Understanding the frequency and magnitude of debris flows on Alpine talus slopes Hérens Valley and Col du Sanetsch (Switzerland) Word count: 18 729 Annelies De Bruyne Student number: 01307441 Supervisor: Dr. Amaury Frankl1 Advisor: MSc. Hanne Hendrickx1, Local supervisor: Prof. dr. Reynald Delaloye2 1Department of Geography, Faculty of Science, Ghent University 2Department of Geosciences, Faculty of Science, University of Fribourg A dissertation submitted to Ghent University in partial fulfilment of the requirements for the degree of Master of Science in Geography. Academic year: 2017 - 2018 PREFACE Writing and working on the dissertation has been a very enriching experience to me. The fulfilment of the dissertation could not have been possible without the support of many people. First of all, I would like to thank my supervisor, Dr. Amaury Frankl, my advisor, MSc. Hanne Hendrickx and the local supervisor Prof. Dr. Reynald Delaloye for the opportunity to carry out the dissertation in the Swiss Alps. They played an important role in the setup of the whole dissertation. Their feedback and instructions were crucial to accomplish this document. Secondly, I would also like to express my gratitude to Bart De Wit and Britt Lonneville for their technical support during the drone campaign. I would also like to thank my family, mountaineering friends and fellow geography students for encouraging me during the five weeks of field work in Switzerland and afterwards when starting to write the thesis. Hopefully, more future students will also see the potential of working and studying in the geomorphological rich environment that is offered by the Alps. 1 TABLE OF CONTENTS POPULARIZED TEXT ............................................................................................................................. 5 SUMMARY .............................................................................................................................................. 6 SAMENVATTING .................................................................................................................................... 7 1. INTRODUCTION ................................................................................................................................. 8 1.1 General introduction ................................................................................................................... 8 1.2 Geomorphological characterization of debris flows and related phenomena ..................... 8 1.3 Triggers and factors influencing magnitude and frequency .................................................. 9 1.3.1 Climatic factors and triggers ................................................................................................ 10 1.3.1.1 Intense rainfall and antecedent rainfall ........................................................................ 10 1.3.1.2 Rapid snowmelt and antecedent snowmelt .................................................................. 10 1.3.1.2 Temperature ................................................................................................................. 11 1.3.1.3 The response of ice and snow bodies .......................................................................... 11 1.3.2 Geomorphic factors ............................................................................................................. 11 1.3.3 The impact of catastrophic events ....................................................................................... 12 1.4 Temporal pattern of debris flows: the return period and debris flow season .................... 13 1.5 The sediment budget ................................................................................................................ 14 1.6 Magnitude-frequency relation and representation ................................................................ 16 1.7 Impact of climate change on debris flow activity .................................................................. 16 1.8 Objectives and research questions ........................................................................................ 17 1.9 About the upcoming dissertation ............................................................................................ 18 2. STUDY AREA ................................................................................................................................... 19 3. METHODOLOGY AND DATA .......................................................................................................... 23 3.1 3D modelling using aerial and terrestrial images .................................................................. 23 3.1.1 Preparation of the field surveys ........................................................................................... 23 3.1.2 Acquisition of UAS images .................................................................................................. 26 3.1.3 Assembling of terrestrial images.......................................................................................... 27 3.1.4 Producing DEM and orthophotomosaics ............................................................................. 27 3.2 Estimating the magnitude of debris flows .............................................................................. 28 3.2.1 Volume based on the volume – inundated area relation ..................................................... 28 3.2.2 Grain size analysis ............................................................................................................... 32 3.2.3 Volume measurement based on morphometric properties ................................................. 33 2 3.2.4 Magnitude estimation based on descriptive properties ....................................................... 34 3.2.5 Volume measurement based on a DEM .............................................................................. 36 3.3 Determining the frequency....................................................................................................... 36 3.4 Map creation .............................................................................................................................. 37 3.5 Tracing the possible lime content of debris flow deposits .................................................. 38 4. RESULTS .......................................................................................................................................... 39 4.1 Frequency .................................................................................................................................. 39 4.2 Magnitude .................................................................................................................................. 45 4.2.1 Case study: comparison of different volume measurements and geomorphology ............. 46 4.2.1.1 Hérens Valley research sites ........................................................................................ 49 4.2.1.2 Col du Sanetsch research site ..................................................................................... 58 4.2.2 Volume-area relations in Val d’Hérens ................................................................................ 63 4.3 Magnitude-frequency relation in Val d’Hérens....................................................................... 65 5. DISCUSSION .................................................................................................................................... 66 5.1 The use of a UAS in Alpine mountain environment .............................................................. 66 5.2 The applicability of volume measurements for debris flow deposits in the field .............. 66 5.3 Frequency via aerial pictures towards other methods ......................................................... 68 5.4 Possible explanations for the observed magnitude-frequency pattern .............................. 68 5.2.1 Climatic factors .................................................................................................................... 68 5.2.2 Geomorphic factors ............................................................................................................. 71 5.2.3 Catastrophic events ............................................................................................................. 72 6. CONCLUSION .................................................................................................................................. 73 7. REFERENCES .................................................................................................................................. 74 7.1 Scientific Articles ...................................................................................................................... 74 7.2 Internet sources and other ....................................................................................................... 77 7.3 Software ..................................................................................................................................... 78 8. ANNEXES ......................................................................................................................................... 80 8.1 Annex 1: GPS measurements .................................................................................................. 80 8.2 Annex 2: Agisoft Photoscan workflow and final data ........................................................... 80 8.3 Annex 3: ArcMap and QGIS workflow to calculate