Debris-flow surges of a very active alpine torrent: a field database

Lapillonne, Suzanne; Fontaine, Firmin; Liebault, Frédéric; Richefeu, Vincent; Piton, Guillaume

doi:https://doi.org/10.5194/egusphere-2022-1297

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https://doi.org/10.5194/egusphere-2022-1297

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03 Jan 2023

Status: this preprint is open for discussion.

Debris-flow surges of a very active alpine torrent: a field database

Suzanne Lapillonne¹, Firmin Fontaine¹, Frédéric Liebault¹, Vincent Richefeu², and Guillaume Piton¹ Suzanne Lapillonne et al.

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¹Univ. Grenoble Alpes, INRAE, CNRS, IRD, Grenoble INP, IGE, Grenoble, France
²Univ. Grenoble Alpes, 3SR, Gières, France

Received: 18 Nov 2022 – Discussion started: 03 Jan 2023

Abstract. This paper presents a protocol to analyze debris flow focusing on the surge scale rather than the full scale of the debris flow event, as well as its application to a French site. Providing bulk surge features like volume, peak discharge, front height, front velocity and Froude numbers allows for numerical and experimental debris flow investigations to be designed with narrower physical ranges and thus, for deeper scientific questions to be explored. We suggest a method to access such features at surge scale that can be applied to a wide variety of monitoring stations. Requirements for monitoring stations for the protocol to be applicable include (i) a flow stage measurements, (ii) a cross section hypothesis and (iii) a velocity estimation. Raw data from three monitoring stations on the Réal torrent (drainage area: 2 km² , South-East France) are used to illustrate an application on 34 surges measured from 2011 to 2020 on the three monitoring stations. Volumes of debris-flow surges on the Réal Torrent are typically sized at a few thousand cubic meters. Peak flow height of surges range from 1 to 2 m. Peak discharge range around a few dozens cubic meters per second. Finally, we show that Froude numbers of such surges are near critical.

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CC1:
'A comment on the availability of debris-flow data', Lorenzo Marchi, 09 Jan 2023 reply

I have enjoyed reading this nice paper and I agree with the authors about the importance of sharing debris-flow data in open databases. This comment aims at providing information on the availability of measurements from two sites (Gadria and Moscardo catchments) instrumented for debris-flow monitoring in Italy.
Regarding the Gadria catchment, the paper under discussion mentions early observations on two debris flows (Comiti et al., 2014) and two more events that were used to compare the results of large scale particle image velocimetry (Theule et al., 2018). A recent paper (Coviello et al., 2021) has extended the dataset by presenting data (flow velocity and bulk volume, separated in surges) from nine debris flows recorded between 2011 and 2017. More data on six debris flows that occurred between 2018 and 2020 will be reported in a contribution accepted for the forthcoming DFHM8 Conference (Torino, June 2023).
Debris-flow data recorded between 1990 and 2019 in the Moscardo Torrent have been published in the repository PANGAEA: https://doi.pangaea.de/10.1594/PANGAEA.919707. In this time interval, 30 debris flows occurred, and 26 of them were monitored by sensors installed on the channel, while four were only documented through post-event observations. The catalog includes flow depth data, measured utilizing ultrasonic sensors, and rainfall. A paper published in NHESS (Marchi et al., 2021) describes the debris-flow dataset of the Moscardo Torrent and presents summary data (event date, number of surges per event, bulk volume, mean velocity, and peak discharge of the main surge). Data from 62 surges, i.e., on average approximately two surges per debris-flow event, were extracted from the records of the flow stage; their analysis enabled describing the shape of the hydrographs.
References
Comiti, F., Marchi, L., Macconi, P., Arattano, M., Bertoldi, G., Borga, M., Brardinoni, F., Cavalli, M., D’Agostino, V., Penna, D., Theule, J.: A new monitoring station for debris flows in the European Alps: first observations in the Gadria basin. Natural Hazards, 73(3), 1175-1198, https://doi.org/10.1007/s11069-014-1088-5, 2014.
Coviello, V., Theule, J.I., Crema, S., Arattano, M., Comiti, F., Cavalli, M., Lucía, A., Macconi, P., Marchi, L.: Combining Instrumental Monitoring and High-Resolution Topography for Estimating Sediment Yield in a Debris-Flow Catchment. Environmental and Engineering Geoscience, 27(1), 95-111, https://doi.org/10.2113/EEG-D-20-00025, 2021.
Marchi, L., Cazorzi, F., Arattano, M., Cucchiaro, S., Cavalli, M., Crema, S: Debris flows recorded in the Moscardo catchment (Italian Alps) between 1990 and 2019. Nat. Hazards Earth Syst. Sci., 21, 87–97, https://doi.org/10.5194/nhess-21-87-2021, 2021.
Theule, J.I., Crema, S., Marchi, L., Cavalli, M., Comiti, F.: Exploiting LSPIV to assess debris-flow velocities in the field. Nat. Hazards Earth Syst. Sci., 18, 1-13, https://doi.org/10.5194/nhess-18-1-2018, 2018.

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Citation: https://doi.org/10.5194/egusphere-2022-1297-CC1
- AC1: 'Reply on CC1 on the availibility of debris flow data', Suzanne Lapillonne, 12 Jan 2023 reply
  
  Dear Lorenzo Marchi,
  We kindly thank you for this insightful comment and your interest in our paper !
  The datasets in the papers you mentioned are indeed very fitting to this study. Volumes from Coviello et al, 2021 and Marchi et al, 2021 will be added in the text as comparison points for the orders of magnitudes of volumes in section 4.1.
  Regarding the Moscardo catchment, dataset and correct references will be added to the introduction. Datapoints from Marchi et al. 2021, coupled with the database of Marchi et al. 2020, provided at https://doi.pangaea.de/10.1594/PANGAEA.919707, will be added to Fig. 7c.
  Thanks a lot for your valuable input. We are looking forward to your contribution to DFHM8 Conference, as we will also be present with a contribution on the Réal torrent monitoring stations.
  
  Reply
  
  Citation: https://doi.org/10.5194/egusphere-2022-1297-AC1
RC1: 'Comment on egusphere-2022-1297', Roland Kaitna, 26 Jan 2023 reply

Lapillone et al. report of results from a debris-flow monitoring station in the Real torrent and suggest a data processing protocol for a more consistent and transparent derivation of debris-flow parameters from field observations. I think this is a well-written and well-structured contribution that will be very valuable for the community.
I have only minor to moderate comments and suggestions:
L 1: “…debris flows” (plural)
L 5: “…at the surge scale” (instead of “at surge scale”)
L 17: I think you refer here to the Nagl et al. (2022) paper (impact forces).
L 16: I think one should write “debris-flow monitoring”, but “debris flow”. Check throughout the manuscript to be consistent.
L 100-101: this sentence is unclear. Please re-formulate.
L 145ff: it is not clear to me, how these hypotheses will be tested. Probably it would be better to term them “assumptions”.
Table 1: I recommend to add a column with the location along the channel (or distance between stations) and a column with the mean slope of the channel reach where stations are located. Both information may help with interpretations given later.
Figure 4: for the reader’s convenience, I suggest to modify this figure for better readability: (1) the labeling is not intuitive and not consistent with labeling in Figure 5. E.g., why is flow stage termed “rad”? What is geo_21 and geo_21ref? (2) change line color/style to allow an easier differentiation between seismic sensors and flow state. You may also consider to plot diagrams above each other (makes them wider and probably easier to read) or show only one.
L 169: what is meant with “the least noisy flow stage signal is chosen”? Please re-formulate.
L 172: bracket is missing.
L 204: Delete “”Finally,”
Section 3.2: it is not clear to me which type of base level change was used (see L 145ff). Did you compare different assumptions? Are the differences small compare to e.g. peak flow?
L 210: write “literature”, not “litterature” throughout the manuscript.
L 212: I recommend to stay more general and replace “viscosity” with “mixture composition”, that’s safer.
L 234: unclear sentence. Please re-formulate.
L 248: unclear. What do you mean by “witnesses”.
Section 4.3: I encourage to add some interpretation of the observations and measurements with regard to channel slope at the stations and distance between stations. E.g., is deposition to be expected at flatter reaches between stations?
Figure 10: I am wondering other symbols color may help to make the time component more readable. Probably grey-scale increasing over time?
Figure 11: can you add the range of uncertainty from volume and peak discharge estimates (assumptions on base level changes, L 145ff) to the diagram? In the Figure caption you may write “debris-flow surge volume”.

Nagl, G., Hübl, J., & Kaitna, R. (2022). Stress anisotropy in natural debris flows during impacting a monitoring structure. Landslides, 19(1), 211–220. https://doi.org/10.1007/s10346-021-01779-2

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Citation: https://doi.org/10.5194/egusphere-2022-1297-RC1
RC2: 'Comment on egusphere-2022-1297', Adam Emmer, 27 Jan 2023 reply

This study presents the results of 10 years of debris flow monitoring in Real catchment, SE France. Presented data are original and novel and have potential to be further exploited, especially in DF modelling studies where physical parameters need to be defined. The authors mention that they present a ‘protocol’ to analyze DF but I don’t see that clearly in present structure of the study (or what do you actually mean by ‘protocol’? is it a methodology how you prepare Table S1? maybe a flowchart figure could help to understand that; please clarify or consider re-framing). Apart from that framing issue, I recommend some additions (see details below) and I have a couple of comments:
L28-30: apart from scientific publications, I wonder whether the DRR authorities or other authorities in charge of DF management / monitoring collect and could provide more data?
L52-54: does this open access database already exist? Is it planned to be created? How would you motivate people to contribute their data?
L57: in a catchment
L62: is this your ‘protocol’? if so, please name it accordingly (please also consider visualization of individual steps in a flowchart figure; see above)
L89: please provide a reference to this hypothesis
L89: Focusing on data processing
L100-103: is this seismic signal analysis something you actually used and presented in the results of your study? If not, it should be removed from Materials and Methods section
Fig. 2: please clarify what is the use of geophone data in your study? How does it contribute to summarizing Table S1
L141-150: I don’t understand what you mean here; if you aim at presenting widely-applicable methodology (protocol), you should be as instructive as possible
Fig. 3: is dotted red line for max (isn’t it rather form min) and vice versa?
L151: please consider separate ‘study area’ section with more details on general physical geographical setting
Fig. 5: please consider adding information about elevation (basic contour lines)
L178-179: not clear, please clarify what you did at this step?
L181: I don’t understand point (ii) -in Table S1, you present rainfall data with precision to 1 decimal place
L184: I suggest to consider re-naming this section (e.g. observed DF, or similar)
L185: how do you defined ‘significant’ evet? Is this where the seismic signal comes into play? Please clarify
L190: how do you know there was natural variability if the measurements didn’t work?
L202: please unify Froude numbers to L194
Fig. 6: does it make sense to plot measurements from three monitoring stations in one curve? Considering erosion / depositional processes on a way, I suggest plotting separate curves for individual monitoring stations
Fig. 7: what is the reason for plotting these values? Would you expect correlation or causality? I suggest you to quantify possible correlations.
L212: lack of trend or no correlation?
L218: this value is beyond what is shown in Fig 7a (max 2 000 m3/km2); please check
L243: viscosity varies
L245-247: this is not clear to me, please reword this sentence
L256: there is no part b-c in Fig. 1
L259-271: I’m not sure I get what you want to say here
L272: insights from multitemporal high-resolution images might help to answer some of the remaining questions raised in this section (e.g., a remobilization of material deposited by previous event(s))
L297-298: see above
L312: ranges of what?
L321: what do you mean by ‘proof of concept for data processing?’; were there any doubts about it?
L323: in the paper, you don’t say much about how this collaboration and common database should like
L329: your data are site-specific rather than representative
- - -
To sum up, some interesting field data are presented and I recommend acceptance of this study as soon as some moderate revisions are made.

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Citation: https://doi.org/10.5194/egusphere-2022-1297-RC2

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Short summary

Debris flow are fast flows most often found in torrential watersheds. They are composed of two phases: a liquid phase which can be mud-like and a granular phase, including large boulders, which are transported along with the flow. Due to their destructive nature, accessing features of the flow, such as velocity and flow height is difficult. We present a protocol to analyze debris flow data and results on the Réal torrent in France. These results will help experts for the design of models.


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