Engineering geomorphology of the widentobel catchment, appenzell and sankt gallen, switzerland. A geomorphologuical inventory system applied to geotechnical appraisal of slope stability

Abstract

The paper presents a method of applied and detailed 1:2000 scale geomorphological and geotechnical surveying, developed in Alpine areas and tested to its engineering consulting value in the Widenbach valley, Appenzell and St. Gallen, Switzerland. The investigation was carried out by a pair of geomorphologists, trained in high relief terrain and well versed in mass movement characteristics. A programme of “Wildbachverbauung” — the controlling of fluviatile erosion by the erection of dams — was already undertaken in the twenties, but this first generation of wooden dams is now being replaced by concrete constructions. The Widenbach valley, situated in the marls and sandstones of the folded molasse zone (untere Susswasserzone) was subjected to glacial scouring and deposition by a branch of the Rhine glacier, transfluencing westwards across the Stosspass, separating the Widenbach valley at 940 m a.s.l. from the Appenzell depression. Postglacially, various earthflows have originated from the instable morainic deposits as well as from the weathered marls and sandstones underlying the glacially oversteepened valley flanks. They have built a two km long earthflow complex filling the Widenbach valley to considerable depth. The townships of Eichberg and Altstätten, situated at the outlet of the system, have since historic times experienced damage to forest and grazing land, houses and roads. In the midstream and upstream valley sections, the Widenbach is flowing on the earthflow morphology, while the downstream part, already constrained by new dams founded in exposed hard rock, is controlled by fluvial processes. The mass movement dynamics of the midstream and upstream sectors make a completely different engineering approach necesssary. When this was made clear by the geomorphological reconnaissance, the lack of data could only be filled by a detailed 1:2000 scale geomorphological site investigation, as time and funds for long-lasting monitoring of the earthflow system were not available. This geomorphological survey contained the following elements: pre-field airphoto-interpretation, mapping of micro-terrainforms, classification of materials, types and order of movement; it placed emphasis on the behaviour of the various discontinuity planes expressed in the terrain.