High magnitude debris-flow event from a proglacial environment, Lachegggraben/Pilatuskar, Austria, 2023

On August 28, 2023, an exceptionally large debris flow occurred in the Rauris Valley, Salzburg, Austria, inundating most of the valley floor, transferring large quantities of sediment to the river system, and causing damage to local infrastructure. As the source area is located in the glacial and paraglacial environment and comparable events of such magnitude seem to have become more frequent in recent years due to changing climate conditions, this contribution investigates the meteorological and geomorphological initiation conditions and documents the flow and deposition behavior of the debris-flow event at Pilatuskar.  We analyzed high-resolution radar-based precipitation records together with local rainfall and temperature data to assess the magnitude and the return period of the rainfall event. The sediment budget was quantified from differential digital elevation models derived from UAV photogrammetry drone flights immediately before (25.08.2023) and after (04.09.2023) the event. The sediment source area was investigated using electrical resistivity tomography and ground penetrating radar to evaluate ground ice occurrence and derive sediment thickness. Geomorphologic features such as levées, lobes and plateaus were manually mapped and samples taken for sedimentological and rheological analysis. Video recordings, seismic records and reports of eye witnesses were used to constrain the dynamics, sequence and time-line of deposition. The event mobilized around 680,000 m³ of sediment in the proglacial area of the Pilatuskees glacier eroding up to 15 m deep into the main discharge channel and about 570,000 m³ were deposited in the valley floor. The triggering precipitation of 145 mm had a duration of 32 hours, a maximum hourly intensity of 21 mm/h, a return period of about 10 years, and was associated with a snow line above 3200 m. The source area is composed of proglacial sediments with a mean thickness of 10 m. Resistivity measurements one year after the event revealed unfrozen conditions in the immediate surroundings of the head scarp, but thick ground ice occurrence covered by coarse supraglacial debris within a distance of 100 m. The sediment-transfer processes lasted about 3 hours and comprised periods of fluvial sediment transport, debris flood, and debris flow. The debris-flow event consisted of a sequence of 4-5 granular surges that deposited material at different locations on the fan due to avulsion. Pebble counts yielded a median grain size ranging between 0.1 and 0.45 m, and a maximum grain size between 0.34 and 1.5 m. The fine fraction consisted of sandy gravel, with only limited clay content. The results of this study shall contribute to the documentation of the geomorphological activity within high-alpine catchments that rapidly respond to changing climate conditions. Future work will focus on monitoring debris flow activity from the newly formed erosion scar at Pilatuskar as a basis for debris-flow simulation model development and testing.