Debris flow modeling for susceptibility mapping at regional to national scale in Norway

Debris flows and related landslide processes occur in many regions all over Norway and pose a significant hazard to inhabited areas. Within the framework of the development of a national debris flows susceptibility map, we are working on a modeling approach suitable for Norway with a nationwide coverage. The discrimination of source areas is based on an index approach, which includes topographic parameters and hydrological settings. For the runout modeling, we use the Flow-R model (IGAR, University of Lausanne), which is based on combined probabilistic and energetic algorithms for the assessment of the spreading of the flow and maximum runout distances. First results for different test areas have shown that runout distances can be modeled reliably. For the selection of source areas, however, additional factors have to be considered, such as the lithological and quaternary geological setting, in order to accommodate the strong variation in debris flow activity in the different geological, geomorphological and climate regions of Norway. typically with abundant unconsolidated sediments, steep gradients and scarce plant cover (Brunsden 1979). Both unconfined and confined debris flows occur in Norway and must be considered in this susceptibility map. Debris flow modeling at medium scale has been subject of various studies in the last decades (e.g., Zimmermann et al., 1997, Horton et al., 2008, Blahut et al., 2010). For a large-area susceptibility assessment, all areas possibly affected by the debris flows have to be identified. The occurrence of debris flows is controlled by the long-term, more or less constant factors such as relief, geological setting, substratum type, availability of debris, but is also strongly influenced by variable factors such as precipitation, heavy snowmelt events, or lake outbursts. However, these variable factors cannot be considered for a country-scale susceptibility analysis, so the focus is on the constant factors. In this study, we use GIS-based approaches incorporating an index-based detection of the source areas and a simple assessment of the debris flow runout to develop a substantial basis for a debris flow susceptibility assessment all over Norway.