the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Jun 2023
Field monitoring of pore-water pressure in fully and partly saturated debris flows at Ohya landslide scar, Japan
Abstract. The characteristics of debris flows (e.g., mobility, sediment concentration, erosion, and deposition of sediment) are dependent on the pore-water pressure in the flows. Therefore, understanding the magnitude of pore-water pressure in debris flows is essential for improving debris flow mitigations measures. Notably, the pore-water pressure in a partly saturated flow, which contains an unsaturated layer in its upper part, has not been understood, due to a lack of data. The monitoring performed in Ohya landslide scar, central Japan, allowed us to obtain the data on the pore-water pressure in fully and partly saturated flows during four debris flow events. In some partly and fully saturated debris flows, the pore-water pressure at the channel bed exceeded the hydrostatic pressure of clean water. The depth gradient of the pore-water pressure in the lower part of the flow, monitored using water pressure sensors at multiple depths, was generally higher than the depth-averaged gradient of the pore-water pressure from the channel bed to the surface of the flow. The low gradient of the pore-water pressure in the upper part of partly saturated debris flow may be affected by the low hydrostatic pressure due to unsaturation of the flow. Additionally, excess pore-water pressure was observed in the lower part of partly saturated surges. The excess pore-water pressure may have resulted from the loading of particles and contraction of interstitial water. The pore-water pressure at the channel bed of fully saturated flow was generally similar to the hydrostatic pressure of clean water, while some saturated surges portrayed higher pore-water pressure than the hydrostatic pressure. The travel distance of debris flows, investigated by the structure from motion technique using unmanned aerial vehicle (UAV-SfM) and the monitoring of time lapse cameras, was long during a rainfall event having high intensity, even though the pore-water pressure in the flow was not significantly high. We conclude that the flow type (fully or partly saturated flows) should be considered to estimate the depth gradient of pore-water pressure in debris flows.
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RC1: 'Comment on egusphere-2023-1032', Anonymous Referee #1, 13 Jul 2023
In this study, the authors present that debris flows exhibit different characteristics during rainfall events, even within the same gully, and the internal stress of debris flow interacts with these variations. The data obtained in this study are highly valuable, including detailed measurements of pore-water pressure, as well as diagrams illustrating the propagation of debris-flow surges with high spatiotemporal resolution in Figs 6 and 7. These findings provide useful insights for predicting debris flow runout and therefore make a significant contribution to the relevant research fields. However, the manuscript requires a few improvements, particularly in the interpretation of the measured excess pore-water pressure.
Major concerns:
Clarification of excess pore water pressure:
The assumption of a potential maximum pore-water pressure of 25970 Pa/m, considering fully suspended sediment within the pore space of debris flow, seems reasonable (L170-171). However, it is important to note that this value also represents the theoretical maximum value for the normal stress in debris flows. The authors should discuss why the measured pore-water pressure in some cases (Figs. 5 & 6) exceeded this theoretical maximum value. It is possible that the pressure gauge detected lateral "dynamic" pressure, which could be influenced by the agitation of sediment particles, despite the authors' aim to measure the "static" pressure using the pressure gauges buried in the sidewall. If this is the case, separating the static excess pore-water pressure from the measured values would be challenging. The authors should provide a justification or at least discuss this discrepancy.Insufficient analyses in section 4.6:
The authors compare dp/dz with flow depth and pore-water pressure on an average-value basis. However, utilizing temporal dp/dz would enable a more detailed and effective comparison by classifying flow conditions (FS, PS) based on the TLS data, in accordance with the classification in Fig. 2b. Additionally, considering the discussion on mobility in section 5.2, dp/dz could be compared with other effective indices besides flow depth and pore-water pressure (Fig. 9). The authors have data on the propagation of each debris-flow surge (Fig. 5b and Fig. 6b), which allows for the determination of debris-flow velocity. By assuming local steady-state conditions and a certain sediment concentration, for example, the friction coefficient could be calculated to evaluate the energy dissipation of the target surges. This type of analysis would enhance/enrich the discussion.Specific minor comments:
L109 and L568: Change "2016a" to "2016."
eq. (1), L171-172: What is the causal factor of the excess pore water pressure?
L179: S1 and S2 are indicated as the set of two sensors, but is S4 also used to calculate dp/dz(lower) in Fig. 7?
eq. (4): Should "pu-pl" be changed to "pl-pu"?
L231: Refer to Table 2 at this point (Table 2 is not mentioned in section 4.1).
L287: “...triggered by a...”
L296 (L381, 382): Change "10:47:46" to "10:47"?
L306 (L433, 455): Remove "h" from the time "10:52 h."
L438-443: Can the influence of rainfall be discussed in the same context as the suspension of fine sediment and Reynolds stress? The relationship between rainfall and pore water pressure is unclear, while the suspension and Reynolds stress reflect the internal stress of debris flow.
L453-460: The debris-flow surges exemplified here were partly or fully saturated at P20 (Fig. 7b), and the pore-water pressure was identical or less than hydrostatic (Fig. 7d, although the data are limited to a few surges). Do the authors consider that these debris-flow surges gained excess pressure in the downstream section where the flows would have been in the deposition phase? If so, what causal factor increased the pore-water pressure?
L562: Is this reference to Imaizumi et al. (2022)?
L569-: Add line breaks for references to Imaizumi et al. (2019) and Itoh et al. (2019).
L615: Place McArdell et al. (2007) in L607.
Citation: https://doi.org/10.5194/egusphere-2023-1032-RC1 -
AC1: 'Reply on RC1', Fumitoshi Imaizumi, 19 Aug 2023
We sincerely thank you for the efforts you have made to review our manuscript. Comments from the reviewer are very helpful for us to improve our manuscript. We have responded to all review comments in the following paragraphs.
[Comment] In this study, the authors present that debris flows exhibit different characteristics during rainfall events, even within the same gully, and the internal stress of debris flow interacts with these variations. The data obtained in this study are highly valuable, including detailed measurements of pore-water pressure, as well as diagrams illustrating the propagation of debris-flow surges with high spatiotemporal resolution in Figs 6 and 7. These findings provide useful insights for predicting debris flow runout and therefore make a significant contribution to the relevant research fields. However, the manuscript requires a few improvements, particularly in the interpretation of the measured excess pore-water pressure.
[Reply] It is our pleasure that the manuscript was acclaimed by the reviewer.[Comment] Major concerns:
Clarification of excess pore water pressure:
The assumption of a potential maximum pore-water pressure of 25970 Pa/m, considering fully suspended sediment within the pore space of debris flow, seems reasonable (L170-171). However, it is important to note that this value also represents the theoretical maximum value for the normal stress in debris flows. The authors should discuss why the measured pore-water pressure in some cases (Figs. 5 & 6) exceeded this theoretical maximum value. It is possible that the pressure gauge detected lateral "dynamic" pressure, which could be influenced by the agitation of sediment particles, despite the authors' aim to measure the "static" pressure using the pressure gauges buried in the sidewall. If this is the case, separating the static excess pore-water pressure from the measured values would be challenging. The authors should provide a justification or at least discuss this discrepancy.
[Reply] As the reviewer points out, the 25970 Pa/m is the theoretical maximum value for the normal stress in debris flow. We will clarify it in the text. We also think the observed pore-water pressure exceeded the maximum value because of the dynamic pressure. We will discuss point in the discussion section[Comment] Insufficient analyses in section 4.6:
The authors compare dp/dz with flow depth and pore-water pressure on an average-value basis. However, utilizing temporal dp/dz would enable a more detailed and effective comparison by classifying flow conditions (FS, PS) based on the TLS data, in accordance with the classification in Fig. 2b. Additionally, considering the discussion on mobility in section 5.2, dp/dz could be compared with other effective indices besides flow depth and pore-water pressure (Fig. 9). The authors have data on the propagation of each debris-flow surge (Fig. 5b and Fig. 6b), which allows for the determination of debris-flow velocity. By assuming local steady-state conditions and a certain sediment concentration, for example, the friction coefficient could be calculated to evaluate the energy dissipation of the target surges. This type of analysis would enhance/enrich the discussion.
[Reply] Based on the comment from the reviewer, we will calculate dp/dz for fully saturated and partly saturated flows in each debris flow surges. The suggestion about analysis on the flow velocity is really interesting. As the reviewer think, flow velocity is different among debris flow events and surges. This might be good material to deepen discussion of the flow characteristics and mechanisms. We will consider additional discussion in 5.2.Specific minor comments:
[Comment] L109 and L568: Change "2016a" to "2016."
[Reply] We will revise the year as pointed out.[Comment] eq. (1), L171-172: What is the causal factor of the excess pore water pressure?
[Reply] Previous studies suggested that the excess pore-water pressure occurs due to the contraction of interstitial water by the surrounding boulders in a high shear stress environment, loading of particles into the interstitial water, Reynolds stress from the turbulence of interstitial water caused by the collision of boulders, and centrifugal force in curved channel sections. We will briefly introduce the causal factors after this sentence.[Comment] L179: S1 and S2 are indicated as the set of two sensors, but is S4 also used to calculate dp/dz(lower) in Fig. 7?
[Reply] In Fig. 7, sensors S1 and S4 were used to calculate dp/dz(lower). We will clarify.[Comment] eq. (4): Should "pu-pl" be changed to "pl-pu"?
[Reply] As pointed by the reviewer, "pu-pl" should be negative value if the numerator is "pu-pl". We will revise the expression.[Comment] L231: Refer to Table 2 at this point (Table 2 is not mentioned in section 4.1).
[Reply] We will refer Table 2 in this sentence.[Comment] L287: “...triggered by a...”
[Reply] We will revise the sentence.[Comment] L296 (L381, 382): Change "10:47:46" to "10:47"?
[Reply] We will remove second as suggested.[Comment] L306 (L433, 455): Remove "h" from the time "10:52 h."
[Reply] We will remove h[Comment] L438-443: Can the influence of rainfall be discussed in the same context as the suspension of fine sediment and Reynolds stress? The relationship between rainfall and pore water pressure is unclear, while the suspension and Reynolds stress reflect the internal stress of debris flow.
[Reply] Currently we do not have an idea on direct effect of rainfall on suspension of fine sediment and Reynolds stress. Importance of the suspension and Reynolds stress on the pore-water pressure would depend on ratio of fine sediment in eroded channel deposits. If the ratio of fine sediment is higher, larger amount of sediment would be suspended. Flow might be turbulence and has Reynolds stress if few course particles are included. We will add discussion about the suspension of fine sediment and Reynolds stress in section.[Comment] L453-460: The debris-flow surges exemplified here were partly or fully saturated at P20 (Fig. 7b), and the pore-water pressure was identical or less than hydrostatic (Fig. 7d, although the data are limited to a few surges). Do the authors consider that these debris-flow surges gained excess pressure in the downstream section where the flows would have been in the deposition phase? If so, what causal factor increased the pore-water pressure?
[Reply] We feel sorry that we cannot directly answer to this question because we observed the pore-water pressure just at P20. Although we are also interested in the pore-water pressure on the debris flow fan, it was impossible to set up monitoring devices because of significant aggradation and degradation of the channel bed and frequent changes in the debris flow path (avulsion). Potential cause is deposition of sediments just below P20 (Fig. 4c). This decreased coarse particles in the flow, and increased mobility of flow. We will add discussion why the travel distance was long during event.[Comment] L562: Is this reference to Imaizumi et al. (2022)?
[Reply]This was published in proceedings in 2023. We will revise.[Comment] L569-: Add line breaks for references to Imaizumi et al. (2019) and Itoh et al. (2019).
[Reply] We will add line breaks.[Comment] L615: Place McArdell et al. (2007) in L607.
[Reply] We will change the order of references as pointed out by the reviewer.Thank you again for reviewing our manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-1032-AC1
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AC1: 'Reply on RC1', Fumitoshi Imaizumi, 19 Aug 2023
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RC2: 'Comment on egusphere-2023-1032', Anonymous Referee #2, 29 Aug 2023
General Comments
This manuscript describes an innovative field measurement program aimed to investigate pore water pressure in fully and partially saturated debris flows. The authors have done a very nice job preparing this manuscript. It is clearly written, the figures are well prepared and clear, and the logic is easy to follow. Consequently, I have very few comments overall because this manuscript is in good shape already.
First, I commend the authors on such a detailed study. The type of data they present is very difficult to obtain, and consequently very valuable to the community in general. I have one suggestion on how rainfall intensity is presented (same comment shown below in the specific comments). It looks like when they discuss the 10 minute rainfall intensity they are showing the total rainfall (mm) in a 10 min period. However, in hydrology, traditionally people use units of mm/hr for intensity. So a 10 minute intensity would be mm/10min, converted to units of mm/hr. I would suggest changing this throughout the paper.
Second, I think at the end of the abstract, they slightly undersell the big conclusions. I think the authors could concentrate on the importance of pore water pressure in the largest surges. The authors could also emphasize the uniqueness of this dataset.
Finally, I’m also curious if you could see if there is a relationship between the peak 10 min intensity preceeding a surge and the peak dpdz? Not sure if there is a relationship there, but it might be worth looking at.
Specific Comments:
23-24: I’m not sure if this is the ‘main point’ with which you want to end the abstract because, realistically, I’m not sure that the flow type is something that can really be “considered” ahead of time. Maybe you could be more specific, and say it needs to be considered in modeling. I think a stronger finding from your paper is that you found that excess pore water pressure was present in many of the biggest surges, and that it appears to be an important mechanism in debris flow surge behavior.
29: change “decrease” to “decreases”
107: Can you label the Hontani torrent on the map?
108: You mention P36, but you haven’t told us what P means yet.
121: change “intermitted” to “intermittent”
123: Is “intermission season” Nov. to March? Can you specify this more clearly?
136: reference the supplement where readers can see this video.
153: Change “accumulations was” to “accumulations were”
232: Here and elsewhere you it looks like when you discuss the 10 minute rainfall intensity you are showing the total rainfall (mm) in a 10 min period. However, in hydrology, traditionally people use units of mm/hr for intensity. So a 10 minute intensity would be mm/10min, converted to units of mm/hr. I would suggest changing this throughout the paper.
276: If possible, can you indicate that 22:20 to 23:00 represents the time of day in hh:mm.
416: after “flows” can you add in the highest dp/dzlower value in ()?
464: you indicate that the volume of material depends on the initiation zone, but there have been a few studies that indicate that the volume of material supplied to channels declines during parts of the year when debris flows are active, and then refills during winter periods. I would suggest considering adding references to these, because that helps to explain your declining volume over time:
Rengers, F. K., Kean, J. W., Reitman, N. G., Smith, J. B., Coe, J. A., & McGuire, L. A. (2020). The influence of frost weathering on debris flow sediment supply in an alpine basin. Journal of Geophysical Research: Earth Surface, 125, e2019JF005369. https://doi.org/10.1029/2019JF005369
Bennett, G., Molnar, P., McArdell, B., Schlunegger, F., & Burlando, P. (2013). Patterns and controls of sediment production, transfer and yield in the Illgraben. Geomorphology, 188, 68–82.
492: Change “thigh” to “high”
Figure 1. Say what “P1 means”
Figure 3a. Can you use an arrow to show the flow direction?
Figure 4. I’m about to suggest something that is 100% a personal preference. I know there are different approaches to blue/red plots of erosion and deposition. I prefer erosion = red and deposition = blue. You may not agree with this, and that’s totally acceptable, but consider switching to that color scheme.
Table 2: Can you show the volume of sediment after the event?
Citation: https://doi.org/10.5194/egusphere-2023-1032-RC2 -
AC2: 'Reply on RC2', Fumitoshi Imaizumi, 08 Sep 2023
We sincerely thank you for the efforts you have made to review our manuscript. Comments from the reviewer are very helpful for us to improve the manuscript. We have responded to all review comments in the following paragraphs.
[Comment] General Comments
This manuscript describes an innovative field measurement program aimed to investigate pore water pressure in fully and partially saturated debris flows. The authors have done a very nice job preparing this manuscript. It is clearly written, the figures are well prepared and clear, and the logic is easy to follow. Consequently, I have very few comments overall because this manuscript is in good shape already.
[Reply] It is our pleasure that the manuscript was acclaimed by the reviewer.[Comment] First, I commend the authors on such a detailed study. The type of data they present is very difficult to obtain, and consequently very valuable to the community in general. I have one suggestion on how rainfall intensity is presented (same comment shown below in the specific comments). It looks like when they discuss the 10 minute rainfall intensity they are showing the total rainfall (mm) in a 10 min period. However, in hydrology, traditionally people use units of mm/hr for intensity. So a 10 minute intensity would be mm/10min, converted to units of mm/hr. I would suggest changing this throughout the paper.
[Reply] As the reviewer comments, our intensity in the rainfall depth in a 1- min period. We will convert the unit to mm/hr[Comment] Second, I think at the end of the abstract, they slightly undersell the big conclusions. I think the authors could concentrate on the importance of pore water pressure in the largest surges. The authors could also emphasize the uniqueness of this dataset.
[Reply] Thank you for your suggestion. We will revise the last part of the sentence in the abstract.[Comment] Finally, I’m also curious if you could see if there is a relationship between the peak 10 min intensity preceeding a surge and the peak dpdz? Not sure if there is a relationship there, but it might be worth looking at.
[Reply] We will add a new figure showing comparison of the 10 min intensity to dp/dz.[Comment] Specific Comments:
23-24: I’m not sure if this is the ‘main point’ with which you want to end the abstract because, realistically, I’m not sure that the flow type is something that can really be “considered” ahead of time. Maybe you could be more specific, and say it needs to be considered in modeling. I think a stronger finding from your paper is that you found that excess pore water pressure was present in many of the biggest surges, and that it appears to be an important mechanism in debris flow surge behavior.
[Reply] As suggested by the reviewer, we will emphasize importance of findings in this study at the end of abstract.[Comment] 29: change “decrease” to “decreases”
[Reply] We will reply as pointed out by the reviewer.[Comment] 107: Can you label the Hontani torrent on the map?
[Reply]We will label the Hontani in Fig. 1[Comment] 108: You mention P36, but you haven’t told us what P means yet.
[Reply]We will revise explanation of the junction.[Comment] 121: change “intermitted” to “intermittent”
[Reply] We will replace “intermitted” to “intermittent”.[Comment] 123: Is “intermission season” Nov. to March? Can you specify this more clearly?
[Reply] This is from November to March. We will clarify.[Comment] 136: reference the supplement where readers can see this video.
[Reply] Link to the video supplement has been described in 502. We will clarify that the video can be seen in video supplement.[Comment] 153: Change “accumulations was” to “accumulations were”
[Reply] We will revise the sentence.[Comment] 232: Here and elsewhere you it looks like when you discuss the 10 minute rainfall intensity you are showing the total rainfall (mm) in a 10 min period. However, in hydrology, traditionally people use units of mm/hr for intensity. So a 10 minute intensity would be mm/10min, converted to units of mm/hr. I would suggest changing this throughout the paper.
[Reply] We will revise the unit as suggested.[Comment] 276: If possible, can you indicate that 22:20 to 23:00 represents the time of day in hh:mm.
[Reply] We will add explanation of hh:mm and hh:mm:ss in the manuscript.[Comment] 416: after “flows” can you add in the highest dp/dzlower value in ()?
[Reply] We will add the value in the sentence[Comment] 464: you indicate that the volume of material depends on the initiation zone, but there have been a few studies that indicate that the volume of material supplied to channels declines during parts of the year when debris flows are active, and then refills during winter periods. I would suggest considering adding references to these, because that helps to explain your declining volume over time:
Rengers, F. K., Kean, J. W., Reitman, N. G., Smith, J. B., Coe, J. A., & McGuire, L. A. (2020). The influence of frost weathering on debris flow sediment supply in an alpine basin. Journal of Geophysical Research: Earth Surface, 125, e2019JF005369. https://doi.org/10.1029/2019JF005369Bennett, G., Molnar, P., McArdell, B., Schlunegger, F., & Burlando, P. (2013). Patterns and controls of sediment production, transfer and yield in the Illgraben. Geomorphology, 188, 68–82.
[Reply] We will add references suggested by the reviewer.[Comment] 492: Change “thigh” to “high”
[Reply] We will revise the “high”[Comment] Figure 1. Say what “P1 means”
[Reply] We will add explanation on P1 to P36 in the figure caption.[Comment] Figure 3a. Can you use an arrow to show the flow direction?
[Reply] We will add an arrow showing flow direction in Fig. 3a.[Comment] Figure 4. I’m about to suggest something that is 100% a personal preference. I know there are different approaches to blue/red plots of erosion and deposition. I prefer erosion = red and deposition = blue. You may not agree with this, and that’s totally acceptable, but consider switching to that color scheme.
[Reply] Based on the reviewer’s comment, we have checked color of erosion/deposition areas in previous papers. We could find both red/blue and blue/red patterns. Papers using red for the erosion would be major. We will think to change the color.[Comment] Table 2: Can you show the volume of sediment after the event?
[Reply] We can add sediment volume after the events.Thank you again for reviewing our manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-1032-AC2
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AC2: 'Reply on RC2', Fumitoshi Imaizumi, 08 Sep 2023
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RC3: 'Comment on egusphere-2023-1032', Anonymous Referee #3, 08 Sep 2023
In this study, valuable field measurement data from the Ohya landslide scar in Japan are presented. Several intriguing and significant phenomena, such as "excess pore pressure" and the high depth gradient of pore-water pressure, have been clearly observed. However, the explanations for these phenomena lack depth and rigor, falling short of the level suggested by the authors in their aim to "reveal the factors that affect the magnitude of the pore-water pressure." The primary concerns are as follows:
Major Concerns:
The author's explanations for "excess pore pressure" and the "high depth gradient of the pore-water pressure" are not sufficiently comprehensive, and the cited reasons lack rigor.
First, regarding the loading of particles, there is a lack of reliable evidence to suggest that this pore-water pressure is induced by the loading of particles. The load of particles becomes part of the pore-water pressure only when particles are fully suspended by fluid, which usually requires a condition typically associated with smaller particle sizes. As indicated in Figure 2, partly saturated debris flows often occur at the head of debris flows where larger particles accumulate. In such conditions, the movement of particles and fluid is markedly different, and the loading of particles does not contribute significantly to the rise in pore-water pressure. Determining whether particles are fully suspended often relies on parameters such as the Rouse number (Rouse, 1937; Dietrich, 1982; Jiménez & Madsen, 2003) and Stokes number (e.g., Ancey et al., 1999). This necessitates knowledge of the mean or median grain size of the debris flow and slurry viscosity, among other parameters. The shear rate can be roughly estimated from surface flow velocity and flow depth. If slurry viscosity has not been measured, it can be estimated based on the typical viscosity range observed in field studies of debris flows in Japan or other regions, such as Illgraben, Chalk Cliff, and Jiangjia Ravine.
Secondly, in addressing the "contraction of interstitial water," it is crucial to analyze the diffusion time of pore pressure, which describes the tendency for pore fluid pressure to develop between moving grains. This can be assessed through parameters such as the Darcy number (Iverson, 1997; Lanzoni et al., 2017) and P number (Iverson et al., 2004), which evaluate the timescale ratio between avalanche motion and the diffusion of disequilibrium pore fluid pressure. Contraction of interstitial water leading to "excess pore pressure" is more likely to occur when this timescale ratio is relatively small. However, these conditions are stringent in natural debris flow, typically requiring finer particle sizes, higher solid-phase concentrations, and greater interstitial fluid viscosity.
"Excess pore pressure" and the "high depth gradient of the pore-water pressure" are central issues in this study. Therefore, when discussing and analyzing these phenomena, the authors should provide quantitative analysis (if feasible, estimates of missing parameters can be approximated to facilitate final calculations) or detailed qualitative discussions, followed by explanations of excess pore pressure, rather than presenting vague concepts.
Relevant references:
Dietrich, W. E. (1982). Settling velocity of natural particles. Water resources research, 18(6), 1615-1626.
Jiménez, J. A., & Madsen, O. S. (2003). A simple formula to estimate the settling velocity of natural sediments. Journal of waterway, port, coastal, and ocean engineering, 129(2), 70-78.
Coussot, P., & Ancey, C. (1999). Rheophysical classification of concentrated suspensions and granular pastes. Physical Review E, 59(4), 4445.
Iverson, R. M., Logan, M., & Denlinger, R. P. (2004). Granular avalanches across irregular three‐dimensional terrain: 2. Experimental tests. Journal of Geophysical Research: Earth Surface, 109(F1).
Minor comments:
Figure 1: The figure displays only three observation points, yet they are labeled from P1 to P20 to P36. If there are additional observation points between P1 and P36 that are relevant and need to be shown, it is advisable to include them on a separate, larger-scale map. If only these three points are pertinent to the study, renumbering is suggested.
Lines 170-175: The statement "the maximum weight per unit volume of the interstitial water was the same as that of the sediments" is not in accordance with reality, as every liquid has its maximum sediment transport capacity.
Lines 175-180: The two methods for calculating the gradient of pore-water pressure (Eq. 3 and Eq. 4) do not differ fundamentally; they mainly vary in the size of the delta_h interval. Since Eq. 4 offers a smaller interval and higher precision, the method relying on bulk flow depth estimation, as represented by Eq. 3, may be less accurate as it overlooks local information within the intermediate flow layer.
Figure 5: What is the unit of rainfall intensity? Is it expressed in mm/day or mm/hour? Although the authors mention "the 10-minute rainfall intensity," this is not a commonly used unit for representing rainfall intensity. Clarification is needed.
Line 492: There is a spelling error in this section.
Citation: https://doi.org/10.5194/egusphere-2023-1032-RC3 -
AC3: 'Reply on RC3', Fumitoshi Imaizumi, 13 Oct 2023
We sincerely thank you for the efforts you have made to review our manuscript. Comments from the reviewer are very helpful for us to deepen our discussion on the physical mechanism of debris flow. We have responded to all review comments in the following paragraphs. Labels and line numbers after our response correspond to those in the revised manuscript with tracked changes.
Major Concerns:
[Comment R3_1] The author's explanations for "excess pore pressure" and the "high depth gradient of the pore-water pressure" are not sufficiently comprehensive, and the cited reasons lack rigor. First, regarding the loading of particles, there is a lack of reliable evidence to suggest that this pore-water pressure is induced by the loading of particles. The load of particles becomes part of the pore-water pressure only when particles are fully suspended by fluid, which usually requires a condition typically associated with smaller particle sizes. As indicated in Figure 2, partly saturated debris flows often occur at the head of debris flows where larger particles accumulate. In such conditions, the movement of particles and fluid is markedly different, and the loading of particles does not contribute significantly to the rise in pore-water pressure. Determining whether particles are fully suspended often relies on parameters such as the Rouse number (Rouse, 1937; Dietrich, 1982; Jiménez & Madsen, 2003) and Stokes number (e.g., Ancey et al., 1999). This necessitates knowledge of the mean or median grain size of the debris flow and slurry viscosity, among other parameters. The shear rate can be roughly estimated from surface flow velocity and flow depth. If slurry viscosity has not been measured, it can be estimated based on the typical viscosity range observed in field studies of debris flows in Japan or other regions, such as Illgraben, Chalk Cliff, and Jiangjia Ravine.
[Reply] Thank you so much for your comments that deepen physical analysis of the pore water pressure. We think our terminology was not very clear in the previous version of the manuscript. We have revised terminology, such as loading of particles. We agree that the suspension of sediments increases ps value in Eq. (1). Based on the comment from the reviewer, we have added analysis of the Rouse number. Our analysis implied that material of suspended particles existed in the debris flow material. At the same time, because the pore-water pressure in some debris flow surges was similar to hydrostatic pressure of clean water, effect of the ps on the pore-water pressure was likely small when fine sediments were poor on the surface of the channel deposits.[Comment R3_2] Secondly, in addressing the "contraction of interstitial water," it is crucial to analyze the diffusion time of pore pressure, which describes the tendency for pore fluid pressure to develop between moving grains. This can be assessed through parameters such as the Darcy number (Iverson, 1997; Lanzoni et al., 2017) and P number (Iverson et al., 2004), which evaluate the timescale ratio between avalanche motion and the diffusion of disequilibrium pore fluid pressure. Contraction of interstitial water leading to "excess pore pressure" is more likely to occur when this timescale ratio is relatively small. However, these conditions are stringent in natural debris flow, typically requiring finer particle sizes, higher solid-phase concentrations, and greater interstitial fluid viscosity.
[Reply] We have added analysis on the P number. The excess-pore water pressure can be maintained over longer timescales because the diffusion of the pore-fluid pressure was not high in the Ichinosawa. We hope analysis of the P number supports our idea that the contraction of interstitial water generated excess pore-water pressure.[Comment R3_3] "Excess pore pressure" and the "high depth gradient of the pore-water pressure" are central issues in this study. Therefore, when discussing and analyzing these phenomena, the authors should provide quantitative analysis (if feasible, estimates of missing parameters can be approximated to facilitate final calculations) or detailed qualitative discussions, followed by explanations of excess pore pressure, rather than presenting vague concepts.
[Reply] Based on the comments from the reviewer, we have calculated Bagnold number, Savage number, and Friction number. New analyses revealed that frictional force dominated in the partly saturated debris flows. Scale of the pore space between boulders due to the high volumetric solid concentration, resulting in the predominance of the frictional force. Additionally, the ∆p/∆zlower was higher in the partly saturated debris flows with larger influence of the frictional force. We have added discussion on the internal force affecting the pore-water pressure. We also added related references in the paper.Minor comments:
[Comment R3_4] Figure 1: The figure displays only three observation points, yet they are labeled from P1 to P20 to P36. If there are additional observation points between P1 and P36 that are relevant and need to be shown, it is advisable to include them on a separate, larger-scale map. If only these three points are pertinent to the study, renumbering is suggested.
[Reply] Because it is difficult to show all of analysis points in the Figure 1, we have displayed their locations in Figure 4. We have clarified it in the caption of Figure 1.[Comment R3_5] Lines 170-175: The statement "the maximum weight per unit volume of the interstitial water was the same as that of the sediments" is not in accordance with reality, as every liquid has its maximum sediment transport capacity.
[Reply] We also think that this value does not have reality as maximum weight per unit volume of the interstitial water. This value is not lower boundary of occurrence of the excess pore-water pressure. But the excess pore-water pressure surely (clearly) exists in debris flows when the pore-water pressure exceeds this value. We have improved explanation and terminology of this value throughout the paper.[Comment R3_6] Lines 175-180: The two methods for calculating the gradient of pore-water pressure (Eq. 3 and Eq. 4) do not differ fundamentally; they mainly vary in the size of the delta_h interval. Since Eq. 4 offers a smaller interval and higher precision, the method relying on bulk flow depth estimation, as represented by Eq. 3, may be less accurate as it overlooks local information within the intermediate flow layer.
[Reply] We think that the spatial difference in the gradient of the pore-water pressure along the depth direction affects disagreement of Eq. 3 and Eq. 4 values. At the same time, as pointed out by the reviewer, difference in the accuracy of estimation also affects disagreements of the two values. We have explained potential effect of the estimation accuracy on the difference in the values from Eq. 3 and Eq. 4.[Comment R3_7] Figure 5: What is the unit of rainfall intensity? Is it expressed in mm/day or mm/hour? Although the authors mention "the 10-minute rainfall intensity," this is not a commonly used unit for representing rainfall intensity. Clarification is needed.
[Reply] We have changed unit of the intensity to mm h-1 throughout the manuscript.[Comment R3_8] Line 492: There is a spelling error in this section.
[Reply] We have revised misspelling in line 492.Thank you again for your review.
Citation: https://doi.org/10.5194/egusphere-2023-1032-AC3
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AC3: 'Reply on RC3', Fumitoshi Imaizumi, 13 Oct 2023
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-1032', Anonymous Referee #1, 13 Jul 2023
In this study, the authors present that debris flows exhibit different characteristics during rainfall events, even within the same gully, and the internal stress of debris flow interacts with these variations. The data obtained in this study are highly valuable, including detailed measurements of pore-water pressure, as well as diagrams illustrating the propagation of debris-flow surges with high spatiotemporal resolution in Figs 6 and 7. These findings provide useful insights for predicting debris flow runout and therefore make a significant contribution to the relevant research fields. However, the manuscript requires a few improvements, particularly in the interpretation of the measured excess pore-water pressure.
Major concerns:
Clarification of excess pore water pressure:
The assumption of a potential maximum pore-water pressure of 25970 Pa/m, considering fully suspended sediment within the pore space of debris flow, seems reasonable (L170-171). However, it is important to note that this value also represents the theoretical maximum value for the normal stress in debris flows. The authors should discuss why the measured pore-water pressure in some cases (Figs. 5 & 6) exceeded this theoretical maximum value. It is possible that the pressure gauge detected lateral "dynamic" pressure, which could be influenced by the agitation of sediment particles, despite the authors' aim to measure the "static" pressure using the pressure gauges buried in the sidewall. If this is the case, separating the static excess pore-water pressure from the measured values would be challenging. The authors should provide a justification or at least discuss this discrepancy.Insufficient analyses in section 4.6:
The authors compare dp/dz with flow depth and pore-water pressure on an average-value basis. However, utilizing temporal dp/dz would enable a more detailed and effective comparison by classifying flow conditions (FS, PS) based on the TLS data, in accordance with the classification in Fig. 2b. Additionally, considering the discussion on mobility in section 5.2, dp/dz could be compared with other effective indices besides flow depth and pore-water pressure (Fig. 9). The authors have data on the propagation of each debris-flow surge (Fig. 5b and Fig. 6b), which allows for the determination of debris-flow velocity. By assuming local steady-state conditions and a certain sediment concentration, for example, the friction coefficient could be calculated to evaluate the energy dissipation of the target surges. This type of analysis would enhance/enrich the discussion.Specific minor comments:
L109 and L568: Change "2016a" to "2016."
eq. (1), L171-172: What is the causal factor of the excess pore water pressure?
L179: S1 and S2 are indicated as the set of two sensors, but is S4 also used to calculate dp/dz(lower) in Fig. 7?
eq. (4): Should "pu-pl" be changed to "pl-pu"?
L231: Refer to Table 2 at this point (Table 2 is not mentioned in section 4.1).
L287: “...triggered by a...”
L296 (L381, 382): Change "10:47:46" to "10:47"?
L306 (L433, 455): Remove "h" from the time "10:52 h."
L438-443: Can the influence of rainfall be discussed in the same context as the suspension of fine sediment and Reynolds stress? The relationship between rainfall and pore water pressure is unclear, while the suspension and Reynolds stress reflect the internal stress of debris flow.
L453-460: The debris-flow surges exemplified here were partly or fully saturated at P20 (Fig. 7b), and the pore-water pressure was identical or less than hydrostatic (Fig. 7d, although the data are limited to a few surges). Do the authors consider that these debris-flow surges gained excess pressure in the downstream section where the flows would have been in the deposition phase? If so, what causal factor increased the pore-water pressure?
L562: Is this reference to Imaizumi et al. (2022)?
L569-: Add line breaks for references to Imaizumi et al. (2019) and Itoh et al. (2019).
L615: Place McArdell et al. (2007) in L607.
Citation: https://doi.org/10.5194/egusphere-2023-1032-RC1 -
AC1: 'Reply on RC1', Fumitoshi Imaizumi, 19 Aug 2023
We sincerely thank you for the efforts you have made to review our manuscript. Comments from the reviewer are very helpful for us to improve our manuscript. We have responded to all review comments in the following paragraphs.
[Comment] In this study, the authors present that debris flows exhibit different characteristics during rainfall events, even within the same gully, and the internal stress of debris flow interacts with these variations. The data obtained in this study are highly valuable, including detailed measurements of pore-water pressure, as well as diagrams illustrating the propagation of debris-flow surges with high spatiotemporal resolution in Figs 6 and 7. These findings provide useful insights for predicting debris flow runout and therefore make a significant contribution to the relevant research fields. However, the manuscript requires a few improvements, particularly in the interpretation of the measured excess pore-water pressure.
[Reply] It is our pleasure that the manuscript was acclaimed by the reviewer.[Comment] Major concerns:
Clarification of excess pore water pressure:
The assumption of a potential maximum pore-water pressure of 25970 Pa/m, considering fully suspended sediment within the pore space of debris flow, seems reasonable (L170-171). However, it is important to note that this value also represents the theoretical maximum value for the normal stress in debris flows. The authors should discuss why the measured pore-water pressure in some cases (Figs. 5 & 6) exceeded this theoretical maximum value. It is possible that the pressure gauge detected lateral "dynamic" pressure, which could be influenced by the agitation of sediment particles, despite the authors' aim to measure the "static" pressure using the pressure gauges buried in the sidewall. If this is the case, separating the static excess pore-water pressure from the measured values would be challenging. The authors should provide a justification or at least discuss this discrepancy.
[Reply] As the reviewer points out, the 25970 Pa/m is the theoretical maximum value for the normal stress in debris flow. We will clarify it in the text. We also think the observed pore-water pressure exceeded the maximum value because of the dynamic pressure. We will discuss point in the discussion section[Comment] Insufficient analyses in section 4.6:
The authors compare dp/dz with flow depth and pore-water pressure on an average-value basis. However, utilizing temporal dp/dz would enable a more detailed and effective comparison by classifying flow conditions (FS, PS) based on the TLS data, in accordance with the classification in Fig. 2b. Additionally, considering the discussion on mobility in section 5.2, dp/dz could be compared with other effective indices besides flow depth and pore-water pressure (Fig. 9). The authors have data on the propagation of each debris-flow surge (Fig. 5b and Fig. 6b), which allows for the determination of debris-flow velocity. By assuming local steady-state conditions and a certain sediment concentration, for example, the friction coefficient could be calculated to evaluate the energy dissipation of the target surges. This type of analysis would enhance/enrich the discussion.
[Reply] Based on the comment from the reviewer, we will calculate dp/dz for fully saturated and partly saturated flows in each debris flow surges. The suggestion about analysis on the flow velocity is really interesting. As the reviewer think, flow velocity is different among debris flow events and surges. This might be good material to deepen discussion of the flow characteristics and mechanisms. We will consider additional discussion in 5.2.Specific minor comments:
[Comment] L109 and L568: Change "2016a" to "2016."
[Reply] We will revise the year as pointed out.[Comment] eq. (1), L171-172: What is the causal factor of the excess pore water pressure?
[Reply] Previous studies suggested that the excess pore-water pressure occurs due to the contraction of interstitial water by the surrounding boulders in a high shear stress environment, loading of particles into the interstitial water, Reynolds stress from the turbulence of interstitial water caused by the collision of boulders, and centrifugal force in curved channel sections. We will briefly introduce the causal factors after this sentence.[Comment] L179: S1 and S2 are indicated as the set of two sensors, but is S4 also used to calculate dp/dz(lower) in Fig. 7?
[Reply] In Fig. 7, sensors S1 and S4 were used to calculate dp/dz(lower). We will clarify.[Comment] eq. (4): Should "pu-pl" be changed to "pl-pu"?
[Reply] As pointed by the reviewer, "pu-pl" should be negative value if the numerator is "pu-pl". We will revise the expression.[Comment] L231: Refer to Table 2 at this point (Table 2 is not mentioned in section 4.1).
[Reply] We will refer Table 2 in this sentence.[Comment] L287: “...triggered by a...”
[Reply] We will revise the sentence.[Comment] L296 (L381, 382): Change "10:47:46" to "10:47"?
[Reply] We will remove second as suggested.[Comment] L306 (L433, 455): Remove "h" from the time "10:52 h."
[Reply] We will remove h[Comment] L438-443: Can the influence of rainfall be discussed in the same context as the suspension of fine sediment and Reynolds stress? The relationship between rainfall and pore water pressure is unclear, while the suspension and Reynolds stress reflect the internal stress of debris flow.
[Reply] Currently we do not have an idea on direct effect of rainfall on suspension of fine sediment and Reynolds stress. Importance of the suspension and Reynolds stress on the pore-water pressure would depend on ratio of fine sediment in eroded channel deposits. If the ratio of fine sediment is higher, larger amount of sediment would be suspended. Flow might be turbulence and has Reynolds stress if few course particles are included. We will add discussion about the suspension of fine sediment and Reynolds stress in section.[Comment] L453-460: The debris-flow surges exemplified here were partly or fully saturated at P20 (Fig. 7b), and the pore-water pressure was identical or less than hydrostatic (Fig. 7d, although the data are limited to a few surges). Do the authors consider that these debris-flow surges gained excess pressure in the downstream section where the flows would have been in the deposition phase? If so, what causal factor increased the pore-water pressure?
[Reply] We feel sorry that we cannot directly answer to this question because we observed the pore-water pressure just at P20. Although we are also interested in the pore-water pressure on the debris flow fan, it was impossible to set up monitoring devices because of significant aggradation and degradation of the channel bed and frequent changes in the debris flow path (avulsion). Potential cause is deposition of sediments just below P20 (Fig. 4c). This decreased coarse particles in the flow, and increased mobility of flow. We will add discussion why the travel distance was long during event.[Comment] L562: Is this reference to Imaizumi et al. (2022)?
[Reply]This was published in proceedings in 2023. We will revise.[Comment] L569-: Add line breaks for references to Imaizumi et al. (2019) and Itoh et al. (2019).
[Reply] We will add line breaks.[Comment] L615: Place McArdell et al. (2007) in L607.
[Reply] We will change the order of references as pointed out by the reviewer.Thank you again for reviewing our manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-1032-AC1
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AC1: 'Reply on RC1', Fumitoshi Imaizumi, 19 Aug 2023
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RC2: 'Comment on egusphere-2023-1032', Anonymous Referee #2, 29 Aug 2023
General Comments
This manuscript describes an innovative field measurement program aimed to investigate pore water pressure in fully and partially saturated debris flows. The authors have done a very nice job preparing this manuscript. It is clearly written, the figures are well prepared and clear, and the logic is easy to follow. Consequently, I have very few comments overall because this manuscript is in good shape already.
First, I commend the authors on such a detailed study. The type of data they present is very difficult to obtain, and consequently very valuable to the community in general. I have one suggestion on how rainfall intensity is presented (same comment shown below in the specific comments). It looks like when they discuss the 10 minute rainfall intensity they are showing the total rainfall (mm) in a 10 min period. However, in hydrology, traditionally people use units of mm/hr for intensity. So a 10 minute intensity would be mm/10min, converted to units of mm/hr. I would suggest changing this throughout the paper.
Second, I think at the end of the abstract, they slightly undersell the big conclusions. I think the authors could concentrate on the importance of pore water pressure in the largest surges. The authors could also emphasize the uniqueness of this dataset.
Finally, I’m also curious if you could see if there is a relationship between the peak 10 min intensity preceeding a surge and the peak dpdz? Not sure if there is a relationship there, but it might be worth looking at.
Specific Comments:
23-24: I’m not sure if this is the ‘main point’ with which you want to end the abstract because, realistically, I’m not sure that the flow type is something that can really be “considered” ahead of time. Maybe you could be more specific, and say it needs to be considered in modeling. I think a stronger finding from your paper is that you found that excess pore water pressure was present in many of the biggest surges, and that it appears to be an important mechanism in debris flow surge behavior.
29: change “decrease” to “decreases”
107: Can you label the Hontani torrent on the map?
108: You mention P36, but you haven’t told us what P means yet.
121: change “intermitted” to “intermittent”
123: Is “intermission season” Nov. to March? Can you specify this more clearly?
136: reference the supplement where readers can see this video.
153: Change “accumulations was” to “accumulations were”
232: Here and elsewhere you it looks like when you discuss the 10 minute rainfall intensity you are showing the total rainfall (mm) in a 10 min period. However, in hydrology, traditionally people use units of mm/hr for intensity. So a 10 minute intensity would be mm/10min, converted to units of mm/hr. I would suggest changing this throughout the paper.
276: If possible, can you indicate that 22:20 to 23:00 represents the time of day in hh:mm.
416: after “flows” can you add in the highest dp/dzlower value in ()?
464: you indicate that the volume of material depends on the initiation zone, but there have been a few studies that indicate that the volume of material supplied to channels declines during parts of the year when debris flows are active, and then refills during winter periods. I would suggest considering adding references to these, because that helps to explain your declining volume over time:
Rengers, F. K., Kean, J. W., Reitman, N. G., Smith, J. B., Coe, J. A., & McGuire, L. A. (2020). The influence of frost weathering on debris flow sediment supply in an alpine basin. Journal of Geophysical Research: Earth Surface, 125, e2019JF005369. https://doi.org/10.1029/2019JF005369
Bennett, G., Molnar, P., McArdell, B., Schlunegger, F., & Burlando, P. (2013). Patterns and controls of sediment production, transfer and yield in the Illgraben. Geomorphology, 188, 68–82.
492: Change “thigh” to “high”
Figure 1. Say what “P1 means”
Figure 3a. Can you use an arrow to show the flow direction?
Figure 4. I’m about to suggest something that is 100% a personal preference. I know there are different approaches to blue/red plots of erosion and deposition. I prefer erosion = red and deposition = blue. You may not agree with this, and that’s totally acceptable, but consider switching to that color scheme.
Table 2: Can you show the volume of sediment after the event?
Citation: https://doi.org/10.5194/egusphere-2023-1032-RC2 -
AC2: 'Reply on RC2', Fumitoshi Imaizumi, 08 Sep 2023
We sincerely thank you for the efforts you have made to review our manuscript. Comments from the reviewer are very helpful for us to improve the manuscript. We have responded to all review comments in the following paragraphs.
[Comment] General Comments
This manuscript describes an innovative field measurement program aimed to investigate pore water pressure in fully and partially saturated debris flows. The authors have done a very nice job preparing this manuscript. It is clearly written, the figures are well prepared and clear, and the logic is easy to follow. Consequently, I have very few comments overall because this manuscript is in good shape already.
[Reply] It is our pleasure that the manuscript was acclaimed by the reviewer.[Comment] First, I commend the authors on such a detailed study. The type of data they present is very difficult to obtain, and consequently very valuable to the community in general. I have one suggestion on how rainfall intensity is presented (same comment shown below in the specific comments). It looks like when they discuss the 10 minute rainfall intensity they are showing the total rainfall (mm) in a 10 min period. However, in hydrology, traditionally people use units of mm/hr for intensity. So a 10 minute intensity would be mm/10min, converted to units of mm/hr. I would suggest changing this throughout the paper.
[Reply] As the reviewer comments, our intensity in the rainfall depth in a 1- min period. We will convert the unit to mm/hr[Comment] Second, I think at the end of the abstract, they slightly undersell the big conclusions. I think the authors could concentrate on the importance of pore water pressure in the largest surges. The authors could also emphasize the uniqueness of this dataset.
[Reply] Thank you for your suggestion. We will revise the last part of the sentence in the abstract.[Comment] Finally, I’m also curious if you could see if there is a relationship between the peak 10 min intensity preceeding a surge and the peak dpdz? Not sure if there is a relationship there, but it might be worth looking at.
[Reply] We will add a new figure showing comparison of the 10 min intensity to dp/dz.[Comment] Specific Comments:
23-24: I’m not sure if this is the ‘main point’ with which you want to end the abstract because, realistically, I’m not sure that the flow type is something that can really be “considered” ahead of time. Maybe you could be more specific, and say it needs to be considered in modeling. I think a stronger finding from your paper is that you found that excess pore water pressure was present in many of the biggest surges, and that it appears to be an important mechanism in debris flow surge behavior.
[Reply] As suggested by the reviewer, we will emphasize importance of findings in this study at the end of abstract.[Comment] 29: change “decrease” to “decreases”
[Reply] We will reply as pointed out by the reviewer.[Comment] 107: Can you label the Hontani torrent on the map?
[Reply]We will label the Hontani in Fig. 1[Comment] 108: You mention P36, but you haven’t told us what P means yet.
[Reply]We will revise explanation of the junction.[Comment] 121: change “intermitted” to “intermittent”
[Reply] We will replace “intermitted” to “intermittent”.[Comment] 123: Is “intermission season” Nov. to March? Can you specify this more clearly?
[Reply] This is from November to March. We will clarify.[Comment] 136: reference the supplement where readers can see this video.
[Reply] Link to the video supplement has been described in 502. We will clarify that the video can be seen in video supplement.[Comment] 153: Change “accumulations was” to “accumulations were”
[Reply] We will revise the sentence.[Comment] 232: Here and elsewhere you it looks like when you discuss the 10 minute rainfall intensity you are showing the total rainfall (mm) in a 10 min period. However, in hydrology, traditionally people use units of mm/hr for intensity. So a 10 minute intensity would be mm/10min, converted to units of mm/hr. I would suggest changing this throughout the paper.
[Reply] We will revise the unit as suggested.[Comment] 276: If possible, can you indicate that 22:20 to 23:00 represents the time of day in hh:mm.
[Reply] We will add explanation of hh:mm and hh:mm:ss in the manuscript.[Comment] 416: after “flows” can you add in the highest dp/dzlower value in ()?
[Reply] We will add the value in the sentence[Comment] 464: you indicate that the volume of material depends on the initiation zone, but there have been a few studies that indicate that the volume of material supplied to channels declines during parts of the year when debris flows are active, and then refills during winter periods. I would suggest considering adding references to these, because that helps to explain your declining volume over time:
Rengers, F. K., Kean, J. W., Reitman, N. G., Smith, J. B., Coe, J. A., & McGuire, L. A. (2020). The influence of frost weathering on debris flow sediment supply in an alpine basin. Journal of Geophysical Research: Earth Surface, 125, e2019JF005369. https://doi.org/10.1029/2019JF005369Bennett, G., Molnar, P., McArdell, B., Schlunegger, F., & Burlando, P. (2013). Patterns and controls of sediment production, transfer and yield in the Illgraben. Geomorphology, 188, 68–82.
[Reply] We will add references suggested by the reviewer.[Comment] 492: Change “thigh” to “high”
[Reply] We will revise the “high”[Comment] Figure 1. Say what “P1 means”
[Reply] We will add explanation on P1 to P36 in the figure caption.[Comment] Figure 3a. Can you use an arrow to show the flow direction?
[Reply] We will add an arrow showing flow direction in Fig. 3a.[Comment] Figure 4. I’m about to suggest something that is 100% a personal preference. I know there are different approaches to blue/red plots of erosion and deposition. I prefer erosion = red and deposition = blue. You may not agree with this, and that’s totally acceptable, but consider switching to that color scheme.
[Reply] Based on the reviewer’s comment, we have checked color of erosion/deposition areas in previous papers. We could find both red/blue and blue/red patterns. Papers using red for the erosion would be major. We will think to change the color.[Comment] Table 2: Can you show the volume of sediment after the event?
[Reply] We can add sediment volume after the events.Thank you again for reviewing our manuscript.
Citation: https://doi.org/10.5194/egusphere-2023-1032-AC2
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AC2: 'Reply on RC2', Fumitoshi Imaizumi, 08 Sep 2023
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RC3: 'Comment on egusphere-2023-1032', Anonymous Referee #3, 08 Sep 2023
In this study, valuable field measurement data from the Ohya landslide scar in Japan are presented. Several intriguing and significant phenomena, such as "excess pore pressure" and the high depth gradient of pore-water pressure, have been clearly observed. However, the explanations for these phenomena lack depth and rigor, falling short of the level suggested by the authors in their aim to "reveal the factors that affect the magnitude of the pore-water pressure." The primary concerns are as follows:
Major Concerns:
The author's explanations for "excess pore pressure" and the "high depth gradient of the pore-water pressure" are not sufficiently comprehensive, and the cited reasons lack rigor.
First, regarding the loading of particles, there is a lack of reliable evidence to suggest that this pore-water pressure is induced by the loading of particles. The load of particles becomes part of the pore-water pressure only when particles are fully suspended by fluid, which usually requires a condition typically associated with smaller particle sizes. As indicated in Figure 2, partly saturated debris flows often occur at the head of debris flows where larger particles accumulate. In such conditions, the movement of particles and fluid is markedly different, and the loading of particles does not contribute significantly to the rise in pore-water pressure. Determining whether particles are fully suspended often relies on parameters such as the Rouse number (Rouse, 1937; Dietrich, 1982; Jiménez & Madsen, 2003) and Stokes number (e.g., Ancey et al., 1999). This necessitates knowledge of the mean or median grain size of the debris flow and slurry viscosity, among other parameters. The shear rate can be roughly estimated from surface flow velocity and flow depth. If slurry viscosity has not been measured, it can be estimated based on the typical viscosity range observed in field studies of debris flows in Japan or other regions, such as Illgraben, Chalk Cliff, and Jiangjia Ravine.
Secondly, in addressing the "contraction of interstitial water," it is crucial to analyze the diffusion time of pore pressure, which describes the tendency for pore fluid pressure to develop between moving grains. This can be assessed through parameters such as the Darcy number (Iverson, 1997; Lanzoni et al., 2017) and P number (Iverson et al., 2004), which evaluate the timescale ratio between avalanche motion and the diffusion of disequilibrium pore fluid pressure. Contraction of interstitial water leading to "excess pore pressure" is more likely to occur when this timescale ratio is relatively small. However, these conditions are stringent in natural debris flow, typically requiring finer particle sizes, higher solid-phase concentrations, and greater interstitial fluid viscosity.
"Excess pore pressure" and the "high depth gradient of the pore-water pressure" are central issues in this study. Therefore, when discussing and analyzing these phenomena, the authors should provide quantitative analysis (if feasible, estimates of missing parameters can be approximated to facilitate final calculations) or detailed qualitative discussions, followed by explanations of excess pore pressure, rather than presenting vague concepts.
Relevant references:
Dietrich, W. E. (1982). Settling velocity of natural particles. Water resources research, 18(6), 1615-1626.
Jiménez, J. A., & Madsen, O. S. (2003). A simple formula to estimate the settling velocity of natural sediments. Journal of waterway, port, coastal, and ocean engineering, 129(2), 70-78.
Coussot, P., & Ancey, C. (1999). Rheophysical classification of concentrated suspensions and granular pastes. Physical Review E, 59(4), 4445.
Iverson, R. M., Logan, M., & Denlinger, R. P. (2004). Granular avalanches across irregular three‐dimensional terrain: 2. Experimental tests. Journal of Geophysical Research: Earth Surface, 109(F1).
Minor comments:
Figure 1: The figure displays only three observation points, yet they are labeled from P1 to P20 to P36. If there are additional observation points between P1 and P36 that are relevant and need to be shown, it is advisable to include them on a separate, larger-scale map. If only these three points are pertinent to the study, renumbering is suggested.
Lines 170-175: The statement "the maximum weight per unit volume of the interstitial water was the same as that of the sediments" is not in accordance with reality, as every liquid has its maximum sediment transport capacity.
Lines 175-180: The two methods for calculating the gradient of pore-water pressure (Eq. 3 and Eq. 4) do not differ fundamentally; they mainly vary in the size of the delta_h interval. Since Eq. 4 offers a smaller interval and higher precision, the method relying on bulk flow depth estimation, as represented by Eq. 3, may be less accurate as it overlooks local information within the intermediate flow layer.
Figure 5: What is the unit of rainfall intensity? Is it expressed in mm/day or mm/hour? Although the authors mention "the 10-minute rainfall intensity," this is not a commonly used unit for representing rainfall intensity. Clarification is needed.
Line 492: There is a spelling error in this section.
Citation: https://doi.org/10.5194/egusphere-2023-1032-RC3 -
AC3: 'Reply on RC3', Fumitoshi Imaizumi, 13 Oct 2023
We sincerely thank you for the efforts you have made to review our manuscript. Comments from the reviewer are very helpful for us to deepen our discussion on the physical mechanism of debris flow. We have responded to all review comments in the following paragraphs. Labels and line numbers after our response correspond to those in the revised manuscript with tracked changes.
Major Concerns:
[Comment R3_1] The author's explanations for "excess pore pressure" and the "high depth gradient of the pore-water pressure" are not sufficiently comprehensive, and the cited reasons lack rigor. First, regarding the loading of particles, there is a lack of reliable evidence to suggest that this pore-water pressure is induced by the loading of particles. The load of particles becomes part of the pore-water pressure only when particles are fully suspended by fluid, which usually requires a condition typically associated with smaller particle sizes. As indicated in Figure 2, partly saturated debris flows often occur at the head of debris flows where larger particles accumulate. In such conditions, the movement of particles and fluid is markedly different, and the loading of particles does not contribute significantly to the rise in pore-water pressure. Determining whether particles are fully suspended often relies on parameters such as the Rouse number (Rouse, 1937; Dietrich, 1982; Jiménez & Madsen, 2003) and Stokes number (e.g., Ancey et al., 1999). This necessitates knowledge of the mean or median grain size of the debris flow and slurry viscosity, among other parameters. The shear rate can be roughly estimated from surface flow velocity and flow depth. If slurry viscosity has not been measured, it can be estimated based on the typical viscosity range observed in field studies of debris flows in Japan or other regions, such as Illgraben, Chalk Cliff, and Jiangjia Ravine.
[Reply] Thank you so much for your comments that deepen physical analysis of the pore water pressure. We think our terminology was not very clear in the previous version of the manuscript. We have revised terminology, such as loading of particles. We agree that the suspension of sediments increases ps value in Eq. (1). Based on the comment from the reviewer, we have added analysis of the Rouse number. Our analysis implied that material of suspended particles existed in the debris flow material. At the same time, because the pore-water pressure in some debris flow surges was similar to hydrostatic pressure of clean water, effect of the ps on the pore-water pressure was likely small when fine sediments were poor on the surface of the channel deposits.[Comment R3_2] Secondly, in addressing the "contraction of interstitial water," it is crucial to analyze the diffusion time of pore pressure, which describes the tendency for pore fluid pressure to develop between moving grains. This can be assessed through parameters such as the Darcy number (Iverson, 1997; Lanzoni et al., 2017) and P number (Iverson et al., 2004), which evaluate the timescale ratio between avalanche motion and the diffusion of disequilibrium pore fluid pressure. Contraction of interstitial water leading to "excess pore pressure" is more likely to occur when this timescale ratio is relatively small. However, these conditions are stringent in natural debris flow, typically requiring finer particle sizes, higher solid-phase concentrations, and greater interstitial fluid viscosity.
[Reply] We have added analysis on the P number. The excess-pore water pressure can be maintained over longer timescales because the diffusion of the pore-fluid pressure was not high in the Ichinosawa. We hope analysis of the P number supports our idea that the contraction of interstitial water generated excess pore-water pressure.[Comment R3_3] "Excess pore pressure" and the "high depth gradient of the pore-water pressure" are central issues in this study. Therefore, when discussing and analyzing these phenomena, the authors should provide quantitative analysis (if feasible, estimates of missing parameters can be approximated to facilitate final calculations) or detailed qualitative discussions, followed by explanations of excess pore pressure, rather than presenting vague concepts.
[Reply] Based on the comments from the reviewer, we have calculated Bagnold number, Savage number, and Friction number. New analyses revealed that frictional force dominated in the partly saturated debris flows. Scale of the pore space between boulders due to the high volumetric solid concentration, resulting in the predominance of the frictional force. Additionally, the ∆p/∆zlower was higher in the partly saturated debris flows with larger influence of the frictional force. We have added discussion on the internal force affecting the pore-water pressure. We also added related references in the paper.Minor comments:
[Comment R3_4] Figure 1: The figure displays only three observation points, yet they are labeled from P1 to P20 to P36. If there are additional observation points between P1 and P36 that are relevant and need to be shown, it is advisable to include them on a separate, larger-scale map. If only these three points are pertinent to the study, renumbering is suggested.
[Reply] Because it is difficult to show all of analysis points in the Figure 1, we have displayed their locations in Figure 4. We have clarified it in the caption of Figure 1.[Comment R3_5] Lines 170-175: The statement "the maximum weight per unit volume of the interstitial water was the same as that of the sediments" is not in accordance with reality, as every liquid has its maximum sediment transport capacity.
[Reply] We also think that this value does not have reality as maximum weight per unit volume of the interstitial water. This value is not lower boundary of occurrence of the excess pore-water pressure. But the excess pore-water pressure surely (clearly) exists in debris flows when the pore-water pressure exceeds this value. We have improved explanation and terminology of this value throughout the paper.[Comment R3_6] Lines 175-180: The two methods for calculating the gradient of pore-water pressure (Eq. 3 and Eq. 4) do not differ fundamentally; they mainly vary in the size of the delta_h interval. Since Eq. 4 offers a smaller interval and higher precision, the method relying on bulk flow depth estimation, as represented by Eq. 3, may be less accurate as it overlooks local information within the intermediate flow layer.
[Reply] We think that the spatial difference in the gradient of the pore-water pressure along the depth direction affects disagreement of Eq. 3 and Eq. 4 values. At the same time, as pointed out by the reviewer, difference in the accuracy of estimation also affects disagreements of the two values. We have explained potential effect of the estimation accuracy on the difference in the values from Eq. 3 and Eq. 4.[Comment R3_7] Figure 5: What is the unit of rainfall intensity? Is it expressed in mm/day or mm/hour? Although the authors mention "the 10-minute rainfall intensity," this is not a commonly used unit for representing rainfall intensity. Clarification is needed.
[Reply] We have changed unit of the intensity to mm h-1 throughout the manuscript.[Comment R3_8] Line 492: There is a spelling error in this section.
[Reply] We have revised misspelling in line 492.Thank you again for your review.
Citation: https://doi.org/10.5194/egusphere-2023-1032-AC3
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AC3: 'Reply on RC3', Fumitoshi Imaizumi, 13 Oct 2023
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Shunsuke Oya
Fumitoshi Imaizumi
Shoki Takayama
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