An analogue Toma Hill formation model for the Tyrolian Fernpass rockslide

Abstract

The Fernpass rockslide occurred about 4100 years ago and is one of the largest rock slope failures in the Alps, the run-out area being characterized by a hummocky surface with Toma Hills that were formed after the rockslide event. These hills, characterized by a cone-like shape with a flattened top, are typical for many rockslides, and the name BToma^is derived from local dialects in Switzerland and Tyrol, Austria. So far, five hypotheses have been proposed for the formation of Toma Hills, the two relating to glacial activity being outdated meanwhile. These hypotheses cannot explain all features observed on site. Therefore, to investigate one of these hypotheses, a qualitative laboratory-scale analogue groundwater flow model with the size 1.5 × 1 × 0.8 m was used to investigate the contribution of internal erosion by suffosion to forming Toma Hills. From the model, hydrogeological calculations of Darcy velocity and hydraulic conductivity were determined to help understand the present study. The average hydraulic gradient of the experiments was close to that in reality. Hydraulic conductivities in the five experiments were comparable to the calculated field hydraulic conductivity (Bialas and Seelheim equations). A limnokrene like one southwest of Biberwier developed in all experiments. Development of lateral and transversal cracks, water-filled lateral depressions of the material in the model and discharge of finer material implicated that internal erosion is a substantial contribution to the Toma Hill formation. Based on the experimental results, we deduce that Toma Hill formation took between 140 and 580 years.

Timing, drivers and impacts of the historic Masiere di Vedana rock avalanche (Belluno Dolomites, NE Italy)

“…6). The stacked subparallel transverse ridges, much like those noted at Tschirgant (Patzelt, 2012;Dufresne et al, 2016;Ostermann et al, 2017), with slight overrunning of the ridges in front by those behind, indicate slowing down of the moving mass due to longitudinal compression (Nicoletti and Sorriso-Valvo, 1991;Dufresne et al, 2016). Outcrop relationships (Fig.…”

“…Tomas, with likely similar origin, are present in the distal deposits at Fernpass in Tyrol (Prager et al, 2009) and at Flims (von Poschinger and Ruegg, 2012). Recently, More and Wolkersdorfer (2019) proposed for the toma hills at Fernpass an alternative ori- gin from internal erosion by suffusion. However, at Masiere di Vedana the fluvial deposition above the rock avalanche suggests that the suffusion process can be ruled out.…”

“…5e). Such morphological structures are "toma" (Turnau, 1906;Abele, 1974;More and Wolkersdorfer, 2019). They are found in association with some large rockslides and are mainly made of landslide material, in many cases showing a gradation from very comminuted fragments in the outer part to less fractured material at the core (see von Poschinger and Ruegg, 2012; More and Wolkersdorfer, 2019).…”

“…Assessment of the potential for future events is distinctly dependent on knowledge of the conditions under which past failures occurred in the immediate vicinity (Samia et al, 2017). This entails detailed analysis and interpretation of driving factors as well as possible triggers of past events (Eisbacher and Clague, 1984;Nicoletti and Sorriso-Valvo, 1991;Hungr, 2006;Hermanns and Longva, 2012). Bedrock bedding, faults, fractures and other discontinuities predispose a rock mass to fail (Stead and Wolter, 2015).…”

Structural and climate drivers of the historic Masiere di Vedana rock avalanche (Belluno Dolomites, NE Italy)

“…Recently, More and Wolkersdorfer (2019) proposed for the Toma Hills at Fernpass an alternative origin from internal erosion by suffusion. However, at Masiere di Vedana the fluvial deposition above the rock avalanche suggests that suffusion process can be ruled out.…”