the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Feb 2026
Potential glacier contributions to the 2024 La Bérarde flood
Abstract. On 20–21 June 2024, an unprecedented flood of the Etançons river caused important damage to the village of La Bérarde (Écrins, France). An analysis of the event showed that the flood was partially caused by the combination of an intense rain-on-snow event at high altitude and the drainage of a supraglacial lake from Glacier de Bonne Pierre. In this study, we quantify the water volume that could have also been trapped beneath the glacier in local minima of the hydraulic head, i.e., in locations that could host so-called glacier water pockets impounded by hydraulic barriers. In the absence of direct observations of water pockets, we use a numerical, steady-state approach that computes the subglacial hydraulic head from surface and bedrock topography of Glacier de Bonne Pierre. As of June 2024, hydraulic barriers at Glacier de Bonne Pierre could, in theory, have impounded water volumes on the order of 105 m3, with the largest modeled water pocket beneath a surface depression that temporarily hosted a supraglacial lake. These results provide a first-order estimate of the potential subglacial water storage capacity prior to the June 2024 flood. We propagate uncertainties in surface elevation, bedrock elevation, and flotation fraction (the ratio of basal water pressure to ice overburden pressure) through a stochastic framework and show that spatial variability in the flotation fraction dominates the uncertainty in the resultant water pocket volumes. This highlights the strong sensitivity of subglacial water-routing results to poorly constrained basal water pressure conditions. While acknowledging that the actual presence and contribution of such water pockets cannot be confirmed from available observations, our study highlights the glacial flood potential of debris-covered glaciers with pronounced surface topographic depressions, which can promote both supraglacial and subglacial water storage.
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Status: open (until 02 Apr 2026)
- RC1: 'Comment on egusphere-2026-466', Anonymous Referee #1, 12 Feb 2026 reply
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RC2: 'Comment on egusphere-2026-466', Anonymous Referee #2, 27 Feb 2026
reply
This study explores the critical role of Glacier de Bonne Pierre as a potential secondary water source that may have further exacerbated the catastrophic June 2024 flood which devastated the French village of Bérarde. The authors model the potential subglacial water storage capacity to investigate whether a hidden “water pocket” outburst could have contributed to the flood. Overall, this is a well written manuscript offering an innovative perspective on a complex disaster. The authors’ transparent approach to accepting the limitations of their study – such as the lack of direct subglacial observations and the use of an arbitrary correlation length of 100 m – is highly appreciated. Their findings suggest the disaster was likely intensified by a glacial outburst, either through the rupture of a large internal cavity or a sudden reorganization of the subglacial drainage network. The manuscript effectively draws attention to invisible glacial reservoirs in mountain catchments and the adverse role they may play under extreme meteorological conditions. Thus, I am in favor of accepting the manuscript with minor revisions. Its subject area falls squarely within the scope of NHESS, and I believe it will make a valuable contribution to the journal once these suggested improvements are addressed.
From my perspective, the primary areas for improvement revolve around contextualizing the study’s limitations earlier and addressing potential discrepancies in the timeline of the data:
- Pre-flood vs post-flood topography: The study relies on post-flood data to model pre-flood states, specifically using a June 28 DEM for surface elevation and an October 28 DEM to derive bedrock elevation. This approach assumes that the surface and bedrock conditions did not change enough between June and October to invalidate their hydraulic barrier models. However, a massive flood – including the drainage of an estimated 100,000 m3 supraglacial lake – occurred in the interim. If the topography indeed remained largely changed (which would be rather curious), this justification should be explicitly stated in the text. If they indeed did change due to the event, that physical alteration should be accounted for. I would expect to see an estimate or discussion of this in the manuscript.
- Limitations of the steady state model: The reliance on a steady-state model to simulate a highly dynamic event is a major limitation, which the authors also recognize. While a steady-state approach provides an effective way to estimate maximum potential storage geometry, it fundamentally falls short of simulating the dynamic, time-dependent processes of the actual flood (e.g. cavity opening, ice creep, or a rapid reorganization of the drainage system). A detailed explanation of this model selection – including its specific pros and cons – earlier in the manuscript (e.g., in the Methods or Introduction sections), rather than being reserved primarily for conclusion (or as a potential future step if I understood it correctly), would be beneficial to the manuscript.
- The hydrological budget: Because critical river discharge estimates became highly uncertain during the intense sediment transport of the flood, the study cannot successfully close the hydrological budget. This context is vital, as it renders the 160,000 m3 calculated subglacial volume strictly a theoretical estimate of potential capacity, rather than a measurable “missing link” definitely proven to be in the flood waters. Ensuring that this distinction remains sharply in focus throughout the text will strengthen the paper’s scientific rigor.
Citation: https://doi.org/10.5194/egusphere-2026-466-RC2
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The article analyzes the water pocket outburst flood (WOP) in the Glacier de Bonne Pierre and its potential impact on the 2024 La Bérarde flood.
The study focuses on the significant research interest in flooding related to climate change, particularly in comparison to glacier lake outburst floods (GLOFs). The article is well-written and easily comprehensible. The discussion analyzes different scenarios in which GLOFs could potentially contribute to flooding in 2024, as well as the uncertainties associated with this analysis.
My opinion is that the article fits the scope and standards of NHESS and should be published.
Minor issue.
Fig. 2: In the axis there aren't X and Y, there should be East and North.