Peer review of: A bedform phase diagram for dense granular currents (2019)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
Review of Nature Communications manuscript: 
A bedform phase diagram for dense granular currents 
Authors: Smith, Rowley, Williams, Giordano, Trolese, Silleni, Parsons, and Capon 
 
Reviewer: Benjamin Andrews, Smithsonian Global Volcanism Program 
 
Smith et al. present experimental results that show how certain depositional structures generally 
attributed to transport and deposition from turbulent particle-laden density currents can be formed 
from granular flows. Specifically, the authors show that regressive dune bedforms (that is, 
upstream propagating dune structures) can be formed by an upstream propagating granular bore 
under waning flow conditions. The conditions at which this process occurs are predictable based 
upon the changes in the flow’s Froude and/or Friction numbers. The authors use these insights to 
suggest that portions of the Pozzolane Rosse ignimbrite featuring steep regressive dune bedforms 
were deposited by granular flows (not by dilute surges). 
 
The implications of this study are very interesting. As Smith et al. point out, much of our 
knowledge of PDCs (and thus our ability to forecast their behavior) is based upon interpretation of 
PDC deposits. Much of our understanding of the processes that form deposits is, however, based 
upon deposition in fluvial systems; studies in recent years have shown that the complex multi-
phase and multi-regime currents that are PDCs exhibit transport and depositional behaviors that 
are not always represented in fluvial systems. To that end, this study is a strong contribution 
towards elucidating how PDC deposits can form. Indeed, the paper is nicely focused and provides a 
good case study of how bedforms that might be attributed to turbulent or tractional processes can 
instead record a purely granular process. In addition, the authors present their findings in the 
context of simple scaling (this is good and elegant!) that shows how laboratory experiments can 
apply to natural currents 1000s of times larger. 
 
The paper could be strengthened with a few minor revisions. First, the methods section should 
explicitly describe the experimental durations, and the authors should mention the upstream 
propagation velocity of the granular bore; together, this will allow the reader to assess how 
steadiness/unsteadiness/impulsiveness of the current affects the bedforms (this will also allow for 
a richer interpretation of natural PDCs and the time recorded by individual depositional units). 
Second, the authors focus on regressive bedforms, which is appropriate, given that is what their 
experiments produced, but a brief mention (even 1 sentence) in the discussion about whether or 
not granular flows can form progressive bedforms would be welcome. 
 
In summary, this is a well-written paper. Smith et al. clearly present experimental results and 
discuss their findings in the context of a natural ignimbrite deposit. The study has important 
implications for the evaluation (or re-evaluation and interpretation) of pyroclastic density current 
deposits around the world. I recommend publication with very minor revision. 
 
Benjamin J. Andrews 
Smithsonian Global Volcanism Program 
 
Specific comments: 
Line 47 – the second “PDCs” should have a possessive apostrophe, “PDC’s” 
Lines 91-95 – were there no progressive dune bedforms observed in your experiments? If so, 
please state that clearly in this paragraph. 
Eq. 2 and Eq. 3 – this is very minor, but please put the terms in the numerators of the equations 
in the same order – this makes it easier to see that NB and NS have the same numerators. 
Fig. 4 – please label the panels a, b, c, d. 



Reviewer #2 (Remarks to the Author): 
 
Dear authors and editor, 
 
I have read with attention the manuscript by Smith et al. entitled "A bedform phase diagram for 
dense granular currents" submitted to Nature. 
 
-I declare here that the main author sent me spontaneously the submitted version of his 
manuscript by email, after we met during EGU2019 and I had asked for news. I do not consider 
this creating any conflict of interest and feel perfectly safe about providing a review.- 
 
This is a nice experimental study with fluidized granular flows that show how concentrated PDCs 
could form backset beddings with cross-lineations or cross-stratifications. It shows how a kind of 
bore is triggered by the forming bedform and becomes unstably steepening. The experimental 
results are then compared to field examples, and a phase diagram based on the experimental 
results is suggested. 
 
In brief, the introduction is in general clear and understandable.The experiments are amazing and 
novel. I agree with the interpretation, and the data is strengthening many ideas that I raised in 
several field based manuscripts about dune bedforms forming from dense granular currents (see 
main comment 2 and line by line 209-213). Something I'd like to suggest to discuss or take into 
account is the possibility of a "granular jamming" in these experiments (it seems described but not 
called). 
 
Some problems with the writing/structure can be corrected quite easily (yet still involving a bit of 
work). In particular, I think it really needs a better transition at the end of introduction. I would 
also put more accent on the bed-aeration story /fluidized-currents. I think the part on the field 
examples can be significantly shortened to focus on the main story delivered by the experiments. I 
have two bigger negative comment: 

1. The manuscript makes a confusion between cross-laminations (i.e. mm scale laminae that are 
not planar) and dune bedforms (anything above the m-scale that shows a non planar surface or 
paleo-surface). It then states that the presence of regressive bedforms is accepted as proving 
parental dilute PDCs. This is, to my opinion, a strong amalgamation of ideas and wrong. Of course, 
regressive bedforms can form from concentrated granular PDCs, and several studies have pointed 
that out (Brown and Branney 2004, Douillet et al. 2013b, 2018b, see main comment 2). I disagree 
that there is a novelty in that part. 

2. I am really unsatisfied by the field pictures, which I don't find are serving as it should the 
manuscript. 
 
All in all, this is a nice study and the comments can be adapted relatively easily (involving quite a 
bit of time though). Please find below my 6 main comments, and then more punctual line-by-line 
comments. I'm honestly not sure that anyone cares about dune bedforms in pyroclastic currents 
apart from me and the authors (but this is only a depressed late Friday thought...), so who knows 
how this study will be received by the broad audience of Nature... I'd vote to give it a chance. 
 
Regards, 
Guilhem Amin Douillet 
 
The rest of the comments are available from the attached pdf file 
 
 
Reviewer #3 (Remarks to the Author): 

This is an interesting paper that makes a decent contribution to understanding of bedforms in an 
experimental setup and some possible implications for pyroclastic density currents. 
 
However at present there are three major problems with this paper which I my opinion must be 
addressed fully before it can be considered for publication: 
 
1. More needs to be done to justify why these experiments are analogous to hot, gas-driven PDCs. 
What about scaling between the experiments and natural PDCs? Also what about the grainsize 
differences. These are clearly mentioned and that there are important differences but not what the 
implications are. Also there is mention that trains of bedforms are not produced (that are seen in 
reality). Why not? These kind of things – grainsize differences, bedform differences, all should be 
discussed more so that the drawbacks and limitations of the experiments are clear when 
comparing to reality. 
 
2. The material on the ‘Field Analogue’ - the Pozzolane Rosse ignimbrite is far too cursory to 
support what is being proposed here. It would take a considerable amount of additional text and 
possibly figures too to justify the ‘bedforms in ignimbrite’ idea. The reader has to be absolutely 
convinced that these are not related to a pulse of the eruption with quite different physical 
characteristics. It is essential to demonstrate that these bedforms are intrinsically related to the 
massive deposits. 
 
One solution which could address this would be the removal of the 'Field analogue' part. I think 
that the paper makes a large enough contribution without the Field Analogue part and a simple 
option would be to remove that and just focus on the bedforms phase diagrams idea. Which is 
what the paper title is. At present it appears that the Field Analogue part is just an afterthought. 
 
3. There are many places in the paper where it would benefit enormously from more detail and/ or 
more extensive discussion (as indicated below in specific points). At present many points, issues 
etc don’t get the discussion they need to and many of the arguments end up appearing cursory 
and not fully discussed. 
 
 
Specific problems 
Line 60 Ignimbrite – a widely used term in volcanology but it needs to be defined carefully here. 
Some of the references given here are for bedforms generated in deposits which would not be 
defined as ignimbrite by many volcanologists. For example I don’t think many Volcanologists would 
term thinly bedded PDC deposits associated with small tuff cones as ignimbrite. So for this reason 
it is essential to define exactly what you mean by ignimbrite. 
 
Line 71 – The regressive / progressive subdivision of bedforms comes from the sedimentology 
literature. Allen (1984) should be cited here. Even in the volcanological literature it didn’t start 
with Brand et al 2016. So I think that the literature can be given a bit more acknowledgement 
here. 
 
Line 81 – Quasi steady currents - this is proposed without any justification as far as I can see. 
Perhaps I am missing something in the odd setup of the journal with methods etc given later, but 
if there is further information please give it, as at present the reader is just left wondering. 
 
Line 84 – don’t think its necessary to say ‘for the first time’ again here. You said that 8 lines back. 
 
Line 99 – Perhaps a few more lines on what the drawbacks might be of not producing ‘trains of 
bedforms’, why are these not produced when they are in nature? This could be developed in the 
discussion a bit more. 
Line 108 – stating > 20o is certainly an over generalisation, some are 90o and some look almost 
‘overturned’ 
 
Line 111 – I think there could be a sentence or two of introduction here. It launches straight in. 
 
Line 114 – ‘…more rapid deaeration’ another term dropped in without any info. The reader is kind 
of left high and dry…More explanation is needed. 
 
Line 162 – Field Analogue 
The first sentence of this section is a bold unsubstantiated statement and is unacceptable. The 
field evidence needs to be presented first, then an interpretation arrived at. The reader needs 
much more descriptive information. The interpretation of these type of deposits is just that an 
interpretation and we need more than ‘matrix supported and fines-depleted’ to reach a reasoned 
conclusion. Its absolutely vital to the understanding. Fines-depleted is a poor term (it also implies 
an interpretation, how do you know there was fines in the first place?). Why not use ‘fines poor’ 
which is a descriptive term. 
 
Line 162 Delete ‘and in places partially lithified by zeolites’ . It’s irrelevant to the story here. 
 
Line 169 Grainsize of 6 samples….. how do we know these are representative of anything? There is 
no information on the location, facies, stratigraphic position as far as I can see. 
 
Line 176 -179 So clearly these natural deposits have different properties to the experimental 
setup. Is this not a serious drawback? This must have some further discussion somewhere. Owing 
to its importance in terms of the differences between experiments and field. 
 
Line 180 – but you state that it isn’t a massive ignimbrite…….How the massive parts relate to the 
dune bedforms is absolutely critical. At present this information is absent 
Line 180 – 185 This paragraph is just far too cursory to address this contentious and complex 
issue. It absolutely cannot be dealt with in 5.5 lines. What is provided is nowhere nearly enough 
information. 
 
Line 187 and Figures 7a and c . These photographs are not clear enough in my opinion, they need 
to be much clearer. In particular 7c could be anything. The upstream beds are much steeper than 
20o. So are your experimental ones. Why not be more specific and give actual angles ? 
 
Line 191 is there a reference for these other bedforms that are found in the Pozzolane Rosse? If so 
please cite it here. 
 
Line 215 In the photographs some parts look like they are ‘overturned’ in places (Fig 9 d and E). 
I’m not sure I have seen this in Field analogues. Perhaps you can expand on the similarities and 
differences more here ? Give more references to those field examples which show these 
similarities perhaps? 
 
Line 228 The field analogue certainly doesn’t support this ‘experimental supposition that dune 
bedforms can be formed by dense granular currents. It could do if there was adequate careful 
description and interpretation of what is present in the field, rather than a couple of overly cursory 
sentences. 
 
Line 251 – generally apart from where the term regressive is introduced there are no progressive 
bedforms mentioned. Presumably as they were not generated in the experiments however perhaps 
there could be more discussion about why you are not seeing/generating progressive bedforms in 
these experiments and the significance of this. 
 
Line 264 - I think the suggestion that dune bedforms can be formed by dense flows is 
demonstrated by this paper. However there are such a wide range of bedforms in reality, in terms 
of size, wavelength etc that more than just grainsize characteristics are needed. Detailed field 
relations must be the most important feature to distinguish the two different end members.