Compound Drivers and Spatial Connectivity led to the Devastating Debris Flood in the Village of La Bérarde, June 2024, French Alps

Abstract. On the evening of June 21, 2024, a debris flood inundated the village of La Bérarde, located at the heart of the Écrins mountain range in the French Alps. More than 200 000 m³ of materials were deposited in place of the village. People were evacuated on time but many buildings were destroyed and buried. The event was understood to be driven by a 10-year return period rain alongside to a 20-year snowmelt, the drainage of a supra-glacial lake, with potentially more internal water storage in the Bonne Pierre glacier. While we do not have direct observation of the supraglacial lake drainage, we found a number of evidences pointing to the role it likely had in destabilizing and triggering sediment transport from the Bonne Pierre fan. This work required an interdisciplinary approach to establish the set of scientific elements to reconstruct the event's chronology and rarity. We found that the combination of moderate magnitude drivers is not sufficient to explain the impacts observed. The location of the village on an alluvial fan directly connected to the source of sediments, was also key to understand the magnitude of the impacts. This event took place in a region particularly sensitive to climate change, where physical processes of the cryosphere at play are subject to alteration in a changing climate (e.g. precipitation amount and phase). The recent paradigm of compound events helps reconsidering the nature of this event and suggests possible approaches in anticipating new up-coming compound events in an era in which the Alps are entering a new paraglacial adjustments. Nevertheless, compound events remain difficult to forecast as they may be generated by diverse set of combination of low to moderate magnitude hazards associated to specific geographical, geomorphological, cryospheric, and meteorological onsets.