the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Sep 2023
Evolution of the Frébouge polygenetic cone during the Holocene (Val Ferret, Mont Blanc Massif)
Abstract. Proglacial settings in the Alps are typically polygenetic, often characterized by a complex and discontinuous interplay between glacial, fluvial, and gravitational processes. These processes yield high volume of sediments, which usually exceeds the transportation capacity. The excessive proglacial sediment load leads to accumulation on slopes, and thus, to subsequent failures such as debris flows. Such failures can occur unexpectedly and harm the villages and infrastructure in the vicinity of proglacial environments. The northern slopes of the Ferret and Veny valleys in the Mont Blanc Massif are home to several polygenetic cones and are a stunning field laboratory for the exploration of the interplay between the glacial, fluvial, and gravitational processes. This study investigates one of the active and well-preserved polygenetic cones in these valleys, namely the Frébouge cone, to disentangle the geomorphic processes that contributed to its formation, and to reconstruct its evolution. To achieve these goals, detailed field, and remote mapping, 10Be surface exposure dating of different geomorphic features, and runout modelling with DAN3D® were applied. The geomorphological map revealed complex interactions of glacial, fluvial, debris flow, and rock and snow avalanche processes. The established chronology indicates two major fluxes of debris flows, the first one at ca. 2 ka, and the second at ca. 1 ka. In addition, a rock mass with a maximum volume of to 12 Mm3 collapsed in the upper reaches of the cone at 1.3 ± 0.1 ka and overran the cone, travelling more than 100 m up onto the opposite valley slope. Afterwards, the Frébouge Glacier overrode the cone several times leaving moraines and till, reaching its maximum extent ca. 300 years ago. This study underscores the untwisting of the complex interaction of surface processes in the Alpine valleys, which are prone to hit the urban areas and infrastructure.
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RC1: 'Comment on egusphere-2023-1873', Anonymous Referee #1, 04 Nov 2023
The manuscript presents the evolution of the Frébouge polygenetic cone in the Ferret Valley based on geomorphological
mapping, 10Be surface exposure ages, and simulations of the rock avalanche. In general, I really like the comprehensive interdisciplinary look at the cone´s evolution, also the writing style is clear, without grammar issues. Some minor typos corrections and modifications are suggested in the attached pdf file.Here I point out some of them:
How did you distinguish the RA and debris flow deposits? Only by the geomorphic context? How looked the RA deposit in detail? Was there any outcrop of the RA sediment? How was the inner fabrics?
L. 243: "The south-eastern side of the valley could be affected by a deep-seated gravitational slope deformation..."
So it was affected or was not? What do you mean by "could be"? In the future? I would suggest rephrasing this sentence.Figure 1: For completeness and reader´s fast orientation, here I miss presentation of the entire source area of the material incorporated in the fan, i.e. the rock avalanche scarp, glacial cirque, etc. Can you please slightly enlarge the area of the detailed terrain map and to label the most important features?
Figure 9: some labels are too small and difficult to be read correctly.
Concluding figure is missing, I would suggest to the authors to prepare a conceptial model schematically presenting the cone´s evolution and main contributing processes.
I propose accepting the manuscript to be published after a minor revision.
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RC2: 'Comment on egusphere-2023-1873', Anonymous Referee #2, 05 Jan 2024
This is a scientifically sound and well written paper that I think may be suitable for publication in EGUsphere after the following minor revisons:
- I suggest improving the inset of Fig. 1 in order to include some elements of geology.
- In the legend of Fig. 3, the text is too small and the boxes should have a black outline.
- A final conceptual figure is required to summarize your findings.
Citation: https://doi.org/10.5194/egusphere-2023-1873-RC2 -
EC1: 'Comment on egusphere-2023-1873', Dirk Scherler, 24 Jan 2024
Dear authors,
first of all, I would like to apologize for the long time it has taken us to evaluate your submission.
I’m happy to see that both reviewers find your manuscript interesting and a valuable contribution to Earth Surface Dynamics. Their comments provide you some guidance to improve your manuscript for submitting a revised version. Both reviewers suggested a final conceptual figure to summarize your findings and I agree with them. I think that such a figure should be accompanied with an additional section in which you try to draw some conclusions regarding the bigger picture. What have you learned from this particular case study, which is relevant for the larger research question of how mountainous landscapes respond to ice retreat and climate change. You should try to pick up some of the topics and questions you raised in the first two paragraphs of your introduction. That helps the reader to place your research in a wider context.
Find below a few additional points to please consider when preparing a revised version:
At the end of the introduction, it would be useful to provide a motivation for the research, ideally a research question, which helps the reader to understand the goal of the research and what difference it makes when you reach this goal. Please try to keep the broader picture in mind, beyond the question of how this particular cone was formed. It would also help if you could provide reasons why geomorphic mapping, surface exposure dating and runout modelling are needed to reach the goal.
Section 3.1: You mention a very high-resolution DEM. What is the resolution and do you show it somewhere in a figure? It was not clear to me how it was useful for the geomorphic mapping. Or was it more the orthoimage that helped you in the geomorphic mapping?
Section 3.3: I found the description of how you evaluated the volume of the deposit a bit short. As this seems to be a key input for the modelling, I suggest to provide more details here. For example, how did you constrain the lower boundary of the deposit? Was it exposed somewhere? If so where and how does it look like? Is it sufficient to constrain the thickness of the deposit across the entire area? Perhaps an additional figure that details the volumetric reconstruction would help the reader, also for the results section. Finally, I can see that adding uncertainties to a volumetric reconstruction is not trivial, but it would help the reader to assess the modeling results if you can provide an estimate of uncertainties. Note that these do not have to be measured uncertainties, but they could be guessed based on some of the assumptions you made in the reconstruction. I’ve seen that you address the uncertainties in the discussion section 5.2, but I think you can provide some percentages, based on your approach and the available data.
Figure 1: Please add a color bar for the main map. The coordinates in the inset figure are very difficult to read.
Figure 2: A scalebar would help those unfamiliar with the setting to better assess the size of the cone on this picture.
Figure 3: It is uncommon to have individual rivers (Doire) get their own signature in a legend. You can use the same signature as for “river channel” and simply make it a bit thicker to indicate that it’s a bigger river. Add the name in the map directly.
Figures 4&5: I can hardly see the panel labels. Please make them bigger and better place them in the upper left corner of your panels.
Table 3: Did you deliberately round many of the ages to hundreds of years? As you also have ages <100 years, I think it is better to keep the decades for all ages.
Figure 6: Better place the subplot labels in the upper left corner of the panels. It would help the readers to somehow indicate how the samples belong to a geomorphic unit, perhaps by combining different line colors and styles.
Section 4.3: In addition to a more detailed description of the method on the volume reconstruction, more details on the results would also be beneficial. I would strongly suggest to provide a figure just on the volume reconstruction, where you could also show the pre-landslide topography and perhaps photos from outcrops you deem important for defining the base of the deposit.
Figure 7: Perhaps add a bigger gap between the PJ and GJ models to indicate their different setups? The V1 models seem to touch the edge of the model and spread laterally. To avoid edge effects, you probably have to make the model domain bigger.
Table 4: “GJ” and “PJ” are only shown for “Multimat”, but they apply to V1-V4, too, right?
Section 5.1: When discussing your results with respect to Figure 8, you can point at specific panels of the figure to help the reader see the raised points.
Figure 8: It would help the reader to see directly in the figure what the graphs are showing. Overall, I find the figure informative when it comes to the rock avalanches, but the moraine ages during the last ~400 years get a little crowded and it is hard to see how they compare to the reference altitudinal/frontal positions. What actually do the numbers on the y-axis represent? Absolute elevations in a and distances from the present-day glacier in b? And what do the horizontal lines indicate (a: 1996, b: 2002)?
Section 5.2: In your discussion of how the Frébouge rock avalanche compares to literature data, you are refering to Figure 9 without calling it. Please do so and point at specific panels. Note the “H7L” typo in line 397
Figure 9: It is not required, but it would make your figure more appealing if you could harmonize the different panels. At present, the different panels all come with different styles. Please make sure to have the required copyright permissions from the original publishers if reusing figures previously published elsewhere.
Best regards,
Dirk ScherlerCitation: https://doi.org/10.5194/egusphere-2023-1873-EC1 -
AC1: 'Comment on egusphere-2023-1873', Catharina Dieleman, 30 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1873/egusphere-2023-1873-AC1-supplement.pdf
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