One of the most dangerous movements in the French Alps is a special type of mudflow sometimes affecting masses of several millions of cubic metres with a high velocity of propagation and large spread if the mudflow is directed by a valley profile.
Forecasting the phenomenon is of great practical importance and for this reason it is necessary to understand the mechanism of formation of this type of mudflow. Generally, these mudflows are observed in larger masses of scree in which there is water circulation. The screes originate from rapidly weathered cliffs of rock. The rocks concerned include sedimentary rocks easily transformed mechanically into silt size particles.
In order to forecast the occurrence of this type of mudflow, the authors have attempted to explain the mechanism of formation of such mudflows in the bed of the River Ravoire (Savoie). One other example is given, for which the above mentioned mechanism seems to be the cause of the mudflows.