Debris flows cause an average 30 million damages and 1-2 fatalities every year in Austria. Detailed documentation of their extent and magnitude is essential for understanding, preventing and mitigating these natural hazard events. The recent development of unmanned aerial systems (UAS) has provided a new possibility for on-demand high-resolution monitoring and mapping. Here, we present a study, where the spatial extent and volume of a large debris flow event were mapped with different UAS, fitted with commercial off-the-shelf sensors. Orthophotos and digital terrain models (DTM) were calculated using structure-from-motion photogrammetry software. Terrain height differences caused by the debris flow in the catchment and valley floor were derived by subtracting the pre-event airborne laser scanning (ALS) DTM from a post-event UAS-DTM. The analysis of the volumetric sediment budget showed, that approximately 265,000 m³ material was mobilised in the catchment, of which 45,000 m³ settled there; of the material, which reached the valley floor, 120,000 m³ was deposited, while another 10,000 m³ was eroded from there. The UAS-results were validated against ALS data and imagery from a traditional manned-aircraft photogrammetry campaign. In conclusion, the UAS-data can reach an accuracy and precision comparable to manned aircraft data, but with the added benefits of higher flexibility, easier repeatability, less operational constraints and higher spatial resolution. Unmanned Aerial Vehicle, Structure-from-motion, Natural Hazards, Volumetric Sediment Budget, Digital Terrain Models, Event Documentation
