Abstract
Abstract. In June 2005, a series of major rockfall events completely wiped out the Bonatti Pillar located in the legendary Drus west face (Mont Blanc massif, France). Terrestrial lidar scans of the west face were acquired after this event, but no pre-event point cloud is available. Thus, in order to reconstruct the volume and the shape of the collapsed blocks, a 3-D model has been built using photogrammetry (structure-from-motion (SfM) algorithms) based on 30 pictures collected on the Web. All these pictures were taken between September 2003 and May 2005. We then reconstructed the shape and volume of the fallen compartment by comparing the SfM model with terrestrial lidar data acquired in October 2005 and November 2011. The volume is calculated to 292 680 m3 (±5.6 %). This result is close to the value previously assessed by Ravanel and Deline (2008) for this same rock avalanche (265 000 ± 10 000 m3). The difference between these two estimations can be explained by the rounded shape of the volume determined by photogrammetry, which may lead to a volume overestimation. However it is not excluded that the volume calculated by Ravanel and Deline (2008) is slightly underestimated, the thickness of the blocks having been assessed manually from historical photographs.
Highlights
The Drus (3754 m a.s.l.) is a mountain with emblematic summits of the Chamonix valley situated in the Mont Blanc massif (France)
Historical photographs of the west face taken from different viewpoints facilitate the estimation of the thickness of the missing elements, which remains the most difficult dimension to determine
The assessment of rock thickness (8 m on average) represents the greatest source of uncertainty since the height and width of the rock avalanche scar could be very accurately measured based on the October 2005 lidar data. Note that these lidar scans correspond to the oldest reference, and no 3-D model is available before the major event of June 2005
Summary
The Drus (3754 m a.s.l.) is a mountain with emblematic summits of the Chamonix valley situated in the Mont Blanc massif (France). In June 2005, a rock pillar (the Bonatti Pillar) estimated to be around 265 000 ± 10 000 m3 by Ravanel and Deline (2008), collapsed, destroying forever numerous climbing routes The assessment of this volume by Ravanel and Deline (2008) was performed in two steps: (a) identification in photos of different rock elements (slabs, dihedrons, overhangs) whose dimensions (height, width, depth) can be compared with compartments collapsed and (b) measurements of these dimensions on terrestrial lidar scans acquired just after the event in October 2005. Historical photographs of the west face taken from different viewpoints facilitate the estimation of the thickness of the missing elements, which remains the most difficult dimension to determine Under this method, the assessment of rock thickness (8 m on average) represents the greatest source of uncertainty since the height and width of the rock avalanche scar could be very accurately measured based on the October 2005 lidar data. In order to get the pre-event topography of the Petit
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