Interactions of hydrometeorological processes and debris-flow activity in two Alpine catchments Olivia Debora Sartorius Department of Physical Geography GE9009 Degree Project in Physical Geography and Quaternary Geology 30 credits, NKA 239 Master's Programme in Hydrology, Hydrogeology and Water Resources (120 credits) Autumn term 2019 Supervisor: Andrew Framton Preface This Master’s thesis is Olivia Debora Sartorius’s degree project in Physical Geography and Quaternary Geology at the Department of Physical Geography, Stockholm University. The Master’s thesis comprises 30 credits (one term of full-time studies). Cooperation with Swiss Federal Institute for Forest, Snow and Landscape Research (WSL). Supervisors have been Andrew Frampton at the Department of Physical Geography, Stockholm University as well as Jacob Hirschberg at WSL. Examiner has been Britta Sannel at the Department of Physical Geography, Stockholm University. The author is responsible for the contents of this thesis. Stockholm, 1 September 2020 Björn Gunnarson Vice Director of studies Supervision Andrew Frampton, SU Jacob Hirschberg, WSL Master in Hydrology, Hydrogeology and Water Resources Department of Physical Geography, Stockholm University In collaboration with the Swiss Federal Institute for Forest, Snow and Landscape Research WSL 2 Abstract Globally, debris flows are one of the most destructive types of natural hazards. Apart from geomorphological conditions, hydrometeorological processes are decisive for the debris flow initiation. So far, hydrometeorological conditions have mainly been studied in large-scale study areas. This thesis focuses on the Dorfbach and the Ritigrabe torrents, which are both located in the Southwestern Swiss Alps. Through establishing rainfall intensity-duration thresholds and the use of the hydrological module of the probabilistic sediment cascade model SedCas, investigations on the hydrological state present before the debris flows as well as on the triggering hydrometeorological processes were done for a time period of 34.5 years. The rainfall intensity-duration threshold curves with the best predictive performance were obtained by � = 1.7 ∗ �!".$% for the Dorfbach and by � = 10.6 ∗ �!&.%' for the Ritigrabe, where D is rainfall duration in hours and I is the mean rainfall intensity in mm/h. The debris-flow trigger classifications indicated that long-lasting rainfall and rain-on-snow events were responsible for the majority of the debris-flow initiations in both catchments. In the Dorfbach 15 debris flows were triggered by long-lasting rainfall and 12 debris flows were related to rain-on-snow events. In the Ritigrabe, four debris flows were initiated by long-lasting rainfall and five of the nine occurring debris flows happened because of rain-on-snow events. Intense snowmelt and short- duration storms could only trigger one, respectively six debris flows in the Dorfbach. It could be observed that if the water storage was already filled up the additional water input which was needed for the debris-flow initiation was only little. The results suggest that many different hydrometeorological conditions can lead to debris-flow initiation. For the first time in the study areas, information on rainfall characteristics promoting debris-flow initiation is provided through the use of intensity-duration thresholds. However, the results also highlight the importance of including hydrometeorological processes such as snowmelt and antecedent soil moisture for the recognition of clearer patterns in the debris-flow activity. 3 Content Abstract 1. Introduction ........................................................................................................................ 1 1.1 Aims of the study ........................................................................................................................... 2 2. Methods .............................................................................................................................. 3 2.1 Study sites ...................................................................................................................................... 3 2.1.1 Dorfbach, Randa .................................................................................................................................... 4 2.1.2 Ritigrabe, St. Niklaus ............................................................................................................................. 5 2.2 Data sources and processing .......................................................................................................... 6 2.2.1 Meteorological data ............................................................................................................................... 6 2.2.2 Debris flow data ..................................................................................................................................... 7 2.3 Intensity-duration thresholds .......................................................................................................... 9 2.4 SedCas model, hydrological module ............................................................................................. 9 2.4.1 Snow module ....................................................................................................................................... 10 2.4.2 Water balance and calibration ............................................................................................................. 11 2.4.3 Validation of the hydrological module from SedCas .......................................................................... 12 2.5 Debris-flow trigger classification ................................................................................................. 13 3. Results .............................................................................................................................. 15 3.1 Intensity-duration thresholds ........................................................................................................ 15 3.1.1 Dorfbach, Randa .................................................................................................................................. 15 3.1.2 Ritigrabe, St. Niklaus ........................................................................................................................... 16 3.1.3 Comparison with established rainfall-intensity thresholds .................................................................. 16 3.2 SedCas modelling ........................................................................................................................ 17 3.2.1 Dorfbach, Randa .................................................................................................................................. 18 3.2.2 Ritigrabe, St. Niklaus ........................................................................................................................... 19 3.3 Debris-flow trigger classification ................................................................................................. 20 3.3.1 Dorfbach, Randa .................................................................................................................................. 21 3.3.2 Ritigrabe, St. Niklaus ........................................................................................................................... 24 4. Discussion ........................................................................................................................ 29 4.1 Uncertainties in input data ........................................................................................................... 29 4.2 Intensity-duration thresholds ........................................................................................................ 29 4.3 SedCas – the hydrological module ............................................................................................... 31 4.4 Debris-flow trigger classification ................................................................................................. 31 4.5 The role of rock glaciers as sediment source ............................................................................... 32 4.5.1 Dorfbach, Randa .................................................................................................................................. 33 4.5.2 Ritigrabe, St. Niklaus ........................................................................................................................... 33 5. Conclusions ...................................................................................................................... 34 6. Acknowledgments ............................................................................................................ 35 References ................................................................................................................................ 36 Appendix .................................................................................................................................. 40 4 1. Introduction Mountainous regions are susceptible to many different natural hazards like avalanches, landslides, debris flows, floods and rockfalls (Badoux et al., 2009). Debris flows represent one of the major hazards (Mostbauer et al, 2018; Prenner et al., 2018a). In the years from 1982 to 2018, debris flows were responsible for a financial damage of 531’116’460 Euros in Switzerland alone (WSL1, 2020; Hilker et al., 2009). In the late 1960s, scientists began