Rhéologie des laves torrentielles

Debris flow rheology

CEMAGREF, Division Protection contre les Erosions, Domaine Universitaire, B.P. 76, 38402 St-Martin-d'Hères, France

Abstract

Debris flows are large masses of water, clay, silt, sand, pebbles and boulders which sometimes flow in mountain streams after long or intense rains and can cause much damages to inhabited areas. We assume that these materials can be considered as homogeneous fluids. Usual hydraulic methods do not allow to predict natural flows of these non-Newtonian fluids. To make progress in this way the basic tool is the constitutive equation of the material. In this paper we review our present knowledge in the field of rheological properties of the various possible water-debris mixtures. Thus we demonstrate that it is possible to distinguish two main debris flow types : muddy debris flows, which contain at least a moderate clay fraction (> 10 %) and exhibit simple shear behaviour which may be fitted by a Herschel-Bulkley model ; granular debris flows, which contain a very low clay fraction (< 1 %) and exhibit a simple shear flow curve with a minimum. All these materials exhibit a yield stress (flow threshold) which is a key rheological parameter. We review the main practical techniques to estimate this yield stress.