Comparison of different techniques to analyse the mobility of debris flows during hazard assessment - Case study in La Comella catchment, Andorra

(ENG) There are many different techniques to estimate the mobility of future debris flows during the runout analysis of a hazard assessment. We compared the usefulness and efficiency of empirical, analytical, probabilistic and numerical methods in the torrent catchment of La Comella, Principality of Andorra. Special attention was given to the results obtained on the densely urbanised debris fan, where the hazard assessment was of extraordinary interest for the urban planning. The comparison of the different runout prediction methods showed that only numerical modelling can provide debris-flow intensity, which is necessary information to determine hazard degree. Other techniques such as empirical relationships can only estimate runout distance directly, but not intensity. However, these relationships may supply preliminary hazard maps with much less effort. The hazard maps created by two-dimensional techniques showed the strong influence of the topographic quality of the digital elevation model. That's why the realization of a geomorphologic study is strongly recommended, in particular on the fan.