Quantifying Aperiodic Cliff Top and Cliff Face Retreat Rates for an Eroding Drumlin on Ireland’s Atlantic Coast Using Structure-from-Motion

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear authors,

Thank you for submitting this interesting paper, based on a good knowledge of the study site, of the methods and based on numerous scientific references.

The study site, the methods and the results are very clearly presented. But the discussion could be improved.

Here are a few remarks and recommendations to improve the quality of the paper.

Best regards.

 

 

2.       Study site

Figure 1: the small maps (a and b) could be enlarged to be more easily readable.

Figure 5 is a bit blur on the pdf file I have downloaded.

 

3.       Materials and Methods

3.1.1         SfM data acquisition

Did you operate a horizontal scan of the cliff to get pictures of the cliff face to be included in the photogrammetry processing?

Could you indicate the fly height of the UAV and the spatial resolution of the aerial photographs?

 

3.2.2         Shoreline changes

L.292: could you define what is Es?

 

4.       Results

Figure 7 (c) and figure 8 (a): please explain more clearly what the "Distance" is.

Figure 8: could be enlarged.

 

5.       Discussion

5.1       Cliff face and cliff top retreat

L.466-473: did you perform any weather conditions analysis on middle term?

 

5.2       Comparison with previous studies

Comparing rates of cliff evolution in such remote and different locations is quite risky. The geology, weather and sea conditions are so different that the comparison is tricky.

Since your paper focuses on morphological evolution and the comparison of measurement methods, I think that the discussion should also focus on methods and on the morphodynamics of the cliffs, for example comparing soft cliffs with and (almost) without talus slopes that seems to be a determining factor of cliff stability.

Figure 13 is a bit blur when zooming.

Author Response

Please see attached file for our response.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Comments for author File: Comments.pdf

Author Response

Please see attached file for our response.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript “Measuring cliff top and cliff face retreat rates of a coastal drumlin using Structure-from-Motion in Galway Bay, Ireland.” addresses an interesting topic that falls within the scope of geosciences. In this work, authors have developed an interesting work, collected and processed a relevant amount of data. However, the work suffers from several shortcomings related to an apparent misalignment between the objectives, collected data and methods. Improving the coherence of the paper requires a more robust integration of available data with realistic objectives.. Following, are some issues that should be considered:

1: Data quality –

a)       The amount and frequency of the available data is very scarce to support some of the conclusions. It is also not clear what is the time-scale for the removal of the unconsolidated talus at the cliff base (a single storm is enough? A decade?) This would frame the required need on the frequency of the data and could be used to support the conclusions. The reader has no insight on that issue.  It is also not clear why the authors did not consider using more maps (they aren’t available?)

b)      Whenever possible, results should be confronted with independent data in order to access the error of the methods and the uncertainties intrinsic to the available data. In this sense, the 1842 map should be represented (or made available to the readers). No information is given on map quality, georeferencing errors or other problems in the methodology used.

c)      Errors in orthophotos processing should be assessed and reported (where there any difficulties or doubts in digitizing the cliff top?).

d)      Available cliff top data and wave data is clearly not sufficient to relate waves with cliff rate. Concerning wave data, the “wave energy climate” should be characterized at the base of the cliff and not offshore. Location of the available wave data point should be represented in a figure and wave transformation (and sheltering effects) to the cliff base described. Should a wave propagation model be used (at least in major storms?).

e)      It is very difficult to access the quality of the data on the cliff face. The ideal case is to make this data available to the reader, but if that is not possible, the figures should represent that data much better.

2: Processing methods

a)      Representation and processing of data of the cliff face should be decoupled from the unconsolidated talus at the cliff base. In fact, the analysis of these two areas could give very interesting results. For example, authors state that “from the cliff face accumulates at its natural angle of repose within the talus slope, which is in turn undercut and steepened by wave action.” but gave no elements to access this ((e.g. it could be represented through a cross-shore profile). I would advise that the talus slope analysis should be separated from the analysis of the  cliff face retreat. The processes and magnitudes are completely different and make the results easier to interpret.

b)      Data analysis should give insights on the spatial distribution of the erosion over the cliff face (volume and size of the eroded material).

c)      Concepts: is not possible to say that “cliff top change is calculated directly from differences in shoreline position” because shoreline is not measured at cliff top.

 

2: Other comments

a)      In conclusion: authors state that “aperiodic formation of unconsolidated talus at the cliff base actively protects the lower face from wave erosion” but figure 12 show a periodic behavior. This figure should be much improved. In my opinion authors should focus on the timescales of the processes.

b)      Representation of the graph in Figure 9 is very strange and potential misleading - points should not be connected with lines.

c)      Abstract should include the objectives explicitly. The abstract introduction should match the introduction section.

d)      Present morphological changes between successive dates, including relevant statistics.

e)      Choice of study site should be justified. For an international reader this site has no particular characteristic.

f)        A digital terrain model of the area could help the reader to understand the geomorphology of the study area.

g)       Figure 1 should be improved. Location of wave data should be represented. The location of the eastern and western sections should also be included here. Results represented in figure 7 suggest that these sections are not exactly contiguous.

h)      In paper introduction an oceanographic setting should be given.

i)        From the abstract and title it is not clear that the most data is quite recent and only a map has been used. This should be justified.

j)        Number of GCPs is very small – any justification? Error should be measured on independent GCPs.

k)       Fig 6 – I don’t understand what is shorelines (clif top?)

l)        Is this “cliff face retreat rate” the average value?

m)    Consider the potential influence of tides and wave direction on the observed phenomena.

n)      Figure 7 c should include the complete range of observed values (i.e. there were no values where the method gave accretion? Even with very small values? )

o)      Figure 8 should be larger and provided with a better color contrast.  Values within uncertainty values should be  cleary separated from areas with the signal is larger.

p)      Direct “Comparison of cliff top and cliff face retreat rates calculated for different studies” seems irrelevant.

 

Author Response

Please see our response in attached document.

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

In my evaluation, I considered the following elements: the importance of the topic, how the article contributes to the current field, the presentation of data and results, and the consistency of the discussion.

The subject addressed in the present paper may be relevant in the domain of coastal erosion on rocky coasts and risk management. However, in my opinion, the current version of the manuscript is not ready to be published.

For the manuscript to be resubmitted, it must be reformulated and several aspects clarified.

The abstract incorporates the main conclusions of the work, but the keywords need to be reviewed because they are not objective enough.

The introduction seems to explore the most important aspects of the rocky coast in Ireland and Great Britain available in the specialized literature. Some statements are unclear, such as cliff types (lines 44 and 45) or the table with previous studies.

The importance and usefulness of the approach applied in his study must be explained and the objectives clarified. For example, why is it important to calculate the cliff face retreat?

Study site. The description of the study area is confusing. What coastal environments characterize the area? A barrier? An estuary? A barrier estuary? A bay? An open tidal flat? What are multiple environmental designations? All the sub-environments seen in the satellite photo have very precise names!

Are the drumlins described in 119 to 127 lines also studied in this work?

From lines 127 to 131, the authors give information about oceanographic settings; however, it is not clear if is applied to the study area. This kind of information is quite important for data interpretation.

Another important piece of information about the study area is the sub-aerial beach characteristics associated with the drumlin geo-form, i.e. i) sediment and textural aspects; ii) beach width (foreshore) in low and high tide situations; iii) if the cliff base is affected by the waves swash in good weather conditions or only in storm events; iv) if the waves are mainly constructive or destructive; v) beach and platform slope.

The Data and Methods section is well exposed but perhaps with many subtitles and sub-subtitles. The methods individually are well described, and they can be easily reproduced. However, the text lacks a better explanation of their integration, given the diversity of data sources.

It's very strange to consider the top of the cliff as the shoreline! The term is objective and widely discussed in the literature. To use the expression shoreline, authors should consider the base of the cliff. However, given the diversity of the origin of the data and its operability, they should consider the designation top of the cliff.

Data relating to maritime unrest needs to be better exposed, in particular, those related to storm events. In line 304, please standardize coordinates with the previous one.

 Results and discussion

In my opinion, the results obtained are not fully explored and some additional aspects should be addressed during the review process. The data are very interesting but are not appropriately integrated with the coastal (and subaerial) processes which are primarily responsible for the cliffs' development. Furthermore, the role and impact of storm events are not adequately discussed.

Sometimes one gets the idea that the authors intend to create an evolutionary reconstruction of the coast when that is not what is at stake (the title and objectives do not point to this aspect).

In this sense, for example, I do not understand the inclusion of lines 403 to 413. It is a temporal scale (geological scale) not comparable to the one used in this work (historical scale).

The western section always presents greater volume losses and cliff face retreats (figures 7 and 8; table 8). Why this behavior, when compared to the east section? Does it have to do with greater exposure to the wave climate? With differences in platform slope? With heterogeneity in the composition of the cliff?

Please clarify the text between lines 462 to 456! The sentence must be a mistake!

Would it be possible for the authors to discuss the meaning of the low cliff-top retreats (not shoreline!)? Are these values ​​affected by operator errors or by the overlay of different source data?

In the subsection, comparison with previous studies, authors must be very careful with some evaluations (table 7, figure 13). In my opinion, they should only compare similar situations, i.e., cliffs with identical lithological composition or origin, and exposure to marine (waves and littoral currents) and subaerial processes.

For example, in lines 489 to 493, on the Algarve coast (Portugal) there was no glaciation, even in the terminal Pleistocene, so the deposits are poorly lithified sands and clays representing fluvial deposits. In this context, the comparison is controversial, given that the contemporary climate is drier when compared with Ireland, windy, and the cliff is associated with a relatively wide sandy beach.

Figures and Tables

The caption for Figure 1 needs to be improved. The source of the data must come immediately after the identification of each map, avoiding repeating the numberings.

Maps a) and b) do not seem to show any connection and the scales are not the most appropriate.

The abbreviations used must be included in the captions of all tables, i.e., reading the tables must be self-sufficient.

 References

Some references listed are not cited in the text.

Some references may be considered gray literature and should therefore be avoided.

 

Author Response

Please see our response in attached document.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The revised version of the manuscript "Measuring cliff top and cliff face retreat rates of a coastal drumlin using Structure-from-Motion in Galway Bay, Ireland." represents a significant improvement over the original submission. However, there are still some shortcomings regarding the available data's apparent limitations in fully supporting the objectives and conclusions, which affects the paper's scientific soundness. Although authors have made a commendable effort to address this issue, these limitations persist. For example, I do not understand why authors state that the study site is surveyed regularly since the 19th century, but they only use three cliff top lines before 2019. Nevertheless, as authors are fully aware of this, I think that should not impede the paper from being published.

Other comments:

The paper title is so down-to-earth that seems the manuscript addresses a technical issue. Moreover it is not fully align with the paper content and fails to catch the attention of an international reader.

Abstract must include the objectives of the work. I’m aware of space limitations but that could not be an excuse to omit an essential element.

Figure 8 requires better color contrast between higher and lower than 0 values to effectively illustrate erosion and accretion areas. The current color ramp is inadequate for this purpose.

 

Author Response

Please see attached file. Regards, Eugene

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

The authors have addressed the comments, clarifying concepts, and stating some limitations of their study.

It is important to consider some minor alterations that need to be made.

The font size of all tables can be smaller, and more importantly, have the same font type (please, see Table 5).
The font sizes of Table 3 labels are different from the others.

Table 6 should be completed or replaced by the new graph proposed by the authors.

Some figures should be improved: i) charts (box and whisker) of Figures 7 and 8: they are not clear enough; ii) the legend of Figure 10 is not clear; iii) Please, align the photos and the graph in Figures 11 and 12, respectively, to the left.

The study design and data support the study’s discussion, but it could benefit from further exploration of the impact of waves under extreme conditions on the rocky cliff.

 

Author Response

Please see attached file. Regards, Eugene

Author Response File: Author Response.docx