Examples of multiple rock-slope collapses from Köfels (Ötz valley, Austria) and western Norway

This paper describes the effects of large massive rock-slope failures on subsequent slope stability. Three examples of large rock-slope failures from the Austrian Alps and Norway demonstrate that failure increases the probability of further collapses. At Köfels, Austria, a Holocene rock-slope failure several km3 in size filled the Ötz valley. The morphology of the deposits indicates that at least one subsequent failure occurred along the head scarp of the first failure, most likely a slide of similar size. Debris of the second landslide slid over the older deposits, forming the famous Köfels frictionite. At least three rock-slope failures, all of them in excess of 106 m3, occurred from the same mountainside within the last 6000–8000 years at Tafjord in western Norway. The most recent of these failures in 1934, triggered a destructive tsunami. Five large failures with volumes ≥ 50,000m3 occurred at Ramnefjell, Norway within 50 years; two of them caused considerable damage and a large number of death due to the formation of destructive tsunamis. Two-dimensional finite element models of rock-slope stability before and after the Köfels and Tafjord landslides show that massive rock-slope failures produce: (a) irregular slopes, parts of which are as steep or steeper than the slope before failure and which represent new zones of instability; and (b) zones of weakness related to slow slope deformation and related cracking.