Climate change and rock fall events in high mountain areas: Numerous and extensive rock falls in 2007 at mittlerer burgstall, central austria

摘要

Landslides in alpine areas are becoming more frequent. In 2007, a number of rock fall events occurred on the sharp SE-ridge of the mountain Mittlerer Burgstall (2933ma.s.l., 47°06′ 07″ N; 12° 42′ 36″ E) completely changing the shape of the mountain. Before the events, the SE-ridge was sharp with steep rock faces on both sides. The mountain was a nunatak surrounded by two glacier tongues of Pasterze Glacier during the Little Ice Age. In this paper we use geomorphological mapping, permafrost distribution modelling, glacier reconstruction, surface and near-surface ground temperature data, air temperature data, and airborne laserscanning data to assess these multiple rock fall events. Results show that the entire area of detachment covers 3100m ~2. The areas of transportation and deposition cover 85000m ~2 partly contributing to the supraglacial debris cover of Pasterze Glacier. The volume of all rock fall deposits is about 56000m ~3. Permafrost modelling and ground temperature monitoring indicate that the area of detachment is located near the lower limit of discontinuous permafrost. Permafrost is relatively warm and thin at the summit area of Mittlerer Burgstall with a mean temperature of only -1.0°C at 1.8m depth in 2007-2010. Substantial surface lowering of the glacier tongues surrounding the mountain on both sides (by -250 and -70m since the Little Ice Age) changed the stress and strain field in the bedrock. Furthermore, the generally highly fractured bedrock favoured slope instability. The triggering event for the rock falls were most likely the effects of the warm winter of 2006/07 which was 2.2-4.8°C warmer compared to the seven winters before. A monitoring programme regarding future rock falls at Mittlerer Burgstall is ongoing.