Post-glacial reshaping of Alpine topography induced by landsliding

Ariagno, Coline; Steer, Philippe; Valla, Pierre; Campforts, Benjamin

doi:https://doi.org/10.5194/egusphere-2025-2088

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https://doi.org/10.5194/egusphere-2025-2088

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26 May 2025

| 26 May 2025

Post-glacial reshaping of Alpine topography induced by landsliding

Coline Ariagno, Philippe Steer, Pierre Valla, and Benjamin Campforts

Abstract. In steep alpine environments, successive glacial-interglacial cycles during the Quaternary led to multiple transient geomorphological phases. In particular, post-glacial periods are key transition phases experiencing rapid geomorphic changes, characterized by intense hillslope processes where ice and permafrost have retreated. Mass wasting is the dominant post-glacial process driving sediment production in steep mountain landscapes. However, its role in shaping topography, particularly in comparison to glacial activity—known for its strong deformational impact—remains poorly understood. By integrating numerical modeling with topographic data, we refine our understanding of how mass wasting shapes evolving landscape and influences sediment dynamics. In the Ecrins massif (French western Alps), we select three catchments, with particular morphological signatures or inheritance (i.e. from fluvial to glacial) to model their associated topographic evolution driven by mass wasting. Using the landscape evolution model ‘HyLands’, we quantitatively assess their individual response to landsliding by exploring the role of different internal or external factors (e.g., bedrock cohesion, return time of landslides). The model is calibrated with the output landslide area-volume scaling law and the massif-averaged denudation rate, inferred from literature. We focus on the cumulative impact of landslides, over a single post-glacial period, on catchment slope distribution, hypsometry, produced sediment volume and erosion rate. Compared to fluvial ones, inherited glacial topography shows a bimodal distribution of elevation for unstable slopes, near the crests and along the U-shape valley walls. The time evolution of this distribution is characterized by a decrease in the number of unstable slopes as well as a lowering in maximum catchment elevations induced by landsliding, usually attributed to the glacial buzzsaw. Indeed, glaciers may be not the only agent controlling mountain elevation, as we discussed in this study. Despite the stochastic nature of landslides, our modeling results also show that landslide activity and induced erosion rates are maximum at the onset of the glacial retreat and then progressive decay during the interglacial period. On the contrary, fluvial catchments show a more stable topography and less intense landslide activity resulting in lower erosion rates. This study quantitatively explores the non-linear interactions between landslides and catchment topographic evolution and questions the role of landslides in the erosion pulse during the Quaternary interglacial periods.

Received: 05 May 2025 – Discussion started: 26 May 2025

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Coline Ariagno, Philippe Steer, Pierre Valla, and Benjamin Campforts

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-2088', Anonymous Referee #1, 30 Jun 2025

Review of ’Post-glacial reshaping of Alpine topography induced by landsliding’ by Ariagno et al.
Summary: Ariano et al. explores how landslides influence landscapes evolution using numerical modelling in combination with topographic analyses. Specifically, they focus on three catchments in the western Alps that exhibit different morphologies (fluvial vs. glacial), to assess how landslide activity and erosion vary spatially and over time depending on the pre-existing landscape. Indeed, the different catchments represent a gradient in glacial imprint and deglaciation timing, allowing these regions to be used as a natural laboratory. The main objective is thus to predict and explore landslide activity and its role in transient landscape evolution during interglacial periods.
In general, this is a well-illustrated paper and well-written, with appropriate references. The scope of the study is well thought out, and I believe the results will be of interest to the wider geomorphological community. However, I do feel that the discussion could use some work (made easier to follow), to clearly communicate the implications of this work to the scientific community.
Particularly, I found that the main hypothesis of the paper is not defined consistently throughout the paper:
1: Line 143-145: “Our working hypothesis is that the different morphological signatures observed for Alpine catchments are evidencing both landslide activity and deglaciation timing.”; So, the scope is to test that landscapes today are a result of both glacial erosion and landslide activity (as well as other processes). Very clear and feasible, and I believe this is indeed shown by the results.
2: Line 559: “Our landscape evolution model […] has been designed to explore the hypothesis that landsliding represent a dominant geomorphological agent during postglacial periods.”; Sort of similar to the first instance; landslides are important. Clear.
3: Line 665: “our initial hypothesis about the capacity of landslides to erase this glacial topographic inheritance over the last post-glacial period”. So, here you state that the hypothesis is to test whether landslides can erase the glacial imprint over an interglacial cycle. This is very different from other two instances, and in my opinion much more difficult to test.
4: Line 704-706: “Here we discuss our initial hypothesis, that all the studied catchments had the same glacial topographic imprint, and show that the three catchments have a distinct erosion dynamics explained by diachronous landslide activity following different glacial retreat times”. Again, this is different, and I am not sure I agree that you can be sure the three catchments all started out with an equal glacial imprint, give their differences e.g., in glacial duration (you do comment on this in the end).
In any case, my point would be that the hypothesis (or hypotheses) to be tested should be clear throughout the manuscript, and more care should be put in developing the rationale behind the 3. and 4. points listed here. These arguments are not trivial, and you are sort of cutting some corners by referring back to a hypothesis (that was stated differently initially).
Can you really infer from this study that the initial glacial landscape of the ‘fluvial’ catchment has been erased by landslide activity, whereas the others are still ongoing? If the upper catchments will not transition into fluvial catchments over timescales of 100 kyr, how has the lower catchment managed to do the transition already? You come to this in the end of section 5.2, but I really think the argumentation for this point should be outlined much more clearly throughout. And it should be clearly stated where you land in terms of your initial hypothesis. You could also present these ideas with more caution: “If the three catchments had the same glacial topographic imprint initially…”.
Another smaller comment I have relates to the time scale of the model simulations. The 100-kyr duration of the models seems a bit odd, i.e., to simulate landscape evolution over such a duration without considering glacial changes. It also seems a bit unnecessary given the overall scope, focused on interglacial timescales. Perhaps this could be justified (or the rationale behind could be explained), for instance by including in the discussion some reflections of how results would differ/be limited given a different choice. Right now, it is simply stated as a fact in the manuscript (line 615-616).
In addition, I think the discussion lacks some perspective related to the fact that the used (present-day) topography has already been influenced by these processes since glacial retreat (i.e., the DEM has already been affected by these processes to some extent – and given your conclusions potentially to a large extent!). Could this suggest that landslide erosion rates would have been even bigger between glacial retreat and now? Specifically, this study predicts a pulse in erosion rate by landslides over a few thousand years. Would this pulse already be done in the real world? Or has it simply been even bigger prior to today? Could such a trend be extrapolated back in time based on the presented results? Reflections on these questions could be added in the discussion.
Finally, I have listed several comments and suggestions below that will hopefully be useful when making the final adjustments of the manuscript. In addition, I suggest going through the manuscript to do a final check of language (e.g., lines 152, 344-345, 444, 503, etc.) and consistency in reference style (e.g., line 51, 72, 351, 580 also using ‘e.g.,’ instead of ‘e.g.’, etc.).
Lines 28-32: the mentioning of ‘the glacial buzzsaw’ might need a little more elaboration. It reads as if the glacial buzzsaw is usually attributed to a decrease in unstable slopes as well as a lowering of maximum topography. But is the mechanism in the glacial buzzsaw not that glaciers increase the steepness of their headwall slopes (i.e., increase in unstable slopes), such that hillslope processes are more active, and therefore by extension reducing the maximum elevation of a catchment? E.g., to quote one of the defining papers: Mitchell and Montgomery, 2006: “The summit altitudes are set by a combination of higher rates of glacial and paraglacial erosion above the ELA and enhanced hillslope processes due to the creation of steep topography.”
Of course, this does not take any value out of the study presented here that focus on how these hillslope processes work.
Line 76: an uncovered landscape or uncovered landscapes
Lines 77-78: consider if this sentence should also be past tense.
Lines 84-85: “leading to a postglacial increase on both the frequency and intensity of hillslope events through time”. I understand that the frequency and intensity go up as the regions deglaciates, but is the point not that it then decrease through time hereafter?
Lines 98-100: maybe start with a ‘while’
Lines 134-138: I would mention the stochastic nature of the model here already.
Line 152: ‘Three’
Line 181: I suggest consistency using ‘three’ versus ‘3’. I would suggest ‘three’.
Lines 205-211: you could consider also citing the new paper by Maxime Bernard here (see below)
Line 222: why V+ ?
Line 263-264: maybe this is the tradition when concerned with the used model. But I find it odd to refer to hillslope height, when talking about elevation/height change between two cells. Would maximum stable slope not be more appropriate?
Line 278: perhaps ‘the erosion scar generates a failure plan’ can be formulated more precisely. Is the erosion scar not generated by the failure and not vice versa?
Lines 282-284: this was not completely clear to me – i.e., whether all DEM cells in the entire catchment above a certain plane would be considered unstable for one specific landslide event?
Line 286: is it necessary to introduce Ff here, when not elaborated further? One could simply state that ‘in this setup, all sediments are instantaneously evacuated.’
Lines 292-324: I would suggest simply to incorporate this section in the Model calibration section 3.3. I see no reason for dividing this into two distinct sections. For instance, the first part of section 3.3.1 gives info/context relevant to the text in 3.2 and right now you refer to back and forth between the sections several times. In addition, I found lines 314-324 difficult to follow. If parameters give rise to few landslides, how do you then generate a large amount of landslides? Maybe it is the ‘Then’ that leads to confusion. Should it be ‘Either we compile multiple simulations … or we reduced the return time…’. But still, it could be clarified how you calibrate t_LSwhile scaling this parameter.
344-345: *did* not display any clear rollover.
346: I guess this is not a matter of visualization but representation. Capitalize ‘we’ or perhaps add. ‘However, we’
Line 350-353: -2.3 is larger than -2.5 ;-)
Lines 397-401: I would expect that the listed three combinations are part of a whole envelope of realistic parameter combinations, where the listed are just some examples. I would highlight that instead of listing specific values explicitly. Also, it would be nice to see an example of the spatial/temporal patterns in landslide activity for this selected catchment, perhaps for a few ‘end-member simulations’, showing the variability possible within reasonable values of parameter values (friction angle, cohesion, return time; e.g., supplementary figure, particularly if they are not very different – but that would be a point in itself).
Lines 425-427: “landsliding results in homogeneous slopes which only slightly exceeds the internal angle of friction (i.e., 0.7, represented by white color in Fig. 6).”. Seems like there are plenty of red colors still? Or do you mean only the regions associated with landslide activity? Again, would be nice with additional panels showing initial and/or changes in slope compared to initial values. Perhaps also comment on the high-slope regions that do not experience landslides.
Lines 452-460. For consistency, I would suggest referencing all figures when referring generally to all catchments, e.g., not only fig. 8 but also the corresponding supp. figs.
Line 483: it seems a bit arbitrary with the selected elevation range of 2400-2800 m, to capture the minimum for both catchments. Why not simply specify a different elevation for each catchment? The large range makes it difficult to see in the left panels that there are fewer red dots in that interval (as it is so wide) – particularly for the glacial catchment.
Lines 487: be careful using the word ‘observations’ in connection with model predictions.
Line 493-495: it is unclear how this is evident from Fig. S5. Should be S4, I assume. Also reference to Fig. 7G-H in the next sentence is unclear.
Line 569: I would suggest specifying rock uplift and sediment transport already in title and throughout.
Lines 584-586: the language of this bit is unclear to me.
Line 593: return time of 150 kyr?
Lines 588-596: I am not sure I understand the rationale behind the need for a model with an average erosion rate of 2-3 mm/yr to compare with the catchment-averaged erosion rate of 1 mm/yr. Do you want to imply that the hillslope erosion rate needs to be higher that the average because other parts of the catchment have lower values? Or is it because you are interested in the predicted longer-term erosion rate to be closer to 1 mm/yr? This is not clear from the text, and then why specifically 2-3 mm/yr was chosen? This would also rely on the assumptions you make about what has happened in the catchments since deglaciation until now (since you use present-day topography that have experienced many landslides already), which is what is reflected in the cosmo-derived rate.
Lines 672-674: I don’t follow the argument here; can you be certain that the ‘glacial’ vs. ‘fluvial’ catchments are due to reshaping through hillslope processes? Given the much lower hillslope activity in the fluvial catchment, could this catchment simply have experiences less glacial modification in the first place? The intended message of this section in general is a bit difficult to follow (lines 668-682), could it be spelled out more clearly?
Line 736: starting a new paragraph with ‘this observation’ is somewhat unclear. Please specify what ‘this observation’ refers to.
Section 5.3.1: this section is somewhat short and could potentially be included elsewhere. In addition, there is some discrepancy here related to other parts on the manuscript – arguing that U-Shaped valleys takes multiple glacial cycles to form, while other parts of the manuscript seem to suggest that the ‘fluvial’ catchment has transitioned from glacial to fluvial during one deglaciation. This will likely be sorted out/become clear if the hypothesis of the paper will be clarified.
Lines 772-797: as mentioned, I believe enhanced hillslope processes are already a recognized component of what has been presented as ‘the glacial buzzsaw’, which could be recognized in this section. This does not make the current study irrelevant in this context.

Figure 3: please specify t_n, t_m, etc. in the caption. Also, for clarity there should be arrows from ‘Trimline zone’ to yellow circles in both sides.
Figure 4, caption. 3. 10^4 m^2 (the period . ) should be fixed, here and throughout the paper.
Figure, 6. I would suggest also to show panels with the change in slope (final slope versus initial slope). It is difficult to assess the changes not having the initial slopes at hand.
Figure 7: I would suggest adding thin horizontal lines at slope 0.7 to mark the internal angle of friction.
Figure 8. unclear why panel B is termed ‘steepest slope’
Figure 9: y-axis label could simply be ‘triggering point elevation (m)’. I think right panel C could be interpreted as bimodal, although I agree it is not as clear..

References:
Bernard, M., van der Beek, P. A., Pedersen, V. K., & Colleps, C. (2025). Production and preservation of elevated low-relief surfaces in mountainous landscapes by Pliocene-Quaternary glaciations. AGU Advances, 6, e2024AV001610. https://doi.org/10.1029/2024AV001610

Citation: https://doi.org/10.5194/egusphere-2025-2088-RC1
- AC1: 'Reply on RC1', Coline Ariagno, 02 Sep 2025
  
  Dear Editors, dear reviewer,
  We thank the reviewer for the insightful comments that greatly helped to rive our manuscript.
  Please find enclose the reviewer’s comments in black italics and our answers in blue.
  Sincerely,
  Coline Ariagno (on behalf of all co-authors)
  
  Citation: https://doi.org/10.5194/egusphere-2025-2088-AC1
RC2:
'Comment on egusphere-2025-2088', Alexander Densmore, 08 Jul 2025

This is a good manuscript that uses an appropriate numerical landscape evolution model (HyLands) and a set of intentionally-simplified numerical experiments to look at patterns of landsliding in mountain catchments after deglaciation, and by extension to look at the evolution and resetting of glacially-eroded catchment topography after deglaciation. The authors have made a number of simplifying assumptions but these are generally well-explained and justified. A few of the points raised in the discussion, such as the importance of landslides as an erosional agent, are at least in part a product of these assumptions, and I've flagged some places where I think this needs to be more clearly acknowledged.
There are a number of statements that are ambiguous or hard to understand, along with some more minor typos and grammatical errors, and I've noted these in the PDF. I won't repeat these here, but one more general point is that the authors refer repeatedly to a set of hypotheses that differ from place to place in the manuscript. I'd strongly suggest that their hypothesis is stated once in the introduction, and then referred back to consistently throughout - as written, though, it's not entirely clear what they're trying to demonstrate.
The suggested edits and clarifications should be fairly straightforward for the authors to address, and so on the whole I'd class this as minor revision. I look forward to seeing this published.
Alex Densmore

Citation: https://doi.org/10.5194/egusphere-2025-2088-RC2
- AC2: 'Reply on RC2', Coline Ariagno, 02 Sep 2025
  
  Dear Editors, dear reviewer,
  We thank the reviewer for the insightful comments, which greatly contributed to improving our manuscript.
  Please find enclose the reviewer’s comments in black italics and our answers in blue.
  Sincerely,
  Coline Ariagno (on behalf of all co-authors)
  
  Citation: https://doi.org/10.5194/egusphere-2025-2088-AC2

Coline Ariagno, Philippe Steer, Pierre Valla, and Benjamin Campforts

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https://doi.org/10.5194/egusphere-2025-2088-supplement

Coline Ariagno, Philippe Steer, Pierre Valla, and Benjamin Campforts

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Short summary

This study explored the impact of landslides on their topography using a landscape evolution model called ‘Hyland’, which enables long-term topographical analysis. Our finding reveal that landslides are concentrated at two specific elevations over time and predominantly affect the highest and steepest slopes, particularly along ridges and crests. This study is part of the large question about the origin of the erosion acceleration during the Quaternary.


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