When the mountain slides down into the valley

Rock avalanches and landslides, rock falls and slope slips are all contained in the concept of mass movements. The ever more intensive usage of the mountainous regions and the climate change are some of the causes for these natural erosion processes from high alpine regions to the hill country, and they are not insignificant causes. Engineering geologists from Vienna University of Technology (TU Vienna) are modelling mass movements with specially adapted computer programs. Their know-how is helpful for the risk assessment of imminent landslides and slope slips. Some of the best-known showplaces are a moving slope located above the Norwegian Geiranger Fjord or the Gschliefgraben at the feet of Traunstein in Gmunden.

A considerable amount of data is necessary for mass movement simulations, from a terrain model above the crash-ready mass to an analysis of the so-called 'silent witnesses.' The latter involves rock masses that have already moved off at a previous point in time. According to Professor Rainer Poisel from the Institute of Engineering Geology of the TU Vienna, there are many of these 'silent witnesses.' 'They help us determine which variables come into play each time there is a crash. We have already adapted existing computer programs in order to be able to better simulate these mass movements,' says Poisel.

The TU research team has worked into the PFC Program (Particle Flow Code) values such as the rolling friction or the leap height of the individual rock bodies. Recently research based on this method has also taken place in Norway, on Mount 'Aknes,' which threatens to crash over the famous Geiranger Fjord and which could trigger tidal waves. Another program (DAN 3D) models the crashing mass as a hard fluid. Poisel asserts: 'We have simulated several falling events both with PFC as well as with the DAN code, and significant similarities have been found between the results of the particle and those of the fluid models.' Each landslide runs on a different pattern. 'A certain mass comes off and crashes down over foothills. Sometimes the mass remains in the same spot. Other times, it travels out for miles and miles into the foothills. We use DAN and PFC to calculate the tracking of these trips,' adds Poisel.

Poisel and his colleagues are also watching major slope movements at Murau and Gmunden in the much-cited Gschliefgraben. Such occurrences may be rarely prevented because they involve masses that are way too large. 'All that is left for us to do is watching, extracting as much water as possible from the slope, warning and also assessing the risks.' The risk is calculated as damages multiplied by the probability of occurrence. 'At Murau, the cost-use ratio has indicated, for example, that, for economic reasons, the valley flanks existing at an early stage can be counteracted only by an improvement of the woodlands. This has as a consequence the fact that the water infiltration into the underground is minimised during precipitations,' says Poisel. Currently, in Gmunden, the water is still pumped out the moved mass through wells in order to extract the lubricant out of the slope in an attempt to counteract the movements.