The temporally varying roles of rainfall, snowmelt and soil moisture for debris flow initiation in a snow-dominated system

Abstract

Abstract. Debris flows represent frequent hazards in mountain regions. Though significant effort has been made to predict such events, the trigger conditions as well as the hydrologic disposition of a watershed at the time of debris flow occurrence are not well understood. Traditional intensity-duration threshold techniques to establish trigger conditions generally do not account for distinct influences of rainfall, snowmelt, and antecedent moisture. To improve our knowledge on the connection between debris flow initiation and the hydrologic system at a regional scale, this study explores the use of a semi-distributed conceptual rainfall–runoff model, linking different system variables such as soil moisture, snowmelt, or runoff with documented debris flow events in the inner Pitztal watershed, Austria. The model was run on a daily basis between 1953 and 2012. Analysing a range of modelled system state and flux variables at days on which debris flows occurred, three distinct dominant trigger mechanisms could be clearly identified. While the results suggest that for 68 % (17 out of 25) of the observed debris flow events during the study period high-intensity rainfall was the dominant trigger, snowmelt was identified as the dominant trigger for 24 % (6 out of 25) of the observed debris flow events. In addition, 8 % (2 out of 25) of the debris flow events could be attributed to the combined effects of low-intensity, long-lasting rainfall and transient storage of this water, causing elevated antecedent soil moisture conditions. The results also suggest a relatively clear temporal separation between the distinct trigger mechanisms, with high-intensity rainfall as a trigger being limited to mid- and late summer. The dominant trigger in late spring/early summer is snowmelt. Based on the discrimination between different modelled system states and fluxes and, more specifically, their temporally varying importance relative to each other, this exploratory study demonstrates that already the use of a relatively simple hydrological model can prove useful to gain some more insight into the importance of distinct debris flow trigger mechanisms. This highlights in particular the relevance of snowmelt contributions and the switch between mechanisms during early to mid-summer in snow-dominated systems.

Trigger characteristics of torrential flows from high to low alpine regions in Austria

“…With SDS triggered events, soil moisture distributes almost to the same fraction (27%, 29%, 27%) over the three most saturated bins (0%-20%, 20%-40%, 40%-60%). This finding supports earlier studies that showed that convective events can start over both, dry and wet soils (Ford et al, 2015;Mostbauer et al, 2018) and may be important to consider when applying antecedent rainfall models to forecast debris flow initiation (Crozier, 1999;Glade et al, 2000).…”

“…Event days on which at least one torrential flow was observed occurred in 10 out of 12 months (except January, February) across all regions. Reflecting results of Mostbauer et al (2018), intense SM trigger appeared comparatively clustered in May (54 % of all SM event days), followed by June 23%and April (15%). A similar cluster is visible for SDS trigger, which favorably occur in July, representing 48% of all event days of this trigger type.…”

“…Glade et al, 2000;Borga et al 2014;Bogaard and Grecco, 2015;). Mostbauer et al (2018) found a varying influence of different sources of water (precipitation, soil moisture, snow melt) at debris flow initiation by analyzing hydro-meteorological variables gained from a hydrological model. In a following work, Prenner et al (2018) analyzed multiple hydro-meteorological watershed variables and found that distinct watershed states exist when debris flows occur.…”

The Value of Using Multiple Hydrometeorological Variables to Predict Temporal Debris Flow Susceptibility in an Alpine Environment

“…Likewise, an unambiguous sign for considerable influence of SM as debris flow trigger during periods of the year when a snowpack is present are prolonged periods of above-freezing temperatures that can, depending on the temperature and water content of the snowpack, lead to sufficiently intense melt to trigger debris flows or to at least contribute to trigger conditions. This study directly follows up on recent results of Mostbauer et al (2018) who hydrologically modeled an Alpine watershed and identified three relevant sources of water input (high-intensity precipitation, SM, and antecedent soil moisture) that are connected to debris flow occurrence with varying seasonal importance. Here we test for the Montafon region in the Austrian Alps the hypotheses that (1) debris flows in such an environment occur at distinct catchment wetness states and are triggered by water input from different weather conditions, (2) signatures of these distinct triggering processes can be found in larger-scale pattern of the hydrometeorological conditions in the days leading up to a debris flow event, without the need for actual precipitation observations and the associated uncertainties and that (3) the hydrometeorological signatures are robust enough to be used for a probabilistic regional prediction of a region's temporal susceptibility to debris flows occurrence.…”

“…Triggering precipitation may origin from long-lasting rainfall (LLR) or short-duration storm (SDS) events that are mostly associated with convective processes (e.g., Berti et al, 1999;Church & Miles, 1987;Mostbauer et al, 2018;Stoffel et al, 2011). In addition, intense snowmelt (SM) can play a considerable role for debris flow initiation in cool regions (Cardinali et al, 2000;Church & Miles, 1987;Mostbauer et al, 2018;Stoffel et al, 2011). Notwithstanding considerable progress over the past years, reliable regional predictions of debris flows remain problematic and need to be improved to reduce human and economic losses (cf.…”