| 13 Apr 2026
Disentangling the contributions of external forcings and internal factors to future alpine sediment fan dynamics
Abstract. Alpine alluvial fans and debris flow cones are central components of mountainous sediment cascades. The projected increase in heavy precipitation due to ongoing climate warming has been shown to intensify sediment redistribution dynamics under transport-limited conditions; however, sediment fan response to heavy precipitation has been shown to be strongly system-specific. The relative importance of external forcings and internal factors for sediment dynamics and their implications for sediment fan evolution in a future changing climate have not been assessed systematically so far. In this study, we compare decadal-scale planimetric dynamics of a mature alpine alluvial fan (“Friedergries”, 5 km2 catchment area) to juvenile debris flow cones (Lake Plansee, catchment areas mostly < 0.5 km2) in the Main Dolomite region of the Northern Calcareous Alps. In both areas, planimetric sediment redistribution dynamics are governed by external forcing by heavy rainfall. Internal system variables such as catchment morphometry transform the external forcing: the juvenile cones corresponding to small and steep catchments are susceptible to moderate precipitation extremes (< 1 year return interval) while floodplain dynamics on the mature fan are only susceptible to extreme precipitation events with supra-regional extent (3–20 years return interval). A detailed analysis of volumetric changes at Friedergries from 2018 to 2024 reveals that total erosion and deposition are best explained by heavy precipitation, while the location and timing of incision and backfill are determined by small-scale autogenic cycles. Based on current projections of climate change in the European Alps, we suggest that future sediment fan dynamics will be characterized by (i) an intensification of sediment redistribution in response to rainfall intensification, (ii) an increase of geomorphic work per rainstorm event in response to rainfall concentration, (iii) an increase in likelihood of system state changes, and (iv) a catchment-dependent seasonal shift of sediment redistribution: Activity of juvenile cones with small, steep catchments will shift towards spring and autumn while mature fans with larger, gentler catchments will continue to experience sediment redistribution mainly in summer. Here we show that catchment morphology and fan maturity control future susceptibility to rainstorms and thus sediment fan evolution in the coming decades.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Earth Surface Dynamics.
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Gewalt et al. have compiled a historic data and new data to study fan evolution of an alluvial fan in the German Alps. The data includes historic orthoiphotos, UAV imagery, precipitation and temperature from nearby catchments. They have processed this data to quantify geomorphologically active areas and for recent years also sediment balances. They find a correlation of active area to cumulative precip above a threshold. Based on some of their results and the literature, they propose a concept of future fan dynamics.
The methods are mostly well described and the results are well presented. The paper is not so easy to follow, for example because a lot of jargon is used and not all results are presented. I also find it problematic to infer future fan dynamics based on one catchment and predicted changes in rainfall patterns. I list my main concerns to be addressed below followed by more specific ones. To summarise, I find the research direction highly relevant and much effort has been put in generating a valuable dataset, but the presentation of it needs some more work.
Major comments
Specific comments
L65-L70: with regard to these aims I see a few things missing in the intro:
- how is sediment redistribution defined? Is it any sediment transport, minor sediment transport between major events, bank failure, etc? how dies it differ from “fan dynamics”?
L190: Do you interpolate the precip to? With a lapse rate?
L241: Statistical modelling of what? Please add to subsection title
Table 2: what is “n”? Information of the timescale is missing here and therefore it is unclear if extremes or climatic means are studied
L252-254: at what temporal scale do you do this analysis? Total precipitation does only make sense over seasonal or annual time scales.
Figure 5: In Table 2, you also mention total precip. Why is this not show here? In addition to the precip sums, have you also considered just counting events which exceed some threshold? Given that you don’t have a local gauge, this could helpful too.
Figure 6: I think this can go to the appendix
Figu 9: Could you please put the dates instead of the epchs? Else it is unclear where to see the seasonal signal
L411: I don’t see why the relaxation time is 15 years. In Fig. 5, the lowest active area is still in the 60’s. Shouldn’t it be less then?
422: this is the first time you use “system state change”. It might be worht introducing it earlier and define more clearly.
L438: please explain what “sediment waves” physically means.
L455: but runoff from snow melt is much more buffered than from a rain storm. Do you have any idea of the discharge in spring and if it’s enough to rework the channel?
L474: Maybe I missed it, but I don’t understand how incision/backfill can be out of sync with net erosion/accumulation. Maybe adding the numbers to Fig. 9 would help. But even then, I don’t get how backfilling can be net erosive. Isn’t backfilling depostional per definition? So it might be worth better explaining this here or in the results.
L483: this seems repetitive to the argument around L422 or is it something elese?
L499: You explain “antecedent conditions” hereafter, but what do mean by memory effects?
L515: there also plenty of studies arguing with decreasing slope stability due to cryosphere degradation. Then there are also arguments for increased slope stability due to greening. These should be mentioned here for the sake of completeness.
L516-524: This theory of incision stabilizing the outside of the channel might only hold for fans with low-magnitude but high-frequency events. Incision will destabilize the banks and favour avulsion of the next high-magnitude event. Why else would one build dams in alpine torrents, if incision would stabilize the areas outside of the channel?
L525-527: This statement hangs a bit loose and is not backed up with the reuslts