The development of susceptibility maps for debris flows is of primary
importance due to population pressure in hazardous zones. However,
hazard assessment by processbased modelling at a regional scale is
difficult due to the complex nature of the phenomenon, the variability
of local controlling factors, and the uncertainty in modelling parameters.
A regional assessment must consider a simplified approach that is
not highly parameter dependant and that can provide zonation with
minimum data requirements. A distributed empirical model has thus
been developed for regional susceptibility assessments using essentially
a digital elevation model (DEM). The model is called Flow-R for Flow
path assessment of gravitational hazards at a Regional scale (available
free of charge under www.flow-r.org) and has been successfully applied
to different case studies in various countries with variable data
quality. It provides a substantial basis for a preliminary susceptibility
assessment at a regional scale. The model was also found relevant
to assess other natural hazards such as rockfall, snow avalanches
and floods. The model allows for automatic source area delineation,
given user criteria, and for the assessment of the propagation extent
based on various spreading algorithms and simple frictional laws.We
developed a new spreading algorithm, an improved version of Holmgren's
direction algorithm, that is less sensitive to small variations of
the DEM and that is avoiding over-channelization, and so produces
more realistic extents. The choices of the datasets and the algorithms
are open to the user, which makes it compliant for various applications
and dataset availability. Amongst the possible datasets, the DEM
is the only one that is really needed for both the source
area delineation and the propagation assessment; its quality is of
major importance for the results accuracy. We consider a 10m DEM
resolution as a good compromise between processing time and quality
of results. However, valuable results have still been obtained on
the basis of lower quality DEMs with 25m resolution.