Randa rockslides

In April and May 1991, two consecutive rockslides occurred from a cliff above the town of Randa in the Matter valley of Switzerland. The rockslides released a cumulative volume of approximately 30 million cubic meters of debris, with each of the rockslide stages occurring over several hours. Slide debris buried key regional transportation lines including the road and railway leading to Zermatt, and dammed the Mattervispa river which eventually flooded a portion of the town of Randa upstream. There were no fatalities resulting from either of the rockslide events, though livestock, farmhouses and holiday homes were destroyed.

The Matter valley in the region of Randa exhibits classical morphology of glacial erosion, with steep rock walls and a wide valley bottom creating the common U-shaped form. Cliffs adjacent to the valley are exceptionally high, rising vertically some 800 m, while the nearby peaks of the Weisshorn and Dom sit 3000 m above the town of Randa.

Crystalline rocks of the slide source region belong to the Siviez–Mischabel nappe and primarily include a competent orthogneiss in the lower half of the slope overlain by highly jointed paragneisses and schists (Willenberg et al., 2008a). Foliation dips gently to the west / southwest (into to the slope at the rockslide), cutting across the north-south trending Matter valley.

The site of the 1991 rockslides sits on a nose of rock on the western wall of the Matter valley, which has been significantly incised to the south by the Bis glacier. To the south and west of the eventual rockslides, an older progressive slope instability had developed leaving a noticeable scarp and debris cone. This scarp would eventually be cut by the 1991 rockslides.

The 1991 rockslides at Randa consisted of two separate collapse events on April 18 and May 9, which released in total a cumulative volume of approximately 30 million cubic meters of rock. The elevation of the top of the scarp is 2320 m (7610 ft), while the elevation of deposit toe is 1320 m (4330 ft).

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