Flooding is one of the most devastating natural disasters worldwide, and has resulted in more than half a million deaths over the past 30 years. Major floods already happened in the past, but as a result of changes in climate, land use, infrastructure, and demographics, more extreme and frequent events with possibly more devastating consequences have to be expected in years to come. Indeed, exposure to floods is forecasted to grow by a factor of three by 2050, owing to a surge in population and economic assets in flood-prone areas. To adequately address such situations and to develop suitable risk reduction strategies, an improved understanding of flood processes, triggers, and multiscale impacts is crucially needed, especially also in the light of global warming. In this context, it is of paramount importance to find enough flood documentation so as to be able to decipher and understand flood activity, especially in mountain regions. Normally, there is a shortage of discharge and precipitation records, which makes it more difficult for the analysis to be statistically significant. To date, this documentation usually relies on historical chronicles or systematic obervations. In fact, documents from gauge stations, historical and natural archives still insufficiently decipher the history of past floods – and a proper merging of sources, although crucial, has not been used sufficiently in combination to disentangle potential interferences between flood activity, human intervations, synoptic weather patterns and socio-economic changes.
The main objective of this thesis is therefore to develop new methods to improve flood risk evaluation in mountain areas. First, data from different instrumental and historical sources were compiled to characterize past floods and to include this combined information in a frequency analysis in the town of Brig-Glis in the Swiss Alps (Saltina river).
In a second paper, dendrogeomorphic reconstructions were used in a Mediterranean catchment (Haut-Asco, Corsica, France). The tree position and growth disturbances were included in a numerical model to simulate flood discharge for each dated event. To investigate the main synoptic patterns and climatic factors that triggered floods at the regional scale of Corsica, we then analyzed Mediterranean cylone tracks.
In a last step, and again in the Swiss Alps, we performed risk analyses by testing changing exposure and vulnerability over time at Aigle (Vaud, Switzerland) where the Third Rhone Correction has a significant impact on risks.
In conclusion, a risk analysis that combines different data, the uses of indirect methods to reconstruct events, the inclusion of historical documents, past images, tree-ring, gauge/meteorological station data, a probabilistic approach, hydraulic modeling and uncertainty and sensitivity analyses can contribute to improvements in the study of (flash) floods in mountain areas. Although the methodology proposed in this PhD thesis has been applied to case studies in the Alps and the Mediterranean, it can be applied in other mountain areas.
