Quantitative spatial analysis of rockfalls from road inventories: a combined statistical and physical susceptibility model

Abstract. Quantitative spatial analyses and statistical susceptibility assessments based on road inventories are often complicated due to the registration of impacts instead of source areas. A rockfall inventory from the Norwegian Directorate of Public Roads is analysed spatially in order to investigate potential controlling parameters in the Norwegian county Sogn and Fjordane. Quantitative spatial relationships are then used to model rockfall susceptibility with the help of the Weights-of-Evidence method. The controlling parameters tectono-stratigraphic position, quaternary geology, geological lineament density, relative relief and slope aspect resulted in the best performing model and thus yielded the basis for the statistical susceptibility map for the entire county of Sogn and Fjordane. Due to registered impacts instead of sources, the important parameter slope angle could not be included in the statistical models. Combining the statistical susceptibility model with a physically based model, restricts the susceptibility map to areas that are steep enough to represent a potential rockfall source. This combination makes it possible to use road inventories, with registered impacts instead of sources, for susceptibility modelling.