Zieher, T.; Rutzinger, M.; Schneider-Muntau, B.; Perzl, F.; Leidinger, D.; Formayer, H. and Geitner, C. (2017b): Sensitivity analysis and calibration of a dynamic physically-based slope stability model. Natural Hazards and Earth System Sciences Discussions 2017, 1 – 31.

Abstract: Physically-based modelling of slope stability at catchment scale is still a challenging task. Applying a physically-based model at such scale (1 : 10,000 to 1 : 50,000), parameters with a high impact on the model result should be calibrated to account for (i) the spatial variability of parameter values, (ii) shortcomings of the selected model, (iii) uncertainties of laboratory tests and field measurements or (iv) if parameters cannot be derived experimentally or measured in the field (e.g. calibration constants). While systematic parameter calibration is a common task in hydrological modelling, this is rarely done using physically-based slope stability models. In the present study a dynamic physically-based coupled hydrological/geomechanical slope stability model is calibrated based on a limited number of laboratory tests and a detailed multi-temporal shallow landslide inventory covering two landslide-triggering rainfall events in the Laternser valley, Vorarlberg (Austria). Sensitive parameters are identified based on a local one-at-a-time sensitivity analysis. These parameters (hydraulic conductivity, specific storage, effective angle of internal friction, effective cohesion) are systematically sampled and calibrated for a landslide-triggering rainfall event in August 2005. The identified model ensemble including 25 “behavioural model runs” with the highest portion of correctly predicted landslides and non-landslides is then validated with another landslide-triggering rainfall event in May 1999. The identified model ensemble correctly predicts the location and the supposed triggering timing of 73.5 % of the observed landslides triggered in August 2005 and 91.5 % of the observed landslides triggered in May 1999. Results of the model ensemble driven with raised precipitation input reveal a slight increase in areas potentially affected by slope failure. At the same time, the peak runoff increases more markedly, suggesting that precipitation intensities during the investigated landslide-triggering rainfall events were already close to or above the soil’s infiltration capacity.
Zieher, T.; Markart, G.; Ottowitz, D.; Römer, A.; Rutzinger, M.; Meißl, G. and Geitner, C. (2017a): Water content dynamics at plot scale – comparison of time-lapse electrical resistivity tomography monitoring and pore pressure modelling. Journal of Hydrology 544, 195 – 209.

Abstract: Physically-based dynamic modelling of shallow landslide susceptibility rests on several assumptions and simplifications. However, the applicability of physically-based models is only rarely tested in the field at the appropriate scale. This paper presents results of a spray irrigation experiment conducted on a plot of 100 m2 on an Alpine slope susceptible to shallow landsliding. Infiltrating precipitation applied at a constant rate (27.5 mm/h for 5.3 h) was monitored by means of 2D time-lapse electrical resistivity tomography, combined with time-domain reflectometry sensors installed at various depths. In addition, regolith characteristics were assessed by dynamic cone penetration tests using a light-weight cone penetrometer. The spray irrigation experiment resulted in a vertically progressing wetting front to a depth of 80–100 cm. Below that, the unconsolidated material was already saturated by rainfall in the previous days. The observed mean resistivity reduction attributed to infiltrating water during irrigation was scaled to pressure head. Mean variations in pore pressure were reproduced by a linear diffusion model also used in physically-based dynamic landslide susceptibility modelling. Sensitive parameters (hydraulic conductivity and specific storage) were tested over selected value ranges and calibrated. Calibrated parameter values are within published and experimentally derived ranges. The results of the comparison of observations and model results suggest that the model is capable of reproducing mean changes of pore pressure at a suitable scale for physically-based modelling of shallow landslide susceptibility. However, small-scale variations in pore pressure that may facilitate the triggering of shallow landslides are not captured by the model.
Zieher, T.; Perzl, F.; Rössel, M.; Rutzinger, M.; Meißl, G.; Markart, G. & Geitner, C. (2016): A multi-annual landslide inventory for the assessment of shallow landslide susceptibility – Two test cases in Vorarlberg, Austria. Geomorphology 259, 40 – 54.

Abstract: Geomorphological landslide inventories provide crucial input data for any study on the assessment of landslide susceptibility, hazard or risk. Several approaches for assessing landslide susceptibility have been proposed to identify areas particularly vulnerable to this natural hazard. What they have in common is the need for data of observed landslides. Therefore the first step of any study on landslide susceptibility is usually the compilation of a geomorphological landslide inventory using a geographical information system. Recent research has proved the feasibility of orthophoto interpretation for the preparation of an inventory aimed at the delineation of landslides with the use of distinctive signs in the imagery data. In this study a multi-annual landslide inventory focusing on shallow landslides (i.e. translational soil slides of 0-2 m in depth) was compiled for two study areas in Vorarlberg (Austria) from the interpretation of nine orthophoto series. In addition, derivatives of two generations of airborne laser scanning data aided the mapping procedure. Landslide scar areas were delineated on the basis of a high-resolution differential digital terrain model. The derivation of landslide volumes, depths and depth-to-length ratios are discussed. Results show that most mapped landslides meet the definition of a shallow landslide. The inventory therefore provides the data basis for the assessment of shallow landslide susceptibility and allows for the application of various modelling techniques.
Zieher, T. & Nicolussi, K. (2015): Rezente Baumgrenz- und Bestandsdynamik im Lafatschertal (Karwendelgebirge, Tirol)

Abstract: In the course of current discussions on climate change, increasing attention is being paid on shifting vegetation boundaries in mountain regions. A great number of studies prove an upward shift. The aim of the study presented is to analyse recent local-scale dynamics of alpine tree- and timberline. Emphasis is given to the spatiotemporally advancement of tree- and timberline and the comparison with measurement series of air temperature. In general, the study was based on the dendrochronological analysis of samples of living trees. The samples were collected within predefined areas below, at and above timberline between 1820 to 2130 m of altitude. The study area is located in the Lafatscher Valley, a tributary valley at the head of the Hinterau valley within the Karwendel Mountains (Tyrol). Primarily the forest stand consists of uncultivated Larch and Stone Pine. Therefore the chosen area is particularly suited for the presented issue of spatiotemporally advancement of treeline as well as developments of the local forest stand. The results show an upward shift of tree line of 117 m since at least 1850. In comparison to a modelling of climatic timberline altitude based on daily means of air temperature good correlations are shown spatially and temporarily. In order to draw conclusions on the development of the local forest stand the year of germination had to be modelled for each measured tree-ring width series. Furthermore history of landuse was concerned according to older literature. Because of the nearly simultaneously germination of Larch around 1700 A.D. a previous absence of high vegetation can be assumed. Subsequently due to the gradually favourable climatic conditions the local forest stand shifted upwards. At the moment (2011) timberline is located at 1988 m a.s.l., treeline at 2118 m a.s.l.
Die Verschiebung von Vegetationsobergrenzen ist aktuell vor dem Hintergrund des Klimawandels ein viel diskutiertes Thema. Eine Vielzahl an Studien belegt eine im Durchschnitt aufwärts gerichtete Verlagerung. Ziel der hier beschriebenen Studie ist es, die raumzeitliche Entwicklung der Baum- und Waldgrenze auf lokalem Mastab nachzuvollziehen und mit Messreihen der Lufttemperatur zu vergleichen. Das Untersuchungsdesign sah die dendrochronologische Auswertung von Bohrproben lebender Individuen vor, die in zuvor abgegrenzten Plots unterhalb, an und oberhalb der Waldgrenze gewonnen wurden. Das gewählte Untersuchungsgebiet befindet sich im Lafatschertal, einem hinteren Seitental des Hinterautales im Karwendelgebirge (Tirol, Österreich) und erstreckt sich zwischen 1820 bis 2130 m Seehöhe. Der dortige Waldbestand (im wesentlichen Karbonat-Lärchen-Zirbenwald) wird aktuell nicht bewirtschaftet und eignet sich deshalb gut, um die raumzeitliche Entwicklung der Baumgrenze sowie des Bestandes nachvollziehen zu können. Die Ergebnisse belegen eine aufwärts gerichtete Verlagerung der Baumgrenze zumindest seit Mitte des 19. Jhd. um 117 Höhenmeter. Der angestellte Vergleich mit einer auf Tagesmitteln der Lufttemperatur basierenden Modellierung der Höhenlage der klimatischen Waldgrenze zeigt sowohl räumlich als auch zeitlich gute Übereinstimmungen. Aussagen über die Waldentwicklung konnten unter Berücksichtigung der lokalen Nutzungsgeschichte mithilfe der gemessenen Jahrringserien, die um die modellierte Anzahl an fehlenden Jahrringen zum Kern bzw. zum Wurzelansatz verlängert wurden, abgeleitet werden. Aufgrund des beinahe zeitgleichen Aufkommens von Lärchen um 1700 n. Chr. kann von einem vorangegangenen Leerstand der aufgenommenen Fläche ausgegangen werden. Der lokale Bestand konnte sich daraufhin infolge verbesserter klimatischer Bedingungen auch in höheren Lagen etablieren. Derzeit (2011) liegt die Waldgrenze auf 1988 m, die Baumgrenze auf 2118 m Seehöhe.
Zieher, T., Rutzinger, M., Vetter, M., Geitner, C., Meißl, G., Perzl, F., Markart, G., Formayer, H. (2014): Climate induced system status changes at slopes and their impact on shallow landslide susceptibility – the project’s research plan.

Abstract: Shallow landslides imminently endanger human living and infrastructure in mountainous regions of the world. As these areas are often densely populated-especially in the Alpine region-it is essential to fully understand the processes involved in order to prevent impacts. Shallow landslides are usually understood as translational, slope-parallel gravitational mass movements comprising of a mixture of earth and debris with a maximum depth of 1-2 m. Under certain circumstances the initial sliding can turn into a flow-like movement leading to high velocities and runout distances. Shallow landslide susceptibility is differentiated into three classes. Predisposing factors involving topography, slope, geology and regolith characteristics (material strength and hydraulic properties) are regarded as a slope’s natural predisposition towards sliding. Variable factors such as meteorological pre-conditions i.e. the soil’s degree of saturation as well as land cover, land use and vegetation period represent the varying system status of a slope. Finally, in the course of triggering events such as heavy or prolonged rainfall (in other regions also earthquakes or volcanic activity) highly susceptible slopes are increasingly destabilized until sliding sets on. The project aims at determining shallow landslide susceptibility in time and space by means of statistical and physically-based modelling with a high degree of automation. For the respective case studies two different scales were defined. Predisposing factors shall be modelled statistically area-wide within the districts Feldkirch, Dornbirn and Bregenzer Wald in Vorarlberg, Austria. Therefore a landslide inventory is set up based on semi-automated classification of multitemporal orthophotos and airborne laser scanning derivatives. In combination with meteorological data this inventory is also used to derive precipitation thresholds for the onset of shallow landslides. The physically-based approach involving the TRIGRS model, demanding for detailed knowledge about regolith characteristics, is applied for a slope in the Laternser valley. Field work and suited laboratory tests yield the necessary geotechnical and hydraulic input parameters. The model is calibrated for past landslide events with known meteorological pre-conditions. In order to quantify the impact of climate change on shallow landslide susceptibility the results of scenario-based simulations are evaluated against current conditions. Finally, potentially affected areas are identified and shown in hazard maps.
This work has been conducted within C3S-ISLS, which is funded by the Austrian Climate and Energy Fund, 5th ACRP Program.
Zieher, T. (2014): Assessing the effects of varying geotechnical parameters on shallow landslide susceptibility modelling with TRIGRS. In: Globaler Wandel – regionale Nachhaltigkeit. Herausforderung für die Geographie?”. Innsbruck, Austria.

Abstract: Shallow landslides are typically understood as slides of loose material triggered by intense and prolonged rainfall, mostly associated with limited depths within the regolith. In mountain regions this type of mass movement endangers infrastructure and human living. Moreover the probability of occurrence of shallow landslides is expected to change due to climate change. Today various approaches aiming at assessing shallow landslide susceptibility exist. However, transferable and repeatable approaches to improve better slope system understanding are still lacking. On the one hand shallow landslide susceptibility is connected to rather stable i.e. invariant site-characteristics involving geology, topography and soil by means of statistical modelling. Physically-based models on the other hand are applied on the basis of spatially high-resolved meteorological data and detailed geotechnical and hydrological information on regolith, demanding for exhaustive field mapping. We present a set-up to simulate the effects of varying geotechnical parameters within the regolith (cohesion and inner friction angle) on shallow landslide susceptibility using the Transient Rainfall Infiltration and Grid-based Regional Slope-stability (TRIGRS) model. Basically TRIGRS combines a hydrologic infiltration model and an infinite slope stability model to estimate the factor of safety in time and space. For the set-up presented all hydrologic input parameters are set constant in accordance to material found to be highly susceptible to sliding in the study area (Laternser Valley, Vorarlberg, Austria). Geotechnical parameters affecting slope stability were modified according to value range for the corresponding material from respective literature. By the use of mathematically explicit digital terrain models the set-up is assured to be reproducible. Moreover, the procedure is automatized by using Python scripting. According to the model results, worst case parameter combinations of cohesion and inner friction angle would lead to unstable conditions at slope angles below 30°. This agrees with field mappings. Therefore the TRIGRS approach is considered suitable for modelling shallow landslide susceptibility in the study area.
This work has been conducted within C3S-ISLS, which is funded by the Austrian Climate and Energy Fund, 5th ACRP Program.
Rutzinger, M., Zieher, T., Vetter, M., Geitner, C., Meißl, G., Perzl, F., Markart, G. & Formayer, H. (2013): Climate induced system status changes at slopes and their impact on shallow landslide susceptibility – a concept. In: Alpine Geomorphologie, CH-AT Mountain Days. Mittersill, Austria.

Abstract: Shallow landslides are gravitative mass movements triggered by long and intense precipitation. The processes considered are translational, slope parallel slides within the regolith layer with a maximum depth of 2 m. Shallow landslides typically occur at lower parts of slopes. Depending on the water content involved shallow landslides can turn into debris flows extending the affected area. Shallow landslide occurrence depends on the basic and variable susceptibility of a slope. The basic susceptibility is controlled by slope, topography, regolith characteristics, and hydraulics properties. Variable susceptibility depends on the system status of a slope regarding to water saturation, land cover and land use, vegetation period and meteorological conditions i.e. snow melt and precipitation patterns. The aim of the project is to model the susceptibility of slopes regarding the occurrence of shallow landslides in location and time. In a first step a landslide database is set up in order to draw conclusions from past occurrence patterns. In further steps shallow landslide events are modeled and scenario analyses are applied to investigate the in fluence of climate change i.e. changes in temperature (snow melt) and precipitation patterns. Finally, social-economic impact of shallow landslides is analyzed under changed triggering conditions. These analyses will help to update hazard maps and serve as input for decision support systems. The project aims at using a high degree of automation by developing mapping and modeling tools integrating remote sensing data, field measurements, and ancillary geo-data of multiple sources. The project will contribute with its developments to existing open source initiatives.

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