Numerical simulation and field observation of debris roll waves in the Illgraben torrent, Switzerland.

In natural debris flows, even if the critical Froude number is almost always exceeded, the formation of waves, which eventually develop into surges, is not always observed. This contribution numerically represent a debris flow event occurred in the Illgraben torrent, Swiss Alps, which consisted of a single surge and of about 28 roll waves on the surge tail. Simulations are carried out with two 1-D codes based on Shallow-Water equations and a wide variety of rheological laws (Herschel Bulkley, Bingham, Dilatant Inertial, Generalised Visco-Plastic, Voellmy). The analysis of flow depth in time, maximum flow depths and velocities along the debris channel shows that a good representation of the main debris features can be obtained with a proper calibration of rheological pa-rameters. More difficult appears to be the representation of flow tail instabilities. The causes of debris wave development based on literature and interpretation of the model results are exam-ined and an original interpretation is provided.