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
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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
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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 -
RC4: 'Comment on egusphere-2025-5479', Anonymous Referee #4, 28 Dec 2025
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In this study, the authors quantitatively examine a giant prehistoric, long runout Luanshibao landslide (CN) in a high-elevation alluvial basin. They combine fieldwork, laboratory tests, and numerical modelling to resolve the dynamics and effect of impact loading. The study indicates - impact forces can near-instantaneously generate high excess pore water pressure in the saturated substrate, reducing basal friction of the landslide mass resulting in extended runout. The authors propose a potential motion pattern, divided into an impact phase and a runout phase to describe a typical erosive landslide behaviour in alluvial basins. As the authors mention, they observe that the farthest runout occurs at an intermediate impact level, when the peak normal impact stress approaches the self-weight of the deposit, akin to substrate liquefaction. In general, the study is interesting and within the scope of the journal NHESS.
However, the writing is poor, odd. The ms requires substantial re-phrasing, re-editing and re-working, in both the content, descriptions, figures, mechanics and dynamics as it is weak in its physical aspect with respect to the advanced understanding of the field available in present days. The ms could be well supported with recent, very relevant referencing. The ms lacks to mention - which models are used, why appropriate for the present study as physically better explained models are available. Some suggestions for improvements are mentioned in the attached annotated ms file.
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CC1: 'Comment on egusphere-2025-5479', Hervé Vicari, 09 Jan 2026
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This manuscript presents a multi-method investigation of the highly mobile Luanshibao landslide. I appreciated the integration of field observations, laboratory experiments, and numerical modelling. However, I have concerns regarding the numerical modelling strategy (in agreement with Comment 3 by Anonymous Referee #1), which I outline below.
Recent studies (Cuomo et al., 2024; Vicari et al., 2025; Zhu et al., 2025) have demonstrated depth-resolved, two-phase simulations of landslides impacting and eroding wet bed sediments. Consistent with current mechanistic understanding of erosion processes (Iverson, 2012), Vicari et al. (2025) and Zhu et al. (2025) showed that some critical geomechanical properties of bed sediments strongly control pore-pressure generation, thereby influencing shear-strength loss, erosion, and runout. In particular, these studies demonstrated that loosely packed, contractive bed materials (i.e., with negative dilatancy, or, in other words, with an initial solid fraction smaller than the critical state solid fraction) tend to develop positive excess pore pressures during shearing induced by the landslide mass. Conversely, densely packed (dilative) bed sediments tend to experience pore-pressure reduction, which limits erosion and runout. This bifurcation behaviour has also been confirmed by recent experiments (Steers et al., 2024).
Furthermore, when excess pore pressures are generated upon impact, their persistence critically depends on the permeability of the bed sediments (Vicari et al., 2025). Excess pore pressures dissipate rapidly in highly permeable materials but may persist in low-permeability beds. The characteristic diffusion timescale of excess pore pressure is given by (Iverson, 2012) T = C η H 2 / k, where C is the bed sediments drained compressibility (inversely proportional to the Young’s modulus E), η is water’s viscosity, H is the bed sediments thickness, and k is the bed sediments permeability.
Unfortunately, the constitutive assumptions underlying the MPM model are not sufficiently described. In Section 2.2, the authors state that a Mohr-Coulomb model is adopted and provide values for the friction angle, but no information is given on the flow rule, i.e., regarding the dilative or contractive behaviour of the landslide and bed sediments. As discussed above, dilatancy fundamentally controls pore-pressure generation, yet this aspect is not addressed. Instead, the authors vary the elastic properties of the landslide and bed materials, an approach that is difficult to justify from a geomechanical perspective. Elastic moduli should be treated as material properties, ideally constrained by geotechnical testing, rather than as tuning parameters.
While the numerical results show (unsurprisingly) that Young’s modulus influences material deformability, its effect on the dissipation timescale of excess pore pressures is neither analysed nor discussed. Moreover, the assumed values of Young’s modulus appear rather low (see, e.g., Iverson and George, 2014). I suspect that this choice may be compensating for unreported assumptions in the flow rule, which prevents a proper assessment of the modelling framework.
Similarly, the permeability values assumed for the bed sediments (and for the landslide material) are not reported, despite permeability being a key parameter governing pore-pressure evolution and erosion processes (Vicari et al., 2025).
In summary, the manuscript does not provide sufficient detail on the geomechanical parameters adopted in the numerical model to allow evaluation of the physical realism of the assumptions. While the parametric exploration of elastic properties is interesting, the overall modelling strategy and its justification remain unclear.
In addition, I am confused by the sketch in Figure 2b: should the peak shear stress along the total stress path not occur at the same level as the peak shear stress along the effective stress path? In your experimental results (Figure 7d-f), the shear stresses for the TSP and ESP indeed coincide.
Finally, I note that Issler et al. (2024) recently showed that the depth-averaged entrainment model proposed by Pudasaini and Krautblatter (2021) is mechanically incorrect. Consequently, the citation at line 27 should be revised to refer to a more fundamental and physically sound treatment of entrainment, such as Iverson (2012).
With my best regards,
Hervé Vicari
References
Cuomo, S., Di Perna, A., Moscariello, M., Martinelli, M., 2024. Possible remediation of impact-loading debris avalanches via fine long rooted grass: an experimental and material point method (MPM) analysis. Landslides 21, 679–696. https://doi.org/10.1007/s10346-023-02178-5
Issler, D., Gauer, P., Tregaskis, C., Vicari, H., 2024. Structure of equations for gravity mass flows with entrainment. Nat Commun 15, 4613. https://doi.org/10.1038/s41467-024-48605-6
Iverson, R.M., 2012. Elementary theory of bed-sediment entrainment by debris flows and avalanches. Journal of Geophysical Research: Earth Surface 117. https://doi.org/10.1029/2011JF002189
Iverson, R.M., George, D.L., 2014. A depth-averaged debris-flow model that includes the effects of evolving dilatancy. I. Physical basis. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 470. https://doi.org/10.1098/rspa.2013.0819
Pudasaini, S.P., Krautblatter, M., 2021. The mechanics of landslide mobility with erosion. Nat Commun 12, 6793. https://doi.org/10.1038/s41467-021-26959-5
Steers, L.J., Beddoe, R.A., Take, W.A., 2024. Propagation velocity of landslide-induced liquefaction and entrainment of overridden loose, saturated sediments. Engineering Geology 334, 107523. https://doi.org/10.1016/j.enggeo.2024.107523
Vicari, H., Tran, Q.-A., Metzsch Juel, M., Gaume, J., 2025. The role of dilatancy and permeability of erodible wet bed sediments in affecting erosion and runout of a granular flow: Two-phase MPM–CFD simulations. Computers and Geotechnics 185, 107307. https://doi.org/10.1016/j.compgeo.2025.107307
Zhu, L., Tang, X., He, S., Yang, Z., Liang, H., Lei, X., Luo, Y., Zhang, L., 2025. Geomorphology and Sedimentology of the Nyixoi Chongco Rock Avalanche and Implications for Emplacement Mechanisms. Journal of Geophysical Research: Earth Surface 130, e2024JF007666. https://doi.org/10.1029/2024JF007666
Citation: https://doi.org/10.5194/egusphere-2025-5479-CC1
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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.