The Identification of Flood-Prone Areas in Accra, Ghana Using a Hydrological Screening Method

Round 1

Reviewer 1 Report (New Reviewer)

Comments and Suggestions for Authors

The paper titled  presents a comprehensive study on identifying flood-prone areas in Accra, Ghana. It highlights the need for improved hydrologic data and local flood management strategies, such as retention basins and better waste management practices. I have the following suggestions for the authors’ consideration:

1.       The paper emphasizes the benefits of using a 10-meter resolution DTM over the previously used 30-meter DTM. Could you provide more details on the specific improvements observed with the higher resolution data, particularly in terms of prediction accuracy and flood mapping?

2.       The study mentions the smoothing of true ground levels due to the DTM processing. How significant is this smoothing, and what impact does it have on the accuracy of flood predictions?

3.       The Arc-Malstrøm hydrologic screening software was used without incorporating detailed time steps or discharge measurements. How do these limitations affect the reliability of the flood predictions? Have you considered integrating real-time data or discharge measurements in future studies?

4.       Considering the diverse soil types in the study area, how confident are you in the infiltration rate estimations? Would obtaining localized infiltration data significantly alter the results?

5.       The paper identifies improper waste management as a significant contributor to flooding. What specific policy recommendations or interventions do you propose to address this issue?

Author Response

Author Response File: Author Response.docx

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

This manuscript is related to identification of flood-prone using a hydrological screening method based on Arc-Malstrøm software and digital terrain model with 10-meter resolution. This approach is applied in order to identify bluespots in flood-prone areas in Accra, Ghana.

Manuscript is quite long, but I need to point out that it is written below the usual standards comparing to other related papers of the corresponding author.

Although the manuscript follows the recommended form, some parts consist of excess data that does not contribute to quality of the paper. On the contrary, it makes the overall structure confusing and difficult to clearly identify the scientific contribution of the proposed approach. 

Authors need to clearly specify why this approach is different in comparison to, for instance, paper entitled “UAV‐borne, LiDAR‐based elevation modelling: a method for improving local‐scale urban flood risk assessment” published by the corresponding Author et al. in 2021.

Quality of English language must be significantly improved in the manuscript. Manuscript had to be better quality-checked before submission. Most of the references are missing in the text (Error! Reference source not found), making it impossible to efficiently track them in any way.

In the Introduction section, a clear explanation of objectives of this research is missing. Is it improvement of hydrological screening process? Impact of trash deposition on hydrological regime and flood magnitude? Or maybe application of more detailed DTM in comparison to previous ones? Without clear definition of objectives and scientific contribution this is more like a project report than a research publication.

This line is repeating in lines 186-188 and 246-216. To get an indication of how often the average rainfall for the 6 weather stations is above 60 mm in a day indicating a significant stormwater event, the data described in section 3.1 were analysed in a Python script fit for the circumstances.

How exactly is data analysed? Do you apply some statistics or some other approach? I suppose that you are using a Phyton script to fit available data to some functional form (regression etc.). Explanation is missing!

How can you explain the discrepancy between the DTM resolution of 10 meters and soil infiltration rates map (Figure 8.) on kilometer (and more) scale? How does the large uncertainty in soil infiltration affects the results?

Comments on the Quality of English Language

Quality of English language must be improved. Sentences are often too long, making it hard to understand the exact point. Manuscript should be edited by a native speaker.

Author Response

See attached file.

Author Response File: Author Response.docx

Reviewer 3 Report (New Reviewer)

Comments and Suggestions for Authors

Dear Authors,

the topic of your manuscript is very interesting although a bit scary since the amount of trash in the Odaw river channel is alarming.

The manuscript itself is a bit messy and in its current form is not suitable for publication. I get the impression that the authors have overloaded the text, which could successfully be divided into at least two separate publications, one on the litter problem and the other methodical about modelling floods in the absence of sufficient data.

The text lacks a hierarchy of the importance of the problems discussed. The first illustrations suggest that the problem of the huge amount of trash in the riverbed will be raised, but subsequent illustrations suggest that infiltration on different soil types or the geology of the area are important. After reading the entire manuscript, on the other hand, one can get the impression that the most important thing for the authors was the modelling of flooding.

Please, find below some minor comments to the text.

Change and correct the citation method. You should cite sources in the order in which they appear in the text, while you put them in alphabetical order, and then use item numbers in the text. Thus, the first cited article has the number 7 instead of 1. Also correct the brackets used for citations numbers.

Check and correct the way of citation in entire text.

The structure of the text must be organized. The introduction lacks the broader background and context of the research presented.

Part of the text from the introduction should be moved to the chapter "Study site" and part to "Materials and methods".

There is a lack of a clearly formulated purpose of the research.

Figure 5 should be moved to the chapter 3.2.

Arrows (indicators) in figures 1 and 3 are in wrong position.

The Odaw river is not indicated in figure 4

It would benefit the text if the chapter Study site were separated into subsections: Location, Climate, Geology and Soils, etc.

I think Table 2 could be illustrated with two figures, for 1-hour rainfall and for 2-hour rainfall. Still, the infiltration value assumed for ferric luvisols remains a question, as this is a densely built-up area as shown in Fig. 7 so infiltration in this area is inhibited by buildings.

The "Materials and Methods" chapter includes a description of the methods, results and a partial discussion of the results. Completely missing is the chapter "Results" in which only the obtained results should be presented. Please rewrite the entire Chapter 3 dividing the information into separate chapters: methods, results and discussion of results.

On the other hand, the “Discussion of results” chapter contains the results, discussions and elements of the methodology.

The "Conclusions" chapter needs significant shortening and rewriting. It is difficult to say whether the authors have achieved their intended goals, because, as I have already mentioned, the purpose of the work is not specified in the introduction to the article. Like the others, this chapter contains excerpts from the discussion. Please clearly list one by one the conclusions drawn from your research.

Author Response

See attached file.

Author Response File: Author Response.docx

Reviewer 4 Report (New Reviewer)

Comments and Suggestions for Authors

Please find attached.

Comments for author File: Comments.pdf

Author Response

See attached file.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

Dear Authors,

improvements have been made in the revised manuscript. However, revised manuscript still does not meet the standards.

You claim that you added additional objectives and conclusions into the manuscript. Please mark them clearly (e.g. using some other color).

Furthermore, references are still missing in the text. This is unacceptable and should be corrected in the revised manuscript!

Considering explanation considering soil infiltration, I appreciate your clarification. I believe that this explanation (at least part of it) should be implemented in the text.

Comments on the Quality of English Language

Minor issues detected.

Author Response

Please see attached document.

Author Response File: Author Response.docx

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

The manuscript aligns well with the perspective of science and its practical applications. Here are some of my comments:

1. 1. The caption of Figure 2 is missing a crucial link.

2.  

3. 2. Could you clarify the significance of the yellow shading in Table 1? Is it indicative of maximum rainfall on certain days, possibly associated with flooding events?

4.  

5. 3. Please ensure the full form of "DTM" is provided, especially if it's the first occurrence in the text.

6.  

7. 4. The conclusion section appears extensive, making it challenging to discern key findings. Perhaps consider renaming it to "Conclusion and Discussion" or condensing its content.

8.  

9. 5. It appears that supplementary materials are referenced by the authors, but I couldn't locate them.

10.  

11. 6. It's unclear from the manuscript what specific datasets or variables were simulated or generated using the Arc-Malstrøm software. How much calibrating time is required for this hydrological model/software. Clarification on this aspect would be helpful.

7. What are the limitation of this study. Because it is a bit difficult to identify the flood-prone areas, which relies heavily on the quality and availability of input data such as topography, land use/land cover, soil type, rainfall patterns, and infrastructure data. In Accra, where data availability and quality might vary across different areas, integrating these datasets into the Arc-Malstrøm software and ensuring their accuracy and consistency could pose a challenge.

Author Response

1. The caption of Figure 2 is missing a crucial link.
   Link deleted.
2. Could you clarify the significance of the yellow shading in Table 1? Is it indicative of maximum rainfall on certain days, possibly associated with flooding events?
   The following sentence was added to the end of the table’s caption for clarification: “Their deviations from the mean values and the value ranges indicate that the events are caused by convection rain appearing randomly, locally and of varying magnitude.”
3. Please ensure the full form of "DTM" is provided, especially if it's the first occurrence in the text.
   The term “digital terrain model” was added to the first occurrence of the abbreviation DTM in the Introduction section.
4. The conclusion section appears extensive, making it challenging to discern key findings. Perhaps consider renaming it to "Conclusion and Discussion" or condensing its content.
   Agree. Some paragraphs were moved to the Discussion section and the conclusion was strengthened.
5. It appears that supplementary materials are referenced by the authors, but I couldn't locate them.
   The supplementary material is a video showing the movement of trash along a minor contributing stream to the Odaw River. The video was uploaded so the journal’s editor is kindly asked to look into the missing resource.
6. It's unclear from the manuscript what specific datasets or variables were simulated or generated using the Arc-Malstrøm software. How much calibrating time is required for this hydrological model/software. Clarification on this aspect would be helpful.
   A couple of sentences were added to section 3.4 clarifying the input and output datasets and the calibration time.
7. What are the limitation of this study. Because it is a bit difficult to identify the flood-prone areas, which relies heavily on the quality and availability of input data such as topography, land use/land cover, soil type, rainfall patterns, and infrastructure data. In Accra, where data availability and quality might vary across different areas, integrating these datasets into the Arc-Malstrøm software and ensuring their accuracy and consistency could pose a challenge.
   We agree, so a conclusion hereon was added.

Reviewer 2 Report

Comments and Suggestions for Authors

In this manuscript, the authors identify areas prone to flooding in Accra, Ghana, which become roadblocks representing an important local problem.

The authors use the Arc-Malstrøm program, based on a Digital Terrain Model (DTM) that required certain corrections or hydro-conditioning (hydraulic infrastructure and Trash/garbage), to identify flood zones (landscape sinks) with water depths ≥ 0, 1 meter and volumes ≥ 5 m³.

The main purpose of the research is to demonstrate the consequences of flooding caused by a randomly placed Trash barrier.

Although the documentation is appropriate, and the effort in creating the manuscript is considerable, unfortunately I cannot highlight the novelty of the manuscript. No new elements are presented (hydrological, spatial analysis, methodology, etc.) that would add to these types of analyses, thus improving research in this field.

The research is elementary, routine, using known and generally applicable methods and approaches, with the possible exception of the use of the Arc-Malstrøm program, which is not such a widespread and well-known program. But similar simulations can be done easily using more widespread programs such as HecRas or Mike. Also, no information is presented regarding the superiority of the program used compared to other, much more widespread and accessible programs. The research results have only local applicability and validity.

Taking into account that the effort made by the authors is considerable, the subject is of medium interest, and the subject is appropriate for GeoHazards, I request the authors to highlight the novelty of the manuscript by introducing a Centralizing Table at the end of the Introduction Section, which will contain all the new elements brought by the manuscript to the scientific community.

It also recommends improving the quality of the Figures.

My decision is MAJOR REVISION.

Author Response

In this manuscript, the authors identify areas prone to flooding in Accra, Ghana, which become roadblocks representing an important local problem.

The authors use the Arc-Malstrøm program, based on a Digital Terrain Model (DTM) that required certain corrections or hydro-conditioning (hydraulic infrastructure and Trash/garbage), to identify flood zones (landscape sinks) with water depths ≥ 0, 1 meter and volumes ≥ 5 m³.

The main purpose of the research is to demonstrate the consequences of flooding caused by a randomly placed Trash barrier.
No, the purpose of this research is to point out the occurences of flood-prone areas causing temporary roadblocks. Moreover, the trash barrier is not placed randomly as it is well-known that trash piles up regularly in the Korle Lagon prior to the outlet in the Gulf of Guinea. However, this was not phrased correctly in the abstract nor elsewhere, but clarified in the resubmission.

Although the documentation is appropriate, and the effort in creating the manuscript is considerable, unfortunately I cannot highlight the novelty of the manuscript. No new elements are presented (hydrological, spatial analysis, methodology, etc.) that would add to these types of analyses, thus improving research in this field.
The novelty of this paper is on the benefits of using of a carefully hydro-conditioned 10 m DTM in predictions of flood prone areas vs. earlier studies having used 30 m DTMs in their research. But also on the benefits of using the Arc-Malstrøm software in a first, quick approximation of the flood prone areas for a region where data on rainfall, soils, land use and more is very sparse, but also because river discharge measurements for calibration of high-end hydro-dynamic models are lacking. Furthermore, we have demonstrated how to simulate the realistic flood consequences from a local piling of trash in the Korle Lagoon at the Odaw River’s outlet into the Gulf of Guinea confirmed by local ground truth observations.

The research is elementary, routine, using known and generally applicable methods and approaches, with the possible exception of the use of the Arc-Malstrøm program, which is not such a widespread and well-known program. But similar simulations can be done easily using more widespread programs such as HecRas or Mike. Also, no information is presented regarding the superiority of the program used compared to other, much more widespread and accessible programs. The research results have only local applicability and validity.
As mentioned above, the superiority of the Arc-Malstrøm software is that in data sparse countries world-wide where only a decent DTM is available, it is possible in a very few minutes to have a first approximation of flood prone areas. The only skill required is to execute a single workflow in ArcGIS Pro pointing the DTM and which uniform rain scenarios to include. Please feel welcome to contacting me to obtain a copy of the software.   

Taking into account that the effort made by the authors is considerable, the subject is of medium interest, and the subject is appropriate for GeoHazards, I request the authors to highlight the novelty of the manuscript by introducing a Centralizing Table at the end of the Introduction Section, which will contain all the new elements brought by the manuscript to the scientific community.
Thanks for the recommendation. Instead of adding a table as suggested, we have re-phrased main parts of the Abstract and the Introduction.

It also recommends improving the quality of the Figures.
As there is no specific comments or enhancement requests to any figures, we have only corrected Figure 15 that by mistake was missing a legend.

My decision is MAJOR REVISION.

Reviewer 3 Report

Comments and Suggestions for Authors

A coherent analysis with significant results.

Author Response

Thank you~

Reviewer 4 Report

Comments and Suggestions for Authors

No critical comments or suggestions for future steps. Your work covers all aspects related to the identification of flood prone areas in the data scarce situation.

Author Response

Thank you~

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Great

Reviewer 2 Report

Comments and Suggestions for Authors

The authors did not respond appropriately to my observations. Their refusal to highlight the possible new elements presented in the manuscript shows that it does not contain new elements. 

Carrying out simulations based on DTM is not a novelty, there are, in the literature, many articles that carry out simulations based on DTM, but which, in the presented methodology, introduce innovative approaches useful to the scientific community (aspects that are not found in the present manuscript). 

Thus, considering the answers of the authors, as a responsible reviewer I cannot agree with the publication of a manuscript that does not have at least one innovative element.