Airborne Scanning

Jakob Rom et al.: Analysing the Large-scale Debris Flow Event in July 2022 in Horlachtal, Austria Using Remote Sensing and Measurement Data 27.03.2023

High-quality in situ measurements are essential for hazard assessment of debris flow events. However, precise data on debris flow triggering thresholds, accumulation volumes and spatial characteristics of large-scale events on catchment scale are scarce due to the rare occurrence of debris flows and the challenges of acquiring accurate data for a larger area. In this study, the authors present quantitative analyses of a single extreme debris flow event in the Horlachtal, Austria, triggered by local high-intensity short-duration precipitation events on 20 and 23 July 2022. Pre- and post-event airborne LiDAR (light detection and ranging) data with a high spatial resolution reveal that 156 different debris flow processes were initiated during these events, with accumulation volumes of up to approximately 40,000 m3. The calculated debris flow deposition volumes also show a power-law relationship with the total amount of rainfall in the respective debris flow catchments. The spatial appearance of the debris flows shows a concentration of processes in a particular area rather than a uniform distribution, suggesting a local nature of the triggering event. This is further supported by the measurements from three meteorological stations and four discharge gauges within the study area. The gridded area-wide INCA (Integrated Nowcasting through Comprehensive Analysis) rainfall data further point to a local convective event on 20 July 2022, with a maximum rainfall intensity of 44 mm/h.

LiDAR data used for the study was acquired in two airborne LiDAR datasets. The pre-event LiDAR data were acquired for the entire study area on 22 September 2021 by the Chair of Physical Geography of the Catholic University of Eichstätt-Ingolstadt using a RIEGL VUX-1LR sensor integrated in a RIEGL VP-1 Helicopter Pod, which was installed on an Airbus Helicopters H125 Ecureuil. The post-event LiDAR data acquisition was carried out on August 3, 2022, as a “special event-based data acquisition” and thus only two weeks after the main debris flow event on July 20, 2022 in Horlachtal. Based on the data of both flights a high-resolution LiDAR-generated DTM before and after the event, covering all parts of the study area. Thus, the topographic changes caused by the debris flow event were analyzed.

The study shows that the combination of several data sets – in this case photogrammetric data of permanently installed monitoring system, LiDAR data of surveys before and directly after the debris flow event, data of three different meteorological stations in the study area, and INCA rainfall data -  can provide valuable information about the spatial characteristics of a debris flow event. It thus contributes to a better understanding of the triggering mechanisms and magnitudes of such events, which is important for hazard assessment of future events.

The full article is published on the platform MDPI, and can be found here.