1 INTRODUCTION

Debris flows occur in mountainous environmentsthroughout the world and may cause devastating effectson the people who live nearby. Beside the main factorof precipitation, the amounts of water released fromthe melting of snow and ice can affect the formation ofdebris flows (Zimmermann 1990). Perennially frozenslopes occurring in the Alps above the timberlineoften consists of ice-rich debris or morainic materialwith temperatures close to the melting point. Therefore,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).

Thus, the occurrence of debris flows arising due tomelting permafrost seems to be related to the amountof water stored within a previously frozen slope(Zimmermann & Haeberli 1992).

This study was carried out within the scope of a sem-inar and is based on a visual interpretation of remotesensing data. It should give a birdseye view of a largerarea by using a low cost method. Zones have beendetected, which show an interrelationship between per-mafrost, debris flows and human infrastructure. Inconsequence, a closer look will be given to these rela-tions in the particular region. The main goals of theinvestigation are to focus on the situation in theCarinthian part of the Hohe Tauern national park inAustria and to provide data for further research.

2 STUDY AREA AND GENERAL CONDITIONS

The study area, part of the Hohe Tauern range, islocated in the northwestern part of Carinthia andbelongs to the Central Alps (Fig. 1a). It concerns a

traditionally cultivated area with settlements andAlpine farming up to high altitude. According to a casestudy in this area (“Seebachtal”), the upper borderlineof extensive seasonal pasture farming is between 2400and 2700 m a.s.l., depending on aspect (Egger 1996).In 1984, a large national park has been established

413

Debris flows in the mountain permafrost zone: Hohe Tauern national park (Austria)

M. HirschmuglInstitute of Geography and Regional Sciences, University of Graz, Austria

ABSTRACT: The existence of permafrost and its degradation can have an important influence on the evolutionof 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 inrelation to threat to humans and infrastructure. The investigation area comprises the Carinthian parts of the HoheTauern national park. Moreover the work should provide data for further research on permanent debris flow-monitoring in a highly sensitive ecosystem.

Permafrost, Phillips, Springman & Arenson (eds)© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

Vienna

Carinthia

Salzburg

Tyrol

East

Figure 1a. Study area (Austria).

riversregional bordersStudy area

national park outside the Study area

Malta

5 kmN

GroßglocknerHeiligen-SonnblickblutMatrei

Lienz

Winklern

Mallnitz

Ankogel

Spittal

Badgastein

Hopf-garten

Nussdorf-Debant Iselsberg-

StronachDölsach

Grosskirch-heim

Mörtschach

Kals

Fusch Rauris

Hüttschlag

Muhr

Carinthia

Gr. Hafner

Salzburg

East-Tyrol

Figure 1b. Study area in detail.

within this region to protect this particular mountain-ous area (Fig. 1b).

Nevertheless, the development of tourism and theuse of hydroelectric power increased the number ofvisitors, and resulted in higher infrastructural facili-ties within the area. Therefore, natural hazards, suchas debris flows can nowadays have more grave effectson human beings than in former times.

The study area stretches from about 1100 m up to3797 m a.s.l. In general, the region mainly consists ofcrystalline parent rocks. Figure 1b shows the area ofthe national park Hohe Tauern, the investigated parthas 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).

Recently, this region has also been involved in otherinvestigations concerning permafrost, which havebeen carried out at the University of Graz (Lieb 1998,Krobath et al. 2003). With altitudes up to 3797 m a.s.l.,the region investigated belongs to a high mountainarea, where both permafrost and debris flows are likelyto occur. In Switzerland, where the population in theAlpine environments is quite dense, debris flows cancause much damage and even loss of life. Hence, thepotential for instability of Alpine permafrost has beenidentified as being an issue of natural importance(Haeberli et al. 1999). In this paper it will be shownhow the hazard of debris flows connected with per-mafrost can have an influence on human beings orinfrastructure.

3 PROCEDURE

3.1 General remarks

Air-borne remote sensing data, namely 103 alreadypre-processed true-colour images (scale 1:5000) fromAugust 1998, have been visually interpreted. Initially,all visually recognizable debris flows in the study areawere digitized by using the GIS software ArcView.Figure 2 shows a sample area with five numbereddebris flows.

In the next phase, a table including appropriateattribute data was created. The characteristics of everyflow, such as the highest point of the catchment area,the minimum and maximum altitude of the debrisflow itself, as well as a figure for the estimatedpermafrost occurrence were included in this table(Table 1).

Finally, the possible hazard to all kind of humaninstallations was estimated and combined with the otherdata to lead to a summarizing statement concerning

the basic question of possible hazards in these criticalzones.

3.2 Debris flows

Debris flows generally occur on steep slopes and havebeen described as rapid viscous flows of granularsolids, water and air. The flows consistency resemblesa fast moving mixture of loose sediment and, mostlyrather small amounts of water (Haeberli 1991). Thedriving forces of slope stability include self-weightgravity, shear resistance and high pore water pressure.

The presence of water in slope materials is animportant cause of their stability or instability,because water causes various forces in the soil. If thepore spaces in the soil matrix are not completely filledwith water – the soil is unsaturated – and a suctionforce is exerted, which tends to draw the soil grainsmore closely together. This suction is caused by aprocess called capillary tension. However, if the porespaces are completely filled with water, then the soilis said to be saturated. In this state, the water exerts apressure within the pore spaces that tends to produceforces that push the grains apart. Since the effective

414

Figure 2. Visual interpretation of remote sensing datawith numbered debris flows.

Table 1. Construction of the table with importantparameters.

H_catchmentH_min H_max area

Flow No. (m a.s.l.) (m a.s.l) (m a.s.l) Pf

200 2010 2320 2880 x201 2050 2320 2820 x202 2200 2600 2800 y203 2180 2660 2800 x204 2220 2560 2800 n

stresses acting between the soil particles directlyinfluence the shear strength of the soil, if pore waterpressures reach high levels in the slopes, these materi-als may become unstable. Based on various classifica-tion schemes, the causes of mass movements can begrouped into the following two categories:

1. So-called permanent factors:Tectonics, changes in stress and strain, weathering,changes in vegetation, root pressure as well as frostand ice with the connected freezing and thawingprocesses.

2. Induction factors:Long term and intense precipitation, snow melt,undercuts, wash-outs, joint water or ground water,earthquakes or human intervention, e.g. construc-tion (Buchroithner & Granica 1995).

So the major causes for the triggering of flows tend tobe the presence of abnormally high amounts of water.As mentioned above, freezing and thawing processesas well as snow melt can be important to provide thecritical amount of water (Arenson & Springman 2000).

Beside the availability of water, large amounts ofmaterial and the factor of mobilization could be thecauses for movements, too. The factor of mobilizationdepends on some parameters, such as grain size andlooseness of the detritus, presence of water leakage,presence and type of vegetation and steepness of theslope. Steep slopes mainly consisting of debris withinclination higher than about 35% tend to instability(Stötter 1994). This is a function of the soil and thegroundwater conditions (Arenson et al. 2002). Slopescontaining permafrost are still stable even if the inclina-tion is far above this threshold. The investigated debrisflows occurring in summer 1987 in the Swiss Alpsshowed inclinations between 23 and 65%, whereby inmost of the cases the theoretical minimal inclination of27% was far exceeded (Haeberli et al. 1991).

3.3 Permafrost

The occurrence of permafrost in the Alps is continu-ous above 3300 m a.s.l., and discontinuous down to2400 m a.s.l. (Dramis et al. 1995). Perennially frozen,loose deposits, often situated on steep slopes are usu-ally supersaturated with ice (Haeberli et al. 1990). Inaddition, these high amounts of underground ice existat temperatures, which are rather close to the meltingpoint. The combination of relatively warm temperaturesand high ice contents on steep slopes makes Alpinepermafrost vulnerable to even small climatic changes.The atmospheric warming during the 20th century hasprobably induced a shifting in altitude of the lowerboundary of Alpine permafrost, causing degradationof underground ice and destabilization of formerly

frozen slopes (Haeberli 1990, 1991, 1992). Therefore,a melting of underground ice can lead to higher insta-bility of the slopes mentioned above.

Concerning the occurrence of permafrost, latestinvestigation results from the study region have beentaken into consideration. In general, discontinuouspermafrost can be expected above 2500 m a.s.l. in thecentral section of the Austrian Alps, which covers theHohe Tauern range (Lieb 1998). However, the altitudeof permafrost occurrence strongly varies with aspect.

Referring to other studies (Haeberli 1975, Lieb1998) surfaces without vegetation can in a first approx-imation be considered as areas of potential permafrostoccurrence in the Alps. This fact has also been provenby current large-scale permafrost investigations (BTSmeasurements) in the Doesen Valley (Lieb 1998) and amodelling of permafrost distribution in the Reisseckmountain range (Krobath et al. 2003).

Simultaneously with altitude and aspect, vegetationcover has been used as the most important indicatorfor the assessment of the occurrence of permafrost inthe adjacent areas of debris flows.

Three categories were established as permafrostprobable (y), no permafrost (n) and occurrence unsure(x). For further projects, small-scale permafrost mod-elling of the entire region, based on an exact digitalterrain model would doubtless lead to a higher accuracyof differentiation.

3.4 Danger

A hazard evaluation has been performed for everysingle debris flow and added to the table (Table 2).Two different types of danger could be identified:

– Column “Danger 1” represents danger to all kinds ofhuman infrastructure, such as huts and forest trails,in the immediate environment of the debris flow.

– Column “Danger 2” concerns debris flows, whichcould reach and destroy permanent settlements ortraffic life-lines.

Two possibilities are provided for both columns: (y)for existing danger and (n) for no danger. To estimatethe possible presence of a danger, following factorshave been taken into consideration. Firstly, the distance

415

Table 2. Estimation of the potential danger & additionalinformation.

Flow no. Danger 1 Danger 2 Information

200 y n long, flat valley201 y n long, flat valley202 y n long, flat valley203 y n long, flat valley204 y n long, flat valley

between the deposition area of the respective debrisflow and the possibly endangered area is a major inputparameter. Secondly, there is a strong interactionbetween distance and slope: in general, the steeper theslope, the farther the runout of the flow. Vegetationcover in the lower parts of the debris flow and the pos-sibility of sediment storage can also play importantroles. The latter, i.e. a lake, a reservoir, a long, flat val-ley or a large hollow, proved to be decisive factors forthe assessment, because these possibilities of sedimentstorage reduce the danger.

The “Information” field in the table reveals the mostimportant and finally decisive factor for the estimationof each debris flow. In addition to this, the kind ofendangered things can also be mentioned in this column.Figure 2, Table 1 and Table 2 show the whole procedureof data capture, whereby Table 2 shows examples of thejust described danger estimation process.

After the interpretation of 103 images, 296 debrisflows and their attributes were integrated into the data-base. At this point, it has to be emphasized that visualinterpretation is always a subjective process, wherebythe quality of the results highly depend on the experi-ence of the interpreter. In consequence, there is defi-nitely a need for the development of automatic andsemi-automatic methods, which would lead to moreobjective results.

4 RESULTS

First results of the investigations are presented in thefollowing section. As research has just started, onlysome basic figures can be given. One example of adebris flow is representative for these results. Furtheranalysis has not been performed recently.

4.1 Distribution above sea level

One of the basic results, the distribution of the debrisflows (highest point of the visible debris flow) abovesea level, is shown in Figure 3.

As shown in this figure, the majority of the debrisflows in the observed area occur in altitudes between2250 and 2750 m a.s.l. Below 2250 m a.s.l., only fewhave been investigated, because in principle, the studyis focused on high mountainous areas.

Above a boundary of about 2750 m a.s.l., glaciersand rock faces in these very high altitudes do not givemuch space or many loose deposits for the formationof debris flows.

4.2 Permafrost and hazardous debris flows

One of the most interesting issues is the correlationbetween possible permafrost occurrence in the

catchment area and the danger to human infrastruc-ture. Before focusing on this topic, the general classi-fication of the debris flows investigated according topermafrost occurrence should be mentioned. Almosthalf of the flows definitely have no connection to per-mafrost. Nearly 30% show signs of permafrost occur-rence in adjacent areas. No distinct statements can bemade for the remaining cases.

Table 3 shows how many debris flows are classifiedas hazardous and how they are distributed over the per-mafrost areas. As the figures demonstrate, 47% of thedebris flows (139) in the investigated area can endangerhuman infrastructure. Less than a third of these (42)can be regarded as being hazardous to the built envi-ronment according to the meaning of “Danger 2”.

Figure 4 displays the distribution of the 42 “Danger 2” evaluated debris flows, in terms of whetherpermafrost is expected at the location.

Most of these cases (30 debris flows, i.e. 72%) aredebris flows without any connection to permafrost,because of their occurrence in lower altitudes. Popula-tion is denser and there is more human infrastructurein these areas. For the categories “existing per-mafrost” (y) and “possible permafrost” (x) six debrisflows fall in each of them. In conclusion, it can bestated that permanent settlements and roads are hardlythreatened by debris flows coming from the “possibly

416

3017

54

64

4735

7

118

70 10 20 30 40 50 60 70

1850-1950

1951-2050

2051-2150

2151-2250

2251-2350

2351-2450

2451-2550

2551-2650

2651-2750

2751-2850

Alti

tude

in m

abo

ve s

ea le

vel

(hig

hest

poi

nt o

f th

e vi

sibl

e de

bris

flow

)

No. of debris flows

Figure 3. Distribution of the 296 debris flows in terms of altitude levels.

Table 3. Distribution of danger-classified debris flowsconnected with the existence of permafrost.

Number of debris flows

Permafrost occurence Danger 1 Danger 2

Permafrost probable (y) 35 6Permafrost possible/unsure (x) 44 6No permafrost expected (n) 60 30

Sum 139 42

permafrost zone”. This case was counted only 12times, which corresponds to 4%.

On the other hand, the investigation also showedthat there is a risk for seasonal settlements and otherhuman infrastructure in high mountain regions. Table3 records that 139 out of 296 debris flows can beregarded as hazardous, analogous to “Danger 1”. Thisnumber still includes debris flows from the region,where no permafrost can be expected. But even aftersubtracting the latter, there remain 79 cases, possiblyoriginating from the permafrost zone. From this, itfollows, that the danger for seasonal settlements, for-est trails or huts is quite high: 27% of the debris flowswere counted in this category.

4.3 Example: Debris flow near the Pasterze glacier

One debris flow has been selected for illustration.This flow is situated on a north-facing slope close to the Pasterze glacier in the Großglockner moun-tains, with a starting zone at about 2500 m a.s.l. Themissing vegetation cover and the nearby glaciers(“Schwerteckkees”) lead to the assumption that thisdebris flow is definitely originating in the permafrostzone. Figure 5 shows the interpreted image data in thisparticular area, Figure 6 displays the surroundings ofthe concerned debris flow.

Permafrost degradation in this area can thereforepartly become a trigger for debris flows. The dangercan be considered as rather low because of the capacityof the “Sandersee” lake for sediment storage. Neverthe-less, the high sediment input from all of the debrisflows in this area and from the Pasterze glacier to the“Sandersee” and further to the “Margaritzenstausee”has been shown to be a problem for the hydropowercompany. Some years ago, this company tried to solvethe sediment difficulties by opening the floodgates toallow large quantities of mud to flow out of the reser-voir. This measure caused an ecological disaster in the “Möll”-river.

This example demonstrates the complexity of therelations between permafrost, debris flows and haz-ard, which will lead to significant economic loss.

5 CONCLUSIONS

Areas near the permafrost boundary, especially steepslopes, are most sensitive to degradation processesand corresponding destabilization (Haeberli 1992).Based on the visual interpretation of the air-borneremote sensing data in this study, the assumption wasconfirmed that permafrost degradation can lead to anincreasing number of debris flows in mountainousregions at high altitudes.

According to this investigation, it could be shownthat huts and forest roads are mostly endangered atthese high altitudes. In some cases the occurrence ofdebris flows increases proportionally with the meltingof underground ice and degradation of formerly frozenslopes, which could endanger settlements or roads.

417

Figure 4. Distribution of the 42 “Danger 2” – classifieddebris flows in terms of existence of permafrost.

Figure 5. Aerial image data (1998): Debris flows near thePasterze glacier, south of the “Sandersee”.

Figure 6. View South-East: Debris flows below the“Schwert-eckkees” glacier, south of the “Sandersee”.

It is foreseen, that semi-automatic methods shouldbe adopted in future, necessitating further research inthe meantime. These methods should at least include apermafrost map of the territory based on a digital ele-vation model, which so far only exists for some partsof the entire study area (Krobath et al. 2003). Firstinvestigations on the automatic detection of the debrisflows have been started, but so far they have notyielded the expected results.

This project can be seen as a first step to providedata for further research for permanent monitoring,forecasting and modelling of debris flows within thepermafrost zone in this particular region.

REFERENCES

Arenson, L. & Springman, S.M. 2000. Slope stability andrelated problems of Alpine permafrost. In Proceedingsof the International Workshop on Permafrost Engineer-ing, Longyearbyen, Svalbard, Norway: 185–196.

Arenson, L., Hoelzle, M. & Springman, S. 2002. BoreholeDeformation Measurements and Internal Structure ofsome Rock Glaciers in Switzerland. Permafrost andPeriglacial Processes 13: 117–135.

Buchroithner, M.F. & Granica, K. 1997. Applications ofImaging Radar in Hydro-Geological Disaster Manage-ment: A Review. Amsterdam: OPA.

Dramis, F., Govi, M., Guglielmin, M. & Mortara, G. 1995.Short Communication: Mountain Permafrost and SlopeInstability in the Italian Alps: the Val Pola Landslide.Permafrost and Periglacial Processes 6: 73–82.

Egger, G. 1996: Vegetationsökologische UntersuchungSeebachtal, Nationalpark Hohe Tauern. Klagenfurt.

Haeberli, W. 1975. Untersuchungen zur Verbreitung vonPermafrost zwischen Flüelapaß und Piz Grialetsch(Graubünden). Mitteilungen der Versuchsanstalt fürWasserbau, Hydrologie und Glaziologie 17: ETHZürich.

Haeberli, W., Rickenmann, D., Zimmermann, M. & Rösli,U. 1990. Investigation of 1987 debris flows in SwissAlps: general concept and geophysical soundings. IAHSPublications 194: 303–310.

Haeberli, W. 1990. Permafrost. In: Schnee, Eis und Wasser der Alpen in einer wärmeren Atmosphäre.

Internationale Fachtagung von 11. Mai 1990.Mitteilungen der Versuchsanstalt für Wasserbau,Hydrologie und Glaziologie 108: 71–88.

Haeberli, W., Rickenmann, D., Zimmermann, M. & Rösli, U.1991. Murgänge. In: Ursachenanalyse der Hochwasser1987 – Ergebnisse der Untersuchungen. Mitteilungendes Bundesamtes für Wasserwirtschaft 4: 77–88.

Haeberli, W. 1991. Permafrost und Murgänge in der alpinenPeriglazialstufe. Bündner Wald 44(6): 59–65.

Haeberli, W. 1992. Possible Effects of Climatic Change onthe Evolution of Alpine Permafrost. CatenaSupplement 22: 23–33.

Haeberli, W., Kääb, A., Hoelzle, M., Bösch, H., Funk, M.,Vonder Mühll, D. & Keller, F. 1999. Eisschwund und Naturkatastrophen im Hochgebirge. Zürich: vdf,Hochschulverlag an der ETH.

Krobath, M., Lieb, G.K. & Pertl, A. 2003. Permafrost in theReisseck mountains (Austria) – a case study. InProceedings of the 8th International Conference onPermafrost, Zurich: this issue.

Lieb, G.K. 1996: Permafrost und Blockgletscher in denösterreichischen Alpen. Beiträge zur Permafrost-forschung in Österreich. Arbeiten aus dem Institut fürGeographie und Raumforschung der Karl-Franzens-Universität Graz 33: 9–125.

Lieb, G.K. 1998. High-Mountain Permafrost in the AustrianAlps (Europe). In Proceedings of the 7th InternationalConference on Permarost, Yellowknife: 663–668.

Stötter, J. 1994. Veränderungen der Kryosphäre in Vergan-genheit und Zukunft sowie Folgeerscheinungen.München.

Veit, H. & Höfner, T. 1993. Permafrost, Gelifluction andFluvial Sediment Transfer in the Alpine/SubnivalEcotone, Central Alps, Austria: Present, Past andFuture. Zeitschrift für Geomorphologie 92: 71–84.

Zimmermann, M. 1990. Periglaziale Murgänge In: Schnee,Eis und Wasser der Alpen in einer wärmerenAtmosphäre – Internationale Fachtagung von 11. Mai1990: Mitteilungen der Versuchsanstalt für Wasserbau,Hydrologie und Glaziologie 108: 89–107.

Zimmermann, M. & Haeberli, W. 1992. Climatic Changeand Debris Flow Activity in High- Mountain Areas –A Case Study in the Swiss Alps. In M. Boer & E.Koster (eds) Greenhouse-Impact on Cold-ClimateEcosystems and Landscapes. Catena Supplement 22:59–72.

418