The vulnerability of buildings to a large-scale debris flow and outburst flood hazard chain that occurred on 30 August 2020 in Ganluo, Southwest China

Wei, Li; Hu, Kaiheng; Liu, Shuang; Ning, Nan; Zhang, Xiaopeng; Zhang, Qiyuan; Rahim, Md Abdur

doi:https://doi.org/10.5194/nhess-2023-75

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https://doi.org/10.5194/nhess-2023-75

Preprints

21 Jul 2023

| 21 Jul 2023

Status: a revised version of this preprint is currently under review for the journal NHESS.

The vulnerability of buildings to a large-scale debris flow and outburst flood hazard chain that occurred on 30 August 2020 in Ganluo, Southwest China

Li Wei, Kaiheng Hu, Shuang Liu, Nan Ning, Xiaopeng Zhang, Qiyuan Zhang, and Md Abdur Rahim

Abstract. In mountainous areas, damage caused by debris flows is often aggravated by subsequent dam-burst floods within the main river confluence zone. On 30 August 2020, a catastrophic disaster chain occurred at the confluence of the Heixiluo Gully and Niri River in Ganluo County, Southwest China, that consisted of a debris flow, the formation of a barrier lake and subsequent dam breach that flooded the community. This study provides a comprehensive analysis of the damage to buildings resulting from the sequential occurrence of debris flow and dam-burst flood. The peak discharge of the debris flow in the gully mouth reached 1937 m³/s, and the change in the main river channel resulting from the dam-burst flood, which had a peak discharge of 2273 m³/s, resulted in a fourfold increase in the extent of flood inundation compared to an ordinary flood. Three hazard zones were established based on the building damage patterns: (I) primary debris flow burial; (II) secondary dam-burst flood inundation and (III) sequential debris flow burial and dam-burst inundation. Vulnerability curves were developed for Zone (II) and Zone (III) using impact pressures and inundation depths, and a vulnerability assessment chart is presented that contains the three damage categories. This research addresses a gap in the vulnerability assessments of debris flow hazard chains and can support in future disaster mitigation within confluence areas.

Received: 15 May 2023 – Discussion started: 21 Jul 2023

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Li Wei, Kaiheng Hu, Shuang Liu, Nan Ning, Xiaopeng Zhang, Qiyuan Zhang, and Md Abdur Rahim

Status: final response (author comments only)

RC1:
'Comment on nhess-2023-75', Anonymous Referee #1, 21 Aug 2023

Dear editor, dear authors,
I read with interest the pre-print of your paper about the vulnerability of buildings within a multi-hazard chain (debris flow - dam breach).
Principally, the idea of this manuscript is certainly interesting as, ... as you say, lots has been written about the hazard chain,
less about the vulnerability of exposed elements.
However, it first seemed strange to me that before getting a complete overview of the situation you present schemes,
which are typically a produc of interpretation .. and should only presented after investigation results.
Also the presentation of the maps in general, also those including flood simulation results are not very clear - so, some
more work is necessary to communicate your results to others.
Then, it is a multi-hazard situation and you (if I understood well) made only simulations of the final flood event, neglecting the
debris flow part. So, you cannot really say that you analysed the full chain as you consider the first part as known and set as a basis.

Therefore, right from the beginning you should indicate that the core analysis is focused on the final part of the chain,
taking the first part as a given element.
This will require a restructuring of the paper and thus a major revision (see also comment about hazard chain schemes).

Sincerely yours
Reviewer H

Citation: https://doi.org/10.5194/nhess-2023-75-RC1
- AC1: 'Reply on RC1', Li Wei, 11 Oct 2023
  
  Thank you very much for your comments. I will revise the article according to your comments.First, we will supplement the results of debris flow simulation and adjust the structure of the paper.
  
  Citation: https://doi.org/10.5194/nhess-2023-75-AC1
CC1:
'Comment on nhess-2023-75', Bin Liu, 20 Nov 2023
Dear editor and authors,
It’s a meaningful work for future disaster mitigation by a detailed case study. In this research, dynamic characteristics of debris flow and building vulnerability analysis were done with some models, risk assessment functions and vulnerability curves. And some more details might be concerned.
Does the parameters of model or data representative in this case study considering different scales? such as rainfall data, some parameters of models in Table1, or three functions used in debris flow risk assessment. how well does the models’results?

A spatial distribution map of damage might be more clearly show the distance from river channel, also the spatial relationship between vulnerability and factors might be more useful. Besides, does some field investigation or actual evidence that can support the vulnerability assessment?

This vulnerability assessment mainly focus on the buildings, is it possible to combine some other factors to analysis , so that might be more comprehesive.

Sincerely yours
B. Liu
Citation: https://doi.org/10.5194/nhess-2023-75-CC1
- AC2: 'Reply on CC1', Li Wei, 21 Nov 2023
  
     Thank you for your valuable advice. Let me answer the questions you raised:
     (1) The formula in Table 1 is an empirical formula, and the parameters in it are based on values that are taken from surveys or experience, so it will not take into account the influence of rainfall and other factors.
       (2) I will consider adding a spatial distribution map of buildings. In addition, because our vulnerability analysis is based on dynamic process simulation and field investigation, the results of the analysis are mainly suitable for the study area of this paper, and have not been verified in other areas.
        (3) Roads and cultivated land in the region were also severely damaged, but it is difficult to obtain the details of their damages in the post-disaster survey due to post-disaster rescue excavation, etc., so only the buildings are analyzed in detail in this paper.
  
  Citation: https://doi.org/10.5194/nhess-2023-75-AC2
RC2:
'Comment on nhess-2023-75', Anonymous Referee #2, 11 Dec 2023
Revision of the manuscript number “nhess-2023-75” entitled “The vulnerability of buildings to a large-scale debris flow and outburst flood hazard chain that occurred on 30 August 2020 in Ganluo, Southwest China”.
This paper uses a specific debris flow event to analyse the vulnerability of buildings considering hazard cascading events as debris flows and floods caused by the previous debris flow event. The structural vulnerability analysis is performed for every single building that has been affected by a specific debris flow event. I recommend handling the recommendations given below before publishing the paper. One of them is to double-check the redaction, especially in some parts specified below.
I think it is more appropriate to use “hazard cascade” instead of “hazard chain”. Consider modifying this in the title and the whole manuscript. See:

Cui, P., Zhou, G. G., Zhu, X. H., & Zhang, J. Q. (2013). Scale amplification of natural debris flows caused by cascading landslide dam failures. Geomorphology, 182, 173-189.

Wei, R., Zeng, Q., Davies, T., Yuan, G., Wang, K., Xue, X., & Yin, Q. (2018). Geohazard cascade and mechanism of large debris flows in Tianmo gully, SE Tibetan Plateau and implications to hazard monitoring. Engineering Geology, 233, 172-182.

However, better to cite the reference given below for having this definition in your manuscript:
Cutter, S. L. (2018). Compound, cascading, or complex disasters: what's in a name?. Environment: Science and Policy for Sustainable Development, 60(6), 16-25.

L15. For this type of process, it is also more common to refer to it as “dam break”, just consider modifying this in the manuscript.

L15-L17. Improve redaction.

L70-L71. Check grammar.

94. Improve the quality of Figure 1. Maybe, “.eps” file type will be better to make the name of the province readable on the map. And make the font size bigger. Same for the legend.

L116. The units must be better written as: “4700 m.a.s.l.”. Check this in the whole manuscript.

L117. Avoid using qualifier words, e.g., “hot”, “humid”, or “abundant rainfall”. Instead, write the numbers, such as “minimum average temperature”, “maximum average temperature”, and so on.

L127-L139. Modify units as mentioned before (m.a.s.l.). And also correct the percentage symbol.

L127-L139. The calculation of the slopes is wrong, I think. Correct this in the whole document. Thinking of this, it is just a typing mistake or were they introduced in Manning’s equation in this way? Please, be completely sure about this.

L131-L132. This is not truly saying something. Consider being very specific about what you describe here or remove the sentence.

L132-L133. Improve redaction, e.g., the “valley” word does not seem to be suitable for this description with slopes of about “600%”, if it is correct.

Improve the figure quality, to see better the date and time.

L160-L162. Improve the redaction of the caption.

L172. Correct to “Digital Elevation Models”.

Table 1. Correct “Rn”.

Why use the Manning equation and HEC-RAS to model this type of natural process? Because there are many different models and codes might help to obtain better results as they account for more variables and parameters. I recommend you settle a discussion that better justifies this selection based on the criteria given in the following reference:

Trujillo-Vela, M. G., Ramos-Cañón, A. M., Escobar-Vargas, J. A., & Galindo-Torres, S. A. (2022). An overview of debris-flow mathematical modelling. Earth-Science Reviews, 104135.

16. Better support how did you select or compute these Manning coefficient values. Be very specific to make the methodology replicable and/or applicable to other circumstances. Maybe the discussion can be based on the following reference:
Barnes, H. H. (1967). Roughness characteristics of natural channels(No. 1849). US Government Printing Office.

17. L217. Check the original paper out for this equation. Be very specific about what this equation describes, which is the “average” total pressure:
Zanchetta, G., Sulpizio, R., Pareschi, M. T., Leoni, F. M., & Santacroce, R. (2004). Characteristics of May 5–6, 1998 volcaniclastic debris flows in the Sarno area (Campania, southern Italy): relationships to structural damage and hazard zonation. Journal of volcanology and geothermal research, 133(1-4), 377-393.

18. L258. Write the percentage of the “relative error” between the two calculations.
19. L413-L415. Improve redaction.
20. Improve the quality of figures 11, 12, 13 and 14, and increase the font size.
21. Discussion has to be improved by emphasizing the benefits and disadvantages of using this methodology to analyse buildings' structural vulnerability, based on debris flows and floods.
Citation: https://doi.org/10.5194/nhess-2023-75-RC2
- AC3:
  'Reply on RC2', Li Wei, 28 Dec 2023
  I think it is more appropriate to use “hazard cascade” instead of “hazard chain”. Consider modifying this in the title and the whole manuscript.
  
  Thanks for your advice, I use “hazard cascade” in the whole manuscript. The reference (Cutter, 2018) was cited in the manuscript.
  For this type of process, it is also more common to refer to it as “dam break”, just consider modifying this in the manuscript.
  
  Thanks for your advice, I modified this in the manuscript.
  L15-L17. Improve redaction.
  
  We rewrite the whole sentence as follows:
  “This study presents a comprehensive analysis of the characteristics of two hazards and the resulting damage to buildings from the cascading hazards”.
  L70-L71. Check grammar.
  
  We rewrite the whole sentence as follows:
  “Our field investigations have revealed that the pattern of damage to buildings in the confluence area of debris flow and flood is distinct from those observed in areas affected by debris flow alone or by flood alone.”
  Improve the quality of Figure 1. Maybe, “.eps” file type will be better to make the name of the province readable on the map. And make the font size bigger. Same for the legend.
  
  We redraw this figure.
  The units must be better written as: “4700 m.a.s.l.”. Check this in the whole manuscript.
  
  We corrected the error in the whole manuscript.
  Avoid using qualifier words, e.g., “hot”, “humid”, or “abundant rainfall”. Instead, write the numbers, such as “minimum average temperature”, “maximum average temperature”, and so on.
  
  We rewrite the whole sentence as follows:
  “The average annual temperature is 16.2 °and the average annual rainfall is 949 mm.”
  L127-L139. The calculation of the slopes is wrong, I think. Correct this in the whole document. Thinking of this, it is just a typing mistake or were they introduced in Manning’s equation in this way? Please, be completely sure about this.
  
  The slope is calculated based on the channel profile, I think it’s right. I modified the slope value as decimals
  L131-L132. This is not truly saying something. Consider being very specific about what you describe here or remove the sentence.
  
  We rewrite the whole sentence as follows:
  “The field investigation indicates that debris flow initiated in the area above elevation of 1990 m a.s.l.”
  L132-L133. Improve redaction, e.g., the “valley” word does not seem to be suitable for this description with slopes of about “600%”, if it is correct.
  
  We use “channel” instead the “valley”.
  Improve the figure quality, to see better the date and time.
  
  We redraw this figure.
  L160-L162. Improve the redaction of the caption.
  
  We rewrite this as follows:
  “Figure 3 Illustration of the hazard cascade process: (a) the normal flow of river flow before the occurrence of debris flow; (b) debris flow blocks the river, creating a dammed lake that destroys the railway, roads, and buildings; (c) the dammed lake bursts, causing a flood that damaged and the road and buildings.”
  Correct to “Digital Elevation Models”.
  
  We correct this mistake.
  Table 1. Correct “Rn”.
  
  We correct this.
  Why use the Manning equation and HEC-RAS to model this type of natural process? Because there are many different models and codes might help to obtain better results as they account for more variables and parameters.
  
  Thanks for your advice. We first use many empirical models based on characteristics of debris flow dam and barrier lake to calculate the peak discharge of the dam break flood, the results vary widely. Considering the uncertainty of the characteristics of the debris flow dam and barrier lake, we used the Manning equation to calculate the peak discharge of dam-break flood. The
  HEC-RAS is often used to model the flood process, See the following articles:
  Butt, M. J. , Umar, M. , & Qamar, R. (2013). Landslide dam and subsequent dam-break flood estimation using hec-ras model in northern Pakistan. Natural Hazards, 65(1), 241-254.
  Mozumder, C. , Tripathi, N. K. , & Tipdecho, T. (2014). Ecosystem evaluation (1989–2012) of ramsar wetland deepor beel using satellite-derived indices. Environmental Monitoring & Assessment, 186(11), 7909-27.
  Better support how did you select or compute these Manning coefficient values. Be very specific to make the methodology replicable and/or applicable to other circumstances.
  
  The Manning coefficient values were determined based on the suggested values in the HEC-RAS 5.0 Reference Manual.
  Check the original paper out for this equation. Be very specific about what this equation describes, which is the “average” total pressure
  
  This equation is calculated as the average total pressure, we rewrite it as follows:
  “Hazard intensity parameters were applied, such as flow depth and average total impact pressure, with average total impact pressure calculated as(Zanchetta et al., 2014) where P is the average total impact pressure, is the flow density, is the velocity, and is the flow depth”
  Write the percentage of the “relative error” between the two calculations.
  
  We rewrite the whole sentence as follows:
  “resulting in a flow discharge of 2273 m3/s with a relative error of 18% which is comparable to the result obtained by Manning's equation.”
  L413-L415. Improve redaction.
  
  We rewrite the whole sentence as follows:
  “The impact pressure thresholds for Zones II and III, where vulnerability is equal to 1, are 75 kPa and 110 kPa, respectively. For the same impact pressure and inundation depth, the damage to buildings in Zone (II) is greater than that in Zone (III). ”
  Improve the quality of figures 11, 12, 13 and 14, and increase the font size.
  
  We redraw this figure.
  Discussion has to be improved by emphasizing the benefits and disadvantages of using this methodology to analyse buildings' structural vulnerability, based on debris flows and floods.
  
  We added this discussion as follows:
  “This study presents a comprehensive analysis of the damage to buildings resulting from a large-scale debris flow and outburst flood hazard cascade. The study develops building vulnerability in different areas of the confluence zone, which is useful for building risk assessment and management along the riverbank. However, some uncertainties and limitations are involved in vulnerability analysis. Firstly, the study did not consider the building's physical characteristics, such as shape, orientation, and maintenance condition. Secondly, in the area affected by the two hazards, the capacity of buildings first damaged by debris flow had declined, leading to a higher failure probability under the impact of sequential flood (Luo et al., 2020). The study analyzed the buildings' structural vulnerability based on debris flows and dam-break flood separately, and did not consider the building response to the primary debris flow or quantify the cumulative effect of the debris flow and the dam-break flood (Luo et al., 2023). A physics-based vulnerability model is required to quantify the dynamic evolution of building vulnerability.”
  In addition, we supplement the results of the debris flow simulation and adjust the structure of the paper.
  
  Citation: https://doi.org/10.5194/nhess-2023-75-AC3

Li Wei, Kaiheng Hu, Shuang Liu, Nan Ning, Xiaopeng Zhang, Qiyuan Zhang, and Md Abdur Rahim

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Short summary

The damage patterns of the buildings were classified into three types: (I) buried by primary debris flow, (II) inundated by secondary dam-burst flood, and (III) buried by debris flow and inundated by dam-burst flood sequentially. The threshold of the impact pressures in Zones II and III where vulnerability is equal to 1 are 88 kPa and 106 kPa, respectively. Heavy damage occurs at an impact pressure greater than 40 kPa, while slight damage occurs below 20 kPa.


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