Geohazard Map Vorarlberg GIS-based geological hazard assessment at

							                                      Geohazard Map Vorarlberg:
   GIS-based geological hazard assessment at the Northern Calcareous Alps, Austria


                                           M. Ruff & K. Czurda


      Department of Applied Geology (AGK), University of Karlsruhe, Kaiserstrasse 12, D-76131 Karlsruhe, Germany.

                                           E-mail: ruff@agk.uni-karlsruhe.de



   Geohazard Assessment, Northern Calcareous Alps, Landslides, Qualitative Index Method



   Introduction

Since 1999 the Department of Applied Geology is working in cooperation with the Vorarlberg Federal
Government and the Vorarlberger Naturschau Museum in Dornbirn on a GIS-based geohazard
assessment at a regional scale. At first, the Pilot-Project Bregenzerwald was started and an area
surrounding the Bregenzerache valley was geologically and geotechnically mapped to identify causes
and mechanism of mass movements. Statistical and empirical methods for GIS-based hazard
assessment were applied and evaluated with the field data. Geohazard maps concerning landslides
were created using a Qualitative Index Method [1]. Recently the project was expanded towards the
Hochtannberg-Arlberg region within the Northern Calcareous Alps. Results of this project are
presented below.

A transparent method of geohazard assessment shall be documented, applicable at other regions of
comparable geological situations. With aid of the geohazard maps, the local authorities are able to
concentrate their safety measurements on geohazard “hotspots”.



    Geology
The study area of the Project Hochtannberg/Arlberg lies in the Northern Calcareous Alps (NCA),
which consist in this part of a up to 3 km thick succession of Mesozoic calcareous and pelitic
sediments. Here the NCA are divided into three tectonic units (Allgäu-, Lechtal- and Inntal-Nappe),
whereas the pelitic formations were used as detachments [2,3]. Topography is characterised by steep
rock faces of limestones, which are folded at various scales. In the northern part of the study area
appear large outcrops of strongly layered flysch sediments. Also Quaternary sediments, like slope
debris and moraine, have local importance.
For the hazard assessment the formations were classified according to their lithological behaviour.
    Mass movements
Because of the prevalence of steep rock faces, rockfall is the main hazard of this area. But also
landslides appear in many places.
For geologists, landslides are movements of coherent masses along a discrete shear surface. Two types
of landslides can be can be differed: rotational landslides have an convoluted shear surface and
therefore the mass is rotated by the movement. This type is common in loose material like slope
debris. Translational landslides have a planar shear surface. The sliding happens mostly along pre-
existing planes like sediment layers or cleavage. Large landslides often show rotational and
translational parts. It must be stated that for both types of landslides different trigger factors must be
considered. To achieve a hazard assessment for both types, extensive abstractions have to be made.
The field works showed small scale rotational landslides mainly within slope debris and glacial filling
with minor depths. Translational slides occurred especially at the strongly layered flysch formations
but also at other formations with penetrating cleavage.

    Hazard assessment

The hazard assessment was made by a Qualitative Index Method [4,5]. For further calculation the field
data was separated in different layers (lithology, tectonic faults, vegetation, etc.) and converted in grid
data. The cell size of the grid was adjusted to the size of the official Digital Elevation Model (DEM),
which is 25 m. Out of the DEM, slope angle and aspect of the terrain were calculated.
Interpreting some exemplary prevailing landslides, following factors have been regarded for hazard
assessment:

    -   slope angle
    -   lithology
    -   orientation of layers (relative to the slope)
    -   tectonic faults
    -   vegetation (stabilisation)
    -   streams (erosion)

Hazard indexes for the layers were defined in a three step iterative method: the index ranges from 0
(low hazard) to 1 (strong hazard). At first, each layer is indexed (i.e. for lithology: slope debris gets
0.8 and limestone 0.2). Second, layers were grouped (DEM-, Geology- and Environment-Group) and
every layer was indexed within the group. Third, the groups were indexed and combined in a hazard
layer (Figure 1). The result was compared with the field observations and the indexes were improved
iteratively.
Fig. 1: Hazard map for landslides at the northern part of the study area. Different layers of factors
concerning landslides have been divided in three groups and were combined by a Qualitative
Index Method to a hazard map.


References
[1] Kassebeer (2002): Georisikokarte Vorarlberg – Pilotprojekt Bregenzer Wald. GIS-gestützte Gefährdungskartierung
einer alpinen Region. Doctoral Thesis University of Karlsruhe (TH)

[2] Linzer, H.-G., Ratschenbacher, L. & Frisch, W. (1995): Transpressional collision structures in the upper crust: the
fold-thrust belt of the Northern Calcareous Alps – Tectonophysics 242, S. 41 – 61

[3] Faupl, P. & Wagreich, M. (1999): Late Jurassic to Eocene Palaeogeography and Geodynamic Evolution of the
Eastern Alps - Mitt. Österr. Geol. Ges. 92, S. 79-94

[4] Juang, C. H., Lee, D. H. Scheu, C. (1992): Mapping slope failure potential using fuzzy sets.- Journal of Geotechnical
Engineering,118: S. 475-494

[5] Reiterer, I. (2001): Gefahrenbeurteilung von Rutschungsbereichen; Versuch der Ausweisung rutschungsgefährdeter
Bereiche im südlichen Salzkammergut mittels Geographischer Informationssysteme (GIS).- Angewandte Geographische
Informationsverarbeitung XIII; Beiträge zum AGIT-Symposium 2001: S. 387-399