The "Salcher" landslide observatory-experimental long-term monitoring in the Flysch Zone of Lower Austria

Shallow landslide processes in geologically prone areas are recognised to pose threat to both human life and property. As precipitation is one of the main triggers for landslides, hydro-meteorological interrelationships and related future changes regarding frequency and magnitude of landslide processes in particular are of major interest. Long-term monitoring investigations of active landslide sites can provide a better understanding of the kinematic behaviour and triggering conditions of slope instabilities induced by hydro-meteorological patterns. In this study, we present the installation and first results of a long-term monitoring setup in the Flysch Zone of Lower Austria equipped with a large variety of combined hydrological and geotechnical measuring techniques. The geological unit of the Flysch Zone, characterised by high contents of clay and the corresponding weathering products, is exceptionally prone to earth and debris slides which are mostly triggered by heavy precipitation events or snow melting. The landslide under investigation is situated in the heterogeneous lithology of Flysch deposits, surrounded by private property and without any agricultural usage. There is evidence of landslide activity since the 1950s. As it is showing only moderate displacement velocities (max. 20 cm in 2009), it represents a predestined study site for a long-term monitoring and the testing of new monitoring techniques. One of the main aims of this study is to characterise the internal structure, assess the current landslide dynamics and to analyse recent process activity by means of surface and subsurface monitoring installations. Surface methods currently include terrestrial laser scanning, GNSS and total station measurements. With these, surface movement rates of approx. 12 cm/6 months have been detected in the most active part of the landslide. Inclinometer measurements together with results from core drillings and penetrations tests suggest a complex, rotational landslide system with different shear zones, consisting of a more active part in the upper 3 m underlain by a less active part down to 9-m depth. As this monitoring site is designed to be operated for at least 10 years, information about its structure and high-resolution, multi-temporal data about its dynamics can be correlated with hydrological cause variables in the future. These insights and the exemplary nature of the study site regarding shallow landslide processes in Flysch deposits will be useful for the development of novel analysis methods for both Lower Austria as well as study sites with similar initial conditions.