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 -
RC3: 'Comment on egusphere-2026-466', Anonymous Referee #3, 12 Mar 2026
reply
The study overall discusses the reasons for a flood event in France. It mainly focuses on the possibility of glacier impact on that flood event. The quantification of the glacier water volume is based on the computational modelling. The results generally indicate the flood-generating potential of the glacier. The following corrections are suggested in order to contribute to the improvement of this study:
Introduction
Particular studies are referred in Introduction, mainly concentrating on this specific glacier. However, other studies exist in the literature, which have focused on the potential contributions of other glaciers in the world for the flood-generation. Their modelling or observation studies should also have been referred to serve as background of the necessity of this study.
Methods
Please refer to the literature for all the formulation of the calculation.
Results and Discussion
Referencing is also valid for the Results. However, they should be discussed within the Discussion section, since it is another separate section. On the other hand, discussion was conducted such as newly introducing the study. In fact, event-based other studies should also be referenced and discussed for sake of comparison with this study results. Otherwise, the study already stands alone as being a solely extreme event.
Citation: https://doi.org/10.5194/egusphere-2026-466-RC3 -
RC4: 'Comment on egusphere-2026-466', Anonymous Referee #4, 16 Mar 2026
reply
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-466/egusphere-2026-466-RC4-supplement.pdf
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RC5: 'Comment on egusphere-2026-466', Martin Lüthi, 23 Mar 2026
reply
Dear Collegues,
A few questions, especially related to the assumption of a temperate
glacier, merit evaluation and discussion:- A large lake forming on a temperate glacier is a quite unusual
occurrence. Usually, in temperate ice, channels and cracks do no
heal (refreeze) easily, and once a channel has been enlarged it will
likely persist for an extended period of time, especially at the low
ice thicknesses of ca 100 m.Could it be, from similarity with Tete Rousse etc, that also this
glacier features cold ice, at least in some areas?- It would be interesting to assess how the supraglacial lake changed
the hydraulic potential landscape. Ponding lake water could easily
create an upstream dam/barrier (in the potential) within which a
subglacial lake could form. Drainage of the surface lake would then
simultaneously lead to a change of the potential barrier, and let
the subglacial lake drain.- If I understand correctly, large parts of the glacier were mapped
with Lidar twice in 2024. Did you look at the differences of the
DEMs? Are there any traces of subglacial collapse features, or
lowering of the surface that cannot be explained with a melt model?- It would be interesting and instructive to show the river discharge
curves, as well as results from the hydraulic modeling.- The conclusions should be considerably shortened, and just highlight
the essential new conclusions. Now it reads like an extended
abstract. Be to the point, and highlight the essence of the paper,
the conclusions!Minor comments
- The units of water volumes are given as 130 x 10^3 m^3. I see the
advantage of using something the is easy to understand, but this has
not been consistently used (Fig 5 shows 10^5). I propose "Olympic
Swimming Pools". Joking aside, settle on one unit and use it
consistently. Personally I would prefer 1.3*10^5, but 10^3 units are
fine, too. But please adopt the mathematical writing with 1.3\cdot 10^5.
The \times cross is ugly and implies an operator.- Check the proper plural forms with a native speaker (e.g. "water
pockets volume" vs. "water pocket volume", etc.). English has
grammar rules that are quite different from French, and yours sound
often wrong to me. But maybe it is just my gut feeling that is wrong.l 5-8 Abstract sounds somewhat repetitive and could be streamlined and
condensed.l 25 Bérarde (consistent with or without accent aigu)
l 28, 94 \sim 10^5 m3
l 39 Déline
l 40ff This sentence and enumeration should either be a proper
enumeration (if allowed by the journal), or rather be split in
four sentences.l 49 "a water pocket"
l 56 "favourable for"
l 61 "allows us"
l 85 3*10^5 m3
l 85 exported -> eroded (?)
l 90 fails
l 117 one obviously also would be interested in differences of the
DEMs, and their relation to melt or subglacial processes.l 126 "vertically": why vertically, electromagnetic waves do not feel
gravity. Better would be perpendicular to the surfacel 135 why not cite a more recent, in-depth analysis of radar signal
speed on alpine type glaciers, especially by comparing with
borehole depths?l 139 \citet{Grab}
l 154 the value of the crucial quantity \rho_i should be given. For
most alpine glaciers it is likley 900 kg/m3 (not 917).l 157 each pixel
l 160 just choose one or two relevant sources (not 10), and use \citep
l 166 "dampened" -> "smoothed / distributed"
l 170 shorten title
Fig 2 a) it would be helpful to indicate the glacier flow direction by
a blue arrow. Also make the green lake really green, and maybe
add a white border, such that it is visibleFig 2 c) make the water lines wider, they are barely discernible from black
Fig 2 d) use a color map where not everything is the same red (e.g.
"jet")Fig 2 all panels: use reasonable numbers for the color bars (not 67/101..)
or the random elevations in panel cFig 2 all panels: you could simplify the coordinates wrt. to an
offset, then the panels would be larger.
1. all in km
2. subtract an offset in x and y
3. call the coordinates "Easting" and "Northing" instead of x,yFig 2 make all panels exactly the same!
Fig 2 legend: streamline the repetitive parts,
remove the double plural (lines intervals)
l 203 "is" -> "was"l 208 In GPR, water is often specular, i.e. the phase of the reflected
signal changes (e.g. Matsuoka). You should see this in the bed
echos, whether you get an inverse first reflection.l 219 "water pocket height"
l 220 distribution ... is shown
l 222 / 227 etc 1.6*10^5, 1.48*10^5
Fig 3 Make the figure better as proposed for Fig 2:
- simple, short coordinate labels (km/offset)Fig 3 use the same color for water reflections as in Fig 2! (maybe
light blue or pink?). Not black!Fig 3 write "water pocket height" everywhere (discard plural 's')
Fig 4 Much like Fig 3 and Fig 2, also use reasoable color bar values
(not 21)254 shorten title (leave away Glacier...., this is the topic of the whole paper)
258 But maybe it would also form another hydraulic dam that blocks the
subglacial water flow upstream of the lake. Such a subglacial lake
might be even bigger?273 But should be very visible after lake drainage.
285 These arguments would be easier to follow if they were illustrated
with some sketches.347 But before you invoked the smoothing effect of ice stress
transfer, which certainly should also somehow enter this
assessment here. Maybe just the elastic field, or some viscous
relaxation?350 And also the detailed stress field at the base. The surface
geometry and boundary conditions lead to a non-hydrostatic stress
field at the base, which differs substantially from the simple
\rho*g*H assumption.353 But then, debris has a void space of 30-50%, such that density is
not that different from compact ice.358 "water pocket volume" (also 359) (be careful with plural in
English! there are lots of these errors everywhere)360 Water pockets alone are not important, but only the fraction that
can drain easily.372 "produces"
Fig 5: Why are 10^5 units shown here, whereas throughout the text
there are 10^3 units. the labels could be considerably shortened
by leaving away ".0" and also 10^5 (could be added to the axis label).Fig 5 caption: leave away "noted" (you mean denoted, but leave it away)
Fig 5 caption: "crosses mark" -> "crosses"
Fig 5 caption: shorten and streamline the text
Acknowledgments: I think you should add the full SNF project number
(starting with 200020_ or similar), not just a part of it.Citation: https://doi.org/10.5194/egusphere-2026-466-RC5
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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.