Rhéologie des laves torrentielles granulaires

The main objective of this paper is to examine the reliability of the phenomenological notion of 'stony debris flow' from a rheological point of view. We demonstrate that, to gain insight into the rheology of particles-fluid mixtures, it is very useful to consider several dimensionless numbers (similitude criteria) defined at the microstructural level (i.e. at the particle level), notably Stokes and Davis numbers. In this paper, we investigate the case where the mixture flow can be regarded as one-phase and granular, which is expected to correspond to the case of fully developed stony debris flows. First, on the basis of a rheometrical analysis, we show that such a mixture does not constitute a simple fluid. We then outline a microstructural theory which accounts for fluid and particle interactions (friction, collision and lubrification contacts). Finally, we present a simple application of this theory by giving the main features of the calculations performed for steady uniform flows along inclined plane : it is shown that (i) the mass flow rate increases linearly with the flow depth, (ii) there exists two possible flow depths for a given mass flow rate, depending on the value of a frictional parameter. All these features are in agreement with the range of our experiments performed on glass beads and sand. / Cet article discute de la notion de lave torrentielle granulaire. Afin de comprendre un peu mieux ce type d'écoulement naturel, on propose d'examiner le matériau mobilisé à l'échelle microstructurelle, c'est-à-dire un mélange d'eau et de particules non colloïdales. A cet effet, on considère plusieurs nombres adimensionnels qui permettent de quantifier les ordres de grandeur des différentes interactions et de les comparer entre eux. On montre que sous certaines conditions, il est possible de considérer le mélange eau-particules à l'échelle macroscopique comme un milieu continu effectif. Les expériences montrent toutefois qu'il ne s'agit pas d'un fluide simple. A partir d'une analyse microstructurelle des interactions, il est possible de déduire la loi de comportement macroscopique du mélange. Nous présentons ensuite quelques applications de notre théorie, notamment dans le cas d'écoulements le long de plan incliné