The Randa Rock Fall in 1991

In May 1991, the Randa rock falls blocked the Matter Valley 10 km downstream of Zermatt. Due to the lake forming upstream of the rock fall deposits, the village of Randa was threatened to be flooded.

The downstream valley was endangered by a possible dam break wave.

Pumps were installed immediately to spill the water over the deposits and thus to stabilise the lake level. However, this measure was not sufficient to prevent the potential lake volume from beeing filled during the snowmelt season.

The VAW was commissioned by the crisis committee to assess the situation and to propose appropriate safety measures. As, by any means, the flooding of Randa was inevitable, the excavation of a crest channel across the coarse deposits promised to be the safest medium-term solution.

As a definitive solution, a 3.6 km long bypass tunnel was completed in 1993. The outlet of this tunnel, located about 30 m above the valley floor, was designed with the help of a hydraulic model.

Similarities can be found between the hydraulic problems related to the Randa rock falls in 1991, the Falli Hoelli earth slide (Canton Fribourg) in 1994 and the Hintersand rock falls (Canton Glarus) in 1996. In all those cases, the Hydraulic Laboratory was involved to support the local authorities.