alps owing to an increase of flood intensity with the warming context. Those regional differences are attributed to different hydro-meteorological processes which trigger the intense precipitation events in each region.. By comparison with the medieval warm period, a general decrease in the frequency of events can be expected throughout the French Alps in the coming decades. This decrease in frequency, however, should be accompanied by a marked increase in the intensity of events in the northern Alps. (author)