25 February 2016

The Vajont Slide: A new event chronology and the importance of geomorphology

Posted by Dave Petley

The Vajont Slide: A new event chronology and the importance of geomorphology

By Andrea Wolter, ETH Zurich

The Vajont Slide has been studied for over 60 years, yet many open questions on the initiation, mechanics, and kinematics of the landslide remain. In our new new paper (Wolter et al. 2015) we have attempted to answer some of these questions using a multi-faceted approach focused on the geological and geomorphological context of this infamous event.  This is an aerial image of the landslide, taken in 1964.  Note the dam in the upper left of the image.  The huge landslide is on the southern side of the valley.  The stripping of vegetation on the northern side gives an impression of the size of the displacement wave generated by the Vajont slide.  It is this displacement wave that caused over 2000 fatalities in the valley below the dam.

Vajont slide

Aerial photograph of the Vajont slide in 1964 (headscarp in South), courtesy of the Friuli-Venezia-Giulia region.


Highlights of our paper includes the first detailed engineering geomorphology maps, pre- and post-event, of the Vajont Slide, allowing:

  • the morphometric characterisation of surface features such as benches, tension cracks, ridges, and depressions;
  • the identification of active processes over time, including gully incision, fluvial erosion, and shallow slope failures;
  • the interaction between structural and geomorphic features; and
  • the delineation of landslide blocks or compartments within the landslide deposits.

The discussion of the origin of the Vajont Gorge just downstream of the landslide, in which we show that there is evidence that suggests this important knickpoint originated due to slope deformation.  In the paper we emphasise the importance of spatial and temporal damage within the prefailure rockmass, and in particular we show that:

  • zones of the rock mass have different mechanical properties, the eastern zone being more damaged than the western zone;
  • damage related to pre-existing faults as well as progressive weakening through geomorphic and internal deformation processes; and
  • the Vajont Slide is structurally controlled, occurring at the intersection of two synclines and bounded by two faults.

All of this allows us to draw together a chronology of events for the Vajont Slide, which is summarised in the figure below:

Vajont slide

Hypothesised schematic chronology of events leading up to the catastrophic Vajont Slide.

Thus, we suggest that he catastrophic event, likely triggered by repeated and rapid reservoir drawdown, was the culmination of slow gravitational slope deformation.

In the paper we also think through the key question of why the landslide occurred at this location.  We propose that there was a unique combination of factors, including structural predisposition (folding and faulting), lithological units (weak clays vs. rock mass), glacial erosion and possible debuttressing, fluvial undercutting, weathering, and reservoir levels.

The broader implications of our paper are:

  • geomorphic and tectonic processes often act in concert to damage rock slopes and bring them to failure;
  • engineering geomorphology, combined with other techniques such as rock mass characterisation, remote sensing, and numerical modelling, is a useful tool to analyse processes influencing rock slope stability and the evolution of slope instabilities; and
  • the geological and geomorphological context of a landslide is a critical consideration in forensic and predictive investigations, allowing the examination of conditioning processes and location.

Wolter, A., Stead, D., Ward, B.C., Clague, J.J., Ghirotti, M. (2015) Engineering geomorphological characterisation of the Vajont Slide, Italy, and a new interpretation of the chronology and evolution of the landslide. Landslides. DOI: 10.1007/s10346-015-0668-0.