Definitions and Concepts for Quantitative Rockfall Hazard and Risk Analysis
Round 1
Reviewer 1 Report
Rockfall is a common phenomenon, but, surprisingly, its full complexity and diversity of its mechanisms are far from being fully understood. The reviewed manuscript makes good contribution to the literature because it summarizes the conceptual understanding of rockfalls. Generally, it is informative and well-written, and, thus, it can be considered for publication in 'Geosciences' after some changes and additions – see below.
- This paper is labeled as a review, it is structured as a review, and, better to say, it is a review. However, its length does not match the MDPI's requirement to become a review (min. 20 pages). The length must be increased, and the best way to do this is to include photos illustrating rockfalls. In my opinion, up to 10 photos can be added.
- The section Introduction cannot be so short. Please, add some general considerations and state your objective clearly.
- The section Conclusion needs extension. It should become more informative.
- I'm a bit surprised that you cite the only work by M. Stoffel, although this specialist and his team published a lot about rockfalls during the past decade.
- If this pretends to be a comprehensive review, why not to consider rockfall deposits and large clasts they consist of? These are highly-specific and allow material for interesting interpretations. Moreover, these deposits permit to judge of those rockfalls that occurred in the geological past. E.g., see here: https://www.mdpi.com/2076-3263/10/2/51 and https://pubs.geoscienceworld.org/gsa/geology/article/49/2/180/590929/A-bigger-splat-The-catastrophic-geology-of-a-1-2-b
- This article can be helpful: https://www.mdpi.com/2076-3417/11/4/1461 . In my opinion, it is sensible to cite it.
- Note that risks also include some other components. This paper is not about rockfalls, but it demonstrates how risks of natural hazards can be evaluated: https://www.sciencedirect.com/science/article/pii/S0959378016300565 Please, cite it and add the relevant discussion.
Author Response
General response: The authors thank the reviewer for its constructive review. Responses to the particular points are given below.
- This paper is labeled as a review, it is structured as a review, and, better to say, it is a review. However, its length does not match the MDPI's requirement to become a review (min. 20 pages). The length must be increased, and the best way to do this is to include photos illustrating rockfalls. In my opinion, up to 10 photos can be added.
Response: The paper doesn't pretend to be a comprehensive review, but a proposition to better define some terms and concepts used for quantitative rockfall hazard and risk assessment. However, additional photos and figures have been added.
- The section Introduction cannot be so short. Please, add some general considerations and state your objective clearly.
Response: Two sentences have been added. The first one at the beginning of the introduction: " There is an increasing need for quantitative rockfall hazard and risk assessment that requires a precise definition of the terms and concepts used for this particular type of landslide." The second one at the end of the introduction: " The objectives of this paper are to suggest terms that appear to be most logic and explicit as possible, and to describe methods to derive some of the main hazard and risk descriptors."
- The section Conclusion needs extension. It should become more informative.
Response: These conclusions have been added: (1)" As a lot of concepts and descriptors can be used for quantitative rockfall hazard and risk analyses, the authors recommend that those be precisely defined." (2) "Using modern surveying techniques should enhance the description of the rock wall structure, the identification of potential failures and the assessment of rockfall frequency." (3) " A better description of the rock wall structure should favor a more realistic modelling of the failure process and should allow a better assessment of the maximal possible rockfall volume and of the in-situ block size distribution. Progress is also needed in fragmentation modelling."
- I'm a bit surprised that you cite the only work by M. Stoffel, although this specialist and his team published a lot about rockfalls during the past decade.
Response: This text has been added (line 291-295): " A rock fragment survey allows to measure the volume of the fragments but the time during which the fragments have fallen may be unknown. Inversely, the observation of damages on trees (Trappman and Stoffel, 2013, 2015; Stoffel and Corona, 2014) allows to date the impacts but there is a large uncertainty on the block volumes."
- If this pretends to be a comprehensive review, why not to consider rockfall deposits and large clasts they consist of? These are highly-specific and allow material for interesting interpretations. Moreover, these deposits permit to judge of those rockfalls that occurred in the geological past. E.g., see here: https://www.mdpi.com/2076-3263/10/2/51 and https://pubs.geoscienceworld.org/gsa/geology/article/49/2/180/590929/A-bigger-splat-The-catastrophic-geology-of-a-1-2-b
Response: Rockfall deposits and they clasts are considered in section 3.3.3. But as you pointed out, rockfalls that occurred in the geological past are not mentioned. So, we have added a mention to the most relevant study for rockfall hazard assessment, which is the one by Hungr and Evans (2004), and to one of the oldest known rockfalls on earth (Killingback et al., 2021). We have added this paragraph: " The bigger their volume is, the more seldom the events are. To assess the spatial-temporal frequency of the biggest rockfall events, one must consider a long period of time and a large area and calculated spatial-temporal frequencies. Hungr and Evans (2004) analyzed rock avalanches bigger than 20 million m3 that occurred in different regions during the post-glacial period and calculated spatial-temporal frequencies. One of the oldest known terrestrial rockfalls on Earth was dated to 1.2 billion years (Killingback et al., 2021), but it is difficult to assess rockfall frequency for periods older than the post-glacial period."
- This article can be helpful: https://www.mdpi.com/2076-3417/11/4/1461 . In my opinion, it is sensible to cite it.
Response: In this paper, the different rockfall modelling methods are briefly presented and the reader is referred to some review papers. The suggested article about the role of digital terrain model grid cell size focuses on a very particular issue and then is not in the scope of our article.
- Note that risks also include some other components. This paper is not about rockfalls, but it demonstrates how risks of natural hazards can be evaluated: https://www.sciencedirect.com/science/article/pii/S0959378016300565 Please, cite it and add the relevant discussion.
Response: We think this paper does not bring more useful information on risk than the cited "Guidelines for landslide susceptibility, hazard and risk zoning for land use planning", which come from an international recognized expert panel. This paper is not an exhaustive review of the different approaches of the risk.
Reviewer 2 Report
L15: most logic instead of the more logic
L18: diffuse instead of diffused
L24/25: However, .... Please rephrase - not clear in abstract.
L147: 1 -> Missing footnote?
L232: has a given failure probability
L258: rather use deposit instead of landing?
general remark: there are also already quite some papers using the term "reach probability" differently than the one proposed here. It might be good if a better, new term could be find for the result of the product of the failure probability and the reach|propagation probability.
Author Response
General response: The authors thank the reviewer for its constructive review. Responses to the particular points are given below.
Comments and Suggestions for Authors
L15: most logic instead of the more logic
Replaced
L18: diffuse instead of diffused
Replaced
L24/25: However, .... Please rephrase - not clear in abstract.
Response: The sentence has been replaced by: " This frequency is relevant for risk assessment when the element at risk can be damaged several times. If it is not replaced, the probability that it is impacted by at least one rockfall is more relevant."
L147: 1 -> Missing footnote?
Sorry, it is not a footnote, it is a reference number. The exponent 1 has been replaced by [1].
L232: has a given failure probability
The word "given" has been added.
L258: rather use deposit instead of landing?
The term "deposit" is also suggested.
general remark: there are also already quite some papers using the term "reach probability" differently than the one proposed here. It might be good if a better, new term could be find for the result of the product of the failure probability and the reach|propagation probability.
Response: OK, we suggest now the term passage probability.
Reviewer 3 Report
The manuscript deals with a comprehensive literature review on concepts to be used for quantitative assessment of rockfall hazard and risk. The manuscript is very well written, and its valuable contents and structure reflect the prominent position which Authors have in the literature of this topic of the Landslide Science.
The only suggestion from my side is to expand, as far as it would be allowed by the manuscript structure, examples of approaches used for estimating the failure frequency and/or susceptibility to rockfall initiation from a rock slope. Moreover, it would be very interesting a brief discussion on how the susceptibility to rockfall initiation could be used (and if) in the assessment of hazard/risk.
By a formal point of view, I just suggest to authors to avoid the repetition of the word "hazards" at the line 18.
Author Response
General response: The authors thank the reviewer for its constructive review. Responses to the particular points are given below.
The only suggestion from my side is to expand, as far as it would be allowed by the manuscript structure, examples of approaches used for estimating the failure frequency and/or susceptibility to rockfall initiation from a rock slope. Moreover, it would be very interesting a brief discussion on how the susceptibility to rockfall initiation could be used (and if) in the assessment of hazard/risk.
Response: An example of topographical inventory using diachronic terrestrial laser scanning is given in Figure 3 and examples of rockfall volume-release frequency relations in Figure 4. We do not know examples where rockfall susceptibility (that does not consider the time) is used in quantitative hazard assessment (that is the scope of the paper).
By a formal point of view, I just suggest to authors to avoid the repetition of the word "hazards" at the line 18.
The word "hazards" has been deleted.
Reviewer 4 Report
9th March 2021
Review of the manuscript ‘Definitions and concepts for quantitative rockfall hazard and risk analysis’ by Hantz, Corominas, Crosta & Jaboyedoff
Nice contribution! I have no major comments, and the authors have a lot of experience on the proposed topic of the rockfall quantification. However, below are some suggestions to be considered, which could improve the paper:
First, add some discussion on the influence of vegetation. Probably it is not possible to quantify it like the other parameters discussed in the text, but this needs to be at least mentioned or discussed.
Second, a graphical example (Figure 2) is a great idea. I suggest also including some examples of calculation for the localized hazard (in the chapter 4.1) and/or diffuse hazard (4.2), as this would help the readers to understand better and perform the calculations themselves.
Also, section 4.3 ‘Different types of risk’ would be better if moved up (I suggest placement after the Rockfall risk introduction and just before the 4.1 section), because now the fourth chapter finishes with this very short three-line paragraph without any calculations and sort of ‘hangs in the air’ at the end. Also, correct the superscript value of 1.
Finally, are there still any open questions that need be addressed at the end of Conclusions?
Some more specific remarks:
- Line 77: I suggest avoiding the term ‘debris’ as a general expression for the deposited material, as this term is already reserved for a more specific type of material.
- Lines 181 and 182: You are suggesting to use the specific expressions, but in fact you are proposing two terms (e.g. failure OR release probability, reach OR passage probability etc.). Please consider suggesting only one expression. Few lines below (191), you have actually decided to use only one (the first) expression (reach, failure and propagation) – I agree. So, why should you leave the decision (this one OR the other one) to the reader, if the paper is basically meant to define the terms?
- Line 316: Calculations of separate volume classes will imply creation of some arbitrary class limit values, which therefore have to be defined manually (how, on what basis?), and this needs some explanation.
- All equations: Please add the units wherever the terms first appear.
- Some references are written as superscript values and look like the footnote number (line 358 and some other places). Please correct.
- Reference list: first three references are duplicated, please correct.
Author Response
General response: The authors thank the reviewer for its constructive review. Responses to the particular points are given below.
First, add some discussion on the influence of vegetation. Probably it is not possible to quantify it like the other parameters discussed in the text, but this needs to be at least mentioned or discussed.
Response: Indeed, the influence of vegetation must be mentioned in the brief review of rockfall modelling methods. A sentence and a reference have been added (line 103-104). However, discussion on the influence of the different parameters on the rockfall modelling is not in the scope of this paper.
Second, a graphical example (Figure 2) is a great idea. I suggest also including some examples of calculation for the localized hazard (in the chapter 4.1) and/or diffuse hazard (4.2), as this would help the readers to understand better and perform the calculations themselves.
Response: An example has been added. An example of calculation of human risk for a 195 m long trail section exposed to a diffuse rockfall hazard is given in Table 1.
Also, section 4.3 ‘Different types of risk’ would be better if moved up (I suggest placement after the Rockfall risk introduction and just before the 4.1 section), because now the fourth chapter finishes with this very short three-line paragraph without any calculations and sort of ‘hangs in the air’ at the end. Also, correct the superscript value of 1.
Response: We have moved up this paragraph as suggested.
Finally, are there still any open questions that need be addressed at the end of Conclusions?
Response: These conclusions have been added: (1)" As a lot of concepts and descriptors can be used for quantitative rockfall hazard and risk analyses, the authors recommend that those be precisely defined." (2) "Using modern surveying techniques should enhance the description of the rock wall structure, the identification of potential failures and the assessment of rockfall frequency." (3) " A better description of the rock wall structure should favor a more realistic modelling of the failure process and should allow a better assessment of the maximal possible rockfall volume and of the in-situ block size distribution. Progress is also needed in fragmentation modelling."
Some more specific remarks:
- Line 77: I suggest avoiding the term ‘debris’ as a general expression for the deposited material, as this term is already reserved for a more specific type of material.
Response: Debris is a relatively well-established term. Several dictionaries define debris as remains of something broken down or destroyed. For geology, an accumulation of fragments of rock (Merriam-Webster). Debris usually applies to the remains of geological activity including landslides, volcanic explosions, lahars, etc (Wkipedia).
- Lines 181 and 182: You are suggesting to use the specific expressions, but in fact you are proposing two terms (e.g. failure OR release probability, reach OR passage probability etc.). Please consider suggesting only one expression. Few lines below (191), you have actually decided to use only one (the first) expression (reach, failure and propagation) – I agree. So, why should you leave the decision (this one OR the other one) to the reader, if the paper is basically meant to define the terms?
Response: We prefer suggesting two or more terms if we consider that they are synonymous. We must respect the terms proposed by other authors if they are consistent. We think a definitive choice of only one expression should be made by an international comity like the JTC1.
- Line 316: Calculations of separate volume classes will imply creation of some arbitrary class limit values, which therefore have to be defined manually (how, on what basis?), and this needs some explanation.
Response: We have added this sentence: "The choice of the class limit values falls within the expert judgement."
- All equations: Please add the units wherever the terms first appear.
Response: The equations can be used with any coherent unit system. A unique unit must be chosen for each physical quantity (time, length, …).
- Some references are written as superscript values and look like the footnote number (line 358 and some other places). Please correct.
Response: It has been corrected.
- Reference list: first three references are duplicated, please correct.
Response: It has been corrected.
Round 2
Reviewer 1 Report
I very like how the authors improved their paper and responded to my questions. So, I think this paper is ready for acceptance.
Reviewer 2 Report
Revised version is good for publication. Merci!
Reviewer 3 Report
Authors have considered satisfactorily all suggestions from my side.
Reviewer 4 Report
All the suggested comments have been considered, and the manuscript can be accepted.
