Landslides are downward movements of rock and/or unconsolidated rocks under gravitational forces (Glade & Dikau, 2001).
Processes associated with landslides include falling, tipping, sliding, drifting and flowing. Various international recognized classification systems exist. The most common being the classifications of Varnes (1978), Cruden & Varnes (1996), Dikau et al. (1996 and 2001) or Dikau & Glade (2002). Often landslides are not associated with a single process, rather a combination of several movement mechanisms making the mapping and inventory of landslides highly subjective. As a result, decisions about the process, classification and assessment of movements are also subjective. Landslide that fail with a high velocity are the most dangerous and have the potential to be fatal. Processes such as debris flows (up to 80 km/h) or rock falls (up to 300 km/h) have a greater potential for fatalities than slower proceeding events such as crawling slope movements with velocities of a few meters to centimetres per year (Glade & Dikau, 2001).
When investigating landslides, three factors need to be separately analysed (Crozier, 1989):
preliminary factors (disposal)
process triggering factors (trigger)
Some important preliminary factors are the bedrock weathering level, changes in vegetation and variations in slope geometry. Most landslides that occur in Lower Austria are triggered by a strong precipitation event causing a change in pore-water pressure. However, landslides can also be triggered by volcanic activity and earthquakes. Controlling factors affect the landslide failure mode and runout distance and hence influence the potential risks. Controlling factors can include slope geometry (concavity, convexity, slope inclination, slope position and exposition), vegetation and surface roughness (Glade & Dikau, 2001). Analysis of the landslide process requires all these factors to be differentiated and separately investigated.
The following subpages address shortly the types of landslides relevant for the NoeSLIDE project in Lower Austria.