General comments

This “Brief communication” presents a novel technique that enables the monitoring of sediment dynamics in remote terrains. It combines photogrammetric processing with UAV and was tested in the Illgraben debris-flow torrent, located in Switzerland. The topic is perfectly fitting with the ones proposed by NHESS and the contents are relevant for researchers and practitioners. I recommend the publication of the ms after some minor revisions.

• I suggest adding some information on recent studies applying UAV (or TLS) in torrential or fluvial areas (not related to Illgraben).
• The (preliminary) results, described between L125 and 131, should be extended and placed in a separate section. The results are brilliant and deserve a longer description. Not only related to the sediment dynamics, but also on basic (more technical) information like the pixel size of the DEM, which is missing.

If the above two points are not possible due to space problems, try to reduce other parts of introduction or discussion-conclusions.

• The description of locations like Illgraben mouth, channel outlet, upper catchment, head of the Illgraben channel, catchment outlet are not always clear. The authors may simplify them and add the most important ones in Figure 1A.

Specific comments:

L30: “DURING debris flows” is not clear. The surveys were before and after debris-flow events, weren’t they?

L73: width not with

L84-85: not totally clear, which was finally used in Illgraben (LAN or/and GSM).

L89: “1-inch Complementary Metal Oxide Semiconductor sensor” only expert may understand it. Please clarify.

L125: Figure 2 (not 1).

Figure 2: Improve the design or layout (e.g. rotate the zoom boxes and make them larger). Clearly indicate the locations of the check dams.

L140: You only mention flight time, but it would also be interesting having some information on the time consumption of the photogrammetric processing.

In their short communication, Walter et al. present a UAV-based semi-automatic system for the multi-temporal topographic monitoring of sediment dynamics in the source areas of a debris-flow basin. The study presents preliminary results on the erosion/deposition processes in the analyzed time period and explores the potential of the technique for investigating debris-flow hazards. The format of the short communication is suitable for this manuscript, the text is well written, and the presented system is definitely unique.

In addition to the comments of the other reviewers, on which I agree, I have few minor remarks that might be considered prior to publication. Section 5 Discussion and Conclusion sounds very optimistic for what concerns the future application of the technique in other contexts. I would suggest to briefly discuss also the potential limitations of the proposed system. For instance, I wonder to what extend the proposed technique is replicable in other locations given the installation, maintenance and data-processing costs. Regarding the assessment of debris-flow hazards in the aftermath of an event, the continuous monitoring of the unstable slope with ground-based systems (with radar, seismic but also photogrammetric sensors) is not, in general, more suitable?