Evaluation of a two-phase debris flow model using field data from the Swiss Alps

We compared a new two-phase model (DBF-1D) of debris flow behavior with front velocity and flow depth data from two automated debris flow observation stations in Switzerland. The depth averaged mass, volume and momentum equations are written for the solid and fluid phases, which are coupled by a momentum transfer function. The Voellmy-fluid flow relation is used to calculate the flowing friction of the debris flow, treating the solid phase as a variably saturated Coulomb-type material. The frictional behavior of the fluid phase is described by a Chezy-type relation. To further constrain parameters used in the model, we estimated the momentum exchange coefficient using video analysis of large (D>0.5m) boulders traveling within coarse and muddy debris flows at the Illgraben observation station. Overall, this two phase approach provides a good approximation of the 1-dimensional behavior of debris flows, while taking into account the complex interactions between solids and fluids. Key words: debris flows, two-phase model, Voellmy-fluid, comparison with field data