Low-regret climate change adaptation in coastal megacities – evaluating large-scale flood protection and small-scale rainwater detention measures for Ho Chi Minh City, Vietnam

Scheiber, Leon; David, Christoph Gabriel; Hoballah Jalloul, Mazen; Visscher, Jan; Nguyen, Hong Quan; Leitold, Roxana; Revilla Diez, Javier; Schlurmann, Torsten

doi:https://doi.org/10.5194/nhess-23-2333-2023

Articles | Volume 23, issue 6

https://doi.org/10.5194/nhess-23-2333-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Future risk and adaptation in coastal cities

https://doi.org/10.5194/nhess-23-2333-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 23, issue 6

Research article

|

26 Jun 2023

Research article |

| 26 Jun 2023

Low-regret climate change adaptation in coastal megacities – evaluating large-scale flood protection and small-scale rainwater detention measures for Ho Chi Minh City, Vietnam

Leon Scheiber, Christoph Gabriel David, Mazen Hoballah Jalloul, Jan Visscher, Hong Quan Nguyen, Roxana Leitold, Javier Revilla Diez, and Torsten Schlurmann

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Final revised paper (published on 26 Jun 2023)
Supplement to the final revised paper
Preprint (discussion started on 13 Oct 2022)

Interactive discussion

Status: closed

RC1:
'Comment on nhess-2022-239', Anonymous Referee #1, 04 Nov 2022
This paper compares the effectiveness of two adaptation strategies: (1) a large-scale flood

protection scheme as currently constructed in HCMC and (2) the widespread installation of small-scale rainwater detention as envisioned in the framework of the Chinese Sponge City Program (SPM). As authors claimed, it is important to explore and evaluate potential options of hazard mitigation as flood risk is becoming a major challenge for many cities in low elevation coastal zones. The topic of this study is valuable. But the quality and innovation of the current manuscript are not satisfactory. In any case, I have a few recommendations that I believe will help the authors to clarify their contribution and improve the readability of the manuscript.

Specific points are:

Most of the figures in the manuscript are very poor in quality and hard to meet the standard for this journal, such as Figs. 2, not clear enough. Generally some of the figures are too small.

How to simulate the small-scale rainwater detention in the HEC-RAS model? What is the limitation or uncertainty?

I am not convinced by the model setup given limited information, more information for the validity of flood simulation by HEC-RAS should be described in the paper.

What is " Flood Severity Index (NFSI) ", how is it measured?

Drainage capacity should be considered for the flood simulation.

In Table 1, Flood depth (dmax) is the average for all the raster cells? This seems very simple. More analysis should be done for different cells, especially considering the spatial distribution.

Spatial distribution of Flood Severity (NFSI) for different cells?

This study only considers three scenarios of the mitigation scenarios, I think more analysis considering different sponge city measures would be very interesting. You will want to discuss this.
Citation: https://doi.org/10.5194/nhess-2022-239-RC1
- AC1: 'Reply on RC1', Leon Scheiber, 23 Dec 2022
  
  Dear Reviewer,
  
  we would like to express our sincere gratitude for the time and effort you invested in studying our manuscript and for the valuable feedback you provided. You may now find our response letter attached to this comment.
  
  Thank you and best regards,
  
  Leon Scheiber
  
  Citation: https://doi.org/10.5194/nhess-2022-239-AC1
RC2:
'Comment on nhess-2022-239', Anonymous Referee #2, 14 Nov 2022

This study presents an interesting topic on low-regret climate change adaption for flood hazard in Ho Chi Minh City of Vietnam. The study has compared the effectiveness of three techniques in flood adaption strategies including: (1) a large-scale protection project using a ring dike; (2) a small-scale rainwater detention scheme; and (3) a combined application of both strategies.

The authors should consider the following specific points:

- Please include a flowchart of data processing in the manuscript for better understanding of the framework of this study.

- How the Normalized Flood Severity Index (NFSI) was generated from the input parameters?

- Line 162: Using reported data in 2010 for validation of the model is out of date. Please use the most recent field data for evaluation.

- Figure 2: all of the legends in this figure should be more clearly presented.

Citation: https://doi.org/10.5194/nhess-2022-239-RC2
- AC2: 'Reply on RC2', Leon Scheiber, 23 Dec 2022
  
  Dear Reviewer,
  
  we would like to express our sincere gratitude for the time and effort you invested in studying our manuscript and for the valuable feedback you provided. You may now find our response letter attached to this comment.
  
  Thank you and best regards,
  
  Leon Scheiber
  
  Citation: https://doi.org/10.5194/nhess-2022-239-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) (04 Jan 2023) by Liang Emlyn Yang

AR by Leon Scheiber on behalf of the Authors (02 Mar 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (16 Mar 2023) by Liang Emlyn Yang

RR by Anonymous Referee #2 (21 Mar 2023)

RR by Anonymous Referee #1 (25 Mar 2023)

RR by Anonymous Referee #3 (10 Apr 2023)

Suggestions for revision or reasons for rejection

This study evaluated and discussed the effectiveness of two flood adaption strategies in Ho Chi Minh City, including the large-scale ring dike protection scheme and decentralized small-scale rainwater detention. Significant reductions of flood hazard, as indicated by Normalized Flood Severity Index (INFS) which considering flood depth and duration, have been revealed for both strategies and their combination. The manuscript is well written, with comprehensive and meaningful analysis and discussions on the implementation of different climate change adaption strategies for HCMC, which also enlightening other areas such as LECZ and ASEAN. I enjoyed reading it. Though I believe the manuscript is publishable in the present form, there are few minor comments for the authors consideration.
1) It seems that certain precipitation scenario was used for the HCMC flood case. The uncertainties of modelling results shall be considered when comparing the effectiveness of different strategies. There are at least two issues related to the choice of precipitation scenario. First, the effectiveness of two strategies at different scales may be aimed for different kinds of flood events, for example, storm surges or pluvial floods. However, certain precipitation scenario shall not be able to reflect such differences. Moreover, how the effectiveness would change with the scale of flood is not clear enough neither.
2) The intra-city heterogeneities shall also be important, especially for the small-scale rainwater detention approach. One advantage of the decentralized rainwater detention is the feasibility to be applied to local scale, for example, the flood prone areas due to the unfavoured topography, land use, drainage system etc.. However, it is a bit hard to identify such information from the “more uniformly distributed” flood reduction of the small-scale rainwater detention strategy.

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ED: Publish subject to minor revisions (review by editor) (16 Apr 2023) by Liang Emlyn Yang

AR by Leon Scheiber on behalf of the Authors (05 May 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (10 May 2023) by Liang Emlyn Yang

ED: Publish as is (13 May 2023) by Paolo Tarolli (Executive editor)

AR by Leon Scheiber on behalf of the Authors (15 May 2023) Manuscript

Short summary

Like many other megacities in low-elevation coastal zones, Ho Chi Minh City in southern Vietnam suffers from the convoluting impact of changing environmental stressors and rapid urbanization. This study assesses quantitative hydro-numerical results against the background of the low-regret paradigm for (1) a large-scale flood protection scheme as currently constructed and (2) the widespread implementation of small-scale rainwater detention as envisioned in the Chinese Sponge City Program.