State of the Vietnamese Coast—Assessing Three Decades (1986 to 2021) of Coastline Dynamics Using the Landsat Archive

Round 1

Reviewer 1 Report

L 38: Reference is missing for the IPCC report. This is a large report with 750+ pages. Suggest citing the appropriate sub-chapter for better orienting the readers. The way to cite each sub-chapter is presented on the first page of the respective section.

Point 7 of the previous comments: The thousand separator is not included in all instances in Tables 4 and 5. Please update accordingly.

Author Response

Dear Reviewer, 

thanks for your comments. We now cited the correct chapter within the IPCC report and also added the missing thousand separators. 

Reviewer 2 Report

I recognise that your manuscript after incorporating the reviewers' comments is significantly improved. Both validation method and also the misunderstandings about the methodology are now clearer and comprehensive.

Author Response

Dear Reviewer, 

thanks for your time and the positive feedback. 

Reviewer 3 Report

The authors have addressed the comments from reviewers. I have no further questions. 

Author Response

Dear Reviewer, 

thanks for your time and the positive feedback. 

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

This manuscript presents a freely available satellite image-based analysis to assess the shoreline variation in Vietnam over the last three decades. Understanding the shoreline variation (and its likely trends) is essential in coastal zone management. Hence, this manuscript will attract the attention of the relevant stakeholders in Vietnam. The method is reasonably explained, and the results are discussed accordingly at eroding, accretion, and stable coasts.

I have a few general comments and suggestions to improve the quality and relevance of the manuscript.

1. The second paragraph of the Introduction section presents an overview of the long-term coastline variation on a global scale. Hence, adding more related references in the global context is highly recommended. Most of the references presented in this paragraph are related to the Vietnam coastline.
2. L79: Adjust the '%' symbols. It should be 50% and 31% (not 50 % and 31 %)
3. L78 - L87: Rephrase the paragraph to clearly show the lack of national scale studies along the Vietnam coast. L78-87 does not reflect this fact properly.
4. L98: Incorrect land area for Vietnam. It should be 331,690 km²
5. L110 - L113: Not clear what coast is referred to. Further, the term 'mainland coast' in L113 could be removed as the study only considers the mainland coast of Vietnam.
6. L289, L298, L307, and L312: Change 'Chapter' to 'Section'
7. Table 4 and Table 5: Use the thousand separators correctly in column 2.
8. Figure 9: Suggest including a hot-spots scale in the legend (for the circle sizes) to better understand the erosion/accretion extent. 
9. L379: 'Figure 7' should be changed to 'Figure 11'.
10. L413: Change the last sentence to "Potential causes for errors are discussed in the following paragraphs."
11. L484 - L487: Re-write the sentence. It does not read well/smoothly.
12. At L497 and L501, Red River's mouth is referred to as 'Bat Lat mouth'. However, at L514, the same is written as 'Ba Lat mouth'. Please be consistent with the terms and phrases used throughout the manuscript.
13. L533: It is better to indicate who the authors are.
14. L555: Use a different term, such as non-erodible materials, rock outcrops, etc., to replace 'solid materials'.
15. L558- L560: Suggest to re-write the sentence. It projects an incorrect meaning, and the use of 'risk' is erroneous. In my understanding, the authors want to mention that the Vietnam coasts are vulnerable to various natural hazards, and hence there are vulnerable sectors. This idea/message is not reflected in the sentence. 
16. Throughout the manuscript, there were numerous occasions where the abbreviations were used without introducing them (e.g., L101: use of M, L131: use of SWIR, and L225: use of GADM). Make sure to introduce every abbreviation at its first appearance in the manuscript and then use the shortened form afterwards. 
17. Suggest updating the language and writing style throughout the manuscript, as there is much room for improvement. 
18. Assessing coastline change is an extensively discussed topic in the global context. Hence, suggest adding more relevant references from such studies, especially in the introduction and discussion, to relate the present research to the context.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

This research presents a new methodology for detection and evaluation of coastal changes. I found the research quite interesting and well presented. My only comment is that on low mean change rates, standard error values are quite large and the authors make no comment about this. I would like a comment about this or even the implementation of another technique only to validate the low mean change rate. I also would prefer a comparison between your results and previous research in some areas like Mekong Delta in order to validate your methodology that as you mention is difficult to validate otherwise. Finally, a minor comment is for figure 8 that I believe is useful to change the colors in order to match the legend of figure 7. 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Abstract

1. In Abstract, the author illustrated Google Earth as the datasets, while Landsat archive appeared in Keywords. It is better to describe the involved datasets clearly in Abstract.
2. Line 14-17, the author wrote three sentences in passive voice. It is better to change it as active voice to express information concisely.
3. I see subpixel coastline extraction in Keywords. It is better to demonstrate your coastline extraction method in Abstract in 1-2 sentences.

 

Introduction

1. What about the previous studies of coastline variations in Vietnam? Did any scientists focus on coastline variations in Vietnam? What method did they use? What did they find? It is better to introduce the past related work in your introduction.
2. The author used OTSU to extract coastline from Landsat dataset. Did any scientists used this method to obtain coastline from remote sensing images? In paragraph 3 in this part, I did not find any information about it.
3. The author gave us details information about coastline change, but the background about recent studies was not enough. Please give us more information about coastline studies in Vietnam and similar coastline extraction methods as you used in this study.

 

 

Materials and Methods

1. Figure 1. What is East Vietnam Sea? Does any international official organization recognize South China Sea also named as East Vietnam Sea?
2. In 2.2 Landsat Data, the author said they used Landsat dataset and only gave the information of total number of Landsat images without any other information. They described different seasons in Vietnam in the brief introduction of the study area. Are there any seasonal differences when you extract coastline from Landsat images in different season? How do you conduct this?
3. In 2.2 Validation data, the author provided the details of involved validation data in this study. As you collected Landsat data in different seasons, is it enough to collected VHR data in only 1 day to validate the Landsat coastlines? Do you calculate seasonal error?
4. Line 225, GADM must be the abbreviation of some words, please add the whole word.
5. Did you use DSAS to calculate coastline change rate? How did you consider coastline uncertainties?

If you used DSAS to calculate coastline change rate, please analyze coastline uncertainties in your article. The following articles would be helpful:

[1] Mirza, R.I.B., et al., 2020. Analysis of shoreline changes in Vishakhapatnam coastal tract of Andhra Pradesh, India: an application of digital shoreline analysis system (DSAS). Ann. Gis 26 (4), 361–376. https://doi.org/10.1080/19475683.2020.1815839.

[2] Mulick, M.R., et al., 2019. Shoreline change assessment using geospatial tools: study on the Ganges deltaic coast of Bangladesh. Earth Sci. Inform. 13, 299–316.

[3] Virdis, S., et al., 2012. A Geomatics Approach to Multitemporal Shoreline Analysis in Western Mediterranean: The Case of Platamona-Maritza Beach (Northwest Sardinia, Italy). J. Coastal Res. 28, 624–640. https://doi.org/10.2307/41508575.

[4] Himmelstoss, E.A., Henderson, R.E., Kratzmann, M.G., and Farris, A.S., 2021, Digital Shoreline Analysis System (DSAS) version 5.1 user guide: U.S. Geological Survey Open-File Report 2021–1091, 104 p., https://doi.org/10.3133/ofr20211091.

[5] Njutapvoui, F.N., Onguene, R., et al., 2021. Seasonal to decadal scale shoreline changes along the Cameroonian coastline, Bay of Bonny (1986 to 2020). Regional Studies in Marine Science. 45, 101798., https://doi.org/10.1016/j.rsma.2021.101798.

[6] Hu, X., & Wang, Y., 2022. Monitoring coastline variations in the Pearl River Estuary from 1978 to 2018 by integrating Canny edge detection and Otsu methods using long time series Landsat dataset. Catena. 209(2), 105840.

 

6. In 2.3.5 Validation, the author also described the data involved in validation in details. For instance, line 290-294. It is better to introduce the data in 2.2.2 validation data.
7. Line 300-306, the author talked about the lack of satellite-based coastline detection studies. It is better to put it in the Introduction. In methodology, just tell us how you figure out validation in this study.

 

 

Results

1. Would you please demonstrate coastline variations from 1986 to 2021? How about the length, the general condition of erosion and accretion? What is the original coastline condition in the beginning of your study? How it looks like in the end of your study?
2. In 3.3 validation, the author showed us they displaced 2167 shore-perpendicular transects between the validation shoreline (Rapid Eye) and the reference shoreline (Landsat 8). But I did not find any information about transects displacements in Materials and Methods.

 

 

Discussion

1. How about the precision of your extraction method? Do you consider coastline uncertainties in your method?
2. The author analyzed erosion hotspots and accretion hotspots in Vietnam. Did you find new hotspots compared with the previous studies?
3. In 3.2, the author described that different provinces showed different coastline conditions, it is better to analyze the reasons why some provinces show coastline accretion and the others show coastline accretion. In Figure 9, which showed the exact position of the hotspots with extreme change rate, nearly all extreme hotspots located in the ends of the coastline. Is there anything influence coastline change rate? Maybe the weather, the current and even the policies led to this. It is a quite interesting point to show the direct and potential driven factors of coastline variations in Vietnam.

 

 

Conclusions

1. This part is not well organized. The author did not conclude the condition of present coastline in Vietnam.

Author Response

Please see the attachment.

Reviewer 4 Report

The paper presents an interesting study on Vietnam coasts using Landsat images during period among 1986-2021.

The paper is well presented and many tests were performed, although the originality from other works already presented in literature is low.

At first, you should clarify the concept of "coastline" and "shoreline" which are used both in the text. As many authors state (Boak, E.H.; Turner, I.L. Shoreline definition and detection: A review. J. Coast. Res. 2005, 21, 688–703; Dominici, D., Zollini, S., Alicandro, M., Della Torre, F., Buscema, P. M., & Baiocchi, V. (2019). High resolution satellite images for instantaneous shoreline extraction using new enhancement algorithms. Geoscience, 9(3), 123.) there is an important difference between coastline and shoreline. This difference should be presented and linked to this experimentation.

About the validation, especially section 2.2 is not really comprehensible, in my opinion. The RapidEye acquires 5 image of the same place at almost the same time every 5,5 days, but then, if I understood well, have you used just one of that images? Could you please better specify how validation process has been carried out? Moreover, I think that 250 km of shoreline is too little compared to all the Vietnam shoreline. It would be good to validate also the north and the south part, as the results in shoreline accretion/erosion are different.

Minor issues:

• Does it make sense to take into account the tidal data? It can be useful if the changes in the shoreline position are greater than 30 meters (the spatial resolution of the Landsat images). Is this the case of Vietnam coasts?
• In fig. 7 and 9 I would add, for completeness, the arrow indicating the north.
• In fig. 8, the x-axis is the Proportion of "transects" or "erosion"?
• In section 3.3, line 366, writing "the reference shoreline (Landsat 8)" may lead to misunderstandings because, in general, the reference shoreline is the one that is used to validate. Maybe, it is better to write reference shoreline for the RapidEye one, and extracted shoreline for the Landsat one.
• line 424: after the : put the t in lowercase letter.
• please rewrite sentence in line 489, I think there is a typing error.

Author Response

Please see the attachment.

Author Response File: Author Response.docx