E3S Web of Conferences 415, 05011 (2023)
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https://doi.org/10.1051/e3sconf/202341505011

Impact of climate change on hydro-meteorological trigger
conditions for debris flows in Austria
Roland Kaitna1*, David Prenner1, Matt Switanek2, Markus Stoffel3, Douglas Maraun2, and Markus Hrachowitz4
1Institute of Mountain Risk Engineering, University of Natural Resources and Life Sciences, Vienna, Austria
2Wegener Center for Climate and Global Change, University of Graz, Austria
3Institute for Environmental Sciences, University of Geneva, Switzerland

4Water Resources Section, Faculty of Civil Engineering and Geosciences, Delft University of Technology, The Netherlands

Abstract. Debris-flow activity is expected to change in a future climate. In this study we connect asusceptibility model for debris-flows on a regional scale with climate projections until 2100. We use this toassess changes of hydro-meteorological trigger conditions for debris flows in six regions in the Austrian
Alps. We find limited changes on an annual basis, but distinct changes when separating between hydrometeorological trigger types and regions. While regions in the east and in the south of Austria may
experience less days susceptible to debris flows in summer, there is a general trend of increasing
susceptibility earlier in the year for both, rainfall-related and snow-related trigger conditions. The outcomesof this study serve as a basis for the development of adaption strategies for future risk management fromthis debris-flow hazard.

1 Introduction
Debris flows initiate by a critical combination ofabundant sediment, steep inclination, and water [1]. The
water-component is mostly provided by rainfall, leading
to slope failure- or runoff-generated debris flows (e.g.
[2, 3]). The triggering rainfall can span from convectivestorms to stratiform precipitation (e.g. [4]). Sometimes
intensive snow-melt or rain-on-snow events can play asignificant role in debris-flow initiation (e.g. [5]).
For all emission scenarios, climate simulations
project an increase of temperature and changes of theseasonal precipitation pattern in the European Alps (e.g.
[6]). A common theme in the assessment of changing
debris-flow activity is that a future climate withincreased temperatures can store more water in theatmosphere, which results in more frequent and intense
rainfall events [7] that may trigger such mass
movements [8].
Up to now, there are only a few studies thatinvestigate the impact of climate change on debris-flow
activity in specific watersheds (e.g. [9]) or regions (e.g.
[10-12]). In this study we aim to assess the impact ofclimate change on debris-flow trigger conditions in theAustrian Alps.

projections on a daily basis until 2100 [13]. The
hydrological model was independently calibrated andvalidated for six study regions in the Austrian Alps overa period of 40+ years ([14, 15]). The resulting timeseries of hydro-meteorological variables of the reference
period (1971-2000) as well as for the near future (2021to 2050) and the far future (2071 to 2100) aresubsequently fed into a trigger model for debris flows[14] that differentiates between the meteorologicaltrigger types “long lasting rainfall” (LLR), “shortduration storm” (SDS), “snow melt” (SM) and “rain-onsnow” (RS). We subsequently assess the change infrequency of days susceptible to debris flows in the six
study regions. We also analyse regional and seasonalchanges of the different trigger types (Figure 1).

2 Methods
2.1 Study design
We feed a semi-distributed hydrological model with28 down-scaled and bias-corrected climate change

Fig. 1. Flowchart of the study design separately applied to all
six study regions.

* Corresponding author: roland.kaitna@boku.ac.at© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

E3S Web of Conferences 415, 05011 (2023)
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https://doi.org/10.1051/e3sconf/202341505011

2.2 Study regions

span during a day. Debris-flow events associated withsnow melt occur on consecutive days increasing soil
moisture due to intensive snowmelt. In the current study
we use a similar set of trigger criteria as presented by[14] and are detailed in [16].

The study regions range from high-altitude valleys
north and south of the alpine chain, to lower elevationsin the east of the Austrian Alps (Figure 2).

3 Results
3.1 Changes of hydrological catchment states
The hydrological response of our study regions
changes significantly by shifting precipitation andtemperature patterns in a future climate, which is similar
to the detailed analysis of changes of runoff signaturesby [15]. Overall, catchments become wetter in winter(increased soil moisture, slightly more precipitation andless snow fall) and drier in summer (less precipitationand drier soils).

Fig. 2. Overview of the six study regions (west to east): AMontafon, B-Pitztal, C-Defereggental, D-Gailtal, E-Paltental,and F-Feistritztal.

The Gailtal region occupies the largest area with 586
km², followed by the Montafon region with 510 km².
The regions Pitztal and Feistritztal are the smallest with133 km² and 115 km², respectively. The elevation rangeof the regions decreases from west to east. While thewestern catchments (Montafon, Pitztal, Defereggental)
have a high-alpine character (exceeding 3000 m above
sea level), the eastern catchments are sub-montane tomontane with a narrower elevation range (e.g.
Feistritztal 400 to 1600 m a.s.l.).

3.2 Changes of debris-flow trigger conditions
Averaged over all regions, we find no strong signal
for changes of any trigger type. However, there aredistinct regional as well as seasonal differences oftrigger conditions. On an annual basis, the most distinct
changes are predicted for the Paltental, a low-alpine
region in the central to eastern part of Austria, and thePitztal, a high-alpine valley in the western part (Figure3). In the Paltental the annual number of days
susceptible to debris flows are projected to decrease by-7 % (-4.3 d/yr) for RCP4.5 and up to 14 % (-5.7 d/y)
for RCP8.5, mainly due to a decrease of LLR and SDS
trigger types (Figure 3a). For the Pitztal region LLR
events are expected to increase by up to 12% (2.7 d/yr)in the far future for emission scenario RCP8.5 (Figure3b). For the regions south of the Alpine main ridge(Gailtal and Defereggental), there is a tendency of snowrelated trigger conditions to decrease.

2.3 Climate projections
Projected station data were derived from regional
climate simulations from the EURO-CORDEX
initiative. The statistical downscaling approachconsiders the spatial correlation of all of the nearby
weather stations within our study regions [13].
The projections are based on the “moderate”
scenario RCP4.5, which assumes an additional forcingof 4.5 W m-2, and the “business as usual” scenario
RCP8.5, with a surplus loading of 8.5 W m-2 by the endof the 21st century. In total we use 28 climate time-series
for each rain and temperature station from 1970 and2100 as input for hydrological modelling.
To assess changes, we define three periods. The first
represents our current and immediate future period(2021 to 2050), which we term “near future”, the second
is the period 2071 to 2100 (“far future”). The historicalperiod from 1971-2000 serves as reference period.
2.4 Trigger model
The trigger model differentiates between the trigger
types long-lasting rainfall (LLR), short-duration storms(SDS), snow melt (SM), and rain-on-snow (RS). Thesetrigger types have shown significantly different
signatures in the hydro-meteorological timeseries ondays when debris flows occurred in the past. For
example, a long-lasting rainfall event is typicallyassociated with decreasing temperatures and increasing
soil moisture on the days preceding a debris-flow eventin the regions. In contrast, a convective “short-durationstorm” occurs when the landscape heats up, leading to adecrease of soil moisture and an increase of temperature

Fig. 3. Annual change of trigger type frequency (LLR, SDS,SM, RS as well as without trigger differentiation) for
emission scenario RCP8.5 for the study regions Paltental (a)and Pitztal (b).

On a seasonal basis, there are strong variations oftrigger types. In most regions, SDS conditions expand to2

E3S Web of Conferences 415, 05011 (2023)
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https://doi.org/10.1051/e3sconf/202341505011

References

spring months and – for some regions show a decreasing
frequency during summer time. LLR conditions show
similar patterns, but are not that distinct as SDS
conditions. RS receives a more prolonged period thanfuture conditions of SM [16].

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4 Discussion and conclusions
For our study, predictions are made on a regional
scale rather than on the scale of individual catchments,
where debris flows actually initiate. This is a trade-off
between resolution and data availability, but also
guarantees that the model chain (climate-hydrologytrigger) is kept on the same scale. We rigorously
accounted for the uncertainties and use 700 time-seriesof future hydrological catchment states that provide a
broad representation of the range of combined
uncertainties of the hydrological and climate model. The
criteria of the trigger model were derived from (usuallyextreme) catchment states where debris-flows wereobserved in the past. The criteria sets were subsequently
applied to both, projected future and past. Hence, results
shown in this study are conditional on the assumptionthat the pattern pointing to debris flows in the past will
also hold in the future.
Debris-flow activity is not only controlled by acertain intensity or duration of rainfall, but also byhydrologicalandgeomorphologicalboundary
conditions that may vary over time. This study covers
the most important aspects of the hydrologicalcomponent on a regional scale, however, thegeomorphological aspects are not included, includingchanges of sediment production due to weathering [9],increased accumulation of debris in the channel during
drier summers [11], or the retreat of glaciers andthawing of permafrost, which releases considerable
amounts of debris. An elevation-resolved quantificationof the extent and temporal variation of thegeomorphological disposition is not yet available, butwould be of importance for a more complete climatechange impact assessment including predictions of
future debris-flow event magnitude (i.e. volume), whichis the most important quantity in engineering hazard
assessment [17].
Our results show seasonal and spatial changes ofhydro-meteorological conditions that were associatedwith past debris-flow occurrence in the Austrian Alps.
We expect an extension of the debris-flow season fromthe summer months into spring. In some regions,rainfall-related triggers become less frequent insummer, but together with snow-melt related trigger
conditions more frequent earlier in the year. Changes are
more pronounced for the far future and RCP8.5 than for
the near future and RCP4.5.
This project received financial support from the Austrian
Climate and Energy Fund and was carried out within theframework of the ACRP Programme. The computational
results presented have been achieved in part using the Vienna
Scientific Cluster (VSC).

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