The manuscript is a re-submission of an earlier version that required very extensive modification to enable clear communication of the conducted study. The authors have greatly improved this updated version, with the most notable changes including:
1. Reorganization to follow a logical flow.
2. Removing superfluous material.
3. Improving the writing for correctness and clarity, although numerous minor errors remain.
4. Streamlining of display items, although several require further improvement for legibility.
With the major issues from the previous submission addressed, some of the finer points of the paper now come through. This also highlights a large number of relatively minor issues, comprising mainly unclear/ambiguous statements or remaining problems with the writing. A number of one-sentence paragraphs also remain. Several figures require additional modification to ensure their legibility.
The site descriptions (of geomorphic markers and OSL dating) stand out as needing further attention. The authors provide only very minimal geomorphic description of the valley-bottom features they study. Unfortunately, stratigraphic and sedimentologic details are almost completely absent. Both the morphology and composition of the landforms/deposits pertinent to the study need to be better, although concisely, characterized.
A large number of minor edits will need to be made. The reference list should be carefully checked against citations in the text. The reference for Elyasi et al. (2014), which is cited on page 4, is missing. Others may be missing as well.
This revised manuscript can be accepted following some further, minor revision outlined below.
TECHNICAL ISSUES (some of which relate to odd or ambiguous wording)
1. Title: Does the title really need to be this long? Also note that this landslide is a ‘rock avalanche’ not a ‘debris avalanche’ (see also comment below for page 2, line 20).
2. Page 1, line 25: I don’t understand the wording ‘…since the beginning of the landslide cut by Seymareh River…’. Is this supposed to mean the initiation of dam breaching?
3. Page 1, line 26: The main post-landslide erosion migrated to the toe of the source slope. Some erosion has certainly occurred in the weaker units of the sources are, but his is on a different scale than that experienced in the valley bottom. Ideally, the wording should reflect this difference, and could even not the different erosion patters (i.e. deep entrenchment of the landslide debris and associated lake deposits in the valley bottom, compared to a shallow, well integrated drainage network in the weakest lithologies of the source area).
4. Page 1, line 28: The text ‘…the possible role of seismic forcing in anticipating the time-to-failure…’ is confusing. I presume the authors mean the role of seismic triggering in prematurely terminating the creep-controlled time-to-failure pathway. If this argument is being made in the paper, consider citing Hermanns and Longva (2012) - as mentioned in the review of the initial manuscript - as they discuss just such a situation (see their Figure 6.1). After reading the rest of the manuscript, however, I see that this topic is really not fleshed out. In that case, should this topic even be part of the abstract?
5. Page 1, line 26: Why is this an ‘end-member case study’? Presumably this is because the landslide was so large. Clarify.
6. Page 2, line 9: The shift from linearly increasing to nonlinearly increasing erosion needs to be clarified. Presumably the authors mean that the nonlinear increase is a response to perturbation of valley-bottom erosion by bedrock landslides, but this is unclear. The nature of the rate of increase – accelerating or deceleration – is also unclear. This would of course be complex as impoundment of lakes will lead to short-lived deposition and storage of sediment, but their later drainage due to dam failure will result in short-lived increases in erosion.
7. Page 2, line 11: What is meant by ‘the ultimate conditions of rock mass creep’? Do the authors mean that rock mass creep culminates in slope failure?
8. Page 2, first paragraph: The introduction of rock mass creep is missing key references to the different stages of creep (i.e. mechanisms of stationary creep and accelerating creep), such as work by Saito and later on that of Petley and colleagues. Citing a couple of these, particularly those directly applicable to bedrock landslides, would be very helpful to the reader.
9. Page 2, line 13: What is meant by ‘generalized failure’?
10. Page 2, line 17: There are two issues with the details in this sentence. Firstly, rock mass creep does not lead directly to rock avalanches, but rather to preliminary transport mechanisms, largely sliding. Large rock slides, if able to accelerate and run out, then transform into rock avalanches. Secondly, although creep may fragment the rock mass some prior to catastrophic failure, rock mass fragmentation largely occurs during transport (see Davies and McSaveney, 2009. The role of rock fragmentation in the motion of large landslides: Eng. Geol. 109). In either case, the fragmentation is not instantaneous as the authors have suggested.
11. Page 2, line 20: The second phase of failure of the Seymareh landslide was a rock avalanche, not a debris avalanche (this also needs to be corrected in the title). See Hungr et al. (2001, cited in this sentence of the manuscript) and further details in Hungr et al. (2014. The Varnes classification of landslide types, an update. Landslides 11). Why is the material type specified in ‘largest subaerial rock landslide’? I am unaware of any subaerial soil/sediments failures larger than this and, unless the authors know of one, ‘rock’ can be dropped. A citation would be good to support the statement – probably Roberts and Evans (2013) since the paper provides the more recent (and accurate) volume estimate for the landslide and demonstrates in its supplementary material that is it larger than the Baga Bogd rockslide, Mongolia.
12. Page 3, line 16: Consider dropping ‘…the emplacement of…’ as this study pertains to valley-bottom evolution due to causation of the landslide as well as subsequent debris emplacement.
13. Page 3, line 17: Is it really necessary to abbreviate ‘Seymareh landslide’ to ‘SL’? Also, capitalization should be ‘the Seymareh landslide’ as this isn’t a truly formal name.
14. Page 5: This is a nice summary of Tucker and Singerland’s (1996) landscape evolution model. However, it is probably a bit detailed for the background section. Considering moving the current, detailed description to the supplementary material and presenting a summary only in section 2. The authors might consider noting very briefly how the chronostratigraphy reported by Homke et al. (2004) agrees or disagrees with this model.
15. Page 6, line 4: As these lakes no longer exist, use ‘included’ not ‘includes’.
16. Page 4, line 8: The meaning of ‘…identified in the merging of Seymareh River with a left tributary as the reason…’ is unclear. Naming the tributary would provide further useful context.
17. Page 7, line 4: What is meant by ‘…state of thickening…’?
18. Page 7, line 4: In identifying locations for OSL sampling, what consideration was giving to selecting sediments that had the highest likelihood to have been fully bleached during transport? Could Seymareh River’s high suspended sediment load prevented full bleaching of sediment? Or is the assumption that any sediment making its way into the river is was fully bleached during slope-wash transport to the river or one of its tributaries? Is not addressed here, this should be noted in the discussion. Sample locations provided in many of the figures are very hard to make out due to the figures’ small scale.
19. Page 7, line 9: The description sounds like the tube may have been inserted vertically into the ground. The typically protocol is to insert the tube horizontally into a vertical face. Clarify what was done.
20. Page 7, line 10: The description of the 30 cm of sediment surrounding the tube is confusing. Does this mean the sampling locations comprised zones of homogeneous sediment wide and thick enough that the tube could be inserted with an at least 30-cm zone of in situ sediment remaining around it? Or is this meant to indicate that when the sample was removed a zone of 30 cm of sediment around it was somehow retained?
21. Page 7, line 22: What is the ‘SAR protocol’? This is neither explained in the text nor supported with a specification citation. Presumably SAR is an abbreviation, and if so will need to be defined.
22. Page 8, line 1: The sentence starting on this line is quite hard to follow. Consider breaking it up based on the two elements it describes: the sediment compositions of the terraces, and relationship of the terraces to landforms (i.e. inactive fans). Also, stating the sediment consist ‘mainly’ of every possible sediment size is not helpful. Persuadably the lacustrine deposits are finer the alluvial ones, but this is not clear from the description. What types of sedimentary structures are present? What types of a depositional environment is suggested by the alluvial deposits – a high-energy braided system or a low-energy meandering system?
23. Page 8, line 4: What is meant by the upstream markers in the midstream section? This is confusing given that the previous line mentions upstream markers (up-valley of the landslide) and downstream markers (down-valley of the landslide).
24. Page 8, line 10: What is meant by the natural damming being ‘extended’?
25. Page 8, line 11: What evidence is there that these features were deposited during drainage of the lake as opposed prior to dam incision?
26. Page 8, line 11: What is the basis for the interpretation of the timing of fan delta formation? Note also that this is an interpretation in the results section.
27. Page 8, line 15: ‘Coherent’ is not the right word here. Presumably the point is that an older alluvial terrain underlying the lake’s fill sequence has been shown to predate the fill sequence. If I have correctly interpreted the implied stratigraphic relationship, then the relative ages are demonstrated by the stratigraphy along. The interesting contribution of the OSL ages here is that they quantify the time spans between deposition of the alluvial and failure of the landslide (ca. 7 ka) and between failure of the landslide and the late phase of the lake (ca. 3 ka).
28. Page 8, line 17: How is the flood plain ‘shaped’ onto the landslide. Presumably this is largely a depositional feature. Is erosion also involved?
29. Page 8, line 17: Stating that ‘clear evidence of MRC’ was observed is unconvincing unless these features are described here, including by citing the relevant figure.
30. Page 8, line 15: ‘Coherent’ is not the right word here. Presumably the point is that an older alluvial terrain underlying the lake’s fill sequence has been shown to predate the fill sequence. If I have correctly interpreted the implied stratigraphic relationship, then the relative ages are demonstrated by the stratigraphy along. The interesting contribution of the OSL ages here is that they quantify the time spans between deposition of the alluvial and failure of the landslide (ca. 7 ka) and between failure of the landslide and the late phase of the lake (ca. 3 ka).
31. Page 9, line 6: Black arrows are mentioned here, but since no figure is cited in this sentence, the location of these arrows is unclear.
32. Page 9, line 14: Why is this sentence set aside as its own paragraph?
33. Page 9, line 26: The relevant figure showing the kickpoint should be cited here.
34. Page 9, line 30: Explain why the base of the landslide debris sheet is important. Is it assumed to rest on the pre-failure river channel (or close to it)? If this contact is being used to suggest that the modern river profile passing through the relict landslide dam – at least in places – is coincident with the pre-failure river profile, the base of the landslide debris should be represented in Fig. 9. The bedrock exposure in Fig. 7 is not visible without the annotation as that part of the figure is small and completely in shadow. Is there a photo of the bedrock and its upper contact that would help convince the reader of this interpretation?
35. Page 10, line 22: The landslide volume was not estimated here. A citation is, therefore, required to attribute this estimate to the proper pervious study, especially since the volume is not quantified anywhere else in the manuscript. Come to think of it, why is that? Volume is a key metric of landslides, and is especially important in this case as the authors emphasize a multiple times that this study addresses a particularly large landslide.
36. Page 10, line 28: This discussion requires some detail on the textural composition of the landslide dam, but such details are missing here, despite being described in several previous geologic studies of the landslide. Internal drainage though the dam – or at least parts of it – is extremely likely given the coarse nature of the debris. It would be helpful to note the role of such drainage in other large damns (see the extensive literature on landslide dams, starting with the early work of John Costa and Robert Schuster). The Usoi landslide dam is probably one of the best examples to consider given its large size (>1 Gm3) and relatively long duration of stability (in a historical context) due to the formation of stable internal drainage.
37. Page 10, line 32: It is unclear whether these ‘fill terraces’ are envisioned as fluvial terraces. This is just one example of the many instances where imprecise description of geomorphic and geologic features has weakened the author’s interpretations and arguments.
38. Page 11, line 8: Is overly narrowly limited to this time? It seems possible that initial overflow could have predated this, and that the fluvial sculpting and deposition noted farther downstream along the dam could have lagged a bit behind, if the initial drainage was minor. Given the very limited geomorphic and geologic descriptions in section 4, it’s hard to interpret such things.
39. Section 5.3: Is a list really necessary here? I don’t see any reason not to use proper paragraph structure.
40. Page 12, line 25: Citations to relevant work are necessary when characterizing the typically creep behaviour of sepcific lithologies.
41. Page 12, line 30: The reduced lateral confining effect needs to be briefly explained.
42. Page 12, line 31: Clarify that the reduction in incision necessary is for kinematic freedom of the landslide is relative to a more steeply dipping sequence, which would require a greater depth of incision. Note that is also suggested based on consideration of this landslide by Roberts (2008, his Figure 4.1).
43. Page 13, line 16: Was does the naming change here to ‘the Seymareh giant landslide’?
FIGURES
1. Figure 3: Erroneous text in caption: ‘…to the Oberlander’s model…’
2. Figure 5: Consider locating photo positions in one of the map figures.
3. Figure 6: Some elements are overly complex and thus hard to read. Can the ‘gully’ symbols be removed? They add a lot of visual complexity, but are of little importance.
4. Figure 7: Consider locating photo positions in one of the map figures. Can a clear photo of the bedrock outcropping below the landslide be added?
5. Figure 8: Erroneous text in caption: ‘…the valley slopes evolution…’
6. Figures 9 & 10: Many features of the figure are far too small to be clearly legible. What is meant in the caption by ‘…upstream and downstream conglomerates; downstream fluvial terrace suite…’? The inset maps are too small and illegible until I zoom in quite far in the PDF. The label font with coloured fill and think black outlines (i.e. the terrace labels) are very hard to read because of the font/colouring used.
7. Figure 11: Indicate the source of the imagery in panel A (including the actual satellite image, not just ‘GoogleEarth’). Represent scale in panel B.
8. Figure 12: Where is the legend for the colour fills? Even if this is a full-page figure, it will be hard to read.
LANGUAGE ISSUES (some examples only)
1. Page 1, line 10: Capitalization should be ‘The Seymareh landslide’. Alternatively, if threating this as a formalized geographic feature, it should be ‘Seymareh Landslide…’.
2. Page 1, line 13: ‘…changes in landscape…’ is incorrect English.
3. Page 2, line 7: ’…by the reaching the threshold…’.
4. Page 3, line 1: ‘…evolution of the dam lake drainage…’. What is ‘dam lake’ meant to be? Dammed lake?
5. Page 3, line 13: ‘the Sichuan’ is imprecise and confusing. This should be ‘the Sichuan River basin’.
6. Page 3, line 18: Placing the compound modifiers ‘pre-failure and post-failure’ after the terraces seems awkward. This sentence is quite long and hard to follow, so consider breaking it up.
7. Page 3, line 25: For clarity, consider ‘…originates convergence since Late Cretaceous time’ as this is ongoing. Also, the current use of hyphens is confusing and, since ‘Late Cretaceous’ is an official part of the geologic timescale, incorrect. Also use of the backslash later in the line is confusing.
NOTE: I have not listed issues beyond this point, but there are many that will need to be addressed. |
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