Extreme Coastal Water Levels Evolution at Dakar (Senegal, West Africa)

Round 1

Reviewer 1 Report

Overall comments:

 The authors use past tidal gage and water level data (20 years, 1994 – 2015) to predict the effect of sea level rise on the increased flood incidence in Dakar, Senegal.  As analyses of sea level rise effects on this region are few, I feel this paper will be a good contribution to the literature.

The authors report an increase of incidence of flooding at an extreme coastal water level (ECWL) at a rate of 8.4 mm/year with wave runup increase of +7mm/year. While I agree with the general conclusions of the model –namely in the magnification of flood impacts due to the combined effects of rising sea levels, tidal and storm effects – I suggest elaborating on the methodology used in the model, namely in the source and analysis (if any) of the data used for the values of runoff and wave breaking (R), sea level anomaly (DAC) and Tidal water levels (T).  I also believe that the authors can employ more statistical trend analyses to show that the various contributions to the extreme water levels are following such a trend at, say, a 95% confidence level.

 In addition, it would be beneficial for the authors to use the model trends to make some predictions on 2020-2030 water levels in Senegal, to report variability and uncertainty, and especially to corroborate with more recent data or local news reports.

 I recommend the paper to be published after some major edits.  Additionally, the paper needs to be proof-read in general as there is some unclear language.

 Specific Comments on the text:

 Page 4:  The ERA data used is at a 0.5 by 0.5 degree grid, which is relatively sparse.  Are these data points at all close to the Dakar shoreline?  Are the significant wave heights and tidal water levels able correspond to particular local points in Dakar?  If they are very far off the coast, there may be a need to perform additional storm surge and wave modeling to see whether the waves and surges will undergo shoaling, wave crest focusing, and refraction, all of which may alter the observed water level in Dakar.

The rationale behind the selection of 1.78m as a critical threshold water level: has this 98% percentile water level (1.78m) been identified as a water level that has caused significant flooding and disruptions in the past (for example from [39])?  What water levels have been recorded in past flooding incidents?

Source [45] appears to be tidal gauge data.  Can the authors explain how the data in Figure 4 are obtained?  Are they a re-analysis of the data presented in source [45]?  If so, this must be stated in the figure caption, or elaborated in the methodology.  In general, I would like to know what the authors have added to Source [45] in terms of additional work that is different from this 2008 source.

Figure 5.  Suggest renaming the horizontal axis to Jan-Dec, as the current axis title could refer to number of months starting from a certain time. 

Page 6: Yes, agreed that it is the magnification of the high spring tide levels with the increased incidence of storm surges and high wave activity that will contribute to the extreme water levels that may cause flooding.  But while Figure 3 tries to show this, I do not find it very convincing, since the magnitude of each contribution appears to be fairly constant throughout time.  Can the authors try to show the variability of the reported trends, and employ some more statistics to show that the trend from 1994-2015 is not trivial?

Table1:  The indicate annual mean trends.  I would like to see some error bars of the variability of the trends.  Also, using these calculated parameters it would be nice for the authors to give a prediction on the possible range of incidence of extreme coastal water levels.  E.g., what is the predicted range of hours exceeding 1.78m water level in the year 2020, 2030 and 2050?  I think this will be a more useful number to report than the rates reported in the Abstracts (e.g. wave runup +7mm/year, ECWL of 8.4 mm/year.

In Equation 1, ECWL = SLA + DAC + T + R.  Why is T not in included in Table 1 so that the reader can see that the four components do indeed add up to the computed ECWL value? This will make it clearer how the evolution of the different contributions to ECWL are varying with time, and are predicted to change.

Finally, please take a read through the paper for language.  Some examples of grammatical errors and cumbersome language:

“..erosion remains a real reality”

“…with its 133, 69 km of coastal line, along which are present two types of coasts...”

“ Noteworthy, this threshold is hydrodynamical…”

“We quantify there the total very high sea levels...”

“… more potential to produce damages...”

Author Response

Hello Dear review;

Attached are our responses to your comments.

Author Response File: Author Response.pdf

Reviewer 2 Report

The presented work "Extreme coastal water levels evolution at Dakar (Senegal, West Africa)" corresponds to the profile of the Climate journal.

The paper analyzes the extreme sea levels over 22 years (1994-2015) in a specific area, the coast of Dakar.

Comments (unfortunately, there is no line numbering to facilitate references to the text).

1. Fig. 1, which is supposed to introduce the study object, is uninformative and completely unacceptable. Why not use a more generalized map, such as this:

Especially since the text mentions differences in the effects of waves on the coast in the Northern and Southern Hemispheres.  In the figure you can immediately see the spatial position of the Dakar in relation to the equator (not every reader will be able to tell you the latitude of Dakar).

 2. Page. 4. It is said that «The energy of the North-South drift current is strongly dissipated by the configuration of the Cape Verde peninsula». From what does it follow? There are no other currents? And is there any point in mentioning currents if they are not analyzed at all later on?

3.  Initial data. An essential point.

It is said that data from the ECMWF analysis at 6-hour discreteness are used to analyze extreme sea levels. Tidal data is at 1-hour discreteness.

Which discreteness is further used? 1 hour? 6 hours? 6 hours is too rough approximation.

Next. At what point of the coast is the data retrieved? Why is this point not on the map?

4. Direct calculation of ECWL extreme levels.

The following formula is used:

ECWL= SLA + DAC +T + R

Here

SLA - sea level anomaly. Satellite data is used.

DAC - height of the storm surge. Where does the data come from?

T - tides. Everything is clear here, the data is available.

R - height of wave breaking. Where exactly does the data come from?

(Besides, there is an error here. Of course, it is not about the height of wave breaking, but about the wave runup. Further in the text everything is correct).

5. Fig. 2b. What is the confidence interval for the linear approximation?  It is not specified. Judging from the graph, it is 95%.

6. Fig. 3. What level does each year represent? The annual average? The maximum for that year?

The contribution of the SLA and DAC parameters is extremely small. Perhaps there is no point in considering them? At least it would remove the question of where DAC comes from.

7. Page 7. It states: «annual extreme sea levels increased by 11 cm between 1993 and 2015 at Dakar (Fig. 5) ». From what does it follow? From Fig. 5? However, Fig. 5 reflects monthly statistics.

8. It is said that Dakar coast is influenced by «North Atlantic swells» and «South Atlantic swells». Clearly, some justification is needed.

Conclusions.

As a whole, the article is interesting and necessary.

However, unfortunately, many of the statements are merely declarative and do not reflect some stages of the work that has been done.

Points that could improve the idea of the article:

1. It is necessary to specify the coast point for which calculations are made.

2. The wave rose (or several, for characteristic years) is required.

3. Why not to divide the waves into two components: directly wind waves and swell? Some reanalysis present wind wave fields with separation of wave systems. In this case, it would be possible to calculate runup separately for wind waves and swell, and estimate the swell role.

Comments for author File: Comments.pdf

Author Response

Hello dear review:

Attached are our responses to your comments

Author Response File: Author Response.pdf

Reviewer 3 Report

Dear Authors,

The field of research in which your manuscript is written is very relevant for the coastal zone of the land, especially in the context of modern climate change. However, I have some comments and suggestions about the manuscript that I hope will help improve the content and readability of your future paper.

(1) You wrote that "Tide is the main contributor to the extremes, when combined with large wave runup, due to wave breaking, which contributes to 38% of the increase in extreme events while sea level rise to 44%". However, it is not clear from the methodological part of the manuscript how you calculated the share of each factor in the results obtained. The methodical section of the manuscript should contain all the explanations for the calculations (the more detailed the better) that you made.

(2) The manuscript is missing a (sub-)section describing the main limitations and uncertainties of your study. You need to be more critical of your results.

(3) The results obtained are model ones. At the same time, are there any model-independent confirmations of the findings? This information is necessary to increase reader confidence in your results.

(4) What practical (applied) recommendations follow from your research?

(5) English needs improvement. First of all, it is recommended to avoid strange phrases such as "In a global context of global warming" and others.

Author Response

Hello dear reviewer;

Attached are our responses to your comments.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Thank you for the updated manuscript -- the methodology, climatic background for the Senegal coastline and results (including variability and limitations) is much clearer now. 

Author Response

Good evening dear reviewer; 
Thank you for your comments and suggestions to improve the quality of the document.
You can receive as an attachment the document in which we have put our answers to your questions. 

Author Response File: Author Response.docx

Reviewer 3 Report

Thank you for revising the manuscript and responding to my comments. I have only one request - it is necessary to improve the quality (readability) of Figure 1.

Author Response

Good evening dear reviewer; 
Thank you for your comments and suggestions to improve the quality of the document.
You can receive as an attachment the document in which we have put our answers to your questions. 

Author Response File: Author Response.pdf