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The Illgraben seen from the Illhorn.
The Pfynwald (dark green plain) with the Illgraben (light cut)

The Illgraben is a deep in the soft Triassic dolomite of the Valais Alps incised torrent of Illbachs near the municipality of Leuk . It begins not far from the summit of the Illhorn and flows into the valley of the Rhone , which it has strongly shaped here.

Erosion processes

The usually small, inconspicuous Illbach swells quickly when there is intense precipitation due to showers, thunderstorms or a combination of meltwater and rain and carries a lot of debris with it due to the continually intensive weathering of the rock material . These debris flows occur several times a year and carry the material into the Rhone. When it rains, this leads to the clouding of the Rhone up to its confluence with Lake Geneva .

The brook has cut 1,500 meters into the rock and with the material heaped a mighty cone of rubble in the valley of the Rhone. As a result, it was pressed against the mountain foot opposite over a length of 5 kilometers. The debris cone is piled up to 200 meters above the valley floor . The eastern part of the debris cone is farmed with meadows and fields, while the western part carries the upper part of the Pfynwald . The lower part, on the other hand, is still characterized by the regular mudflows, the material of which is partly washed from the Rhone to Sierre .

Due to the strong erosion, there is no permanent vegetation in the Illgraben.

Protective measures

Torrent control

After the devastating mudslide events in 1961, a comprehensive protection concept was developed and implemented for the village of Susten. This consists of a dam as the main dam, which is supposed to hold back the largest masses of debris, and 29 other smaller dams.

Debris flow warning system

The optical and acoustic warning system at Illgraben warns of debris flows

The Illgraben is continuously monitored by the Swiss Federal Research Institute for Forests, Snow and Landscape (WSL) using precipitation seesaws , geophones , echo sounders , radar and laser measuring devices as well as video cameras . This has made it possible since 2007 to detect floods and mudflows up to 15 minutes in advance. Acoustic and optical warning systems were installed at the three frequently frequented channel crossings (footpaths) .

Bhutan Bridge

Bhutan bridge that spans the Illgraben

The 134-meter-long Bhutan bridge spans the Illgraben in the area where it enters the Rhone valley in the Pfynwald ( Lage ) . It connects the German-speaking Upper Valais with the French-speaking Lower Valais . In addition, the Bhutan Bridge, which is designed in the style of a Bhutanese suspension bridge with prayer flags, is intended to symbolize the bond with the Kingdom of Bhutan , which supported the construction.

Individual evidence

  1. a b eLexikon - Geographical Lexicon of Switzerland, 1902
  2. P. Missbauer, Schweizerische Bauzeitung Volume 89 (1971), retaining wall for the torrent control in the Illgraben, available from the Swiss Electronic Library Service (PDF, 4.9 MB)
  3. Information on the debris flow observation station in Illgraben near Leuk of the Federal Research Institute for Forests, Snow and Landscape (WSL)
  4. Leuctourism - Bhutan Bridge (from web archive)

Web links

Commons : Illgraben  - collection of images, videos and audio files