Natural Hazards and Earth System Science www.nat-hazards-earth-syst-sci.net 1561-8633 1684-9981 10 7 2010 10.5194/nhess-10-1635-2010 http://www.nat-hazards-earth-syst-sci.net/10/1635/2010/ http://www.nat-hazards-earth-syst-sci.net/10/1635/2010/nhess-10-1635-2010.html http://www.nat-hazards-earth-syst-sci.net/10/1635/2010/nhess-10-1635-2010.pdf 1635 1645 2010-07-30 Description and analysis of the debris flows occurred during 2008 in the Eastern Pyrenees M. Portilla modesto.portilla@upc.edu G. Chevalier M. Hürlimann Department of Geotechnical Engineering and Geosciences, Technical University of Catalonia, Barcelona, Spain Geosciences Department – Faculty of Sciences, National University of Colombia, Bogotá, Colombia Sediment Transport Research Group, Technical University of Catalonia, Catalonia, Barcelona Rainfall-triggered landslides taking place in the Spanish Eastern Pyrenees have usually been analysed on a regional scale. Most research focussed either on terrain susceptibility or on the characteristics of the critical rainfall, neglecting a detailed analysis of individual events. In contrast to other mountainous regions, research on debris flow has only been performed marginally and associated hazard has mostly been neglected. In this study, five debris flows, which occurred in 2008, are selected; and site specific descriptions and analysis regarding geology, morphology, rainfall data and runout were performed. The results are compared with worldwide data and some conclusions on hazard assessment are presented. The five events can be divided into two in-channel debris flows and three landslide-triggered debris flows. The in-channel generated debris flows exceeded 10 000 m3, which are unusually large mass movements compared to historic events which occurred in the Eastern Pyrenees. In contrast, the other events mobilised total volumes less than 2000 m3. The geomorphologic analysis showed that the studied events emphasize similar patterns when compared to published data focussing on slope angle in the initiation zone or catchment area. Rainfall data revealed that all debris flows were triggered by high intensity-short duration rainstorms during the summer season. Unfortunately, existing rainfall thresholds in the Eastern Pyrenees consider long-lasting rainfall, usually occurring in autumn/winter. Therefore, new thresholds should be established taking into account the rainfall peak intensity in mm/h, which seems to be a much more relevant factor for summer than the event's total precipitation. The runout analysis of the 2008 debris flows confirms the trend that larger volumes generally induce higher mobility. The numerical simulation of the Riu Runer event shows that its dynamic behaviour is well represented by Voellmy fluid rheology. A maximum front velocity of 7 m/s was back-analysed for the transit section and even on the fan velocities larger than 2 m/s were obtained. This preliminary analysis of the major Eastern Pyrenean debris flows represents the first background for future studies. Additional research on other events is necessary to support the results presented herein, and to properly assess and reduce hazard related to debris flows. Aleotti, P.: A warning system for rainfall-induced shallow failures, Eng. Geol., 73, 247–265, 2004 Baeza, C. and Corominas, J.: Assessment of shallow landslide susceptibility by means of multivariate statistical techniques, Earth Surf. Proc. Land., 26, 1251–1263, 2001. Bardou, E.: Methodologie de diagnostic des laves torrentiells sur un bassin versant, Ecole Polytechnique de Lausanne, thèse doctorale, 188~pp., 2002 (in French). Bathurst, J. C., Burton, A., Clarke, B. G., and Gallart, F.: Application of the SHETRAN basin-scale, landslide sediment yield model to the Llobregat basin, Spanish Pyrenees, Hydrol. Process., 20, 3119–3138, 2006. Brochot, S. and Marchi, L.: Les cônes de déjection torrentiels dans les Alpes françaises. Morphométrie et processus de transport solide torrentiel, Revue de géographie alpine, 88, 23–38, 2000 (in French). Coussot, P. and Meunier, M.: Recognition, classification and mechanical description of debris flows, Earth-Sci. Rev., 40, 209–227, 1996. Corominas, J.: The angle of reach as a mobility index for small and large landslides, Can. Geotech. J., 33(2), 260–271, 1996. Corominas, J. and Moya, J.: Reconstructing recent landslide activity in relation to rainfall in the Llobregat River basin, Eastern Pyrenees, Spain, Geomorphology, 30, 79–93, 1999. Corominas, J., Moya, J., and Hürlimann, M.: Landslide rainfall triggers in the Spanish Eastern Pyrenees, Editrice, Mediterranean Storms, Proceedings of the 4th EGS Plinius Conference held at Mallorca, Spain, 2002. Crosta, G.: Regionalization of rainfall thresholds: an aid to landslide hazard evaluation, Environ. Geol., 35, 131–145, 1998. Cuadrat, J. M. and Pita, M. F.: Climatología. Ediciones Cátedra, Madrid, España, 496~pp., 1997. D'Agostino, V. and Marchi, L.: Debris Flow Magnitude in the Eastern Italian Alps: Data Collection and Analysis, Phys. Chem. Earth Pt C, 26(9), 657–663, 2001. Di Crescenzo, G. and Santo, A.: Debris slides-rapid earth flows in the carbonate massifs of the Campania region (Southern Italy): morphological and morphometric data for evaluating triggering susceptibility, Geomorphology, 66, 255–276, 2005. ECORS Pyrenees Team: The ECORS deep reflection seismic survey across the Pyrenees, Nature, London, 331, 508–510, 1988 Fitzgerald, P. G., Muñoz, J. A., Coney, P. J., and Baldwin, S. L.: Asymmetric exhumation across the Pyrenean orogen: implications for the tectonic evolution of a collisional orogen, Earth Planet. Sc. Lett., 173(3), 157–170, 1999. Gallart, F. and Clotet, N.: Some aspects of the geomorphic processes triggered by an extreme rainfall event: The November 1982 flood in The Eastern Pyrenees, Catena Supp., 13, 79–95, 1988. Guzzetti, F., Peruccacci, S., Rossi, M., and Stark, C. P.: Rainfall thresholds for the initiation of landslides in central and southern Europe, Meteorol. Atmos. Phys., 98(3–4) , 239–267, doi:10.1007/s00703-007-0262-7, 2007. Hungr, O.: A model for the run-out analysis of rapid flow slides, debris flows, and avalanches, Can. Geotech. J., 32, 610–623, 1995. Hungr, O., Evans, S. G., Bovis, M. J., and Hutchinson, J. N.: A review of the classification of landslides of the flow type, Environ. Eng. Geosci., 3, 221–238, 2001. Hürlimann, M. and Baeza, C.: Analysis of debris-flow events in the eastern Pyrenees, Spain, 1st European Conference on Landslides, Balkema, Prague, 213–220, 2002. Hürlimann, M., Copons, R., and Altimir, J.: Detailed debris flow hazard assessment in Andorra: A multidisciplinary approach, Geomorphology, 78, 359–372, 2006. Hürlimann, M., Rickenmann, D., Medina, V., and Bateman, A.: Evaluation of approaches to calculate debris-flows parameters for hazard assessment, Eng. Geol., 102, 152–163, 2008. ICC, Institut Cartogràfic de Catalunya: Mapa geològic de Catalunya 1:250 000, Barcelona, 2003. Jackson, L. E., Kostaschuk, R. A., and MacDonald, G. M.: Identification of debris flow hazard on alluvial fans in the Canadian rocky mountains, Eng. Geol., 7, 115–124, 1987. Kostaschuk, R. A., MacDonald, G. M., and Putnam, P. E.: Depositional process and alluvial fan-drainage basin morphometric relationships near Banff, Alberta, Canada, Earth Surf. Proc. Land., 11, 471–484, 1986. Liu, X., Yue, Z. Q., Tham, L. G., and Lee, C. F.: Empirical assessment of debris flow risk on a regional scale in Yunnan Province, southwestern China, Environ. Manage., 30, 249–264, 2002. Lynn, G.: Macrogeomorphology and Erosional History of the Post-Orogenic Pyrenean Mountain Belt, Ph.D thesis, The University of Edinburgh, Edinburgh, 388~pp., 2005 Marco, P. L.: Determinació de llindars de pluja desencadenants d'esllavisades a Catalunya, Tesina de Grau, Departament d'Enginyeria del Terreny, Cartogràfica i Geof\'isica, Universitat Politècnica de Catalunya, Barcelona, Espanya, 150~pp., 2007 (in Catalan). Martín, V. J. and Olcina, C. J.: Climas y Tiempos de España, Alianza Editorial, S.A., Madrid, España, 258~pp., 2001. Melton, M. A.: The morphologic and paleoclimatic significance of alluvial deposits in southern Arizona, J. Geol., 73, 1–38, 1965. Muñoz, J. A.: Evolution of a continental collision belt: ECORS-Pyrenees crustal balanced cross-section, in: Thrust Tectonics, edited by: McClay, K. R., Chapman and Hall, 235–246, 1992. Naef, D., Rickenmann, D., Rutschmann, P., and McArdell, B. W: Comparison of flow resistance relations for debris flows using a one-dimensional finite element simulation model, Nat. Hazards Earth Syst. Sci., 6, 155–165, doi:10.5194/nhess-6-155-2006, 2006. Rickenmann, D.: Empirical relationships for debris flows, Nat. Hazards, 19, 47–77, 1999. Rickenmann, D.: Run-out prediction methods, in: Debris-flow Hazards and Related Phenomena, edited by: Jakob, M. and Hungr, O., Springer, Berlin, 305–324, 2005. Santacana, N., Baeza, B., Corominas, J., Paz, A. D., and Marturiá, J.: A GIS-Based Multivariate Statistical Analysis for Shallow Landslide Susceptibility Mapping in La Pobla de Lillet Area (Eastern Pyrenees, Spain), Nat. Hazards, 30, 281–295, 2003. Teixell, A.: Crustal structure and orogenic material budget in the west-central Pyrenees, Tectonics, 17, 395–406, 1998. VAW: Murgänge 1987: Dokumentation und Analyse, in: Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie (VAW), No 97.6, ETH Zürich, unpublished, 620~pp., 1992. Zimmermann, M., Mani, P. Gamma, P., Gsteiger, P., Heiniger, O., and Hunziker, G.: Murganggefahr und Klimaänderung – ein GIS-basierter Ansatz, in: Schlussbericht NFP~31, vdf-ETH Zürich, Switzerland, 161~pp., 1997 (in German).