Signature of the 2005 Flood Event in the Sediments of Lake Brienz: Implications for Flood Reconstruction Master’s Thesis Faculty of Science, University of Bern presented by Danae Motta Oeschger Centre for Climate Change Research OCCR 2016 Supervisor: Prof. Flavio Anselmetti Institute of Geology and Oeschger Centre for Climate Change Research Abstract In August 2005, an extreme precipitation event lead to episodes of erosion, flooding, landslides and debris flows over Switzerland. Strong rainfall initiated on the 20th of August, peaking on August 22 and finally decreasing on the 23rd of August. Between 20-23rd of August, more than 300 mm of rain fell in some regions. Lake Brienz (Bernese Oberland) is considered to be the one of the most representative examples of the destructive power of the event. In the night between the 22nd and 23rd of August, two debris flows from two mountain torrents (Trachtbach and Glyssibach) hit the villages of Brienz and Schwanden bei Brienz, damaging and partly destroying the area and causing the death of two inhabitants. During the event, flooding was also observed for the two main tributaries of the lake, Lütschine and Aare, with discharges increasing by an order of magnitude of 3-4 compared to normal levels. The present study focuses on a sedimentological and hydrological analysis of the 2005 flood event in Lake Brienz. The 2005 layer was identified in the sediment record and the contribution of the different inflows during the event was assessed based on sedimentary differences within the layers and the geochemical composition of the sediments. Geochemical analysis provided insights on the two main controlling factors of the sediments, such as lithology and grain size. The lithological signal allowed to differentiate the contribution of Aare and Lütschine inflows, which present a different catchment geology and lithology. Cores presenting a Lütschine signal show a carbonate and siliciclastic composition, while cores reflecting the Aare signal mainly present a siliciclastic composition. Calcium was therefore used as proxy for the distinction of Aare and Lütschine input. Grain size and carbon content analysis were also applied to better differentiate the different riverine contributions, as well as to identify the two debris flows within the sediment record. Trachtbach and Glyssibach presented a higher carbonate content in respect to the two major inflows due to sedimentary composition of the two catchments, and their related debris flows were identified in two cores. The extent of the different riverine contributions during the flood was assessed, however missing layers in the depocentre precluded a complete reconstruction of the extent of the turbidic plumes deriving from the event. Missing layers have been interpreted to be a result of erosion or unsuccessful core recovery due to very coarse and thick material accumulated in the centre of the lake. Discharge and precipitation time series were also analysed in order to reconstruct the succession of events during the 2005 flood. Finally, an estimation of the volume of sediments transported and accumulated in the lake basin was presented for all the studied inflows. It was estimated that around 1 Mio m3 of material deposited during the event. Overall, Lake Brienz provided valuable insights on sedimentary processes characterizing the flood of August 2005 in the area, allowing to extend the flood record in the Holocene and to better interpret climatic conditions of floods triggered by extreme precipitation events. II Table of Contents Abstract ............................................................................................................................................................................................. II List of Figures ............................................................................................................................................................................. VI List of Tables ............................................................................................................................................................................ VIII Acknowledgements .................................................................................................................................................................. X 1 Introduction .............................................................................................................................................................................. 1 1.1 Research Motivation .......................................................................................................................................................... 1 1.2 Floods ....................................................................................................................................................................................... 2 1.2.1 Floods in Switzerland ............................................................................................................................................... 2 1.2.2 Floods in Mountain Regions .................................................................................................................................. 3 1.2.3 Precipitation ................................................................................................................................................................. 4 1.2.3.1 Precipitation in Alpine Regions .................................................................................................................. 4 1.2.4 Vegetation ..................................................................................................................................................................... 5 1.2.5 Topographic Relief .................................................................................................................................................... 6 1.2.6 Soil .................................................................................................................................................................................... 6 1.2.7 Runoff .............................................................................................................................................................................. 6 1.3 Floods and Climate ............................................................................................................................................................. 7 1.4 Floods in the Future ........................................................................................................................................................... 7 1.5 Lake Sediments as Archives ........................................................................................................................................... 8 1.6 August 2005 Flood Event ............................................................................................................................................. 10 1.6.1 Meteorological Situation ...................................................................................................................................... 11 1.6.2 Lake Brienz ................................................................................................................................................................ 14 1.6.2.1 Trachtbach & Glyssibach Debris Flows ................................................................................................ 17 1.7 Project Aim ......................................................................................................................................................................... 19 1.8 Project Design: .................................................................................................................................................................. 19 2 Study Site .................................................................................................................................................................................. 21 2.1 Geography ........................................................................................................................................................................... 21 2.2 Geology and Catchment Lithology ............................................................................................................................ 21 2.3 Climate .................................................................................................................................................................................. 24 2.4 Vegetation and Soil .......................................................................................................................................................... 25 2.5 Hydrology and Lake System ........................................................................................................................................ 26 2.6 Anthropogenic Impact on Lake Brienz ................................................................................................................... 29 3 Materials and Methods .................................................................................................................................................. 31 III 3.1 Fieldwork ............................................................................................................................................................................ 31 3.2 Core Opening and Description ..................................................................................................................................