Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 Debris flows in the mountain permafrost zone: Hohe Tauern national park (Austria) M. Hirschmugl Institute of Geography and Regional Sciences, University of Graz, Austria ABSTRACT: The existence of permafrost and its degradation can have an important influence on the evolution of debris flows in high mountain areas. Areas have been selected by visual interpretation of remote sensing data, which show an interrelation between permafrost and debris flows. Their hazard potential has been estimated in relation to threat to humans and infrastructure. The investigation area comprises the Carinthian parts of the Hohe Tauern national park. Moreover the work should provide data for further research on permanent debris flow- monitoring in a highly sensitive ecosystem. 1 INTRODUCTION traditionally cultivated area with settlements and Alpine farming up to high altitude. According to a case Debris flows occur in mountainous environments study in this area (“Seebachtal”), the upper borderline throughout the world and may cause devastating effects of extensive seasonal pasture farming is between 2400 on the people who live nearby. Beside the main factor and 2700 m a.s.l., depending on aspect (Egger 1996). of precipitation, the amounts of water released from In 1984, a large national park has been established the melting of snow and ice can affect the formation of debris flows (Zimmermann 1990). Perennially frozen slopes occurring in the Alps above the timberline often consists of ice-rich debris or morainic material with temperatures close to the melting point. Therefore, Vienna these localities, especially those near the lower bound- ary of permafrost, are expected to be the most sensi- tive to degradation processes (Haeberli 1992, Veit & Höfner 1993). Salzburg Thus, the occurrence of debris flows arising due to East melting permafrost seems to be related to the amount Tyrol Carinthia of water stored within a previously frozen slope (Zimmermann & Haeberli 1992). Figure 1a. Study area (Austria). This study was carried out within the scope of a sem- inar and is based on a visual interpretation of remote sensing data. It should give a birdseye view of a larger Fusch Rauris area by using a low cost method. Zones have been detected, which show an interrelationship between per- Hüttschlag mafrost, debris flows and human infrastructure. In Salzburg Badgastein consequence, a closer look will be given to these rela- Muhr tions in the particular region. The main goals of the Großglockner Gr. Hafner Heiligen-Sonnblick Ankogel investigation are to focus on the situation in the Matrei blut Carinthian part of the Hohe Tauern national park in Kals Grosskirch- Mallnitz Malta heim Austria and to provide data for further research. East- Mörtschach Hopf- Tyrol garten Winklern 2 STUDY AREA AND GENERAL Nussdorf- Debant Iselsberg- Stronach Carinthia CONDITIONS Lienz N 5 km Dölsach Spittal The study area, part of the Hohe Tauern range, is Study area regional borders national park outside the Study area rivers located in the northwestern part of Carinthia and belongs to the Central Alps (Fig. 1a). It concerns a Figure 1b. Study area in detail. 413 within this region to protect this particular mountain- ous area (Fig. 1b). Nevertheless, the development of tourism and the use of hydroelectric power increased the number of visitors, and resulted in higher infrastructural facili- ties within the area. Therefore, natural hazards, such as debris flows can nowadays have more grave effects on human beings than in former times. The study area stretches from about 1100 m up to 3797 m a.s.l. In general, the region mainly consists of crystalline parent rocks. Figure 1b shows the area of the national park Hohe Tauern, the investigated part has been shaded and consists of two sections: – parts of the Ankogel-Mountains (ca. 303 km2), and – parts of the Großglockner- and Schober-Mountains (ca. 301 km2). Figure 2. Visual interpretation of remote sensing data Recently, this region has also been involved in other with numbered debris flows. investigations concerning permafrost, which have been carried out at the University of Graz (Lieb 1998, Table 1. Construction of the table with important Krobath et al. 2003). With altitudes up to 3797 m a.s.l., parameters. the region investigated belongs to a high mountain H_catchment area, where both permafrost and debris flows are likely H_min H_max area to occur. In Switzerland, where the population in the Flow No. (m a.s.l.) (m a.s.l) (m a.s.l) Pf Alpine environments is quite dense, debris flows can cause much damage and even loss of life. Hence, the 200 2010 2320 2880 x potential for instability of Alpine permafrost has been 201 2050 2320 2820 x identified as being an issue of natural importance 202 2200 2600 2800 y (Haeberli et al. 1999). In this paper it will be shown 203 2180 2660 2800 x 204 2220 2560 2800 n how the hazard of debris flows connected with per- mafrost can have an influence on human beings or infrastructure. the basic question of possible hazards in these critical zones. 3 PROCEDURE 3.2 Debris flows 3.1 General remarks Debris flows generally occur on steep slopes and have been described as rapid viscous flows of granular Air-borne remote sensing data, namely 103 already solids, water and air. The flows consistency resembles pre-processed true-colour images (scale 1:5000) from a fast moving mixture of loose sediment and, mostly August 1998, have been visually interpreted. Initially, rather small amounts of water (Haeberli 1991). The all visually recognizable debris flows in the study area driving forces of slope stability include self-weight were digitized by using the GIS software ArcView. gravity, shear resistance and high pore water pressure. Figure 2 shows a sample area with five numbered The presence of water in slope materials is an debris flows. important cause of their stability or instability, In the next phase, a table including appropriate because water causes various forces in the soil. If the attribute data was created. The characteristics of every pore spaces in the soil matrix are not completely filled flow, such as the highest point of the catchment area, with water – the soil is unsaturated – and a suction the minimum and maximum altitude of the debris force is exerted, which tends to draw the soil grains flow itself, as well as a figure for the estimated more closely together. This suction is caused by a permafrost occurrence were included in this table process called capillary tension. However, if the pore (Table 1). spaces are completely filled with water, then the soil Finally, the possible hazard to all kind of human is said to be saturated. In this state, the water exerts a installations was estimated and combined with the other pressure within the pore spaces that tends to produce data to lead to a summarizing statement concerning forces that push the grains apart. Since the effective 414 stresses acting between the soil particles directly frozen slopes (Haeberli 1990, 1991, 1992). Therefore, influence the shear strength of the soil, if pore water a melting of underground ice can lead to higher insta- pressures reach high levels in the slopes, these materi- bility of the slopes mentioned above. als may become unstable. Based on various classifica- Concerning the occurrence of permafrost, latest tion schemes, the causes of mass movements can be investigation results from the study region have been grouped into the following two categories: taken into consideration. In general, discontinuous permafrost can be expected above 2500 m a.s.l. in the 1. So-called permanent factors: central section of the Austrian Alps, which covers the Tectonics, changes in stress and strain, weathering, Hohe Tauern range (Lieb 1998). However, the altitude changes in vegetation, root pressure as well as frost of permafrost occurrence strongly varies with aspect. and ice with the connected freezing and thawing Referring to other studies (Haeberli 1975, Lieb processes. 1998) surfaces without vegetation can in a first approx- 2. Induction factors: imation be considered as areas of potential permafrost Long term and intense precipitation, snow melt, occurrence in the Alps. This fact has also been proven undercuts, wash-outs, joint water or ground water, by current large-scale permafrost investigations (BTS earthquakes or human intervention, e.g. construc- measurements) in the Doesen Valley (Lieb 1998) and a tion (Buchroithner & Granica 1995). modelling of permafrost distribution in the Reisseck So the major causes for the triggering of flows tend to mountain range (Krobath et al. 2003). be the presence of abnormally high amounts of water. Simultaneously with altitude and aspect, vegetation As mentioned above, freezing and thawing processes cover has been used as the most important indicator as well as snow melt can be important to provide the for the assessment of the occurrence of permafrost in critical amount of water (Arenson & Springman 2000). the adjacent areas of debris flows. Beside the availability of water, large amounts of Three categories were established as permafrost material and the factor of mobilization could be the probable (y), no permafrost (n) and occurrence unsure causes for movements, too. The factor of mobilization (x). For further projects, small-scale permafrost mod- depends on some parameters, such as grain size and elling of the entire region, based on an exact digital looseness of the detritus, presence of water leakage, terrain model would doubtless lead to a higher accuracy presence and type of vegetation and steepness of the of differentiation. slope. Steep slopes mainly consisting of debris with inclination higher than about 35% tend to instability 3.4 Danger (Stötter 1994). This is a function of the soil and the groundwater conditions (Arenson et al.