the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Assessment of coastal inundation triggered by multiple drivers in Ca Mau Peninsula, Vietnam
Abstract. The Ca Mau Peninsula plays a critical role in the agricultural and aquacultural productivity of the Vietnam Mekong Delta (VMD), central to regional food security and the population’s economic and social welfare. Unfortunately, this region has also historically been a hotspot for natural disasters, particularly from flooding, which is initiated by seasonal river flux upstream and heightened sea levels downstream, but also exacerbated by global climate change (e.g., increased rainfall and sea-level rise, tropical storm surges) and human activities (e.g. river bed lowering, land subsidence). The potential risks associated with rising inundation levels is important information for the future sustainability of the region and its ability to adapt to both current and forthcoming changes. The research around the influence of such drivers on future flood risk, in the Ca Mau Peninsula, is incomplete, primarily due to the absence of a quantitative coastal inundation map corresponding to future compounded scenarios. In this study, we therefore evaluate flooding dynamics in the Ca Mau peninsula using a fully calibrated 1D model, to represent a range of anthropogenic and climate change compound scenarios. Our findings indicate that factors such as increased high-flows upstream, alterations in the riverbed of the main Mekong channel, and occurrences of storm surges effecting the mainstream Mekong River, are unlikely to significantly affect inundation dynamics in this region. However, land subsidence, rising sea levels, and their combined effects emerge as the primary drivers behind the escalation of inundation events in the Ca Mau peninsula, both in terms of their extent and intensity, in the foreseeable future. These results serve as vital groundwork for strategic development and investment as well as for emergency decision-making and flood management planning, providing essential insights for shaping development policies and devising investment strategies related to infrastructure systems in an area which is rapidly developing.
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Status: final response (author comments only)
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CC1: 'Suggested improvements for nhess-2024-107', Philip S.J. Minderhoud, 24 Jul 2024
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AC1: 'Reply on CC1 Dr. Philip S.J. Minderhoud', N.N. Hung, 07 Aug 2024
Dear Dr. Philip Minderhoud,
Thank you for your kind reading and very helpful feedback, which will considerably improve our manuscript. I will review and edit the manuscript based on your suggestions.
Best Regards
Hung
Citation: https://doi.org/10.5194/nhess-2024-107-AC1 -
AC4: 'Reply on CC1', N.N. Hung, 05 Feb 2025
Dear Dr. Philip,
We would like to sincerely thank you for your time and thoughtful consideration in reviewing our manuscript. We found that your feedback has been extremely helpful in guiding us to finalize and enhance the quality of the paper.
Please find the attached file, where we have provided a point-by-point response to your comments.
Once again, we deeply appreciate your valuable insights and contributions.
Kind regards,
Hung
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AC1: 'Reply on CC1 Dr. Philip S.J. Minderhoud', N.N. Hung, 07 Aug 2024
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RC1: 'Comment on nhess-2024-107', Anonymous Referee #1, 19 Aug 2024
The authors present a well-designed study modeling various flooding scenarios in the Ca Mau Peninsula. They have identified a research gap dealing with regional flooding dynamics, including compound flooding, and through the simulation plan, aim to contribute to filling this gap. Simulations were defined by discharge, storm surge occurrence, lowering of the river bed, land subsidence, and sea level rise, considering SSP5-8.5 scenarios. They conclude that land subsidence, rising sea levels, and their combined effects are the principal drivers behind increased inundation events. Overall I think the manuscript is of high quality with impactful results, my feedback deals mainly with the discussion of the modeling approach and the discussion section in general. I support the suggestions outlined in the community comment (CC1) and there is no need to add repeat suggestions. My review is structured as major points and line-by-line suggestions/feedback.
Major points:
The limitations of 1D hydrodynamic modeling and not considering sediment transport should be discussed or further justified.
What were the roughness values used (lines 182)? Where were they different? And how do they compare to hydraulic manuals in terms of used values vs. guideline values (e.g. Chow, 1959)? If they are different from guideline values this should be explained.
Line 454-458: Potential mitigation measures, if discussed, require a much more thorough assessment with references. I think expanding on this section and connecting it to the natural (nature-based) storm surge buffering effects of mangrove forests would enhance this section while connecting to the importance of the ecological system. How would one establish a freshwater ecological zone within saline environments (lines 449-450)? Additionally, model limitations should be addressed in greater detail in the discussion section.
De Dominicis, M., Wolf, J., van Hespen, R. et al. Mangrove forests can be an effective coastal defence in the Pearl River Delta, China. Commun Earth Environ 4, 13 (2023). https://doi.org/10.1038/s43247-022-00672-7
Line by Line Suggestions:
Line 16: I would remove historically since been implies past tense.
Line 32: Remove comma after area
Line 47: Ward et al., 2018 after Wahl et al., 2018
Line 55: Suggest amplify instead of worsen
Lines 57-58: I would add and before “to facilitate”
Line 66: References in chronological order
Line 66: Suggest: …, which is a major concern… (keep as 1 sentence)
Line 69: Suggest: I would remove penetration since intrusion inland describes the same thing or you could say the extent of the inland tidal intrusion
Line 73: the IPCC report
Line 86: reference prior research
Line 116: Currently
Line 304: DEM of the region
Line 305: ArcGIS model builder?
Line 405: why not put the units with the number in figure 5 (and 6)?
Line 477: delete with after with
Citation: https://doi.org/10.5194/nhess-2024-107-RC1 -
AC3: 'Reply on RC1', N.N. Hung, 05 Feb 2025
Dear Referee #1,
We would like to sincerely thank you for your time and thoughtful consideration in reviewing our manuscript. Your feedback has been extremely helpful in guiding us to finalize and enhance the quality of the paper.
Please find the attached file, where we have provided a point-by-point response to your comments.
Once again, we deeply appreciate your valuable insights and contributions.
Kind regards,
Hung
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AC3: 'Reply on RC1', N.N. Hung, 05 Feb 2025
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RC2: 'Comment on nhess-2024-107', Anonymous Referee #2, 13 Dec 2024
This study aims to investigate multiple drivers of the compound flood risk in Ca Mau Peninsula, Vietnam using a 1D hydrodynamic model under different simulation scenarios. Results indicated that the primary drivers for the escalation of inundation events in the study area are land subsidence, rising sea levels, and their combined effects. Overall, the study is comprehensive, and the findings are meaningful. The community member and the other reviewer have provided constructive comments, upon which I have several additional concerns and suggestions regarding the methodology and results of this study as follows.
Methodology
1) 1D hydrodynamic models have some limitations compared to 2D models especially when we are interested in the flood inundation extents for relatively flat floodplains and urban areas. How did you justify the validity of applying a 1D model to the study area?
2) The definition of risks may be slightly different in literature, either including three primary factors, i.e., hazard, vulnerability, and exposure, or the probability of extreme events and the consequences due to their occurrence. However, this study mainly focused on the possible scenarios of flood inundation depths and extents (or hazards) rather than the consequences due to the floods. I suggest authors keep the terms (e.g., risk and vulnerability) consistent with what is commonly used in relevant literature.
3) What is the time interval used in the model simulation? Hourly and Daily, or adjustable time steps based on the stability of the hydrodynamic model?
4) Previous studies have shown that the roughness parameter is a key factor in 1D flood modeling and roughness coefficients tend to change at different water depths. Even though the model used in this study has been fully calibrated, how did you make sure the calibrated parameters are still applicable under extreme flood events?
5) NSE, deviation, and R2 were employed in the flood model calibration and validation processes. However, the weaknesses of these metrics should be noted, and it is suggested to apply the metrics to the flow periods of interest (high flows in the case) and present the values of metrics with a statistical distribution instead of a fixed number given the sampling uncertainty. The authors can refer to the article below for more information about the limitations of these evaluation metrics.
Reference: “Beyond a fixed number: Investigating uncertainty in popular evaluation metrics of ensemble flood modeling using bootstrapping analysis” (https://doi.org/10.1111/jfr3.12982)
6) In Figure 4, what is the (linear) regression model the R2 is measuring? Also, please note that R^2 may not capture the bias in the model prediction.
7) Line 304: “Areas with a depth < 0.1 m are classified as unflooded.” What is the basis of this assumption? The water depth in Figure 1 (right) may be less than 0.1 m. Flood water with a depth of less than 0.1 m but with a high velocity can be dangerous in urban areas.
Results
8) Figure 4: Why are some data points of water discharge negative?
9) Figure 5: The maps for S4_a and S4_b scenarios are missing?
10) Table 6: The accumulated increase in flooded areas for S1 at the level of 0.1-0.4 m is 43.0%? At least it is not true based on the results in Table 5. Please double-check the results presented in the tables, which will affect your conclusions.
Minor Issues
11) It is suggested to add a north arrow and a scale bar to Figure 2, Figures 5-7, and Figure A1. What are the units of the numbers along the box in Figures 2(a) and A1?
12) Line 273: What do the numbers in “B1.5” and “B2” stand for?
13) Table 4: The caption above S6 should be “Scenarios based on multiple drivers” instead of “individual drivers”.
Citation: https://doi.org/10.5194/nhess-2024-107-RC2 -
AC2: 'Reply on RC2', N.N. Hung, 05 Feb 2025
Dear Referee #2,
We would like to sincerely thank you for your time and thoughtful consideration in reviewing our manuscript. Your feedback has been invaluable in helping us finalize and improve the manuscript paper.
Please find the attached file, where we provide a point-by-point response to your comments.
Once again, we deeply appreciate your insightful contributions.
Kind regards,
Hung
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AC2: 'Reply on RC2', N.N. Hung, 05 Feb 2025
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The paper presents some of the first insights into compound flooding in the Mekong Delta. By providing a range of scenarios, including variations in discharge, storm surge, lowering of the riverbed and integration of (extraction-induced) land subsidence and sea-level rise, the authors aim to attribute modelled inundation to the drivers investigated. They conclude that relative sea-level rise (i.e. land subsidence and absolute sea-level rise) constitutes the most to increase inundation in the delta.
We think this paper addresses a so far huge research gap and can make a significant contribution to the advancement of exposure and risk assessment in the Mekong Delta and the investigation of compound flooding and their local specific characteristics. However, we found some critical aspects that have not been touched by the authors but which are necessary to be at least discussed if they cannot be considered in the processing. We briefly address the major points by listing them as follows and also provide a section-wise feedback:
Details and references to the datasets used are missing. The datasets and their quality also need to be reflected in the discussion section.
Minor comments:
Introduction
Methods
Results
Discussion
Conclusions