Cost-Benefit Analysis of Mixing Gray and Green Infrastructures to Adapt to Sea Level Rise in the Vietnamese Mekong River Delta
Round 1
Reviewer 1 Report
This is an interesting and topical paper. You are addressing a critical topic on which more funding will be deployed in the future. As such, robust evidence base assessing climate and socioeconomic pathways is needed and you contribute to this. This is done in a convincing manner with, however, some limitations - see below.
Major comments that are interconnected between them:
- RCP 8.5 was selected as the only scenario to be explored. please justify and link to point three. Worse case scenario is not necessarily the best approach, particularly when it has huge cost implications.
- yet, later on, different heights of dikes are discussed. could this be a way of addressing uncertainty around RCPs? (are other RCPs implicit to the different adaptation heights?)
- is there a benefit from gradual adaptation until further information is available (regarding climate change and SSPs)? see adaptation pathways methodologies, e.g. we do not know now if RCP2.5 or RCP8.5 is going to take place.
- what are the possible implications of an adaptation pathways approach to the BCR? they would be even more favourable by reducing expenditure, i.e. the denominator in the ratio
Overall, excellent and innovative use of SSPs, good discussion and comparison between green, grey and green-grey, great use of BCR and analysis of costs and benefits, as well as uncertainty (in terms of scenarios and the discount rate). Limitations relating to Multiple Benefits well pointed out.
Although some additional information on the confidence over the costs would be useful, it is understood that available information is hard to get on this. It is clear you have made efforts to be thorough.
Author Response
We sincerely appreciate the referee’ comments and the editor’s kind arrangements regarding our manuscript. We believe that our manuscript has benefited considerably from their helpful suggestions. In addition, we have studied their comments very carefully and have made the necessary revisions.
Point 1: RCP 8.5 was selected as the only scenario to be explored. please justify and link to point three. Worst case scenario is not necessarily the best approach, particularly when it has huge cost implications.
Response 1: First, we added more information about our previous study in which already assessed the uncertainty of RCP2.6 and RCP8.5 (line 225-232). Second, unfortunately, since it appears that the global community is moving along a trajectory between RCP6.0 and RCP8.5, we selected RCP8.5 for the main analysis. It is also relevant your second and third comments below.
Point 2: Yet, later on, different heights of dikes are discussed. could this be a way of addressing uncertainty around RCPs? (are other RCPs implicit to the different adaptation heights?)
Response 2: We established different SLR scenarios based on differences in the heights of dikes, which were in turn based on the current adaptation strategies proposed by the Vietnamese government. The current strategies divided the VMRD coastal area into two sides; an east side and a west side. The east side, which extends from Long An to Ca Mau, will have a dike system that has an average height of 3-4 meters. However, the maximum dike height requirement for the west side, which extends from Ca Mau to Kien Giang, is only 2.5-3 meters; thus, it is the intention of the Vietnamese Government to construct a heterogeneous dike system in the VMRD. Therefore, our discussion addresses the protection function of the sea dike system in the VMRD in the 21st century. The different dike height may be a good option to deal with the uncertainty of RCPs.
Point 3: Is there a benefit from gradual adaptation until further information is available (regarding climate change and SSPs)? see adaptation pathways methodologies, e.g. we do not know now if RCP2.5 or RCP8.5 is going to take place.
Response 3: The benefit is that extensive damage could be avoided by implementing comprehensive adaptation methods in a timeous manner As in the first comment, we added some additional background and context about our previous study, which already assessed the uncertainty associated with RCP2.6 and RCP8.5 (line 225-232). If we rely only on the current climate commitments of the Paris Agreement, temperatures can be expected to rise by 3.2°C this century, which puts us on a trajectory between around RCP6.0 and RCP8.5. We added a similar explanation in line 511-513.
Point 4: What are the possible implications of an adaptation pathways approach to the BCR? they would be even more favourable by reducing expenditure, i.e. the denominator in the ratio
Response 4: We suggested that we can reduce the expenditure of adaptation (i.e., the denominator of BCR) through careful optimization of grey and green infrastructures.
Point 5: Overall, excellent and innovative use of SSPs, good discussion and comparison between green, grey and green-grey, great use of BCR and analysis of costs and benefits, as well as uncertainty (in terms of scenarios and the discount rate). Limitations relating to Multiple Benefits well pointed out.
Response 5: We appreciate your positive comments.
Best regards,
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper tackle important and current topic. It is well organized. However, some work should be carried out to improve the manuscript. The discussion section needs to be expanded a bit. The author should pay attention to the fact the larger part of references to sources is usually found in the discussion section. The authors should compare their results with those from other studies related to the study area, to show its originality, e.g. “Using a Risk Cost-Benefit Analysis for a Sea Dike to Adapt to the Sea Level in the Vietnamese Mekong River Delta” by Vo Thanh Danh and Huynh Viet Khai of 2014.
Other comments: keywords should not repeat words already used in the title of the paper. The function of keywords is to supplement the information given in the title. Words in the title are automatically included to databases, and keywords serve as additional pointers.
Author Response
We sincerely appreciate the referee’ comments and the editor’s kind arrangements regarding our manuscript. We believe that our manuscript has benefited considerably from their helpful suggestions. In addition, we have studied their comments very carefully and have made the necessary revisions.
Point 1: The paper tackle important and current topic. It is well organized. However, some work should be carried out to improve the manuscript. The discussion section needs to be expanded a bit. The author should pay attention to the fact the larger part of references to sources is usually found in the discussion section. The authors should compare their results with those from other studies related to the study area, to show its originality, e.g. “Using a Risk Cost-Benefit Analysis for a Sea Dike to Adapt to the Sea Level in the Vietnamese Mekong River Delta” by Vo Thanh Danh and Huynh Viet Khai of 2014.
Response 1: Thank you for these comments. We expanded on the originality of the study in lines 480-485 of Section 4.1.
Point 2: Other comments: keywords should not repeat words already used in the title of the paper. The function of the keywords is to supplement the information given in the title. Words in the title are automatically included to databases, and keywords serve as additional pointers.
Response 2: We changed keywords as you suggested.
Best regards,
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Thank you for adequately addressing the previous round of comments.
Please mention explicitly that "adaptation pathways can reduce the expenditure of adaptation through careful deployment of interventions over time, i.e. by taking low regrets decisions that are viable across multiple possible futures (Kapetas, 2020). This method of adaptive pathways was first tested for delta environments by Haasnoot (2012)"
https://doi.org/10.1098/rsta.2019.0204
https://doi.org/10.1007/s10584-012-0444-2
Author Response
Point 1: Please mention explicitly that "adaptation pathways can reduce the expenditure of adaptation through careful deployment of interventions over time, i.e. by taking low regrets decisions that are viable across multiple possible futures (Kapetas, 2020). This method of adaptive pathways was first tested for delta environments by Haasnoot (2012)"
Response 1: We mentioned your points in line 515-518.
Author Response File: Author Response.pdf
