Review Reports - What If an Intense Rain Event Should Trigger Diffuse Shallow Landslides in a Small Mediterranean Catchment? Numerical Modeling Through Remote Sensing Techniques

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This article uses the 'Tsunami Squares' model to simulate the impact range of debris flows in small watersheds at a regional scale and then establishes risk scenarios for disaster-affected bodies exposed within the disaster impact range. The author organized this article well, so the logic of the article is smooth. The methods in the article are mature and the data is abundant. The results have reference significance for the protection of relics and the planning of basic facilities in similar areas of the study area and even the Mediterranean area.I have no revision suggestions for this article.

Author Response

Reviewer #1

The methods in the article are mature and the data is abundant. The results have reference significance for the protection of relics and the planning of basic facilities in similar areas of the study area and even the Mediterranean area.

I have no revision suggestions for this article.

We wish to thank the Reviewer for the comments.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This paper discusses the numerical simulation results of shallow earthquake landslides triggered by rainfall in a small watershed in Liguria, located on the northern coast of the Mediterranean Sea in northern Italy. It discusses the simulation results of the disaster chain of rainfall-landslide-mudslide in a small watershed under the background of heavy rainfall under the instantaneous rainstorm scenario, and points out the potential impact of local human activities on the occurrence process of the disaster chain. The main problems of this paper are:

1. The scale of the study area is relatively small. For the representativeness of the northern Italian coastal area (Table 2) discussed in this paper, the geological and hydrological disasters in the study area mentioned in the literature in Table 2 mainly occurred in two other areas, and the number of disaster events involving the POR area was relatively small. Therefore, the reviewer believes that the scale of the study area discussed in this paper is really relatively small. Small disaster chain events are not typical. The discussion of landslides and rainfall-induced flood events in smaller basins may lead to greater uncertainty in the assessment of regional-scale geological disasters. Under the condition that data permits, it is recommended that the author take the three study areas mentioned in Table 2 as the study areas of this paper to discuss the occurrence mechanism of disaster chains at the regional multi-basin scale;

2. The research object of this paper is unclear. When discussing disasters, the paper involves landslides, debris flows, storm surges, and extreme rainfall. While the title of the paper is about shallow landslides, and the simulation results are mainly based on debris flows and instantaneous rainfall. Therefore, the research object is relatively confusing. The possible shallow landslide locations interpreted for the source of each secondary tributary in the basin (as shown in Figure 5) have been compared by the reviewer with Google at the same location. Earth, whether these areas can be regarded as shallow landslides is questionable, because the main lithology of this area is bedrock mountainous area, and the surface may develop weathering crust, but most of it is covered by shrubs and forests. The possibility of large-scale landslides is low. Therefore, the reviewer doubts the role of the interpreted landslides in the disasters caused by instantaneous floods, and it is recommended that the author clarify the research object of this article;

3. Then the second question is that all the simulation work of the author does not have a field photo of a real and reliable landslide or bedrock collapse event in a small watershed. The only two photos mainly reflect that the flood discharge capacity is irrelevant to the landslide disaster discussed in this article. It is recommended that the author supplement the relevant field survey or historical documentary image data to confirm the universality of rainfall-triggered landslides in this area;

4. The author emphasizes the significant impact of the remains of human activities in the historical period on regional geological disasters in the overview, but it is not fully expanded in the discussion and conclusion. It is recommended to expand the study area, the impact of terraces and artificial buildings built by human activities in these historical periods on the formation of disasters can be shown;

5. Other minor issues are listed as follows:

a) Regarding the map section, the main map of Figure 1 lacks necessary local names, such as the study area of this article, which is recommended to be supplemented. Figure 2 also lacks necessary place names. The boundaries of the landslides and the boundary range of small-scale landslides listed in Figure 2 are not clear on the remote sensing image. If there are more detailed LiDAR image data, it is recommended that the author add local examples of evidence; Figure 3 should indicate the data source; Please check the interpretation of potential landslides in Figure 5. The source of the river cannot be simply understood as the landslide area. It is very likely that this is just a catchment area, not a potential landslide area. There is a lack of place name annotations. Figures 6-10 have the same problem;

b) What is most lacking in the article is a detailed review of previous research literature on the occurrence of landslides in this study area, including relevant statistical analysis of floods caused by extreme rainstorms to major settlements.

Author Response

Reviewer #2

We wish to thank the Reviewer for his comments and requests: modifications and integrations have been implemented in the new version of the manuscript. Besides, hereafter specific answers are following.

The main problems of this paper are:

The scale of the study area is relatively small. For the representativeness of the northern Italian coastal area Table 2) discussed in this paper, the geological and hydrological disasters in the study area mentioned in the literature in Table 2 mainly occurred in two other areas, and the number of disaster events involving the POR area was relatively small. Therefore, the reviewer believes that the scale of the study area discussed in this paper is really relatively small. Small disaster chain events are not typical. The discussion of landslides and rainfall-induced flood events in smaller basins may lead to greater uncertainty in the assessment of regional-scale geological disasters. Under the condition that data permits, it is recommended that the author take the three study areas mentioned in Table 2 as the study areas of this paper to discuss the occurrence mechanism of disaster chains at the regional multi-basin scale;

In Figure 1 the Paraggi catchment is shown together with the Camogli (CAM), Portofino (POR) and Santa Margherita Ligure (SML) municipalities, while Table 1 presents its main geomorphometric features. Half of the catchment belongs to the POR Municipality and half to the SML one. As it may be seen from the inlet in Figure 1 the catchment extension compared to the whole promontory, then the CAM, POR and SML areas, may be considered as significant. Locations and names have been added to the Figure and to the relative caption.

Then, the events in table 2 belongs to the whole promontory (the sum of CAM, POR and SML municipalities) and, as it has been already published in other cited papers [47, 48], they belong to similar geomorphometric and geological features.

Finally, as detailed throughout the manuscript, the simulation has been intended as a “What if” statement considering what happened in 2011 in the similar Cinque Terre and Val di Vara area [20, 36, 37]: after an intense rain event in a steep and largely terraced small catchment many small shallow landslides were triggered simultaneously. Then, considering that the present manuscript represents the last step of a research path conducted in the framework of the Horizon 2020 RECONECT project, the results obtained in the previous published steps have been used in the present one. To furtherly clarify this aspect an explaining text has been added at lines 392-411.

Finally, we wish to underline the potentiality in upscaling the approach to other areas that share similar geomorphologic features and the diffuse presence of terraces. Besides, similar simulations/approach may be performed at a sub-catchment scale.

The research object of this paper is unclear. When discussing disasters, the paper involves landslides, debris flows, storm surges, and extreme rainfall. While the title of the paper is about shallow landslides, and the simulation results are mainly based on debris flows and instantaneous rainfall. Therefore, the research object is relatively confusing. The possible shallow landslide locations interpreted for the source of each secondary

tributary in the basin (as shown in Figure 5) have been compared by the reviewer with Google at the same location. Earth, whether these areas can be regarded as shallow landslides is questionable, because the main lithology of this area is bedrock mountainous area, and the surface may develop weathering crust, but most of it is covered by shrubs and forests. The possibility of large-scale landslides is low. Therefore, the reviewer doubts the role of the interpreted landslides in the disasters caused by instantaneous floods, and it is recommended that the author clarify the research object of this article;

As stated at lines 139-158, 314-321 and 444-447 the research represents a simulation of a possible event hitting the studied area, assuming the 2011 event that hit the similar territory of the Cinque Terre [20, 36, 37]. Then, the aim is a “What if” simulation which is conducted after the research path followed along the Horizon 2020 RECONECT project whose results have been already published and represents the basis for the present research work [47, 48, 52, 53, 57, 76]. Considering the Cinque Terre event only small shallow landslides have been considered (table 4)

Then, considering the past events in the area and along the Ligurian region, the attention has been focused on the role played by man-made terraces in terms of possible source of shallow landslides, which may assume features similar to debris or mud flows as it happened in 2011 in the Cinque Terre area [20, 36, 37]. In that case terraces collapse happened simultaneously in different zones of the catchment and involving both cultivated and shrubs or vegetated terraces.

The possible triggering from wild vegetated terraces has been added as text at lines 321-323.

Then the second question is that all the simulation work of the author does not have a field photo of a real and reliable landslide or bedrock collapse event in a small watershed. The only two photos mainly reflect that the flood discharge capacity is irrelevant to the landslide disaster discussed in this article. It is recommended that the author supplement the relevant field survey or historical documentary image data to confirm the universality of rainfall-triggered landslides in this area;

We did not consider to add other images to the actual paper as it already includes 13 figures and 5 tables.

Papers of the previous researche steps conducted along the RECONECT project include some photo and we have referred to them in the manuscript [47, 48, 52, 53, 57, 76].

In fact, some significant images can be found in previous works, such as the following two [47].

The author emphasizes the significant impact of the remains of human activities in the historical period on regional geological disasters in the overview, but it is not fully expanded in the discussion and conclusion. It is recommended to expand the study area, the impact of terraces and artificial buildings built by human activities in these historical periods on the formation of disasters can be shown;

The presence of historical elements in the area assumes two main meanings:

Terraces, as anthropogenic modification of the slopes, have been realized since ancient times, due to the subsistence agriculture and to cultivate olives and produce oil to be exchanged with other goods; the ancient origin and actual significant of terraces have been referred to at lines: 122-123, 253-271 and 298-302.
Cultural heritage: the ancient Abbey in the nearby San Fruttuoso Bay is an exposed element to possible slope collapse, as it already happened in 1915 [57]. The presence of the cultural heritage and its spatial relationships with the geo-hydrological hazard has been underlined at lines: 166-167, 358-360 and 442-444.

Text has been added at lines 569-576.

Other minor issues are listed as follows:
a) Regarding the map section, the main map of Figure 1 lacks necessary local names, such as the study area of this article, which is recommended to be supplemented.

Local names and locations have been added to the figure and to the relative caption.

Figure 2 also lacks necessary place names. The boundaries of the landslides and the boundary range of small-scale landslides listed in Figure 2 are not clear on the remote sensing image. If there are more detailed LiDAR image data, it is recommended that the author add local examples of evidence;

Local name has been added to the figure, which frames the same area of figure 1.

Figure 3 should indicate the data source;

Data source has been added in the figure caption.

Please check the interpretation of potential landslides in Figure 5. The source of the river cannot be simply understood as the landslide area. It is very likely that this is just a catchment area, not a potential landslide area.

The possible landslides have been identified according to the previous researches that have been conducted through the years as part of the project Horizon2020 RECONECT and that have been published in previous papers, as stated at lines 363-368. Then, the present manuscript represents the last step of a research path that is schematically resumed as follows: i) precise identification of terraces, which in the recent years have often played the role of source areas for shallow landslides at the regional scale, and the study of terraces spatial relationships with possible risk exposed elements as buildings, infrastructures and cultural heritage [52, 76]; ii) the shallow landslides susceptibility assessment, which is based on the previous events that happened in the whole promontory in the last 110 years [48]; iii) the identification of rainfall threshold for shallow landslides triggering and the rainfall long-term trend analysis [47]; iv) a numerical modeling of debris and soil anthropogenically trapped into terraces [76]; v) the back analysis numerical modeling of a shallow landslide that hit and partially destroyed the cultural heritage in the nearby San Fruttuoso bay in 1915 [57].

Then, the possible shallow landslides that could be triggered from terraced areas, which represent the few debris and soil significant cover in the area due to the gradient slope and geological asset, have been identified after the entire research path and considering recent past events that hit the Ligurian coastal area. In particular the event that hit the Cinque Terre and Val di Vara area in 2011 [20, 36, 37], when in the similar Vernazza catchment about two hundreds small shallow landslides were triggered almost simultaneously by heavy rains. In that case, and other similar one along the region area, the source area of shallow landslides assumed shapes similar to the ones that have been assumed in the present research [36, 37].

Text has been added at lines 392-411.

Previous researches have been focusing on:

Terraces precise identification and their evaluation in terms of possible source of shallow landslides: Paliaga, G.; Luino, F.; Turconi, L.; De Graff, J.V.; Faccini, F. Terraced Landscapes on Portofino Promontory (Italy): Identification, Geo-Hydrological Hazard and Management. Water 2020, 12, 435. https://doi.org/10.3390/w12020435.
Shallow landslide susceptibility assessment: “Roccati, A., Paliaga, G., Luino, F., Faccini, F., Turconi, L. GIS-Based Landslide Susceptibility Mapping for Land Use Planning and Risk Assessment. Land 2021, 10, 162. https://doi.org/10.3390/land10020162.”

Identification of a rainfall threshold for shallow landslides triggering and long-term rainfall trend: Roccati, A.; Paliaga, G.; Luino, F.; Faccini, F.; Turconi, L. Rainfall Threshold for Shallow Landslides Initiation and Analysis of Long-Term Rainfall Trends in a Mediterranean Area. Atmosphere 2020, 11, 1367. https://doi.org/10.3390/atmos11121367

Debris and soil volume trapped into terraces numerical evaluation and the relationships with the geo-hydrological risk: Paliaga, G., Luino, F., Turconi, L., Profeta, M., Vojinovic, Z., Cucchiaro, S., & Faccini, F. Terraced landscapes as NBSs for geo-hydrological hazard mitigation: towards a methodology for debris and soil volume estimations through a LiDAR survey. Remote Sensing, 2022, 14(15), 3586.
Back analysis and numerical modeling of the 1915 event that hit the nearby San Fruttuoso bay and partially destroyed the cultural heritage: Paliaga, G., Ward, S., Luino, F., Turconi, L., Faccini, F. The 1915 mud-debris flow at San Fruttuoso di Camogli: modeling the collapse effects in the Portofino pilot area of the H2020 RECONECT project. SimHydro 2021: Models for complex and global water issues - Practices and expectations, 16-18 June 2021, Sophia Antipolis, 2022.

There is a lack of place name annotations. Figures 6-10 have the same problem;

Reference squares for figures 6, 8, 10 and 7, 9, 11 have been added in figure 5.

b) What is most lacking in the article is a detailed review of previous research literature on the occurrence of landslides in this study area, including relevant statistical analysis of floods caused by extreme rainstorms to major settlements.

In the Introduction section (lines 60-101) a framework of previous studies has been commented. Specific literature on the occurrence of shallow landslides in the region has been addressed at section 2 at lines 235-244 and 270-281 and then in section 2.2.

The authors wish to underline that the present manuscript represents the last step of a research path conducted through the years as part of the project Horizon2020 RECONECT: results regarding the analysis of past events have been already published and properly cited throughout the present manuscript [47, 48]. Rainfall conditions for the occurrence of shallow landslides and rainfall trends were investigated in the Portofino promontory in the article cited [47]; in this area where heavy rainfall and related damage often occur. In [47] the empirical intensity-duration threshold was estimated, highlight that the rainfall intensity required to trigger landslides is lower for the same duration than those expected in other similar environments. This result suggesting a high susceptibility to rainfall-induced landslides in the Portofino territory. The Mann-Kendall test and Hurst exponent were used for detecting potential trends. The Authors in [47] have analyzed of long-term rainfall time series showed statistically significant increasing trends in short duration precipitation occurrence and rainfall rates, suggesting a possible future scenario with a more frequent exceedance of the threshold triggering value and an increase of landslide risk.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

please see attached

Comments for author File: Comments.pdf

Author Response

Reviewer #3

This paper offers an interesting study simulating potential rain event triggering shallow landslides in a catchment area.

We wish to thank the Reviewer for his comments and requests that have been accepted: the modifications have been implemented in the new version of the manuscript. Hereafter the replay comments are following.

The following are some suggestions to the paper:

Line 362 mentioned about 27 small slides. Are these predefined?

The text has been added at lines 389-408.

Previous researches have been focusing on:

Terraces precise identification and their evaluation in terms of possible source of shallow landslides: Paliaga, G.; Luino, F.; Turconi, L.; De Graff, J.V.; Faccini, F. Terraced Landscapes on Portofino Promontory (Italy): Identification, Geo-Hydrological Hazard and Management. Water 2020, 12, 435. https://doi.org/10.3390/w12020435.
Shallow landslide susceptibility assessment: “Roccati, A., Paliaga, G., Luino, F., Faccini, F., Turconi, L. GIS-Based Landslide Susceptibility Mapping for Land Use Planning and Risk Assessment. Land 2021, 10, 162. https://doi.org/10.3390/land10020162.”

Identification of a rainfall threshold for shallow landslides triggering and long-term rainfall trend: Roccati, A.; Paliaga, G.; Luino, F.; Faccini, F.; Turconi, L. Rainfall Threshold for Shallow Landslides Initiation and Analysis of Long-Term Rainfall Trends in a Mediterranean Area. Atmosphere 2020, 11, 1367. https://doi.org/10.3390/atmos11121367

Debris and soil volume trapped into terraces numerical evaluation and the relationships with the geo-hydrological risk: Paliaga, G., Luino, F., Turconi, L., Profeta, M., Vojinovic, Z., Cucchiaro, S., & Faccini, F. Terraced landscapes as NBSs for geo-hydrological hazard mitigation: towards a methodology for debris and soil volume estimations through a LiDAR survey. Remote Sensing, 2022, 14(15), 3586.
Back analysis and numerical modeling of the 1915 event that hit the nearby San Fruttuoso bay and partially destroyed the cultural heritage: Paliaga, G., Ward, S., Luino, F., Turconi, L., Faccini, F. The 1915 mud-debris flow at San Fruttuoso di Camogli: modeling the collapse effects in the Portofino pilot area of the H2020 RECONECT project. SimHydro 2021: Models for complex and global water issues - Practices and expectations, 16-18 June 2021, Sophia Antipolis, 2022.

2) Is there more detail regarding the tsunamic square model that you can offer, example,

what is the friction function assumed in the model?

The details about friction function are explained in reference 88 (Wang J, Ward SN, Xiao L Numerical modelling of rapid, flow-like landslides across 3-D terrains: a tsunami squares approach to El Picacho landslide, El Salvador, 19 September 1982. Geophys J Int. 2015, 201, 1534–1544). Hereafter we clarify this point which is referred to in the paper.

There are two frictions in TS.

Frictions in TS are actually accelerations in the opposite direction as velocity.

(1) A basal friction. Equation (19) in reference 88. This is just a constant times g.

Basal friction applies all the time after a few seconds from the slide beginning. We "turn if off" for the first few seconds to get things moving.

It is the conventional "sliding friction" (or "static friction").

(2) A dynamic friction Equation (20) in reference 88. This grows like velocity square and decreases with flow thickness H.

We call it "dynamic" because it is not constant, but rather depends on velocity.

Dynamic friction gives a "terminal velocity" to landslides depending on slope and flow thickness.

We assign this to keep the maximum landslide speed reasonable.

Dynamic friction cannot actually stop a landslide. However, when velocity falls below a certain level,

basal friction will stop it provided the stopping slope is not too steep.

3) Please be consistent with the number reporting: Ex. Line 158, “14,000 people”, line 214,

“1,300 mm”

The corrections have been done throughout the whole manuscript.

4) While English language is not a major problem for the paper, there are several misspelled

words. A thorough spell check can be helpful. Some examples are as follows:

Line 20, “natur”, “cohexist,”.

The correction has been done.

Line 32: “threathens”

The correction has been done.

Line 82: “leding”

The correction has been done.

Line 136, “an earth”

The correction has been done.

Line 167: “1000 A.C.”?

The correction has been done.

Line 206: “hierarchical”

The correction has been done.

Line 235: “whit”

The correction has been done.

Line 265: “woodden”

The correction has been done.

Line 319, “20m-2”?

The correction has been done.

Line 496: “manutantion”

The correction has been done.

Throughout the paper: “Tab.” versus “Table”.

The correction has been done.

Similarly, “fig. “ versus “figure”

The correction has been done.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

No comments