Sediments of Hallstätter See as a palaeoflood archive

Among all natural hazards, extreme flood events after extensive summer rains represent by far the largest societal and economic threat in Central Europe. Nevertheless, knowledge about the spatio-temporal variabil- ity of past flood occurrence and its connection to climatic changes is still limited due to the small number of long, precisely dated palaeoflood records. In general, palaeoflood time series based on instrumental and doc- umentary data rarely exceed 500 years. However, this is too short to reliably assess natural flood variability under different climatic boundary conditions and consequently estimate future flood occurrence with respect to global climate change. In this regard, lake sediments with event layers of detrital material from flood- triggered catchment erosion can provide long, precisely dated and continuous records of past flood activity, allowing a substantial extension of instrumental and documentary flood time series and thus an improved understanding of the natural recurrence frequency, regional distribution and climatic triggers of extreme floods. Albeit the recent progress in utilizing lake sediments as palaeoflood archives, the spatial coverage of such records across Central Europe is still very heterogeneous. While quite many long lake sediment pal- aeoflood records are already available from the Western Alps, their number in the Eastern Alps is still lim- ited. The resulting uncertainties about past flood activity in this region are particularly invidious as flooding in river catchments located there (e.g. Danube, Elbe) is likely to affect large parts of Central and Southeast- ern Europe. As the main objective of the proposed project Sediments of Hallstätter See as a palaeoflood archive, a new palaeoflood record for the Eastern Alps will be established by investigating the sedimentary record of Hallstätter See (Upper Austria). As indicated by a pilot study, the Hallstätter See sediments contain frequent event layers of detrital material from catchment erosion, which is transported to the lake during flood events of the inflowing river Traun, a major tributary of the Danube. These layers can thus be considered as indica- tors of past extreme flood events. Within the proposed project, 20-m-long sediment cores will be raised from Hallstätter See and investigated regarding the occurrence of detrital flood layers by combining detailed mi- croscopic analysis of the sediment structure with high-resolution geochemical analyses. This will finally yield a precisely dated record of flood activity in the Eastern Alps for at least the last 5000 years. The com- parison with other Alpine palaeoflood records will ultimately improve the understanding of the spatio- temporal variability of past flood occurrence in Central Europe as well as the underlying climatic trigger mechanisms and thus allow a more reliable assessment of future flood scenarios with regard to global cli- mate change.

Based on the analysis of the sedimentary deposits of Hallstätter See, the project aimed at reconstructing extreme flood events in the Eastern Alps during the last ca. 5000 years. By comparing the results with other regional studies, the climatic trigger mechanisms behind extreme flood occurrence should be identified, providing the basis for improved projections of flood occurrence under future climate change scenarios. To achieve the project goal, a continuous lake sediment sequence of 15.63 m length has been recovered from Hallstätter See (Upper Austria), which in contrast to the initial plan only covers the depositional history of the last ca. 2300 years. Most of the recovered sediments are characterized by a distinct sub-mm- to cm-scale lamination, reflecting the seasonally variable input of clastic-detrital material by the tributaries. Extremely thick and/or coarse-grained layers thereby indicate strong clastic-detrital input through high runoff following extreme precipitation events. These layers can be used to reconstruct the occurrence of flood events in the past. The results of sedimentological and geochemical analyses reveal that different types of clastic- detrital input layers can be related to individual tributaries. However, comparison with instrumental runoff data indicates that their thickness does not necessarily correlate with the strength of the runoff event. Nevertheless, this part of the project is not finished yet and requires further analyses and a critical evaluation of the already available data. As another important result of the project, the deposits of three large-scale mass movements with thicknesses of up to several meters could be identified in the recovered sediment cores. These date to ca. 100, 1050 and 2300 years before present (AD 1950) and are possibly related to large landslides and/or rock falls that entered the lake or maybe to sub-aquatic collapses of a tributary delta. Given the remarkable thickness of these deposits, they must have been caused by extremely large mass movements that most likely also influenced local human settlement activity, e.g. through the destruction of lake shore settlements by tsunami-like waves or the destruction of salt mines in the Hallstatt high valley by landslides. There is a remarkable temporal agreement between the emplacement of the oldest mass transport deposit in the lake and the archaeologically documented destruction of mining facilities in the Hallstatt high valley by massive landslides during the Iron Age. Although a direct spatial connection between both events is difficult to prove at the moment, it could be possible that both have been caused by a common trigger, for example a large earthquake. The investigation of the Hallstätter See sediments therefore not only provides new information on past climate development and flood occurrence in the Eastern Alps but also on human-environment relationships in one of the oldest cultural landscapes worldwide.