A rock avalanche is a large bulk of mostly dry rock debris deriving from the collapse of a slope or cliff and moving at a high velocity and for a long distance, even on a gentle slope.
What is a rockfall avalanche?
(Extract from Dikau et al., 1996)
A rock avalanche is a large bulk of mostly dry rock debris deriving from the collapse of a slope or cliff and moving at a high velocity and for a long distance, even on a gentle slope. Its speed can be in the order of tens of meters per second, the travel distance in the order of kilometres. In the area of accumulation, its volume can exceed 1 x 106 m3, covering a total surface of over 0.1 km². Owing to its velocity and dimensions, this kind of landslide can be extremely costly in terms of human lives.
The rock avalanche can develop in two ways: first, by the fall or slide of a rock body which during movement progressively looses its cohesion by turning into dry debris and thus continues its advancement as a debris avalanche; secondly, by the sudden mobilisation of a debris avalanche, debris flow, either because of the fall of an overhanging rock mass or because of a seismic shock.
The Valpola rockfall avalanche (Italy): 28 July 1987
For the latter it is important to describe the phases that occurred on 28 July 1987 which led to be sliding of a portion of Mont Zandila in Valtellina (Italian Central Alp). At 7:25 a.m., after a preliminary phase lasting less than four days and without any sign of preceding seismic events, the detachment of a large rock volume of about 34 million cubic meters suddenly occurred. Although the collapse took place at a great velocity, its main phases of development can nevertheless be reconstructed thanks to direct witnesses and morphological evidence recorded after the event:
- The first displacements, which were relatively limited in volume, happened because of a progressive uphill widening of the crown generated by the falls occurring an hour earlier;
- After a few seconds, the entire rock mass started to slide along two main shear panes: the first was known to have 45° dip to the east, the second was a neo-formation plane dipping 35° to the north;
- Along the latter plane, the translational movements to the north, that is, toward the deep valley-floor of Valpola, took place initially with a series of short successive impulses and progressively increasing accelerations until it eventually came to a stop after the impact with a rock bluff which bordered the unstable slope. The thickness of the rock body that came into collision was estimated to be in excess of 70 m;
- Following the impact, the displaced rock, which up to that moment had remained fairly compact, was subdivided into several fragments of various dimensions, falling to the valley-floor in an easterly direction from altitudes ranging from 600 to 850 m. Therefore during this phase the gravitational event which had started as a slide, rapidly turned into rock avalanche, involving in its movement also the wood cover and the debris deposits distributed along the underlying slope. The fragmented collapsed rocks, after having obstructed a large area of the valley-floor, went up the opposite slope to a height of about 300 m, preceded by a cloud of dust which raised an altitude of 200 m.
A portion of the collapsed material fell into a previously formed barrier-lake, causing a mud wave which destroyed the village of Aquilone, located a few kilometres upstream, and claimed several lives. The volume of the accumulation deposit was estimated as 40 million m3, with a maximum thickness of 90 m. On the surface of the landslide deposit the finer fraction was composed of millimetre to decimetre fragments, while the coarser material was concentrated along the flanks of the mass movement route.
Fig. 1: The Valpola rock avalanche occurred on 28 July 1987 in the Italian Central Alps: left, aerial view to the west; right, general view to the south (from Dikau et al., 1996)