the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Dec 2024
Capture of near-critical debris flows by flexible barriers: an experimental investigation
Abstract. This study addresses the key issue of the interaction between debris flows and flexible barriers based on small scale experiments for which both the flowing mixture and the barrier were designed to achieve similitude with real situations in Alpine environments. The considered debris consisted of a large solid fraction mixture with large and angular particles, flowing down a moderately inclined flume and resulting in near critical flows, with a Froude number in the 0.9–2 range. The flexible barrier model consisted in 3D printed cables and net. The flow characteristics, evolution and deposition after contact with the barrier as well as the deformation and the loading experienced by the barrier were addressed varying the flume inclination and released mass. Four different interaction modes between the flow and the barrier are identified increasing the flow kinematics. A model based on the hydrostatic pressure assumption reveals relevant for estimating the total force exerted on the barrier when all the released material is trapped. This force doubles in case there was barrier overflow.
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RC1: 'Comment on egusphere-2024-3575', Anonymous Referee #1, 09 Jan 2025
This study examines the impact loads and modes of flow-flexible barriers using small-scale flume tests. By varying flow materials and inclinations, different flow dynamics (Fr) and energies were produced. Although the range of Fr tested is limited (0.9-2) and many results align with previous findings, the paper's extensive data make it valuable for publication. However, some statements and conclusions lack rigor and may mislead readers due to the complex nature of flow-flexible barrier interactions. The manuscript can be accepted after careful revisions and minor clarifications as detailed below:
- While there are numerous experimental and numerical studies on debris flows impacting flexible barriers, the specific research question addressed by this work is unclear. Please clarify the novelty of this study at the very beginning of the introduction.
- Page 12, Line 230: The two impact modes are crucial for theoretical work and engineering practice. It is recommended to include the following statement: "Kong et al. (2021, Engineering Geology, 289: 106188; 2022a, JGR Earth Surface 127(6): e2021JF006587) provide systematic investigations and a unique dataset on flow and flexible barrier behavior in pile-up and run-up modes and their transitions. This is highly relevant to this study and may offer valuable insights to readers."
- Page 13, Line 285: The authors identify critical Fr and energy thresholds for different impact modes. However, real-world scenarios are highly complex. To avoid misleading conclusions, consider adding: "Flow-flexible barrier interaction modes are too complex to be quantitatively distinguished by simple signatures. For example, Kong et al. (2021, Engineering Geology, 289: 106188; 2022a, JGR Earth Surface, 127(6): e2021JF006587; 2022b, JGR Earth Surface, 127(11): e2022JF006870; 2024, Géotechnique, 74(5): 486-498) reported that factors such as flow dynamics, solid fractions, flow types, barrier types, and geometry, including flow-barrier height ratios, significantly influence interaction modes and are key considerations in flexible barrier design. This is directly relevant to this study."
- Page 18, Line 340: The author presents the 'force within the barrier Fw' in Figures 12 and 13, defined as the sum of forces in three cables at rest. Please clarify why this force is considered more critical than their peak values. If not, please provide the peak values. Additionally, it is expected that Fw would initially increase and then decrease with increasing inclination. The observed negative trend is likely due to the limited inclination range (11 to 15 degrees).
- Equation for Fb: I could not locate the equation for Fb. Is it simply 3*Fs? Fb is crucial for designing flexible barriers, but this assumption is quite strong. Please provide more details. Kong et al. (2022b, JGR: Earth Surface, 127(11): e2022JF006870) conducted a comprehensive study on analytical models for estimating total impact loads on flexible barriers. They found that the ratio between peak impact load and static load (at rest) varies with barrier types, flow materials, and dynamics. For example, Kong (2022b) reported peak-static load ratios of approximately 4 for debris flows and 1.5 for rock avalanches at a pre-impact Fr of around 4. This ratio also changes with different flow materials and barrier types (Kong et al., 2021, Engineering Geology, 289: 106188; 2022a, JGR Earth Surface, 127(6): e2021JF006587). Including these insights could prevent misunderstandings.
- Appendix A: Note that Kong et al. (2022b, JGR: Earth Surface, 127(11): e2022JF006870) reported that cable-based solutions might introduce significant errors in estimating total impact loads on flexible barriers. This finding is relevant to the analytical model discussed and may benefit readers.
Citation: https://doi.org/10.5194/egusphere-2024-3575-RC1 -
AC1: 'Reply on RC1', miao huo, 16 Jan 2025
The authors would like to warmly thank the reviewer for the time dedicated for reading and commenting the proposed manuscript with the aim of improving it. It is clear reading these comments that the reviewer has a strong expertise in this field.
All the comments made have been thoroughly considered and a response to each is provided in the following.
The authors would like to remind that the presented research focus on lab experiments of a realistic type of debris flows (in terms of material and Froude number) when compared to torrents encountered in the European Alps or similar settings (volume of several thousand of m3, intense rainfalls related to thunderstorms but not as intense as under typhoons, very high sediment concentration, i.e. 50-80%). Other types of debris flows exist elsewhere but we explained in our introduction the type of debris flow we restricted our study to. This type of flow has rarely been addressed in previously published research and, for this reason, current knowledge may not be relevant to real debris flows occurring in Alpine environments, in terms of flow-barrier interaction in particular. Because the type of flow considered in this research significantly differs from that in most of other studies, choice was made to keep minimum the number of references to other studies, and to refer to works from research teams all over the world.
C1. While there are numerous experimental and numerical studies on debris flows impacting flexible barriers, the specific research question addressed by this work is unclear. Please clarify the novelty of this study at the very beginning of the introduction.
R1. As mentioned in the introduction, the first novelty in this research lies in the fact that it focuses on the interaction of near critical debris flows (i.e. with F r = 0.9 – 2) with a flexible barrier while the vast majority of previous research addressed flows at much wider range of Froude numbers toward high values. Froude numbers lower than 2 are typically uncovered or just the lower end of a spectrum and covered only by a few simulations. This is clearly highlighted in the introduction. Addressing a wide range of Froude number is interesting but because it covers a wide range of regimes of impact but also consequently does not allow to comprehensively cover each regime. Here we focus on near critical debris flows. Another novelty relates to the use of a flexible barrier close to perfect mechanical similitude with the real scale. It is also important highlighting that, by contrast with previously published studies, this one considered a high solid fraction flow which contained a large amount of coarse material, as usually observed in nature. The novelty also lies in some of the main conclusions drawn from this work and in particular: (i) Four different interaction modes between the flow and the barrier are identified when the flow kinematic is varied, while two are generally considered based on studies at higher Froude numbers and (ii) the total force exerted on the barrier when all the released material is at rest is doubled in case barrier overflow occurred. All these points are mentioned in the abstract and/or the introduction. There is no doubt all this is novel and of great interest to academics and engineers.
C2. Page 12, Line 230: The two impact modes are crucial for theoretical work and engineering practice. It is recommended to include the following statement: "Kong et al. (2021, Engineering Geology, 289: 106188; 2022a, JGR Earth Surface 127(6): e2021JF006587) provide systematic investigations and a unique dataset on flow and flexible barrier behavior in pile-up and run-up modes and their transitions. This is highly relevant to this study and may offer valuable insights to readers."
R2: As mentioned in R1 and as detailed in the article, observations made during the experiments reveal that the flow-barrier interaction may follow four different modes. This conclusion specifically concerns this type of flowing material and this realistic range of Froude numbers, which both differ from that in previously published studies. See Line 281 where one can read “It also revealed it could hardly be described in a binary way as often done in the literature, where pile-up and run-up are proposed as the two processes by which an obstacle modifies the flow kinematics.” In addition, the detailed description of what was observed (i) do not allow asserting that ‘pile up’ occurred strictly speaking (in particular read the comments in Line256 and Line 265) and (ii) run-up was observed in one case only and it was considered as not representative as explained in the text (read Lines 269-277). The two references of Kong et al. are relevant numerical studies and the first one was actually yet cited in our paper, Line 227.
C3. Page 13, Line 285: The authors identify critical Fr and energy thresholds for different impact modes. However, real-world scenarios are highly complex. To avoid misleading conclusions, consider adding: "Flow-flexible barrier interaction modes are too complex to be quantitatively distinguished by simple signatures. For example, Kong et al. (2021, Engineering Geology, 289: 106188; 2022a, JGR Earth Surface, 127(6): e2021JF006587; 2022b, JGR Earth Surface, 127(11): e2022JF006870; 2024, Géotechnique, 74(5): 486-498) reported that factors such as flow dynamics, solid fractions, flow types, barrier types, and geometry, including flow-barrier height ratios, significantly influence interaction modes and are key considerations in flexible barrier design. This is directly relevant to this study."
R3: We agree with the general statement made by the reviewer, but we believe that these elements are yet partially, explicitly or implicitly stated in the introduction. In addition, we do not claim that our findings are relevant to other situations (in terms of flowing material and Froude numbers), actually we suspect that these findings differ in relation with the increase of kinetic energy or the other parameters listed by the reviewer. Besides, it is definitely true that the flow-barrier interaction is extremely complex and depends on many parameters. To some extent, the differences between the results derived from this research and that from other studies emphasize this complexity! Nevertheless, the reviewer points on Section 3 which concerns the results presentation and where all comments made relate to the specific material and Froude number range we addressed. For this reason, the authors prefer not to include any reference to some specific works dealing with other cases in this Section.
C4. Page 18, Line 340: The author presents the 'force within the barrier Fw' in Figures 12 and 13, defined as the sum of forces in three cables at rest. Please clarify why this force is considered more critical than their peak values. If not, please provide the peak values. Additionally, it is expected that Fw would initially increase and then decrease with increasing inclination. The observed negative trend is likely due to the limited inclination range (11 to 15 degrees).
R4: Fw is the sum of the forces in the three cables when at rest. Indeed, it was not possible to give a precise value at peak, as explained from Line 167 “The force sensors were mainly used to determine the force at rest, after the material has stopped flowing. Indeed, obtaining a precise measurement of the force during the event was not possible due to some technical limitations with the system.”. Besides, we do not see any negative trend in figure 12 and 13 of Fw vs inclination.
C5. Equation for Fb: I could not locate the equation for Fb. Is it simply 3*Fs? Fb is crucial for designing flexible barriers, but this assumption is quite strong. Please provide more details. Kong et al. (2022b, JGR: Earth Surface, 127(11): e2022JF006870) conducted a comprehensive study on analytical models for estimating total impact loads on flexible barriers. They found that the ratio between peak impact load and static load (at rest) varies with barrier types, flow materials, and dynamics. For example, Kong (2022b) reported peak-static load ratios of approximately 4 for debris flows and 1.5 for rock avalanches at a pre-impact Fr of around 4. This ratio also changes with different flow materials and barrier types (Kong et al., 2021, Engineering Geology, 289: 106188; 2022a, JGR Earth Surface, 127(6): e2021JF006587). Including these insights could prevent misunderstandings.
R5: Please see the appendix for the computation, of Fb. This is mentioned from Line 365: “As detailed in Appendix A, the model used for computing the barrier loading assumed an uniform load distribution and considered both the circular and parabolic barrier deformation assumptions. These models were used to compute the total force exerted at rest on the barrier, Fb.”. Even though the comment by the reviewer is highly relevant and based on very nice research, the fact is that it is out of purpose here, as the mentioned ratio compares the residual force with the peak force, which is not computed here.
C6. Appendix A: Note that Kong et al. (2022b, JGR: Earth Surface, 127(11): e2022JF006870) reported that cable-based solutions might introduce significant errors in estimating total impact loads on flexible barriers. This finding is relevant to the analytical model discussed and may benefit readers.
R6: The reviewer suggests referring to an excellent article where the raised issue is mentioned in one of the conclusions. More precisely, this conclusion mentions that this problem mainly concerns fast impact dynamics while in the presented research, the analytical solutions are used for slow impact dynamics and focus on the load at rest, and not at peak. In spite of this, the reviewer is true: the analytical computation of the loading exerted on the barrier based on other measurements (deflection, force in the cables) relies on many assumptions which may biases the computed result. This is also demonstrated by the proposed manuscript where two assumptions as for the barrier shape are accounted for, resulting in a ratio of 1:2 in terms of loading exerted on the barrier. Because an appendix is expected to be concise to the point and it is generally not the place where comments are required a reference to this article will be made in section 3.3.3.
Citation: https://doi.org/10.5194/egusphere-2024-3575-AC1
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RC2: 'Comment on egusphere-2024-3575', Anonymous Referee #2, 17 Jan 2025
This manuscript has addressed the interaction problem between debris flow and flexible barrier with a focus on the near-critical flow regimes. The physical experiments are well designed, and with the obtained data, the deformation and loading behaviors of flexible barrier are analyzed in detailed. I would recommend this manuscript after a proper revision. Several main comments are listed below.
- The authors have emphasized the importunate of near-critical debris flows, but the main reason is that the most debris flows in nature fall into this flow regime but without enough investigations. I think this motivation cannot justify the contribution of the manuscript. There are quite many researches that try to establish impact models suitable for a wide range of Froude regimes. For such a reason, the authors are supposed to defend their contributions by declaring the fundamental differences between the near-critical and supercritical debris flow impact.
- I didn’t understand why only the loading characteristics of the barrier after impact are considered in this manuscript. Now that the cable tension force has been measured, why not show the time-history data of impact force? I think at least for the Mode Ⅳ, peak force may not appear at the end of impact stage. This aspect should be better explained.
- It is questionable that whether the barrier model could represent the dynamic responses of prototype flexible barriers. At least, without the energy dissipators the deformation of flexible barriers has been fundamentally changed. Because the loading and deformation response of barrier are the main points of this manuscript, it is necessary to explain this discrepancy.
- The proposed impact model would be quite site-specific, e.g., the Illgraben torrent (Switzerland), because only the data of near-critical debris flows is included. It would be more interesting and useful to develop a more unified load model by incorporating the data cross a wide range of Froude regimes.
- The flexible barrier is open-type with the net allowing the pass of debris flow materials. But this part has not been discussed. I think it is also important for loading response of barrier in addition to the overflow process.
6. Some minor comments also are provided for reference: 1) the details about the used cameras, e.g., the frame rate and resolution; 2) the strength similarity design of model barrier; 3) the figure quality; 4) the specific value of the solid fraction of the modelled debris flows.
Citation: https://doi.org/10.5194/egusphere-2024-3575-RC2
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