Evaluation of approaches to calculate debris-flow parameters for hazard assessment Marcel Hürlimann a, ⁎, Dieter Rickenmann b,c , Vicente Medina d , Allen Bateman d a Department of Geotechnical Engineering and Geosciences, Technical University of Catalonia, Jordi Girona 1-3, 08034 Barcelona, Spain b Swiss Federal Research Institute WSL, Mountain Hydrology and Torrents, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland c University of Natural Resources and Applied Life Sciences, Department Civil Engineering and Natural Hazards, Institute of Mountain Risk Engineering, Peter Jordan-Strasse 82, 1190 Vienna, Austria d Sediment Transport Research Group, Department of Engineering Hydraulic, Marine and Environmental Engineering, Technical University of Catalonia, Jordi Girona 1-3, 08034 Barcelona, Spain abstract article info Article history: Accepted 4 March 2008 Available online 19 July 2008 Keywords: Debris flow Runout Hazard assessment Pyrenees Many different runout prediction methods can be applied to estimate the mobility of future debris flows during hazard assessment. The present article reviews the empirical, analytical, simple flow routing and numerical techniques. All these techniques were applied to back-calculate a debris flow, which occurred in 1982 at La Guingueta catchment, in the Eastern Pyrenees. A sensitivity analysis of input parameters was carried out, while special attention was paid to the influence of rheological parameters. We used the Voellmy fluid rheology for our analytical and numerical modelling, since this flow resistance law coincided best with field observations. The simulation results indicated that the “basal” friction coefficients rather affect the runout distance, while the “turbulence” terms mainly influence flow velocity. A comparison of the velocity computed on the fan showed that the analytical model calculated values similar to the numerical ones. The values of our rheological parameters calibrated at La Guingueta agree with data back-calculated for other debris flows. Empirical relationships represent another method to estimate total runout distance. The results confirmed that they contain an important uncertainty and they are strictly valid only for the conditions, which were the basis for their development. With regards to the simple flow routing algorithm, this methods could satisfactorily simulate the total area affected by the 1982 debris flow, but it was not able to directly calculate total runout distance and velocity. Finally, a suggestion on how different runout prediction methods can be applied to generate debris-flow hazard maps is presented. Taking into account the definition of hazard and intensity, the best choice would be to divide the resulting hazard maps into two types: “final hazard maps” and “preliminary hazard maps”. Only the use of numerical models provided final hazard maps, because they could incorporate different event magnitudes and they supplied output-values for intensity calculation. In contrast, empirical relationships and flow routing algorithms, or a combination of both, could be applied to create preliminary hazard maps. The present study only focussed on runout prediction methods. Other necessary tasks to complete the hazard assessment can be looked up in the “Guidelines for landslide susceptibility, hazard and risk zoning” included in this Special Issue. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Besides the probability of occurrence and magnitude of a future debris flow, the determination of its dynamics is one of the most important tasks during hazard assessment (Jakob, 2005; JTC1, 2008-this issue). The total runout distance, the area affected by the event and the energy along the flow path are necessary information for hazard mapping and should be ideally determined by a dynamic method. The selection of the adequate method during runout analysis, however, is a difficult task. The methods available for runout analysis can be divided into different classes, such as empirical, analytical, simple flow routing and numerical ones (Dai et al., 2002; Hungr et al., 2005; Rickenmann, 2005a). Moreover, these techniques can be classified concerning the dimension of the calculation. Herein we make use of the hydraulic definition and the term one-dimensional (1D) for calculations along a previously selected topographic profile. In contrast, two-dimensional (2D) methods determine debris-flow dynamics over an area typically represented by a digital elevation model (DEM). Thus, 1D methods must be extrapolated into two dimensions to obtain a hazard map, while 2D techniques can be used to directly create a hazard map. The present publication complements the article entitled “Guide- lines for landslide susceptibility, hazard and risk zoning” (JTC1, 2008- this issue). Our review and evaluation of different debris-flow runout and intensity calculation methods only deals with one single task during hazard assessment. The main objective would be then to help the experts in charge of a runout analysis to select the appropriate Engineering Geology 102 (2008) 152–163 ⁎ Corresponding author. Tel.: +34 93 401 73 77; fax: +34 93 401 72 51. E-mail address: marcel.hurlimann@upc.edu (M. Hürlimann). 0013-7952/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2008.03.012 Contents lists available at ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo