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SEDRUN LANDSLIDE (Swiss Alps)

1. Description of the site

The North Slope on the left side of the high Vorderrhein Valley and of the Urseren Valley between Realp and Andermatt is marked by a series of rock slope instabilities of a very peculiar type. Among these landslides in a broad sense, the zone of Cuolm da Vi above the touristic resort of Sedrun (Vorderrhein Valley, Eastern Switzerland) presents a high level of activity since some thirty years. The movements reach quite worrying values from 1983 on (up to 60 cm/year). The unstable rock mass extends over an area of approximately 1 km², between El. 2458 and 1700 m. Its base is drained by a temporary torrent with a very steep river bed which crosses the village of Sedrun.

2. Geology, investigations and monitoring

The related instability phenomenon affects the geological zone of Urseren that is the metamorphic permo-triassic cover of the cristalline Aar Massif. The rock layers are set vertically there, so that they are very exposed to the toppling mechanism under the effect of gravity. The instability process results from a more or less marked evolution of the toppling mechanism, which evolves into real rockslides. The progressive inclination of the layers in the downstream direction induces "uphill facing scarp" type morphologies with sometimes open "graben" which have induced some geologists to believe that it was an active alpine tectonic process, as this zone is located between the two most seismic areas in Switzerland.

Geodetic investigations have been carried out to confirm such an hypothesis, within the design phase of the base Gotthard Tunnel, as a part of a research financed by the Swiss National Science Foundation (NSF).

The Cuolm da Vi site has been first studied by a local geological firm (voluminous report in 1991), then within the National Research Programme of the NSF, between 1992 and 1995. Numerous determinations of movements by GPS, geodetic and photogrammetric studies have been done by several administrations and scientific institutions, showing displacements of several meters in some years.

3. Risks and present impact of the landslide

The danger for the resort of Sedrun does not result directly from the movements of the unstable mass, as it does not dominate directly the village, but from the very important sediment flows in the Drun Torrent. The formation of a dam in its stream by a localised slide at the toe of the large instability phenomenon, followed by its sudden failure, would have dramatic consequences: destruction of the regional railway line and of the national road connecting the Rhine and Rhone Valleys, and devastation of the centre of the touristic resort of Sedrun.

Engineering protection measures of this stream should be taken soon; unfortunately the misjudgement on the exact nature of the phenomenon has delayed a proper consciousness of the dangers. The exact and complete nature of this peculiar rockslide mechanism is not yet fully established, so that the real importance of the risks induced by the measured movements has still to be determined; thus the local authorities will be in a position to take appropriate measures like warning systems or hazard zonation.