the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Oct 2024
Applicability and effectiveness of structural measures for subsidence (risk) reduction in urban areas
Abstract. Managing subsidence and its impacts on cities in coastal and delta areas is a global challenge that requires comprehensive risk reduction policies, including both mitigation and prevention strategies. Urban areas often lack systematic methodologies for determining appropriate countermeasures. This paper proposes a twofold strategy for selecting subsidence reduction measures in urban areas – which refer to structural (i.e., technical) measures to prevent and mitigate subsidence and its physical consequences - based on their applicability and performance. The Question-and-Response (Q&R) system serves as a decision tree to identify suitable subsidence countermeasures based on their applicability to specific cases. Four indicators of effectiveness – i.e., reduction potential, operational reliability, negative impact and service life – are then used to assess the performance of subsidence reduction measures. The proposed procedure was applied to 49 cases derived from a review of 52 scientific publications and additional expert sessions and surveys involving five academic scholars and 13 experts. Also, the method was applied to examples from Shanghai (China), Jakarta (Indonesia) and San Joaquin Valley (USA, California). The strategies proposed in this paper proved suitable for an initial screening of subsidence reduction measures applicable in different urban areas, after which a site-specific assessment can follow. Furthermore, this study shows the need to collect and share experiences in evaluating the performance of subsidence reduction measures more systematically, and gives a first framework to do so.
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Status: open (until 09 Dec 2024)
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RC1: 'Comment on egusphere-2024-2537', Samar Momin, 18 Nov 2024
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General Comments:
The paper presents a novel and systematic approach for addressing subsidence risks in urban areas, combining the Question-and-Response (Q&R) system with performance indicators to identify and evaluate structural mitigation measures. The methodology's application to global case studies, including Shanghai, Jakarta, and San Joaquin Valley, demonstrates its practical utility. The integration of expert surveys, literature reviews, and real-world data enriches the analysis.
The manuscript is well-organized, with a logical flow that facilitates understanding. Key concepts are defined early, and the proposed methodology is clearly explained. The difference in the use of the terms Structural and Non-structural from those in Structural Engineering applications provides clarity. The findings provide valuable insights for decision-makers, engineers, and urban planners addressing subsidence in diverse geotechnical and socio-economic contexts.
Strengths:
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Comprehensive Framework:
- The twofold strategy, combining the Q&R system with effectiveness indicators, fills a critical gap in subsidence risk management literature.
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Broad Application:
- The analysis spans 49 cases across 18 countries, providing a robust validation of the methodology.
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Clarity in Presentation:
- Figures, tables, and case study descriptions are detailed and informative, supporting the narrative effectively.
Specific Comments:
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Definitions of Key Terms:
- The definitions of "mitigation," "prevention," and "adaptation" are clear. However, these could be linked to more concrete examples from the 49 cases to further ground theoretical explanations.
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Limitations of Current Data:
- The paper acknowledges that some indicators lack sufficient data. It would benefit from emphasizing the potential bias this introduces and suggesting how future studies could address these gaps.
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Comparative Effectiveness:
- While the Q&R system identifies suitable measures, the discussion could delve deeper into why certain measures perform better in specific contexts (e.g., why aquifer recharge was prioritized over injection wells in certain cases).
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Broader Impacts:
- The paper lightly touches on the indirect consequences of subsidence measures, such as their environmental and social impacts. Expanding this discussion would strengthen the study’s utility for policymakers.
Technical Comments:
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Grammar and Style:
- Minor grammatical inconsistencies were noted, such as:
- Line 263: “If subsidence exceeds this threshold” could clarify what “this threshold” specifically refers to.
- Minor grammatical inconsistencies were noted, such as:
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Citations and Formatting:
- Some citations lack consistent formatting. For instance, references to multiple authors (e.g., Akbar et al., 2019; Herrera-García et al., 2021) are not uniform across the text.
Citation: https://doi.org/10.5194/egusphere-2024-2537-RC1 -
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RC2: 'Comment on egusphere-2024-2537', Anonymous Referee #2, 19 Nov 2024
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Nappo and Korff have presented a novel framework for evaluating the applicability and effectiveness of different subsidence countermeasures derived from existing literature. Except the exclusion of cost in their framework, I believe that it fills a major gap in the ability to undertake a structured approach to mitigating subsidence impacts.
A major concern for me is the authors’ approach to concluding the validity of their methodology based on 3 example studies. I do not have the same confidence as them in their conclusions due to data leakage and over-reliance on one category - size of the impacted region. As a suggestion for improving this part of the manuscript, instead of using the example studies as evidence for their framework’s validity, they could be used as motivating and detailed examples for developing the framework.
- The authors have not included cost of a countermeasure in their Q&R system. This is a major gap and limitation of their system as cost is often a primary factor in determining the appropriate countermeasure.
- The Q&R system has been presented as a decision tree approach, however, decision trees were not employed in the 3 example study regions in the manuscript.
- While the example regions used by the authors are geographically diverse, their subsidence category of large/regional scale is the same. In order to draw conclusions about the effectiveness of authors’ methodology, a more diverse set of example cases should have been reviewed.
- The authors have used existing literature to populate the results of their Q&R system in Figure 6. Then the system has been applied to 3 of the regions from the same literature and presented as evidence of the validity of the authors’ methodology. As a result of this data leakage, I don’t believe that the conclusions on the effectiveness of the authors’ methodology can be drawn here.
- In Section 5, the results are presented as a percentage of the 49 cases. While percentages can be useful in larger datasets, for a small dataset, I would suggest additionally including the counts in the results.
- Line 32 - The sentence would be clearer by separately classifying groundwater, for example, groundwater extraction…
- Line 36 - *Unlike*
- Line 110 - Please cite Table 1 for this sentence.
- Line 149 - Could the authors clarify what metric is used for measuring reduction - is it subsidence risk that would include the hazard along with exposure and vulnerability?
- Line 154 - Could the authors clarify if “time” refers to the entire service life of the countermeasure?
- Line 164 - *causes*
- Line 164 - Could the authors provide examples of detrimental impacts?
- Line 171 - Could the authors elaborate on how 10 years was selected as the serviceable life threshold, as it appears quite low for structural countermeasures? Typically most non-critical structures are engineered for 50 years, and critical structures for longer than that.
- Line 184 - 32% of the 49 cases *have* a secondary cause. “Also” is not necessary given the “additionally”.
- Fig 2 - *Tertiary*
- Fig 2 - Since the authors have mentioned “anthropogenic and natural” in the figure caption, it would be clearer if these were also identified in the figure.
- Fig 3 caption - geological types *of* the investigated
- Fig 5 - It is an impressive figure that makes it very easy to see which measures are used in conjunction. As a minor suggestion, the figure could be further improved by assigning edge weight based on the number of times that edge is present in the case studies.
- Section 5.2 - The authors have mentioned the number of experts as 13 elsewhere but it will also be helpful to mention it in this section.
- Section 5.2 - Did the number of occurrences of a particular countermeasure across the 49 case studies influence the expert responses? How did the authors ensure that frequency of countermeasure did not influence its performance assessment?
- Line 236 - How is the average computed? Given the qualitative nature of all responses, it would not be ideal if 2 responses of High and Low are averaged as medium. Rather, mode may be a better metric.
- Line 238 - What is the definition of “insufficient information”? What is the minimum number of responses required to populate each cell in Table 4?
- Line 255 - It would be helpful to elaborate why mitigation is needed over a “large/regional scale”, for example, by providing the area of subsidence region.
- Line 257 - It is not clear which effectiveness parameters led to removal of compartmentalization as a countermeasure.
- Line 267 - It is not clear how the authors reached their conclusion. They only used one category - regional scale - for their analysis and did not consider any of the other categories. Additionally, as the authors highlight on Line 268 that viability primarily depends on cost considerations, why was cost of a countermeasure excluded from their Q&R system?
- Line 275 - Wouldn’t “lowering of the groundwater level” be a cause of subsidence instead of its impact?
- Line 277 - Does the peak refer to 160 cm/year subsidence or total subsidence of 160 cm over some time period, at a particular site?
- Line 283 - It would be helpful to elaborate why and how the two regional scales are determined, for example, by providing the extent of subsidence.
- Line 287 - It is not clear which effectiveness parameters led to removal of compartmentalization and building jacking as countermeasures.
- Line 295 - Similar to my previous comment for Shanghai, I am not sure if the author’s conclusion can be effectively drawn from their limited analysis. Especially as 2 (building jacking, exfiltration sewers) out of 6 implemented or proposed countermeasures are not among the authors’ recommendations.
- Line 296 - I was unable to find any supporting information in the manuscript for the ineffectiveness of building jacking in Jakarta.
- Line 303 - What is the time period over which the subsidence of 8.53 m was observed? Additionally, was it observed over a broad area or was this the peak at a single site?
- Line 308 - Similar to my previous comments, please provide supporting evidence for selecting large/regional scale; and the determination of ineffectiveness of compartmentalization.
- Line 318 - Similar to my previous comments, the conclusion drawn by the authors seems quite subjective as 1 (sustainable water use practices) of 2 implemented countermeasures was not part of their suggestions.
- Line 350 - It is not clear to me why the authors’ methodology could only be evaluated for the 3 example studies, since all the countermeasures presented in the manuscript and their applicability in Figure 6 are derived from existing literature.
Citation: https://doi.org/10.5194/egusphere-2024-2537-RC2
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