|Comments to manuscript esurf-2020-79-ATC1 entitled: The Relative Influence of Dune Aspect Ratio and Beach Width on Dune Erosion as a function of Storm Duration and Surge Level|
The new version of the manuscript submitted by Itzkin et al. shows a clear effort to accommodate the concerns raised by the reviewers in relation to the previous version, I appreciate this effort and I see that the authors have addressed some of those concerns in a very elegant and convincing manner. In this regard, I congratulate the authors for this improved version of their manuscript where they have been able to reorganize the same data in order to present and explain the observations from the modelling in a way that can be more easily transferable and comparable.
Despite the improvements made on the manuscript, I maintain some of my concerns, namely regarding the major contribution of this work, and new concerns related to the way some of the data is presented in this new version. I maintain my view that demonstrating that a wider beach protects more the dune should not be the major output or fact to demonstrate within a work, as I insist, that is the basis of mitigation practices involving beach nourishment. So, I insist that it should not be shown as a novelty fact, as we have all learned from before. I see that the authors prefer to maintain their point of view and insist that this fact has not been demonstrated before, then I ask the authors what principles the engineers have used so far in order to plan these measures as the dimension of the berm is a key element?
It is not easy to understand why the authors insist also in looking at the ratio to understand different erosion patterns and as the way they present it, mechanism. From their data, and previous knowledge that they also have, the only major factor that seem to influence the mechanism or pattern of dune erosion is the stoss slope. As they mention, very high dunes with scarps (similar to the high ratio in the toe-fixed profiles) could have the tendency to collapse, and then add a different mechanism into the dune dismantling when compared to the low ratio dunes. Other than this, I cannot see how the shape of the dune influences the erosion mechanism as the authors express. The width of the beach determines the amount of volume eroded from the dune, but the author affirm that the width of the dune is also an important factor, I would rephrase this as what the latter determines is longevity of the dune to survive successive erosive events, as the dune will erode in the same manner but the wider the dune, the longer it will last under erosive processes.
One more very important point that raised doubts regarding the approach, is the way the dune changes in the dune are evaluated for the case of the crest- and heel-aligned profiles. The authors have opted for presenting the volume change of the latter relative to the change for the toe-aligned in order to isolate the effect of the ratio and focus on the beach width. As a result, the variation in volume will be always positive because of the wider beach that the latter profiles present relative to the toe ones. I find this way of presenting the data all but intuitive, I believe that the authors can find a way to show the results in a manner that does not raise doubts as what one reads from the graphs is that the larger storm accumulated larger volumes of sand as the beach width or dune ratio increases. It is obvious that with a greater beach the volume of erosion from the dune will be always smaller than if you buffer zone is reduced, then the comparison among the volumes will be always positive (figures 9 and 10) but has a very different meaning from the results in figure 8. Then I suggest changing the name of the axis and thus of this estimate, as it is a delta volume but a relative one or in other words, the volume preserved in the dune because of the increase in the beach width. And the same applies to the dune toe.
Below I will transfer some more specific comments from the reading of the manuscript that I hope can help the authors to further improve the manuscript.
From the abstract:
I believe the authors have not updated the abstract to incorporate the slightly different outcomes that they got within this new version. In this line, I would suggest the authors to adapt this new version to what is inside the manuscript, namely the authors state:
“We find that low aspect ratio (low and wide) dunes lose less volume than high aspect ratio (tall and narrow) dunes during longer storms, especially if they are fronted by a narrow beach.”
This sentence was fitting the previous version but need adaptation as from my understanding the beach width is the major factor and then minor factor the ratio. Also, for the particular case of the toe aligned and low surge case, the different in volume change is not so dramatically different as the authors try to express.
“During more intense storms, low aspect ratio dunes experience greater erosion as they are more easily overtopped than high aspect ratio dunes.”
First, I cannot see how this affirmation is novel or bring something we did not know. Second, this affirmation tries to fit what is observed in figure 8C, where the very low dunes are easily eroded, however there is not a gradient in the sense that if the ratio increases, then erosion drops, but a jump in the dune response related to the magnitude of the surge. In fact, the affirmation refers only to dunes with ratios very close to zero while high ratio dunes AGAIN erode more during longer storms, in this regard, how is this different from the low or from the approach by Sallenger?
Finally and also in the abstract, I was a bit disappointed to see that the authors maintained the fenced profiles. I wonder why they have not separated the effect of the fenced dune from the wider beach as well in this case? Because, the fenced profile does not only present a fenced dune but a wider beach. so, again, what are we actually looking at here? the combined effect of wider beach with the addition of a small dune, so additional volume of sand. The final result of a reduction in 50% might depend on the size of the fenced dune but also on the width of the backshore that you had to add to fit in the fenced dune. Your results also show that the increase of beach width may reduce up to 100% in dune erosion, therefore, why do you show the case of the fenced dune? I here would give this a second thought because of the message that these results are going to pass.
Lines 30 and 31: The authors call our attention to the fact that long-term nourishment may have a negative effect on the capacity of the system to maintain its elevation. I believe that the authors are totally forgetting or dismissing the aeolian sediment transport, do you have any special reason? I agree with the authors that nourishing is not an eternal solution but because of its un-sustainability, but I do believe that the systems if they have positive budgets do not need to rollover through overwash to adapt the sea-level rise if they can also maintain it through aeolian transport. Of course the elevation of the berm will be controlled by the runup levels and then the sand only needs to be transferred inland by the wind.
From the Introduction section:
Line 60: “while the role that dune height…”.
I disagree with the authors. I rather understand that the overwash potential has been widely explored and in this line the dune height appears as a passive parameter. I cannot find many works where the dune erosion depending on the storm is analysed as you are doing with the ratio. what i mean is that you seem to explore how the shape of the dune determines the degree of erosion, and not only the impact as other works (Sallenger) did.
Lines 68-69: “While vegetation zonation controls the positioning and height of the dune, the dominant plant species can influence overall dune shape”, this is a bit confusing. Most works point to the sediment budget as the main factor controlling foredune height (Moore et al., 2016, Psuty).
Line 102-103: “Beach nourishment may also be used to widen the beach (and decrease its slope), limiting wave impacts to the dune and stimulating dune growth”
Here you are using what is widely known about the effect of beach width, then why this need to prove it again?
Line 116: “the main goal…”
I believe the goal of the work is to assess dune erosion and you use a model to do so, but not all the way around.
Line 122: “the relative role of beach width in dune erosion processes requires further investigation (Davidson et al., 2020).”
This is not what the authors are stating, in fact the authors have not doubt and they present the beach width as a major factor preventing dune scarping, what the authors mention is that further research could be done on: “Surfzone–beach type controls on beach erosion and the degree of scarping. Observations and tests that record the difference in the magnitude or degree of scarping occurring on reflective versus intermediate versus dissipative beaches under the same storm and offshore swell conditions would be very useful.” Any reader can understand that this is quite different from the effect of beach width, which effect is well-known.
From the Methods section:
Line 136: “dune toe erosion”
Do you mean retreat or erosion?
Line 142: I would not include any mention to the beach slope. First because you are not using it in any of your plots and second because the beach slope computation should not include the backshore beach. any equation that you apply to estimate the runup etc. so, the processes to which the slope of the beach could have any effect are related to waves, and therefore they apply the foreshore slope or the combined foreshore and nearshore or only nearshore, but never the backshore because that is only important for the overwash potential, not for the runup, in fact you could induce to error because you may have a wide dry beach (as it is the case of your crest- and heel-aligned profiles) but the foreshore is the same and therefore the runup should not change.
From section 3.1 Erosion on synthetic Dunes:
Lines 258-261: “Foredunes erode under most simulated conditions, except when they have a high dune aspect ratio, are situated farther from the shoreline, or when the storm is of low intensity, in which case there is slight accretion at the dune toe due to wave processes (e.g., Cohn et al., 2019, Figure 8).”
If referring to fig. 8, please review this affirmation as I cannot see what you describe to see. How do you notice the wave processes? could you explain it using the graph? Because you mention this here but not in the subsections, so it is disconnected from the data. If you refer to all your experiments, then you should refer to them or to cite all the figures, not only to fig.8. I cannot agree that high aspect ratio do not erode, at least from your results and here you should only refer to fig.8 experiments as otherwise you are looking at the effect of the beach width. So, please fix this as it might be the case that remained from the previous version.
Line 277: “..especially pronounced..”
I would rather say “clear” or I would try to lower the importance as it is not so pronounced. Also, for the intense storm and long duration the difces are not obvious anymore as well. What about the low ratio dunes which have the greatest losses?
Line 306: “In some cases, the tall/narrow dune toes prograde seaward by up to ~12m”
If this is related to avalanching, it should be more clearly stated. If that is the case, maybe I would give it a second thought and see if the dune actually eroded? Are you computing the avalanched dune sand as erosion? Could you explain this with more detail?
Lines 333-334: “… no appreciable increase in the amount of protection offered by the narrowest beaches as storm duration increases, …”
Is this needed? Why don’t you also include what is left to be eroded from the beach to exemplify this? If the beach was already eroded because it is narrow, how it could increase the amount of protection as the storm duration increases? This affirmation or perspective is missing the beach morphodynamics and therefore it seems rather pointless.
Line 341: “…that wider beaches lead to a greater seaward migration of the dune toe.”
This again needs further explanation, why when you subtract the volumes then the change in volume relative to the toe-aligned profiles is volume loss prevented and the dune toe is interpreted as additional progradation? are not you just subtracting the retreat of the alternative experiments from the toe-aligned? If so, then is again distance prevented to migrate inland, but not progradation. Besides, how would you explain dune progradation? or is this related again to avalanching? But is it normal to have 30m (around double) progradation due to avalanching? Is this because the erosion is less intense? Or in other words, the energy reaching the base of the dune decreased.
Lines 342-343: “…while the toe of the lowest aspect ratio dune retreated by ~10m during the same storm during the toe-aligned simulations (Figure 7F).”
This makes sense as the beach is wider, but how do you explain that the same dune (low ratio) with a wider beach shows progradation? could you explain the process?
Also, I imagine you intended to refer to Figure 8, please fix.
Line 345: “progrades 15m (30m) landward compared”
What do you mean by landward progradation?
Lines 347-349: “Wave energy reaching the dune is reduced by up to 6000 Nm/m2 for the high aspect ratio dunes during the most intense storms with the widest beaches (heel-aligned; Figure 10I) while the energy impacting the dune is reduced by 1000 Nm/m2.”
You are still mixing up ratios and beach widths. The energy reduction is because of the widest beach, so do not mention the ratio as it is not the factor controlling that, and then could you explain better the second part of the sentence in relation to the first part? Is something missing in the sentence?
General comment: I still do not get the need of this simulations, also, as you were discussing the effect of beach width, could you explain how much different is this from enlarging the beach?? or how much extra-protection is due to the fenced dune and how much to the wider beach width?
Lines 358-359: “Additionally, the aspect ratio of the dune behind the fence plays a minimal role in influencing volume loss except in the case of the most intense storms,”
Is there a need to state this? it is too obvious that if the width of the beach is enlarged, and even you add a small dune in front, you would have to have a very long group of storms to reach the dune behind, i only see the point of this experiment in long-term simulations, not at the same temp scales as the previous as you have already shown that the beach width already prevents erosion.
Lines 364-366: “… 21C). While the presence of a fenced dune prevents volume loss from the natural dune, there is little to no change (<10 m) in the dune toe position relative to the toe-aligned simulations where the fenced dune was not present (Figure 365 21D, E, F).”
Please fix the reference to the figure to 12.
Could you explain this better, it is not clear what you are trying to express. How do you explain that if the dune is not eroded (you say “prevented volume loss”) the position of the dune toe changes less than for the toe-aligned profiles without fenced dunes in front?
Section Comparison with Field Surveys:
Line 397: “…a weak relationship…”
I cannot see this relation, not even weak. But you could plot the aspect ratio vs the volume change in order to make your point clearer. I see high aspect ratio are the ones showing smaller vol change and lower aspect with the greatest vol change. Is this actually in agreement with what you showed…not very clear.
Lines 399-400: “those profiles with a lower aspect ratio dune experience similar or even less erosion than high aspect ratio dunes with the same beach width (i.e., at a beach width of 40 m in 400 Fig. 13)”.
Once again, this is not clear to me, the diff for the 40m is that you have only one point with low aspect ratio, but the high aspect ratio shows a large variability for the 40m, so this is not clear at all.
Line 435: “accretion occurring at the dune toe for the high aspect ratio dunes that weren’t…”
Accretion at the toe, but dune crest retreat, right?
Line 445: “resilient”
Do you mean resistant?
Line 448-449: “While dune morphology plays a primary role in describing how dunes erode, particularly with respect to whether or not sediment is piled at the toe of the dune (high aspect ratio dunes) or transported offshore”
Why particularly? From your results and interpretation, I can only see with respect to this, so if the aspect is important with respect to additional aspects it should be clearly stated.
Line 453: “Wider beaches lead to less sediment loss from the dune and more progradation of the dune toe.”
This has not been explained, which process is this? Is only avalanching? If so, how do you explain that low aspect ratio dunes also prograde???
Line 457-458: “ beach width is the primary control on dune erosion, followed by dune width, and then dune height.”
I agree with the first part of the sentence, but i do not agree with the affirmation that dune width controls dune erosion, it may control the relative erosion, meaning that if your dune is narrow, it will disappear faster, but the width does not influence the process of erosion itself as does the beach width, which determines the wave energy impacting the dune. alternatively, i agree that high aspect may help erode the dune.
Lines 469-471: “Thus, the aspect ratio of the natural dune is secondary to the morphology of the fenced dune in providing protection to back-barrier environments (a taller fenced dune would offer even greater protection).”
I think that here there are mixing concepts, i think that if you would address this as volumes of sand as defence, distributed within the fenced dune and the additional beach width because of the space needed to place an artificial dune, it would make much more sense to me.
Line 471: “can reform”
Do you actually mean reform? From their work I understood form or develop.
Line 472: “can sand fences effectively prevent storm-induced erosion,”
Sand fences do not prevent storm erosion, but the volume of sand accumulated or the sand fencing or the fenced dune.
Line 476: “are still present following the storm or are re-built.”
In their work, they also mention nourishment in those areas, so is not only the fences, but the extra sand that helps, don´t you agree?
Lines 478-479: “widens the dune but does not add to its elevation will cause the dune to assume a lower aspect ratio than it had in its pre-management state.”
Of course, and I insist, this implies widening the beach as you state below, and in fact i would change the order and start stating that any management practice aiming to enlarge the dune needs to be paired with nourishment.
Lines 491-492: “slope (βf), which lowers incident band swash (e.g., Ruggiero et al., 2004) and total wave runup (Stockdon)”
You could explain this better and make sure you explain what slope you are refereeing to. Usually, these works use the foreshore slope, and the foreshore+nearshore, i understand that the beach width increase in your case is related to the backshore width, should not you compute the slope of the intertidal beach? how is the backshore included in the equation from stockdon? the slope of the beach should be constant if you change the position of the toe of the dune with not additional changes in the morphology of the beach.
Line 492: “reducing the likelihood of dune erosion.”
Not because of the slope, but because of the beach width of the dry beach. If your beach is widening because you move the dune behind in your synthetic profiles, it means that what actually grows is the width of the beach berm, which implies large volume of sediment but that does not affect the wave runup.
Line 508: “thereby increasing the likelihood of eventual barrier drowning”
I still don’ t get this point. If you are adding sand to your beach/dune, you may not have rollover because the system is able to cope with sea-level rise and maintained, why should it drown? The nourishment can even help build the backbarrier through aeolian transport. I do agree and I am glad to read that the main problem is the sustainability of these type of measures.
Line 518: “dunes, although high aspect ratio dunes offer greater protection against more intense storms.”
Can you expand this? If i got it right, it is not the case, from the results section as you do not properly explain the case of the overwashed dunes. The volume of erosion is of the same order of magnitude for the high ratio dunes (Fig. 8).
Lines 520-522: “eroded sediment is lost offshore whereas the high aspect ratio dunes lose greater amounts of sediment through persistent scarping, more of the sediment is preserved at the toe of the dune as a result of avalanching.”
You mention this here, but how did you see this? it cannot be easily taken from the results.
Lines 528-529: the sentence is missing something.
Lines 544-545: “management initiatives reduce overwash flux, which is essential for barrier islands to maintain elevation as sea level continues to rise”
Not only because of this, but also because of sustainability of the resources needed. You may also need to keep in mind that if you nourish a beach, what also may happen is that the dune will rise and likely the backbarrier because of aeolian sed transport, which is not mentioned here but also contributes to keep the elevation of the coastal barrier.