the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Aug 2024
Causes, consequences and implications of the 2023 landslide-induced Lake Rasac GLOF, Cordillera Huayhuash, Peru
Abstract. Glacierized Peruvian mountain ranges are experiencing accelerated, climate change-driven glacier ice loss. Peru’s second highest mountain range, the Cordillera Huayhuash, has lost about 40 % (~34 km2) of its glacier cover since the 1970s. Newly exposed landscapes are prone to a number of hazard processes including the formation and evolution of glacial lakes, changing stability conditions of mountain slopes, and rapid mass movements. In this study, we integrate the analysis of meteorological data, remotely sensed images and field observations in order to document the most recent (February 2023) large mass movement-induced glacial lake outburst flood (GLOF) from moraine-dammed Lake Rasac. The GLOF was triggered by a mass movement from the failure of an arête ridge with an estimated volume of 1.1 to 1.5 x 106 m3; this occurred from the frozen rock zone with cold, deep permafrost, and was preceded by several small-magnitude precursory rockfall events. The reduced stability of the frozen rocks in the detachment zone most likely relates to deep warming, but not to especially critical conditions of warm permafrost with higher amounts of unfrozen water. Further, we describe the surprisingly short-distanced process chain (attenuated by the Lake Gochacotan located 3.5 km downstream from the detachment zone) and analyze the transport of large boulders with the use of hydrodynamic modelling, revealing that flow velocities > 5 m/s must have been reached in the case of translational motion, and > 10 m/s in the case of rotational motion of the largest transported boulders (diameter > 3.5 m). In addition, we analyze climate trends over past seven decades as well as meteorological conditions prior to the GLOF, revealing a statistically significant atmospheric temperature rise and thermal anomaly before the event. Climate change effects (warming air and permafrost temperatures) served to hasten the failure of an already critical geological situation. This study helps us to understand (i) mechanisms, amplification and attenuation elements in GLOF process chains; and (ii) altering frequency-magnitude relationships of extreme geomorphic processes in rapidly changing high-mountain environments on a regional scale (both large magnitude rockfalls and GLOFs). This study supports earlier work that indicated an increasing frequency of large mass movement-induced GLOFs originating from ice-related effects of glacial de-buttressing and warming permafrost in recent decades.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-2316', Fabrizio Troilo, 24 Sep 2024
General Comment:
The paper by Adam Emmer et Al. presents the description and the analysis of a major mass movement event involving cascading processes that happened in February 2023 in the Peruvian Andes.
The paper is well written and well structured; the text is supported by high quality figures and the references cited highlight a good literature review.
The topic of GLOFs, cascading processes triggering GLOFs, and the evolution of such events in the present climate change scenarios are relevant.
I believe that the prompt data and informations gathering after such large events is crucial for the better understanding of these processes which are still difficult to model and to predict. The rapid publishing of such observations and studies is essential for the scientific community that studies such processes.
The hydrodynamical analysis performed in the study is interesting and can serve as a basis for future, more in-depth modeling work on this event. Another relevant point is the understanding of features or conditions that can reduce the propagation of gravitational phenomena (section 5.1) which is crucial to be correctly interpreted and integrated in dynamic gravitational modeling and eventually in risk assessment studies.
The discussion of the attribution of the event to climate change and the analysis of the variation in the likelihood of the occurrence of extreme temperature anomalies is particularly interesting, and could be the subject of future work on this and other case studies.
I believe this is an interesting paper and I hereby introduce some specific comments which i consider all as minor revisions and suggestions that could nonetheless further improve an already very good work.
Specific comments:
L20 it might be better to express the value with the estimated uncertainity. 1.3 +/- 0.2 or so. Also Check and homogeneize with L383 and L387 notation.
L31 to make it more clear and fluent i would slightly rephrase: (ii) frequency-magnitude relationships of extreme geomorphic processes that undergo alteration because of rapidly changing ...
L33 the wording of the phrase is a bit complicate...because glacial debuttressing is implicitly an ice related effect, why dont contract into : GLOFs originating in recent decades from glacial de-buttressing and warming permafrost (areas?)
L62 just remove "then", "if so, how this event can be attributed" is clear enough
Fig1 the Cordillera Huayhuash outline can hardly be seen. Why don't indicate it in black in the legend and just stick to the rectangular bounding box? Then you can give the more accurate perimeter in (b). It could be a good idea to change outline color in (b) to make it more visible. I would also write the full C. HUAYHUASH in legend (b) C.H. could be not really intuitive.
L103 Can you give a value of uncertainity for the volume estimation?
Table1 Could you add a column that refers to the native satellite or sensor type? For example on the first row it could be: mosaicking of worldview and Pléiades imagery...For the 2nd row Planetscope satellite imagery etc... In the reference column you could add a detail if freely available or of restrained/commercial access. For the Sentinel images it could be good to add a reference on how to access original imagery from The copernicus data space.
A column with the number of images actually used in the study could make it more complete
L146 I would put two commas: "the event, as defined above, has changed ... "
L202 It would be good to introduce the Ravanel 2023 reference here as well, just after mentioning the ice aprons, as many geoscientists might not be aware of the definition of ice aprons.
L223 Here you name it Rasac Ridge. Elsewhere in the paper you also refer to the Rasac Arete Ridge. It should be named in the same way in the whole text. Wouldn't it be more straightforward to name it Rasac Ridge everywhere in the text? (But I might be missing something about local topography naming)
Figure 3 It would be really interesting for the reader to see the slope on the other side of the ridge as well if an image is available. From this side of the slope it is hard to tell that the morphology of the ridge is the one highlighted in the topographic profile in Figure 5.
L243 Just a typo, the reference appears in red.
Figure 4 The north arrow is quite small, as long as you are showing images with the north pointing to the left (which is good for the composition of the panels of the image sequence), I would highlight it more clearly for the reader; I would add a bigger north arrow on the august 2017 panel for example, or make it bigger and/or colored to differenciate it from the main legend elements in the actual location.
L256 Permafrost thawing might be more appropriate?
L315 In this paraghraph you highlight the fact that the Rasac Ridge is very sharp and horizontal heat flow component is present.
Because of this setting, wouldn't it be reasonable to discuss (maybe in 5.2) the possibility that glacial debuttressing from the elevation loss of the big glacier tongue located to the East of the Rasac ridge could have played a role in the destabilization process? Maybe the recent exposure of part of the eastern flank of the Rasac ridge have also had consequences in the variation of the thermal condition of the slope as well as water percolation od circulation? Glacial de-buttressing effects are introduced generally in the paper and it could be worth to contextualize it to the local setting. The data from the paper of (Hugonnet at Al. 2021) could maybe give an idea of the elevation loss rates on this Glacier. Maybe historical data or geomorphologic evidence can at least give an idea of the LIA glacier extentions in the area?
L324 "interested by the detachment" instead of "with the detachment" probably sounds better to the reader.
L325 Temperature rise "is" from both sides of the ridge. It might sound better: temperature rise "have origin" or "comes" .
L330 In 4.2.3, the content of figure 8 is very clear, but the text is very short. You could perhaps describe this data a little more, for example: skin temperature reached the highest anomaly peaking at +1.4°C etc… the temperature anomaly started from mid-january and returned to less extreme values by end of February etc etc …
A more general consideration on the analysis of meteorological conditions is the following:
You show that a strong positive temperature anomaly was present at the time of failure of the rock avalanche of 12th of February, which is really interesting, but wouldn’it be interesting to show a bit more data from the previous months? Or maybe from the previous 2 or 3 years? It seems that the reader would need a bit larger temporal outlook on these data. Maybe another 2 panels in figure 8 could highlight a little larger time window? In this direction you could also highlight in colored lines years 2021 and 2022 for example in the panels A, C and E of figure 6.
Figure 9 i would reproduce the (c) outline in (A) also, to better compare A and B panels
In (B) the label "GLOF impact area" covers the area itself, if you shift it to the north, the reader can better appreciate the change in the impact area occurred in between (A) and (B) panels
It could be good to have another panel highlighting the (D) area showing the same detail of this area prior to the GLOF.
Remember to add a North arrow
L364 "apparently"
L366 isn't it referring to fig 9?
L385 add a reference to fig 9. maybe it is also worth better highlighting the fact that there are 2 different overtopping locations on fig 9 with 2 additional circles or better separate the 2 arrows
L387 don't express "millions" but 0.8 x 106 m3
L389 it is not clear .. lake rasac persisted in a more limited extent .. compared to pre-glof extent?or refers to temporal extent?
L393 just say unusual if it is so, dont say quite. You should also explain why it is unusual.
L397 why is it rather unlikely?
L403 it would be more fluent by merging the two sentences: a partial role, in the sense ...
This way you eliminate the repetition of the word partial.
L418 "prior to" sounds better than "leading up" if I interpreted correctly the meaning.
Figure 11 Orange and purple coloring might be hard to differenciate.
(a) and (c) panels have the x axis label truncated on the 10000 label
L456 Typo: event , not evet
L465 Building on the regional
L474 Maybe a little table with some BASIC data on the events you mention could make the paper more complete at this point.
Hoping that my comments will be useful for the publication of your manuscript,
Best regards,
Fabrizio
Citation: https://doi.org/10.5194/egusphere-2024-2316-RC1 - RC2: 'Comment on egusphere-2024-2316', Anonymous Referee #2, 14 Oct 2024
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