Complex, slow-moving landslide dynamics: implications from a long-term monitoring setup on the Hofermühle landslide in Lower Austria

Slow-moving landslides play an important role in both theoretical slope evolution and practical landslide hazard and risk research. Their process rates impede the quantitative analysis of related dynamics over short time periods, given that the actual changes are often lying within the error margins of the respective methodological approaches. In this study, current results are presented for a long-term monitoring setup of a slow-moving earth slide – earth flow system in the Flysch and Klippen Zone of Lower Austria. The aim is to further assess surface and subsurface characteristics, their interrelations, and implications on spatio-temporal landslide dynamics.

The research strategy comprises the utilization and analysis of both surface and subsurface monitoring data. The methodology includes the application of Terrestrial Laser Scanning (TLS) and Unmanned Aerial Vehicle (UAV) based Structure from Motion (SfM). Geotechnical methods, such as penetration tests, percussion drilling and inclinometer measurements are used to gain information about subsurface characterization. A meteorological station and piezometer measurements provide information on hydro-meteorological conditions. Surface monitoring data is available since 2015, subsurface monitoring started in 2018.

Results suggest that a) very high-resolution surface data is necessary to capture real surface changes and that TLS is more suited for processes such as these than UAV based SfM, b) the interpretation of morphological features based on multi-temporal mapping can increase the DoD based level of surface change detection, c) only prolonged observation periods can reveal interrelations on surface and sub-surface dynamics and d) that in-depth knowledge on the study area is important to interpret results and that the impact of natural, but especially artificial disturbances of the hillslope system more or less temporarily close to recent process activities remains difficult to evaluate.

Current monitoring results reveal the complexity and non-linearity of slow-moving, complex landslide behaviour. Both high spatial and temporal resolution of on-going monitoring data enables an assessment of low rates and changes. However, the slower the process, the longer the observation needs to be. Otherwise the actual process dynamics might be misinterpreted, e.g. the data might be superimposed by technical restrictions.