Review Reports - Weakening of Coastlines and Coastal Erosion in the Gulf of Guinea: The Case of the Kribi Coast in Cameroon

Round 1

Reviewer 1 Report

The authors have assessed the coastal kinematics or even the weakening of the Kribi coast and quantified the levels of retreat, accretion and stability between 1973 and 2020. In addition, the factors of the evolution of the coastline have also been discussed. While this paper seems interesting, it suffers from some weaknesses that lead me to recommend its major revision from publication. The following are examples of such concerns.

First, this study employed different types of remote sensing images to detect the changes in the coastline over a long period. However, the spatial resolution of the Landsat images is 30 m, and the Google Earth images of 2013, 2018 and 2020 have a spatial resolution of 0.5 m. Therefore, if two types of remote sensing images are used simultaneously to detect shoreline changes, a specific scientific algorithm is required for spatial alignment and fusion due to the different spatial resolutions of the two images, which is not seen in this paper to be handled by the authors. In addition, the analysis results of this work show that the average decline is estimated at +1.9 m/year over the entire area. However, the resolution of the Landsat satellite images is around 30 m, and it isn't easy to conclude 1.9 m/year from 1973 to 2020 with the only available Google Earth images of the years 2013, 2018 and 2020.

Second, it is suggested that the authors add a literature review of previous studies on this topic in the first part of the manuscript.

Third, equation 2 and equation 3 in the text look like low-resolution pictures and need improvement.

Author Response

Response to Reviewer 1

Dear Reviewer, Thank you for your remarkable contribution to enhancing the quality of this article.

First, this study employed different types of remote sensing images to detect the changes in the coastline over a long period. However, the spatial resolution of the Landsat images is 30 m, and the Google Earth images of 2013, 2018 and 2020 have a spatial resolution of 0.5 m. Therefore, if two types of remote sensing images are used simultaneously to detect shoreline changes, a specific scientific algorithm is required for spatial alignment and fusion due to the different spatial resolutions of the two images, which is not seen in this paper to be handled by the authors. In addition, the analysis results of this work show that the average decline is estimated at +1.9 m/year over the entire area. However, the resolution of the Landsat satellite images is around 30 m, and it isn't easy to conclude 1.9 m/year from 1973 to 2020 with the only available Google Earth images of the years 2013, 2018 and 2020.

We have taken into account this valuable comments well, showing that the choice of the two image sources is justified by the fact that the Landsat (30 m) are available and cover a long period of observations on the Kribi coast (1973-2020 ). And, given their sufficiently low spatial resolution, we used Pleiades images, acquired with the support of Professor Ferrari from the University of Liège. It is therefore not a question of making a comparative analysis, but rather of having a clear and fairly precise idea of the rates of retreat and accretion of the Kribian coast in full dynamics. The work having taken place within the framework of a doctoral thesis. The elements of the response are provided on page 15-16

Second, it is suggested that the authors add a literature review of previous studies on this topic in the first part of the manuscript.

To avoid lengthening the article too much, and taking into account your judicious remark, we have added a small non-exhaustive literature review to the first part of the article, (p.2).

Third, equation 2 and equation 3 in the text look like low-resolution pictures and need improvement.

Thank you for the suggestion. We have re-written equations 2 and 3, as you can see below (p. 19)

(2)

(3)

Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript presents the results of a study that employs satellite-imagery analysis to evaluate coastal erosion and stability along the Kribi coast of Cameroon. The rise of agro-industrial plantations is considered as a cause of substantial coastal degradation.

My feedback, comments, and questions are indicated below:

Grammatical errors. The manuscript needs significant copy edits from this perspective. I indicate some of the issues over the Introduction, but I did not correct all of them. There are tense issues, incorrect word usage, repeated words and phrases, omitted words, and sentences that need to be reworded and restructured. These issues detract from understanding the content of the manuscript. It needs to be proofread and edited.

Line 29: “coastline” instead of “linear”?

Line 40: “The African continent . . .”

Line 41: consider different word choice for “polarize”

Line 43: “This strong growth of the urban . . .”

Line 44: “. . . migratory phenomenon as its main driver [2].”

Line 47: What do you mean by “weakening”? Be more specific.

Line 48: delete “the situation”

Lines 49-50: “Coastal erosion is the primary concern of the present study.”

Lines 53-54: Delete the last sentence of this paragraph.

Line 55: “. . . Guinea also experience various natural . . .”

Lines 61-62: Explain how deposition of materials leads to erosion and significant deterioration of the coastline.

Lines 62-67: What types of visible erosion are occurring?

Lines 68-69: “. . . experiencing recent shoreline adjustment, resulting in . . .”

Line 69: What do you mean by loss of weight? Loss of sediment?

Line 71: “This explains . . .”

Line 73: “. . . this coastal area around . . .”

Lines 78-80: Reconstruct these sentences.

Lines 83-84: “has orchestrated” instead of “have orchestrating”

Line 87: “. . . as is the case . . .”

Lines 88-90: This sentence is confusing and seems like multiple ideas are expressed at once. Consider adding a second sentence.

Line 102: “The coastal section observed in this study . . .”

Line 104: What do you mean by “second lung”?

Figure 1: Some of the text is difficult to read. Include fewer localities. Capitalize “Ocean”.

Line 116: What do you mean by “less than 100 meters”? In elevation? In width?

Lines 151-152: This sentence is confusing and needs to be reconstructed.

Lines 157-158: What is the approximate slope angle?

Lines 155-162: Significant grammar and sentence construction issues in this paragraph.

Line 169: Can more detailed information be provided? How narrow is the beach? How steep is the intertidal zone?

Figure 4: How were the profiles generated?

Lines 175-177: Not necessary.

2.3.2-2.3.4: What is the average channel gradient of these rivers?

2.3.2: Provide similar description about upstream topography and land cover as you do with sections 2.3.3-2.3.4. Why is this river so silty?

2.3.3-2.3.4: What is the sediment discharge for these rivers, similar to what you expressed for the Kienké in section 2.3.2?

Figure 6: Scale is needed. What do 1^st, 2^nd, and 3^rd measurements refer to? Are these different times of the year?

Figures (general comment): The size of some of the text on the figures is often too small to read. Figures need to be reviewed at the scale they will be published to determine if the text is legible.

2.8: Does the shape of the coastline influence the direction of currents? The 3^rd measurement of Figure 6 appears to indicate such an effect.

Lines 298-299: How does this graph show the impact of the port if construction only began in 2007? How does the port exacerbate sedimentation?

Line 347: “ not needed before However

3.5.1: Exactly how was the coastline determined then? Are you stating that you used different definitions to demark the coastline depending on available data and visual evidence?

Lines 370-371: What factors determined transect spacing and length?

Lines 404-406: This sentence is taken word-for-word from Temporal (1948-2012) and Dynamic Evolution of the Wouri Estuary Coastline within the Gulf of Guinea by Fotsi et al. (2019).

At this point, I will no longer continue to review the manuscript. Plagiarism in any form is simply unacceptable. How am I to trust that other portions of the manuscript are not also plagiarized in a similar manner? For now, I will not recommend rejecting the manuscript, but a major revision is needed that addresses the extensive grammatical errors, sentence construction issues, and plagiarism concern.

Author Response

Response to Reviewer 2

Dear Reviewer, Thank you for the deep and proof reading of our paper and the way you help us to enhance this piece of work by your valuable comments.

My feedback, comments, and questions are indicated below:

Thank you for this valuable comment: First of all, it is through that we are not English speaking authors. We try to submit the text to a trilingual (French-English-Spanish) colleague who has proofread it entirely.

Figure 4: How were the profiles generated?

Thank:

We observed the coast profile and we tried to apply and match it as well as possible with Hegge (1994) classification as you can see explanation below (p.6-7)

According to Hegge's [43] classification of coastal morphotypes, this rocky coast has a very narrow beach and a relatively steep intertidal zone sometimes not exceeding 2 m. It is characterized by a strong step-like break in the subtidal zone. From north to south, the first break in slope is located towards the Lobé Falls. It increases from almost 2% to almost 5%. Another break is present to the north of the Kribi seaport. The coastline profiles shown in Figure 4 illustrate the complex configuration of the Kribi coast.

And also the comments provided under the figure 4 (p.7)

Profile 1 characterises these relatively small beaches, with a forebeach of less than 10 m and a swash zone of less than 5 m, and moderate slope breaks along the profile. Profile 2 corresponds to the staircase morphotype. This is a very narrow beach with a relatively steep intertidal zone. It is characterised by a strong break in slope in the subtidal zone. This type of profile can be found at the Lobé Falls in Kribi. Finally, the third profile presents a flat morphotype characteristic of wide beaches, with a uniform profile with no break in slope. These profiles are characterised by the flattest swash and surf zones. The flattening extends into the submerged area.

Lines 175-177: Not necessary.

Thanks for the suggestion. These lines have been removed

2.3.2-2.3.4: What is the average channel gradient of these rivers?

2.3.2: Provide similar description about upstream topography and land cover as you do with sections 2.3.3-2.3.4. Why is this river so silty?

Your valuable suggestion help us to improve this section in two ways: (page 8-9)

We give and in-depth description of upstream topography and land cover as you require.

We have given a broad comment on it and one thing to know is that these rivers are flowing from forestry hinterland made up of gneissic rocks with poor quartz

2.3.3: What is the sediment discharge for these rivers, similar to what you expressed for the Kienké in section 2.3.2?

thank you: Sediment transport for the Lobe River is estimated at 11.1 m3/year, with a strong seasonal variation (p 9)

Figure 6: Scale is needed. What do 1^st, 2^nd, and 3^rd measurements refer to? Are these different times of the year?

2.8: Does the shape of the coastline influence the direction of currents? The 3^rd measurement of Figure 6 appears to indicate such an effect.

Thanks Dear Reviewer

A series of three measurements were made around the Kribi port site (1^st, 2st, 3st). The results obtained show little variation in the speed of the currents. They vary from 0.3 m/s to 0.6 m/s. (figure 9). It can be concluded from the third measurement that the shape of the Kribi coastline influences the direction and speed of the currents (p 13).

Lines 298-299: How does this graph show the impact of the port if construction only began in 2007? How does the port exacerbate sedimentation?

Indeed, sedimentation was generally present on the Kribian and Cameroonian coastline [53]. However, since the beginning of the port construction in 2007, this sedimentation seems to have increased, given the hydro sedimentary disturbances observed in situ. Thus, there is a high accumulation of sediment to the south of the harbour, due to the presence of the harbour's protection dam. This should be complete by the analysis of the bathymetric configuration of the Cameroonian coast to understand its influence on the sequence of marine processes (p. 14)

3.5.1: Exactly how was the coastline determined then? Are you stating that you used different definitions to demark the coastline depending on available data and visual evidence?

We opted for the wet sand boundary, as did in a similar study based on Landsat images by some authors [63]. The rectilinear nature of the Kribi coastal profile allows this boundary line to be well visualized on the images, whether at high or low spatial resolution (p17)

Lines 370-371: What factors determined transect spacing and length?

The main factor that determines the "transect spacing and length" is the spatial resolution of the images used and the gap between the baseline and the oldest coastline.

Lines 404-406: This sentence is taken word-for-word from Temporal (1948-2012) and Dynamic Evolution of the Wouri Estuary Coastline within the Gulf of Guinea by Fotsi et al. (2019).

Valuable reviewer, this is a misfortune. The paper is based on a PhD thesis defended in January 2022 at the University of Liege by Mr Mbevo under the co-supervision of Prof Hubert Ferrari & Tchindjang. University of Liège in its policy fight against plagiarism and check well any manuscript from Master of PhD students before acceptance for defence. Perhaps when writing the paper and with it length, our attention was drawn by the fact we wanted to give the best finding of his work. We strongly apologize for this inattention and thank you for have raising it up.

The new phrase is

There are uncertainties in this type of processing, given that the satellite data are second-hand sources. They can be grouped into five types: errors related to the pixel (Ep), those related to the extraction of coastlines, digitizing (Ed), and planimetric (EP) [11]. These types of errors seem random and cannot appear automatically on all images. Their sum is determined by the total value of the errors (Et), which is equal to the square root of the different errors (equation from [67]). (p. 19).

Author Response File: Author Response.pdf

Reviewer 3 Report

Please see the attachment

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 3

Dear Reviewer, Thank you for the deep and proof reading of our paper and the way you help us to enhance this piece of work by your valuable suggestions that have been integrated.

suggestions:
1. L. 11 and 15. two or four decades?

Four decades (p1)

L. 18. Not only “vegetation cover”. The erosion also leads to
degradation of coast’s land/ground cover, as well human made infrastructure.

The markers of this erosion are, among others, the presence of falling trees, destroyed houses, and damaged dykes. (p.1-2)..

L. 29. “The results show that erosion is in its initial phase in Kribi”,
please make a sentence clear.

Off course, because significant retreats of the coastline are noticeable over the period 2015-2020.

L. 31. “The average decline is estimated at +1.9 m/year over the entire
area”. please make a sentence clear.

Thank you, it has been corrected

L. 33. “a clear growth in plantations, which have fallen”?

Please, see page 2 and three, everything corrected is in read

L. 36. Please avoid the repeating “words” in Keywords, which were
already mentioned in the Title.

Thank you, we have taken into account

Figure 1. Does “national parks” also located inside Atlantic Ocean?
And better use English (instead French) for names in Figure, for example
“Atlantique”.

A part of the Kribi Natural Marine park is located into the ocean. The figure have been corrected.

LL. 124-129. Please give also extra names of soil types according to
WRB (IUSS Working Group WRB. World Reference Base for Soil Resources
2014, update 2015. International soil classification system for naming soils and
creating legends for soil maps. In World Soil Resources Reports; FAO: Rome,
Italy, 2015; p. 182.).

We have found the document indicated. It makes a brief description of the types of land encountered in the geographical areas in Central Africa and Cameroon. Only that a spatialization of these types of soil is not made.

Nevertheless, we have quoted as follow (page 6).

The 2014 Global Baseline for Soil Resources updated in 2015 ([41]) classifies for tropical African countries, such as Kenya, DR Congo, and Cameroon, Nitisols, Plinthosols (soils with high pisolite content up to 80%), Andosols (derived from parent materials other than glass-rich volcanic products are located in humid regions). (p6)

Figure 3. Is an average Kribi elevation profile? If no, please indicate
the section line on Figure 1.

C’est un profil d’élévation du littoral kribien généré à partir du modèle numérique de terrain (ligne 171).

It is an elevation profile of the Kribi coast generated from the digital terrain model (p.6).

L. 149. Please give a full name of abbreviation (DEM). Also here, it
is not a DEM (digital elevation model), is a elevation profile.
11. LL. 175-177. Could be omitted.

Figure 3: Kribi elevation profile, generated from the Digital Terrain Model (DTM) (P.6)

L. 181. Could you describe the land use/cover content and distribution
(the proportion of forest, agricultural lands and etc) of catchment (as well for Ntem
River Lobé River). Or provide a land use/cover maps.

Please Dear reviewer, We have not adopted a watershed approach because a recent study has already done so (Ebodé, 2020) and we have cited it in this work. This author has dealt with all the watersheds on the Kribi coast. nevertheless, we have added a land use and land cover map (figure 5) (Page 9)

LL. 220-227. What about vegetation coverage (density)?

A land use and land cover map is proposed for 2000, 2015 and 2020 (Figures 5-6) with a short comments:

There is a clear degradation of the vegetation cover and an increase in agro-industrial plantations and built-up areas. Between 2000 and 2010, the Atlantic forest declined. This decline will be consolidated between 2010 and 2020. Urbanization and agro-industrial activity are therefore the main factors of degradation on the Kribian coast (Figure 6). (p.9-10)

LL. 230-237. What about rainfall intensity? You know that intensive
rainfalls provide a high both solid and liquid runoff. Your region subject to the
typical monsoons. Similar to your region, for example like most subtropical
islands, Japan is characterized by high-intensity downpours, when about 10 mm of
precipitation can fall within 30 min with a frequency of 50 or more times a year.
Probably your study area also should have intensive rainfalls. For example, the
runoff start few minutes after rainfall onset (sure runoff formation also depends
from soil erodibility) when rainfall intensity exceed 4-6 mm/min (Effect of shrubgrass vegetation coverage and slope gradient on runoff and sediment yield under
simulated rainfall; Effect of rainfall intensity and slope steepness on the
development of soil erosion in the Southern Cis-Ural region (A model experiment);
Duration and intensity of rainfall events with the same erosivity change sediment
yield and runoff rates and etc.).

On the Kribian coast, rainfall has little effect on soil erosion because it is a forested area where the soils are sufficiently protected. The only places where rainfall has an effect are the areas that have suffered from degradation/deforestation. With approximately 2900 mm of rain per year, the Kribi coastline remains one of the least watered, compared to Douala, which receives between 3500 and 4000 mm of rain per year, and Debunscha with approximately 13,000 mm per year. For this reason, the main factors that amplify coastal erosion on the Kribi coast are anthropogenic forcings. (page 10).

L. 247. Which type of soil erosion is more expressed on sediment
transport in your study area? Wind or water, their proportions?

Sediment transport is more influenced by winds blowing from the ocean to the coast, since winds blowing from the interior to the coast are blocked by the Atlantic forest. These winds give speed to the littoral drift and the swell that affect the coast. When it is a soft coast, the erosion is more important, contrary to rocky coasts ‘(p. 11).

L. 568 and 610. What does mean NGO?

Thnak you : NGO is an acronym of Non-governmental organizations

L. 797. Please describe more in detail how climate change affect on
soil erosion development in your study area. Does wind speed increased/decreased,
precipitation amount, and its intensity from your research period? How such
changes impacted on coastal erosion? Does your area was affected by strong
typhoons? For example, in Japan extreme floods, strong soil and coastal erosion
were associated with typhoons Etau and Hagibis (A comparative study of riverine
137Cs dynamics during high-flow events at three contaminated river catchments in
Fukushima; Coastal impacts of super typhoon Hagibis on Greater Tokyo and
Shizuoka areas, Japan; The impact of typhoons on sediment connectivity: lessons
learnt from contaminated coastal catchments of the Fukushima Prefecture (Japan);
Erosion and Redeposition of Sediments and Sediment-Associated Radiocesium on
River Floodplains (the Niida River Basin and the Abukuma River as an Example);
Radionuclide contamination in flood sediment deposits in the coastal rivers
draining the main radioactive pollution plume of Fukushima Prefecture, Japan
(2011–2020) and etc.)

In both developed and developing countries, climate change is having an impact on coastal erosion [97]. Sea level rise, increased swell speeds, and rising sea surface temperatures are all climate drivers that exacerbate coastal erosion [98, 99]. In Kribi, the relative effects of climate changes are exacerbated by anthropogenic forcing because climate seems more stable in Kribi than in Douala or West Cameroonian coast (i.e. Limbe, Idenau and Debunscha). In certain regions of the world, one can witness a radical change in the morphology of the coastline [100]. (p. 35)

The reference list consists with many French sources; I suggest adding
and cite some extra papers from international journals.

Thank you, it is like a defect of French speaking writer and we plead you to excuse us from this. We have tried to improve by citing some at the beginning in a short literature review and at the end of the discussion and these papers are marked in read in the final references.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The author has well answered the questions previously asked. Therefore, this article can be accepted after minor revision.

Author Response

We are grateful to the reviewer for the time he took to review our manuscript once again before conclude on minor revision.

With the precious help of Academic Editors, we have try to check all possible errors and defaults of the paper and correct it.

So thanks once more

Author Response File: Author Response.docx