Manuscript: The effects of storms and a transient sandy veneer on the interannual planform evolution a low-relief coastal cliff and wavecut platform at Sargent Beach, Texas, USA

General Comments:

This manuscript looks at the occurrence and mechanisms of coastal cliff erosion, using Sargent Beach as a case study. The study is interesting and reveals some good relationships between rates of erosion, storm events, sediment cover, roughness, and sinuosity. My main comments on this manuscript can be addressed by some restructuring and expanding the results. In terms of restructuring, the introduction is lacks theory but some of the background information I was looking for then appeared in the study site section. There is also some study site information that was in the introduction. There is a good dataset behind the results, but I don’t feel the authors go into enough detail and then it feels there are big leaps to some of the discussion points and conclusions. I’ve made some more specific comments below and some minor suggestions/corrections. I hope my comments helps to focus the first half of the paper and draw out some more of the interesting results in the second half.

Specific Comments:

1. Intro

- I was looking for a bit more in the intro/lit review before the study site, e.g. move L120-126, 135-142 into the intro – but these need revising so I know why you’re telling me all of this. It reads a bit like a list of ‘facts’ rather than it setting the stage for your research.

- L50 “Although shoreline change at Sargent has been historically analyzed using measurements spaced 50 meters or more apart 50” – can we have some more info on this?

- L54: “Our measurements show”, are you talking about results from this study? Shouldn’t be in the intro. I’d like more literature/background on this in the intro though.

2. Methods

- Did you do any estimation of error with your method of detecting the shoreline. It would be good for this to be acknowledge. Was it always really clear where the shoreline/base of the cliff was?

- Figure 4 could do with being zoomed out – or having a zoomed out map as well so we can get the context. It’s hard to see what exactly we’re looking at because of how cropped it is.

3. Results

- Can you add some values to section 4.1.

- L212 : “Using the fitted empirical model, we calculate a recovery timescale” – please add the results of this calculation – later I read that it’s 5 years?

- L216: What is the consequence of the erosion pins falling out and only being able to plot the smallest possible retreat? If May-July was actually much larger, this might change the theory that there is little to no retreat for sediment cover >90%

4. Discussion

- You make some quite big statements without, I think, the data to support – or at least we need a little more explanations/acknowledgement of the limitations.

- L246: “Here we show that cliff retreat occurs when sediment cover is <90%” – cliff retreat still ours above 90%, it’s just slower. I don’t feel you have enough datapoints to identify 90% as a threshold.

- L246: “Additionally, our observations of cliff morphology can be linked to erosion by sediment abrasion” – there needs to be more in section 4.1, we don’t really see much of the results relating to morphology of the cliff face. I’m not sure where in the results this statement stems from?

- L247: “If sediment cover is 90% or greater, no cliff erosion occurs and there is no change in morphology” I don’t agree the results show this – erosion still occurs >90% even if small and I can’t see clearly in the results how this links to morphology? It could become more evident if there was more in results section 4.1.

- L249: “erosion is focused on the sides and back of the embayments, increasing the sinuosity and roughness of the cliff face” – not sure where this is shown in the results?

- L268: “Storm occurrence is clearly not sufficient to infer net erosional processes,” – storms can have a significant impact!

- L284: “at Sargent Beach is the recovery timescale, ~5 years after a storm or shoreline roughening event” – this wasn’t shown in the results (see earlier comment).

Minor comments/Technical Corrections:

L42: please can you define Type-A

L44: “the largest concentrated extreme of shoreline erosion globally”, this sentence doesn’t really make sense without reading it a couple of time, although I get the gist. Consider revising this phrasing.

L48: “Similar cohesive coastal cliffs exist globally… London, UK (Hutchinson, 1973).” This is broad and I’m not aware of cliffs in London itself? I could be wrong…

L59: “and other larger coastal cliff systems” – you only talk about Sargent, so this isn’t true. You can only infer?

L63: “foreseeable timescale” – do you mean that it is likely to breach more regularly, or that it is likely to breach in the foreseeable future?

L66: “150 m strip of barrier coast that separates the GIWW” …from the Gulf of Mexico.

L129: “allows sediment particles to act as tools of abrasion” - add 'to'

L261: “Sediment cover is often highest on the cliff face at Sargent Beach in the embayments and  decreases to little or none in front of the headlands.” – Can this be revised to be a little clearer – e.g. Sediment cover is often highest in the embayments at Sargent Beach, and lowest at the headlands.

L278: “smooth the roughened the sea cliff and with initially high erosion rate” - remove 'the'

- Gulf Intracoastal Waterway – sometimes you abbreviate this and others you don’t.

- Be careful of the length of your sentence – many are 3+ lines long which makes them harder to read and digest.

- Figure 9c: the colours confused me initially as I was expecting red to be negative change. Consider changing this.

- Abbreviation of TX throughout either needs to be defined or just stated a Texas, for those not in the states.

- Sometimes you use ‘five-year’ and other time ‘5-year’ – make this consistent.

- Figure 7 in the manuscript text comes before figure 8?

General Comments

I had the opportunity of reviewing the preprint (esurf-2021-29) entitled “The effects of storms and a transient sandy veneer on the interannual planform evolution a low-relief coastal cliff and wave-cut platform at Sargent Beach, Texas, USA” by Dr. R. Palermo et al., under consideration for publication in the journal Earth Surface Dynamics of Copernicus-EGU. The preprint investigates soft cliff and platform retreat at Sargent Beach (Texas, USA) to understand erosion mechanisms that drive the high retreat at this location. The authors quantify cliff retreat between 2009 and 2017 from 7 aerial photographs using the transects separated alongshore by 1 m. The authors complemented these calculations with a terrain change quantification from lidar-derived digital elevation models of 2016 and 2017. Also, they quantified sediment cover and local cliff erosion (from erosion pins) every 6-8 weeks during 2015. Based on these remote sensing data and field campaigns, the authors quantify soft-cliff retreat rates, shoreline sinuosity, and shoreline roughness, which they use to drive conclusions and propose a conceptual model of retreat.

The authors focus on: (1) showing that storm events increase shoreline roughness and sinuosity, which in turn drive annual high erosion rates; (2) evaluating the relationship between sediment cover and cliff retreat; and (3) evaluating the temporal and spatial relationship between shoreline roughness and sinuosity with storms and sediment cover. These objectives are interesting from basic and applied standpoints. For example, the authors highlight that understanding cliff retreat at Sargent Beach matters to the local community because it constitutes the barrier that separates the Gulf Intracoastal Waterway from the Gulf of Mexico.

I commend the authors for collecting remote sensing and in situ data to assess shoreline retreat, including relatively novel ways of quantifying cliff erosion (roughness, sinuosity), and providing an insightful discussion on soft cliff retreat. I find the article well written, with sound science and precisely and delicately edited figures. I, therefore, find this article publishable after minor revisions.

I might provide a couple of suggestions worthy of attention from the authors from the stylistic perspective, which I include under “Technical corrections.” However, I’d like to highlight a potentially incorrect concept that the authors use profusely in the text: wave-cut platform. As the authors recognize in Line 102 (beginning of section 2.3), subaerial processes may be as crucial as marine (waves) action in driving soft cliff retreat. I suggest replacing the term “wave-cut platform” with “shore platform,” which encompasses subaqueous and subaerial drivers. This change shouldn’t affect the authors’ findings and discussion.

Specific comments

    Title: I suggest replacing “wave-cut platform” with “shore platform” here and throughout the text.

    Abstract: I suggest including a statement on how did you perform your study (methodology).

    Line 165: I suggest including how did you calculate the uncertainty (e.g., Genz et al., 2007)

    Line 167: Regarding the linear regression, I cannot find Figs. S3-S4. Could you please make sure they are available? Also, why using the linear regression and not using, e.g., the angle related to the principal components or a low pass filter?

    Line 170: I suggest showing the equations you used to quantify the sinuosity and roughness.

    Line 210: I suggest showing the exponential model in Fig. 6a.

    Line 213: I suggest including the value you obtain for t_r.

    Line 241: this difference is interesting. Any idea why it occurred?

    Line 244: I suggest changing “coastal” with “cliff” to avoid potential ambiguities related to the statement of sediment being exogenic. This statement could be interpreted as “sediment being exogenic from the littoral cell”.

    Line 252: there is a potential feedback occurring (not yet studied) when muddy cliffs erode and the sediment settles during calm wave periods. This settled mud could dampen wave energy, particularly at long (infragravity) periods (Elgar and Raubenheimer, 2008).

Technical corrections

• Line 32: I suggest using “cohesive cliffs” instead of “clay cliffs”.
• Line 121: please fix the name “Sunamera” in the references.
• Line 129: maybe a “to” is missing after “linked”?
• Line 136: I suggest checking the writing style of the phrase “Rocky sea cliffs…”
• Line 137: there is a pen-slip in “replated”, maybe you mean “related”?
• Line 188: please add a space between “1” and “m”.
• Line 191: I cannot find Fig. S1. Please make sure to include it.
• Line 305: I suggest including a short account of your study in the beginning of the conclusions.
• Line 319: I suggest checking References format.
• Figure 6: I recommend indicating when Ida and Havey occurred. Also, please separate the units and variables in labels.
• Figure 7: it seems you don’t mention this figure in the correct order…
• Line 469: you mean black and brown lines?
• Line 475: you mean Fig. 9.

References suggested

Elgar, S. and Raubenheimer, B., 2008. Wave dissipation by muddy seafloors. Geophysical Research Letters, 35(7).

Genz, A.S., Fletcher, C.H., Dunn, R.A., Frazer, L.N. and Rooney, J.J., 2007. The predictive accuracy of shoreline change rate methods and alongshore beach variation on Maui, Hawaii. Journal of Coastal Research, 23(1 (231)), pp.87-105.