the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Oct 2025
Brief communication: First field observations of basal slip velocities in natural debris flows
Abstract. The internal dynamics of field-scale debris flows are challenging to observe. Constitutive equations for debris flows, often base on simplified shear models that assume a no-slip condition at the bed. However, laboratory experiments suggest that basal slip can occur under fixed bed conditions, influenced by the variable material composition of debris flows, which includes grains and boulders interacting with the basal boundary. This study presents results from a novel monitoring system that has been designed to directly measure basal slip velocities in natural debris flows using paired conductivity sensors. The system, which was installed in the Lattenbach catchment (Tyrol, Austria), captures high-resolution temporal data from debris-flow events. A detailed examination of two events that occurred in June 2025 has revealed the presence of persistent slip velocities, particularly during surge phases and granular flow fronts. Basal sliding was found to be consistently lower in comparison to surface velocities. These findings call into question the no-slip assumption of traditional rheological models and underscore the significance of basal slip in the dynamics of debris flows. Future research will focus on refining the detection depth of the sensors, analysing additional events, and conducting comparative studies across different catchments to further understand the role of basal slip in debris flows.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-4872', Dongri Song, 27 Oct 2025
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RC2: 'Comment on egusphere-2025-4872', Anonymous Referee #2, 18 Nov 2025
Review: "First field observations of basal slip velocities in natural debris flows"
General Assessment
The study presents interesting initial field observations of basal slip velocities in debris flows, addressing an important question in debris flow rheology. The authors demonstrate that basal slip occurs continuously and is not limited to specific phases, which challenges established no-slip assumptions.
Major Concerns
Method Not Applied for the First Time
The authors present the method as a "novel monitoring system" (lines 15-16), but obscure the fact that the same measurement method was already applied in 2020 by Nagl et al. at the Gadria station, Italy (lines 53-54, line 138). The method was originally developed for laboratory experiments (Kaitna et al., 2014) and then transferred to the field. The present study thus does not represent a methodological innovation, but rather an application at another location. This should be clearly stated in the abstract and introduction. The actual contribution lies in the analysis of two new events, not in the development of the method.
Critical Limitation Due to Artificial Channel
A fundamental problem of the study is the heavily modified measurement environment. The sensors are installed in an artificial, concrete-reinforced channel supported by a series of check dams (lines 48-51). The authors themselves mention that the sensors are located "directly above a check dam" (lines 132-133), which could influence acceleration effects.
It is extremely difficult to infer natural basal friction conditions from measurements in such an artificial channel, because:
- The roughness of the concrete channel bed is fundamentally different from natural substrates
- Hydraulic jumps and turbulence at check dams drastically alter flow dynamics
- The geometric constraint of the channel suppresses natural lateral dynamics
- Erosion and deposition processes that dominate basal interaction in natural channels are largely eliminated
The authors do not discuss this limitation with sufficient critical depth.
Urgent Need for Further Investigation
The study is based on only two events at a single, heavily modified location. For robust conclusions, the method urgently requires:
a) Investigations at different locations:
- The authors mention a planned comparative study at Illgraben, Switzerland (lines 153-154), but without these data, the results remain site-specific
- Ideally, measurements should be conducted in natural, undisturbed channels to test transferability
b) Different configurations:
- The study shows clear differences between sensor pairs (lines 99-106), indicating methodological uncertainties
- The penetration depth of the sensors is completely unclear (lines 123-126), which massively complicates interpretation
c) Different debris flow characteristics:
- Different material compositions: The two events already show different behaviors (granular vs. viscous-turbulent)
- Various volumes: Both events are relatively short (25-35 minutes) – larger, longer events might behave differently
- Different velocities: The v_slip/v_surf ratio varies between 0.3 and 0.5 – a systematic analysis across a broader velocity spectrum is missing
Supplementary Materials Not Accessible
A significant issue is that the supplementary materials, including the webcam videos of both events (mentioned in line 70), are currently not accessible. The authors reference the supplementary material multiple times (lines 70, 157, 159, 161) with a Zenodo DOI (https://doi.org/10.5281/zenodo.17249344), but this material cannot be reviewed. These videos and data would be essential for:
- Verifying the qualitative descriptions of flow behavior (lines 71-79)
- Understanding the visual differences between granular and viscous phases
- Assessing the plausibility of the measured slip velocities
- Evaluating the flow conditions in the artificial channel
The manuscript cannot be fully evaluated without access to these crucial materials. The authors should ensure the supplementary materials are publicly accessible before publication.
Additional Methodological Concerns
- Uncertain detection depth: The authors acknowledge that the sensors may not measure basal velocity but rather the velocity of the lowermost layers (lines 123-126). Without knowledge of this depth, the "basal slip velocities" in the title may be misleading.
- Low correlation rate: Only 12-18% of possible correlations are valid (lines 94-98), indicating substantial data gaps.
- Methodological artifacts: The higher velocities of the central sensor pair (larger spacing) suggest systematic errors that are not yet understood (lines 99-106, 127-133).
Positive Aspects
- The study provides valuable field data on a difficult-to-access research question
- The continuous presence of basal slip is an interesting finding
- The discussion of limitations (Section 4) is partially transparent
- The planned laboratory investigations of penetration depth are sensible
Recommendations
- Revise title and abstract: Remove "First" and clarify that this represents further applications of an existing method
- Make supplementary materials accessible: Ensure all referenced videos, data, and code are publicly available for review
- Emphasize limitations more clearly:
- Artificial channel as a fundamental constraint
- Difficult transferability to natural conditions
- Preliminary character of the results
- Prioritize method validation: The planned laboratory tests and Illgraben study should be completed before drawing far-reaching rheological conclusions
- Specify outlook more concretely: Explicitly state that measurements in undisturbed, natural channels and for various debris flow types are necessary
Conclusion
The study is an interesting exploratory contribution, but is severely constrained by the artificial measurement site, limited data basis (two events), methodological uncertainties, and currently inaccessible supplementary materials. The method urgently requires further validation at different locations, ideally in natural channels, and for different material compositions, volumes, and velocities before the results can be considered representative of "natural debris flows."
Citation: https://doi.org/10.5194/egusphere-2025-4872-RC2 -
RC3: 'Comment on egusphere-2025-4872', Anonymous Referee #3, 06 Dec 2025
The basal boundary dynamics; particularly, the basal velocity; of the particle-fluid mixture debris flows is one of the fundamental mechanisms controlling the motion, run-out and deposition morphology, and the devastating impact forces of such flows. These are mechanically/dynamically known, as their importance. However, the measurements of the basal velocity of such flows is rare, or as claimed by the authors, non-existing for the field debris flows. The authors mention that, this study introduces a novel technique to measure basal sliding velocity and report some first results, based on two recent natural debris flows in Austria. I appreciate the authors efforts in obtaining the basal velocity, for different types of mixtures and different parts of the flow body. These results already provide some important basic information as one would anticipate. However, in parts, the writing appears to be odd. It contains invalid, inaccurate statements and claims. It is not clear how the basal velocity was obtained, and are these for the particle, fluid or for the mixture? Please make it clear. Flow dynamical and bed morphological conditions should be mentioned before explaining the velocity results. The main velocity results are a bit difficult to perceive: how these results can be physically explained with respect to the velocity of surface and the flow depth changes as they do not directly match to each other? In general, I would suggest to properly re-formulate the ms. Please see the attached file for detailed suggestions for improvements.
Data sets
First field observations of basal slip velocities in natural debris flows Georg Nagl and Maximilian Ender https://doi.org/10.5281/zenodo.17249344
Model code and software
First field observations of basal slip velocities in natural debris flows Georg Nagl and Maximilian Ender https://doi.org/10.5281/zenodo.17249344
Video supplement
First field observations of basal slip velocities in natural debris flows Georg Nagl and Maximilian Ender https://doi.org/10.5281/zenodo.17249344
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Reliable measurement of basal velocity is rather difficult. This is partly due to the destructiveness of debris flows. Another obvious reason is how to define the boundary layer of debris flow, and how to detect the field of variables within this layer, without probing out of the boundary layer (as discussed in this communication). This manuscript is therefore a welcome trial on this topic. Please see my detailed comments below.
General comments
Goto, K., Itoh, T., Nagayama, T., Utsunomiya, R., Tsutsumi, D., & Mizuyama, T. (2016). Development and installation of bedload monitoring systems with submerged load cells. Journal of Mountain Science, 13(2), 369-376.
Some specific comments