Twenty-Five Years of Geomorphological Evolution in the Gokurakudani Gully (Unzen Volcano): Topography, Subsurface Geophysics and Sediment Analysis

Round 1

Reviewer 1 Report

I have read thoroughly the manuscript, and I apologize for the delay. 

The paper gathers the research conducted by the authors at the Unzen volcano in Japan, and it is sort of intermediate between original research and review. I do not have much criticism to offer, since I could not find inaccuracies or poor explanations. In fact, the manuscript is detailed in the results and techniques, and only minor things should be fixed (e.g., line 420; I recommend the authors to read again the manuscript). A minor thing! I believe that the cited Rachmaninov is a piano concerto, not a symphony.

Author Response

To the reviewer:

Thank you for taking the time to review our manuscript.

Line 420, the sentence was a bit long and I have rephrased it, so that it is easier to understand:

They present a sloping angle comprised between values below 35.4 degrees (the angle of repose or friction angle), up to subvertical angles (71 degrees). The distribution of these angles is controlled by the status of the wall: whether it has collapsed or not, and also by its position on right or left side of the gully (Fig. 7). 

In the conclusion, it is indeed a piano concerto, rather than a symphony, and the editor has asked for I remove this "non-scientific" quote, so that this part will disappear from the final version.

Best Regards,

The authors' team

Reviewer 2 Report

Dear Authors,

I went through your manuscript and I found it very interesting for a wide audience. Nevertheless, I suggest moderate revisions before accept the work.

Best regards

Comments for author File: Comments.pdf

Author Response

To the reviewer:

Thank you for your review, I have made the changes you suggest, and they appear in "green" in the corrected version.

In the abstract, I changed "Pyroclastic-flow" for Pyroclastic density current as you suggested.

The title "Introduction" was modified to "Introduction and state-of-the-art of volcanic process-geomorphology

For the title of 1.4, I also changed it to pyroclastic density currents' deposits as you suggested.

Then line 154 and in the paragraph I changed the terms as well to pyroclastic density currents.

Line 178-179: Indeed, the basement of the volcano is made of sedimentary formation.

Line210: I added the missing verb (However, during the latest stage of the eruption, pyroclastic density currents are also flowing northbound. )

I have moved the 1.7 to the beginning of the introduction as you suggested, so that the rest of the sections (1.2 to 1.6) are now the "state of the art section, as you also suggested.

This paragraph was rephrased based on the numerous suggestions made throughout:

The GPR was used in order to investigate the internal structure and find the presence (or not) of bedrock that may not be visible from the surface following the methods used on PDCDs [39,40] and lahar deposits [31, 42, 44]. The method was also aimed to better understand the evolution of the gullies: if the channel is going through a purely erosive phase, one is expected to find solely PDCDs or the pre-eruptive material, however if a mix of lahar deposits, wall collapsed-material, etc. is present, it shows that a non-linear evolution. This is essential, because even yearly topographic surveys are blind to topographic variations that occur at the minute to hour scale.

Starting Line 333, you suggested to rephrase the paragraph because the sentences were too long and hard to understand; assessment I agree with and I have rephrased and made shorter sentences as follows:

For the present study, the GPR is a Ramac® Pro-Ex mounted with shielded antennas 500 MhZ and 800 MhZ. , The distance was measured using a calibrated coding wheels, and a measuring tape to confirm the wheel’s data, as well as a hand-held GNSS to mark the track position in the Gokurakudani gully. The data used in the present research follows semi-horizontal topographies for which no complex topography inversion was necessary (cf. Figure 1 for location).

Line 349 - 356: I had explained the reason between choosing the hyperbola method, but you suggested I just present the method I used. I have therefore cut this paragraph to a couple of sentences presenting my method solely:

- Measure of the velocity through the ground. As the common mid-point method is not possible with the shielded antenna [42], the authors have used the slopes of the hyperbola curvesvto calculate the velocity profiles.

In paragraph lines 400-409, you proposed several cuts and better way to present the data. You also noticed a mistake in my write up where there are not 2 eroding zones but 3. I have then made those changes. This is now how it reads.

There are 3 unstable zones between 250 – 300 m and between 550 and 600 m and at 1100 m. These features appear as topographical step, with the top of the step being stable, while the lower part erodes. The recorded change has been 1.5 m to 2 m per year. There are three zones of such erosion, creating several meters drops in the gully floor (Fig. 6) for the studied period. On top of the three steps with erosion > 1.5 m, there are smaller secondary steps that also exist with erosion values between 0.5 m and 1 m.

The paragraph between 420 - 427 was in need of rephrasing as you suggested, so I did:

Dividing the walls that have shown signs of collapses from those that have not, the data shows that the true-left walls can sustain angle of 56.7 degrees, while the true-right walls can withstand angles up to 68 degrees. The collapsing mechanisms are arguably different on the true right and the true left of the gully. This is further evidenced by the distribution of collapsed or partially collapsed wall profiles that can be straighten up to 71 degrees on the true-right, while the value only reaches 45 degrees for the true-left (Fig. 7). The “not collapsed” walls show the same pattern with maxima at 68 and 56.7 degrees for the true left and the true right respectively.

Between lines 442 and 448, all the squares had become "2". I have reshaped them into exponents.

Lines 465, 472, 475, 476 I have replaced PFD and PDC for PDCD (Pyroclastic Density Current Deposit) as used in the introduction.

Line 472, I rephrased this sentence to:

The Tansandani gully is also much lower than the Gokurakudani gully. 

L497 I replaced PDC by PDCD

L498 I replaced the reference by its number [4]

L518, 519, 524: I replaced PFDs for PDCD

L532: As you suggested, I have rephrased this sentence:

The Tansandani gully is also much lower than the Gokurakudani gully.

Line 548: I have rephrased the sentence for it is less confusing:

Then, located on top of these units, layers 8 to 10 show a downstream prograding pattern, which have the typical morphology of a foreset. 

Lines 585, 607 and 622 the reviewer suggested that the paragraphs are restructured with titles 4.1, 4.2 and 4.3. I followed these advices and transformed these paragraphs into sub-parts of the discussion.

I have also erased and made the corrections the reviewer has proposed for this section of the discussion (in green in the text).

In the conclusion, I have replaced pyroclastic-flow by pyroclastic density current and changed terragenic by sedimentary. I also removed the reference to the symphony as suggested by the reviewer and the editor as well.

Reviewer 3 Report

Review report for sensors-690464-peer-review-v1

1. Summary

The manuscript studied a long term geomorphological evolution in the Gokurakudani Gully. In general, the paper is well documented and up to the standards of published international journals; while, the conclusion section is not appropriate in description. I listed my suggestions for the authors to improve their research.

2. Major issues
3. The conclusion section should be rewritten for the sentences are not suitable for a research paper (e.g. P22L650-655). Please draw your important finding s here.
4. More minor issues
5. In P2L60, please check the instructions for authors to prepare your manuscript, e.g. the symbol used “*”.
6. In P2L85 and other places (e.g. P8L300), please check the instructions for authors to prepare your manuscript, e.g. the form of citation of references.
7. In P4L172, please check the slope for the term (2 to 2 degrees)?
8. In Fig. 1, please add scale bar for (a) and (b) images.
9. In P7L265, no new segment for the paragraph, I think.
10. In P8 in Eqns (3)and (4), please explain what are ϕ84 and others.
11. P9L137, use subscript for symbol V1.
12. In P9L318 add symbol for velocity, e.g. velocity (V); also, what are the differences for ξ? , 1 and 2?
13. In P9L325, delete Mala-ProEx (no commercial meaning).
14. In P10L366 and the same paragraph, use superscript for the unit.
15. In Fig. 4, I suggest to add X- and Y-axis at the bottom of the figure (e.g. for Year 2016).
16. In Fig. 7, please check the text on the figure (e.g. ~ have not (yet)).
17. In P17L499 and p21L604, revised the form of cited reference.
18. P17L607, check this sentence.
19. Opinion

In general, the manuscript shows well documented but some crucial parts that need to rewrite the contents.

Author Response

To the reviewer,

Thank you for your detailed review, I concur with your comments and made the changes accordingly.

In the conclusion, I have removed the sentences referring to Rachmaninoff and refocused it on scientific results.

In the equation, I have removed the "*" sign as a multiplier

I have removed the names and year of citation where I had forgotten it (two locations) and replaced it by their number in the citation list.

The paragraph PL265 had been moved

 P9L318, the missing (V) for the velocity was added line 318.

I have kept the Mala-ProEx, because the different GPR produce controllable or set voltages, and specifying which type of GPR control provides important information on the voltage, but also the shape of the wave differs from instruments.

In P10L366, I have replaced all the "2" to the superscript as it should indeed be.

In Fig. 7, please check the text on the figure (e.g. ~ have not (yet)).

I simplified the text underneath the figure. It was all together too long and a repetition of what I was writing in the text, so that the "oddities" you found have been removed all together.

I have cut the sentence in shorter ones and rephrased the end of the paragraph. It now reads as:

Indeed, as the skeleton of the sediment structure is holding the general shape of the deposit, fine fractions in between may – for given concentration – not play any role in this skeleton. They can then leave the deposit, without any change in the topography. It is just changing the density of the deposit. 

(it is in orange in the text with "track change")