More information on fall


Generally the mass is detached from a very steep slope along a surface on which little or no shear displacement takes place. It occurs mainly in the air and the phenomenon includes the material’s free fall, saltation, bouncing and rolling.

What is a fall phenomenon?

(Extract from Maquaire and Malet, 2006)

Falls (and also topples) comprise a free movement of material from steep slopes or cliffs. A topple is very similar to a fall in many aspects, but normally involves a pivoting action rather than a complete separation at the base of the failure.

Their general characteristics are as follows: the shape of the rupture surface is usually smooth and vertical; the material falls suddenly from a main scarp following a preparation phase during which a slice of material is separated, damaging the intact mass; the volume and size of the fallen material are extremely variable, depending on the morpho-structural and lithological conditions of the slope. These phenomena occur on cliffs when the base is eroded by the action of the sea or of rivers. The falls are always sudden and very quick, while topples vary in speed from extremely slow to extremely quick, with acceleration and deceleration phases (Maquaire and Malet, 2006).

A fall starts with the detachment of rock or soil from a steep slope along a surface on which little or no shear displacement takes place. The trajectory of the fallen material is rectilinear and vertical for slopes with an inclination of more than 70° (Cruden et Varnes, 1996).

For a rock fall, material on slopes with an inclination of between 45 and 70° moves by successive rebounds, depending on the size of the material, the restitution coefficient and the angle between the slope and the trajectory of the falling mass. Throughout the length of slopes of less than 45° the falling material may roll. In the latter two cases, the material, which is very mobile, may move considerable distances from the source zone. Debris falls and soil falls are typically shallow landslides. Debris and soil falls are triggered in loose materials and their volume varies from a few cubic metres to tens of cubic metres. The accumulated material, which is not consolidated, may be easily mobilized and transported.

Fig. 1: Block diagram of a typical coastal rock fall (from Dikau et al., 1996)

Fig. 2: Maè Valley (NE Italian Alps): January 1985 rock fall (Archivio CNR – IRPI, Padova)

Fig. 3: Rock fall in progress at Piz Sompluf peak, Dolomites, NE italian Alps (photos Summer 2006, from Archivio Provincia di Bolzano)

Fig. 4: Pietra di Bismantova (Northern Apennines, Italy): big boulders of a relict rock fall (photo by D. Castaldini)

Fig. 5: Randa rockfalls, Switzerland, occurred on 18th April, 9th May 1991. The events blocked the road, rail way and river which have subsequently been diverted (from Dikau et al., 1996)

Fig. 6: Rockfall at the west coast of Zakynthos Island Greece (Photo by M. Soldati)

Fig. 7: Big boulders due to rockfalls at the northwestern coast of Malta Island (Photo by M. Soldati)
Fig. 8: Big boulders due to rock fall at Ericeira, central coast of Portugal (photo by D. Castaldini)


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