General comments:

The authors investigate how the grain-size distribution of debris flows affects the fan-forming processes. For this, flume tests have been conducted to compare the debris-flow fan morphology under varying sediment source grain-size distributions. The obtained results associated with the debris-flow runout and space-time variations in the fan morphology provide important insights into how the grain-size distribution affects the fan formative processes. The topic is interesting, investigation is novel, and is within the scope of esurf. However, the ms requires a thorough revision in concept, mechanics and dynamics. Detailed suggestions and comments are provided below hopping that they will help to improve the quality, consistency and clarity of the revised ms.

Abstract can be improved. E.g., “Grain-size distribution was closely related to spatial diversity in fan morphology and stratigraphy.” Should be other way round, “Spatial diversity in fan morphology and stratigraphy were found to be closely related to grain-size distribution.”

Introduction is not that much concerned about the main topic on how the grain-size distribution influences the debris flow fan morphology. So, it needs to be substantially expanded focusing on how the grainsize of the source material affects the deposition and fan formation process including the important and often dominant dynamical processes - the material separation, erosion and run-out dynamics.

Often the writing is not explicit, not smooth and difficult to follow. It seems if the ms was made for very short paper, less for a professional journal, requiring clearer and smoother presentation. Particular attention should be given on these issues.   

Specific comments:

L36-37: “Changes in the physical parameters (e.g., flow rate, duration, and sediment concentration)”: The writing must be significantly improved, conceptually: flow rate, duration, and sediment concentration are not the physical parameters, rather they are the dynamical quantities. Physical parameters include densities, viscosities, frictions, slope geometry, curvature, etc.

L38-39: “and sediment entrainment rate (Egashira et al., 2001; De Haas and Van Woerkom, 2016).” better change to “and sediment entrainment rate (Egashira et al., 2001; De Haas and Van Woerkom, 2016; Pudasaini and Fischer, 2020: https://doi.org/10.1016/j.ijmultiphaseflow.2020.103416) and separation between the particles and fluid in the mixture (https://doi.org/10.1016/j.ijmultiphaseflow.2020.103292).”

L39-40: “and depending on the topographic complexity, could produce varying functional changes in subsequent debris flows.”: not clear how topography produces functional changes and which?

“functional and structural changes” what are the functional and structural changes? General readers may not be able to follow the text.

L50: “A straight flume (8 m long and 0.1 m wide, with a uniform 15° bed slope”: This is not true, and the text around must be improved appropriately, consistent with the corresponding figure.

L50-67: “erodible bed conditions”: Here comes the great thing! I see two major aspects in this ms. First, as the authors say, the effect of particle size in the erodible bed and how it will affect the deposition fan. I understand this differently than the author, it is not the particle size in initial debris mass (initially water is released) that will influence the deposition fan, but it is how the particle size of the erodible bed that affects the deposition fan when that bed substrate is eroded and entrained by the water flood released from upstream, consequently forming a debris mixture that ultimately flows down and deposits in the gentle open flat slope forming debris fan. The text does not mention this.  Second, probably even more important, is the fact that the flood erodes and entrains the granular bed converting it in to the debris flow. So, the physical process of erosion, entrainment and the associated mobility must be described. This could however be done with respect to the mechanical erosion rate models and the mechanical model for the mass flow mobility with erosion (Pudasaini and Krautblatter, 2021: https://www.nature.com/articles/s41467-021-26959-5). I would focus on these governing aspects of experiments.    

Writing style needs to be made more appropriate with better physical understanding. “The supplied water generated a granular flow that imitated a single debris-flow surge and then entrained the erodible bed to the deposition area” This is difficult to follow, probably not representing reality. Does the water flow first generate the granular front by entraining the granular bed? I guess, as the water front impacts the granular bed it will erode and entrain the grain, mixing will take place resulting in the subsequent debris flow. Not that the way the authors explained. Moreover, you mixed up erosion and entrainment, which are clearly two different mechanical processes as proven by the reference mentioned above. So, the process of erosion and entrainment should be carefully investigated/discussed.

“the fan morphology gradually formed in accordance with the runout and inundation of the released granular flow”: This is rather the fan of the granular-water mixture debris flow. For a granular fan, you must only have the dry material without water, for which the fan will be substantially different than what it is now.

“Two types of granular flow, namely mono-granular and multi-granular, were used to determine the impact of grain-size distribution within a debris flow on the fan-forming processes.”: This is not right. You have two types of granular materials in the erodible bed, resulting in two types of debris flows, one consisting of water and mono-granular material, another composed of water and multi-granular [this term needs to be defined carefully, as mono-granular and multi-granular are not the usual terms, usual terms are the mono-dispersed and poly-dispersed, with respect to grain size, etc.].

Not enough information on the material and channel are provided, e.g., the basal and internal friction angles of the granular material, viscosity of water, their densities, and so on. This information is crucial in understanding erosion-entrainment and mobility, the mixing and separation between particles and fluids, and the transport/deposition of debris mixture. 

Another principle concern is the representative grainsize, the two granular materials, mono-dispersed and poly-dispersed are represented by the same average grain size (D50). This does not help to physically clearly study the erosion-entrainment, transport and deposition fans, except that you can say – we observe this and that for the mono-dispersed and poly-dispersed erodible bed. But, we don’t know how small and big particles in the mixture influence the erosion, mixing or separation, dynamics and deposition processes. Moreover, different grains might need to be represented by different rheological equations. These are crucial aspects the authors should discuss. Otherwise the results can not be understood mechanically and dynamically clearly, and these data cannot that easily be used in model validation and parameter calibration.

L69-70: “sediment was released to the deposition area”. A bit strange writing. First, it is not sediment, it is the debris material. Second, it is not released to the deposition area, it is the transported material in the deposition area. So, the dynamical perspectives are weak.

L70-73: “The flow depth of a generated granular flow cannot be measured in the flume because the thickness of the erodible bed decreases sequentially in response to the sediment entrainment. Therefore, the displacement of the flow surface at three positions in the flume (upper, middle, and lower, Fig. 1a) was measured to account for this shortcoming, using ultrasonic displacement meters”:

I agree with the first sentence, it is a really complex process, however, there are some literature in this direction with some success (Lanzoni et al., 2017: https://doi.org/10.1002/2016JF004046). The authors should put some efforts to review relevant literature. The bed erosion process is an under-investigated process, and I respect any attempt in this direction. The second sentence is not the solution to the first, because, the measured flow depth cannot be split into the material from the flow and from the bed. Thus, it cannot be straightforward connected to the erosion depth. Furthermore, the involved energy associated with erosion is the dominant factor to decisively defining the dynamics, runout and the associated impact forces of the erosive mass flows. This needs to be discussed with respect to the references mentioned above. 

I stop suggesting and commenting on the mechanical and dynamical aspects of the ms, and hope that the authors will improve the text while revising it.

Fig. 2: Figures are difficult to follow. It should be self-explanatory. For example, Run 1-4, are they repeated exps.?

L98-99: “The thickness of the erodible bed decreased monotonically with time, probably because the entrainment rate was the same in all the test runs, irrespective of the grain-size distribution of the granular flows”: This cannot be true, could only be a speculation. Because, as proved in the above-mentioned references, erosion rate is a complex phenomenon, and changes with the dynamic load applied by the flow and resisted by the bed. This needs to be discussed. 

L100-102: “Overall, the results from the flume experiment showed that the difference in the grain-size distribution did not lead to substantial changes in the hydrograph and arrival time of the granular flows.”: I can’t fully agree with this. E.g., if you take the mean of four runs in C and F and plot them in one figure, you will see discernible difference.    

L113: Grain size separation is one aspect, but separation between particles and fluid (as seen in the experimental results) is another, even more complex mechanical phenomenon in debris flow. However, the authors did not discuss anything on it.

L123: “avulsed obviously”: it is better also to put orthophoto to clearly see avulsion. The quality of Fig. 5-8 should be improved, with filters, or whatever means such that we can clearly see avulsion. The problem I have seen is that avulsion cannot be predicted, or was not possible with the present setup. We should understand why this is happening. This needs to be discussed, because, one of the main aims of experiments should be to generate reproducible results.

L142-144: “Some equations that describe debris flows assume that multi-granular debris flows can be approximated to mono-granular debris flows with the same average grain-size (e.g., Egashira et al., 1997; Takahashi, 2007).”: This is not the state-of-the art. The multi-mechanical, multi-phase mass flow model by Pudasaini and Mergili (2019: https://doi.org/10.1029/2019JF005204) has proven the necessity of simulating debris flows as mixture of different materials, that has been used in accurately simulating complex multi-phase natural events (Mergili et al., 2020: https://doi.org/10.5194/nhess-20-505-2020; Shugar et al., 2021: DOI: 10.1126/science.abh4455). The ms should be up dated with relevant, recent literature. 

L144-145: “However, the mono-granular and multi-granular flows with the same average grain-size produced fans with different morphologies”: This is probably the most important aspect of this ms, and I like it. However, it has not yet been clearly discussed for why this is so. The authors should put some energy to explore why it is happening, that will lift the importance of this paper to a higher level.

L146-147: “which indicates that existing models that assume a mono-granular approximation may provide ambiguous simulations of the debris-flow deposition and inundation ranges.”: This proves the need of multi-phase mass flow models (mentioned above) in properly simulating debris flows. This should be discussed.   

Discussion and Conclusion, References:

Needs re-working, including the above suggestions. E.g., multi-phase flow simulations, erosion-entrainment and mobility, separating particles and fluid, and so on. Important point why the flow with the poly-dispersed erodible bed has shorter travel distance and run-out reveals that more energy has been consumed for this than the bed with mono-dispersed particles. This exclusively depends on the erosion velocity controlling the mobility of the mass flow, this fact has been proven by the mechanical model for the mobility of erosive mass flows by Pudasaini and Krautblatter (2021). The discussion and conclusion should give proper space for these important mechanical and dynamical aspects also observed in this ms.

Technical comments:

L22: “sinks”, the meaning was not that clear, better would be “deposits”?

L29: Please check English.

L85: “SfM-MVS”: is its meaning clear?

L97-98: “while, apart from run 1, those of the multi-granular flows were around ~0.03 m”: Not true. Please check all the technical details carefully.

Reviewer Report of Tsunetaka et al 2021;

The effect of debris-flow sediment grain size distribution on fan forming processes

In this manuscript the authors used a flume to analyse how the grain size distribution of a debris flow may impact the morphology of the resulting fan. The authors ran 2 sets of experiment runs in their flume, all parameters inside the experiments were kept constant except the grainsize distribution. One set of experiment runs used monogranular sediment while the other used multigranular. Both sets had the same average grain size so the authors could identify the impact of variations in grain size on the debris flow fan. They quantified these changes by measuring the surge height in the flume, the speed of the surge within the deposition area, the runout distance and a DEM of the final debris flow. The authors discovered that multigranular debris flows were more likely to produce alluviations in the debris flow resulting in asymmetrical fans. They postulate that these alluviations are the result of grain size segregation occurring within the flow where coarse sediment is forced to the front of the flow where it can produce an obstacle for any following surges.

I think this study is an interesting addition to the literature on debris flows. The experiments seem well thought-out and the results aim to fill a clear knowledge gap. However, there are several areas where I feel the manuscript needs to be improved before it is ready for publication. The manuscript is very short and as a result I feel that significant detail is missing, particularly from the description of the experiment design, results and discussion. I also found the figures poorly cited and discussed throughout. I have provided more specific comments below for the authors to read through. I hope the authors find my comments useful and I look forward to seeing the revised manuscript.

Detailed Comments:

Introduction

The introduction is too short and vague to be of use to the reader. Despite the research statement at the end of the section it is not completely clear how the authors see the study contributing to the literature. It is also not obvious from these paragraphs why the authors have chosen to focus on grain size distribution for this study rather than many of the other controlling factors highlighted here. Finally, this section would be greatly improved if there was better separation between discussing debris flow physics from debris flow fans. Currently it is very confusing whether the authors are referring to how a certain parameter might affect a debris flow or how it may affect the autogenic fan forming processes or the links between the two.

Line by line comments:

Lines 25 – 26: What specific climate and sediment dynamics information can be identified from debris flow fans? And how is it derived? How is the form (which is what is investigated here) important?

Lines 26 – 29: If debris flow fans are primarily formed by autogenic processes how can information on any external forcing be derived from them?

Lines 31-33: Shifting the focus of the paragraph from a geological perspective of fans to one about hazard is confusing to the reader particularly as neither focus is well covered.

Lines 34-36: It would be useful to the reader if the authors would elaborate on how these physical factors affect morphology and stratigraphy of the fans.

Lines 36 – 38: From this section the reader cannot tell how these changes will affect the debris flows. The authors do not define the property that is changing carefully nor do they describe the impact of these changes on debris flow behaviour. Without this information the readers can not make the link between debris flows and the resulting fan.

Lines 38-43: Again, the links between debris flow behaviour and resulting fan form are not well made. Further, the terms functional and structural changes are poorly defined and it is not obvious how these affect autogenic processes, which are also not defined.

Lines 44 – 48: This is a good succinct research statement however it is completely disconnected from the preceding 2 paragraphs. It does not mention why the authors have chosen to focus on grain size distributions nor how they expect them to change the debris flow fan.

Methods

The experiment design is reasonably well explained, however I struggled to understand what exactly was being measured. The authors have gone to great lengths to capture the vast amounts of data generated by the experiments; however, they do not discuss why they collected these particular datasets or what they plan to do with them. A better motivating statement within the introduction will help to improve this section.

Line by line comments:

Lines 50-60: The authors have not explained how the debris flow surge is generated. The paragraph could be separated to first describe how and where the surge is generated before discussing the depositional area.

Lines 61-67: How fans are produced in this study? Is the fan a result of a single surge triggered by the outlet of water? Or the result of multiple surges? Is the erodible layer rebuilt between surges? What is going to be measured as a result of these experiments?

Lines 69-74: Why is flow displacement being measured? Why is the flow height of the surge important to the authors?

Lines 79-80: What is being measured and how?

Line 86: It is not clear what is meant by “the SfM-MVS photogrammetry could not measure locations where granular flows descended”. Does this mean that the photogrammetry cannot measure the flow when it is moving?

Results 

The results section also suffers from the same problems as the previous sections. What is being measured and compared between the different runs is not specifically stated and as a result it is hard to understand some of the findings of the manuscript. Many of the result figures are poorly explained and some are cited out of order or not cited at all.

Line by line comments:

Line 95: What is the lower portion of the flume? How is this defined?

Line 96: Same with arrival point and upper position.

Line 97: What is a run? This refers back to the earlier point that it is not clear whether the experiment is single or multiple surges.

Line 101: Unclear how the arrival time is measured.

Lines 103-106: This should be in the discussion or introduction rather than in the results

Lines 110 – 113: It is unclear which panel is figure 3 is being referred to. Panel 3c is also not cited at all in this section.

Figures S2 and S3 seem important to the overall narrative of the manuscript and therefore the authors should consider including them as part of the main text.

Lines 119-121: State how the locations of the lobes differ, are they closer to the flume exit? Does the slope differ between the 2 locations?

Line 124: Why is the series of events being described in terms of time? Time is not likely to be a controlling factor in how the debris flow behaves. The slope over which it is traveling is much more likely to be the control (along with the grain size distribution).

Figures 5 – 9 are very poorly explained and hard to interpret. This is not helped by there being no explanation of what is meant by “Run” in the experiment.

Lines 217-129: A numerical metric would help to compare the shape of the mono vs multigranular flows. Perhaps the angle of deviation from directly straight or a ratio of the left vs right side length?

Line 135: The difference in shape at the 2.2m line could be due to the difference in runout. While in the monogranular flow the authors are measuring the apex of the flow height in the multigranular it after the apex. As the debris flows are producing a fan like shape you would expect the fan to be wider after the apex regardless of the granular structure.

Discussion

The discussion, similarly to the introduction, is lacking in detail and is too vague in some of its points to make an impact on the reader. Currently the discussion spends too much time focusing on areas the authors did not investigate (pore fluid seepage) and not enough time putting their results back into the context of the literature. To help the reader the authors should put their results, which are interesting and novel, front and centre and discuss the processes that they actually recorded before moving on to areas they do not have direct evidence for.

Line by line comments:

Lines 141 – 142: The term “processes” is too vague and the results do not mention stratigraphy at all so this seems like a strange sentence to start the discussion with.

Lines 142 – 147: This section is poorly linked to the previous opening statement of the discussions.

Lines 150 – 151: This would also apply if the flow was monogranular with coarse grains.

Lines 152 – 155: The authors previously mentioned that there was minimal difference between the thicknesses of the mono and multi-granular flows. This idea of the coarser grains forming an obstacle which diverts the tail of the flow should be expanded upon further with more descriptions if the authors believe it to be significant.

Lines 158 – 164: It is unclear what the authors are suggesting here. How can there be moisture differences in the bed of the depositional area? The depositional area is the same for all of the test runs? Unless they are discussing deposition on a previously deposited fan? This is very underdeveloped.

Lines 165 – 169: This section is a strange ending to the manuscript. It focuses on two areas which the authors did not measure in their study; stratigraphy and moisture content of the fan.

Conclusions

The conclusions does not contain any references to the discussion and therefore it feels disconnected from the rest of the manuscript.