Compound Drivers and Spatial Connectivity led to the Devastating Debris Flood in the Village of La B&eacute;rarde, June 2024, French Alps

Filhol, Simon; Misset, Clément; Bontemps, Noélie; Cusicanqui, Diego; Paquet, Emmanuel; Dumont, Marie; Gagliardini, Olivier; Lacroix, Pascal; Gascoin, Simon; Thirel, Guillaume; Brondex, Julien; Hagenmuller, Pascal; Larose, Eric; Schoeneich, Philipp; Roy, Denis; Thibert, Emmanuel; Eckert, Nicolas; de Montety, Félix; Mainieri, Robin; Hauet, Alexandre; Gottardi, Frédéric; Berthet, Johan; Baratier, Alexandre; Liébault, Frédéric; Chmiel, Małgorzata; Piton, Guillaume; Chambon, Guillaume; James, Guillaume; Frey, Philippe; Deline, Philip; Astrade, Laurent; Vincent, Christian; Laigle, Dominique; Recking, Alain; Karbou, Fatima; Mauss, Adrien; Bonnefoy-Demongeot, Mylène; Fontaine, Firmin; Langlais, Mickael; Berthier, Etienne; Blanc, Antoine

doi:10.5194/egusphere-2026-971

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https://doi.org/10.5194/egusphere-2026-971

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06 Mar 2026

| 06 Mar 2026

Status: this preprint is open for discussion and under review for Natural Hazards and Earth System Sciences (NHESS).

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

Simon Filhol, Clément Misset, Noélie Bontemps, Diego Cusicanqui, Emmanuel Paquet, Marie Dumont, Olivier Gagliardini, Pascal Lacroix, Simon Gascoin, Guillaume Thirel, Julien Brondex, Pascal Hagenmuller, Eric Larose, Philipp Schoeneich, Denis Roy, Emmanuel Thibert, Nicolas Eckert, Félix de Montety, Robin Mainieri, Alexandre Hauet, Frédéric Gottardi, Johan Berthet, Alexandre Baratier, Frédéric Liébault, Małgorzata Chmiel, Guillaume Piton, Guillaume Chambon, Guillaume James, Philippe Frey, Philip Deline, Laurent Astrade, Christian Vincent, Dominique Laigle, Alain Recking, Fatima Karbou, Adrien Mauss, Mylène Bonnefoy-Demongeot, Firmin Fontaine, Mickael Langlais, Etienne Berthier, and Antoine Blanc

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.

How to cite. Filhol, S., Misset, C., Bontemps, N., Cusicanqui, D., Paquet, E., Dumont, M., Gagliardini, O., Lacroix, P., Gascoin, S., Thirel, G., Brondex, J., Hagenmuller, P., Larose, E., Schoeneich, P., Roy, D., Thibert, E., Eckert, N., de Montety, F., Mainieri, R., Hauet, A., Gottardi, F., Berthet, J., Baratier, A., Liébault, F., Chmiel, M., Piton, G., Chambon, G., James, G., Frey, P., Deline, P., Astrade, L., Vincent, C., Laigle, D., Recking, A., Karbou, F., Mauss, A., Bonnefoy-Demongeot, M., Fontaine, F., Langlais, M., Berthier, E., and Blanc, A.: Compound Drivers and Spatial Connectivity led to the Devastating Debris Flood in the Village of La Bérarde, June 2024, French Alps, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-971, 2026.

Received: 19 Feb 2026 – Discussion started: 06 Mar 2026

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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RC1: 'Comment on egusphere-2026-971', Lorenzo Marchi, 20 Mar 2026 reply

General comment
This manuscript provides a comprehensive analysis of a complex event that resulted in an extreme debris flood in a small catchment of the Fench Alps in June 2024.

The results point out that the severity of the event stems from its compound nature, i.e. the co-occurrence of different drivers. Such a result is undoubtedly relevant for the interpretation of cascading events in alpine catchments under current climate changes. The conclusions could stress the wider interest of these findings beyond the interpretation of this study case.

This work could become a valuable contribution to NHESS after moderate revision aimed at improving the structure and organization of the manuscript.

I would like to invite the authors to perform a thorough revision of paper structure and organization aimed at making it smoother and easier to follow. The present manuscript, because of its structure, looks like a research report instead of a journal paper. Moreover, although an important part of the work is presented in Appendices, the text is very long, and it has some redundancies. For instance, the Introduction includes a rather long text and some figures that would be more appropriate in the description of the study site. I would recommend checking the entire manuscript for repetitions and redundant text.

Specific comments
Figure 2 does not provide significant information: it could be expunged or moved to the Appendix.
The methods applied to the study of this event would deserve to be described in a separate section, instead of being part of Section 2, which also discusses site and data (Section 2.5 is very short and does not present all methods applied in the study). All relevant methods should be described or referred to by indicating the Appendices where they are described.
Section 2.1

The Melton ruggedness number of the Étançon at La Berarde could be computed: its value, together with the fan slope, would provide some clues about long-term flow processes (debris flow, debris flood, mixed processes?) in the distal part of the torrent (see Bertrand et al., 2013 for thresholds between debris flows and fluvial processes). Of course, no ultimate answers on the type of flow processes that occurred during a specific event can be expected from these simple morphometric indices, but they could be of some interest in site description.

Reference: Bertrand, M., Liébault, F., & Piégay, H. (2013). Debris-flow susceptibility of upland catchments. Natural Hazards, 67(2), 497-511.
Could Sections from 3 to 6 be pooled under the general heading “Results”? This would imply a three-levels heading, but it would result in a better-defined structure of the work.
Section 3.1. Some details on the warning procedures, which permitted rescuing people and avoiding loss of lives, should be reported.
“Drivers of the event” (now Section 4) should be described before “Impact of the event” (now Section 3).
The reconstruction of event chronology, now in section 5.4 and part of the section on river response, should be one of the first reported results. Moreover, event chronology should not be limited to fluvial processes, and it should cover the entire spectrum of the processes involved in the studied event.
Lines 41-43. “The detached… mobile debris flow”. This sentence is redundant: the references to the Himalayan event are sufficient.
Line 72. Baud-Bovy, 1907 is not on the refrences list.
Lines 103-106. I wish to suggest reporting the areas of upper Étançons and Bonne Pierre catchments. The current sentence “Its [upper Étançons] catchment is about 2.5 larger than Bonne Pierre Torrent” does not clarify the extent of these two subcatchments.
Line 130 “alluvion fan” should be corrected to “alluvial fan”.
Lines 130-138. Some rephrasing and more details are necessary. What is the alluvial fan area? Reporting it would permit appreciation that the fan is small in comparison to the catchment size (line 130). The truncation of the fan toe by the Vénéon river should be reported immediately after the second sentence, which mentions the evacuation of sediment by the Vénéon river.
Lines 139-143. Why has a 48-hours duration been considered? Does it correspond to the duration of the rainstorm and snowmelt that triggered the 2024 event (see lines 59 and 211.213)? Please specify. The authors could consider moving here the text at lines 211-213.
Lines 163-164. A reference to the Glacioclim program is necessary.
Line 242. “leading to many damages in the Bérarde village”: damage to the village has already been mentioned. No need to say it again.
Lines 639-641. Two possible references to papers that propose different interpretations on the formation of a cluster of megafans in the eastern Alps:

Jarman, D., Agliardi, F., & Crosta, G. B. (2011). Megafans and outsize fans from catastrophic slope failures in Alpine glacial troughs: the Malser Haide and the Val Venosta cluster, Italy.

Brardinoni, F., Picotti, V., Maraio, S., Bruno, P. P., Cucato, M., Morelli, C., & Mair, V. (2018). Postglacial evolution of a formerly glaciated valley: Reconstructing sediment supply, fan building, and confluence effects at the millennial time scale. Bulletin, 130(9-10), 1457-1473.
Lines 609-620 and A12

The authors note that the observed flow depth at La Bérarde is much smaller than would have been assuming 200,000 m³ debris flow triggered by the sudden drainage of the superglacial lake. Could we argue that the also the type of flow processes observed in the distal part of the channel (i.e. a debris flood instead of a debris flow at La Berarde) indicate that a sudden lake drainage is highly unlikely?
Line 1327 (Torricelli law)

If the authors wish to cite the original work of E. Torricelli, which is probably not strictly necessary, the year of publication (1644) should be reported.

The reference would be Torricelli, E. Opera geometrica, typis A. Masse et L. de Landis, Florentiæ, 1644.

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Citation: https://doi.org/10.5194/egusphere-2026-971-RC1

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Short summary

On June 21 2024, the village of La Bérarde, in the French Alps, was devastated by a flood destroying centuries old buildings. This study is an interdisciplinary work to decipher the causes and chronology of the event. The flood started with decadal rain falling on a thick snowpack. A lake observed on top of a glacier few days prior, had drained post event. With climate change, should we expect more similar compound events for alpine communities?


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