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

RR2
 Reviewers' comments:
 Reviewer #1 (Remarks to the Author):
  The authors describe how different bedforms can be generated in dense granular currents, use
 their results to construct a diagram predicting what bedforms should be generated as a function of
 current parameters. Finally, they apply their results and diagram to a natural deposit to estimate
 the flow parameters of the pyroclastic density current that formed the deposit.
 The authors addressed nearly of the comments or concerns raised by the reviewers. I should note
 that the authors attempted to address the few remaining items but they need to go a little bit
 further. If the items listed below can be address, I recommend the manuscript for publication.
 1) Approximately line 57. I recommend including the original references (27-31) as even though
 they are not about volcanoes, they have definitely informed much of our understanding and
 interpretation of volcanic deposits and processes.
 2) Reviewer 3 raises a good point about comparing experiments and reality (approximately lines
 189-203). The new text about grain differences helps, but does not fully explain the relationship
 between the two. I suggest adding in the range of Savage numbers Ns for the experiments and
 natural deposits. This will help to show that the experimental and natural systems have similar
 scaling.
 3) Reviewer 3 had questions about “quasi-steady currents” (lines 123-124). The authors have
 partly addressed this, but it would be useful to add in a single sentence or phrase describing what
 “quasi-steady” actually means (i.e., “currents with variation in mass flux occurring over a
 characteristic timescale longer than or comparable to XXXXXXXXXX.”)
 4) Fig. 5 gran size data – I can tell that the different samples (a-f) have different distributions, but
 they are presently hard to compare. Could they be combined into a single panel with each line a
 different color and still labeled a-f?
 Best wishes,
 Benjamin Andrews, Smithsonian Global Volcanism Program
 Reviewer #2 (Remarks to the Author):
Two fundamental things need modifications to my opinion:
  1. I still don't really get from the main text where exactly the velocity measurements (and thus
 Froude numbers etc...) have been taken... Directly at the point of deposition or just before (and
 thus
 at a mobile spot)? at a fixed point after release? Is it for the initial front or for the body of the
 current?
 2. The last paragraph states that backset bedforms are taken as diagnostic for dilute PDCs. I
 strongly
 disagree. It's the organization of deposits as cross-stratification (i.e. with sets of mm to cm scale
 lamina) in general that is taken as diagnosis for dilute (turbulent) flow-bed boundary conditions...
 Many cross-laminated deposits show backset beds, but this is not the feature that is calling dilute
 conditions... (e.g. lines 56 and conclusion from 308). This misleading statement kind of oversell
 the
 outcomes although its not necessary.
 A few recurrent phrasings I'd recommend to adapt:
 -I find the introduction of figures heavy sometimes (e.g. 111, 195-208),
 -I'd avoid the use of "ballotini" and replace by "particles". (I don't know what ballotini are)

 -In several occurrences, you list first the steep backset data, then shallow, then flat. This is
 reversed
 in term of chronology of deposition, and deceleration of the flow. I'd follow the chronology (L136,
 L164, etc..)
 -Sample description (granulometry etc...): I'd rephrase, see text.
 -Your field examples are at Pozzolane Rosso, this is clear from one sentence. No need to repeat
 "Pozzolane Rosso" that often (17 occurrences...)
 Generally, I'd suggest to shift part of the main text in the captions (see line by line)
 -Lines 211, 249-252, 272-285, 286-289 all deal with the question of the massive/backset bedded
 question and interpretation of dense granular flows. As they come, it feels repetitive, and they
 should be arranged together in a strong chapter (at the very beginning of the discussion).
 Little things I did not like but figure its the editor choice:
 -I guess the editors and other reviewers wanted more field details... I still disagree with this, but
 that's only my opinion, so fine.
 -Does the Method section really have to go at the end? (and figure 10 be last instead of first?)
 Figures:
 FIgure 2: Please find a way to get all images centered on the aeration limit at 1 m. The way it is
 presented here is misleading and structures cannot be compared. I'd rather crop-out part of a
 proximal/distal end than get confused on the deposition onset. vs deaeration limit..
 Figure 5: Can you put all your cumulative curves on a single graph? I don't see much difference
 between samples, neither a fundamental use of these curves so I would minimize the space it
 takes...
 Figure 6: I think there's little reason to have figure 6 detached from figure 5, could you do it figure
 5c instead?
 Captions: see line by line
 Line by line:
 33: is "by" correct or should it be through an upstream propagating? (not native question)
 41: is there any reference for this first sentence?
 42: Deposit from deep see turbidity currents form... (it refers to the sediment, not the flow here)
 41-44: "PDCs" should be introduced at the first occurrence (41 rather than secon in 44
 46: seafloor infrastructure(s?) => isn't it plural (not native)?
 50: internal dynamics? (instead of interior dynamics)
 56: I'd use cross-stratified/cross-laminated bedforms (Very coarse and massive bedforms can
 result
 from a dense pyroclastic flow...)
 61: maybe you can state somewhere here what are backset structures (i.e. beds deposited on the
 upstream side of an obstacle)? I doubt it is clear to every reader what backsets are, and it is so
 central that you might want to be sure that the definition is clear and vulgarized...
 63 which are "generally thought to be" formed under supercritical [...]
 85, 118, 190: I do not know the word "ballotini", not sure it is commonly known... Could you put
 "fine particles" instead and simply state in the section "Experimental material" at line 339 "the
 experiments were performed using PARTICLES OF spherical soda lime ballotini..."
 87: As deposition aggrades... I'd say either "as deposit aggrades" or "As deposition occurs"
 88: same, either "recorded using a high speed camera" or "recorded on high speed videos"
 94: rapid deaeration "hypothesized" to occur instead of "we know"?
 95: coarser instead of courser (not native)?
 106 either "very shallow backsets" or "very shallowly backsetting"
 109: where steep and shallow backset bedforms are present... unclear to me, you mean "where
 there
 are both steep sided and shallow sided backsets beds occurring in a single bedform"? or that "for
 both the steep backset type and the shallow backset type", a group of 3-4 stoss-side lamina

 merge
 into a single laminae on the lee side?
 110-111: not sure a new paragraph is needed here (and the 109-110 paragraph is a single
 sentence
 right now)
 110: I suggest "dipping at varying angle and converging/merging into a single corresponding
 leeside laminae.
 111: I would just add (Tab 1) at the end of line 110 in brackets and remove sentence in 111:
 "Characteristics of steep and shallow backset bedforms are given in Table 1."
 112: Similarly, I think this sentence is not needed, since you introduced the figure in line 106:
 "The
 experimental deposits are presented in figure 2"
 118: "ballotini", replace by particles
 135-138: I suggest reversing the order of listing to keep the chronology of deposition: first explain
 the planar beds, then the shallow backsets, and end up with the steep ones.
 140: "As a result of thickening", is this within a single flow or between experimental runs of
 variable thicknesses?
 151: Nf is a ratio instead of the ratio?
 162: by definition of Fr= U/(gH)^0.5, isn't it normal that U is correlated to Fr?
 164-166: again, I recommend to reverse the order of listing to begin with the onset of deposition
 and follow the deceleration chronology.
 167: no need for a new paragraph, this concludes on the data just listed... (and I find 1-sentence
 paragraphs are not very elegant)
 176-184: I suggest rephrasing to shorten and reorganize the structure of appearance (first where
 samples come from, then how they look like, then what's the particle size distribution, and finally
 what it means). Here a suggestion:
 "Six samples were taken from three localities (within 18-24 km of the vent) and two facies
 (massive, and ‘undulated’ bedding as described in Giordano & Doronzo63, Fig. 5). Grains are
 dominantly poorly vesicular scoria with compositions plotting in the tephrite/basanite field62. The
 grain size analysis shows that distribution of all samples is are dominated by lapilli-sized grains
 and
 poor in the < 63 μm fraction (Fig. 5), and are consistent with samples from other studies (Fig. 6).
 All plot in the ‘fines-depleted flow’ field of Walker25, close to Pozzolane Rosse samples taken
 18-24 km from the vent, analysed by Giordano & Dobran58 (Fig. 6). Hence, no differences in
 differences in grain size distribution at localities where the ignimbrite is massive vs where it
 contains bedforms was observed and the Pozzolane Rosse ignimbrite is considered a valid example
 of for an analogue dense granular current."
 187: suggest to delete "on the six samples taken from the Pozzolane Rosse": the entire chapter is
 about Pozzolane Rosse, no need to repeat where they come (also at former line 176)
 190: replace ballotini with "particles"
 192: I am lost, grain size does not appear in the scaled equations, so what do you mean with this
 scaling? They have similar Nf?
 195: mixing flow and deposits here. the flows are interpreted to have left the plain and run up, but
 the ignimbrite IS there, not moving. Please rephrase.
 195-198: I suggest to keep this minimal in the text and keep details in the caption. The natural
 examples (Fig 7) share similarities with our experiments ( e.g. stoss sides for fig 7a vs 2a-c, lee in
 7c vs 2d).
 205: keep the text in brackets for the caption and just put (Fig 7d): "the example in Figure 7d is
 thicker by ~15 cm over the stoss and crest compared to the lee"
 207: Again a sentence just to call a figure, and reminding that you talk about Pozzolane Rosso... It
 is
 to my opinion non-aesthetic and heavy for the reader. Maybe something like "Measured stoss
 angles
 spam the same range as experimental deposits, yet the lee beds are much steeper (Fig 7b)"
 209: grades laterally? suggest "evolves" instead of grade, which may be connoted as a grain size

 evolution. "laterally"? could you say "downflow/ towards paleovalleys/ towards depocenters?"
 210: suggest to add (Fig5-6) at the end of the sentence
 211: Careful, this belongs to the interpretation (and is developed at line 279 and onward already).
 It
is out of place and creates a repetition with later statements. Also, I'm not sure this "it is reasonable
to infer..." is very scientific... Maybe something like "It is interpreted here that the same event deposited both facies without significant changes in dynamics, such as particle concentration or turbulence level." Again, this would belong to the interpretation (at least move it just below the "Discussion" title?)
214:I would begin the discussion with a paragraph about the fact that your granular flows produce bedforms, thus backset bedforms are not only related to dilute PDCs, that the two facies of your field examples have similar grain sizes, and that you don't interpret a change in dynamics between both facies. This is probably the priority of your message, and first thing to understand. You could do so by using lines 249-252, this sentence line 211, and the two paragraphs 272-285
(for example just shifting lines249-252 here, removing the parts on the Froude jump). Then you directly follow that your experiments supports that the same event and dynamics can create the massive and undulated beds in the field examples from the red Pozzolane.
215-217: The two first sentences are descriptive and belong to the data (move them just before the
"Discussion" title?). The third sentence belongs to the part developed on Froude numbers at lines223-230 (Maybe simply write after 230 "The absence of downstream propagating structures (progressive) is interpreted as the quasi absence of subcritical conditions in the experiments"?) 222-223: No need to change paragraph, it's all about Froude numbers?
223: add (Fig 4) at end of sentence?
225: I suggest to rephrase as"An apparent Froude jump occurs during deposition of the steep backset beds." If the planar and shallow backset beds are deposited at Fr>1, they cannot experience
a Froude jump, so that the jump is only valid for the steep backsets...
234: [...] to mean the upstream propagation of the "FREEZING / DEPOSITION FRONT" of the granular material
240: [...] steepening "OF STOSS FACES/BEDS" well beyond the repose angle
250 [...] Froude jumps within "DILUTE" PDCs into question [...]. Also, this is a one sentence paragraph, could you put it somewhere else?
266 [...] close to the fluid-escape "AND GRANULAR-BASED" end member"S" of Branney & Kokelaar (2002)
278-279: keep in a single paragraph, beginning sentence in 279 with "INSTEAD, THE CHANGE IN DACIES COULD BE DUE TO" the onset of [...]
286-289: it becomes very repetitive to me (or I don't see the subtleties?) suppress?
289-291: maybe better after line 296 with slight rephrasing? I suggest: [...] until it collapses upstream by avalanching (Fig 9d-e). "Our work "FURTHER" provides evidences that this process can create bedforms with "dense granular" PDC deposits. These mechanisms support the stoss- side
blocking process suggested by Douillet based on field deposits. "
300: Froude number(S) (remove S?)
301: the backset bedforms are technically recording "the transition from supercritical to subcritical (transcritical) flow"?
304-305: maybe not necessary to mention, the reader can judge by himself (could be seen as arrogant by some readers?).
308: I disagree. It's the presence of cross-stratification/cross-lamination that is used as diagnostic evidence for dilute turbulent currents, not backsets.
570:I suggest"Sketches of backset bedforms from PDC deposits". Also, please remove all "idealised"
613: flow direction is clear from the arrow (no need to write?)
614 I suggest: "b. Stoss vs lee beds angle for the Pozzolane and experimental bedforms." Could all
                                                    
 coordinates go in the supplementary data? They are not fundamental in your study and there's
 already a location map. (And they need a reference system (WGS84?), and could be easier to
 understand in degrees minutes seconds...)