the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Jun 2024
Equilibrium distance from long-range dune interactions
Abstract. Flow perturbations induced by dune topography affect sediment transport locally, but can also be felt over long distances altering the dynamics of isolated neighbouring dunes downstream. In order to work under optimal conditions that eliminate transverse flow components, collisions and mass exchange between dunes, we study here these long-range interactions using a 2D numerical model where two equal-sized dunes lying on a non-erodible bed are exposed to a symmetric reversing flow. Depending on the initial spacing, dunes either attract or repel each other, to eventually converge towards a steady-state spacing. This equilibrium distance decreases with flow strength and increases with period of flow reorientation and dune size. It is mainly controlled by the reversing dune shape and the structure of the turbulent wake it generates, which continuously modulates the mean shear stress on the downstream dune. Under multi-directional wind regimes, these long-range flow perturbations offer an alternative mechanism for wavelength selection in dune fields with non-erodible interdune areas. Within these dune fields, estimates of mean shear stress could be used to assess the relative migration rate and the state of attraction/repulsion between neighbouring dunes.
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RC1: 'Comment on egusphere-2024-1634', Anonymous Referee #1, 22 Jul 2024
Equilibrium distance from long-range dune interactions
Vérité et al. (2024) preprint
General comments
The authors present an application of a well-established and widely-used numerical model (ReSCAL) to uncover the influence of long-range interactions on two-body (and briefly several-body) dynamics of simulated bedforms exposed to different flow regimes. The manuscript is well-structured and well-written, and the figures are of an excellent standard and the authors should be congratulated. The work will be of interest to those who study aeolian systems and compliments some of the existing literature, particularly the properties of systems of linear dunes and quasi-two-dimensional flume experiments.
However, there are some aspects of the work that this reviewer believes require additional investigation of clarification before the work is ready for publication. Chief among these concerns is that the authors have highlighted in this manuscript the importance of the “dune” shape in governing the interactions that they observe. The shape of the dune, in particular the aspect ratio, is found to govern much of the dynamical behaviour which is observed. However, to the knowledge of this reviewer, the effect of changing the elementary aspect ratio (i.e. height to length or width) of the sand slabs (cells) from which the bedforms are formed has never been investigated (other than a brief mention in Appendix A of Narteau et al. (2009)). Thus, one cannot be certain whether the results presented here are universal outputs of the model or hold only in the specific case of a 1:1 cell height:width ratio.
Changing the elementary aspect ratio may affect coupling between the bedform and lattice gas since the bedforms are likely to be comprised of a greater number of sand slabs if the cell height is less than and so the removal of a single slab due to erosion/transport will have less of an impact on the shape/size of the bedform and thus result in a smaller perturbation of the flow. Changing the height of the slabs would also affect the avalanche procedure which could in turn influence the shape (and migration etc) of the bedforms.
Although this reviewer concedes that there is a precedent for publication of studies applying ReSCAL without an investigation of the affect of changing the cell height:width ratio, this manuscript places a significant emphasis on the role of shape and bedform aspect ratio on governing the interactions. As such, this reviewer feels that it is important to ascertain how the findings of this study would change if the elementary aspect ratio is varied before publication.
Below follow some more specific comments regarding the manuscript as presented.
Specific comments
- Introduction
Lines 34-38 “Instead, another set…” – Génois et al., 2013 presents an agent-based model of barchan systems in which there is no consideration of the flow and so it is not accurate to say that the work “indirectly highlights the long-range flow-induced interactions”. Long-range interactions in that work occur only through the sand-flux exchange/shadowing i.e. an upwind dune prevents a downwind one from absorbing incoming flux. This has nothing to do with the fluid flow – it is a sediment-induced interaction.
Lines 39-42 “Under unidirectional flows…” – It is not yet established whether this shearing at a long-distance takes place in aeolian dunes or solely in subaqueous experiments and highly turbulent numerical simulations. Some authors have claimed that in aeolian settings it is the case that it results from increased erosion induced by an upwind dune (e.g. Bourke Icarus (2010)) but others (e.g Elbelrhiti Geomorphology (2012)) have instead suggested that is it due to sand flux absorption and not due to erosion. It is worth mentioning that this debate for aeolian dunes has not been resolved.
- Methods
Line 80 “The physical environment…” – This sentence makes it clear that this work considers only the case that the height and length of the cells are equal (i.e. a square lattice) and changing this aspect ratio (e.g. a rectangular lattice) has not been considered.
Line 85 “To take into account…” – How is this angle of repose achieved, surely at an angle of 35 degrees two adjacent cells that differ by a single slab in height would avalanche and so the only stable state would be a flat bed. If two adjacent cells are able to have a difference of a single slab (which is what this reviewer believes was applied in e.g. Zhang et al. JGR (2010)) then the angle of repose is in fact 45 degrees, which is higher than stated here and what it should be for aeolian bedforms.
Line 114 “The elementary length…” – in Narteau et al. (2009) the height:length ratio of the cells in all simulations shown was set to unity and it is not clear in that work how changing the ratio would impact the results due to both flow-coupling changes and avalanche changes as previously mentioned.
Line 121 “Theoretically, this rescaling…” – same comment as above.
Line 126 “To prevent collisions…” – presumably this means that below this initial separation collisions do occur. Since collisions are observed in nature, does this mean that dunes which collide must have formed closer to each other than this distance?
Line 127 “Except for the simulations…” – many linear dunes e.g. those shown in figure 1 form with an angular separation of less than 180 degrees. If the 2D bedforms shown in this work are taken to be cross-sections of these linear dunes then that would mean that different parts of the dunes would be interacting in the primary and secondary modes (since there is a also a longitudinal component to the wind). How is this likely to affect the results presented here?
Line 136 “We save the…” – again, these morphological characteristics might be influenced by choice of the cell shape.
- Results
Line 154 “Regardless of the…” – provided that the initial spacing is above a predetermined length to avoid collisions.
Line 158 “Under these conditions…” – again, if the angular separation of the wind modes is less than 180 degrees different cross-sections of the dunes will be interacting under each mode.
Line 164 “Since this equilibrium…” – in Appendix A of Narteau et al. (2009) it is conceded that changing the aspect ratio of the cells in the model would “allow for the modeling of smaller structures of the flow”. Thus the results here are certain to be influenced by the choice of a square lattice.
Line 172 “In the reattachment…” – the bedforms shown in this work are around 30 cells high, how would this “a few dune heights” change if the cells in the model were e.g. 1/10 as high as they are long and so dunes were 300 cells high?
Section 3.3 (whole thing) – This section goes into detail about the reshaping timescale of the dunes and their shape. All of these processes are likely to be affected by the shape of the cells. If, it turns out that they are unaffected then that is an interesting result worth including.
Line 185 “In the limit…” – As before, how is avalanching achieved at 35 degrees in a square lattice?
Line 219 “The drop 220 observed…” – again, how do the cells affect these aspect ratios and dune size?
Line 226 “Wake flow perturbations…” – what are the characteristic wavelengths in barchan dune field which have non-erodible interdune areas? If it is the flow then surely there would be a coupling between the size of the dunes and their spacing but this has been shown not to be the case in Durán et al. Granular Matter (2009)
Figure 6 – This is an excellent figure.
- Discussion
Line 238 “The mechanism of…” – Given this, it is even more important to understand how the cell shapes are influencing the bedform shape.
Line 262 “This could be…” – as mentioned, how is the distance of the reattachment changed by changing the cell height in this model?
Line 280 “These changes in…” – same comment as previously
Line 290 “For example, before…” – why then do we see collisions between barchans? Is it simply that the fact that you have a three-dimensional dune rather than a two-dimensional cross-section changes the whole behaviour, in which case why should the results of two-dimensional simulations matter for studying barchans?
Line 309 “However, the results…” – this is a really important point and could be emphasised further.
Citation: https://doi.org/10.5194/egusphere-2024-1634-RC1 -
AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
We thank the two anonymous invited reviewers, the voluntary commenter Yuanwei Lin and the associate editor Andreas Baas for their very useful comments which have helped us to improve the quality and the precision of the manuscript.
The main comment made by Reviewer #1 concerns the aspect ratio of the elementary cells of the model: these have a height:length ratio of 1:1 (i.e. square) whereas other cellular automata models use an aspect ratio of 1:3 (i.e. rectangular). Reviewer 1 suggested that new simulations might be carried out to test its impacts on the mechanisms proposed in this article. In this review we provide a detailed discussion of the physical basis of the numerical model used in this paper, arguing that the aspect ratio of cells should not be a parameter of interest in a model integrating the dynamic interactions between topography, basal shear stress and sediment transport. We therefore assume the choice not to carry out new simulations, which would require fundamental changes to the model, implying a timing that is incompatible with that of a review, as well as being unnecessary.
On the other hand, the main comment made by Reviewer #2 focused on the ability of the model to accurately simulate the required flow dynamics and the influence of flow depth. We provide an additional discussion of this point in the manuscript based on new simulations.
More generally, we have provided detailed answers to all major and specific comments from Reviewers #1 and #2, as well as Yuanwei Lin. These replies to reviewer’s comments are addressed in the attached document in red.
Kind regards,
The author team.
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RC2: 'Comment on egusphere-2024-1634', Anonymous Referee #2, 25 Jul 2024
Overall comments
This is a really nice study on dune interactions. The problem is well posed and the simulations address it properly. The discussion feels complete and balanced. The scope is appropriate, but there is plenty of further questions the manuscript produces that leave the reader hoping for more! I have some comments below which should be addressed, but none undermine the quality and novelty of the work. Looking forward to the final publication – I hope my review is useful.
General comments
Can you verify with the cross-sectional area of the dunes that there is no between-dune transport influencing the result? It’s not clear how an initial spacing of 120l_0 is a sufficient guarantee.
How do your results depend on the height of the domain? 100l_0 is about 1/3 of the dune height for the S=2e+3l_0^2 case (as per Figure 2) – there is almost certainly ‘pinching’ of the flow over the dunes in this case. For lower S values, maybe the pinching is less important, but the varying levels of flow pinching between the dune crest and the domain top (akin to fluvial dunes), and it’s role in influencing the stress across the dune stosses may make these results deviate from what could be observed in real aeolian dune systems. I think it’s especially important to test this and discuss it since most of the results in the paper depend on it.
I think it would be worth mentioning if the simulated dune aspect and shape ratios are comparable to observed dunes in nature, to give more weight to the results shown here.
Line comments
Line 20: Flow perturbation by aeolian dunes over long distances (i.e., beyond pair-wise interaction, at the dune-field scale) has been observed if the authors want a reference: https://doi.org/10.1029/2020GL088773
Line 28: This sentence is true for dunes with a sufficiently steep lee side, it might be worth qualifying that a separation bubble does not form for all dunes.
Line 45: “In other” should be “Another”.
Line 109: Controlling flow strength indirectly by \tau_1 is ok, but can you confirm that when you vary it, if you also vary \tau_2 commensurately? A little more explanation of how this isn’t quite the same as explicitly changing flow strength could be useful, e.g., non-linearity of flux-speed relationship, implicit Re-invariance of separation bubble size and speed-up effect, etc.
Line 146: Is \langle\tau_{flat}\rangle measured on the flat topography of an identical domain with no dunes, or on flat topography in a domain with dunes in it? If it’s the latter, then it should be clear what the condition for “away from any topography” is.
Figure 2: “Unidirectionnal” typo in top left plot title. X-axis label on panel b says “x-ct”, while panel f this is omitted.
Figure 3: It would be useful to know what timestep in which experiment panels a and b correspond to, and if it is currently repulsing, attracting, or in equilibrium. In panel b, where does the dispersion band come from? Is it from the 10 random seeds or from multiple timesteps? Panels d and e aren’t labelled.
Line 172: This sentence is critical, yet I don’t think it’s reflected that well in Figure 3b. Here it says that downwind of a dune, once the flow has reattached, the stress is higher than if the dune wasn’t there. That is only shown for the downwind dune, not the upwind dune – it’s probably worth mentioning that when referencing the figure here.
Line 174: Since \Delta\langle\tau_S\rangle is normalised by \langle\tau_{flat}\rangle I think maybe it’s useful to put a \hat{} over it so it’s clear it’s normalised. I know they are all already non-dimensional but this one doubly so. Readers may take the quantities of ~1e-1 on the y-axis of Fig 3e literally.
Section 3.3: This whole section and Figures 4 & 5 are fantastic. Very well explained.
Figure 5: caption for panel a should say “shape ratio (bottom)”. The panel b is really nice, I wonder if it’s possible to draw an analogy to pair-wise interaction potentials (e.g., Lennard-Jones) – especially since at even shorter ranges (i.e., in the recirculation bubble) the curve will be attractive again, right? But for panel b, I still do not think that it’s demonstrated anywhere how the long-range attractive regime can be seen in the surface stress profile downwind of a dune. Shouldn’t you see effectively a vertically flipped version of this curve in the \tau profile downwind of an isolated single dune? It would be great to see that – since in Figure 3b that attractive regime is missing.
Figure 6: It would be very useful to see if there’s any way of collapsing these curves onto a master curve given the control parameters – it may not be simple though. Also, why is the control parameter set sparse? E.g., reversing flow \tau_1/\tau_0=30 data, and (\tau_1/\tau_0<30)&(S<2e+3l_0^2) data, are not shown. Now also, it should be stated when exactly the aspect ratio shown on this graph is being measured – is this the time-averaged aspect ratio, or at the flow reversal, or at t/\DeltaT=1/4, etc.? Figure 5a demonstrates well that this specificity is required.
Line 246: isn’t this perturbation decay the simplest explanation for the slope change in Figure 3e?
Line 253: The sentence “Our model…” should probably come with an explanation of why you think your model is not accurate enough – i.e., provide its limitations.
Line 262: For the sentence “This could be…”, I feel like this could’ve been tested using the simulations. You did not quantify the lengthscales of the recirculation bubble or of the upstream shear-stress drop, or show how they depend on dune shape or size.
Section 4.3: This is an interesting discussion, but I think it’s definitely worth noting earlier on (rather than at the end) that we expect from various data (ReSCAL simulations included) that dune spacing during dune field formation is predominantly controlled by a pattern coarsening process, even when resultant fluxes are small. A ‘steady state’ dune field—or one where coarsening rate is low enough that pair-wise potential between similarly-sized dunes is the dominant control on dune spacing—may not be very common on Earth since climate changes faster than large dunes can keep up with (e.g., https://doi.org/10.2110/jsr.2018.55 and https://doi.org/10.1130/G50837.1). Overall though I believe it’s a provocative and balanced section.
Citation: https://doi.org/10.5194/egusphere-2024-1634-RC2 -
AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
We thank the two anonymous invited reviewers, the voluntary commenter Yuanwei Lin and the associate editor Andreas Baas for their very useful comments which have helped us to improve the quality and the precision of the manuscript.
The main comment made by Reviewer #1 concerns the aspect ratio of the elementary cells of the model: these have a height:length ratio of 1:1 (i.e. square) whereas other cellular automata models use an aspect ratio of 1:3 (i.e. rectangular). Reviewer 1 suggested that new simulations might be carried out to test its impacts on the mechanisms proposed in this article. In this review we provide a detailed discussion of the physical basis of the numerical model used in this paper, arguing that the aspect ratio of cells should not be a parameter of interest in a model integrating the dynamic interactions between topography, basal shear stress and sediment transport. We therefore assume the choice not to carry out new simulations, which would require fundamental changes to the model, implying a timing that is incompatible with that of a review, as well as being unnecessary.
On the other hand, the main comment made by Reviewer #2 focused on the ability of the model to accurately simulate the required flow dynamics and the influence of flow depth. We provide an additional discussion of this point in the manuscript based on new simulations.
More generally, we have provided detailed answers to all major and specific comments from Reviewers #1 and #2, as well as Yuanwei Lin. These replies to reviewer’s comments are addressed in the attached document in red.
Kind regards,
The author team.
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AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
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CC1: 'Comment on egusphere-2024-1634', Yuanwei Lin, 26 Jul 2024
This paper studied the long-range interactions of two sand dunes (and briefly multiple dunes) using a widely-used 2D cellular model (ReSCAL). The current manuscript is well-organized and insightful. However, some aspects of this work require further clarification and revision before publication.
Specific comments:
Fig. 1d & Line 45-49: I recommend further emphasizing that these materials originate from the flume experiments. Also, this figure looks so great that I suspect it may be the result of post-processing water flume experiment data. Therefore, I suggest including the post-processing methods and procedures for clarification.
Line 61: ...or barchans (Zhang et al., 2014; Lin et al., 2024).
Line 64: One sentence (To eliminate...) is obviously insufficient to explain the reason for conducting only 2D simulations. The authors may provide further clarification on this issue by referring Javis et al. (2023).
Line 119-121: It is suggested to mention examples of physical scaling in underwater experiments in this sentence by referring Javis et al. (2023).
Lin 127-129: Since previous publications have studied long-range dune interactions under unidirectional flow (Bacik et al., 2020; He et al., 2023), I was initially confused by the setting of bidirectional flow when I read this sentence. After reading this manuscript several times, I understood that the symmetrical bidirectional flow here corresponds to the evaluation of linear dunes under multi-directional flow regimes, as illustrated in Fig. 1c and d. Therefore, I suggest bridging the setting of bidirectional (or multi-directional) flow with Fig. 1c and d through a few explanatory sentences to enhance readers’ understanding.
Line 129-131: I would like to understand the reason behind implementing 10^4 iterations after each flow reversal and why this specific number 10^4 was selected. Additionally, it is worth explaining whether this parameter setting has been validated through pre-simulation testing.
Line 168-174 & Figs. 3a & b: To my knowledge, this concept and similar figures have been presented many times in previous papers (Wang et al., 2018; Cai et al., 2021), making it unnecessary to emphasize them further in the current manuscript.
Line 216-217: According to this sentence and Fig. 6, the equilibrium distance is affected by three factors: dune size, flow strength and flow reorientation periods. There is an open question regarding whether flow strength and reorientation periods can be integrated into single variable termed flow conditions, i.e., flow intensity with positive and negative values indicating flow reversals.
Line 225: “underlining the long-term morphological impact of crest reversal on dune shapes.” It is recommended to supplemented few sentences to explain in detail the impact of crest reversal on dune shapes.
Fig. 6: The relationships depicted in this figure are highly consistent. Consequently, I anticipate the development of an intriguing unifying law and a compelling figure featuring dimensionless λ_D on the y-axis and a parameter combination of flow conditions and dune size on the x-axis. Certainly, I acknowledge that establishing this law in the short term is challenging and requires further discussion. However, the authors could consider including this idea as part of the prospects in the conclusion of the current manuscript.Typo:
Line 196: “so dunes maintain high aspect ratio and low aspect ratio…”. “aspect ratio” here is repeated.
Line 216: two equal-sized dune –> two equal-sized dunes
Last paragraph of Section 4: There are citation errors here in the sub-figures of Fig. 8.Reference:
Zhang, D., Yang, X., Rozier, O., & Narteau, C. (2014). Mean sediment residence time in barchan dunes. Journal of Geophysical Research: Earth Surface, 119(3), 451-463.
Lin, Y., Guan, K., Zhang, Y., Gao, X., Yang, B., & He, N. (2024). The impact of inter-dune space and volume ratio on barchan dune collision patterns: A numerical simulation study. Physics of Fluids, 36, 011703.
Jarvis, P. A., Narteau, C., Rozier, O., & Vriend, N. M. (2023). The probabilistic nature of dune collisions in 2D. Earth Surface Dynamics, 11, 803-815.
Bacik, K. A., Lovett, S., Caulfield, C. P., & Vriend, N. M. (2020). Wake induced long range repulsion of aqueous dunes. Physical Review Letters, 124(5), 054501.
He, N., Lin, Y., Zhang, Y., Yang, B., & Gao, X. (2023). Self-stabilization of barchan dune chasing. Physics of Fluids, 35, 106609.
Wang, C., & Anderson, W. (2018). Large-eddy simulation of turbulent flow over spanwise-offset barchan dunes: Interdune vortex stretching drives asymmetric erosion. Physical Review E, 98(3), 033112.
Cai, D., Li, S., Gao, X., & Lei, J. (2021). Wind tunnel simulation of the aeolian erosion on the leeward side of barchan dunes and its implications for the spatial distribution patterns of barchan dunes. Catena, 207, 105583.Citation: https://doi.org/10.5194/egusphere-2024-1634-CC1 -
AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
We thank the two anonymous invited reviewers, the voluntary commenter Yuanwei Lin and the associate editor Andreas Baas for their very useful comments which have helped us to improve the quality and the precision of the manuscript.
The main comment made by Reviewer #1 concerns the aspect ratio of the elementary cells of the model: these have a height:length ratio of 1:1 (i.e. square) whereas other cellular automata models use an aspect ratio of 1:3 (i.e. rectangular). Reviewer 1 suggested that new simulations might be carried out to test its impacts on the mechanisms proposed in this article. In this review we provide a detailed discussion of the physical basis of the numerical model used in this paper, arguing that the aspect ratio of cells should not be a parameter of interest in a model integrating the dynamic interactions between topography, basal shear stress and sediment transport. We therefore assume the choice not to carry out new simulations, which would require fundamental changes to the model, implying a timing that is incompatible with that of a review, as well as being unnecessary.
On the other hand, the main comment made by Reviewer #2 focused on the ability of the model to accurately simulate the required flow dynamics and the influence of flow depth. We provide an additional discussion of this point in the manuscript based on new simulations.
More generally, we have provided detailed answers to all major and specific comments from Reviewers #1 and #2, as well as Yuanwei Lin. These replies to reviewer’s comments are addressed in the attached document in red.
Kind regards,
The author team.
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AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
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EC1: 'Comment on egusphere-2024-1634', Andreas Baas, 12 Aug 2024
Dear authors,
Your manuscript has received two encouraging and extensive reviews from the invited referees as well as another detailed and extensive review from the wider community (Yuanwei Lin). All reviewers agree that the work holds a great potential and could make a significant contribution to our understanding of aeolian dune dynamics. The comments, questions, and suggestions should be of great help for revising the work. The key concern from referee #1 regarding the aspect ratio of the sand cells in Rescale is quite crucial however, and may require some further simulation work. I agree with the referee that since the findings presented here are so specifically linked to the cross-sectional shape of the dunes it is really crucial that the potential effect of the 1:1 ratio of the slabs is fully investigated. At the very least, it does appear rather strange that the current 1:1 ratio only allows for a minimum slip face angle of 45 degrees, while a more realistic slope of ~30 degrees would presumably have an impact on the separation envelope and the interdune spacings, for example.
Referee #2, meanwhile, has focused a number of key questions on the ability of the model to accurately simulate the required flow dynamics, as for example the concern about the limited vertical height of the simulation domain relative to dune heights. I believe these are also important questions to address since your findings rely specifically on the veracity of the flow dynamics.
The voluntary comments from Yuanwei Lin are also clearly relevant and useful.
For the final phase of the current interactive discussion, please post a brief general reply to the three reviews indicating how you intend to respond to the detailed questions and suggestions. After that we encourage you to submit a fully revised manuscript (potentially including further simulation and analysis work if required to address the referee comments), together with a detailed author-response document, which will be sent out to the two reviewers for a second round of evaluation.
Please do not hesitate to request any deadline extensions where needed,
Andreas Baas, handling editor
Citation: https://doi.org/10.5194/egusphere-2024-1634-EC1 -
AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
We thank the two anonymous invited reviewers, the voluntary commenter Yuanwei Lin and the associate editor Andreas Baas for their very useful comments which have helped us to improve the quality and the precision of the manuscript.
The main comment made by Reviewer #1 concerns the aspect ratio of the elementary cells of the model: these have a height:length ratio of 1:1 (i.e. square) whereas other cellular automata models use an aspect ratio of 1:3 (i.e. rectangular). Reviewer 1 suggested that new simulations might be carried out to test its impacts on the mechanisms proposed in this article. In this review we provide a detailed discussion of the physical basis of the numerical model used in this paper, arguing that the aspect ratio of cells should not be a parameter of interest in a model integrating the dynamic interactions between topography, basal shear stress and sediment transport. We therefore assume the choice not to carry out new simulations, which would require fundamental changes to the model, implying a timing that is incompatible with that of a review, as well as being unnecessary.
On the other hand, the main comment made by Reviewer #2 focused on the ability of the model to accurately simulate the required flow dynamics and the influence of flow depth. We provide an additional discussion of this point in the manuscript based on new simulations.
More generally, we have provided detailed answers to all major and specific comments from Reviewers #1 and #2, as well as Yuanwei Lin. These replies to reviewer’s comments are addressed in the attached document in red.
Kind regards,
The author team.
-
AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
-
AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
We thank the two anonymous invited reviewers, the voluntary commenter Yuanwei Lin and the associate editor Andreas Baas for their very useful comments which have helped us to improve the quality and the precision of the manuscript.
The main comment made by Reviewer #1 concerns the aspect ratio of the elementary cells of the model: these have a height:length ratio of 1:1 (i.e. square) whereas other cellular automata models use an aspect ratio of 1:3 (i.e. rectangular). Reviewer 1 suggested that new simulations might be carried out to test its impacts on the mechanisms proposed in this article. In this review we provide a detailed discussion of the physical basis of the numerical model used in this paper, arguing that the aspect ratio of cells should not be a parameter of interest in a model integrating the dynamic interactions between topography, basal shear stress and sediment transport. We therefore assume the choice not to carry out new simulations, which would require fundamental changes to the model, implying a timing that is incompatible with that of a review, as well as being unnecessary.
On the other hand, the main comment made by Reviewer #2 focused on the ability of the model to accurately simulate the required flow dynamics and the influence of flow depth. We provide an additional discussion of this point in the manuscript based on new simulations.
More generally, we have provided detailed answers to all major and specific comments from Reviewers #1 and #2, as well as Yuanwei Lin. These replies to reviewer’s comments are addressed in the attached document in red.
Kind regards,
The author team.
Status: closed
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RC1: 'Comment on egusphere-2024-1634', Anonymous Referee #1, 22 Jul 2024
Equilibrium distance from long-range dune interactions
Vérité et al. (2024) preprint
General comments
The authors present an application of a well-established and widely-used numerical model (ReSCAL) to uncover the influence of long-range interactions on two-body (and briefly several-body) dynamics of simulated bedforms exposed to different flow regimes. The manuscript is well-structured and well-written, and the figures are of an excellent standard and the authors should be congratulated. The work will be of interest to those who study aeolian systems and compliments some of the existing literature, particularly the properties of systems of linear dunes and quasi-two-dimensional flume experiments.
However, there are some aspects of the work that this reviewer believes require additional investigation of clarification before the work is ready for publication. Chief among these concerns is that the authors have highlighted in this manuscript the importance of the “dune” shape in governing the interactions that they observe. The shape of the dune, in particular the aspect ratio, is found to govern much of the dynamical behaviour which is observed. However, to the knowledge of this reviewer, the effect of changing the elementary aspect ratio (i.e. height to length or width) of the sand slabs (cells) from which the bedforms are formed has never been investigated (other than a brief mention in Appendix A of Narteau et al. (2009)). Thus, one cannot be certain whether the results presented here are universal outputs of the model or hold only in the specific case of a 1:1 cell height:width ratio.
Changing the elementary aspect ratio may affect coupling between the bedform and lattice gas since the bedforms are likely to be comprised of a greater number of sand slabs if the cell height is less than and so the removal of a single slab due to erosion/transport will have less of an impact on the shape/size of the bedform and thus result in a smaller perturbation of the flow. Changing the height of the slabs would also affect the avalanche procedure which could in turn influence the shape (and migration etc) of the bedforms.
Although this reviewer concedes that there is a precedent for publication of studies applying ReSCAL without an investigation of the affect of changing the cell height:width ratio, this manuscript places a significant emphasis on the role of shape and bedform aspect ratio on governing the interactions. As such, this reviewer feels that it is important to ascertain how the findings of this study would change if the elementary aspect ratio is varied before publication.
Below follow some more specific comments regarding the manuscript as presented.
Specific comments
- Introduction
Lines 34-38 “Instead, another set…” – Génois et al., 2013 presents an agent-based model of barchan systems in which there is no consideration of the flow and so it is not accurate to say that the work “indirectly highlights the long-range flow-induced interactions”. Long-range interactions in that work occur only through the sand-flux exchange/shadowing i.e. an upwind dune prevents a downwind one from absorbing incoming flux. This has nothing to do with the fluid flow – it is a sediment-induced interaction.
Lines 39-42 “Under unidirectional flows…” – It is not yet established whether this shearing at a long-distance takes place in aeolian dunes or solely in subaqueous experiments and highly turbulent numerical simulations. Some authors have claimed that in aeolian settings it is the case that it results from increased erosion induced by an upwind dune (e.g. Bourke Icarus (2010)) but others (e.g Elbelrhiti Geomorphology (2012)) have instead suggested that is it due to sand flux absorption and not due to erosion. It is worth mentioning that this debate for aeolian dunes has not been resolved.
- Methods
Line 80 “The physical environment…” – This sentence makes it clear that this work considers only the case that the height and length of the cells are equal (i.e. a square lattice) and changing this aspect ratio (e.g. a rectangular lattice) has not been considered.
Line 85 “To take into account…” – How is this angle of repose achieved, surely at an angle of 35 degrees two adjacent cells that differ by a single slab in height would avalanche and so the only stable state would be a flat bed. If two adjacent cells are able to have a difference of a single slab (which is what this reviewer believes was applied in e.g. Zhang et al. JGR (2010)) then the angle of repose is in fact 45 degrees, which is higher than stated here and what it should be for aeolian bedforms.
Line 114 “The elementary length…” – in Narteau et al. (2009) the height:length ratio of the cells in all simulations shown was set to unity and it is not clear in that work how changing the ratio would impact the results due to both flow-coupling changes and avalanche changes as previously mentioned.
Line 121 “Theoretically, this rescaling…” – same comment as above.
Line 126 “To prevent collisions…” – presumably this means that below this initial separation collisions do occur. Since collisions are observed in nature, does this mean that dunes which collide must have formed closer to each other than this distance?
Line 127 “Except for the simulations…” – many linear dunes e.g. those shown in figure 1 form with an angular separation of less than 180 degrees. If the 2D bedforms shown in this work are taken to be cross-sections of these linear dunes then that would mean that different parts of the dunes would be interacting in the primary and secondary modes (since there is a also a longitudinal component to the wind). How is this likely to affect the results presented here?
Line 136 “We save the…” – again, these morphological characteristics might be influenced by choice of the cell shape.
- Results
Line 154 “Regardless of the…” – provided that the initial spacing is above a predetermined length to avoid collisions.
Line 158 “Under these conditions…” – again, if the angular separation of the wind modes is less than 180 degrees different cross-sections of the dunes will be interacting under each mode.
Line 164 “Since this equilibrium…” – in Appendix A of Narteau et al. (2009) it is conceded that changing the aspect ratio of the cells in the model would “allow for the modeling of smaller structures of the flow”. Thus the results here are certain to be influenced by the choice of a square lattice.
Line 172 “In the reattachment…” – the bedforms shown in this work are around 30 cells high, how would this “a few dune heights” change if the cells in the model were e.g. 1/10 as high as they are long and so dunes were 300 cells high?
Section 3.3 (whole thing) – This section goes into detail about the reshaping timescale of the dunes and their shape. All of these processes are likely to be affected by the shape of the cells. If, it turns out that they are unaffected then that is an interesting result worth including.
Line 185 “In the limit…” – As before, how is avalanching achieved at 35 degrees in a square lattice?
Line 219 “The drop 220 observed…” – again, how do the cells affect these aspect ratios and dune size?
Line 226 “Wake flow perturbations…” – what are the characteristic wavelengths in barchan dune field which have non-erodible interdune areas? If it is the flow then surely there would be a coupling between the size of the dunes and their spacing but this has been shown not to be the case in Durán et al. Granular Matter (2009)
Figure 6 – This is an excellent figure.
- Discussion
Line 238 “The mechanism of…” – Given this, it is even more important to understand how the cell shapes are influencing the bedform shape.
Line 262 “This could be…” – as mentioned, how is the distance of the reattachment changed by changing the cell height in this model?
Line 280 “These changes in…” – same comment as previously
Line 290 “For example, before…” – why then do we see collisions between barchans? Is it simply that the fact that you have a three-dimensional dune rather than a two-dimensional cross-section changes the whole behaviour, in which case why should the results of two-dimensional simulations matter for studying barchans?
Line 309 “However, the results…” – this is a really important point and could be emphasised further.
Citation: https://doi.org/10.5194/egusphere-2024-1634-RC1 -
AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
We thank the two anonymous invited reviewers, the voluntary commenter Yuanwei Lin and the associate editor Andreas Baas for their very useful comments which have helped us to improve the quality and the precision of the manuscript.
The main comment made by Reviewer #1 concerns the aspect ratio of the elementary cells of the model: these have a height:length ratio of 1:1 (i.e. square) whereas other cellular automata models use an aspect ratio of 1:3 (i.e. rectangular). Reviewer 1 suggested that new simulations might be carried out to test its impacts on the mechanisms proposed in this article. In this review we provide a detailed discussion of the physical basis of the numerical model used in this paper, arguing that the aspect ratio of cells should not be a parameter of interest in a model integrating the dynamic interactions between topography, basal shear stress and sediment transport. We therefore assume the choice not to carry out new simulations, which would require fundamental changes to the model, implying a timing that is incompatible with that of a review, as well as being unnecessary.
On the other hand, the main comment made by Reviewer #2 focused on the ability of the model to accurately simulate the required flow dynamics and the influence of flow depth. We provide an additional discussion of this point in the manuscript based on new simulations.
More generally, we have provided detailed answers to all major and specific comments from Reviewers #1 and #2, as well as Yuanwei Lin. These replies to reviewer’s comments are addressed in the attached document in red.
Kind regards,
The author team.
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RC2: 'Comment on egusphere-2024-1634', Anonymous Referee #2, 25 Jul 2024
Overall comments
This is a really nice study on dune interactions. The problem is well posed and the simulations address it properly. The discussion feels complete and balanced. The scope is appropriate, but there is plenty of further questions the manuscript produces that leave the reader hoping for more! I have some comments below which should be addressed, but none undermine the quality and novelty of the work. Looking forward to the final publication – I hope my review is useful.
General comments
Can you verify with the cross-sectional area of the dunes that there is no between-dune transport influencing the result? It’s not clear how an initial spacing of 120l_0 is a sufficient guarantee.
How do your results depend on the height of the domain? 100l_0 is about 1/3 of the dune height for the S=2e+3l_0^2 case (as per Figure 2) – there is almost certainly ‘pinching’ of the flow over the dunes in this case. For lower S values, maybe the pinching is less important, but the varying levels of flow pinching between the dune crest and the domain top (akin to fluvial dunes), and it’s role in influencing the stress across the dune stosses may make these results deviate from what could be observed in real aeolian dune systems. I think it’s especially important to test this and discuss it since most of the results in the paper depend on it.
I think it would be worth mentioning if the simulated dune aspect and shape ratios are comparable to observed dunes in nature, to give more weight to the results shown here.
Line comments
Line 20: Flow perturbation by aeolian dunes over long distances (i.e., beyond pair-wise interaction, at the dune-field scale) has been observed if the authors want a reference: https://doi.org/10.1029/2020GL088773
Line 28: This sentence is true for dunes with a sufficiently steep lee side, it might be worth qualifying that a separation bubble does not form for all dunes.
Line 45: “In other” should be “Another”.
Line 109: Controlling flow strength indirectly by \tau_1 is ok, but can you confirm that when you vary it, if you also vary \tau_2 commensurately? A little more explanation of how this isn’t quite the same as explicitly changing flow strength could be useful, e.g., non-linearity of flux-speed relationship, implicit Re-invariance of separation bubble size and speed-up effect, etc.
Line 146: Is \langle\tau_{flat}\rangle measured on the flat topography of an identical domain with no dunes, or on flat topography in a domain with dunes in it? If it’s the latter, then it should be clear what the condition for “away from any topography” is.
Figure 2: “Unidirectionnal” typo in top left plot title. X-axis label on panel b says “x-ct”, while panel f this is omitted.
Figure 3: It would be useful to know what timestep in which experiment panels a and b correspond to, and if it is currently repulsing, attracting, or in equilibrium. In panel b, where does the dispersion band come from? Is it from the 10 random seeds or from multiple timesteps? Panels d and e aren’t labelled.
Line 172: This sentence is critical, yet I don’t think it’s reflected that well in Figure 3b. Here it says that downwind of a dune, once the flow has reattached, the stress is higher than if the dune wasn’t there. That is only shown for the downwind dune, not the upwind dune – it’s probably worth mentioning that when referencing the figure here.
Line 174: Since \Delta\langle\tau_S\rangle is normalised by \langle\tau_{flat}\rangle I think maybe it’s useful to put a \hat{} over it so it’s clear it’s normalised. I know they are all already non-dimensional but this one doubly so. Readers may take the quantities of ~1e-1 on the y-axis of Fig 3e literally.
Section 3.3: This whole section and Figures 4 & 5 are fantastic. Very well explained.
Figure 5: caption for panel a should say “shape ratio (bottom)”. The panel b is really nice, I wonder if it’s possible to draw an analogy to pair-wise interaction potentials (e.g., Lennard-Jones) – especially since at even shorter ranges (i.e., in the recirculation bubble) the curve will be attractive again, right? But for panel b, I still do not think that it’s demonstrated anywhere how the long-range attractive regime can be seen in the surface stress profile downwind of a dune. Shouldn’t you see effectively a vertically flipped version of this curve in the \tau profile downwind of an isolated single dune? It would be great to see that – since in Figure 3b that attractive regime is missing.
Figure 6: It would be very useful to see if there’s any way of collapsing these curves onto a master curve given the control parameters – it may not be simple though. Also, why is the control parameter set sparse? E.g., reversing flow \tau_1/\tau_0=30 data, and (\tau_1/\tau_0<30)&(S<2e+3l_0^2) data, are not shown. Now also, it should be stated when exactly the aspect ratio shown on this graph is being measured – is this the time-averaged aspect ratio, or at the flow reversal, or at t/\DeltaT=1/4, etc.? Figure 5a demonstrates well that this specificity is required.
Line 246: isn’t this perturbation decay the simplest explanation for the slope change in Figure 3e?
Line 253: The sentence “Our model…” should probably come with an explanation of why you think your model is not accurate enough – i.e., provide its limitations.
Line 262: For the sentence “This could be…”, I feel like this could’ve been tested using the simulations. You did not quantify the lengthscales of the recirculation bubble or of the upstream shear-stress drop, or show how they depend on dune shape or size.
Section 4.3: This is an interesting discussion, but I think it’s definitely worth noting earlier on (rather than at the end) that we expect from various data (ReSCAL simulations included) that dune spacing during dune field formation is predominantly controlled by a pattern coarsening process, even when resultant fluxes are small. A ‘steady state’ dune field—or one where coarsening rate is low enough that pair-wise potential between similarly-sized dunes is the dominant control on dune spacing—may not be very common on Earth since climate changes faster than large dunes can keep up with (e.g., https://doi.org/10.2110/jsr.2018.55 and https://doi.org/10.1130/G50837.1). Overall though I believe it’s a provocative and balanced section.
Citation: https://doi.org/10.5194/egusphere-2024-1634-RC2 -
AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
We thank the two anonymous invited reviewers, the voluntary commenter Yuanwei Lin and the associate editor Andreas Baas for their very useful comments which have helped us to improve the quality and the precision of the manuscript.
The main comment made by Reviewer #1 concerns the aspect ratio of the elementary cells of the model: these have a height:length ratio of 1:1 (i.e. square) whereas other cellular automata models use an aspect ratio of 1:3 (i.e. rectangular). Reviewer 1 suggested that new simulations might be carried out to test its impacts on the mechanisms proposed in this article. In this review we provide a detailed discussion of the physical basis of the numerical model used in this paper, arguing that the aspect ratio of cells should not be a parameter of interest in a model integrating the dynamic interactions between topography, basal shear stress and sediment transport. We therefore assume the choice not to carry out new simulations, which would require fundamental changes to the model, implying a timing that is incompatible with that of a review, as well as being unnecessary.
On the other hand, the main comment made by Reviewer #2 focused on the ability of the model to accurately simulate the required flow dynamics and the influence of flow depth. We provide an additional discussion of this point in the manuscript based on new simulations.
More generally, we have provided detailed answers to all major and specific comments from Reviewers #1 and #2, as well as Yuanwei Lin. These replies to reviewer’s comments are addressed in the attached document in red.
Kind regards,
The author team.
-
AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
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CC1: 'Comment on egusphere-2024-1634', Yuanwei Lin, 26 Jul 2024
This paper studied the long-range interactions of two sand dunes (and briefly multiple dunes) using a widely-used 2D cellular model (ReSCAL). The current manuscript is well-organized and insightful. However, some aspects of this work require further clarification and revision before publication.
Specific comments:
Fig. 1d & Line 45-49: I recommend further emphasizing that these materials originate from the flume experiments. Also, this figure looks so great that I suspect it may be the result of post-processing water flume experiment data. Therefore, I suggest including the post-processing methods and procedures for clarification.
Line 61: ...or barchans (Zhang et al., 2014; Lin et al., 2024).
Line 64: One sentence (To eliminate...) is obviously insufficient to explain the reason for conducting only 2D simulations. The authors may provide further clarification on this issue by referring Javis et al. (2023).
Line 119-121: It is suggested to mention examples of physical scaling in underwater experiments in this sentence by referring Javis et al. (2023).
Lin 127-129: Since previous publications have studied long-range dune interactions under unidirectional flow (Bacik et al., 2020; He et al., 2023), I was initially confused by the setting of bidirectional flow when I read this sentence. After reading this manuscript several times, I understood that the symmetrical bidirectional flow here corresponds to the evaluation of linear dunes under multi-directional flow regimes, as illustrated in Fig. 1c and d. Therefore, I suggest bridging the setting of bidirectional (or multi-directional) flow with Fig. 1c and d through a few explanatory sentences to enhance readers’ understanding.
Line 129-131: I would like to understand the reason behind implementing 10^4 iterations after each flow reversal and why this specific number 10^4 was selected. Additionally, it is worth explaining whether this parameter setting has been validated through pre-simulation testing.
Line 168-174 & Figs. 3a & b: To my knowledge, this concept and similar figures have been presented many times in previous papers (Wang et al., 2018; Cai et al., 2021), making it unnecessary to emphasize them further in the current manuscript.
Line 216-217: According to this sentence and Fig. 6, the equilibrium distance is affected by three factors: dune size, flow strength and flow reorientation periods. There is an open question regarding whether flow strength and reorientation periods can be integrated into single variable termed flow conditions, i.e., flow intensity with positive and negative values indicating flow reversals.
Line 225: “underlining the long-term morphological impact of crest reversal on dune shapes.” It is recommended to supplemented few sentences to explain in detail the impact of crest reversal on dune shapes.
Fig. 6: The relationships depicted in this figure are highly consistent. Consequently, I anticipate the development of an intriguing unifying law and a compelling figure featuring dimensionless λ_D on the y-axis and a parameter combination of flow conditions and dune size on the x-axis. Certainly, I acknowledge that establishing this law in the short term is challenging and requires further discussion. However, the authors could consider including this idea as part of the prospects in the conclusion of the current manuscript.Typo:
Line 196: “so dunes maintain high aspect ratio and low aspect ratio…”. “aspect ratio” here is repeated.
Line 216: two equal-sized dune –> two equal-sized dunes
Last paragraph of Section 4: There are citation errors here in the sub-figures of Fig. 8.Reference:
Zhang, D., Yang, X., Rozier, O., & Narteau, C. (2014). Mean sediment residence time in barchan dunes. Journal of Geophysical Research: Earth Surface, 119(3), 451-463.
Lin, Y., Guan, K., Zhang, Y., Gao, X., Yang, B., & He, N. (2024). The impact of inter-dune space and volume ratio on barchan dune collision patterns: A numerical simulation study. Physics of Fluids, 36, 011703.
Jarvis, P. A., Narteau, C., Rozier, O., & Vriend, N. M. (2023). The probabilistic nature of dune collisions in 2D. Earth Surface Dynamics, 11, 803-815.
Bacik, K. A., Lovett, S., Caulfield, C. P., & Vriend, N. M. (2020). Wake induced long range repulsion of aqueous dunes. Physical Review Letters, 124(5), 054501.
He, N., Lin, Y., Zhang, Y., Yang, B., & Gao, X. (2023). Self-stabilization of barchan dune chasing. Physics of Fluids, 35, 106609.
Wang, C., & Anderson, W. (2018). Large-eddy simulation of turbulent flow over spanwise-offset barchan dunes: Interdune vortex stretching drives asymmetric erosion. Physical Review E, 98(3), 033112.
Cai, D., Li, S., Gao, X., & Lei, J. (2021). Wind tunnel simulation of the aeolian erosion on the leeward side of barchan dunes and its implications for the spatial distribution patterns of barchan dunes. Catena, 207, 105583.Citation: https://doi.org/10.5194/egusphere-2024-1634-CC1 -
AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
We thank the two anonymous invited reviewers, the voluntary commenter Yuanwei Lin and the associate editor Andreas Baas for their very useful comments which have helped us to improve the quality and the precision of the manuscript.
The main comment made by Reviewer #1 concerns the aspect ratio of the elementary cells of the model: these have a height:length ratio of 1:1 (i.e. square) whereas other cellular automata models use an aspect ratio of 1:3 (i.e. rectangular). Reviewer 1 suggested that new simulations might be carried out to test its impacts on the mechanisms proposed in this article. In this review we provide a detailed discussion of the physical basis of the numerical model used in this paper, arguing that the aspect ratio of cells should not be a parameter of interest in a model integrating the dynamic interactions between topography, basal shear stress and sediment transport. We therefore assume the choice not to carry out new simulations, which would require fundamental changes to the model, implying a timing that is incompatible with that of a review, as well as being unnecessary.
On the other hand, the main comment made by Reviewer #2 focused on the ability of the model to accurately simulate the required flow dynamics and the influence of flow depth. We provide an additional discussion of this point in the manuscript based on new simulations.
More generally, we have provided detailed answers to all major and specific comments from Reviewers #1 and #2, as well as Yuanwei Lin. These replies to reviewer’s comments are addressed in the attached document in red.
Kind regards,
The author team.
-
AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
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EC1: 'Comment on egusphere-2024-1634', Andreas Baas, 12 Aug 2024
Dear authors,
Your manuscript has received two encouraging and extensive reviews from the invited referees as well as another detailed and extensive review from the wider community (Yuanwei Lin). All reviewers agree that the work holds a great potential and could make a significant contribution to our understanding of aeolian dune dynamics. The comments, questions, and suggestions should be of great help for revising the work. The key concern from referee #1 regarding the aspect ratio of the sand cells in Rescale is quite crucial however, and may require some further simulation work. I agree with the referee that since the findings presented here are so specifically linked to the cross-sectional shape of the dunes it is really crucial that the potential effect of the 1:1 ratio of the slabs is fully investigated. At the very least, it does appear rather strange that the current 1:1 ratio only allows for a minimum slip face angle of 45 degrees, while a more realistic slope of ~30 degrees would presumably have an impact on the separation envelope and the interdune spacings, for example.
Referee #2, meanwhile, has focused a number of key questions on the ability of the model to accurately simulate the required flow dynamics, as for example the concern about the limited vertical height of the simulation domain relative to dune heights. I believe these are also important questions to address since your findings rely specifically on the veracity of the flow dynamics.
The voluntary comments from Yuanwei Lin are also clearly relevant and useful.
For the final phase of the current interactive discussion, please post a brief general reply to the three reviews indicating how you intend to respond to the detailed questions and suggestions. After that we encourage you to submit a fully revised manuscript (potentially including further simulation and analysis work if required to address the referee comments), together with a detailed author-response document, which will be sent out to the two reviewers for a second round of evaluation.
Please do not hesitate to request any deadline extensions where needed,
Andreas Baas, handling editor
Citation: https://doi.org/10.5194/egusphere-2024-1634-EC1 -
AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
We thank the two anonymous invited reviewers, the voluntary commenter Yuanwei Lin and the associate editor Andreas Baas for their very useful comments which have helped us to improve the quality and the precision of the manuscript.
The main comment made by Reviewer #1 concerns the aspect ratio of the elementary cells of the model: these have a height:length ratio of 1:1 (i.e. square) whereas other cellular automata models use an aspect ratio of 1:3 (i.e. rectangular). Reviewer 1 suggested that new simulations might be carried out to test its impacts on the mechanisms proposed in this article. In this review we provide a detailed discussion of the physical basis of the numerical model used in this paper, arguing that the aspect ratio of cells should not be a parameter of interest in a model integrating the dynamic interactions between topography, basal shear stress and sediment transport. We therefore assume the choice not to carry out new simulations, which would require fundamental changes to the model, implying a timing that is incompatible with that of a review, as well as being unnecessary.
On the other hand, the main comment made by Reviewer #2 focused on the ability of the model to accurately simulate the required flow dynamics and the influence of flow depth. We provide an additional discussion of this point in the manuscript based on new simulations.
More generally, we have provided detailed answers to all major and specific comments from Reviewers #1 and #2, as well as Yuanwei Lin. These replies to reviewer’s comments are addressed in the attached document in red.
Kind regards,
The author team.
-
AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
-
AC1: 'Comment on egusphere-2024-1634 - General reply to review's comments', Jean Vérité, 30 Aug 2024
We thank the two anonymous invited reviewers, the voluntary commenter Yuanwei Lin and the associate editor Andreas Baas for their very useful comments which have helped us to improve the quality and the precision of the manuscript.
The main comment made by Reviewer #1 concerns the aspect ratio of the elementary cells of the model: these have a height:length ratio of 1:1 (i.e. square) whereas other cellular automata models use an aspect ratio of 1:3 (i.e. rectangular). Reviewer 1 suggested that new simulations might be carried out to test its impacts on the mechanisms proposed in this article. In this review we provide a detailed discussion of the physical basis of the numerical model used in this paper, arguing that the aspect ratio of cells should not be a parameter of interest in a model integrating the dynamic interactions between topography, basal shear stress and sediment transport. We therefore assume the choice not to carry out new simulations, which would require fundamental changes to the model, implying a timing that is incompatible with that of a review, as well as being unnecessary.
On the other hand, the main comment made by Reviewer #2 focused on the ability of the model to accurately simulate the required flow dynamics and the influence of flow depth. We provide an additional discussion of this point in the manuscript based on new simulations.
More generally, we have provided detailed answers to all major and specific comments from Reviewers #1 and #2, as well as Yuanwei Lin. These replies to reviewer’s comments are addressed in the attached document in red.
Kind regards,
The author team.
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