Dynamic characterization of a slow-moving landslide system – Assessing the challenges of small process scales utilizing multi-temporal TLS data

Abstract Slow-moving landslides play an important role in both theoretical slope evolution and practical landslide hazard and risk research. Related dynamics therefore are of major interest, including the investigation of surficial changes. Yet, very slow process rates imped their quantitative analysis over short time periods, the actual change not uncommonly lying within the error margins of the respective methodological approaches. This study aims in investigating the surface dynamics of a small, retrogressive, slow-moving (cm-dm/a) earth slide-earth flow system in the Flysch Zone of Lower Austria via multi-temporal, high-resolution terrestrial laser scanning (TLS) data. 7 epochs covering a 10 years’ observation period were utilized to apply both detailed morphological mapping and the computation of digital elevation models (DEMs) of difference (DoDs). Data was analysed to 1) determine and delineate (recently active) process areas, 2) to describe their characteristics, rates and tendencies comparatively via mapping and DoDs - but also 3) to assess the applicability of TLS regarding vegetation cover and to 4) evaluate the added value of this comparative approach when it comes to interpreting landslide dynamics on such detailed scale. Two small Subsystems of the respective landslide, I (~3 300 m2) and II (2 100 m2), exhibit the highest activity within the observation period. Results show 1) areas of changes in surface height correspond with changes in the distribution and characterisation of morphological landslide features, indicating landslide activity. 2) Both Subsystems exhibit different results regarding the magnitude of changes in surface height (DoDs) and feature assembly (mapping), but show similarity regarding the frequency of both changes in surface height and feature evolution, identifying them as rotational and translational process types interrelated with the main landslide system. 3) Findings suggest TLS based DoD computations to be able to detect real surface change on detailed scale (0.05 m raster, ±0.05 error range, 0.05 m steps) in areas of optimum conditions regarding vegetation cover, but also that 4) real surface change could be assessed in areas of less optimum conditions (±020 m error range, 0.20 m steps) where real surface change was overshadowed by changes in vegetation cover via comparatively analysing both DoD and mapping results.