Nat. Hazards Earth Syst. Sci., 17, 1207–1220, 2017 https://doi.org/10.5194/nhess-17-1207-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Brief communication: 3-D reconstruction of a collapsed rock pillar from Web-retrieved images and terrestrial lidar data – the 2005 event of the west face of the Drus (Mont Blanc massif) Antoine Guerin1, Antonio Abellán2, Battista Matasci3, Michel Jaboyedoff1, Marc-Henri Derron1, and Ludovic Ravanel4 1Risk Analysis Group, Institute of Earth Sciences, University of Lausanne, Lausanne, Switzerland 2Scott Polar Research Institute, University of Cambridge, Cambridge, UK 3Bureau d’Etudes Géologiques SA, Aproz, Switzerland 4EDYTEM, University Savoie Mont Blanc – CNRS, Le Bourget du Lac, France Correspondence to: Antoine Guerin ([email protected]) Received: 27 September 2016 – Discussion started: 7 October 2016 Revised: 6 June 2017 – Accepted: 12 June 2017 – Published: 18 July 2017 Abstract. In June 2005, a series of major rockfall events 1 Introduction completely wiped out the Bonatti Pillar located in the leg- endary Drus west face (Mont Blanc massif, France). Ter- The Drus (3754 m a.s.l.) is a mountain with emblematic sum- restrial lidar scans of the west face were acquired after this mits of the Chamonix valley situated in the Mont Blanc mas- event, but no pre-event point cloud is available. Thus, in or- sif (France). Since the middle of last century, the Petit Dru der to reconstruct the volume and the shape of the collapsed west face (1000 m high, 3730 m a.s.l.) has been affected by blocks, a 3-D model has been built using photogrammetry intense erosion which has significantly modified the mor- (structure-from-motion (SfM) algorithms) based on 30 pic- phology of this peak (Ravanel and Deline, 2006, 2008; Fort et tures collected on the Web. All these pictures were taken al., 2009). In June 2005, a rock pillar (the Bonatti Pillar) esti- between September 2003 and May 2005. We then recon- mated to be around 265 000 ± 10 000 m3 by Ravanel and De- structed the shape and volume of the fallen compartment by line (2008), collapsed, destroying forever numerous climb- comparing the SfM model with terrestrial lidar data acquired ing routes. The assessment of this volume by Ravanel and in October 2005 and November 2011. The volume is cal- Deline (2008) was performed in two steps: (a) identification culated to 292 680 m3 (±5.6 %). This result is close to the in photos of different rock elements (slabs, dihedrons, over- value previously assessed by Ravanel and Deline (2008) for hangs) whose dimensions (height, width, depth) can be com- this same rock avalanche (265 000 ± 10 000 m3/. The differ- pared with compartments now collapsed and (b) measure- ence between these two estimations can be explained by the ments of these dimensions on terrestrial lidar scans acquired rounded shape of the volume determined by photogramme- just after the event in October 2005. Historical photographs try, which may lead to a volume overestimation. However it of the west face taken from different viewpoints facilitate the is not excluded that the volume calculated by Ravanel and estimation of the thickness of the missing elements, which re- Deline (2008) is slightly underestimated, the thickness of the mains the most difficult dimension to determine. Under this blocks having been assessed manually from historical pho- method, the assessment of rock thickness (8 m on average) tographs. 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. Thus, in order to get the pre-event topography of the Petit Published by Copernicus Publications on behalf of the European Geosciences Union. 1208 A. Guerin et al.: 3-D reconstruction of a collapsed rock pillar Dru west face, we collected several pictures dating from 2003 natural disasters. More specifically, this short note reports the to 2005 from different online picture-hosting services, and a results of the 3-D reconstruction of the Drus west face before 3-D photogrammetric model was reconstructed. Such an ap- the Bonatti Pillar collapse in June 2005. proach has already been used in different research areas, such as cultural heritage conservation: a precursor of this “crowd- Geological and structural setting sourced” technics, Grün et al. (2004, 2005) reproduced in 3- D the statue of the Great Buddha of Bamiyan (Afghanistan) From a geological point of view, the Mont Blanc crystalline using a series of pictures obtained from the Internet. More range describes a broad ellipse elongated in the NE–SW di- recently, many historians, archaeologists and architects (e.g., rection extending from the Val Ferret (Valais, Switzerland) Furukawa et al., 2010; Doulamis et al., 2013; Ioannides et al., to the Chapieux valley (Savoie, France) (Fig. 1a). The cen- 2013; Kyriakaki et al., 2014; Santos et al., 2014) have taken tral part of the massif develops in the Aosta valley (Italy) and advantage of the large amount of images available online to Haute-Savoie (France), and it consists of two major petrolog- preserve and keep a digital record of cultural and histori- ical units: plutonic rocks (granites), mainly, and metamor- cal heritage using structure-from-motion (SfM) algorithms phic rocks (gneiss and mica schists) which merge near the (Snavely et al., 2008). According to the New York Times (Es- summit of Mont Blanc (Fig. 1a and b). From southwest to trin, 2012), over 380 million pictures are uploaded on Face- northeast, granite moves from an intrusive position in gneiss book every day, and other authors such as Stathopoulou et to a tectonic contact materialized by the faille de l’Angle al. (2015) or Vincent et al. (2015) have used crowdsourced (“de l’Angle fault”, Fig. 1b) (Epard, 1990). This fault sep- imagery to virtually replicate heritage objects destroyed by arates the Mont Blanc massif into two sections: an internal natural disasters, armed conflict or terrorism. Examples in- part that is essentially granitic and a more metamorphic ex- clude the stone bridge of Plaka (Greece), the city of Kath- ternal part (Epard, 1990; Steck et al., 2000, 2001). The Petit mandu before and after the 2015 earthquake and several art- Dru west face presents a coarse-grained calc-alkaline granite, works at the Mosul Museum (Iraq). which was formed during the Hercynian orogeny and dated In geosciences, conventional photogrammetry has long from 305 ± 2 million years (Bussy et al., 1989; von Raumer been used for digital elevation model (DEM) generation, but and Bussy, 2004; Egli and Mancktelow, 2013). The steep rock cliff (average dip angle of 75◦) is cut by a set of two it is only recently that SfM has popularized the use of 3-D ◦ ◦ ◦ ◦ point clouds in this field (e.g., Firpo et al., 2011; Salvini et large sub-vertical fractures oriented 238 /85 and 303 /79 which form wedges and by four other joint sets (especially al., 2013; James and Robson, 2014; Lucieer et al., 2014). The ◦ ◦ review conducted by Eltner et al. (2016) shows that the an- 106 /33 ) which form deep overhangs (Ravanel and Deline, nual number of publications that refer to SfM has really ex- 2008; Matasci et al., 2015). These very persistent dihedral ploded since 2014, particularly in the fields of soil erosion, structures (mean trace length of 80 m) promote the collapse glaciology and fluvial morphology. This method is surpris- of large compartments and played a major role (Matasci et ingly straightforward to implement and also relatively ac- al., 2015) during the large rockfall events of summer 2005 curate when compared to other techniques such as ground- and fall 2011 (Fig. 1c). based lidar data. In 2013, Fonstad et al. (2013) obtained dif- ferences of about 0.1 m (in X, Y and Z) between these two 2 Material and methods methods. In addition, new technologies such as unmanned aerial vehicles (UAVs) combined with SfM have modern- The 3-D reconstruction of the Drus west face was carried ized and revolutionized investigations on several Earth sur- out using 30 Web-retrieved images from different picture- face phenomena (Abellán et al., 2016; Smith et al., 2016). For hosting services (Flickr.com, SummitPost.org and Campto- instance, Turner et al. (2012) and Lucieer et al. (2014) ob- camp.org; see Appendix A) and a commercial photogram- tained 4 cm errors when comparing DEMs from UAV-SfM to metric (Agisoft PhotoScan, 2014) software (version 1.0.3). differential Global Positioning System (dGPS) ground con- The georeferencing–alignment procedure of point clouds trol points. In 2016, Bakker and Lane (2016) innovated by was done with CloudCompare (Girardeau-Montaut, 2015) showing the potential to couple archival aerial photographs software (version 2.7.0), and an estimation of the missing and SfM algorithms to quantify morphological changes in volume was then performed using 3DReshaper (Technodigit, a river–floodplain system at a decadal scale. However, de- 2014) software (2014 MR1 version) by comparing the SfM spite all these recent advances, paleotopographic reconstruc- point cloud with terrestrial lidar scans acquired after the tion based on old terrestrial images or orthophotos has rarely event. been used in the field of geohazards to improve erosion rate quantification (Oikonomidis et al., 2016). For this reason, the 2.1 Selection of photographs from the Internet aim of this brief communication is to illustrate the potential to merge ground-based lidar measurements with terrestrial Before the 30 June 2005 rock avalanche, the Drus west SfM point clouds made from publicly available images.