Bad Goisern, Stambach Valley, Austria

Zwerchwand rockfall/topple and Stambach mud slide

   

 

Introduction

About forty-five km east of Salzburg, Stambach valley in the Austrian Alps is situated at the southern part of a giant anticlinal system undergoing tectonic uplift. The anticline consists of rocks of the Hallstädter nappe ( Triassic limestones, dolomites and halites-evaporites) with a thickness of about 2000 m. Along the southern wing of the anticline the strongly deformed and easily soluble "Haselgebirge" reaches the surface. This characteristic formation (consisting of a series of lower Mesozoic halites,shales,anhydrites and other evaporites) when in contact with water becomes highly unstable and moves easily. The overlying carbonate blocks ( upper Triassic ) that kind of "float" on it undergo noticeable displacements then (owing to increased pore pressure and to wathering). Uplift has strongly fractured the Triassic units, promoting easy access of meteoric water to the Haselgebirge Formation, and continues gradually to steepen the surface angle of slope. As a result, the valley is subject to frequent slope movement, and dated landslides have occurred for at least 17,000 years.

Many rockfalls and landslides are caused by these two factors:

  1. The lithological situation described above and
  2. Neotectonic movements of the Hallstadt nappe.

Since 1978, displacement of the Haselgebirge Formation has destabilized more than 10 million m3 of rock and generated about 300,000 m3 of slow mudslides, some advancing over 100 m a day. Detected slide planes in the source area lie about 40 m below the surface. More than 60,000 m2 of primary forest has already been destroyed and further movement will threaten villages along the valley floor. Hydrological measurements indicate that the water table is rising by 0.5-1.5 m a year and seismic refraction studies have confirmed a corresponding weakening of the water-laden horizons. Preventive measures taken against new mudslides include artificial drainage of the sliding area and the planting of light trees and bushes with strong roots (e.g. willow and alder). These measures alone are expected only to slow the rate of slope movement, rather than to arrest motion completely.

Investigations including interpretation of geological maps and hydrological data, ground checks and GIS-analysis and areal image and satellite image interpretationwhere undertaken to get a very broad idea of what is going on in this area.