Physical Modelling of Arctic Coastlines—Progress and Limitations

Round 1

Reviewer 1 Report

I think the authors will not be surprised I still think the paper should be rejected. There are not so many new results in the paper. It is in between a review paper and a paper on the first results of some experiments. The paper is very long, even much longer than the previous version, but does not contain concrete results. This can already be seen from the abstract and the conclusions, which are very descriptive and talk more about plans for the future than about the main findings.

I am not an expert in the field of erosion of arctic coastlines. Also after reading the paper I still have no feeling about magnitudes and I am missing a quantitative understanding of how a certain parameter influences erosion, something that would be needed in a review paper. However, I do like Figures 1 and 2 and they give me a sense of the processes that are important, but also these Figs are missing numbers, like length and time scales. I would have loved to see the paper go into depth instead of staying very generic and superficial. It is too broad. Also it seems to me that the presented framework of experimental set-up is not very helpful. I don’t see how you will ever reach generation 5. It is almost as complex as nature itself and there are many issues. It would be better to observe the processes in nature in that case. I do see the value of isolating the effect of certain processes and parameters on erosion, but I don’t see why you would like to achieve the same level of complexity as nature.  You should never bet on one horse. So field data is also needed, as well as numerical model development

But the decision is with the editor. I also noted that the other reviewer is happy with the work and thinks it is an important advancement. If he/she is really from the field he is better suited to judge the content. But for an interested reader not from the field the paper is too much an 'internal'piece of work and is not helping me to grasp the topic.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear Authors,

I have prepared the following comments for your consideration and invite you to prepare a detailed point-by-point reply.  My compliments to you and I hope you find these comments useful to you.

Abstract

Please polish to better reflect the objectives of your manuscript and the research needs you list in lines 276-288

Introduction

line 37-44.  This paragraph could be reordered to more clearly orient readers to first the general processes that are occurring, the primary or dominant process, and detailed mechanics of that dominant process.  A note here about seasonal and local circumstances and their influence on this mechanism would be helpful for the following paragraph.  This paragraph is also not linked to Section 2.2 where you have a focus on erosion due to waves.  Please strengthen.

line 45 - 'While significant work and insight of the thermal erosional mechanism has been developed through observational studies, process-based knowledge remains limited as..."

line 47 - please clarify or provide details regarding this limitation.  A reader may assume that a site-specific observational study would have data for a numerical model of that specific site but perhaps the point you are trying to make is focused on a process-based model and our limitations in crafting that model, not in the local information for boundary conditions, etc.

lines 53-59.  It may be appropriate to simply provide a bullet list of your objectives since the current paragraph is not clearly written.

line 60.  Would it be appropriate to re-title this section?  "Current state-of-art" or "Current Knowledge and Approach"?  Something more appropriate than 'literature review' is needed.

lines 67-88.  Fig. 1 includes vegetation.  Please consider noting the role of vegetation in this paragraph, as flora may exacerbate or reduce local erosion.

lines 75, 84,85, Fig 1.  Consider replacing soils with sediments or a more inclusive term, as not all polar shorelines are only soil but a variety of materials may be present.

line 102.  please reword mechanical strength as there is a wide spectrum of structural properties of rock.  Perhaps a reference?

lines 110-118.  Please consider noting several key or pioneering studies on the recession of coastlines due to erosion.  It seems a bit strange to only site Madsen et al. (1976) and in the context of Shields Diagram.

line 125 - what type of soft protection?

line 135 - Thermomechanical Erosion?

line 157.  Please add a photograph illustrating this scenario to Fig. 2.  Please consider a Fig. 2a and Fig. 2b approach.

lines 160-163.  It seems for this to be a comprehensive framework for experimental modelling of permafrost erosion, the effects of ice noted here should not be excluded.  Furthermore, a great deal of work by Ettema and colleagues on ice effects on sediment transport has been completed, although it is focused on freshwater and rivers.  Nevertheless there is valuable information that can be applied to arctic coastlines.  Please expand to address this aspect with regards to your manuscript objectives.

Line 200.  Please consider being very specific on the primary limitations found among these models, as this would greatly help readers as they then read about what current models are using and their capabilities.  Leading with limitations is in line with the focus of this manuscript.

Lines 311-317.  Yes it is true Froude is often applied for open channel flows, but there are many other aspects beyond surface tension and viscosity for what you are describing.  I am not sure what value this paragraph provides that could not be noted in the previous paragraph or that is already discussed in lines 327-337

Line 443.  Please consider how future technological advances play a role in instrumentation schemes for hydraulic modeling efforts.  This consideration will help keep your proposed scheme relevant in future years as these advancements occur.

Line 462.  Have you considered testing field samples?  There are some very creative approaches that could be done - if the sample is fully frozen could one not simply saw-cut a large sample to be loaded onto a lorry or truck? The overtopping community also has a mobile facility concept for testing in the field the erosion of waves on river banks, levees, etc. which is not dissimilar.  Please  expand to consider these points as it is indeed very difficult to 'bake the cake' in the lab.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

I was intrigued by the topic. It is a very important research topic, but is indeed overlooked by the coastal research community.  I am also not an expert in this field, although I do work on coastal systems, erosion, sediment transport etc. I have read the paper from the perspective of a novice, willing to learn but not mastering the topic. But then I was a little bit disappointed. Because there are hardly any results and also the paper is not presenting an overview of existing knowledge. To me the paper reads like a report that was developed as part of experimental set-up and project design. It is missing clear research objectives, is very long, contains many details that cannot be fully understood and seem to be irrelevant at this stage (why is Eq 1 relevant? I thought thermomechanical erosion was dominant?). Why bother about six stages of experimental set-up, while stages 3- 6 are not feasible and even stage 1 and 2 are already difficult. It seems you expect you can achieve at some point the same complexity in the lab as in the field, while the idea of lab experiments is to simplify and get understanding of first order processes. All in all, the paper is too much an internal report and the audience seems to be their own research group. This can for example be recognized in the second bullet point in the conclusions of the paper. It is basically saying that initial tests have shown that temperature sensors should be placed differently.  Ok, let’s do that! It is important information for the project, but not for the general audience of this scientific journal.

In the present form the paper cannot be published. The paper could go in two ways. The first option is to make it a review paper on erosion of permafrost coastlines. But then it needs to pay more attention to observations of permafrost erosion, to discuss the mechanisms and their sensitivity to forcing and material properties in much more detail, to address numerical modeling efforts of permafrost erosion and what we have learned from that and discuss the possibilities  of physical scale models. The second option is to focus the paper on only one topic and what can be concluded from first experiments. But for that more results are needed. I understand it is a challenge to do experiments that represent nature and also doing measurements in the field can be hard. So there is no need to do it all at once in one grand experiment or so. One could define several topics of interest and study them separately, for example study the thermodynamics of mixed permafrost systems as function of ice content, inhomogeneity, vegetation, types of sediment etc.  

For future work it seems to be important to measure erosion rates as well. This could be achieved with LiDAR devices.  Also, one could try to measure the important parameters in the field. At a site with high erosion rates, one could measure waves, thermal properties etc with ‘cheap’ devices. Or test the predictive capability of existing models.

Reviewer 2 Report

The manuscript addresses the main conceptual and practical challenges dealing with physical modelling of costal processes in permafrost environments. The topic is interesting and comprises different interconnected physical processes, along with the difficulties to correctly simulate them in a laboratory facility.

A great effort was devoted by the authors to collect and organize a huge amount of information from literature. The analysis of the different and complex issues related to the properties of permafrost, interaction with sea, scaling issues, conceptualization of physical model and technological constraints is detailed and clearly presented.

The generational model approach proposed in this manuscript is interesting and I am confident that it will be used as a valid reference framework by other researchers for further studies in this field.

I have some concerns on the physical meaning of some experimental results presented in section 5, as detailed in my comments below. However, I think that the main interest of the manuscript is on conceptual and technological issues, rather than on the experiments reported.

Overall, the manuscript is well organized and sufficiently easy to understand.

In conclusion, my impression is that the manuscript is good, and I would recommend it to Editors for publication, with minor revisions.

I add my comments and suggestions in the following.

Citation style

I suggest using a numerical style for citations of references in the text. A specific application (e.g. Zotero, that is free) could be of help.

Section 3

Lines 286-298: The Nusselt number is described in equation (2). I would suggest spending some words also for the other parameters of interest for scaling, at least for the Prandtl number, that may be less common for some readers.

Line 391: Please, check the term “within”. Do you mean “without”?

Line 392: Please, check “Section 4.0”.

Lines 395-396: The whole sentence seems not clear.

Line 415: The reference to Table 2 seems not clear.

Figure 4: Some of the technologies or sections of the experimental facility indicated with capital letter in the figure are not described in the text (A, B, J…). I would suggest adding a list or a table with definitions of all the items.

Section 4

How did you choose the characteristics of the permafrost (grain size, density, etc.) for your tests? Do they correspond to a particular site?

Figure 6: I would suggest adding a side view of the experimental setup, in which the ramp with reducing depth is visible.

Lines 488-489: please, check the interruption of the line.

At lines 489 and 535 the number of waves is specified (N=69). Considering that wave period is 1.7 s, the resulting duration of wave attack would be 69x1.7=117.3 s, i.e. about 2 minutes. However, the reported duration of the test no. 3 exceeds 50 minutes. I suggest clarifying this aspect.

Section 5

Lines 540 and following. The installation of a barrier behind the sample after the fully development of the niche induces wave reflection, with effects on the erosion of the sample. My question is the physical meaning of this. Did you intend to simulate a phenomenon that can occur in nature or your intention was just to describe a possible laboratory effect? I would suggest spending some words to discuss this point.

I would suggest improving the resolution and the shape of the graphs plotted in Figures 8, 11 and 12. Maybe the use of different styles or indicators for the lines could be of help.

I would suggest considering the possibility to improve the resolution or increase the size of the pictures in Figure 9 and 10, if possible. Moreover, if you have pictures in which the wave breaking at the ramp in front of the sample is visible, I would suggest including one or two of them in the manuscript.

If possible, I would also suggest including a graph of measured wave heights and/or the wave profile at one gauge (e.g. WG 7).

Finally, the chapter "Conclusion" is numbered as 5, it should be 6. 

Kind regards