RECONSTRUCTION OF DEBRIS FLOW FREQUENCY USING LAKE SEDIMENTS OF LAGO DI BRAIES/N-ITALY AND THE INFLUENCE OF VEGETATION AND CLIMATE

In the catchment of the high-alpine lake Lago di Braies debris flows frequently occur. Thereby the fine grained material of the debris flow events is flushed into the lake. This material forms characteristic layers, quite different than normal, laminated sediments. Via thin sections it was possible to date different debris flow events of the last 3 centuries with an annual resolution. In this period, the frequency of events increases, while the intensity decreases. Up to now it is not possible to distinguish between climate or human impact as reasons. By comparing the results, gained from the lacustrine archive, with an independent dendrogeomorphic archive of same catchment, we achieve a good correlation. Therefore we assume that it is possible to reconstruct a debris flow calendar with this lacustrine archive, which probably covers the last 7000 years. Palynological investigations show that the debris flow process is correlated with an increase of anthropogenic indicators in this area. Especially plantcommunities of meadows and poor grassland are strengthened during in this period. The palynological results reveal another increase of anthropogenic influence in the core section around 550 cm depth (Bronze Age?). Further investigations try to connect high resolution analysis of varves and pollen to investigate changes in climate or human influence and their effect on the debris flow activity.