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…

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.