the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Massive sediment pulses triggered by a multi-stage alpine cliff fall (Hochvogel, DE/AT)
Natalie Barbosa
Johannes Leinauer
Juilson Jubanski
Michael Dietze
Ulrich Münzer
Florian Siegert
Michael Krautblatter
Abstract. Massive sediment pulses in catchments are a key alpine multi-risk component. Substantial sediment redistribution in alpine catchments frequently causes flooding, river erosion, and landsliding, and affects infrastructure such as dam reservoirs as well as aquatic ecosystems and water quality. While systematic rock slope failure inventories have been collected in several countries, the subsequent cascading sediment redistribution is virtually unaccessed. This contribution reports for the first time the massive sediment redistribution triggered by the multi-stage failure of more than 150,000 m3 from the Hochvogel dolomite peak during the summer of 2016. We applied change detection techniques on seven 3D-coregistered high-resolution true-orthophotos and digital surface models (DSM) obtained through digital aerial photogrammetry later optimized for precise volume calculation in steep terrain. The analysis of seismic information from surrounding stations revealed the temporal evolution of the cliff fall. We identified the proportional contribution of > 600 rockfall events (>1 m3) from 4 rock slope catchments with different aspects and their volume estimates. In a sediment cascade approach, we evaluated erosion, transport, and deposition from the rockface to the upper channelized erosive debris flow channel, then to the widened dispersive debris flow channel, and finally to the outlet into the braided sediment-supercharged Jochbach river. We observe the decadal flux of more than 400,000 m3 of sediment with massive sediment pulses that (i) respond with reaction times of 0–4 years and relaxation times beyond 10 years, (ii) with faster response times of 0–2 years in the upper catchment and more than 2 years response times in the lower catchments, (iii) the inversion of sedimentary (102–103 mm/a) to massive erosive regimes (102 mm/a) within single years and the (iv) dependency of redistribution to rainfall frequency and intensity. This study provides generic information on spatial and temporal patterns of massive sediment pulses in highly-charged alpine catchments.
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Natalie Barbosa et al.
Status: final response (author comments only)
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RC1: 'Comment on esurf-2023-10', Anonymous Referee #1, 10 Jul 2023
Dear editor,
Please find below my review of the paper esurf-2023-10:
Massive sediment pulses triggered by a multi-stage alpine cliff fall (Hochvogel, DE/AT)
by
Natalie Barbosa, Johannes Leinauer, Juilson Jubanski, Michael Dietze, Ulrich Münzer, Florian Siegert, Michael Krautblatter.
This paper is related to the link between large rockfalls and subsequent debris-flow activity. This is based on 2 years surveys by imaging and creating DEMs. The rockfall event sequences is based on seismic stations using an equation provided by Le Roy et al., (2019). The reaction time of the catchment varies from 0 to 4 years. This is illustrated by DoDs.
General Comments
The subject is of great interest and the response of a catchment to a large contribution of landslides is important especially nowadays with climate change. The observations are worth to be published, but unfortunately there is a large overinterpretation of the results, low resolution in temporality is bad for interpretation, but enough for a short paper about observations. The pulses of sediment delivery in the sedimentary cascade need more time resolution and also better spatial resolution in space, the use of potential energy with distance stations seems not really relevant looking at the table 13 sup. Mat. It shows a very low percentage of less than 1% of total volume (I guess it is not percent), some mostly below 50%, which is not relevant to give interpretation, but maybe I misunderstood. In addition, Le Roy et al. used very close seismic stations.
By the way many tables have no units (4, 5, 13 sup mat.) and axis legends in figs. sup mat 4, 5, 6.
The descriptions of the volumes form DoDs have less than 1% of error which is very small, consider some other factor, especially using eq. 7. A flow chart can improve and simplify the description for the DoD recreation. In addition, how do you create the grid, which interpolator is used… this brings some errors…
Some figures form the supplementary material can be included or merged with others in the main manuscript.
The paper needs to be simplified and restructured and focused mainly on observation and less on speculations, for instance the relationship with precipitations can be just mentioned but as the rainfall intensity thresholds are very local and depends very much on the geometry of the deposit, its position, and the material granulometry that such analysis is useless.
Specific Comments
- Lines 31-32 is not clear…
- Line 66: blank after “system”
- Line 152: two dots at the end
- Line 157: meaning of M unclear to me!
- Line 160: r means point-to-point or points to surface?
- Lines 170: 2.5 what?
- Eq 3 and 4: are you sure of the necessity of n?
- From lines 210 to 218 unclear, what doe mean v’? You use only one sigma?
- Line 223: site of the paper of Williams…
- Line 285: error less than 1 %?
- Figure 3, is not sufficient to illustrate the events…
- Table 2 and line 317: there must be a discussion between these two numbers.
- Figure 8: difficult to understand
- Lines 457-459: unclear
- Line 471: that is true, but some authors have already worked on that (Williams…)
- Line 479: “regimen” I am not sure it is correct…
- Lines 500-503: unclear…
Citation: https://doi.org/10.5194/esurf-2023-10-RC1 -
AC1: 'Reply on RC1', Natalie Barbosa, 09 Aug 2023
We gratefully thank the Reviewer for their comments and time devoted to the manuscript. Here we comment on the main aspects risen by the reviewer in general. In the attached file we answer in detail point-by-point the specific reviewers’ comments.
We would also always wish for a higher temporal resolution, but up to our understanding, this is the first work that has measured entire catchment sediment dynamics at a high spatial resolution (20 cm) before and after a rock slope failure big (>105 m³) and one out of few papers that illustrate the spatio-temporal patterns of sediment waves at a catchment scale in a temporal resolution of 2-to-3 years. We do not intend to resolve single debris flows but we want to decipher sediment dynamics on a (multi-) annual time scale. In fact, the temporal resolution of 7-time steps in 10 years is high enough to demonstrate the catchment wide-propagation of sediment waves, despite the coalesce of erosional events. We consider the temporal resolution a relevant time scale because sediment waves after a high sediment pulse occur in the range of years to a few decades.
We are aware of the greater distance between the Hochvogel and the seismic stations used to characterize the 2016 cliff fall compared to the original source Le Roy et al. (2019), however, the energy released by the event was easily sufficient to be recorded and discriminated. Table 13 sup. summarizes the volume percentage of each identified sub-event from the total estimated volume. Our results show that regardless of the distance of the seismic stations, the proportional contribution of the sub-events is consistent. The 2016 cliff fall followed a multi-stage behavior with almost half of the total volume being detached in the last event (sub-event #6). The total volume presented in the manuscript (1.02*105) is taken from the closest possible seismic station (Oberstdorf) (line 304) and the supplementary material aims to illustrate the uncertainty of the volume estimation when using farther stations (Line 224-225 Sup material). We also think that the number of 1% has been mistaken because even if sub-event 2 and sub-event 3 are considerably smaller than the other sub-events, block falls with a magnitude of 103 (Table 2) are relevant in the context of the assessment of natural hazards and rockfall slope dynamics. Note that further interpretation and conclusions are based only on the total volume of the multi-stage 2016 event in coherence with the temporal resolution of the aerial imagery.
Finally, we carefully revise the interpretation of the results and strengthen the argumentation that leads to our conclusions. In the attached file we answer in detail point-by-point the specific reviewers’ comments.
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RC2: 'Comment on esurf-2023-10', Georgina Bennett, 13 Jul 2023
This paper investigates the sediment cascade following a large alpine rockfall in Switzerland. It uses digital photogrammetry to detect volumes of erosion and deposition of sediment through the sediment cascade over a multi-year period. It uses seismology to detect individual rockfall events within the periods of analysis. It finds that 97% of sediment is delivered by a large rockfall event in 2016 that delivers sediment into the downstream debris flow channel. This sediment is remobilised from the upper part of the channel within 1-2 years but then moves more slowly through the catchment and results in overall deposition within the lower catchment even 4 years afterwards. The fluvial system below the outlet incises in response to a reduction in sediment supply creating large terraces. The authors suggest that the response time of the system will be much longer i.e to fully export all the sediment from the rockfall event from the system but needs ongoing monitoring.
I really enjoyed reading this manuscript, having worked on the Illgraben sediment cascade during my PhD, also using digital photogrammetry to extract valuable information on sediment production and transfer from aerial photographs. The team use state of the art photogrammetric techniques to carefully quantify volumes of sediment through the system and accounting for the various uncertainties resulting from topography, shadowing etc. The use of seismology to try to identify individual rockfall events and complement the photogrammetric record is excellent, though I can’t comment on the techniques here as a non-seismologist. The combination of photogrammetry and seismology to deal with the issue of coalescence I think is quite novel. Overall, the analysis seems very rigorous and robust and is well presented with a well-designed series of figures. The findings are interesting, particularly the timing of rockfall events leading up to the main failure will be interesting to the rockfall hazard and prediction community. The increase in erosion (by debris flows) in the upper catchment will also be of interest to hazard managers though longer-term monitoring is needed to capture full response of the system to the large rockfall event.
Main comments
Whilst the resolution of rockfalls achieved through the combination of photogrammetry, visual inspection and seismology is very good, the information on debris flow events is less detailed, with only bulk measurements of sediment volumes remobilised over the multi-year periods, presumably mostly by debris flows. Was it not possible to identify individual debris flow events from the seismic records as well as the rockfalls or would this require closer seismometers?
Analysis of rainfall data to explain patterns of sediment remobilisation through the system are somewhat of an afterthought in the discussion and more could have been made of this to try to explain patterns of remobilisation (i.e. debris flow activity) in figures 6 and 8.
Some of the literature on the topic of quantifying sediment cascades has been overlooked – see suggestions below. Additionally, a paragraph on the utility of numerical models for untangling controls on debris flows, i.e. relative importance of sediment supply versus transport capacity, and predictive power into the future under climate change would be useful in the discussion.
The start of the introduction could do with some restructuring. Instead of emphasizing several times near the beginning the lack of studies on sediment cascades (there have been quite a few), I would introduce the key processes of landslides, rockfalls debris flows and define the sediment cascade, say what has been done before and then identify gaps, e.g. lack of research on impacts of extreme events on sediment transfer in following years and why this is important e.g. under climate change…Also you could mention some of the technological gaps a bit more, e.g. the challenge of constructing DEMs from aerial photographs in mountainous landscapes (I’m not sure how many studies have achieved this) and therefore unlocking the spatial coverage and historical perspective that aerial photographs offer, the problem of temporal resolution and coalescence when considering photogrammetric records alone, which you target by using seismic records together with photogrammetry.
Otherwise, more minor comments throughout as annotated on the attached PDF.
I think my comments constitute minor to moderate revisions to this paper before I think it is ready for publication in ESurf. All the best with your revisions, Georgie Bennett.
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AC2: 'Reply on RC2', Natalie Barbosa, 09 Aug 2023
Thanks for these very encouraging comments! We appreciate the time spent on the review of the manuscript and the detailed and constructive comments. We will implement all the suggestions to the best of our ability in a revised version of the manuscript. In the attached file we answer in detail point-by-point the specific reviewers’ comments.
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AC2: 'Reply on RC2', Natalie Barbosa, 09 Aug 2023
Natalie Barbosa et al.
Natalie Barbosa et al.
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