Assessing the relationship between weather conditions and rockfall using terrestrial laser scanner to improve risk management

Birien, Tom; Gauthier, Francis

doi:https://doi.org/10.5194/egusphere-2022-326

Preprints

https://doi.org/10.5194/egusphere-2022-326

Preprints

16 May 2022

| 16 May 2022

Assessing the relationship between weather conditions and rockfall using terrestrial laser scanner to improve risk management

Tom Birien and Francis Gauthier

Abstract. Since 1987, more than 13,200 rockfalls have been inventoried by the Québec Ministry of Transport (MTQ) as having impacted the National Road 132 in northern Gaspésie. This natural hazard represents a nearly permanent danger for users. Traditional mitigation measures can be ineffective on poorly consolidated, deformed and highly fractured rockwall such as those found in northern Gaspésie. To address this issue, implementing a preventive risk management based on the factors that trigger rock instabilities could be the most effective method. Earthquake, rainfall and freeze-thaw cycles are commonly considered to be the main rockfall triggering factors. This study aims to better understand the climatic conditions conducive to rockfalls in northern Gaspésie in order to provide knowledge to implement an appropriate risk management strategy. Three rockwalls were scanned with a terrestrial laser scanner (TLS) during specific pre-targeted weather conditions. Over a period of 18 months, 17 surveys have allowed to identify 1287 rockfalls with a magnitude above 0.005 m³ on a scanned surface of 12 056 m². In addition, meteorological instruments and a 550 cm thermistor string have been installed directly on a vertical rockwall. It appears that some weather conditions influence occurrence, frequency, and magnitude of rockfalls. In winter, rockfall frequency is 12 times higher during a superficial thaw than during a cold period in which temperature remains below 0 °C. In summer, rockfall frequency is 22 times higher during a heavy rainfall event than during a period mainly dry. Superficial freeze-thaw cycle (< 50 cm) causes mostly a high frequency of small magnitude events while deeper spring thaw (> 100 cm) results in a high frequency of large magnitude events. Influence of weather conditions on rockfall frequency and magnitude is crucial in order to improve risk management since large magnitude events represent higher potential hazards. This study provides a classification of weather conditions based on their ability to trigger rockfalls of different magnitudes. This knowledge could be used to implement a risk management strategy.

Received: 11 May 2022 – Discussion started: 16 May 2022

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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Journal article(s) based on this preprint

27 Jan 2023

Assessing the relationship between weather conditions and rockfall using terrestrial laser scanning to improve risk management

Tom Birien and Francis Gauthier

Nat. Hazards Earth Syst. Sci., 23, 343–360, https://doi.org/10.5194/nhess-23-343-2023,https://doi.org/10.5194/nhess-23-343-2023, 2023

Short summary

Tom Birien and Francis Gauthier

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-326', Anonymous Referee #1, 27 Jun 2022

Please, find attached the .PDF with the review.

Citation: https://doi.org/10.5194/egusphere-2022-326-RC1
- AC1:
  'Reply on RC1 and RC2', Tom Birien, 30 Sep 2022
  Please, note that this answer is common for the 2 reviewers.
  Thank you for your relevant comments and for your in-depth reading of this paper. Your comments will for sure improve the quality of our manuscript. I will not give you a long answer because we have agreed with most of your advices and so we applied them. We have corrected all the minor errors that you had identified and we have also done these major changes:
  We described with lots of more details our methodology from LiDAR acquisition to rockfall volume calculation.
  
  In order to improve the reading fluency, we have reorganised in-depth the section “methods”. We have added some subsections and we have moved Figures 3 and 4 (and their description) in the “result” section because, they actually are some results. This major new structure seems to us more consistent and helps the understanding of the text: all the codes/acronyms are now only used at the end of the result section and sometimes in the discussion section.
  
  With the new structure of this paper, we first of all discuss of raw data (included for site by site analysis) and then we discuss about the relation between rockfalls and weather conditions.
  
  We also have added some paragraphs in all sections to prepare the reader to our figure 10 (the matrix which is one of the aims of this paper).
  
  As you noticed, a paragraph to explain why we believe that a per phenomenon analysis is better than a periodic one was missing in the “discussion” section. We acknowledge you for this comment that we have corrected!
  
  We have not included two of your propositions but we clarified our approach into the paper:
  We understand that our last figure (Fig. 11) is located in the “discussion” section even if it could be viewed as a result. Nevertheless, this picture (and its description) allows us to discuss about the efficiency of our results summarized in the matrix which is our main and final result. Moreover, it is not a result of this study since it does not come from our LiDAR surveys. We would appreciate to keep it in this section because we believe that it is interesting and useful to discuss about this event at the end of our paper, also to support our results.
  
  We understand your advice to present our results separately for the three study areas. First of all, we want to mention that we have already presented individually for the three study sites, 1) the rockfall spatial distribution (appendices) as well as 2) its frequency and 3) the rock slopes erosion rates (Fig. 5). Nevertheless, we used the whole database to study the influence of weather conditions on rockfalls. This approach has proved necessary to avoid misinterpretation of the occurrence or non-occurrence of rockfalls during short microclimatic periods (e.g. heavy and high intensity rainfall or winter freeze-thaw cycles). We have added a paragraph in the “method” sections to explain our choice for the reader.
  
  For the “minor changes at authors’ choice” (reviewer1), we all considered them but we have sometimes chosen to keep our initial version. Attached, please find our version of the revised manuscript with all the visible changes with the tracking tool (figures in low quality for this version).
  Thank you,
  The authors
  
  Citation: https://doi.org/10.5194/egusphere-2022-326-AC1
RC2:
'Comment on egusphere-2022-326', Anonymous Referee #2, 04 Aug 2022

In this manuscript, the authors use repeat TLS surveys at targeted times to explore the link between a variety of weather conditions and rockfall activity. Overall, I found this an interesting and relevant study, and I really like the approach of timing surveys to specifically target certain weather conditions. I found the freeze-thaw discussion particularly interesting - nice to see the link between thaw characteristics and timing and the rockfall occurrence and magnitude. The risk assessment matrix in fig. 10 is also interesting – it gives a succinct overview of your findings it’s clear how this provides a nice framework for risk management based on weather conditions. The manuscript is well-written, clear, and nice to read. The previous reviewer mentioned the heavy use of the codes for the weather conditions in places; I do agree that this can take some getting used to and be a little clunky in some places, but I’m not sure what a good alternative would be.

My only major comment is with the description of the methods. I agree with the previous reviewer that there is important information missing about how the volumes were calculated, how the level of detection was determined (what are the thresholds for positive and negative change in fig. 6?), what the uncertainties are, and how real rockfalls were distinguished from other changes (i.e. there are positive changes from snow accumulation, so there must be negative snow-related changes as well in some scan pairs?).

For the precipitation and temperature gauges, can you give the locations? Were the borehole/thermistors in one of the scanned walls? If so, which one?

You might mention in the introduction that there are also an increasing number of seismic-based studies that link rockfall activity to triggering conditions. These can get very precise timing, but not precise volumes.

Figure 1: the site photos are pretty dark – could these be lightened a bit, to better see the cliffs?

Figure 3: I really like this figure!

Figure 11: this might also be nice for the introduction/motivation - based on the dates, I would guess that this event did provide some motivation for the study!

Some typos:

Line 145: slope

Line 180: from

Line 363: 2019

Citation: https://doi.org/10.5194/egusphere-2022-326-RC2
- AC2:
  'Reply on RC1 and RC2', Tom Birien, 30 Sep 2022
  Please, note that this answer is common for the 2 reviewers.
  Thank you for your relevant comments and for your in-depth reading of this paper. Your comments will for sure improve the quality of our manuscript. I will not give you a long answer because we have agreed with most of your advices and so we applied them. We have corrected all the minor errors that you had identified and we have also done these major changes:
  We described with lots of more details our methodology from LiDAR acquisition to rockfall volume calculation.
  
  In order to improve the reading fluency, we have reorganised in-depth the section “methods”. We have added some subsections and we have moved Figures 3 and 4 (and their description) in the “result” section because, they actually are some results. This major new structure seems to us more consistent and helps the understanding of the text: all the codes/acronyms are now only used at the end of the result section and sometimes in the discussion section.
  
  With the new structure of this paper, we first of all discuss of raw data (included for site by site analysis) and then we discuss about the relation between rockfalls and weather conditions.
  
  We also have added some paragraphs in all sections to prepare the reader to our figure 10 (the matrix which is one of the aims of this paper).
  
  As you noticed, a paragraph to explain why we believe that a per phenomenon analysis is better than a periodic one was missing in the “discussion” section. We acknowledge you for this comment that we have corrected!
  
  We have not included two of your propositions but we clarified our approach into the paper:
  We understand that our last figure (Fig. 11) is located in the “discussion” section even if it could be viewed as a result. Nevertheless, this picture (and its description) allows us to discuss about the efficiency of our results summarized in the matrix which is our main and final result. Moreover, it is not a result of this study since it does not come from our LiDAR surveys. We would appreciate to keep it in this section because we believe that it is interesting and useful to discuss about this event at the end of our paper, also to support our results.
  
  We understand your advice to present our results separately for the three study areas. First of all, we want to mention that we have already presented individually for the three study sites, 1) the rockfall spatial distribution (appendices) as well as 2) its frequency and 3) the rock slopes erosion rates (Fig. 5). Nevertheless, we used the whole database to study the influence of weather conditions on rockfalls. This approach has proved necessary to avoid misinterpretation of the occurrence or non-occurrence of rockfalls during short microclimatic periods (e.g. heavy and high intensity rainfall or winter freeze-thaw cycles). We have added a paragraph in the “method” sections to explain our choice for the reader.
  
  For the “minor changes at authors’ choice” (reviewer1), we all considered them but we have sometimes chosen to keep our initial version. Attached, please find our version of the revised manuscript with all the visible changes with the tracking tool (figures in low quality for this version).
  Thank you,
  The authors
  
  Citation: https://doi.org/10.5194/egusphere-2022-326-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-326', Anonymous Referee #1, 27 Jun 2022

Please, find attached the .PDF with the review.

Citation: https://doi.org/10.5194/egusphere-2022-326-RC1
- AC1:
  'Reply on RC1 and RC2', Tom Birien, 30 Sep 2022
  Please, note that this answer is common for the 2 reviewers.
  Thank you for your relevant comments and for your in-depth reading of this paper. Your comments will for sure improve the quality of our manuscript. I will not give you a long answer because we have agreed with most of your advices and so we applied them. We have corrected all the minor errors that you had identified and we have also done these major changes:
  We described with lots of more details our methodology from LiDAR acquisition to rockfall volume calculation.
  
  In order to improve the reading fluency, we have reorganised in-depth the section “methods”. We have added some subsections and we have moved Figures 3 and 4 (and their description) in the “result” section because, they actually are some results. This major new structure seems to us more consistent and helps the understanding of the text: all the codes/acronyms are now only used at the end of the result section and sometimes in the discussion section.
  
  With the new structure of this paper, we first of all discuss of raw data (included for site by site analysis) and then we discuss about the relation between rockfalls and weather conditions.
  
  We also have added some paragraphs in all sections to prepare the reader to our figure 10 (the matrix which is one of the aims of this paper).
  
  As you noticed, a paragraph to explain why we believe that a per phenomenon analysis is better than a periodic one was missing in the “discussion” section. We acknowledge you for this comment that we have corrected!
  
  We have not included two of your propositions but we clarified our approach into the paper:
  We understand that our last figure (Fig. 11) is located in the “discussion” section even if it could be viewed as a result. Nevertheless, this picture (and its description) allows us to discuss about the efficiency of our results summarized in the matrix which is our main and final result. Moreover, it is not a result of this study since it does not come from our LiDAR surveys. We would appreciate to keep it in this section because we believe that it is interesting and useful to discuss about this event at the end of our paper, also to support our results.
  
  We understand your advice to present our results separately for the three study areas. First of all, we want to mention that we have already presented individually for the three study sites, 1) the rockfall spatial distribution (appendices) as well as 2) its frequency and 3) the rock slopes erosion rates (Fig. 5). Nevertheless, we used the whole database to study the influence of weather conditions on rockfalls. This approach has proved necessary to avoid misinterpretation of the occurrence or non-occurrence of rockfalls during short microclimatic periods (e.g. heavy and high intensity rainfall or winter freeze-thaw cycles). We have added a paragraph in the “method” sections to explain our choice for the reader.
  
  For the “minor changes at authors’ choice” (reviewer1), we all considered them but we have sometimes chosen to keep our initial version. Attached, please find our version of the revised manuscript with all the visible changes with the tracking tool (figures in low quality for this version).
  Thank you,
  The authors
  
  Citation: https://doi.org/10.5194/egusphere-2022-326-AC1
RC2:
'Comment on egusphere-2022-326', Anonymous Referee #2, 04 Aug 2022

In this manuscript, the authors use repeat TLS surveys at targeted times to explore the link between a variety of weather conditions and rockfall activity. Overall, I found this an interesting and relevant study, and I really like the approach of timing surveys to specifically target certain weather conditions. I found the freeze-thaw discussion particularly interesting - nice to see the link between thaw characteristics and timing and the rockfall occurrence and magnitude. The risk assessment matrix in fig. 10 is also interesting – it gives a succinct overview of your findings it’s clear how this provides a nice framework for risk management based on weather conditions. The manuscript is well-written, clear, and nice to read. The previous reviewer mentioned the heavy use of the codes for the weather conditions in places; I do agree that this can take some getting used to and be a little clunky in some places, but I’m not sure what a good alternative would be.

My only major comment is with the description of the methods. I agree with the previous reviewer that there is important information missing about how the volumes were calculated, how the level of detection was determined (what are the thresholds for positive and negative change in fig. 6?), what the uncertainties are, and how real rockfalls were distinguished from other changes (i.e. there are positive changes from snow accumulation, so there must be negative snow-related changes as well in some scan pairs?).

For the precipitation and temperature gauges, can you give the locations? Were the borehole/thermistors in one of the scanned walls? If so, which one?

You might mention in the introduction that there are also an increasing number of seismic-based studies that link rockfall activity to triggering conditions. These can get very precise timing, but not precise volumes.

Figure 1: the site photos are pretty dark – could these be lightened a bit, to better see the cliffs?

Figure 3: I really like this figure!

Figure 11: this might also be nice for the introduction/motivation - based on the dates, I would guess that this event did provide some motivation for the study!

Some typos:

Line 145: slope

Line 180: from

Line 363: 2019

Citation: https://doi.org/10.5194/egusphere-2022-326-RC2
- AC2:
  'Reply on RC1 and RC2', Tom Birien, 30 Sep 2022
  Please, note that this answer is common for the 2 reviewers.
  Thank you for your relevant comments and for your in-depth reading of this paper. Your comments will for sure improve the quality of our manuscript. I will not give you a long answer because we have agreed with most of your advices and so we applied them. We have corrected all the minor errors that you had identified and we have also done these major changes:
  We described with lots of more details our methodology from LiDAR acquisition to rockfall volume calculation.
  
  In order to improve the reading fluency, we have reorganised in-depth the section “methods”. We have added some subsections and we have moved Figures 3 and 4 (and their description) in the “result” section because, they actually are some results. This major new structure seems to us more consistent and helps the understanding of the text: all the codes/acronyms are now only used at the end of the result section and sometimes in the discussion section.
  
  With the new structure of this paper, we first of all discuss of raw data (included for site by site analysis) and then we discuss about the relation between rockfalls and weather conditions.
  
  We also have added some paragraphs in all sections to prepare the reader to our figure 10 (the matrix which is one of the aims of this paper).
  
  As you noticed, a paragraph to explain why we believe that a per phenomenon analysis is better than a periodic one was missing in the “discussion” section. We acknowledge you for this comment that we have corrected!
  
  We have not included two of your propositions but we clarified our approach into the paper:
  We understand that our last figure (Fig. 11) is located in the “discussion” section even if it could be viewed as a result. Nevertheless, this picture (and its description) allows us to discuss about the efficiency of our results summarized in the matrix which is our main and final result. Moreover, it is not a result of this study since it does not come from our LiDAR surveys. We would appreciate to keep it in this section because we believe that it is interesting and useful to discuss about this event at the end of our paper, also to support our results.
  
  We understand your advice to present our results separately for the three study areas. First of all, we want to mention that we have already presented individually for the three study sites, 1) the rockfall spatial distribution (appendices) as well as 2) its frequency and 3) the rock slopes erosion rates (Fig. 5). Nevertheless, we used the whole database to study the influence of weather conditions on rockfalls. This approach has proved necessary to avoid misinterpretation of the occurrence or non-occurrence of rockfalls during short microclimatic periods (e.g. heavy and high intensity rainfall or winter freeze-thaw cycles). We have added a paragraph in the “method” sections to explain our choice for the reader.
  
  For the “minor changes at authors’ choice” (reviewer1), we all considered them but we have sometimes chosen to keep our initial version. Attached, please find our version of the revised manuscript with all the visible changes with the tracking tool (figures in low quality for this version).
  Thank you,
  The authors
  
  Citation: https://doi.org/10.5194/egusphere-2022-326-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (further review by editor and referees) (01 Oct 2022) by Nadav Peleg

AR by Tom Birien on behalf of the Authors (03 Oct 2022) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (07 Oct 2022) by Nadav Peleg

RR by Anonymous Referee #1 (22 Oct 2022)

RR by Anonymous Referee #2 (08 Nov 2022)

ED: Publish as is (08 Nov 2022) by Nadav Peleg

ED: Publish subject to technical corrections (21 Nov 2022) by Bruce D. Malamud (Executive editor)

AR by Tom Birien on behalf of the Authors (29 Nov 2022) Manuscript

Journal article(s) based on this preprint

27 Jan 2023

Assessing the relationship between weather conditions and rockfall using terrestrial laser scanning to improve risk management

Tom Birien and Francis Gauthier

Nat. Hazards Earth Syst. Sci., 23, 343–360, https://doi.org/10.5194/nhess-23-343-2023,https://doi.org/10.5194/nhess-23-343-2023, 2023

Short summary

Tom Birien and Francis Gauthier

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Short summary

On highly fractured rockwall such as those found in northern Gaspésie, most rockfalls are triggered by weather conditions. This study highlights that in winter, rockfall frequency is 12 times higher during a superficial thaw than during a cold period in which temperature remains below 0 °C. In summer, rockfall frequency is 22 times higher during a heavy rainfall event than during a period mainly dry. This knowledge could be used to implement a risk management strategy.


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