Zwerchwand rockfall/topple and Stambach mud slide

Zwerchwand - some recent shear planes (lighter) are visible

Geologic situation

The about 1 km long rockwall of the Zwerchwand with an altitude of 50 m on average is part of the Northern Limestone Alps, more exactly of the Hallstätter Zone. The following table (after the geologist A. Tollmann, 1976) shows a simplified stratigraphy of the area.

The permian Haselgebirge is a mixture of halite, shale and evaporites. Due to diapirism these permian saltclays have lifted the uplaying sediments. Those sediments are now strongly fractured and allow a high infiltration of water which leads to a self intensifying process. The strong tectonic disturbation lead to a very instable condition of Zwerchwand. The visible results are the distinguishable rockfalls beyond Zwerchwand. The oldest proven mass movement must have happened after the deglaciation of the Traun valley (after 17 000 BP). This is proven by old halites and marls that are located about 1300 m away from its origin and at their borders peat bogs have been dated.

Schematic view of the Haselgebirge lifting the uplaying sediments.

Zwerchwand (triassic limestone wall) rockfalls coincide with typical joint and fracture zones and is definitely tectonic enforced. The geomorphological system in this region at least since deglaciation always worked like this:

After a collapse of a part of a limestone wall the underlying "Haselgebirge" became mobile leading to huge clay slides/flows. The initial movement is tectonically enforced.

The "Haselgebirge" and the old mudslide was mobilised again when in the 20^th century new rockfalls forced it to move again. Most prominent movements took place from 1978onwards. The mass transport was about 380 m in the upper part of the slide and about 180 m in the lower part. So the movement became slower downhill. It stopped at a rock barrier that was uplifted at a tectonic line.

Looking at a map of lineaments we find a correspondence of tectonic lines with the rock wall above and the edge of the mass movement. By seismic sounding the depth of the slide was found to be 45 m (30-40 m average). The highest speed of the movement was up to 120 m/day.

Hydrology

The marls which are usually aquicludes show signs of doline growth therefore one might conclude again that this area is in tectonic tensional stress and defragmentation.

Due to the rather loose consistency of the landslide material and the many new fissures caused by the movement a large amount of water is able to infiltrate this area. Some water intakes that were measured took between 5-20 lit/sec. Even the lower value of 5 lit/sec would amount to about 150 000 m³ per year. For the whole area (32 ha) this would be a water level of about 50 cm . But probably 2 - 3 times as much is the reality.

The mechanical properties of the rocks will definitely be altered by that large amount of water drastically. Seismic refraction analyses has proven a dramatic worsening of the physical properties of these already stressed and loosened rocks. The especially soft and weak stratum was lowered to 10-15 m in two years ( after the slides of 1981/82).

To cope with the mudslide that endangers a village downslope the main important task is to remove the water from the sliding area. This was done by a series of pipes and by biological countermeasures. Vegetation was planted on the slide area that uses a large amount of water. Of course one tried not to use plants like trees that would exert a heavy weight on the area when maturing but instead bushes like willow and alder which will be cut every few years and regrow from the sprouts.

Mostly alder is growing in the parts where the mudslide occured. The path of the mudslide can now easily be seen due to the difference in vegetation.

The displacements and the slide are evidently gravitational and hydrologically induced. But the breakoff of the limestone wall on top is definitely due to tectonic young movements which was also proven by insitu stress measurements in 1984.

Precipitation at nearby Bad Goisern from 1931 to 1990