Triggering conditions and mobility of debris flows associated to complex earthflows J.-P. Malet a, * , D. Laigle b , A. Remaı ˆtre a , O. Maquaire a a School and Observatory of Earth Sciences, Institute of Global Physics, UMR 7516 CNRS-ULP, 5 rue Rene ´ Descartes, F-67084 Strasbourg Cedex, France b Cemagref - Snow Avalanche and Torrent Control Research Unit, B.P. 76, 2, rue de la Papeterie, F-38042, Saint-Martin d’He `res, France Received 9 December 2002; received in revised form 28 February 2004; accepted 15 September 2004 Available online 19 November 2004 Abstract Landslides on black marl slopes of the French Alps are, in most cases, complex catastrophic failures in which the initial structural slides transform into slow-moving earthflows. Under specific hydrological conditions, these earthflows can transform into debris flows. Due to their sediment volume and their high mobility, debris flow induced by landslides are far much dangerous than these resulting from continuous erosive processes. A fundamental point to correctly delineate the area exposed to debris flows on the alluvial fans is therefore to understand why and how some earthflows transform into debris flow while most of them stabilize. In this paper, a case of transformation from earthflow to debris flow is presented and analysed. An approach combining geomorphology, hydrology, geotechnics and rheology is adopted to model the debris flow initiation (failure stage) and its runout (postfailure stage). Using the Super-Sauze earthflow (Alpes-de-Haute-Provence, France) as a case study, the objective is to characterize the hydrological and mechanical conditions leading to debris flow initiation in such cohesive material. Results show a very good agreement between the observed runout distances and these calculated using the debris flow modeling code Cemagref 1-D. The deposit thickness in the depositional area and the velocities of the debris flows are also well reproduced. Furthermore, a dynamic slope stability analysis shows that conditions in the debris source area under average pore water pressures and moisture contents are close to failure. A small excess of water can therefore initiate failure. Seepage analysis is used to estimate the volume of debris that can be released for several hydroclimatic conditions. The failed volumes are then introduced in the Cemagref 1-D runout code to propose debris flow hazard scenarios. Results show that clayey earthflow can transform under 5-year return period rainfall conditions into 1-km runout debris flow of volumes ranging between 2000 to 5000 m 3 . Slope failures induced by 25-year return period rainfall can trigger large debris flow events (30,000 to 50,000 m 3 ) that can reach the alluvial fan and cause damage. D 2004 Elsevier B.V. All rights reserved. Keywords: Earthflow; Debris flow; Modeling; Rheology; Runout; Hazard assessment 0169-555X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2004.09.014 * Corresponding author. Now at: Faculty of Geosciences, University of Utrecht, The Netherlands. E-mail address: J.Malet@geog.uu.nl (J.-P. Malet). Geomorphology 66 (2005) 215 – 235 www.elsevier.com/locate/geomorph