Exploring the dynamics of a complex, slow-moving landslide in the Austrian Flysch Zone with 4D surface and subsurface information

Complex, slow-moving landslides are common in deeply weathered soils with clay-rich layers, as can be found in the Flysch Zone of Lower Austria. Complex process behaviour on differing spatio-temporal scales calls for longterm measurement series on both surface and subsurface parameters. Only then, dynamics and interrelations with possible triggering mechanisms might be further understood. This study investigates a small, slow-moving earth slide - earth flow system. Information on surface changes, based on digital elevation models (DEMs) of Difference (DoDs) using terrestrial laser scanning (TLS), is available since 2015 (2009, including ALS). Subsurface monitoring started in 2018. Data was analysed comparatively for overlapping periods: A conceptual subsurface model was generated with geotechnical information (penetration tests/ drill core samples), incorporating regolith thickness and potential shear surfaces. For four periods between 2018-11-14 and 2020-11-18 (-2 years), interrelations of hydrometeorological input data (precipitation/ temperature), changes in the spatio-temporal development of subsurface hydrology (piezometers), subsurface dynamics (inclinometer) and surface dynamics (DoDs) were analysed in detail. Total dynamics in the 2-year period were marginal. Total inclinometer displacements were max. -1.65 cm; these changes were too small to be depictable on the surface with TLS based DoDs. Nonetheless, several relations of present state and known former dynamics were found: Regolith thickness could be reasonably described via penetration tests, being relatively shallower in areas of known process activity. Interpolated shear surfaces based on drill core analyses, penetration tests and inclinometer data are highly simplified; nevertheless, they depict relations to locations of known activity with respect to depth and geometry. Locations of shallowest groundwater levels and highest fluctuations correspond with areas of recent minor and former larger dynamics. Inclinometer displacement rates exhibit relations to hydro-meteorological input data and are in locations known from former bigger process magnitudes.