Regional mapping and frequency- magnitude analysis of debris avalanches and debris flows in Hordaland, Norway Kamilla Skåre Sandboe Master thesis in Geosciences GEO5960 Geohazards 60 credits Department of Geosciences The Faculty of Mathematics and Natural Sciences UNIVERSITY OF OSLO June 2019 © Kamilla Skåre Sandboe 2019 Regional mapping and frequency-magnitude analysis of debris avalanches and debris flows in Hordaland, Norway Kamilla Skåre Sandboe http://www.duo.uio.no/ Trykk: Reprosentralen, Universitetet i Oslo II Summary With its large topographic differences and location at the west coast of Norway, Hordaland county is susceptible to weather-induced landslides such as debris flows and debris avalanches. The temporal and spatial occurrence of these landslides are predicted by the landslide warning system established by the Norwegian Water Resources and Energy Directorate (NVE). The warnings are disseminated to the public as text messages and in the form of four color-coded levels. Each level describes the number of landslides that are expected from a meteorological event within an area with a 10.000-15.000 km2 extension. However, the warnings do not include expected landslide size, except a general assumption as “large landslides that disturb infrastructure and roads may occur” for the orange level 3. The observations, however, show that the same warning level might cause several smaller slides instead, causing widespread damage. Although the Norwegian landslide database (NLDB) is quite extensive and with a large number of historical and recent landslides recorded the information of landslide magnitude is lacking. This is a consequence of a lack of systematic mapping of landslides in Norway. This study aimed to prepare event-inventory maps for rain- and snowmelt episodes, that have triggered landslides in the Hordaland county in the period 2011-2017 by controlling the quality of landslides registered in the database, mapping their extension and by performing frequency- magnitude (FM) analyses, resulting in FM-curves. A list of 20 weather events that caused landslides were prepared, and six weather events were investigated further. 78 of 205 landslides caused by these weather events were then mapped with polygons in ArcMap. The mapping was conducted using different sources of information like aerial photos, lidar imagery, photos, and written descriptions. Frequency-magnitude curves were prepared for the different event- inventories, resulting in a cumulative distribution with a power-law relationship and rollover point. The mapped landslides in Hordaland varied greatly, between 206 m2 to 140.359 m2, with most confirmed landslides between 4600 m2 and 56000 m2. A threshold analysis was also performed and confirmed that the thresholds did not perform well during the weather events. Results from the adjacent county Sogn og Fjordane were also compared, due to their proximity and similarities in weather and topography. This thesis is part of a project by the Norwegian scientific community that aims to improve the quality of the landslide database and improve the thresholds used in the warning system. III IV Prologue This thesis is part of an internal project at NVE lead by senior geologist Graziella Devoli, with work done by MSc students and summer students. The project has the aim of improving the quality of the landslide database to improve the landslide thresholds used in Norway. It was introduced to me through conversations with Graziella Devoli and piqued my interest due to my interest in landslides and the landslide warning system at NVE. Karianne Staalesen Lilleøren also joined as a supervisor for the project. I want to give a big thank you to both of my supervisors Graziella Devoli and Karianne Staalesen Lilleøren. Thank you, Graziella for the idea for the thesis, and the guidance, ideas, and feedback during the work. Thank you to Karianne for your help with using GIS, understanding the results I got, and the encouragement and honest feedback during the past year. Thank you both for motivating me to attend both the “Geofaredagen” conference in Lillestrøm and the “EGU” conference in Vienna to present my thesis. They were both great experiences I am happy to have. I also want to thank Michel Heeremans for his help with accessing and use of data needed for the work. A big thank you as well to Søren Boje at NVE for excellent assistance with data for the threshold analysis, and informative conversations about the thresholds used in Norway. Thank you to Jess Joar Andersen as well, for quick and helpful access to the data. And finally, thank you to my parents for help and encouragement during the past year and their understanding of my constant preoccupation. Kamilla Skåre Sandboe V VI Table of contents 1 Introduction ……………………………………………………………………………….1 1.1 Background …………………………………………………………………………..1 1.2 Motivation for the thesis……………………………………………………………...3 1.3 Objectives …………………………………………………………………………….4 2 Theory …………………………………………………………………………………….6 2.1 Landslides…………………………………………………………………………….6 2.1.1 Classification based on triggering causes and kinematics ………………………6 2.2 Assessment of landslide hazard……………………………………………………..10 2.2.1 Frequency-magnitude…………………………………………………………..11 2.2.2 Landslides in a changing climate ………………………………………………13 2.2.3 Thresholds ……………………………………………………………………...14 2.3 Landslide Early Warning Service…………………………………………………...16 2.3.1 Norwegian landslide forecasting and warning service (NLFWS) ……………..17 2.4 Extreme weather events……………………………………………………………..20 3 Study area………………………………………………………………………………..21 3.1 Meteorological conditions …………………………………………………………..24 3.2 Landforms and landscape …………………………………………………………...28 3.3 Geology ……………………………………………………………………………..31 3.4 Landslides and climate change scenarios in Hordaland …………………………….33 3.4.1 Climate change scenarios ………………………………………………………37 4 Data and source of data ………………………………………………………………….39 4.1 Rainfall data ………………………………………………………………………...39 4.2 Landslide data……………………………………………………………………….39 4.2.1 Additional information about landslides ……………………………………….42 4.3 Aerial photos and lidar imagery …………………………………………………….42 4.4 Information about landslide warning and thresholds ……………………………….42 4.5 Geological and topographical information ………………………………………….43 5 Method …………………………………………………………………………………..44 5.1 Analysis of weather events ………………………………………………………….45 5.2 Landslide control and mapping landslide area ……………………………………...47 5.2.1 Collection and selection of landslide data: “Dataset 1” ………………………..47 VII 5.2.2 Quality control: received polygons …………………………………………….50 5.2.3 Drawing of the new polygons: “Dataset 2” ……………………………………51 5.3 Terrain and geological parameters ………………………………………………….59 5.4 Frequency-magnitude analysis ……………………………………………………...61 5.5 Threshold analysis …………………………………………………………………..62 5.6 Analysis of warning levels ………………………………………………………….63 6 Results ………………………………………….………………………………………..64 6.1 Weather events that triggered landslides in the period 2011-2017 …………………64 6.1.1 Spatial pattern and characteristics of the selected weather events ……………..67 6.2 Spatial and temporal distribution of landslide events ………………………………72 6.3 Landslide characterization…………………………………………………………..77 6.3.1 Landforms and susceptibility …………………………………………………..77 6.3.2 Topography and geology ………………………………………………………79 6.3.3 Landslide parameters …………………………………………………………..82 6.4 Frequency-magnitude analysis ……………………………………………………...84 6.5 Time of occurrence and threshold analysis …………………………………………86 6.6 Warning levels ………………………………………….…………………………91 7 Discussion ………………………………………….……………………………………94 7.1 Rainfall events and landslide distribution………………………………………….. 94 7.2 Mapping and landslide characterization …………………………………………….99 7.2.1 Selection and quality control of landslide data ………………………………...99 7.2.2 The mapped polygons: Dataset 2 ……………………………………………..100 7.2.3 Challenges during mapping …………………………………………………..101 7.3 Landslide characteristics in Hordaland ……………………………………………105 7.3.1 Evaluating the performance of susceptibility maps …………………………..105 7.3.2 Characterization of landslides in terms of topography and geology………….106 7.3.3 Typical landslide parameters …………………………………………………107 7.4 Frequency-magnitude analysis …………………………………………………….110 7.5 Threshold analysis …………………………………………………………………115 7.6 Evaluation of warning levels ……………………………………………………....117 7.7 Comparison of Hordaland and Sogn og Fjordane………………………………… 117 7.7.1 Frequency-magnitude analysis comparison with Sogn og Fjordane………… 118 7.7.2 Threshold analysis comparison with Sogn og Fjordane……………………... 120 VIII 8 Conclusion…………………………………………………………………………….. 122 References…………………………………………………………………………………... 125 Appendix…………………………………………………………………………………….140 Appendix 1: Polygons from other students……………………..………...………....140 Appendix 2: List of sources used to map the polygons…….………..………………143 Appendix 3: Aerial photos……………………………………..……………………146 Appendix 4: Lidar images….………………………………………..………………147 Appendix 5: Spatial distribution of landslides…..……………..……………………148 Appendix 6: Dataset 2 Terrain parameters and landslide dimensions………...….…149 Appendix 7: Dataset 2 Geological information…………………………...…………152 Appendix 8: List of frequency-magnitude values………….………..………………158 Appendix 9: Values of soil water used in threshold analysis…..……………………161 Appendix 10: Poster for Geofaredagen 2018………………………..………………164 Appendix 11: Poster for EGU 2019..…………………………..……………………165 IX X 1 Introduction 1.1 Background Climate change with increased temperatures and more intense rainfall is expected