Geospatial Analysis of Flood Susceptibility in Nigeria’s Vulnerable Coastal States: A Detailed Assessment and Mitigation Strategy Proposal

Round 1

Reviewer 1 Report (Previous Reviewer 1)

Comments and Suggestions for Authors

Comments for author File: Comments.pdf

Author Response

Point to point Response to the Reviewers-1 Comments

Reviewer’s Comment 1. The manuscript presents a framework for geospatial analysis of flood risk in extensive coastal delta areas. The study area includes two coastal states in Nigeria, characterized by high vulnerability. The proposed framework relies on processing a composite dataset comprising topographic data and satellite images, by means of Google Earth Engine (GEE) to define risk levels resulting from proximity to water bodies, vegetation health, and surface moisture. The authors present the analysis results through traditional thematic maps, which identify areas at different levels of risk and frequency histograms that highlight the quantitative distribution of risk levels in the specific region. The type of representation helps to assess areas that could be prioritized for mitigation interventions.

General impressions include that the introduction is well written and quite understandable. The narrative is flowing and the motivations of the work are clearly presented. Figures and Tables are well readable. Please ensure that the figure numbers cited in the text correspond accurately to the captions (after Figures 6 and 7). The results are well depicted, thoroughly commented on, and adequately discussed. After Rejection, the Authors have revised the manuscript’s contents, taking into account the observations made by the Reviewer after the initial review. Overall, the resubmitted paper’s contents are now more concise. The revised flowchart is certainly improved compared to its first version. However, a concern rises about the transition (if any) from hazard maps to risk levels. I argue about the need of addressing to “flood hazard risk”. Clarification of this point is essential. Are the maps “hazard maps” or “risk maps”? It seems to me that too much expression has been used to address in some cases the same thing. If Risk has intended, as usual, as the result of Hazard factors, Exposure, and Vulnerability, please clarify what are the factors related to Hazard, what are the Vulnerability factors, and what are the elements exposed to risk. Could impact-based descriptions be associated with risk levels? What consequences are associated with each risk level?

Response 1: We sincerely appreciate your comprehensive review and valuable feedback, which has helped us refine our manuscript further. Please find below the actions we have taken to address your concerns: -

We have revised the manuscript to clarify the terminology used to describe our maps and analyses. The term "flood susceptibility" is now consistently used throughout to describe our maps, which focus on the potential for flooding based on environmental and geographical data alone. This term is used to reflect that our assessments do not incorporate detailed risk evaluations involving economic losses or human casualties, which would traditionally fall under "risk assessments.

The "Flood Hazard Score Risk Map" has been renamed to "Flood Susceptibility Map" to avoid confusion, ensuring that our terminology aligns with the scope of our data and analytical focus.

 

 

 

Specific comments

Reviewer’s Comment 2. The Authors refer to “high resolution data”. What is the resolution of the data? What is the overall resolution of the output maps?

Response 2: We have provided detailed information on the spatial resolution of the datasets used in our analyses. The primary datasets, including the Joint Research Centre (JRC) Global Surface Water dataset and the Shuttle Radar Topography Mission (SRTM) data, are processed at a 30-meter resolution. This is consistent throughout our analysis to maintain data integrity.

Output Maps: The flood susceptibility maps are also generated at a 30-meter resolution, providing detailed and actionable insights for local government and disaster management authorities. This detail has been added to Section 2.2.1 of the Materials and Methods.

Minor issues

Some articles appear multiple times in the bibliography. I believe it is necessary to correct this. (e.g. L. 504 and 550 , 502 and 554)

We have corrected all instances where terms or references were redundantly used or cited in the bibliography, ensuring each citation is unique and appropriately placed.

 

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

The work is interesting, and the text is well based on the scientific scope of quantitative assessment of flood risks, which impacts on the Nigerian coast deserve further study in the context of the intensification of climate extremes on a planet undergoing climate change. The results demonstrated a relevant contribution to risk mapping based on state-of-the-art geospatial technology, which, there is no doubt, can be useful for planning and implementing mitigation and adaptation strategies in the face of escalating extreme climate events.

 

Figure 3 was not mentioned in the text of the study area (item 2.1).

 

It is important to clarify the reference years of some geospatial data. What year is the surface water dataset? Does NDVI cover any period of years? or did you use a specific year? (it is not coherent to use a rainy or dry year). Please clarify these points.

 

In the methodology, items 2.2.1 to 2.2.5, there was no mention of the spatial resolutions in km of each database used. And the final products with the risk maps are at what spatial resolution?

 

In item 2.2.7 and in the Flowchart in figure 4: it was not clear how the final product was calculated into categories, i.e., what metric was used to categorize the risk? Please clarify this point.

 

The authors highlight the use of hydrological modeling... What model? This was not described in the methodological procedures. Please check this point.

 

The discussion and conclusion are appropriate and coherent with the objectives proposed in the work.

 

Author Response

Point to point Response to the Reviewers-2 Comments

The work is interesting, and the text is well based on the scientific scope of quantitative assessment of flood risks, which impacts on the Nigerian coast deserve further study in the context of the intensification of climate extremes on a planet undergoing climate change. The results demonstrated a relevant contribution to risk mapping based on state-of-the-art geospatial technology, which, there is no doubt, can be useful for planning and implementing mitigation and adaptation strategies in the face of escalating extreme climate events.

 

Comment 1. Figure 3 was not mentioned in the text of the study area (item 2.1).

 Response 1. Cited in the text

 

 

Comment 2. It is important to clarify the reference years of some geospatial data. What year is the surface water dataset? Does NDVI cover any period of years? or did you use a specific year? (it is not coherent to use a rainy or dry year). Please clarify these points.

Response 2. Thank you for your insightful comments regarding the clarification of the reference years for the geospatial data used in our study. Your feedback has prompted us to provide additional details to enhance the transparency and reproducibility of our research.

1. Reference Years for Geospatial Data:
• Surface Water Dataset: The Joint Research Centre (JRC) Global Surface Water dataset, as used in our study, spans the years 2015 to 2020. This period was selected to capture recent water occurrence trends essential for assessing flood risk.
• Normalized Difference Vegetation Index (NDVI): For NDVI, we used Landsat 8 imagery from the year 2022. This single year was specifically chosen to avoid the confounding effects of inter-annual climatic variability and to provide a snapshot of vegetation health reflective of current conditions.

We acknowledge that the selection of specific years, especially for NDVI, must carefully consider seasonal variations to avoid skewing the analysis towards a particularly wet or dry year. Therefore, we ensured that the NDVI data represented a composite of images throughout the year to mitigate seasonal bias.

 

 

Comment 3. In the methodology, items 2.2.1 to 2.2.5, there was no mention of the spatial resolutions in km of each database used. And the final products with the risk maps are at what spatial resolution?

 

Response 3. Thank you for your comment highlighting the need for clarity regarding the spatial resolutions of the datasets and the resolution of the final flood risk maps used in our study. We acknowledge that this information is crucial for understanding the scope and detail of our analysis, and apologize for the oversight in our initial submission.

Revisions Made:
We have updated Section 2.2 "Methodology for Flood Hazard Assessment Using Google Earth Engine" to include the specific spatial resolutions of each dataset used in our analysis as well as the spatial resolution of the final products. Here are the key updates:

• JRC Global Surface Water dataset is processed at a spatial resolution of 30 meters, which is critical for accurately identifying and classifying permanent water bodies.
• Landsat 8 imagery, used for deriving NDVI and NDWI, is also utilized at a 30-meter resolution. This resolution is adequate for assessing vegetation health and surface moisture effectively.
• SRTM data, employed for elevation and topography analysis, is available at a 30-meter resolution, ensuring detailed terrain vulnerability assessments.
• The final flood risk maps are produced and visualized at a 30-meter resolution. This resolution was selected to provide a detailed and actionable output, suitable for local-scale planning and risk management.

These amendments are intended to enhance the manuscript by providing comprehensive details on the methodologies and technologies applied, thereby improving the transparency and reproducibility of our work.

 

 

Comment 4. In item 2.2.7 and in the Flowchart in figure 4: it was not clear how the final product was calculated into categories, i.e., what metric was used to categorize the risk? Please clarify this point.

 

Response 4. Thank you for your feedback. We have updated Section 2.2.7 and Figure 4 in the manuscript to clearly explain the metrics used for categorizing flood risks. The updates specify that risk categorization is based on a composite score derived from key factors including distance from water bodies, elevation, vegetation health, and surface moisture. Each factor is scored from 1 to 5, aggregating to a total score that categorizes risk from very low to very high. These revisions ensure the methodology behind the flood risk categorization is transparent and comprehensible.

 

Comment 5. The authors highlight the use of hydrological modeling... What model? This was not described in the methodological procedures. Please check this point.

Response 5. Thank you for your observation regarding the mention of hydrological modeling in our study. Upon reviewing your comment, we realized that the term "hydrological modeling" may have been misleading as it traditionally refers to specific simulation software or models. In our manuscript, the analysis conducted through Google Earth Engine, though methodologically rigorous and involving hydrological factors like water occurrence, distance from water bodies, and topographic data, does not utilize a conventional hydrological model such as HEC-HMS or SWAT.

 

Comment 6. The discussion and conclusion are appropriate and coherent with the objectives proposed in the work.

Response 6. Thank you for your positive feedback on the coherence and appropriateness of the discussion and conclusion sections of our manuscript. We are pleased to hear that these sections align well with the objectives we set out to achieve

 

 

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

Comments and Suggestions for Authors

Comments for author File: Comments.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors,

I have attached below my suggestions for your article.

Comments for author File: Comments.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript entitled “Geospatial Analysis of Flood Hazard Risks in Nigeria's Vulnerable Coastal States: A Detailed Assessment and Mitigation Strategy Proposal”constitutes a general description of the flood risk analysis performed for Flood Hazard Risks in Nigeria's Vulnerable Coastal States. In its current version, the designation of areas at risk of flooding is performed with measures which are not known, and the criteria for dividing of these areas into five categories are not provided. Two indicators, NDVI and NDWI, appear in the text which are calculated on the basis of Landsat-8 images. However, no definition or interpretation of them are presented in this manuscript.

 

Unfortunately, the presented aim of the manuscript "research is to develop a comprehensive and scalable flood risk assessment model aimed at improving disaster management practices, informing mitigation strategies, and guiding policy decisions across diverse geographical settings" has  not been executed in the manuscript. Moreover, assessing by the layout of the manuscript itself, over 25% of the text is devoted to the introduction, 1% to the methodology, 25% to the results (presented in the form of a quantitative evaluation without qualitative criteria), and 5% to the discussion. Since the methodology is a basis for the scientific evaluation of the article and the current version is missing, my assessment of the article is negative.