22 November 2019
Deep-seated gravitational slope deformations: Piz Dora
Deep-seated gravitational slope deformation (DGSD) is one of the most poorly recognised, but most dramatic, types of landslide. Mauro Soldati defined them as follows:-
A deep-seated gravitational slope deformation (DGSD) is a gravity-induced process affecting large portions of slopes evolving over very long periods of time. A DGSD may displace rock volumes of up to hundreds of millions of cubic meters, with thicknesses of up to a few hundred meters…Deep-seated gravitational slope deformations (DGSDs) are not considered hazardous phenomena because they evolve very slowly. However, they must not be neglected when defining slope instability in a territory and the related hazard implications. Despite their slow deformation rates, DGSDs may cause damage to surface and underground (e.g., tunnels) structures. In addition, they may evolve into faster mass movements or favor collateral landslide processes.
A really nice example is described in an article in Engineering Geology (Agliardi et al. 2019) at Piz Dora in Switzerland. This landslide, which is located at 46.601, 10.350, is shown very nicely in the Google Earth image below:-
As the map shows, the entire mountain side is moving on a deep-seated shear surface (or on multiple shear surface), creating a set of scarps and counter scarps high on the hillside. One of the largest ones is seen below:-
Downslope from the scarp is a large displaced mass. Smaller scarps extend across the slope – at the main peak (Piz Dora itself, there is a pin to mark it in the first image) the scarps extend right the way through the crest of the slope:-
The lower part of the mobile slope consists of a series of smaller, but from insignificant, rockslides, mostly covered in forest.
have measured the rate of movement of the Piz Dora deep-seated gravitational slope deformation. They found that, as is usual for this type of landslide, the mass creeps continuously at low rates. Movement rates vary across the landslide, with a maximum velocity in the order of 30 – 100 mm per year.
These types of landslides are very common in high mountain areas. Because they do not generate significant levels of hazard, they are poorly investigated and are rarely reported. However, they are one of the most fascinating types of landslide, and they deserve more attention.
Federico Agliardi, Federico Riva, Marta Barbarano, Stefano Zanchetta, Riccardo Scotti and Andrea Zanchi. 2019. Effects of tectonic structures and long-term seismicity on paraglacial giant slope deformations: Piz Dora (Switzerland). Engineering Geology, 263, 105353. https://doi.org/10.1016/j.enggeo.2019.105353.