the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Nov 2025
Buried and displaced: moving characteristics of building fragments in debris flows
Abstract. Buildings can be destroyed and displaced from their original position in large-scale debris flows and flow-type landslides. Accurate prediction of the relocated position of buildings within debris-flow deposits is urgently needed for emergency rescue. This has been proven to be challenging due to the intricate nature of physical processes. In this study, an elucidation of the complicated physical mechanisms associated with the movement of building fragments within debris flows is provided. Well-controlled flume experiments are conducted to verify the theoretical predictions, and an inertial measurement unit is embedded within the model of the building block to monitor the block's movement mode. An analytical model considering the hydrodynamic drag force, earth pressure, and basal friction is further established. Dimensionless parameters are derived to clarify the underlying physical mechanisms. The results demonstrate that the deposition position of building fragments is predominantly governed by the basal sliding velocity of debris flow. The dimensionless parameter αFr2 informs optimal model selection to enhance predictive accuracy within this framework. These findings provide actionable guidance for post-disaster emergency rescue by enabling precise positioning of buried structures.
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Status: open (until 24 Dec 2025)
- RC1: 'Comment on egusphere-2025-5236', Anonymous Referee #1, 26 Nov 2025 reply
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RC2: 'Comment on egusphere-2025-5236', Anonymous Referee #2, 04 Dec 2025
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This study investigates the physical mechanisms associated with the movement of building fragments within debris flows through an integrated experimental-theoretical approach. The authors conducted controlled flume experiments to captured the force and kinematic responses of the moving blocks within debris flows. On this basis, they developed an analytical dynamics model formulated within a leading-edge framework, introduced a set of dimensionless groups (e.g., D*, K*, G*), and proposed the criterion αFr² to distinguish the dominant mechanical regimes. Model predictions were quantitatively evaluated against the ratio L*/S* (block displacement versus depositional front displacement). The experimental and theoretical results exhibit good overall agreement across three debris flows with different solid concentrations. They found that the deposition position of building fragments is predominantly governed by the basal sliding velocity of debris flow. The study is innovative and well-designed; however, major revisions are required regarding the model assumptions, parameter sensitivity analysis, and the scalability of the proposed framework. Please refer to the following comments for detailed suggestions.
General comments:
The study treats the building fragments as passive blocks that do not exert feedback on the surrounding flow, and it represents the depositional region as a single body undergoing uniform deceleration within a leading-edge framework. This approximation may be valid under the experimental scales and parameter ranges provided by the authors. However, at field scale, or when the block size ratio, the number of blocks, or the block volume becomes significantly larger, the feedback of the blocks on local flow velocity and pore-pressure distribution may become important. Therefore, the authors should specify the parameter ranges in which this approximation remains applicable, such as the block-to-flow depth ratio or the areal ratio of the block. Also, they should discuss the limitations of this approximation in the manuscript.
The authors relate the ratio of D* to K* to the Froude number and introduce αFr² as a criterion. However, the selection of parameters within α (such as Cd, ka, kp, λ, and h/H) is not sufficiently transparent for both the experimental and field conditions. The authors should perform a parameter sweep for the key variables (Cd, kp-ka, λ, h/H, and 1/n), or at least provide sensitivity curves showing how uncertainties in their magnitudes influence the prediction of L*/S*. This would help demonstrate the robustness of the conclusions to variations in these parameters. In addition, the manuscript contains many symbols. A complete notation table at the end of the paper would assist readers in checking definitions.
Regarding the IMU data, the authors acknowledge that direct integration of acceleration can lead to significant errors, as cited from Harding et al. (2014). Nevertheless, they use short-time integration over the initial 0.2 seconds to obtain the initial block velocity m (0.9-1.0). If the IMU signals were filtered through some approaches, the authors should describe the full processing procedure in this manuscript.
The authors conclude that the depositional position of the block is controlled by the basal sliding velocity of the debris flow. This conclusion may result from the assumptions used in the theoretical model, where the block is submerged in the flow and remains in contact with the bed. In addition, the experimental block has a density greater than that of the debris flows. In real events, however, building fragments may be carried within the debris-flow front and may not remain in direct contact with the bed. The authors should clarify this distinction in the discussion.
In the experiment, the authors attached particles at the flume bottom that were identical to the solid phase of the debris flow. In dry granular flows, this approach is commonly used to approximate a no-slip boundary. It is unclear, however, what the intended effect is in two-phase debris flows. If the purpose is similarly to enforce a no-slip condition, the presence of substantial slip velocities in the reported velocity profiles raises questions. Clarification or discussion of this apparent discrepancy would strengthen the manuscript.
Specific comments:
The manuscript contains many detail errors that require careful correction:
1. Line 25: The reference “F Zhao et al., 2025” should be formatted in accordance with the journal’s reference style.
2. Line 25: The conjunction “However” is inappropriate here because the preceding and following sentences do not express a contrast relationship. Please choose a more suitable transition or remove it.
3. In line49: “debris flow mobility” should be changed to “debris-flow mobility”.
4. In line 57: The citation should be: “For both dry granular (Faug, 2015) and two-phase granular-fluid flows (Sturm et al., 2018)”.
5. Lines 63-66: The manuscript cites studies of boulder transport in tsunamis but does not explain how those studies inform debris-flow boulder transport. This citation appears abrupt, I recommend reorganizing the text to clarify the relevance or removing the citation.
6. Figure 2: “flume set up” should be revised to “flume set-up”.
7. line 117: Replace “liquid phase” with “fluid phase” to maintain consistency with the terminology used elsewhere in the manuscript.
8. Figure 4: “pore water pressure” should be revised to “pore-water pressure”.
9. line 215: “coefficient of friction” should be changed to “friction coefficient”.
10. Figure 8: for state 1, the inequality shown as “vb>vd/n” is incorrect and should be “vb<vd/n”. Please revise this label carefully so it is consistent with the textual description in lines 258-262.
11. Line 263: Equation (6) represents the block motion equation, not the debris-flow motion equation. Consider rephrasing, for example: “Substituting Equation (7) into the block-movement governing Equation (6)”.
12. line 265: The equation reference is incorrect; “Equation (7)” should be “Equation (6)”.
13. line 289, There is an extraneous left parenthesis “(”. Please remove it.
14. line 294: “Table 2)” should be removed.
Citation: https://doi.org/10.5194/egusphere-2025-5236-RC2 -
RC3: 'Comment on egusphere-2025-5236', Georg Nagl, 06 Dec 2025
reply
Review of: Buried and displaced: moving characteristics of building fragments in debris flows
The manuscript presents laboratory experiments investigating the transport and deposition behavior of building fragments in debris flows. An analytical model is applied to predict the deposition locations of these fragments. Overall, the manuscript is well-structured and clearly written, though some sections could be more concise. The methods section would benefit from additional experimental details. Specific comments are provided below:
Line 75: Would the displacement of objects by snow avalanches also be considered by this assumption? Some object like cars?
Line 95: how many basal sensing modules?
Line 105: Is the IMU sensor time triggered? Is the system synchronized with the other setup?
Why did you used this shape of the imu block? Please describe.
Line 100-105: Company of the sensors and camera is missing, please specify.
Line 117: how do you kept the viscosity constant? What fluid did you use?
Line 119: What is L (liter)?
Tabe 1: Was this the maximum flow depth?
Tabe1: Is this the solid concentration by volumetric or the mass? Please specify
Line 129: What PIV program did you use?
Please describe the data processing (e.g. filtering).
Movie S3 is not working at the end
Describe the other datasets of test 53-40, B2 and B3 (or are the corrupt) (Figure 4)
Line 160 and Figure 5 describe L/S
Line 179: How do you adopt?
Tabel 2 : The variables (G*,Gd*, and so one ) should be described in the text.
Citation: https://doi.org/10.5194/egusphere-2025-5236-RC3
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The study of debris flow-structure interaction is a complex topic, since many physical processes are involved. Given the recent high frequency of large debris flows, rock-ice avalanches, and snow avalanches in Europe and high mountain Asia, this study is a very timely research towards understanding this complex and practical problem. This manuscript carried out flume experiments to investigate the trajectory of building fragment under the action forces of debris flows. IMU was adopted to monitoring the state of block; PIV technology was used to reveal the velocity field prior interacting with the block. Analytical approach is further adopted to predict the deposition position of the displaced block. I have the following comments for further consideration on the limitation (simplification) of this study.
Some specific comments: