the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Dec 2025
Runout mechanism of landslides in alluvial basins with emphasis on the impact and erosion effects
Abstract. Landslide runout is a critical factor in risk assessment, and runout distance is the most widely used indicator of mobility. Runout distance is determined both by the landslide's initial conditions and through interactions with erodible substrates, which can affect momentum by altering basal friction or by increasing overall flow volume, generally increasing runout distance. After initiation, landslide processes can be separated into two phases: an impact phase and a runout phase. While erosion during the runout phase has been considered in prior studies, impact forces themselves have been overlooked. Here, we combine fieldwork in SE Tibet, laboratory tests, and numerical modelling to resolve the dynamics and effect of impact loading on landslides in alluvial basins. Impact-loading ring-shear tests and numerical simulations, backed up by field evidence, indicate that impact forces can near-instantaneously generate high excess pore water pressure within a saturated substrate, reducing basal friction of the landslide mass and extending runout. Both impactor and substrate properties, including stiffness and compressibility, control the impact load and duration, leading to different runout patterns and landslide mobilities. We find that the farthest runout occurs at an intermediate impact level, when the normal component of peak impact stress matches the self-weighted stress of the final deposits, as this condition most effectively liquefies the substrate. The findings highlight the importance of considering substrate properties for both erosion and impact during landslide runout analyses, particularly those occurring in alluvial basins.
Status: open (until 12 Jan 2026)
- RC1: 'Comment on egusphere-2025-5479', Anonymous Referee #1, 20 Dec 2025 reply
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RC2: 'Comment on egusphere-2025-5479', Anonymous Referee #2, 21 Dec 2025
reply
Review of “Runout mechanism of landslides in alluvial basins with emphasis on the impact and erosion effects” by Chen et al.
A very interesting and useful study. A major revision is recommended.
Major comments
- While this study provides valuable mechanistic insights into impact-induced liquefaction and its role in landslide runout within alluvial basins, its broader impact remains circumscribed by the highly specific geomorphic and material conditions examined. The authors are advised to highlight the broader impact of mechanical understanding on society (like risk reduction or hazard assessment) and academia (e.g., simulation software development).
- A significant limitation of this study is its constrained material and environmental scope, which restricts the broader applicability of its conclusions. The experimental and numerical framework almost exclusively centers on a single soil type—the Luanshibao (LSB) sand—and assumes fully saturated conditions for both the sliding mass and the erodible substrate. This overlooks the critical influence of soil type diversity (e.g., clays, silts, or gravelly soils with differing permeability, cohesion, and liquefaction potential) and variable soil moisture conditions (from unsaturated to partially saturated states) that dominate many real-world alluvial basins, especially in seasonal or arid climates. By not conducting comparative tests across a spectrum of sediment types or saturation degrees, the study cannot confirm whether the proposed impact-liquefaction mechanism is a general principle or a phenomenon specific to clean, saturated sands, thereby limiting its utility for comprehensive regional hazard assessment. Adding extra lab experiments with different soil types and saturation levels would be a big plus to this study.
Minor comments
- Title page and page 1, please consistency in author’s name. “Maximillian Van Wyk de Vries”
- Line 56. “graduate” or “gradual”?
Citation: https://doi.org/10.5194/egusphere-2025-5479-RC2 -
RC3: 'Comment on egusphere-2025-5479', Anonymous Referee #3, 22 Dec 2025
reply
This work investigates how landslide impact on saturated soils can enhance mobility through liquefaction-like processes. This is very interesting, but more precisions must be provided on how the numerical and laboratory experiments contribute to prove the hypothesis of the authors, and more detailed analysis would enhance the manuscript.
- How was tha angle alpha in Fig. 2 chosen for the experiments? I would draw the graphs a, b and c in Fig. 7 as in Fig 2b to present more clearly your experiments design. Besides, the link between the laboratory and numerical experiments is not clear to me. To what extent do the numerical simulations illustrate what you show with the laboratory experiments? Or how do the laboratory experiments help interpret the simulation results?
- You should provide some reference on the MPM model you use, and explain what rheology / constitutive equations are used to model the propagation of the landslide and the entrainement of the underlying bed.
- You should provide some more theoretical or at least phenomenological explanations to that fact that, in your experiments, kd=1.7 yields a faster compelte strength loss that kd=2.5.
-l.270 : "This impact process involves the disintegration of the sliding mass and modifies the mechanical behaviour of the underlying materials". I agree, but to what extent is desintegration taken into account in your simulation? More generally, you should discuss the processes that are not taken into account in your model but that could however play a role in entrainement and propagation.
-l.279: "For a given landslide geometry, both the mass and velocity at impact are fixed". I agree for the mass, but not for the velocity, it depends on the initiation mechanism and on the initial propagation processes before the impact on the loose sediments (e.g. through the friction coefficient at the interface between the landslide and the topography).
-l.283: "Notably, when the impact load equals ∆W cosα/sinα as in the test with kd=1.7, resulting in a constant normal stress during the unloading stage, the specimen reaches complete liquefaction in the most efficient manner". Well, it is the most effective of the three different tests you carried out, but is it in general the case? Meaning, can you prove the value ∆W cosα/sinα for the impact load will always yield the fastest liquefaction?
Detailed remarks:
l.42-52 : I would add that physically-based erosion models are difficult to derive. Many models (in particular thin-layer models) use empirical relations between momentum and erosion rates, but preserving energy in the resulting equations is not straight-forward.
*Bouchut, F., E. D. Fernández-Nieto, A. Mangeney, et P.-Y. Lagrée. 2008. « On New Erosion Models of Savage–Hutter Type for Avalanches ». Acta Mechanica 199 (1‑4): 181‑208. https://doi.org/10.1007/s00707-007-0534-9.
*Iverson, Richard M., et Chaojun Ouyang. 2015. « Entrainment of Bed Material by Earth-Surface Mass Flows: Review and Reformulation of Depth-Integrated Theory ». Reviews of Geophysics 53 (1): 27‑58. https://doi.org/10.1002/2013RG000447.
l.66 : contribute à contributes
Fig 2 : It is not clear to me why $\theta$ on Figures (a) and (b) are necessarily the same. You must also explain in the legend what the notations are. In particular, I may have missed it but I’m not sure you explain in the text (from l.89 to l.130) what $\phi_p$ and $\phi_m$ stand for.
Fig 6 : add a scale to images.
l.138-142: How did you chose the Poisson’s ratio and the effective Young’s modulus? How did you choose the initial porosities?
l.145-146 : I would imagine that the porosity of the bed also has a significant impact on the results.
Figure 7 : X label for figures d, e and f is missing. You must explain in the legend what TSP and ESP stand for.
Citation: https://doi.org/10.5194/egusphere-2025-5479-RC3
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This manuscript, taking the Luanshibao landslide, illustrates how the impact induced liquefaction affects the mobility of hug landslides. The hypothesis is clear to the reviewer. However, the field investigation, ring shear experiments, and numerical simulation are not consistent with each other, and therefore fail to prove the proposed idea. Below is my detailed comments.