Landslide Hazard Assessment in a Monoclinal Setting (Central Italy): Numerical vs. Geomorphological Approach

Round 1

Reviewer 1 Report

Dear authors, you may find the review in the attached file.

Kind regards

Comments for author File: Comments.pdf

Author Response

MAJOR COMMENTS

Please, I would suggest to consider for enriching this part to consider:

• Cascini, L. (2008). Applicability of landslide susceptibility and hazard zoning at different scales. Engineering Geology, 102, 164–177.
• Cascini L (2015) Geotechnics for urban planning and land use management. Ital Geotech J 49(4):7–62
• Cotecchia, F., Santaloia F., Lollino, P., Vitone, C., Pedone, G. & Bottiglieri O. 2016a. From a phenomenological to a geomechanical approach to landslide hazard analysis. European Journal of Environmental and Civil Engineering, 20(9): 1004–1031.
• Cotecchia, F.; Santaloia, F.; Tagarelli, V. Towards A Geo-Hydro-Mechanical Characterization of Landslide Classes: Preliminary Results. Appl. Sci. 2020, 10, 7960. https://doi.org/10.3390/app10227960
• Salvati, P., Rossi, M., Bianchi, C., & Guzzetti, F. 2016. Landslide risk to the population of Italy and its geographical and temporal variations; M. Chavez, M. Ghil, J. Urrutia‐Fucugauchi (Eds.), Extreme Events: Observations, Modeling, and Economics, Geophysical Monograph Series, John Wiley & Sons, Inc, Hoboken, NJ: 177‐194.
• Wilde, M., Günther, A., Reichenbach, P., Malet J. & Hervás, J. 2018. Pan‐European landslide susceptibility mapping: ELSUS Version 2. Journal of Maps, 14:2, 97‐104.

R: Accepted. Many of these references have been cited within the paragraph 1

Moreover, as regards the numerical section of the paper, I would recommend the authors to be more specific in the overall description of the modelling procedures also because in this paper, the modelling results are used to build up some conclusive remarks, also making statement about the landslide hazard.

There are no information about the numerical procedure adopted for carrying out the analysis. The hydraulic boundary conditions (i.e. transient or steady state; saturated or unsaturated, with or without suction; hydrostatic or seepage etc.) and the initialization procedure of the stress state in the slope (in situ k0, geostructural features, excavation and/or river erosion, OCR values, pre-existing landslide bodies etc.); some of the such issues have been highlighted in (among the others):

• Potts, D.M., Kovacevic, N. & Vaughan, P.R. 1997. Delayed collapse of cut slopes in stiff clay. Géotechnique, 47(5): 953‐982.
• Tagarelli, V. & Cotecchia, F. 2020. The effects of slope initialization on the numerical model predictions of the slope‐vegetation‐atmosphere interaction. Geosciences, 10‐85, doi:10.3390/geosciences10020085.

 

R: Accepted. All these aspects have been summarized (together with other suggestions coming from the other referees) within the paragraph 2.2

The quality of figure 7 should be improved; even if I have to say that I do not see the usefulness of that plots. In fact, if the low GSI and/or high slope angles determine the FoS, there would be no need for slope stability and landslide hazard assessment. And this is not the case because, generally speaking, the FoS depends on several factors, which Terzaghi (1950) described those classifying it in “internal” and “external” factors.

R: Accepted. Figure 7 has been deleted because considered not very significant.

As for the conclusion, I would suggest the authors to try to better describe the results obtained; in particular several of the reasons which are given in the final part are consequent to the hypothesis adopted for carrying out the numerical analysis.

R: Accepted. Conclusions have been improved (Lines 498-503)

SPECIFIC MINOR REVISION IN THE FINAL TABLE HAVE BEEN ACCEPTED AND MODIFIED THROUGHOUT THE MANUSCRIPT

Author Response File: Author Response.docx

Reviewer 2 Report

My comments are in the attached file.

Comments for author File: Comments.pdf

Author Response

MINOR COMMENTS

R: Accepted. All the minor correction throughout the text has been properly modified following the referee indication

General aspects to be improved:

1. Authors declare to compare the numerical vs geomorphological approach, but the paper appears too much oriented towards the numerical analyses. It is opinion of the reviewer that the description of the study area should be improved before the application of the numerical analysis to a limited number of selected slopes. In particular:

1.1. It is necessary to add to Figure 1 a geological section of the area of interest, possibly east-west, demonstrating the monoclinal setting of the area at an appropriate scale of representation. The section of Fig. 1b is at a regional scale, while a more detailed cross-section, at the same scale of the geological map, is needed.

R: Accepted. Figure 1 has been improved. The cross section with W-E direction of Figure 1b has been enlarged. A cross section showing the monoclinal setting with higher detail has been add in a proper figure within the paragraph 3

1.2. The geological map of Fig.1c includes several landslides. It is supposed that these landslides have been pointed out by the geomorphological approach whose main elements should be highlighted. Moreover, all the landslides indicated in the map should be classified based on geomorphological evidence. It is suggested to introduce a section named “results of the geomorphological approach” (or similar).

R: Accepted. A specific paragraph (3. Results of the geomorphological approach) with more information about the geological and the geomorphological setting of the study area has been inserted.

1.3. Again, in the geological map of Fig. 1c the sections analyzed by numerical method in the second part of the paper have been traced. No explanations about the location of these sections are provided and they should be added. It is possible to infer that the Authors intend to study some area affected by landslides and some other area not affected by landslides. These choices have to be clearly explained.

 

R: Accepted. Explanation about choice and location of the cross-section used for the numerical analyses is discussed within the paragraph 4 – Model implementation (Lines 324-326)

2. Numerical analyses:

2.1. The properties of the 10 sections on which numerical analyses have been performed should be carefully described, one by one (structural settings, ground model, average slope, hints coming from surveys).

2.2. Being both soils and rock masses, the numerical approach has been based on different failure criteria: Mohr-Coulomb, Modified Hoek and Brown. Moreover, an Ubiquitous Joints Model has been used to model formations characterized by anisotropy along some specific directions (sedimentation joints, discontinuities of what origin?). It is necessary to include in Figure 6 a picture of the numerical model used for each analysis, including the mesh used to model the slope.

2.3. In Table 1, it is unusual to indicate the cohesion and in Pa. Please use kPa or MPa to make clearer the assumed figures. Please, specify the association between cohesion and friction angle (if you assume 3 friction angle and 3 values for cohesion, it seems that 9 different associations, by combining all the values, have been performed. On the contrary, being the cohesion and the friction angle linked in every failure envelope, it is probable that only 3 analysis have been performed. Please, specify this aspect.

 

R: Accepted. Table 1 has been modified. A picture of the numerical model used for each analysis, including the mesh used to model the slope has been included in Figure 7 and 8. A clarifications on the relationship between friction angle and cohesion and their use within the simulations have been provided in the paragraph 4 together with a detailed description of the different failure criteria (Lines 324-334 and Lines 338-342).

 

2.4. It is suggested to add a representation of all assumed failure envelopes (may be by indicating only the average envelope) in a Mohr-Coulomb plane or in the plane 1-3. In this way it is possible to recognize and compare the strength level of the different soil and rock formations. On the contrary, it is difficult to recognize which are the strongest and weakest formations.

 

R: Not Accepted. Unfortunately, the software is not able to represent failure envelopes. Nevertheless, the fact to have inserted the mesh for each cross-section makes possible to differentiate among formations with different strength level  

 

2.5. When dealing with landslides in clayey formations, it is usual to recognize some layer at residual. Also, many times, the surface between colluvial and clayey formation is at residual. The existence of this layer can affect severely the results of the numerical analysis. Please, add a comment about this point.

 

R: Accepted. The presence of residual layers within the clayey formation has been specified within the paragraph 3 (Lines 280-283).

 

2.6. When dealing numerical analysis of slopes, it is fundamental to define the initial state of stress. How the Authors address this problem? The horizontal state of stress is lithostatic or not? (please see, Piernicola Lollino, Federica Cotecchia, Gaetano Elia, Giuseppina Mitaritonna & Francesca Santaloia (2014): Interpretation of landslide mechanisms based on numerical modelling: two case-histories, European Journal of Environmental and Civil Engineering, DOI: 10.1080/19648189.2014.985851)

 

R: Accepted. This aspect has been pointed out within the paragraph 4 (Lines 331-334)

2.7. It is not clear how the used software calculates the Factor of Safety. Based on the description at the beginning of section 2.2 it seems that a -c reduction method is considered. Please specify this aspect, by referring to appropriate literature (see, among others: Dawson, E. M., Roth, W. H., & Drescher, A. (1999). Slope stability analysis by strength reduction. Géotechnique, 49(6), 835–840. doi:10.1680/geot.1999.49.6.835)

 

R: Accepted. This aspect has been pointed out within the paragraph 2.2 (Lines 203-206). More references have been included in the text and in the reference list.

2.8. By observing the shape of the failure masses obtained by numerical analysis, some concerns arise about the representativeness of the results. In particular:

2.8.1. Sect. 2V and sect. 3V: the failure mechanism is clearly determined by the boundaries of the numerical model, so they did not represent a reliable result. Try to wide the model, it is probable to observe the triggering of a different critical mechanism of failure.

2.8.2. Sect. 5V, sect. 6V, 1M, 2M, 3M, 4M: all the failure mechanisms activated along these sections present a weird shape, they are very short and deep. Some in-depth examinations are necessary to validate the reliability of these results. It is possible that the numerical model indicates a local failure for numerical reasons (e.g., the presence of a sharp angle at the soil surface, or some elements of the mesh with size or proportion not optimal). Please, verify these results by using a constant slope surface equal to the average slope of the real situation. In any case, a detailed comment about the localization, shape and dimension of the failure mechanisms is necessary.

 

R: Accepted. The boundaries of Sect. 2V and 3V have been modified and the results updated. Likewise, also the failure mechanisms shown in Sect. 5V, 6V, 1M, 2M, 3M, 4M, have been checked, modified if needed and the results updated. (See Figure 7 and 8 and Table 2)

 

2.9. It is not clear the meaning of the plots in Figure 7. How the GSI can affect the FoS irrespectively the considered slope angle? Or, how can the slope angle alone be relevant for the FoS?. Please, carefully check the relevance of the plots.

 

R: Accepted. Figure 7 has been deleted because considered not very significant.

Author Response File: Author Response.docx

Reviewer 3 Report

Dear authors,

thank you for your interesting study regarding the analysis of landslides in a monoclinal setting.

The study is well done and organized, but there are some aspects to be significantly improved before the manuscript can be considered for publication.

The main aspect to improve is the comparison between the geomorphological and numerical approaches, which is suggested by the title but is not highlighted within the manuscript.

In the attached pdf, you will find more detailed suggestions regarding this main point, and other points to improve.

Best regards.

Comments for author File: Comments.pdf

Author Response

MINOR COMMENTS

R: Accepted. All the minor correction throughout the text has been properly modified following the referee indication

MAJOR COMMENTS

• Figure 1 – problems of readability

R: Accepted .Figure 1 has been totally modified following mostly the indication of referee #2

• Figure 2 – problems of readability

R: Accepted. Figure 2 has been modified and improved

• Figures 3-4-5 – orientation of the photos

R: Accepted. Orientation has been add to Figures 3 -4 and 5

• Paragraph 3: Model implementation. In the text, especially in the following paragraphs, it is explained (correctly, in my opinion) that the sections used in modeling have been calibrated, positioned, and chosen following a preliminary geomorphological survey; also the title of the manuscript suggests a comparison between a geomorphological and a numerical approach. However, this point is not highlighted in the text.

I would suggest to slightly modify the structure of the manuscript in this way: 1) in paragraph 2.1 I would leave only a geological and geomorphological framework on a larger scale than the study area, highlighting the monoclinal arrangement of the area; 2) I would add a paragraph in the results which highlights the results of the area’s detailed geomorphological survey and the criteria on the basis of which the sections to be modelled were selected. Adding this paragraph, I would add a geomorphological map of the study area and move here the traces of the sections highlighted in Figure 1c (maybe I would just move Figure 1c here).

 

R: Accepted. Following the indication of the referee #2 too, the organization of the manuscript has been deeply revised. Paragraph 2.1 contains only a general framework of the geological and geomorphological setting. The results of the detailed geomorphological surveys have been discussed in detail in a new paragraph (paragraph 3 – Results of the geomorphological approach). Moreover, a geomorphological map of the study area with a detailed geological cross-section has been included.

• Figures 6 a and b to be modified

R: Accepted. Following the indication of other referees these figures have been strongly modified and implemented

• Figure 7: usefulness..

R: Accepted. Following the indication of all the referees, this figure has been deleted because not significant

 

• References

R: Accepted. The style of references has been formatted following the journal rules

Author Response File: Author Response.docx

Reviewer 4 Report

The paper is very interesting and well written and leaves the ,beaten path' of landslide hazard assessment for a more complex process-oriented approach focusing on expert knowledge supported by numerical modelling.

I have no specific criticisms but I find the overall structure of the paper worth improving in the following suggested ways:

Section 2. Data and Methods includes a mixture of descriptive parts, such as Geological and Geomorphological Setting (2.1) and Numerical Modelling (2.2). which does not correspond to the content one would expect from the heading of section 2.

My suggestion is to separate the description of the study area from the data and methods section in the following form:

2. Geological and geomorphological setting of the study area

to be filled with current content of section 2.1

Here I also would like to suggest adding the derivation of the typical structural-geological environments which are used later for the definition of the position of the cross sections as the basis for the subsequent numerical modelling.

In the current version of the paper, there is also no consistent separation between methods and results. In the following you find my suggestions for a modified structure with the goal of better accommodating this separation.

3. Data and methods

3.1 Geomorphological approach

So far, in the paper the methodology of the geomorphological approach is missing. I expect that it might include analysis of geological information sources (maps, profiles, stratigraphic sections etc.) and field surveys in order to map certain categories which need to be presented in a conceptional form. Such a description should also include the underlying information sources and to what degree they are typical for all of Italy and who provides these information (geological survey or others). In this context it might also be interesting when these sources have been created and whether they are up to date or would require an update.

3.2 Numerical approach

as it is included in the current version of the paper. Here I would consider extending this section by implementation aspects in order to have a more consistent separation between methods and results later on in the paper.

4. Results

4. 1 Geomorphological approach

to be filled with content of 3.

4. 2 Numerical approach

to be filled with the result related aspects of the current section 4. destriped from the model implementation aspects which I would suggest being described under methods in section 3.2.

5. Discussion and conclusions

In the current form of the paper a structured discussion is missing comparing the different approaches related to certain criteria, such as information content, effort, costs, reproducibility, reliability and possibility for updates.

The discussion should also contain a more profound comparison to the standard (statistical?) LHA approach(es) used in Italy in order to derive the pros and cons of the different methods in order to give recommendations which go beyond using the presented approaches in case of absence of other regulations.

The conclusion section should then be based on the main findings of the discussion.

 

 

Round 2

Reviewer 1 Report

My comments are in the attached file

Comments for author File: Comments.pdf

Author Response

[…….]  Very often in fact, numerical results could be misleading. In this view, please clarify the role of the perched or static water table in your analysis; this may be as example of how a difference that may seem slight, could be so relevant in the FoS value.

Lines 239-241 and Chapter 4. The authors said that perched w.t. are present in the area, and are potentially strongly impacting the stability conditions; but in the numerical computation a static water table seems to be implemented. Could you discuss on this, even in the paper?

R: Accepted. A better explanation of the role of water table and how it has been modeled in the simulation has been added (Lines 428-431 of manuscript “Land-1138475” or Lines 388-391 of manuscript “Land-1138475_completerevision”)

Lines 73-74. It is still not clear. Please rewrite. What does it mean that the “geomorphological surveys highlighted both the presence of gravitational phenomena or stability conditions”? It does not make sense… Moreover “both” it is used when

 

R: Accepted. The sentence has been modified (Line 76 of manuscript “Land-1138475” or Lines 74 of manuscript “Land-1138475_completerevision”)

Lines 78-83. If this is a conclusion statement, please move it away from the introduction. Do not you think it is the case to move it? You should maybe change the tense of the verbs, or saying that “the purpose is to use the results in order to…..” instead of saying that “the obtained results clarify the role, usefulness etc….”. Please reply to this.

R: Accepted. The sentence has been modified (Lines 81-87 of manuscript “Land-1138475” or Lines 79-85 of manuscript “Land-1138475_completerevision”)

Lines 234-241. It is not clear how it is possible to have mainly low permeability material overcropping the area, but the runoff is “reduced or absent”. It is a not very rainy area? It seems to me that those two circumstances are incoherent.

R: Accepted. The sentence has been modified (Lines 292-299 of manuscript “Land-1138475” or Lines 250-252 of manuscript “Land-1138475_completerevision”)

 

 

 

Author Response File: Author Response.docx

Reviewer 2 Report

Dear Authors, you can find my second review in the attached file.

Comments for author File: Comments.pdf

Author Response

1. The introduction of the section n.3 “Results of the geomorphological approach” has been very appreciated. However, this section does not include a detailed description of the geomorphological aspects of ALL THE SLOPES analysed by numerical analysis. To be clear, I think that every slope considered in the following, one by one, must be described here to point out if it hosts a landslide, what type of landslide is recognised, where is supposed to be the sloping plane, etc… . The pictures of figure 6 are not enough informative in this sense, in my opinion. Differently, I cannot understand the last column of Table 2 where the Authors say if the numerical results agree with the geomorphological model, as the geomorphological results for each slope have been never pointed out.

R: Accepted: The paragraph 3 has been strongly implemented with a detailed geomorphological description for each cross-section (Lines 327-405 of manuscript “Land-1138475” or Lines 275-360 of manuscript “Land-1138475_completerevision”)

  

2. According to the title of the paper I expected to observe a relationship between the monoclinal settings of the area and the number and orientation of the landslides. Differently, the Authors say “although the stratigraphic setting is favorable to the activation of slides (rotational/planar) particularly on the eastern side, these are less frequent, while flows are dominant” (L247-248). Some more words are necessary to explain this counterintuitive observation.

R: Accepted: A short sentence has been added to clarify the concept (Lines 307-312 of manuscript “Land-1138475” or Lines 262-265 of manuscript “Land-1138475_completerevision”)

 

3. It is not clear for which soil/rock has been used the ubiquitous Joint model and, in any case, what is the orientation of the weak layer.

R: Accepted. A short sentence has been added within paragraph 4 (Lines 421-424 of manuscript “Land-1138475” or Lines 379-384 of manuscript “Land-1138475_completerevision”)

 

4. I don’t understand why the Authors don’t want to add a plot in which all the failure envelopes adopted for the soils/rocks are compared. The large tables of material properties obscure the ranking of the soil/rock resistances. I think it could be very useful to add a Mohr-Coulomb plane - where to represent all the failure envelopes. Please note that the Hoek-Brown criterion can be represented on the Mohr Coulomb plane by use the free software RocLab 1.0 (Rocscience).

R: Accepted. A new Figure 8 has been added and the text of the manuscript has been implemented (Lines 444-453 of manuscript “Land-1138475” or Lines 403-412 of manuscript “Land-1138475_completerevision”)

   

5. L307-309 and Figure 7: please note that the failures observed for section 1V, 2V and 3V are related to the assumed boundary conditions and do not represent any realistic sloping mass (try to enlarge the boundary and you will obtain different failure surfaces and different FoS). So, this point should be clearly expressed in the text. I think that for these sections you can conclude that the high value of the FoS able to activate the first landslide indicate a very stable condition. The obtained number of FoS in meaningless.

R: Accepted. The sentence has been modified (Lines 492-496 of manuscript “Land-1138475” or Lines 442-446 of manuscript “Land-1138475_completerevision”)

 

Other aspects to take into account:

6. L147-151: I don’t understand why the Authors cite the exact words of the FLAC manual. Are they not confident about the analyses performed by the software? Please rephrase by using your own words.

R: Accepted. The sentence has been modified (Lines 204-211 of manuscript “Land-1138475” or Lines 155-163 of manuscript “Land-1138475_completerevision”)

 

7. L164: Please check the notations. Why the Authors say the normal stress is negative in compression? I think it is the opposite. c is both used to indicate normal stress and uniaxial compressive stress. Please change one symbol to avoid confusion (i.e. n in Mohr Coulomb?)

R: Accepted. The symbol used in the Mohr-Coulomb formulas and within the table has been modified

 

8. L274-275: “The model used for each simulation (“Mohr-Coulomb” or “Hoek-Brown”) is automatically chosen by the software based…”. What is the meaning of this sentence? Please clarify.

R: Accepted. The sentence has been modified (Lines 419-424 of manuscript “Land-1138475” or Lines 377-384 of manuscript “Land-1138475_completerevision”)

 

9. L278-280: I suggest indicating that the initial state of stress has been assumed as lithostatic as a first approximation. To mention significant erosion phenomena, suggests the unreliability of the lithostatic hypothesis.

R: Accepted. The sentence has been modified (Lines 425-428 of manuscript “Land-1138475” or Lines 387-388 of manuscript “Land-1138475_completerevision”)

 

 

Author Response File: Author Response.docx

Reviewer 3 Report

Dear authors,

thank you for evaluating and responding positively to my revisions.

The manuscript is greatly improved, especially in its structure. In my opinion, the manuscript can now be accepted.

Best regards

Author Response

Thank you for your positive comment.

Round 3

Reviewer 1 Report

Dear Author, 

I have no more specific comments to give to you. I think that I am glad to see that the paper has benefited a lot from the whole review process it has been subjected to.

The paper now is scientifically sound, and despite it is not a very rigorous slope modelling paper, the literature and the issues to be considered when dealing with slope stability are at least mentioned if not directly faced; as such I am satisfied.

I feel that now the paper is ready to be published in the journal.

Kind regards

Reviewer 2 Report

Dear Authors,

I have appreciated the last revisions introduced in the paper (now the geomorphological approach is structured) and I think it now deserves the publication in Land.

Some last suggestions:

• be careful about the notation of effective stress (') in the formulations 1, 2 and 5. They do not appear consistent, at the moment.
• Fig. 8: please, use the same maximum values for the axes of all the plots. It should be evident which soil/rock are stronger.
• Table 1: please check the symbol for tension (T?)
• L365-367: please clarify this sentence about the modeling of the groundwater