Large landslides associated with a diapiric fold in Canelles Reservoir (Spanish Pyrenees): Detailed geological–geomorphological mapping, trenching and electrical resistivity imaging Francisco Gutiérrez a, ⁎, Rogelio Linares b , Carles Roqué c , Mario Zarroca b , Domingo Carbonel a , Joan Rosell d , Mateo Gutiérrez a,1 a Departamento de Ciencias de la Tierra, Universidad de Zaragoza, C/. Pedro Cerbuna 12, E-50009 Zaragoza, Spain b Departamento de Geología, Universidad Autónoma de Barcelona, E-08193 Barcelona, Spain c Àrea de Gerodinàmica Externa i Geomorfologia, Universitat de Girona, Campus Montilivi, E-17071 Girona, Spain d C/. de la Penya 3, Àger, E-25691 Lleida, Spain abstract article info Article history: Received 16 October 2014 Received in revised form 13 April 2015 Accepted 15 April 2015 Available online 22 April 2015 Keywords: Rapid landslides Retrodeformation analysis Geophysics Reservoir Impulse water wave Detailed geomorphological–geological mapping in Canelles Reservoir, the Spanish Pyrenees, reveals the presence of several large landslides overlooked in previous cartographic works. One of the slope movements, designated as the Canelles landslide, corresponds to a 40 × 10 6 m 3 translational landslide reactivated in 2006 by a severe decline in the reservoir water level. The geomorphic features mapped in the upper part of the Canelles landslide, including surface ruptures corroborated by electrical resistivity imaging and trenching, indicate multiple displacement episodes pre- vious to the 2006 human-induced event. Consistently, the stratigraphic and structural relationships observed in a trench record at least two displacement events older and larger in magnitude than the 2006 reactivation. The oldest recorded event occurred in the 6th to 7th Centuries and the second in 1262–1679 yr AD. This latter episode might be correlative to the 1373 Ribagorza earthquake (M w 6.2), which caused the reactivation of a landslide and the conse- quent destruction of a village in the adjacent valley. The available data indicate that over more than one millennium the kinematics of the landslide has been characterised by discrete small-displacement episodes. These data, together with the available literature on rapid rockslides, do not concur with the acceleration predicted by modelling in a pre- vious investigation, which foresees a speed of 16 m s -1 despite the low average dip of the sliding surface (9–10°). This case study illustrates that the trenching technique may provide valuable practical information on the past be- haviour of landslides, covering a much broader time span than instrumental and historical records. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Landslides are one of the main geomorphological problems in build- ing and maintaining dams and reservoirs. Valley constrictions are fre- quently searched for dam sites, but these may correspond to narrowings related to undetected landslides (Schuster, 1979; Záruba and Mencl, 1982; Ramírez-Reynaga, 1998; Schuster, 2006; Ertunç and Çetin, 2007). Schuster (2006) compiled 254 case histories of large dams (at least 10 m high) that have interacted directly with landslides. According to this thorough review, a great portion of the slope move- ments that have impinged on dams correspond to reactivations of pre-existing undetected landslides. This fact reveals the importance of producing sound geomorphological maps in the earliest stages of the dam projects. Geological maps, which focus on the bedrock geology, fre- quently overlook large landslides that are usually represented as zones of local structural complexity. This may have a detrimental impact on the quality of landslide hazard assessments and geological maps (e.g. erroneous structures and strike and dip data; McCalpin, 2008). Large landslides affecting the slopes of reservoirs, which may under- go reactivations induced by the impoundment of the basin and the sub- sequent water level changes (Jones et al., 1961; Schuster, 1979; Millet et al., 1992; Wu et al., 2001; Liu et al., 2004; Wang et al., 2004, 2005, 2007), may generate problems and hazardous situations, including: (1) damage on human structures that may lead to the relocation of vil- lages (Deng et al., 2000; Wu et al., 2001; Liu et al., 2004; Qi et al., 2006); (2) reduction of the storage capacity of reservoirs; (3) creation of a nat- ural landslide dam upstream of an artificial dam; and (4) generation of impulse water waves by extremely rapid landslides, which may cause catastrophic flooding in reservoir shores and downstream of an artificial lake if a dam is overtopped or breached. In 1959 and 1963, landslide- generated water waves in the nearby Pontesei and Vaoint Reservoirs, Italy, killed 1 and around 2000 people, respectively (Panizzo et al., 2005). Geomorphology 241 (2015) 224–242 ⁎ Corresponding author. Tel: +34 976 761090; fax: +34 976 761106. E-mail address: fgutier@unizar.es (F. Gutiérrez). 1 Tel.: +34 976 761090. http://dx.doi.org/10.1016/j.geomorph.2015.04.016 0169-555X/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph