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

• Main Author: Wilhelm, Bruno
• Other Authors: Environnements, Dynamiques et Territoires de Montagne (EDYTEM), Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry )-Centre National de la Recherche Scientifique (CNRS), Université de Grenoble, Fabien Arnaud, Jean-Jacques Delannoy
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2012
• Subjects: Sedimentology; Lakes; Millenium; High elevation; Extreme floods; Climatic changes; Sédimentologie; Lacs; Millénaires; Haute altitude; Crues extrêmes; Changement climatique; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; North Atlantic
• Online Access: https://theses.hal.science/tel-00734248; https://theses.hal.science/tel-00734248/document; https://theses.hal.science/tel-00734248/file/36022_WILHELM_2012_archivage.pdf

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 and the cold Little Ice Age. 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 identify a large number of flood deposits, distinguish deposits resulting from floods from gravity processes and 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 Little Ice Age. 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. Finally the most extreme events occurred during the warmer periods in the northern Alps while they occur during the Little Ice Age 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 ...