Randa rockslides

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.

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).

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.

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.

Precursory events noted immediately prior to the April, 1991 rockslide included explosive ruptures of rock slabs and new forceful water discharges from the face (Schindler et al., 1993).

18 April 1991: This primary rockslide event occurred over the span of a few hours time, producing a large steep debris cone and a thick layer of dust over the valley.

A warm period producing ample snow melt occurred in the days prior to the April rockslide, and water could be seen emanating from springs on the rock face.

Work digging through the blockage was begun immediately, but heavy rain and snow melt eventually resulted in flooding of a portion of the town of Randa upstream.

Since 2001, researchers from the Department of Earth Science at the Swiss Federal Institute of Technology (ETH, Zurich) have conducted studies into the cause and nature of the ongoing deformations.