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LANDSLIDE SUSCEPTIBILITY ANALYSIS AT A REGIONAL SCALE - A QUALITATIVE APPROACH AT THE EASTERN ALPS
(GEOHAZARD MAP VORARLBERG)
Dr. Michael Ruff1, Prof. em. Kurt Czurda2
The aim of the project “Geohazard Map Vorarlberg” was landslide susceptibility assessment at a regional scale. It was carried out by the Department of Applied Geology (AGK) at Karlsruhe University in cooperation with the INATURA Museum Dornbirn and the Federal Government of Vorarlberg. The project startet in 1999 with the following key tasks:
x Usage of GIS-Technology
x Collection of data with geological and geotechnical mappings x Analysis of recent events (landslide inventory)
x Proposing of a method at a scale of 1:25’000 x Method understandable for non-geologists
x Method easy to use and adeptable to various data sources
A special focus was laid on the presentation of the results. The susceptibility map should be understandable for spatial planners as well as local people, municipal employees and politicians. The map should give an overview of the present situation and be used as a tool for risk management in the Vorarlberg communities.
FIELD STUDIES AND DATA
In the years 1999 – 2005 several study areas in Vorarlberg (Bregenzerwald, Hochtannberg-Arlberg and Walgau regions) were geologically and geotechnically mapped to identify causes and mechanisms of active mass movements. All geological information was collected in a database and the geological formations were divided into geotechnical classes. The field data were implemented into a Geographical Information System (ArcGIS) and analysed as grid data with a cell size of 25 meters (Fig. 1).
1 Ingenieure Bart AG, Waisenhausstrasse 15, 9000 St. Gallen, Switzerland, Phone +41 71 2280175, Fax: +41 71 2280171, Email: ruff@bart.ch
2 Department of Applied Geology (AGK), University of Karlsruhe (TH), Kaiserstrasse 12, 76128 Karlsruhe, Germany. Website: www.agk.uni-karlsruhe.de/georisiko
Fig. 1: Data Management of the project.
Analog Data Fieldwork Existing Maps
Literature
Digital Data Orthophotos
DEM Topography
Database
GIS Vector
Layers Grid Layers
Susceptibility Slides
Susceptibility Rockfall
0 5 10 15 20
Statistics
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The field studies concentrated on two main types of mass movements: sliding and falling.
Because of the different mechanical behaviour, both types had to be considered separately.
Using a qualitative approach, the susceptibility was analysed according to five categories (very low, low, medium, high, very high).
SUSCEPTIBILITY ASSESSMENT
The susceptibility assessment for slides was accomplished using an index method. Comparing the preparatory factors to the landslide inventory with bivariate statistics, susceptibility indices for the data layers lithology, bedding conditions, faults, slope angle, slope aspect, vegetation and erosion were estimated. In a three step iterative method the layers were combined into a susceptibility map.
Areas prone to rockfall were estimated with a cost analysis of the movement of rolling rock samples. Potential source areas of falling material were extracted out of the Digital Elevation Model (DEM). A cost grid was calculated on the basis of slope angle and rolling friction.
Interpreting the cost calculation of all possible rockfall trajectories strating from the source cells, a susceptibility map could be constructed.
RESULTS
The results of the susceptibility assessement were presented by plotting the five susceptibility classes with a green-yellow-red colormap (Fig. 2). Because of the simple color definition (green for “good” and red for”bad”) these maps are self explaining without any geological knowledge needed. Therefore these maps represent a useful tool for different kinds of users.
As a first step of risk management people can be informed about geological hazards of their homelands.
Fig. 2: An example for susceptibility maps at the Hochtannberg area (Warth, Vorarlberg). The maps are usually printed in color to make them self-explanatory.
Rockfall Scarp Mapped Source Area Calculated Rockfall Debris Cone
very low lowmedium highvery high
KARHORN
MOHNENFLUH
Warth
-18000
-18000
-16000
-16000
-14000
-14000
-12000
-12000
233000 233000
235000 235000
2200 2100 19002000 1800 1600 1700 1800 1800 1900 2000 2100 2200 B200 B198 Susceptibility Rockfall KARHORN MOHNENFLUH Warth -18000
-18000
-16000
-16000
-14000
-14000
-12000
-12000
233000 233000
235000 235000
2200 2100 19002000 1800 1600
1700 1800
1800 1900 2000 2100 2200
B200
B198
Susceptibility Slides
Contour Line Road Slide Area
Keywords: Susceptibility, Slides, Rockfall
